Produced by Sue Asscher
Coral Reefs, Volcanic Islands, South American Geology
by Charles Darwin
EDITORIAL NOTE
Although in some respects more technical in their subjects and style
than Darwin’s “Journal,” the books here reprinted will never lose their
value and interest for the originality of the observations they
contain. Many parts of them are admirably adapted for giving an insight
into problems regarding the structure and changes of the earth’s
surface, and in fact they form a charming introduction to physical
geology and physiography in their application to special domains. The
books themselves cannot be obtained for many times the price of the
present volume, and both the general reader, who desires to know more
of Darwin’s work, and the student of geology, who naturally wishes to
know how a master mind reasoned on most important geological subjects,
will be glad of the opportunity of possessing them in a convenient and
cheap form.
The three introductions, which my friend Professor Judd has kindly
furnished, give critical and historical information which makes this
edition of special value.
G.T.B.
CONTENTS
THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
CRITICAL INTRODUCTION
INTRODUCTION
Chapter I—ATOLLS OR LAGOON-ISLANDS.
_Section I_—DESCRIPTION OF KEELING ATOLL.
Corals on the outer margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.
_Section II_—GENERAL DESCRIPTION OF ATOLLS. General form and size of
atolls, their reefs and islets.—External slope.—Zone of
Nulliporæ.—Conglomerate.—Depth of lagoons.—Sediment.—Reefs submerged
wholly or in part.—Breaches in the reef.—Ledge-formed shores round
certain lagoons.—Conversion of lagoons into land.
_Section III_—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK.
Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
depths in the lagoons of the southern atolls.—Reefs in the lagoons all
rising to the surface.—Position of islets and breaches in the reefs,
with respect to the prevalent winds and action of the
waves.—Destruction of islets.—Connection in the position and submarine
foundation of distinct atolls.—The apparent disseverment of large
atolls.—The Great Chagos Bank.—Its submerged condition and
extraordinary structure.
Chapter II—BARRIER REEFS.
Closely resemble in general form and structure atoll-reefs.—Width and
depth of the lagoon-channels.—Breaches through the reef in front of
valleys, and generally on the leeward side.—Checks to the filling up of
the lagoon-channels.—Size and constitution of the encircled
islands.—Number of islands within the same reef.—Barrier-reefs of New
Caledonia and Australia.—Position of the reef relative to the slope of
the adjoining land.—Probable great thickness of barrier-reefs.
Chapter III—FRINGING OR SHORE-REEFS.
Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
up.—Currents of water formed within it.—Upraised reefs.—Narrow
fringing-reefs in deep seas.—Reefs on the coast of E. Africa and of
Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
and on worn-down islands.—Fringing-reefs affected by currents of the
sea.—Coral coating the bottom of the sea, but not forming reefs.
Chapter IV—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.
_Section I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.
_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS.
_Section III_—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
Chapter V—THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS.
The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment.—Immense areas interspersed with
atolls.—Recent changes in their state.—The origin of barrier-reefs and
of atolls.—Their relative forms.—The step-formed ledges and walls round
the shores of some lagoons.—The ring-formed reefs of the Maldiva
atolls.—The submerged condition of parts or of the whole of some
annular reefs.—The disseverment of large atolls.—The union of atolls by
linear reefs.—The Great Chagos Bank.—Objections, from the area and
amount of subsidence required by the theory, considered.—The probable
composition of the lower parts of atolls.
Chapter VI—ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION.
Description of the coloured map.—Proximity of atolls and barrier-
reefs.—Relation in form and position of atolls with ordinary
islands.—Direct evidence of subsidence difficult to be detected.—Proofs
of recent elevation where fringing-reefs occur.—Oscillations of
level.—Absence of active volcanoes in the areas of
subsidence.—Immensity of the areas which have been elevated and have
subsided.—Their relation to the present distribution of the land.—Areas
of subsidence elongated, their intersection and alternation with those
of elevation.—Amount and slow rate of the subsidence.—Recapitulation.
Appendix
Containing a detailed description of the reefs and islands in Plate
III.
GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
CRITICAL INTRODUCTION
Chapter I—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series.—A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent.—Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriæ.—Ancient and
obliterated orifices of eruption of small size.—Difficulty of tracing
over a bare plain recent streams of lava.—Inland hills of more ancient
volcanic rock.—Decomposed olivine in large masses.—Feldspathic rocks
beneath the upper crystalline basaltic strata.—Uniform structure and
form of the more ancient volcanic hills.—Form of the valleys near the
coast.—Conglomerate now forming on the sea beach.
Chapter II—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.
FERNANDO NORONHA.—Precipitous hill of phonolite. TERCEIRA.—Trachytic
rocks: their singular decomposition by steam of high temperature.
TAHITI.—Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype. MAURITIUS.—Proofs of its recent
elevation.—Structure of its more ancient mountains; similarity with St.
Jago. ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular
mineralogical composition.
Chapter III—ASCENSION.
Basaltic lavas.—Numerous craters truncated on the same side.—Singular
structure of volcanic bombs.—Aeriform explosions.—Ejected granite
fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
formation.—Concretions in pumiceous tuff.—Calcareous deposits and
frondescent incrustations on the coast.—Remarkable laminated beds,
alternating with, and passing into obsidian.—Origin of
obsidian.—Lamination of volcanic rocks.
Chapter IV—ST. HELENA.
Lavas of the feldspathic, basaltic, and submarine series.—Section of
Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous
Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge
and a parapet.—Cones of phonolite.—Superficial beds of calcareous
sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the
land.—Denudation.—Craters of elevation.
Chapter V—GALAPAGOS ARCHIPELAGO.
Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
basaltic craters, with hollows at their bases.—Albemarle Island; fluid
lavas, their composition.—Craters of tuff; inclination of their
exterior diverging strata, and structure of their interior converging
strata.—James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected
fragments.—Concluding remarks on the craters of tuff, and on the
breached condition of their southern sides.—Mineralogical composition
of the rocks of the archipelago.—Elevation of the land.—Direction of
the fissures of eruption.
Chapter VI—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.
The sinking of crystals in fluid lava.—Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation.—Obsidian.—Apparent non-separation of the elements of
plutonic rocks.—Origin of trap-dikes in the plutonic
series.—Distribution of volcanic islands; their prevalence in the great
oceans.—They are generally arranged in lines.—The central volcanoes of
Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
volcanic islands, and their elevation in mass.—Eruptions on parallel
lines of fissure within the same geological period.
Chapter VII—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.
New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
Land.—Palæozoic formation.—Newer formation with volcanic
rocks.—Travertin with leaves of extinct plants.—Elevation of the
land.—New Zealand.—King George’s Sound.—Superficial ferruginous
beds.—Superficial calcareous deposits, with casts of branches; its
origin from drifted particles of shells and corals.—Their extent.—Cape
of Good Hope.—Junction of the granite and clay-slate.—Sandstone
formation.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.
CRITICAL INTRODUCTION
Chapter I—ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.
Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.—Step-formed plains of Patagonia, with upraised
shells.—Terrace-bounded valley of Santa Cruz, formerly a
sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of
the elevated area.—Equability of the movements, as shown by the similar
heights of the plains.—Slowness of the elevatory process.—Mode of
formation of the step-formed plains.—Summary.—Great shingle formation
of Patagonia; its extent, origin, and distribution.—Formation of
sea-cliffs.
Chapter II—ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.
Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions
of the inhabitants on this subject.—Concepcion, earthquake and
elevation of.—VALPARAISO, great elevation of, upraised shells, earth or
marine origin, gradual rise of the land within the historical
period.—COQUIMBO, elevation of, in recent times; terraces of marine
origin, their inclination, their escarpments not horizontal.—Guasco,
gravel terraces of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique,
and Arica.—Lima, shell-beds and sea-beach on San Lorenzo.—Human
remains, fossil earthenware, earthquake debacle, recent subsidence.—On
the decay of upraised shells.—General summary.
Chapter III—ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL
DEPOSITS.
Basin-like plains of Chile; their drainage, their marine origin.—Marks
of sea-action on the eastern flanks of the Cordillera.—Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin.—Boulders in the valley of
Cachapual.—Horizontal elevation of the Cordillera.—Formation of
valleys.—Boulders moved by earthquake-waves.—Saline superficial
deposits.—Bed of nitrate of soda at Iquique.—Saline
incrustations.—Salt-lakes of La Plata and Patagonia; purity of the
salt; its origin.
Chapter IV—ON THE FORMATIONS OF THE PAMPAS.
Mineralogical constitution.—Microscopical structure.—Buenos Ayres,
shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—S.
Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta,
shells, bones, and infusoria of; co-existence of the recent shells and
extinct mammifers.—Buenos Ayres to St. Fe.—Skeletons of
Mastodon.—Infusoria.—Inferior marine tertiary strata, their
age.—Horse’s tooth. BANDA ORIENTAL.—Superficial Pampean
formation.—Inferior tertiary strata, variation of, connected with
volcanic action; Macrauchenia Patachonica at S. Julian in Patagonia,
age of, subsequent to living mollusca and to the erratic block period.
SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of
sediment.—Estuary origin.—Contemporaneous with existing
mollusca.—Relations to underlying tertiary strata. Ancient deposit of
estuary origin.—Elevation and successive deposition of the Pampean
formation.—Number and state of the remains of mammifers; their
habitation, food, extinction, and range.—Conclusion.—Supplement on the
thickness of the Pampean formation.—Localities in Pampas at which
mammiferous remains have been found.
Chapter V—ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.
Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P.
Gallegos.—Eastern Tierra del Fuego; leaves of extinct
beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary
formations of the Western Coast.—Chonos and Chiloe groups, volcanic
rocks of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age of the tertiary
formations.—Lines of elevation.—Silicified wood.—Comparative ranges of
the extinct and living mollusca on the West Coast of S.
America.—Climate of the tertiary period.—On the causes of the absence
of recent conchiferous deposits on the coasts of South America.—On the
contemporaneous deposition and preservation of sedimentary formations.
Chapter VI—PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of
foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.
Ventana.—Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes.—Falkland Islands, palæozoic fossils
of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
successive formation of.—Central and Northern Chile.—Concluding remarks
on cleavage and foliation.—Their close analogy and similar
origin.—Stratification of metamorphic schists.—Foliation of intrusive
rocks.—Relation of cleavage and foliation to the lines of tension
during metamorphosis.
Chapter VII—CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
Central Chile.—Basal formations of the Cordillera.—Origin of the
porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section
of the Cordillera by the Peuquenes or Portillo Pass.—Great gypseous
formation.—Peuquenes line; thickness of strata, fossils of.—Portillo
line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks
of.—Concluding remarks on the denudation and elevation of the Portillo
line.—Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous
strata.—Section near the Puente del Inca; fossils of.—Great
subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the
Uspallata chain.—Structure and nature of the strata.—Silicified
vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding
remarks on the Uspallata range; origin subsequent to that of the main
Cordillera; two periods of subsidence; comparison with the Portillo
chain.
Chapter VIII—NORTHERN CHILE.—CONCLUSION.
Section from Illapel to Combarbala; gypseous formation with silicified
wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley;
fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas,
silicified wood.—Conglomerates, nature of former land, fossils,
thickness of strata, great subsidence.—Valley of Despoblado, fossils,
tufaceous deposit, complicated dislocations of.—Relations between
ancient orifices of eruption and subsequent axes of injection.—Iquique,
Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the
porphyritic conglomerate and gypseous formations.—Great subsidence with
partial elevations during the cretaceo-oolitic period.—On the elevation
and structure of the Cordillera.—Recapitulation on the tertiary
series.—Relation between movements of subsidence and volcanic
action.—Pampean formation.—Recent elevatory movements.—Long-continued
volcanic action in the Cordillera.—Conclusion.
Index to “Coral-Reefs”
Index to “Volcanic Islands”
Index to “South American Observations”
THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
CRITICAL INTRODUCTION
A scientific discovery is the outcome of an interesting process of
evolution in the mind of its author. When we are able to detect the
germs of thought in which such a discovery has originated, and to trace
the successive stages of the reasoning by which the crude idea has
developed into an epoch-making book, we have the materials for
reconstructing an important chapter of scientific history. Such a
contribution to the story of the “making of science” may be furnished
in respect to Darwin’s famous theory of coral-reefs, and the clearly
reasoned treatise in which it was first fully set forth.
The subject of corals and coral-reefs is one concerning which much
popular misconception has always prevailed. The misleading comparison
of coral-rock with the combs of bees and the nests of wasps is perhaps
responsible for much of this misunderstanding; one writer has indeed
described a coral-reef as being “built by fishes by means of their
teeth.” Scarcely less misleading, however, are the references we so
frequently meet with, both in prose and verse, to the “skill,”
“industry,” and “perseverance” of the “coral-insect” in “building” his
“home.” As well might we praise men for their cleverness in making
their own skeletons, and laud their assiduity in filling churchyards
with the same. The polyps and other organisms, whose remains accumulate
to form a coral-reef, simply live and perform their natural functions,
and then die, leaving behind them, in the natural course of events, the
hard calcareous portions of their structures to add to the growing
reef.
While the forms of coral-reefs and coral-islands are sometimes very
remarkable and worthy of attentive study, there is no ground, it need
scarcely be added, for the suggestion that they afford proofs of design
on the part of the living builders, or that, in the
words of Flinders, they constitute breastworks, defending the workshops
from whence “infant colonies might be safely sent forth.”
It was not till the beginning of the present century that travellers
like Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others,
began to collect accurate details concerning the forms and structure of
coral-masses, and to make such observations on the habits of
reef-forming polyps, as might serve as a basis for safe reasoning
concerning the origin of coral-reefs and islands. In the second volume
of Lyell’s “Principles of Geology,” published in 1832, the final
chapter gives an admirable summary of all that was then known on the
subject. At that time, the ring-form of the atolls was almost
universally regarded as a proof that they had grown up on submerged
volcanic craters; and Lyell gave his powerful support to that theory.
Charles Darwin was never tired of acknowledging his indebtedness to
Lyell. In dedicating to his friend the second edition of his
“Naturalist’s Voyage Round the World,” Darwin writes that he does so
“with grateful pleasure, as an acknowledgment that the chief part of
whatever scientific merit this journal and the other works of the
author may possess, has been derived from studying the well-known and
admirable ‘Principles of Geology.’”
The second volume of Lyell’s “Principles” appeared after Darwin had
left England; but it was doubtless sent on to him without delay by his
faithful friend and correspondent, Professor Henslow. It appears to
have reached Darwin at a most opportune moment, while, in fact, he was
studying the striking evidences of slow and long-continued, but often
interrupted movement on the west coast of South America. Darwin’s acute
mind could not fail to detect the weakness of the then prevalent theory
concerning the origin of the ring-shaped atolls—and the difficulty
which he found in accepting the volcanic theory, as an explanation of
the phenomena of coral-reefs, is well set forth in his book.
In an interesting fragment of autobiography, Darwin has given us a very
clear account of the way in which the leading idea of the theory of
coral-reefs originated in his mind; he writes, “No other work of mine
was begun in so deductive a spirit as this, for the whole theory was
thought out on the west coast of South America, before I had seen a
true coral-reef. I had therefore only to verify and extend my views by
a careful examination of living reefs. But it should be observed that I
had during the two previous years been incessantly attending to the
effects on the
shores of South America of the intermittent elevation of the land,
together with the denudation and deposition of sediment. This
necessarily led me to reflect much on the effects of subsidence, and it
was easy to replace in imagination the continued deposition of sediment
by the upward growth of corals. To do this was to form my theory of the
formation of barrier-reefs and atolls.”
On her homeward voyage, the _Beagle_ visited Tahiti, Australia, and
some of the coral-islands in the Indian Ocean, and Darwin had an
opportunity of testing and verifying the conclusion at which he had
arrived by studying the statements of other observers.
I well recollect a remarkable conversation I had with Darwin, shortly
after the death of Lyell. With characteristic modesty, he told me that
he never fully realised the importance of his theory of coral-reefs
till he had an opportunity of discussing it with Lyell, shortly after
the return of the _Beagle_. Lyell, on receiving from the lips of its
author a sketch of the new theory, was so overcome with delight that he
danced about and threw himself into the wildest contortions, as was his
manner when excessively pleased. He wrote shortly afterwards to Darwin
as follows:—“I could think of nothing for days after your lesson on
coral-reefs, but of the tops of submerged continents. It is all true,
but do not flatter yourself that you will be believed till you are
growing bald like me, with hard work and vexation at the incredulity of
the world.” On May 24th, 1837, Lyell wrote to Sir John Herschel as
follows:—“I am very full of Darwin’s new theory of coral-islands, and
have urged Whewell to make him read it at our next meeting. I must give
up my volcanic crater forever, though it cost me a pang at first, for
it accounted for so much.” Dr. Whewell was president of the Geological
Society at the time, and on May 31st, 1837, Darwin read a paper
entitled “On Certain Areas of Elevation and Subsidence in the Pacific
and Indian oceans, as deduced from the Study of Coral Formations,” an
abstract of which appeared in the second volume of the Society’s
proceedings.
It was about this time that Darwin, having settled himself in lodgings
at Great Marlborough Street, commenced the writing of his book on
“Coral-Reefs.” Many delays from ill-health and the interruption of
other work, caused the progress to be slow, and his journal speaks of
“recommencing” the subject in February 1839, shortly after his
marriage, and again in October of the same year. In July 1841, he
states that he began once more “after more than thirteen month’s
interval,” and the last proof-sheet of
the book was not corrected till May 6th, 1842. Darwin writes in his
autobiography, “This book, though a small one, cost me twenty months of
hard work, as I had to read every work on the islands of the Pacific,
and to consult many charts.” The task of elaborating and writing out
his books was, with Darwin, always a very slow and laborious one; but
it is clear that in accomplishing the work now under consideration,
there was a long and constant struggle with the lethargy and weakness
resulting from the sad condition of his health at that time.
Lyell’s anticipation that the theory of coral-reefs would be slow in
meeting with general acceptance was certainly not justified by the
actual facts. On the contrary the new book was at once received with
general assent among both geologists and zoologists, and even attracted
a considerable amount of attention from the general public.
It was not long before the coral-reef theory of Darwin found an able
exponent and sturdy champion in the person of the great American
naturalist, Professor James D. Dana. Two years after the return of the
_Beagle_ to England, the ships of the United States Exploring
Expedition set sail upon their four years’ cruise, under the command of
Captain Wilkes, and Dana was a member of the scientific staff. When, in
1839, the expedition arrived at Sydney, a newspaper paragraph was found
which gave the American naturalist the first intimation of Darwin’s new
theory of the origin of atolls and barrier-reefs. Writing in 1872, Dana
describes the effect produced on his mind by reading this passage:—“The
paragraph threw a flood of light over the subject, and called forth
feelings of peculiar satisfaction, and of gratefulness to Mr. Darwin,
which still come up afresh whenever the subject of coral islands is
mentioned. The Gambier Islands in the Paumotus, which gave him the key
to the theory, I had not seen; but on reaching the Feejees, six months
later, in 1840, I found there similar facts on a still grander scale
and of a more diversified character, so that I was afterward enabled to
speak of his theory as established with more positiveness than he
himself, in his philosophic caution, had been ready to adopt. His work
on coral-reefs appeared in 1842, when my report on the subject was
already in manuscript. It showed that the conclusions on other points,
which we had independently reached, were for the most part the same.
The principal points of difference relate to the reason for the absence
of corals from some coasts, and the evidence therefrom as to changes of
level, and the distribution of the oceanic regions of
elevation and subsidence—topics which a wide range of travel over the
Pacific brought directly and constantly to my attention.”
Among the Reports of the United States Exploring Expedition, two
important works from the pen of Professor Dana made their
appearance;—one on “Zoophytes,” which treats at length on “Corals and
Coral-Animals,” and the other on “Coral-Reefs and Islands.” In 1872,
Dana prepared a work of a more popular character in which some of the
chief results of his studies are described; it bore the title of
“Corals and Coral-Islands.” Of this work, new and enlarged editions
appeared in 1874 and 1890 in America, while two editions were published
in this country in 1872 and 1875. In all these works their author,
while maintaining an independent judgment on certain matters of detail,
warmly defends the views of Darwin on all points essential to the
theory.
Another able exponent and illustrator of the theory of coral-reefs was
found in Professor J. B. Jukes, who accompanied H.M.S. _Fly_, as
naturalist, during the survey of the Great Barrier-Reef—in the years
1842 to 1846. Jukes, who was a man of great acuteness as well as
independence of mind, concludes his account of the great Australian
reefs with the following words:—“After seeing much of the Great
Barrier-Reefs, and reflecting much upon them, and trying if it were
possible by any means to evade the conclusions to which Mr. Darwin has
come, I cannot help adding that his hypothesis is perfectly
satisfactory to my mind, and rises beyond a mere hypothesis into the
true theory of coral-reefs.”
As the result of the clear exposition of the subject by Darwin, Lyell,
Dana, and Jukes, the theory of coral-reefs had, by the middle of the
present century, commanded the almost universal assent of both
biologists and geologists. In 1859 Baron von Richthofen brought forward
new facts in its support, by showing that the existence of the thick
masses of dolomitic limestone in the Tyrol could be best accounted for
if they were regarded as of coralline origin and as being formed during
a period of long continued subsidence. The same views were maintained
by Professor Mojsisovics in his “Dolomit-riffe von Südtirol und
Venetien,” which appeared in 1879.
The first serious note of dissent to the generally accepted theory was
heard in 1863, when a distinguished German naturalist, Dr. Karl Semper,
declared that his study of the Pelew Islands showed that uninterrupted
subsidence could not have been going on in that region. Dr. Semper’s
objections were very carefully
considered by Mr. Darwin, and a reply to them appeared in the second
and revised edition of his “Coral-Reefs,” which was published in 1874.
With characteristic frankness and freedom from prejudice, Darwin
admitted that the facts brought forward by Dr. Semper proved that in
certain specified cases, subsidence could not have played the chief
part in originating the peculiar forms of the coral-islands. But while
making this admission, he firmly maintained that exceptional cases,
like those described in the Pelew Islands, were not sufficient to
invalidate the theory of subsidence as applied to the widely spread
atolls, encircling reefs, and barrier-reefs of the Pacific and Indian
Oceans. It is worthy of note that to the end of his life Darwin
maintained a friendly correspondence with Semper concerning the points
on which they were at issue.
After the appearance of Semper’s work, Dr. J. J. Rein published an
account of the Bermudas, in which he opposed the interpretation of the
structure of the islands given by Nelson and other authors, and
maintained that the facts observed in them are opposed to the views of
Darwin. Although, so far as I am aware, Darwin had no opportunity of
studying and considering these particular objections, it may be
mentioned that two American geologists have since carefully re-examined
the district—Professor W. N. Rice in 1884 and Professor A. Heilprin in
1889—and they have independently arrived at the conclusion that Dr.
Rein’s objections cannot be maintained.
The most serious opposition to Darwin’s coral-reef theory, however, was
that which developed itself after the return of H.M.S. _Challenger_
from her famous voyage. Mr. John Murray, one of the staff of
naturalists on board that vessel, propounded a new theory of
coral-reefs, and maintained that the view that they were formed by
subsidence was one that was no longer tenable; these objections have
been supported by Professor Alexander Agassiz in the United States, and
by Dr. A. Geikie, and Dr. H. B. Guppy in this country.
Although Mr. Darwin did not live to bring out a third edition of his
“Coral-Reefs,” I know from several conversations with him that he had
given the most patient and thoughtful consideration to Mr. Murray’s
paper on the subject. He admitted to me that had he known, when he
wrote his work, of the abundant deposition of the remains of calcareous
organisms on the sea floor, he might have regarded this cause as
sufficient in a few cases to raise the summits of submerged volcanoes
or other mountains
to a level at which reef-forming corals can commence to flourish. But
he did not think that the admission that under certain favourable
conditions, atolls might be thus formed without subsidence,
necessitated an abandonment of his theory in the case of the
innumerable examples of the kind which stud the Indian and Pacific
Oceans.
A letter written by Darwin to Professor Alexander Agassiz in May 1881
shows exactly the attitude which careful consideration of the subject
led him to maintain towards the theory propounded by Mr. Murray:—“You
will have seen,” he writes, “Mr. Murray’s views on the formation of
atolls and barrier-reefs. Before publishing my book, I thought long
over the same view, but only as far as ordinary marine organisms are
concerned, for at that time little was known of the multitude of minute
oceanic organisms. I rejected this view, as from the few dredgings made
in the _Beagle_, in the south temperate regions, I concluded that
shells, the smaller corals, etc., decayed and were dissolved when not
protected by the deposition of sediment, and sediment could not
accumulate in the open ocean. Certainly, shells, etc., were in several
cases completely rotten, and crumbled into mud between my fingers; but
you will know whether this is in any degree common. I have expressly
said that a bank at the proper depth would give rise to an atoll, which
could not be distinguished from one formed during subsidence. I can,
however, hardly believe in the existence of as many banks (there having
been no subsidence) as there are atolls in the great oceans, within a
reasonable depth, on which minute oceanic organisms could have
accumulated to the depth of many hundred feet.”
Darwin’s concluding words in the same letter written within a year of
his death, are a striking proof of the candour and openness of mind
which he preserved so well to the end, in this as in other
controversies.
“If I am wrong, the sooner I am knocked on the head and annihilated so
much the better. It still seems to me a marvellous thing that there
should not have been much, and long-continued, subsidence in the beds
of the great oceans. I wish some doubly rich millionaire would take it
into his head to have borings made in some of the Pacific and Indian
atolls, and bring home cores for slicing from a depth of 500 or 600
feet.”
It is noteworthy that the objections to Darwin’s theory have for the
most part proceeded from zoologists, while those who have fully
appreciated the geological aspect of the question, have been
the staunchest supporters of the theory of subsidence. The desirability
of such boring operations in atolls has been insisted upon by several
geologists, and it may be hoped that before many years have passed
away, Darwin’s hopes may be realised, either with or without the
intervention of the “doubly rich millionaire.”
Three years after the death of Darwin, the veteran Professor Dana
re-entered the lists and contributed a powerful defence of the theory
of subsidence in the form of a reply to an essay written by the ablest
exponent of the anti-Darwinian views on this subject, Dr. A. Geikie.
While pointing out that the Darwinian position had been to a great
extent misunderstood by its opponents, he showed that the rival theory
presented even greater difficulties than those which it professed to
remove.
During the last five years, the whole question of the origin of
coral-reefs and islands has been re-opened, and a controversy has
arisen, into which, unfortunately, acrimonious elements have been very
unnecessarily introduced. Those who desire it, will find clear and
impartial statements of the varied and often mutually destructive views
put forward by different authors, in three works which have made their
appearance within the last year,—“The Bermuda Islands,” by Professor
Angelo Heilprin; “Corals and Coral-Islands,” new edition by Professor
J. D. Dana; and the third edition of Darwin’s “Coral-Reefs,” with Notes
and Appendix by Professor T. G. Bonney.
Most readers will, I think, rise from the perusal of these works with
the conviction that, while on certain points of detail it is clear
that, through the want of knowledge concerning the action of marine
organisms in the open ocean, Darwin was betrayed into some grave
errors, yet the main foundations of his argument have not been
seriously impaired by the new facts observed in the deep-sea
researches, or by the severe criticism to which his theory has been
subjected during the last ten years. On the other hand, I think it will
appear that much misapprehension has been exhibited by some of Darwin’s
critics, as to what his views and arguments really were; so that the
reprint and wide circulation of the book in its original form is
greatly to be desired, and cannot but be attended with advantage to all
those who will have the fairness to acquaint themselves with Darwin’s
views at first hand, before attempting to reply to them.
JOHN W. JUDD.
CORAL-REEFS
INTRODUCTION
The object of this volume is to describe from my own observation and
the works of others, the principal kinds of coral-reefs, more
especially those occurring in the open ocean, and to explain the origin
of their peculiar forms. I do not here treat of the polypifers, which
construct these vast works, except so far as relates to their
distribution, and to the conditions favourable to their vigorous
growth. Without any distinct intention to classify coral-reefs, most
voyagers have spoken of them under the following heads:
“lagoon-islands,” or “atolls,” “barrier” or “encircling reefs,” and
“fringing” or “shore-reefs.” The lagoon-islands have received much the
most attention; and it is not surprising, for every one must be struck
with astonishment, when he first beholds one of these vast rings of
coral-rock, often many leagues in diameter, here and there surmounted
by a low verdant island with dazzling white shores, bathed on the
outside by the foaming breakers of the ocean, and on the inside
surrounding a calm expanse of water, which from reflection, is of a
bright but pale green colour. The naturalist will feel this
astonishment more deeply after having examined the soft and almost
gelatinous bodies of these apparently insignificant creatures, and when
he knows that the solid reef increases only on the outer edge, which
day and night is lashed by the breakers of an ocean never at rest. Well
did François Pyrard de Laval, in the year 1605, exclaim, “C’est une
mérueille de voir chacun de ces atollons, enuironné d’un grand banc de
pierre tout autour, n’y ayant point d’artifice humain.” The
accompanying sketch of Whitsunday island, in the South Pacific, taken
from Captain Beechey’s admirable “Voyage,” although excellent of its
kind, gives but a faint idea of the singular aspect of one of these
lagoon-islands.
Whitsunday Island is of small size, and the whole circle has been
converted into land, which is a comparatively rare circumstance. As the
reef of a lagoon-island generally supports many separate small islands,
the word “island,” applied to the whole, is often the cause of
confusion; hence I have invariably used in this volume the term
“atoll,” which is the name given to these circular groups of
coral-islets by their
inhabitants in the Indian Ocean, and is synonymous with “lagoon-
island.”
[Illustration: Whitsunday Island]
Barrier-reefs, when encircling small islands, have been comparatively
little noticed by voyagers; but they well deserve attention. In their
structure they are little less marvellous than atolls, and they give a
singular and most picturesque character to the scenery of the islands
they surround. In the accompanying sketch, taken from the “Voyage of
the _Coquille_,” the reef is seen from within, from one of the high
peaks of the island of Bolabola.[1] Here, as in Whitsunday Island, the
whole of that part of the reef which is visible is converted into land.
This is a circumstance of rare occurrence; more usually a snow-white
line of great breakers, with here and there an islet crowned by
cocoa-nut trees, separates the smooth waters of the lagoon-like channel
from the waves of the open sea. The barrier-reefs of Australia and of
New Caledonia, owing to their enormous dimensions, have excited much
attention: in structure and form they resemble those encircling many of
the smaller islands in the Pacific Ocean.
[1] I have taken the liberty of simplifying the foreground, and
leaving out a mountainous island in the far distance.
[Illustration: Island of Bolabola]
With respect to fringing, or shore-reefs, there is little in their
structure which needs explanation; and their name expresses their
comparatively
small extension. They differ from barrier-reefs in not lying so far
from the shore, and in not having within a broad channel of deep water.
Reefs also occur around submerged banks of sediment and of worn-down
rock; and others are scattered quite irregularly where the sea is very
shallow; these in most respects are allied to those of the fringing
class, but they are of comparatively little interest.
I have given a separate chapter to each of the above classes, and have
described some one reef or island, on which I possessed most
information, as typical; and have afterwards compared it with others of
a like kind. Although this classification is useful from being obvious,
and from including most of the coral-reefs existing in the open sea, it
admits of a more fundamental division into barrier and atoll-formed
reefs on the one hand, where there is a great apparent difficulty with
respect to the foundation on which they must first have grown; and into
fringing-reefs on the other, where, owing to the nature of the slope of
the adjoining land, there is no such difficulty. The two blue tints and
the red colour[2] on the map (Plate III), represent this main division,
as explained in the beginning of the last chapter. In the Appendix,
every existing coral-reef, except some on the coast of Brazil not
included in the map, is briefly described in geographical order, as far
as I possessed information; and any particular spot may be found by
consulting the Index.
Several theories have been advanced to explain the origin of atolls or
lagoon-islands, but scarcely one to account for barrier-reefs. From the
limited depths at which reef-building polypifers can flourish, taken
into consideration with certain other circumstances, we are compelled
to conclude, as it will be seen, that both in atolls and barrier-reefs,
the foundation on which the coral was primarily attached, has subsided;
and that during this downward movement, the reefs have grown upwards.
This conclusion, it will be further seen, explains most satisfactorily
the outline and general form of atolls and barrier-reefs, and likewise
certain peculiarities in their structure. The distribution, also, of
the different kinds of coral-reefs, and their position with relation to
the areas of recent elevation, and to the points subject to volcanic
eruptions, fully accord with this theory of their origin.[3]
[2] Replaced by numbers in this edition.
[3] A brief account of my views on coral formations, now published in
my Journal of Researches, was read May 31st, 1837, before the
Geological Society, and an abstract has appeared in the Proceedings.
In the several original surveys, from which the small plans on this
plate have been reduced, the coral-reefs are engraved in very different
styles. For the sake of uniformity, I have adopted the style used in
the charts of the Chagos Archipelago, published by the East Indian
Company, from the survey by Captain Moresby and Lieutenant Powell. The
surface of the reef, which dries at low water, is represented by a
surface with small crosses: the coral-islets on the reef are marked by
small linear spaces, on which a few cocoa-nut trees, out of all
proportion too large, have been introduced for the sake of clearness.
The entire _annular reef_, which when surrounding an open expanse of
water, forms an “atoll,” and when surrounding one or more high islands,
forms an encircling “barrier-reef,” has a nearly uniform structure. The
reefs in some of the original surveys are represented merely by a
single line with crosses, so that their breadth is not given; I have
had such reefs engraved of the width usually attained by coral-reefs. I
have not thought it worth while to introduce all those small and very
numerous reefs, which occur within the lagoons of most atolls and
within the lagoon-channels of most barrier-reefs, and which stand
either isolated, or are attached to the shores of the reef or land. At
Peros Banhos none of the lagoon-reefs rise to the surface of the water;
a few of them have been introduced, and are marked by plain dotted
circles. A few of the deepest soundings are laid down within each reef;
they are in fathoms, of six English feet.
_Plate I_—Map showing the resemblance in form between barrier
coral-reefs surrounding mountainous islands, and atolls or lagoon
islands.
[Illustration: Map showing the resemblance in form.]
Fig. 1—VANIKORO, situated in the western part of the South Pacific;
taken from the survey by Captain D’Urville in the _Astrolabe_; the
soundings on the southern side of the island, namely, from thirty to
forty fathoms, are given from the voyage of the Chev. Dillon; the other
soundings are laid down from the survey by D’Urville; height of the
summit of the island is 3,032 feet. The principal small detached reefs
within the lagoon-channel have in this instance been represented. The
southern shore of the island is narrowly fringed by a reef: if the
engraver had carried this reef entirely round both islands, this figure
would have served (by leaving out in imagination the barrier-reef) as a
good specimen of an abruptly-sided island, surrounded by a reef of the
fringing class.
Fig. 2—HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the
“Atlas of the Voyage of the _Astrolabe,_” compiled from the surveys of
Captains Duperrey and D’Urville; the depth of the immense lagoon-like
space within the reef is not known.
Fig. 3—RAIATEA, in the Society Archipelago; from the map given in the
quarto edition of “Cook’s First Voyage;” it is probably not accurate.
Fig. 4—BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago,
from the survey by Captain Beechey, R.N.; the lagoon is choked up with
reefs, but the average greatest depth of about twenty fathoms, is given
from the published account of the voyage.
Fig. 5—BOLABOLA, in the Society Archipelago, from the survey of Captain
Duperrey in the _ Coquille_: the soundings in this and the following
figures have been altered from French feet to English fathoms; height
of highest point of the island 4,026 feet.
[Illustration: Map showing the resemblance in form.]
Fig. 6.—MAURUA, in the Society Archipelago; from the survey by Captain
Duperrey in the _ Coquille_: height of land about eight hundred feet.
Fig. 7—POUYNIPÈTE, or SENIAVINE, in the Caroline Archipelago; from the
survey by Admiral Lutké.
Fig. 8—GAMBIER ISLANDS, in the southern part of the Low Archipelago;
from the survey by Captain Beechey; height of highest island, 1,246
feet; the islands are surrounded by extensive and irregular reefs; the
reef on the southern side is submerged.
Fig. 9—PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in
the Indian Ocean; from the survey by Captain Moresby and Lieutenant
Powell; not nearly all the small submerged reefs in the lagoon are
represented; the annular reef on the southern side is submerged.
Fig. 10—KEELING, or COCOS ATOLL (or lagoon-island), in the Indian
Ocean; from the survey by Captain Fitzroy; the lagoon south of the
dotted line is very shallow, and is left almost bare at low water; the
part north of the line is choked up with irregular reefs. The annular
reef on the north-west side is broken, and blends into a shoal
sandbank, on which the sea breaks.
Chapter I ATOLLS OR LAGOON-ISLANDS
_Section I_—KEELING ATOLL
Corals on the outer margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.
KEELING or COCOS atoll is situated in the Indian Ocean, in 12° 5′ S.,
and longitude 90° 55′ E.: a reduced chart of it was made from the
survey of Captain Fitzroy and the Officers of H.M.S. _Beagle_, is given
in Plate I, Fig. 10. The greatest width of this atoll is nine miles and
a half. Its structure is in most respects characteristic of the class
to which it belongs, with the exception of the shallowness of the
lagoon. The accompanying woodcut represents a vertical section,
supposed to be drawn at low water from the outer coast across one of
the low islets (one being taken of average dimensions) to within the
lagoon.
[Illustration: Vertical section of one of the low islets.]
A.—Level of the sea at low water: where the letter A is placed, the
depth is twenty-five fathoms, and the distance rather more than one
hundred and fifty yards from the edge of the reef.
B.—Outer edge of that flat part of the reef, which dries at low water:
the edge either consists of a convex mound, as represented, or of
rugged points, like those a little farther seaward, beneath the water.
C.—A flat of coral-rock, covered at high water.
D.—A low projecting ledge of brecciated coral-rock washed by the waves
at high water.
E.—A slope of loose fragments, reached by the sea only during gales:
the upper part, which is from six to twelve feet high, is clothed with
vegetation. The surface of the islet gently slopes to the lagoon.
F.—Level of the lagoon at low water.
The section is true to the scale in a horizontal line, but it could not
be made so in a vertical one, as the average greatest height of the
land is only between six and twelve feet above high-water mark.
I will describe the section, commencing with the outer margin. I must
first observe that the reef-building polypifers, not being tidal
animals, require to be constantly submerged or washed by the breakers.
I was assured by Mr. Liesk, a very intelligent resident on these
islands, as well as by some chiefs at Tahiti (Otaheite), that an
exposure to the rays of the sun for a very short time invariably causes
their destruction. Hence it is possible only under the most favourable
circumstances, afforded by an unusually low tide and smooth water, to
reach the outer margin, where the coral is alive. I succeeded only
twice in gaining this part, and found it almost entirely composed of a
living Porites, which forms great irregularly rounded masses (like
those of an Astræa, but larger) from four to eight feet broad, and
little less in thickness. These mounds are separated from each other by
narrow crooked channels, about six feet deep, most of which intersect
the line of reef at right angles. On the furthest mound, which I was
able to reach by the aid of a leaping-pole, and over which the sea
broke with some violence, although the day was quite calm and the tide
low, the polypifers in the uppermost cells were all dead, but between
three and four inches lower down on its side they were living, and
formed a projecting border round the upper and dead surface. The coral
being thus checked in its upward growth, extends laterally, and hence
most of the masses, especially those a little further inwards, had
broad flat dead summits. On the other hand I could see, during the
recoil of the breakers, that a few yards further seaward, the whole
convex surface of the Porites was alive; so that the point where we
were standing was almost on the exact upward and shoreward limit of
existence of those corals which form the outer margin of the reef. We
shall presently see that there are other organic productions, fitted to
bear a somewhat longer exposure to the air and sun.
Next, but much inferior in importance to the Porites, is the _
Millepora complanata._[1]
[1] This Millepora (Palmipora of Blainville), as well as the _M.
alcicornis_, possesses the singular property of stinging the skin
where it is delicate, as on the face and arm.
It grows in thick vertical plates, intersecting each other at various
angles, and forms an exceedingly strong honeycombed mass, which
generally affects a circular form, the marginal plates alone being
alive. Between these plates and in the protected crevices on the reef,
a multitude of branching zoophytes and other productions flourish, but
the Porites and Millepora alone seem able to resist the fury of the
breakers on its upper and outer edge: at the depth of a few fathoms
other kinds of stony corals live. Mr. Liesk, who was intimately
acquainted with every part of this reef, and likewise with that of
North Keeling atoll, assured me that these corals invariably compose
the outer margin. The lagoon is inhabited by quite a distinct set of
corals, generally brittle and thinly branched; but a Porites,
apparently of the same species with that on the outside, is found
there, although it does not seem to thrive, and certainly does not
attain the thousandth part in bulk of the masses opposed to the
breakers.
The woodcut shows the form of the bottom off the reef: the water
deepens for a space between one and two hundred yards wide, very
gradually to twenty-five fathoms (A in section), beyond which the sides
plunge into the unfathomable ocean at an angle of 45°.[2] To the depth
of ten or twelve fathoms the bottom is exceedingly rugged, and seems
formed of great masses of living coral, similar to those on the margin.
The arming of the lead here invariably came up quite clean, but deeply
indented, and chains and anchors which were lowered, in the hopes of
tearing up the coral, were broken. Many small fragments, however, of _
Millepora alcicornis_ were brought up; and on the arming from an
eight-fathom cast, there was a perfect impression of an Astræa,
apparently alive. I examined the rolled fragments cast on the beach
during gales, in order further to ascertain what corals grew outside
the reef. The fragments consisted of many kinds, of which the Porites
already mentioned and a Madrepora, apparently the _M. corymbosa_, were
the most abundant. As I searched in vain in the hollows on the reef and
in the lagoon, for a living specimen of this Madrepore, I conclude that
it is confined to a zone outside, and beneath the surface, where it
must be very abundant. Fragments of the _Millepora alcicornis_ and of
an Astræa were also numerous; the former is found, but not in
proportionate numbers, in the hollows on the reef; but the Astræa I did
not see living. Hence we may infer, that these are the kinds of coral
which form the rugged sloping surface (represented in the woodcut by an
uneven line), round and beneath the external margin. Between twelve and
twenty fathoms the arming came up an equal number of times smoothed
with sand, and indented with coral: an anchor and lead were lost at the
respective depths of thirteen and sixteen fathoms. Out of twenty-five
soundings taken at a greater depth than twenty fathoms, every one
showed that the bottom was covered with sand; whereas, at a less depth
than twelve fathoms, every sounding showed that it was exceedingly
rugged, and free from all extraneous particles. Two soundings were
obtained at the depth of 360 fathoms, and several between two hundred
and three hundred fathoms. The sand brought up from these depths
consisted of finely triturated fragments of stony zoophytes, but not,
as far as I could distinguish, of a particle of any lamelliform genus:
fragments of shells were rare.
[2] The soundings from which this section is laid down were taken with
great care by Captain Fitzroy himself. He used a bell-shaped lead,
having a diameter of four inches, and the armings each time were cut
off and brought on board for me to examine. The arming is a
preparation of tallow, placed in the concavity at the bottom of the
lead. Sand, and even small fragments of rock, will adhere to it; and
if the bottom be of rock it brings up an exact impression of its
surface.
At a distance of 2,200 yards from the breakers, Captain Fitzroy found
no bottom with a line of 7,200 feet in length; hence the submarine
slope of this coral formation is steeper than that of any volcanic
cone. Off the mouth of the lagoon, and likewise off the northern point
of the atoll, where the currents act violently, the inclination, owing
to the accumulation of sediment, is less. As the arming of the lead
from
all the greater depths showed a smooth sandy bottom, I at first
concluded that the whole consisted of a vast conical pile of calcareous
sand, but the sudden increase of depth at some points, and the
circumstance of the line having been cut, as if rubbed, when between
five hundred and six hundred fathoms were out, indicate the probable
existence of submarine cliffs.
On the margin of the reef, close within the line where the upper
surface of the Porites and of the Millepora is dead, three species of
Nullipora flourish. One grows in thin sheets, like a lichen on old
trees; the second in stony knobs, as thick as a man’s finger, radiating
from a common centre; and the third, which is less common, in a
moss-like reticulation of thin, but perfectly rigid branches.[3] The
three species occur either separately or mingled together; and they
form by their successive growth a layer two or three feet in thickness,
which in some cases is hard, but where formed of the lichen-like kind,
readily yields an impression to the hammer: the surface is of a reddish
colour. These Nulliporæ, although able to exist above the limit of true
corals, seem to require to be bathed during the greater part of each
tide by breaking water, for they are not found in any abundance in the
protected hollows on the back part of the reef, where they might be
immersed either during the whole or an equal proportional time of each
tide. It is remarkable that organic productions of such extreme
simplicity, for the Nulliporæ undoubtedly belong to one of the lowest
classes of the vegetable kingdom, should be limited to a zone so
peculiarly circumstanced. Hence the layer composed by their growth
merely fringes the reef for a space of about twenty yards in width,
either under the form of separate mammillated projections, where the
outer masses of coral are separate, or, more commonly, where the corals
are united into a solid margin, as a continuous smooth convex mound (B
in woodcut), like an artificial breakwater. Both the mound and
mammillated projections stand about three feet higher than any other
part of the reef, by which term I do not include the islets, formed by
the accumulation of rolled fragments. We shall hereafter see that other
coral reefs are protected by a similar thick growth of Nulliporæ on the
outer margin, the part most exposed to the breakers, and this must
effectually aid in preserving it from being worn down.
[3] This last species is of a beautiful bright peach-blossom colour.
Its branches are about as thick as crow-quills; they are slightly
flattened and knobbed at the extremities. The extremities only are
alive and brightly coloured. The two other species are of a dirty
purplish-white. The second species is extremely hard; its short
knob-like branches are cylindrical, and do not grow thicker at their
extremities.
The woodcut represents a section across one of the islets on the reef,
but if all that part which is above the level of C were removed, the
section would be that of a simple reef, as it occurs where no islet has
been formed. It is this reef which essentially forms the atoll. It is a
ring, enclosing the lagoon on all sides except at the northern end,
where there are two open spaces, through one of which ships can enter.
The reef varies in width from two hundred and fifty to five
hundred yards, its surface is level, or very slightly inclined towards
the lagoon, and at high tide the sea breaks entirely over it: the water
at low tide thrown by the breakers on the reef, is carried by the many
narrow and shoal gullies or channels on its surface, into the lagoon: a
return stream sets out of the lagoon through the main entrance. The
most frequent coral in the hollows on the reef is _Pocillopora
verrucosa_, which grows in short sinuous plates, or branches, and when
alive is of a beautiful pale lake-red: a Madrepora, closely allied or
identical with _M. pocillifera_, is also common. As soon as an islet is
formed, and the waves are prevented breaking entirely over the reef,
the channels and hollows in it become filled up with cemented
fragments, and its surface is converted into a hard smooth floor (C of
woodcut), like an artificial one of freestone. This flat surface varies
in width from one hundred to two hundred, or even three hundred yards,
and is strewed with a few large fragments of coral torn up during
gales: it is uncovered only at low water. I could with difficulty, and
only by the aid of a chisel, procure chips of rock from its surface,
and therefore could not ascertain how much of it is formed by the
aggregation of detritus, and how much by the outward growth of mounds
of corals, similar to those now living on the margin. Nothing can be
more singular than the appearance at low tide of this “flat” of naked
stone, especially where it is externally bounded by the smooth convex
mound of Nulliporæ, appearing like a breakwater built to resist the
waves, which are constantly throwing over it sheets of foaming water.
The characteristic appearance of this “flat” is shown in the foregoing
woodcut of Whitsunday atoll.
The islets on the reef are first formed between two hundred and three
hundred yards from its outer edge, through the accumulation of a pile
of fragments, thrown together by some unusually strong gale. Their
ordinary width is under a quarter of a mile, and their length varies
from a few yards to several miles. Those on the south-east and windward
side of the atoll, increase solely by the addition of fragments on
their outer side; hence the loose blocks of coral, of which their
surface is composed, as well as the shells mingled with them, almost
exclusively consist of those kinds which live on the outer coast. The
highest part of the islets (excepting hillocks of blown sand, some of
which are thirty feet high), is close to the outer beach (E of the
woodcut), and averages from six to ten feet above ordinary high-water
mark. From the outer beach the surface slopes gently to the shores of
the lagoon, which no doubt has been caused by the breakers the further
they have rolled over the reef, having had less power to throw up
fragments. The little waves of the lagoon heap up sand and fragments of
thinly-branched corals on the inner side of the islets on the leeward
side of the atoll; and these islets are broader than those to windward,
some being even eight hundred yards in width; but the land thus added
is very low. The fragments beneath the surface are cemented into a
solid mass, which is exposed as a ledge (D of the woodcut), projecting
some yards in front of the outer shore and from two to four feet high.
This ledge is just reached by the waves at ordinary high-water:
it extends in front of all the islets, and everywhere has a water-worn
and scooped appearance. The fragments of coral which are occasionally
cast on the “flat” are during gales of unusual violence swept together
on the beach, where the waves each day at high-water tend to remove and
gradually wear them down; but the lower fragments having become firmly
cemented together by the percolation of calcareous matter, resist the
daily tides longer, and hence project as a ledge. The cemented mass is
generally of a white colour, but in some few parts reddish from
ferruginous matter; it is very hard, and is sonorous under the hammer;
it is obscurely divided by seams, dipping at a small angle seaward; it
consists of fragments of the corals which grow on the outer margin,
some quite and others partially rounded, some small and others between
two and three feet across; and of masses of previously formed
conglomerate, torn up, rounded, and re-cemented; or it consists of a
calcareous sandstone, entirely composed of rounded particles, generally
almost blended together, of shells, corals, the spines of echini, and
other such organic bodies; rocks, of this latter kind, occur on many
shores, where there are no coral reefs. The structure of the coral in
the conglomerate has generally been much obscured by the infiltration
of spathose calcareous matter; and I collected a very interesting
series, beginning with fragments of unaltered coral, and ending with
others, where it was impossible to discover with the naked eye any
trace of organic structure. In some specimens I was unable, even with
the aid of a lens, and by wetting them, to distinguish the boundaries
of the altered coral and spathose limestone. Many even of the blocks of
coral lying loose on the beach, had their central parts altered and
infiltrated.
The lagoon alone remains to be described; it is much shallower than
that of most atolls of considerable size. The southern part is almost
filled up with banks of mud and fields of coral, both dead and alive,
but there are considerable spaces, between three and four fathoms, and
smaller basins, from eight to ten fathoms deep. Probably about half its
area consists of sediment, and half of coral-reefs. The corals
composing these reefs have a very different aspect from those on the
outside; they are very numerous in kind, and most of them are thinly
branched. Meandrina, however, lives in the lagoon, and great rounded
masses of this coral are numerous, lying quite or almost loose on the
bottom. The other commonest kinds consist of three closely allied
species of true Madrepora in thin branches; of _Seriatapora subulata_;
two species of Porites[4] with cylindrical branches, one of which forms
circular clumps, with the exterior branches only alive; and lastly, a
coral something like an Explanaria, but with stars on both surfaces,
growing in thin, brittle, stony, foliaceous expansions, especially in
the deeper basins of the lagoon. The reefs on which
these corals grow are very irregular in form, are full of cavities, and
have not a solid flat surface of dead rock, like that surrounding the
lagoon; nor can they be nearly so hard, for the inhabitants made with
crowbars a channel of considerable length through these reefs, in which
a schooner, built on the S.E. islet, was floated out. It is a very
interesting circumstance, pointed out to us by Mr. Liesk, that this
channel, although made less than ten years before our visit, was then,
as we saw, almost choked up with living coral, so that fresh
excavations would be absolutely necessary to allow another vessel to
pass through it.
[4] This Porites has somewhat the habit of _P. clavaria_, but the
branches are not knobbed at their ends. When alive it is of a yellow
colour, but after having been washed in fresh water and placed to dry,
a jet-black slimy substance exuded from the entire surface, so that
the specimen now appears as if it had been dipped in ink.
The sediment from the deepest parts in the lagoon, when wet, appeared
chalky, but when dry, like very fine sand. Large soft banks of similar,
but even finer grained mud, occur on the S.E. shore of the lagoon,
affording a thick growth of a Fucus, on which turtle feed: this mud,
although discoloured by vegetable matter, appears from its entire
solution in acids to be purely calcareous. I have seen in the Museum of
the Geological Society, a similar but more remarkable substance,
brought by Lieutenant Nelson from the reefs of Bermuda, which, when
shown to several experienced geologists, was mistaken by them for true
chalk. On the outside of the reef much sediment must be formed by the
action of the surf on the rolled fragments of coral; but in the calm
waters of the lagoon, this can take place only in a small degree. There
are, however, other and unexpected agents at work here: large shoals of
two species of Scarus, one inhabiting the surf outside the reef and the
other the lagoon, subsist entirely, as I was assured by Mr. Liesk, the
intelligent resident before referred to, by browsing on the living
polypifers. I opened several of these fish, which are very numerous and
of considerable size, and I found their intestines distended by small
pieces of coral, and finely ground calcareous matter. This must daily
pass from them as the finest sediment; much also must be produced by
the infinitely numerous vermiform and molluscous animals, which make
cavities in almost every block of coral. Dr. J. Allan, of Forres, who
has enjoyed the best means of observation, informs me in a letter that
the Holothuriæ (a family of Radiata) subsist on living coral; and the
singular structure of bone within the anterior extremity of their
bodies, certainly appears well adapted for this purpose. The number of
the species of Holothuria, and of the individuals which swarm on every
part of these coral-reefs, is extraordinarily great; and many shiploads
are annually freighted, as is well-known, for China with the trepang,
which is a species of this genus. The amount of coral yearly consumed,
and ground down into the finest mud, by these several creatures, and
probably by many other kinds, must be immense. These facts are,
however, of more importance in another point of view, as showing us
that there are living checks to the growth of coral-reefs, and that the
almost universal law of “consumed and be consumed,” holds good even
with the polypifers forming those massive bulwarks, which are able to
withstand the force of the open ocean.
Considering that Keeling atoll, like other coral formations, has been
entirely formed by the growth of organic beings, and the accumulation
of their detritus, one is naturally led to inquire how long it has
continued, and how long it is likely to continue, in its present state.
Mr. Liesk informed me that he had seen an old chart in which the
present long island on the S.E. side was divided by several channels
into as many islets; and he assures me that the channels can still be
distinguished by the smaller size of the trees on them. On several
islets, also, I observed that only young cocoa-nut trees were growing
on the extremities; and that older and taller trees rose in regular
succession behind them; which shows that these islets have very lately
increased in length. In the upper and south-eastern part of the lagoon,
I was much surprised by finding an irregular field of at least a mile
square of branching corals, still upright, but entirely dead. They
consisted of the species already mentioned; they were of a brown
colour, and so rotten, that in trying to stand on them I sank halfway
up the leg, as if through decayed brushwood. The tops of the branches
were barely covered by water at the time of lowest tide. Several facts
having led me to disbelieve in any elevation of the whole atoll, I was
at first unable to imagine what cause could have killed so large a
field of coral. Upon reflection, however, it appeared to me that the
closing up of the above-mentioned channels would be a sufficient cause;
for before this, a strong breeze by forcing water through them into the
head of the lagoon, would tend to raise its level. But now this cannot
happen, and the inhabitants observe that the tide rises to a less
height, during a high S.E. wind, at the head than at the mouth of the
lagoon. The corals, which, under the former condition of things, had
attained the utmost possible limit of upward growth, would thus
occasionally be exposed for a short time to the sun, and be killed.
Besides the increase of dry land, indicated by the foregoing facts, the
exterior solid reef appears to have grown outwards. On the western side
of the atoll, the “flat” lying between the margin of the reef and the
beach, is very wide; and in front of the regular beach with its
conglomerate basis, there is, in most parts, a bed of sand and loose
fragments with trees growing out of it, which apparently is not reached
even by the spray at high water. It is evident some change has taken
place since the waves formed the inner beach; that they formerly beat
against it with violence was evident, from a remarkably thick and
water-worn point of conglomerate at one spot, now protected by
vegetation and a bank of sand; that they beat against it in the same
peculiar manner in which the swell from windward now obliquely curls
round the margin of the reef, was evident from the conglomerate having
been worn into a point projecting from the beach in a similarly oblique
manner. This retreat in the line of action of the breakers might
result, either from the surface of the reef in front of the islets
having been submerged at one time, and afterward having grown upwards,
or from the mounds of coral on the margin having continued to grow
outwards. That an outward growth of this part is in process, can hardly
be doubted from the fact already mentioned of the mounds of Porites
with their summits apparently lately killed, and their sides only three
or four inches lower down thickened by a fresh layer of living coral.
But there is a difficulty on this supposition which I must not pass
over. If the
whole, or a large part of the “flat,” had been formed by the outward
growth of the margin, each successive margin would naturally have been
coated by the Nulliporæ, and so much of the surface would have been of
equal height with the existing zone of living Nulliporæ: this is not
the case, as may be seen in the woodcut. It is, however, evident from
the abraded state of the “flat,” with its original inequalities filled
up, that its surface has been much modified; and it is possible that
the hinder portions of the zone of Nulliporæ, perishing as the reef
grows outwards, might be worn down by the surf. If this has not taken
place, the reef can in no part have increased outwards in breadth since
its formation, or at least since the Nulliporæ formed the convex mound
on its margin; for the zone thus formed, and which stands between two
and three feet above the other parts of the reef, is nowhere much above
twenty yards in width.
Thus far we have considered facts, which indicate, with more or less
probability, the increase of the atoll in its different parts: there
are others having an opposite tendency. On the south-east side,
Lieutenant Sulivan, to whose kindness I am indebted for many
interesting observations, found the conglomerate projecting on the reef
nearly fifty yards in front of the beach: we may infer from what we see
in all other parts of the atoll, that the conglomerate was not
originally so much exposed, but formed the base of an islet, the front
and upper part of which has since been swept away. The degree to which
the conglomerate, round nearly the whole atoll, has been scooped,
broken up, and the fragments cast on the beach, is certainly very
surprising, even on the view that it is the office of occasional gales
to pile up fragments, and of the daily tides to wear them away. On the
western side, also, of the atoll, where I have described a bed of sand
and fragments with trees growing out of it, in front of an old beach,
it struck both Lieutenant Sulivan and myself, from the manner in which
the trees were being washed down, that the surf had lately recommenced
an attack on this line of coast. Appearances indicating a slight
encroachment of the water on the land, are plainer within the lagoon: I
noticed in several places, both on its windward and leeward shores, old
cocoa-nut trees falling with their roots undermined, and the rotten
stumps of others on the beach, where the inhabitants assured us the
cocoa-nut could not now grow. Captain Fitzroy pointed out to me, near
the settlement, the foundation posts of a shed, now washed by every
tide, but which the inhabitants stated, had seven years before stood
above high watermark. In the calm waters of the lagoon, directly
connected with a great, and therefore stable ocean, it seems very
improbable that a change in the currents, sufficiently great to cause
the water to eat into the land on all sides, should have taken place
within a limited period. From these considerations I inferred, that
probably the atoll had lately subsided to a small amount; and this
inference was strengthened by the circumstance, that in 1834, two years
before our visit, the island had been shaken by a severe earthquake,
and by two slighter ones during the ten previous years. If, during
these subterranean disturbances, the atoll did subside, the downward
movement must have been very small, as we must
conclude from the fields of dead coral still lipping the surface of the
lagoon, and from the breakers on the western shore not having yet
regained the line of their former action. The subsidence must, also,
have been preceded by a long period of rest, during which the islets
extended to their present size, and the living margin of the reef grew
either upwards, or as I believe outwards, to its present distance from
the beach.
Whether this view be correct or not, the above facts are worthy of
attention, as showing how severe a struggle is in progress on these low
coral formations between the two nicely balanced powers of land and
water. With respect to the future state of Keeling atoll, if left
undisturbed, we can see that the islets may still extend in length; but
as they cannot resist the surf until broken by rolling over a wide
space, their increase in breadth must depend on the increasing breadth
of the reef; and this must be limited by the steepness of the submarine
flanks, which can be added to only by sediment derived from the wear
and tear of the coral. From the rapid growth of the coral in the
channel cut for the schooner, and from the several agents at work in
producing fine sediment, it might be thought that the lagoon would
necessarily become quickly filled up. Some of this sediment, however,
is transported into the open sea, as appears from the soundings off the
mouth of the lagoon, instead of being deposited within it. The
deposition, moreover, of sediment, checks the growth of coral-reefs, so
that these two agencies cannot act together with full effect in filling
it up. We know so little of the habits of the many different species of
corals, which form the lagoon-reefs, that we have no more reasons for
supposing that their whole surface would grow up as quickly as the
coral did in the schooner-channel, than for supposing that the whole
surface of a peat-moss would increase as quickly as parts are known to
do in holes, where the peat has been cut away. These agencies,
nevertheless, tend to fill up the lagoon; but in proportion as it
becomes shallower, so must the polypifers be subject to many injurious
agencies, such as impure water and loss of food. For instance, Mr.
Liesk informed me, that some years before our visit unusually heavy
rain killed nearly all the fish in the lagoon, and probably the same
cause would likewise injure the corals. The reefs also, it must be
remembered, cannot possibly rise above the level of the lowest
spring-tide, so that the final conversion of the lagoon into land must
be due to the accumulation of sediment; and in the midst of the clear
water of the ocean, and with no surrounding high land, this process
must be exceedingly slow.
_Section II_—GENERAL DESCRIPTION OF ATOLLS.
General form and size of atolls, their reefs and islets.—External
slope.—Zone of Nulliporæ.—Conglomerate.—Depth of
lagoons.—Sediment.—Reefs submerged wholly or in part.—Breaches in the
reef.—Ledge-formed shores round certain lagoons.—Conversion of lagoons
into land.
I will here give a sketch of the general form and structure of the many
atolls and atoll-formed reefs which occur in the Pacific and Indian
Oceans, comparing them with Keeling atoll. The Maldiva atolls
and the Great Chagos Bank differ in so many respects, that I shall
devote to them, besides occasional references, a third section of this
chapter. Keeling atoll may be considered as of moderate dimensions and
of regular form. Of the thirty-two islands surveyed by Captain Beechey
in the Low Archipelago, the longest was found to be thirty miles, and
the shortest less than a mile; but Vliegen atoll, situated in another
part of the same group, appears to be sixty miles long and twenty
broad. Most of the atolls in this group are of an elongated form; thus
Bow Island is thirty miles in length, and on an average only six in
width (See Fig. 4, Plate I), and Clermont Tonnere has nearly the same
proportions. In the Marshall Archipelago (the Ralick and Radack group
of Kotzebue) several of the atolls are more than thirty miles in
length, and Rimsky Korsacoff is fifty-four long, and twenty wide, at
the broadest part of its irregular outline. Most of the atolls in the
Maldiva Archipelago are of great size, one of them (which, however,
bears a double name) measured in a medial and slightly curved line, is
no less than eighty-eight geographical miles long, its greatest width
being under twenty, and its least only nine and a half miles. Some
atolls have spurs projecting from them; and in the Marshall group there
are atolls united together by linear reefs, for instance Menchikoff
Island (See Fig. 3, Plate II), which is sixty miles in length, and
consists of three loops tied together. In far the greater number of
cases an atoll consists of a simple elongated ring, with its outline
moderately regular.
The average width of the annular wreath may be taken as about a quarter
of a mile. Captain Beechey[5] says that in the atolls of the Low
Archipelago it exceeded in no instance half a mile. The description
given of the structure and proportional dimensions of the reef and
islets of Keeling atoll, appears to apply perfectly to nearly all the
atolls in the Pacific and Indian Oceans. The islets are first formed
some way back either on the projecting points of the reef, especially
if its form be angular, or on the sides of the main entrances into the
lagoon—that is in both cases, on points where the breakers can act
during gales of wind in somewhat different directions, so that the
matter thrown up from one side may accumulate against that before
thrown up from another. In Lutké’s chart of the Caroline atolls, we see
many instances of the former case; and the occurrence of islets, as if
placed for beacons, on the points where there is a gateway or breach
through the reef, has been noticed by several authors. There are some
atoll-formed reefs, rising to the surface of the sea and partly dry at
low water, on which from some cause islets have never been formed; and
there are others on which they have been formed, but have subsequently
been worn away. In atolls of small dimensions the islets frequently
become united into a single horse-shoe or ring-formed strip; but Diego
Garcia, although an atoll of considerable size, being thirteen miles
and a half in length, has its lagoon entirely surrounded, except at the
northern end, by a belt of land, on an average a third of a mile in
width. To show how small the total area of the annular reef and the
land is in islands of this class,
I may quote a remark from the voyage of Lutké, namely, that if the
forty-three rings, or atolls, in the Caroline Archipelago, were put one
within another, and over a steeple in the centre of St. Petersburg, the
whole world would not cover that city and its suburbs.
[5] Beechey’s “Voyage to the Pacific and Beering’s Straits,” chapter
viii.
The form of the bottom off Keeling atoll, which gradually slopes to
about twenty fathoms at the distance of between one and two hundred
yards from the edge of the reef, and then plunges at an angle of 45°
into unfathomable depths, is exactly the same[6] with that of the
sections of the atolls in the Low Archipelago given by Captain Beechey.
The nature, however, of the bottom seems to differ, for this officer[7]
informs me that all the soundings, even the deepest, were on coral, but
he does not know whether dead or alive. The slope round Christmas atoll
(Lat. 1° 4′ N., 157° 45′ W.), described by Cook,[8] is considerably
less, at about half a mile from the edge of the reef, the average depth
was about fourteen fathoms on a fine sandy bottom, and at a mile, only
between twenty and forty fathoms. It has no doubt been owing to this
gentle slope, that the strip of land surrounding its lagoon, has
increased in one part to the extraordinary width of three miles; it is
formed of successive ridges of broken shells and corals, like those on
the beach. I know of no other instance of such width in the reef of an
atoll; but Mr. F. D. Bennett informs me that the inclination of the
bottom round Caroline atoll in the Pacific, is like that off Christmas
Island, very gentle. Off the Maldiva and Chagos atolls, the inclination
is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell[9]
found fifty and sixty fathoms close to the edge of the reef, and at 300
yards distance there was no bottom with a 300-yard line. Captain
Moresby informs me, that at 100 fathoms from the mouth of the lagoon of
Diego Garcia, he found no bottom with 150 fathoms; this is the more
remarkable, as the slope is generally less abrupt in front of channels
through a reef, owing to the accumulation of sediment. At Egmont
Island, also, at 150 fathoms from the reef, soundings were struck with
150 fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef,
no bottom was obtained, as I am informed by Captain Moresby, with a
line of 200 fathoms! The currents run with great force round these
atolls, and where they are strongest, the inclination appears to be
most abrupt. I am informed by the same authority, that wherever
soundings were obtained off these islands, the bottom was invariably
sandy: nor was there any reason to suspect the existence of submarine
cliffs, as there was at Keeling Island.[10] Here then occurs a
difficulty; can sand accumulate on a slope, which, in some cases,
appears to exceed fifty-five degrees? It must be observed, that I speak
of slopes where soundings were obtained, and not of such cases, as that
of Cardoo, where the nature of the bottom is unknown, and where its
inclination must be nearly vertical. M. Elie de Beaumont[11] has
argued, and there is no higher authority on this subject, from the
inclination at which snow slides down in avalanches, that a bed of sand
or mud cannot be formed at a greater angle than thirty degrees.
Considering the number of soundings on sand, obtained round the Maldiva
and Chagos atolls, which appears to indicate a greater angle, and the
extreme abruptness of the sand-banks in the West Indies, as will be
mentioned in the Appendix, I must conclude that the adhesive property
of wet sand counteracts its gravity, in a much greater ratio than has
been allowed for by M. Elie de Beaumont. From the facility with which
calcareous sand becomes agglutinated, it is not necessary to suppose
that the bed of loose sand is thick.
[6] The form of the bottom round the Marshall atolls in the Northern
Pacific is probably similar: Kotzebue (“First Voyage,” vol. ii, p. 16)
says: “We had at a small distance from the reef, forty fathoms depth,
which increased a little further so much that we could find no
bottom.”
[7] I must be permitted to express my obligation to Captain Beechey,
for the very kind manner in which he has given me information on
several points, and to own the great assistance I have derived from
his excellent published work.
[8] Cook’s “Third Voyage,” vol. ii, chap. 10.
[9] This fact is taken from a MS. account of these groups lent me by
Captain Moresby. See also Captain Moresby’s paper on the Maldiva
atolls in the _Geographical Journal_, vol. v, p. 401.
[10] Off some of the islands in the Low Archipelago the bottom appears
to descend by ledges. Off Elizabeth Island, which, however, consists
of raised coral, Captain Beechey (page 45, 4to ed.) describes three
ledges: the first had an easy slope from the beach to a distance of
about fifty yards: the second extended two hundred yards with
twenty-five fathoms on it, and then ended abruptly, like the first;
and immediately beyond this there was no bottom with two hundred
fathoms.
[11] “Memoires pour servir à une description Geolog. de France,” tome
iv, p. 216.
Captain Beechey has observed, that the submarine slope is much less at
the extremities of the more elongated atolls in the Low Archipelago,
than at their sides; in speaking of Ducie’s Island he says[12] the
buttress, as it may be called, which “has the most powerful enemy (the
S.W. swell) to oppose, is carried out much further, and with less
abruptness than the other.” In some cases, the less inclination of a
certain part of the external slope, for instance of the northern
extremities of the two Keeling atolls, is caused by a prevailing
current which there accumulates a bed of sand. Where the water is
perfectly tranquil, as within a lagoon, the reefs generally grow up
perpendicularly, and sometimes even overhang their bases; on the other
hand, on the leeward side of Mauritius, where the water is generally
tranquil, although not invariably so, the reef is very gently inclined.
Hence it appears that the exterior angle varies much; nevertheless in
the close similarity in form between the sections of Keeling atoll and
of the atolls in the Low Archipelago, in the general steepness of the
reefs of the Maldiva and Chagos atolls, and in the perpendicularity of
those rising out of water always tranquil, we may discern the effects
of uniform laws; but from the complex action of the surf and currents,
on the growing powers of the coral and on the deposition of sediment,
we can by no means follow out all the results.
[12] Beechey’s “Voyage,” 4to ed., p. 44.
Where islets have been formed on the reef, that part which I have
sometimes called the “flat” and which is partly dry at low water,
appears similar in every atoll. In the Marshall group in the North
Pacific, it may be inferred from Chamisso’s description, that the reef,
where islets have not been formed on it, slopes gently from the
external margin to the shores of the lagoon; Flinders states that the
Australian barrier has a similar inclination inwards, and I have no
doubt it is of general occurrence, although, according to Ehrenberg,
the reefs of the Red Sea offer an exception. Chamisso observes that
“the red colour of the reef (at the Marshall atolls) under the breakers
is caused by a Nullipora, which covers the stone _wherever the waves
beat_; and, under favourable circumstances, assumes a stalactical
form,”—a description perfectly applicable to the margin of Keeling
atoll.[13] Although Chamisso does not state that the masses of
Nulliporæ form points or a mound, higher than the flat, yet I believe
that this is the case; for Kotzebue,[14] in another part, speaks of the
rocks on the edge of the reef “as visible for about two feet at low
water,” and these rocks we may feel quite certain are not formed of
true coral,[15] Whether a smooth convex mound of Nulliporæ, like that
which appears as if artificially constructed to protect the margin of
Keeling Island, is of frequent occurrence round atolls, I know not; but
we shall presently meet with it, under precisely the same form, on the
outer edge of the “barrier-reefs” which encircle the Society Islands.
[13] Kotzebue’s “First Voyage,” vol. iii, p. 142. Near Porto Praya, in
the Cape de Verde Islands, some basaltic rocks, lashed by no
inconsiderable surf, were completely enveloped with a layer of
Nulliporæ. The entire surface over many square inches, was coloured of
a peach-blossomed red; the layer, however, was of no greater thickness
than paper. Another kind, in the form of projecting knobs, grew in the
same situation. These Nulliporæ are closely related to those described
on the coral-reefs, but I believe are of different species.
[14] Kotzebue’s “First Voyage,” vol. ii, p. 16. Lieutenant Nelson, in
his excellent memoir in the Geological Transactions (vol. ii, p. 105),
alludes to the rocky points mentioned by Kotzebue, and infers that
they consist of Serpulæ, which compose incrusting masses on the reefs
of Bermudas, as they likewise do on a sandstone bar off the coast of
Brazil (which I have described in _London Phil. Journal,_ October
1841). These masses of Serpulæ hold the same position, relatively to
the action of the sea, with the Nulliporæ on the coral-reefs in the
Indian and Pacific Oceans.
[15] Captain Moresby, in his valuable paper “on the Northern atolls of
Maldivas” (_Geographical Journal_, vol. v), says that the edges of the
reefs there stand above water at low spring-tides.
There appears to be scarcely a feature in the structure of Keeling
reef, which is not of common, if not of universal occurrence, in other
atolls. Thus Chamisso describes[16] a layer of coarse conglomerate,
outside the islets round the Marshall atolls which “appears on its
upper surface uneven and eaten away.” From drawings, with appended
remarks, of Diego Garcia in the Chagos group and of several of the
Maldiva atolls, shown me by Captain Moresby,[17] it is evident that
their outer coasts are subject to the same round of decay and
renovation as those of Keeling atoll. From the description of the
atolls in the Low Archipelago, given in Captain Beechey’s “Voyage,” it
is not apparent that any conglomerate coral-rock was there observed.
[16] Kotzebue’s “First Voyage,” vol. iii, p. 144.
[17] See also Moresby on the Northern atolls of the Maldivas,
_Geographical Journal_, vol v, p. 400.
The lagoon in Keeling atoll is shallow; in the atolls of the Low
Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall
Group, according to Chamisso, from 30 to 35; in the Caroline atolls it
is only a little less. Within the Maldiva atolls there are large spaces
with 45 fathoms, and some soundings are laid down of 49 fathoms. The
greater part of the bottom in most lagoons, is formed of sediment;
large spaces have exactly the same depth, or the depth varies so
insensibly, that it is evident that no other means, excepting aqueous
deposition, could have leveled the surface so equally. In the Maldiva
atolls this is very conspicuous, and likewise in some of the Caroline
and Marshall Islands. In the former large spaces consist of sand and
_soft clay_; and Kotzebue speaks of clay having been found within one
of the Marshall atolls. No doubt this clay is calcareous mud, similar
to that at Keeling Island, and to that at Bermuda already referred to,
as undistinguishable from disintegrated chalk, and which Lieutenant
Nelson says is called there pipe-clay.[18]
[18] I may here observe that on the coast of Brazil, where there is
much coral, the soundings near the land are described by Admiral
Roussin, in the _Pilote du Brésil_, as siliceous sand, mingled with
much finely comminuted particles of shells and coral. Further in the
offing, for a space of 1,300 miles along the coast, from the Abrolhos
Islands to Maranham, the bottom in many places is composed of “tuf
blanc, mêlé ou formé de madrépores broyés.” This white substance,
probably, is analogous to that which occurs within the above-mentioned
lagoons; it is sometimes, according to Roussin, firm, and he compares
it to mortar.
Where the waves act with unequal force on the two sides of an atoll,
the islets appear to be first formed, and are generally of greater
continuity on the more exposed shore. The islets, also, which are
placed to leeward, are in most parts of the Pacific liable to be
occasionally swept entirely away by gales, equalling hurricanes in
violence, which blow in an opposite direction to the ordinary
trade-wind. The absence of the islets on the leeward side of atolls, or
when present their lesser dimensions compared with those to windward,
is a comparatively unimportant fact; but in several instances the reef
itself on the leeward side, retaining its usual defined outline, does
not rise to the surface by several fathoms. This is the case with the
southern side of Peros Banhos (Plate 1, Fig. 9) in the Chagos group,
with Mourileu atoll,[19] in the Caroline Archipelago, and with the
barrier-reef (Plate I, Fig. 8) of the Gambier Islands. I allude to the
latter reef, although belonging to
another class, because Captain Beechey was first led by it to observe
the peculiarity in the question. At Peros Banhos the submerged part is
nine miles in length, and lies at an average depth of about five
fathoms; its surface is nearly level, and consists of hard stone, with
a thin covering of loose sand. There is scarcely any living coral on
it, even on the outer margin, as I have been particularly assured by
Captain Moresby; it is, in fact, a wall of dead coral-rock, having the
same width and transverse section with the reef in its ordinary state,
of which it is a continuous portion. The living and perfect parts
terminate abruptly, and abut on the submerged portions, in the same
manner as on the sides of an ordinary passage through the reef. The
reef to leeward in other cases is nearly or quite obliterated, and one
side of the lagoon is left open; for instance, at Oulleay (Caroline
Archipelago), where a crescent-formed reef is fronted by an irregular
bank, on which the other half of the annular reef probably once stood.
At Namonouito, in the same Archipelago, both these modifications of the
reef concur; it consists of a great flat bank, with from twenty to
twenty-five fathoms water on it; for a length of more than forty miles
on its southern side it is open and without any reef, whilst on the
other sides it is bounded by a reef, in parts rising to the surface and
perfectly characterised, in parts lying some fathoms submerged. In the
Chagos group there are annular reefs, entirely submerged, which have
the same structure as the submerged and defined portions just
described. The Speaker’s Bank offers an excellent example of this
structure; its central expanse, which is about twenty-two fathoms deep,
is twenty-four miles across; the external rim is of the usual width of
annular reefs, and is well-defined; it lies between six and eight
fathoms beneath the surface, and at the same depth there are scattered
knolls in the lagoon. Captain Moresby believes the rim consists of dead
rock, thinly covered with sand, and he is certain this is the case with
the external rim of the Great Chagos Bank, which is also essentially a
submerged atoll. In both these cases, as in the submerged portion of
the reef at Peros Banhos, Captain Moresby feels sure that the quantity
of living coral, even on the outer edge overhanging the deep-sea water,
is quite insignificant. Lastly, in several parts of the Pacific and
Indian Oceans there are banks, lying at greater depths than in the
cases just mentioned, of the same form and size with the neighbouring
atolls, but with their atoll-like structure wholly obliterated. It
appears from the survey of Freycinet, that there are banks of this kind
in the Caroline Archipelago, and, as is reported, in the Low
Archipelago. When we discuss the origin of the different classes of
coral formations, we shall see that the submerged state of the whole of
some atoll-formed reefs, and of portions of others, generally but not
invariably on the leeward side, and the existence of more deeply
submerged banks now possessing little or no signs of their original
atoll-like structure, are probably the effects of a uniform
cause,—namely, the death of the coral, during the subsidence of the
area, in which the atolls or banks are situated.
[19] Frederick Lutké’s “Voyage autour du Monde,” vol. ii, p. 291. See
also his account of Namonouito, at pp. 97 and 105, and the chart of
Oulleay in the Atlas.
There is seldom, with the exception of the Maldiva atolls, more than
two or three channels, and generally only one leading into the lagoon,
of sufficient depth for a ship to enter. in small atolls, there is
usually not even one. Where there is deep water, for instance above
twenty fathoms, in the middle of the lagoon, the channels through the
reef are seldom as deep as the centre,—it may be said that the rim only
of the saucer-shaped hollow forming the lagoon is notched. Mr.
Lyell[20] has observed that the growth of the coral would tend to
obstruct all the channels through a reef, except those kept open by
discharging the water, which during high tide and the greater part of
each ebb is thrown over its circumference. Several facts indicate that
a considerable quantity of sediment is likewise discharged through
these channels; and Captain Moresby informs me that he has observed,
during the change of the monsoon, the sea discoloured to a distance off
the entrances into the Maldiva and Chagos atolls. This, probably, would
check the growth of the coral in them, far more effectually than a mere
current of water. In the many small atolls without any channel, these
causes have not prevented the entire ring attaining the surface. The
channels, like the submerged and effaced parts of the reef, very
generally though not invariably occur on the leeward side of the atoll,
or on that side, according to Beechey,[21] which, from running in the
same direction with the prevalent wind, is not fully exposed to it.
Passages between the islets on the reef, through which boats can pass
at high water, must not be confounded with ship-channels, by which the
annular reef itself is breached. The passages between the islets occur,
of course, on the windward as well as on the leeward side; but they are
more frequent and broader to leeward, owing to the lesser dimensions of
the islets on that side.
[20] “Principles of Geology,” vol. iii, p. 289.
[21] Beechey’s “Voyage,” 4to ed., vol. i, p. 189.
At Keeling atoll the shores of the lagoon shelve gradually, where the
bottom is of sediment, and irregularly or abruptly where there are
coral-reefs; but this is by no means the universal structure in other
atolls. Chamisso,[22] speaking in general terms of the lagoons in the
Marshall atolls, says the lead generally sinks “from a depth of two or
three fathoms to twenty or twenty-four, and you may pursue a line in
which on one side of the boat you may see the bottom, and on the other
the azure-blue deep water.” The shores of the lagoon-like channel
within the barrier-reef at Vanikoro have a similar structure. Captain
Beechey has described a modification of this structure (and he believes
it is not uncommon) in two atolls in the Low Archipelago, in which the
shores of the lagoon descend by a few, broad, slightly inclined ledges
or steps: thus at Matilda atoll,[23] the great exterior reef, the
surface of which is gently inclined towards and beneath the surface of
the lagoon, ends abruptly in a little cliff three fathoms deep; at its
foot, a ledge forty yards wide extends, shelving gently inwards
like the surface-reef, and terminated by a second little cliff five
fathoms deep; beyond this, the bottom of the lagoon slopes to twenty
fathoms, which is the average depth of its centre. These ledges seem to
be formed of coral-rock; and Captain Beechey says that the lead often
descended several fathoms through holes in them. In some atolls, all
the coral reefs or knolls in the lagoon come to the surface at low
water; in other cases of rarer occurrence, all lie at nearly the same
depth beneath it, but most frequently they are quite irregular,—some
with perpendicular, some with sloping sides,—some rising to the
surface, and others lying at all intermediate depths from the bottom
upwards. I cannot, therefore, suppose that the union of such reefs
could produce even one uniformly sloping ledge, and much less two or
three, one beneath the other, and each terminated by an abrupt wall. At
Matilda Island, which offers the best example of the step-like
structure, Captain Beechey observes that the coral-knolls within the
lagoon are quite irregular in their height. We shall hereafter see that
the theory which accounts for the ordinary form of atolls, apparently
includes this occasional peculiarity in their structure.
[22] Kotzebue’s “First Voyage,” vol. iii, p. 142.
[23] Beechey’s “Voyage,” 4to ed., vol. i, p. 160. At Whitsunday Island
the bottom of the lagoon slopes gradually towards the centre, and then
deepens suddenly, the edge of the bank being nearly perpendicular.
This bank is formed of coral and dead shells.
In the midst of a group of atolls, there sometimes occur small, flat,
very low islands of coral formation, which probably once included a
lagoon, since filled up with sediment and coral-reefs. Captain Beechey
entertains no doubt that this has been the case with the two small
islands, which alone of thirty-one surveyed by him in the Low
Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15 deg
S.) is described by Chamisso[24] as formed by a dam of madreporitic
rock inclosing a flat space, thinly covered with trees, into which the
sea on the leeward side occasionally breaks. North Keeling atoll
appears to be in a rather less forward stage of conversion into land;
it consists of a horse-shoe shaped strip of land surrounding a muddy
flat, one mile in its longest axis, which is covered by the sea only at
high water. When describing South Keeling atoll, I endeavoured to show
how slow the final process of filling up a lagoon must be;
nevertheless, as all causes do tend to produce this effect, it is very
remarkable that not one instance, as I believe, is known of a
moderately sized lagoon being filled up even to the low water-line at
spring-tides, much less of such a one being converted into land. It is,
likewise, in some degree remarkable, how few atolls, except small ones,
are surrounded by a single linear strip of land, formed by the union of
separate islets. We cannot suppose that the many atolls in the Pacific
and Indian Oceans all have had a late origin, and yet should they
remain at their present level, subjected only to the action of the sea
and to the growing powers of the coral, during as many centuries as
must have elapsed since any of the earlier tertiary epochs, it cannot,
I think, be doubted that their lagoons and the islets on their reef,
would present a totally different appearance from what they now do.
This consideration leads to the suspicion that some renovating agency
(namely subsidence) comes into play at intervals, and perpetuates their
original structure.
[24] Kotzebue’s “First Voyage,” vol. iii, p. 221.
_Section III_—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK
Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
depths in the lagoons of the southern atolls.—Reefs in the lagoons all
rising to the surface.—Position of islets and breaches in the reefs,
with respect to the prevalent winds and action of the
waves.—Destruction of islets.—Connection in the position and submarine
foundation of distinct atolls.—The apparent disseverment of large
atolls.—The Great Chagos Bank.—Its submerged condition and
extraordinary structure.
Although occasional references have been made to the Maldiva atolls,
and to the banks in the Chagos group, some points of their structure
deserve further consideration. My description is derived from an
examination of the admirable charts lately published from the survey of
Captain Moresby and Lieutenant Powell, and more especially from
information which Captain Moresby has communicated to me in the kindest
manner.
The Maldiva Archipelago is 470 miles in length, with an average breadth
of about 50 miles. The form and dimensions of the atolls, and their
singular position in a double line, may be seen, but not well, in the
greatly reduced chart (Fig. 6) in Plate II. The dimensions of the
longest atoll in the group (called by the double name of
Milla-dou-Madou and Tilla-dou-Matte) have already been given; it is 88
miles in a medial and slightly curved line, and is less than 20 miles
in its broadest part. Suadiva, also, is a noble atoll, being 44 miles
across in one direction, and 34 in another, and the great included
expanse of water has a depth of between 250 and 300 feet. The smaller
atolls in this group differ in no respect from ordinary ones; but the
larger ones are remarkable from being breached by numerous deep-water
channels leading into the lagoon; for instance, there are 42 channels,
through which a ship could enter the lagoon of Suadiva. In the three
southern large atolls, the separate portions of reef between these
channels have the ordinary structure, and are linear; but in the other
atolls, especially the more northern ones, these portions are
ring-formed, like miniature atolls. Other ring-formed reefs rise out of
the lagoons, in the place of those irregular ones which ordinarily
occur there. In the reduction of the chart of Mahlos Mahdoo (Plate II,
Fig. 4), it was not found easy to define the islets and the little
lagoons within each reef, so that the ring-formed structure is very
imperfectly shown; in the large published charts of Tilla-dou-Matte,
the appearance of these rings, from standing further apart from each
other, is very remarkable. The rings on the margin are generally
elongated; many of them are three, and some even five miles, in
diameter; those within the lagoon are usually smaller, few being more
than two miles across, and the greater number rather less than one. The
depth of the little lagoon within these small annular reefs is
generally from five to seven fathoms, but occasionally more; and in Ari
atoll many of the central ones are twelve, and some even more than
twelve fathoms deep. These rings rise abruptly from the platform or
bank, on which they are placed; their outer margin is
invariably bordered by living coral[25] within which there is a flat
surface of coral rock; of this flat, sand and fragments have in many
cases accumulated and been converted into islets, clothed with
vegetation. I can, in fact, point out no essential difference between
these little ring-formed reefs (which, however, are larger, and contain
deeper lagoons than many true atolls that stand in the open sea), and
the most perfectly characterised atolls, excepting that the ring-formed
reefs are based on a shallow foundation, instead of on the floor of the
open sea, and that instead of being scattered irregularly, they are
grouped closely together on one large platform, with the marginal rings
arranged in a rudely formed circle.
[25] Captain Moresby informs me that _Millepora complanata_ is one of
the commonest kinds on the outer margin, as it is at Keeling atoll.
The perfect series which can be traced from portions of simple linear
reef, to others including long linear lagoons, and from these again to
oval or almost circular rings, renders it probable that the latter are
merely modifications of the linear or normal state. It is conformable
with this view, that the ring-formed reefs on the margin, even where
most perfect and standing furthest apart, generally have their longest
axes directed in the line which the reef would have held, if the atoll
had been bounded by an ordinary wall. We may also infer that the
central ring-formed reefs are modifications of those irregular ones,
which are found in the lagoons of all common atolls. It appears from
the charts on a large scale, that the ring-like structure is contingent
on the marginal channels or breaches being wide; and, consequently, on
the whole interior of the atoll being freely exposed to the waters of
the open sea. When the channels are narrow or few in number, although
the lagoon be of great size and depth (as in Suadiva), there are no
ring-formed reefs; where the channels are somewhat broader, the
marginal portions of reef, and especially those close to the larger
channels, are ring-formed, but the central ones are not so; where they
are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed. Although their presence is thus contingent on
the openness of the marginal channels, the theory of their formation,
as we shall hereafter see, is included in that of the parent atolls, of
which they form the separate portions.
The lagoons of all the atolls in the southern part of the Archipelago
are from ten to twenty fathoms deeper than those in the northern part.
This is well exemplified in the case of Addoo, the southernmost atoll
in the group, for although only nine miles in its longest diameter, it
has a depth of thirty-nine fathoms, whereas all the other small atolls
have comparatively shallow lagoons; I can assign no adequate cause for
this difference in depth. In the central and deepest part of the
lagoons, the bottom consists, as I am informed by Captain Moresby, of
stiff clay (probably a calcareous mud); nearer the border it consists
of sand, and in the channels through the reef, of hard sand-banks,
sandstone, conglomerate rubble, and a little live coral. Close outside
the reef and the line joining its detached portions (where intersected
by many channels), the bottom is sandy, and it slopes abruptly into
unfathomable depths.
In most lagoons the depth is considerably greater in the centre than in
the channels; but in Tilla-dou-Matte, where the marginal ring-formed
reefs stand far apart, the same depth is carried across the entire
atoll, from the deep-water line on one side to that on the other. I
cannot refrain from once again remarking on the singularity of these
atolls,—a great sandy and generally concave disc rises abruptly from
the unfathomable ocean, with its central expanse studded and its border
symmetrically fringed with oval basins of coral-rock, just lipping the
surface of the sea, sometimes clothed with vegetation, and each
containing a little lake of clear water!
In the southern Maldiva atolls, of which there are nine large ones, all
the small reefs within the lagoons come to the surface, and are dry at
low water spring-tides; hence in navigating them, there is no danger
from submarine banks. This circumstance is very remarkable, as within
some atolls, for instance those of the neighbouring Chagos group, not a
single reef comes to the surface, and in most other cases a few only
do, and the rest lie at all intermediate depths from the bottom
upwards. When treating of the growth of coral I shall again refer to
this subject.
Although in the neighbourhood of the Maldiva Archipelago the winds,
during the monsoons, blow during nearly an equal time from opposite
quarters, and although, as I am informed by Captain Moresby, the
westerly winds are the strongest, yet the islets are almost all placed
on the eastern side of the northern atolls, and on the south-eastern
side of the southern atolls. That the formation of the islets is due to
detritus thrown up from the outside, as in the ordinary manner, and not
from the interior of the lagoons, may, I think be safely inferred from
several considerations, which it is hardly worth while to detail. As
the easterly winds are not the strongest, their action probably is
aided by some prevailing swell or current.
In groups of atolls, exposed to a trade-wind, the ship-channels into
the lagoons are almost invariably situated on the leeward or less
exposed side of the reef, and the reef itself is sometimes either
wanting there, or is submerged. A strictly analogous, but different
fact, may be observed at the Maldiva atolls—namely, that where two
atolls stand in front of each other, the breaches in the reef are the
most numerous on their near, and therefore less exposed, sides. Thus on
the near sides of Ari and the two Nillandoo atolls, which face S. Mãle,
Phaleedoo, and Moloque atolls, there are seventy-three deep-water
channels, and only twenty-five on their outer sides; on the near side
of the three latter named atolls there are fifty-six openings, and only
thirty-seven on their outsides. It is scarcely possible to attribute
this difference to any other cause than the somewhat different action
of the sea on the two sides, which would ensue from the protection
afforded by the two rows of atolls to each other. I may here remark
that in most cases, the conditions favourable to the greater
accumulation of fragments on the reef and to its more perfect
continuity on one side of the atoll than on the other, have concurred,
but this has not been the case with the Maldivas; for we have seen that
the islets are placed on the eastern or south-eastern
sides, whilst the breaches in the reef occur indifferently on any side,
where protected by an opposite atoll. The reef being more continuous on
the outer and more exposed sides of those atolls which stand near each
other, accords with the fact, that the reef of the southern atolls is
more continuous than that of the northern ones; for the former, as I am
informed by Captain Moresby, are more constantly exposed than the
northern atolls to a heavy surf.
The date of the first formation of some of the islets in this
Archipelago is known to the inhabitants; on the other hand, several
islets, and even some of those which are believed to be very old, are
now fast wearing away. The work of destruction has, in some instances,
been completed in ten years. Captain Moresby found on one water-washed
reef the marks of wells and graves, which were excavated when it
supported an islet. In South Nillandoo atoll, the natives say that
three of the islets were formerly larger: in North Nillandoo there is
one now being washed away; and in this latter atoll Lieutenant Prentice
found a reef, about six hundred yards in diameter, which the natives
positively affirmed was lately an island covered with cocoa-nut trees.
It is now only partially dry at low water spring-tides, and is (in
Lieutenant Prentice’s words) “entirely covered with live coral and
madrepore.” In the northern part, also, of the Maldiva Archipelago and
in the Chagos group, it is known that some of the islets are
disappearing. The natives attribute these effects to variations in the
currents of the sea. For my own part I cannot avoid suspecting that
there must be some further cause, which gives rise to such a cycle of
change in the action of the currents of the great and open ocean.
Several of the atolls in this Archipelago are so related to each other
in form and position, that at the first glance one is led to suspect
that they have originated in the disseverment of a single one. Mãle
consists of three perfectly characterised atolls, of which the shape
and relative position are such, that a line drawn closely round all
three, gives a symmetrical figure; to see this clearly, a larger chart
is required than that of the Archipelago in Plate II; the channel
separating the two northern Male atolls is only little more than a mile
wide, and no bottom was found in it with 100 fathoms. Powell’s Island
is situated at the distance of two miles and a half off the northern
end of Mahlos Mahdoo (see Fig. 4, Plate II), at the exact point where
the two sides of the latter, if prolonged, would meet; no bottom,
however, was found in the channel with 200 fathoms; in the wider
channel between Horsburgh atoll and the southern end of Mahlos Mahdoo,
no bottom was found with 250 fathoms. In these and similar cases, the
relation consists only in the form and position of the atolls. But in
the channel between the two Nillandoo atolls, although three miles and
a quarter wide, soundings were struck at the depth of 200 fathoms; the
channel between Ross and Ari atolls is four miles wide, and only 150
fathoms deep. Here then we have, besides the relation of form, a
submarine connection. The fact of soundings having been obtained
between two separate and perfectly characterised atolls is in itself
interesting, as it has never, I believe, been effected in any of the
many
other groups of atolls in the Pacific and Indian seas. In continuing to
trace the connection of adjoining atolls, if a hasty glance be taken at
the chart (Fig. 4, Plate II) of Mahlos Mahdoo, and the line of
unfathomable water be followed, no one will hesitate to consider it as
one atoll. But a second look will show that it is divided by a
bifurcating channel, of which the northern arm is about one mile and
three-quarters in width, with an average depth of 125 fathoms, and the
southern one three-quarters of a mile wide, and rather less deep. These
channels resemble in the slope of their sides and general form, those
which separate atolls in every respect distinct; and the northern arm
is wider than that dividing two of the Mãle atolls. The ring-formed
reefs on the sides of this bifurcating channel are elongated, so that
the northern and southern portions of Mahlos Mahdoo may claim, as far
as their external outline is concerned, to be considered as distinct
and perfect atolls. But the intermediate portion, lying in the fork of
the channel, is bordered by reefs less perfect than those which
surround any other atoll in the group of equally small dimensions.
Mahlos Mahdoo, therefore, is in every respect in so intermediate a
condition, that it may be considered either as a single atoll nearly
dissevered into three portions, or as three atolls almost perfect and
intimately connected. This is an instance of a very early stage of the
apparent disseverment of an atoll, but a still earlier one in many
respects is exhibited at Tilla-dou-Matte. In one part of this atoll,
the ring-formed reefs stand so far apart from each other, that the
inhabitants have given different names to the northern and southern
halves; nearly all the rings, moreover, are so perfect and stand so
separate, and the space from which they rise is so level and unlike a
true lagoon, that we can easily imagine the conversion of this one
great atoll, not into two or three portions, but into a whole group of
miniature atolls. A perfect series such as we have here traced,
impresses the mind with an idea of actual change; and it will hereafter
be seen, that the theory of subsidence, with the upward growth of the
coral, modified by accidents of probable occurrence, will account for
the occasional disseverment of large atolls.
_Plate II_—Great Chagos Bank, New Caledonia, Menchikoff Atoll, etc.
[Illustration: Great Chagos Bank]
Fig. 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey
by Captain Moresby and Lieutenant Powell; the parts which are shaded,
with the exception of two or three islets on the western and northern
sides, do not rise to the surface, but are submerged from four to ten
fathoms; the banks bounded by the dotted lines lie from fifteen to
twenty fathoms beneath the surface, and are formed of sand; the central
space is of mud, and from thirty to fifty fathoms deep.
Fig. 2.—A vertical section, on the same scale, in an eastern and
western line across the Great Chagos Bank, given for the sake of
exhibiting more clearly its structure.
[Illustration: New Caledonia, Menchikoff Atoll, etc.]
Fig. 3.—Menchikoff Atoll (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the
Pacific;” originally surveyed by Captain Hagemeister; the depth within
the lagoons is unknown.
Fig. 4.—MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
Powell; the white spaces in the middle of the separate small reefs,
both on the margin and in the middle part, are meant to represent
little lagoons; but it was found not possible to distinguish them
clearly from the small islets, which have been formed on these same
small reefs; many of the smaller reefs could not be introduced; the
nautical mark (dot over a dash) over the figures 250 and 200, between
Mahlos Mahdoo and Horsburgh atoll and Powell’s island, signifies that
soundings were not obtained at these depths.
Fig. 5.—NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern’s “Atlas,” compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the “Atlas
of the Voyage of the _Astrolabe_.” In Krusenstern’s “Atlas,” the reef
is represented by a single line with crosses; I have for the sake of
uniformity added an interior line.
Fig. 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
Captain Moresby and Lieutenant Powell.
The Great Chagos bank alone remains to be described. In the Chagos
group there are some ordinary atolls, some annular reefs rising to the
surface but without any islets on them, and some atoll-formed banks,
either quite submerged, or nearly so. Of the latter, the Great Chagos
Bank is much the largest, and differs in its structure from the others:
a plan of it is given in Plate II, Fig. 1, in which, for the sake of
clearness, I have had the parts under ten fathoms deep finely shaded:
an east and west vertical section is given in Fig. 2, in which the
vertical scale has been necessarily exaggerated. Its longest axis is
ninety nautical miles, and another line drawn at right angles to the
first, across the broadest part, is seventy. The central part consists
of a level muddy flat, between forty and fifty fathoms deep, which is
surrounded on all sides, with the exception of some breaches, by the
steep edges of a set of banks, rudely arranged in a circle. These banks
consist of sand, with a very little live coral; they vary in breadth
from five to twelve miles, and on an average lie about sixteen fathoms
beneath the surface;
they are bordered by the steep edges of a third narrow and upper bank,
which forms the rim to the whole. This rim is about a mile in width,
and with the exception of two or three spots where islets have been
formed, is submerged between five and ten fathoms. It consists of
smooth hard rock, covered with a thin layer of sand, but with scarcely
any live coral; it is steep on both sides, and outwards slopes abruptly
into unfathomable depths. At the distance of less than half a mile from
one part, no bottom was found with 190 fathoms; and off another point,
at a somewhat greater distance, there was none with 210 fathoms. Small
steep-sided banks or knolls, covered with luxuriantly growing coral,
rise from the interior expanse to the same level with the external rim,
which, as we have seen, is formed only of dead rock. It is impossible
to look at the plan (Fig. 1, Plate II), although reduced to so small a
scale, without at once perceiving that the Great Chagos Bank is, in the
words of Captain Moresby,[26] “nothing more than a half-drowned atoll.”
But of what great dimensions, and of how extraordinary an internal
structure? We shall hereafter have to consider both the cause of its
submerged condition, a state common to other banks in the group, and
the origin of the singular submarine terraces, which bound the central
expanse: these, I think, it can be shown, have resulted from a cause
analogous to that which has produced the bifurcating channel across
Mahlos Mahdoo.
[26] This officer has had the kindness to lend me an excellent MS.
account of the Chagos Islands; from this paper, from the published
charts, and from verbal information communicated to me by Captain
Moresby, the above account of the Great Chagos Bank is taken.
Chapter II BARRIER REEFS
Closely resemble in general form and structure atoll-reefs.—Width and
depth of the lagoon-channels.—Breaches through the reef in front of
valleys, and generally on the leeward side.—Checks to the filling up of
the lagoon-channels.—Size and constitution of the encircled
islands.—Number of islands within the same reef.—Barrier-reefs of New
Caledonia and Australia.—Position of the reef relative to the slope of
the adjoining land.—Probable great thickness of barrier-reefs.
The term “barrier” has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to
that on the western coast of New Caledonia. At one time I thought it
convenient thus to restrict the term, but as these reefs are similar in
structure, and in position relatively to the land, to those, which,
like a wall with a deep moat within, encircle many smaller islands, I
have classed them together. The reef, also, on the west coast of New
Caledonia, circling round the extremities of the island, is an
intermediate form between a small encircling reef and the Australian
barrier, which stretches for a thousand miles in nearly a straight
line.
The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high
land rising from within their central expanse. The general resemblance
between the reefs of the barrier and atoll classes may be seen in the
small, but accurately reduced charts on Plate I,[1] and this
resemblance can be further shown to extend to every part of the
structure. Beginning with the outside of the reef; many scattered
soundings off Gambier, Oualan, and some other encircled islands, show
that close to the breakers there exists a narrow shelving margin,
beyond which the ocean becomes suddenly unfathomable; but off the west
coast of New Caledonia, Captain Kent[2] found no bottom with 150
fathoms, at two ships’ length from the reef; so that the slope here
must be nearly as precipitous as off the Maldiva atolls.
[1] The authorities from which these charts have been reduced,
together with some remarks on them are given in a separately appended
page, descriptive of the Plates.
[2] Dalrymple, “Hydrog. Mem.” vol. iii.
I can give little information regarding the kinds of corals which live
on the outer margin. When I visited the reef at Tahiti, although it was
low water, the surf was too violent for me to see the living masses;
but, according to what I heard from some intelligent native chiefs,
they resemble in their rounded and branchless forms, those on the
margin of Keeling atoll. The extreme verge of the reef, which was
visible between the breaking waves at low water, consisted of a
rounded, convex, artificial-like breakwater, entirely coated with
Nulliporæ, and absolutely similar to that which I have described at
Keeling atoll. From what I heard when at Tahiti, and from the writings
of the Revs. W. Ellis and J. Williams, I conclude that this peculiar
structure is common to most of the encircled islands of the Society
Archipelago. The reef within this mound or breakwater, has an extremely
irregular surface, even more so than between the islets on the reef of
Keeling atoll, with which alone (as there are no islets on the reef of
Tahiti) it can properly be compared. At Tahiti, the reef is very
irregular in width; but round many other encircled islands, for
instance, Vanikoro or Gambier Islands (Figs 1 and 8, Plate I), it is
quite as regular, and of the same average width, as in true atolls.
Most barrier-reefs on the inner side slope irregularly into the
lagoon-channel (as the space of deep water separating the reef from the
included land may be called), but at Vanikoro the reef slopes only for
a short distance, and then terminates abruptly in a submarine wall,
forty feet high,—a structure absolutely similar to that described by
Chamisso in the Marshall atolls.
In the Society Archipelago, Ellis[3] states, that the reefs generally
lie at the distance of from one to one and a half miles, and,
occasionally, even at more than three miles, from the shore. The
central mountains are generally bordered by a fringe of flat, and often
marshy, alluvial
land, from one to four miles in width. This fringe consists of
coral-sand and detritus thrown up from the lagoon-channel, and of soil
washed down from the hills; it is an encroachment on the channel,
analogous to that low and inner part of the islets in many atolls which
is formed by the accumulation of matter from the lagoon. At Hogoleu
(Fig. 2, Plate I), in the Caroline Archipelago,[4] the reef on the
south side is no less than twenty miles; on the east side, five; and on
the north side, fourteen miles from the encircled high islands.
[3] Consult, on this and other points, the “Polynesian Researches,” by
the Rev. W. Ellis, an admirable work, full of curious information.
[4] See “Hydrographical Mem.” and the “Atlas of the Voyage of the
_Astrolabe_,” by Captain Dumont D’Urville, p. 428.
The lagoon channels may be compared in every respect with true lagoons.
In some cases they are open, with a level bottom of fine sand; in
others they are choked up with reefs of delicately branched corals,
which have the same general character as those within the Keeling
atoll. These internal reefs either stand separately, or more commonly
skirt the shores of the included high islands. The depth of the
lagoon-channel round the Society Islands varies from two or three to
thirty fathoms; in Cook’s[5] chart of Ulieta, however, there is one
sounding laid down of forty-eight fathoms; at Vanikoro there are
several of fifty-four and one of fifty-six and a half fathoms
(English), a depth which even exceeds by a little that of the interior
of the great Maldiva atolls. Some barrier-reefs have very few islets on
them; whilst others are surmounted by numerous ones; and those round
part of Bolabola (Plate I, Fig. 5) form a single linear strip. The
islets first appear either on the angles of the reef, or on the sides
of the breaches through it, and are generally most numerous on the
windward side. The reef to leeward retaining its usual width, sometimes
lies submerged several fathoms beneath the surface; I have already
mentioned Gambier Island as an instance of this structure. Submerged
reefs, having a less defined outline, dead, and covered with sand, have
been observed (see Appendix) off some parts of Huaheine and Tahiti. The
reef is more frequently breached to leeward than to windward; thus I
find in Krusenstern’s “Memoir on the Pacific,” that there are passages
through the encircling reef on the leeward side of each of the seven
Society Islands, which possess ship-harbours; but that there are
openings to windward through the reef of only three of them. The
breaches in the reef are seldom as deep as the interior lagoon-like
channel; they generally occur in front of the main valleys, a
circumstance which can be accounted for, as will be seen in the fourth
chapter, without much difficulty. The breaches being situated in front
of the valleys, which descend indifferently on all sides, explains
their more frequent occurrence through the windward side of
barrier-reefs than through the windward side of atolls,—for in atolls
there is no included land to influence the position of the breaches.
[5] See the chart in vol. i of Hawkesworth’s 4to ed. of “Cook’s First
Voyage.”
It is remarkable, that the lagoon-channels round mountainous islands
have not in every instance been long ago filled up with coral and
sediment; but it is more easily accounted for than appears at first
sight. In cases like that of Hogoleu and the Gambier Islands, where a
few
small peaks rise out of a great lagoon, the conditions scarcely differ
from those of an atoll, and I have already shown, at some length, that
the filling up of a true lagoon must be an extremely slow process.
Where the channel is narrow, the agency, which on unprotected coasts is
most productive of sediment, namely the force of the breakers, is here
entirely excluded, and the reef being breached in the front of the main
valleys, much of the finer mud from the rivers must be transported into
the open sea. As a current is formed by the water thrown over the edge
of atoll-formed reefs, which carries sediment with it through the
deep-water breaches, the same thing probably takes place in
barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up. The low alluvial border, however,
at the foot of the encircled mountains, shows that the work of filling
up is in progress; and at Maura (Plate I, Fig. 6), in the Society
group, it has been almost effected, so that there remains only one
harbour for small craft.
If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many
points already noticed of resemblance, or rather of identity in
structure with atolls, there is a close general agreement in form,
average dimensions, and grouping. Encircling barrier-reefs, like
atolls, are generally elongated, with an irregularly rounded, though
sometimes angular outline. There are atolls of all sizes, from less
than two miles in diameter to sixty miles (excluding Tilla-dou-Matte,
as it consists of a number of almost independent atoll-formed reefs);
and there are encircling barrier-reefs from three miles and a half to
forty-six miles in diameter,—Turtle Island being an instance of the
former, and Hogoleu of the latter. At Tahiti the encircled island is
thirty-six miles in its longest axis, whilst at Maurua it is only a
little more than two miles. It will be shown, in the last chapter in
this volume, that there is the strictest resemblance in the grouping of
atolls and of common islands, and consequently there must be the same
resemblance in the grouping of atolls and of encircling barrier-reefs.
The islands lying within reefs of this class, are of very various
heights. Tahiti[6] is 7,000 feet; Maurua about 800; Aitutaki 360, and
Manouai only 50. The geological nature of the included land varies: in
most cases it is of ancient volcanic origin, owing apparently to the
fact that islands of this nature are most frequent within all great
seas; some, however, are of madreporitic limestone, and others of
primary formation, of which latter kind New Caledonia offers the best
example. The central land consists either of one island, or of several:
thus, in the Society group, Eimeo stands by itself; while Taha and
Raiatea (Fig. 3, Plate I), both moderately large islands of nearly
equal size, are included in one reef. Within the reef of the Gambier
group there are four large and some smaller islands (Fig. 8, Plate I);
within that of
Hogoleu (Fig. 2, Plate I) nearly a dozen small islands are scattered
over the expanse of one vast lagoon.
[6] The height of Tahiti is given from Captain Beechey; Maurua from
Mr. F. D. Bennett (_Geograph. Journ._ vol. viii, p. 220); Aitutaki
from measurements made on board the _Beagle_; and Manouai or Harvey
Island, from an estimate by the Rev. J. Williams. The two latter
islands, however, are not in some respects well characterised examples
of the encircled class.
After the details now given, it may be asserted that there is not one
point of essential difference between encircling barrier-reefs and
atolls: the latter enclose a simple sheet of water, the former encircle
an expanse with one or more islands rising from it. I was much struck
with this fact, when viewing, from the heights of Tahiti, the distant
island of Eimeo standing within smooth water, and encircled by a ring
of snow-white breakers. Remove the central land, and an annular reef
like that of an atoll in an early stage of its formation is left;
remove it from Bolabola, and there remains a circle of linear
coral-islets, crowned with tall cocoa-nut trees, like one of the many
atolls scattered over the Pacific and Indian Oceans.
The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions. The reef on the west coast of New
Caledonia (Fig. 5, Plate II) is 400 miles in length; and for a length
of many leagues it seldom approaches within eight miles of the shore;
and near the southern end of the island, the space between the reef and
the land is sixteen miles in width. The Australian barrier extends,
with a few interruptions, for nearly a thousand miles; its average
distance from the land is between twenty and thirty miles; and in some
parts from fifty to seventy. The great arm of the sea thus included, is
from ten to twenty-five fathoms deep, with a sandy bottom; but towards
the southern end, where the reef is further from the shore, the depth
gradually increases to forty, and in some parts to more than sixty
fathoms. Flinders[7] has described the surface of this reef as
consisting of a hard white agglomerate of different kinds of coral,
with rough projecting points. The outer edge is the highest part; it is
traversed by narrow gullies, and at rare intervals is breached by
ship-channels. The sea close outside is profoundly deep; but, in front
of the main breaches, soundings can sometimes be obtained. Some low
islets have been formed on the reef.
[7] Flinders’ “Voyage to Terra Australis,” vol. ii, p. 88.
There is one important point in the structure of barrier-reefs which
must here be considered. The accompanying diagrams represent north and
south vertical sections, taken through the highest points of Vanikoro,
Gambier, and Maurua Islands, and through their encircling reefs. The
scale both in the horizontal and vertical direction is the same,
namely, a quarter of an inch to a nautical mile. The height and width
of these islands is known; and I have attempted to represent the form
of the land from the shading of the hills in the large published
charts. It has long been remarked, even from the time of Dampier, that
considerable degree of relation subsists between the inclination of
that part of the land which is beneath water and that above it; hence
the dotted line in the three sections, probably, does not widely differ
in inclination from the actual submarine prolongation of the land. If
we now look at the outer edge of the reef (AA), and bear in mind that
the plummet on the right hand represents a depth of 1,200 feet, we must
conclude that the vertical thickness of these barrier coral-reefs is
very great.
[Illustration: Vertical thickness of Vanikoro, Gambier and Maurua.]
1. VANIKORO, from the “Atlas of the Voyage of the _Astrolabe_,” by D.
D’Urville.
2. GAMBIER ISLAND, from Beechey.
3. MAURUA, from the “Atlas of the Voyage of the _ Coquille_,” by
Duperrey.
The horizontal line is the level of the sea, from which on the right
hand a plummet descends, representing a depth of 200 fathoms, or 1,200
feet. The vertical shading shows the section of the land, and the
horizontal shading that of the encircling barrier-reef: from the
smallness of the scale, the lagoon-channel could not be represented.
AA.—Outer edge of the coral-reefs, where the sea breaks.
BB.—The shore of the encircled islands.
I must observe that if the sections had been taken in any other
direction across these islands, or across other encircled islands,[8]
the result would have been the same. In the succeeding chapter it will
be shown that reef-building polypifers cannot flourish at great
depths,—for instance, it is highly improbable that they could exist at
a quarter of the depth represented by the plummet on the right hand of
the woodcut. Here there is a great _apparent_ difficulty—how were the
basal parts of these barrier-reef formed? It will, perhaps, occur to
some, that the actual reefs formed of coral are not of great thickness,
but that before their first growth, the coasts of these encircled
islands were deeply eaten into, and a broad but shallow submarine ledge
thus left, on the edge of which the coral grew; but if this had been
the case, the shore would have been invariably bounded by lofty cliffs,
and not have sloped down to the lagoon-channel, as it does in many
instances. On this view,[9] moreover, the cause of the reef springing
up at such a great distance from the land, leaving a deep and broad
moat within, remains altogether unexplained. A supposition of the same
nature,
and appearing at first more probable is, that the reefs sprung up from
banks of sediment, which had accumulated round the shore previously to
the growth of the coral; but the extension of a bank to the same
distance round an unbroken coast, and in front of those deep arms of
the sea (as in Raiatea, see Plate II, Fig. 3) which penetrate nearly to
the heart of some encircled islands, is exceedingly improbable. And
why, again, should the reef spring up, in some cases steep on both
sides like a wall, at a distance of two, three or more miles from the
shore, leaving a channel often between two hundred and three hundred
feet deep, and rising from a depth which we have reason to believe is
destructive to the growth of coral? An admission of this nature cannot
possibly be made. The existence, also, of the deep channel, utterly
precludes the idea of the reef having grown outwards, on a foundation
slowly formed on its outside, by the accumulation of sediment and coral
detritus. Nor, again, can it be asserted, that the reef-building corals
will not grow, excepting at a great distance from the land; for, as we
shall soon see, there is a whole class of reefs, which take their name
from growing closely attached (especially where the sea is deep) to the
beach. At New Caledonia (see Plate II, Fig. 5) the reefs which run in
front of the west coast are prolonged in the same line 150 miles beyond
the northern extremity of the island, and this shows that some
explanation, quite different from any of those just suggested, is
required. The continuation of the reefs on each side of the submarine
prolongation of New Caledonia, is an exceedingly interesting fact, if
this part formerly existed as the northern extremity of the island, and
before the attachment of the coral had been worn down by the action of
the sea, or if it originally existed at its present height, with or
without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but
fronting its sides, in the same line with the reefs which front the
shores of the lofty island? We shall hereafter see, that there is one,
and I believe only one, solution of this difficulty.
[8] In the fifth chapter an east and west section across the Island of
Bolabola and its barrier-reefs is given, for the sake of illustrating
another point. The unbroken line in it (woodcut No. 5) is the section
referred to. The scale is .57 of an inch to a mile; it is taken from
the “Atlas of the Voyage of the _Coquille_,” by Duperrey. The depth of
the lagoon-channel is exaggerated.
[9] The Rev. D. Tyerman and Mr. Bennett (“Journal of Voyage and
Travels,” vol. i, p. 215) have briefly suggested this explanation of
the origin of the encircling reefs of the Society Islands.
One other supposition to account for the position of encircling
barrier-reefs remains, but it is almost too preposterous to be
mentioned; namely, that they rest on enormous submarine craters,
surrounding the included islands. When the size, height, and form of
the islands in the Society group are considered, together with the fact
that all are thus encircled, such a notion will be rejected by almost
every one. New Caledonia, moreover, besides its size, is composed of
primitive formations, as are some of the Comoro Islands;[10] and
Aitutaki consists of calcareous rock. We must, therefore, reject these
several explanations, and conclude that the vertical thickness of
barrier-reefs, from their outer edges to the foundation on which they
rest (from AA in the section to the dotted lines) is really great; but
in this, there is no difficulty, for it is not necessary to suppose
that the coral has sprung up from an immense depth, as will be evident
when the theory of the upward growth of coral-reefs, during the slow
subsidence of their foundation, is discussed.
[10] I have been informed that this is the case by Dr. Allan of
Forres, who has visited this group.
Chapter III FRINGING OR SHORE-REEFS
Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
up.—Currents of water formed within it.—Upraised reefs.—Narrow
fringing-reefs in deep seas.—Reefs on the coast of East Africa and of
Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
and on worn-down islands.—Fringing-reefs affected by currents of the
sea.—Coral coating the bottom of the sea, but not forming reefs.
Fringing-reefs, or, as they have been called by some voyagers,
shore-reefs, whether skirting an island or part of a continent, might
at first be thought to differ little, except in generally being of less
breadth, from barrier-reefs. As far as the superficies of the actual
reef is concerned this is the case; but the absence of an interior
deep-water channel, and the close relation in their horizontal
extension with the probable slope beneath the sea of the adjoining
land, present essential points of difference.
The reefs which fringe the island of Mauritius offer a good example of
this class. They extend round its whole circumference, with the
exception of two or three parts,[1] where the coast is almost
precipitous, and where, if as is probable the bottom of the sea has a
similar inclination, the coral would have no foundation on which to
become attached. A similar fact may sometimes be observed even in reefs
of the barrier class, which follow much less closely the outline of the
adjoining land; as, for instance, on the south-east and precipitous
side of Tahiti, where the encircling reef is interrupted. On the
western side of the Mauritius, which was the only part I visited, the
reef generally lies at the distance of about half a mile from the
shore; but in some parts it is distant from one to two, and even three
miles. But even in this last case, as the coast-land is gently inclined
from the foot of the mountains to the sea-beach, and as the soundings
outside the reef indicate an equally gentle slope beneath the water,
there is no reason for supposing that the basis of the reef, formed by
the prolongation of the strata of the island, lies at a greater depth
than that at which the polypifers could begin constructing the reef.
Some allowance, however, must be made for the outward extension of the
corals on a foundation of sand and detritus, formed from their own
wear, which would give to the reef a somewhat greater vertical
thickness, than would otherwise be possible.
[1] This fact is stated on the authority of the Officier du Roi, in
his extremely interesting “Voyage à l’Isle de France,” undertaken in
1768. According to Captain Carmichael (Hooker’s _Bot. Misc._ vol. ii,
p. 316) on one part of the coast there is a space for sixteen miles
without a reef.
The outer edge of the reef on the western or leeward side of the island
is tolerably well defined, and is a little higher than any other part.
It chiefly consists of large strongly branched corals, of the genus
Madrepora, which also form a sloping bed some way out to sea: the
kinds of coral growing in this part will be described in the ensuing
chapter. Between the outer margin and the beach, there is a flat space
with a sandy bottom and a few tufts of living coral; in some parts it
is so shallow, that people, by avoiding the deeper holes and gullies,
can wade across it at low water; in other parts it is deeper, seldom
however exceeding ten or twelve feet, so that it offers a safe coasting
channel for boats. On the eastern and windward side of the island,
which is exposed to a heavy surf, the reef was described to me as
having a hard smooth surface, very slightly inclined inwards, just
covered at low-water, and traversed by gullies; it appears to be quite
similar in structure to the reefs of the barrier and atoll classes.
The reef of Mauritius, in front of every river and streamlet, is
breached by a straight passage: at Grand Port, however, there is a
channel like that within a barrier-reef; it extends parallel to the
shore for four miles, and has an average depth of ten or twelve
fathoms; its presence may probably be accounted for by two rivers which
enter at each end of the channel, and bend towards each other. The fact
of reefs of the fringing class being always breached in front of
streams, even of those which are dry during the greater part of the
year, will be explained, when the conditions unfavourable to the growth
of coral are considered. Low coral-islets, like those on barrier-reefs
and atolls, are seldom formed on reefs of this class, owing apparently
in some cases to their narrowness, and in others to the gentle slope of
the reef outside not yielding many fragments to the breakers. On the
windward side, however, of the Mauritius, two or three small islets
have been formed.
It appears, as will be shown in the ensuing chapter, that the action of
the surf is favourable to the vigorous growth of the stronger corals,
and that sand or sediment, if agitated by the waves, is injurious to
them. Hence it is probable that a reef on a shelving shore, like that
of Mauritius, would at first grow up, not attached to the actual beach,
but at some little distance from it; and the corals on the outer margin
would be the most vigorous. A shallow channel would thus be formed
within the reef, and as the breakers are prevented acting on the shores
of the island, and as they do not ordinarily tear up many fragments
from the outside, and as every streamlet has its bed prolonged in a
straight line through the reef, this channel could be filled up only
very slowly with sediment. But a beach of sand and of fragments of the
smaller kinds of coral seems, in the case of Mauritius, to be slowly
encroaching on the shallow channel. On many shelving and sandy coasts,
the breakers tend to form a bar of sand a little way from the beach,
with a slight increase of depth within it; for instance, Captain
Grey[2] states that the west coast of Australia, in latitude 24°, is
fronted by a sand bar about two hundred yards in width, on which there
is only two feet of water; but within it the depth increases to two
fathoms. Similar bars, more or less perfect, occur on other coasts. In
these cases I suspect that the shallow channel (which no doubt during
storms is occasionally obliterated) is scooped out by the flowing away
of the
water thrown beyond the line, on which the waves break with the
greatest force. At Pernambuco a bar of hard sandstone,[3] which has the
same external form and height as a coral-reef, extends nearly parallel
to the coast; within this bar currents, apparently caused by the water
thrown over it during the greater part of each tide, run strongly, and
are wearing away its inner wall. From these facts it can hardly be
doubted, that within most fringing-reefs, especially within those lying
some distance from the land, a return stream must carry away the water
thrown over the outer edge; and the current thus produced, would tend
to prevent the channel being filled up with sediment, and might even
deepen it under certain circumstances. To this latter belief I am led,
by finding that channels are almost universally present within the
fringing-reefs of those islands which have undergone recent elevatory
movements; and this could hardly have been the case, if the conversion
of the very shallow channel into land had not been counteracted to a
certain extent.
[2] Captain Grey’s “Journal of Two Expeditions,” vol. i, p. 369.
[3] I have described this singular structure in the _Lond. and Edin.
Phil. Mag.,_ October 1841.
A fringing-reef, if elevated in a perfect condition above the level of
the sea, ought to present the singular appearance of a broad dry moat
within a low mound. The author[4] of an interesting pedestrian tour
round the Mauritius, seems to have met with a structure of this kind:
he says “J’observai que là, où la mer étale, indépendamment des rescifs
du large, il y à terre _une espèce d’effoncement_ ou chemin couvert
naturel. On y pourrait mettre du canon,” etc. In another place he adds,
“Avant de passer le Cap, on remarque un gros banc de corail élevé de
plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné,
il regne au pied une longue flaque d’eau, dont on pourrait faire un
bassin pour de petits vaisseaux.” But the margin of the reef, although
the highest and most perfect part, from being most exposed to the surf,
would generally during a slow rise of the land be either partially or
entirely worn down to that level, at which corals could renew their
growth on its upper edge. On some parts of the coast-land of Mauritius
there are little hillocks of coral-rock, which are either the last
remnants of a continuous reef, or of low islets formed on it. I
observed that two such hillocks between Tamarin Bay and the Great Black
River; they were nearly twenty feet high, about two hundred yards from
the present beach, and about thirty feet above its level. They rose
abruptly from a smooth surface, strewed with worn fragments of coral.
They consisted in their lower part of hard calcareous sandstone, and in
their upper of great blocks of several species of Astræa and Madrepora,
loosely aggregated; they were divided into irregular beds, dipping
seaward, in one hillock at an angle of 8°, and in the other at 18°. I
suspect that the superficial parts of the reefs, which have been
upraised together with the islands they fringe, have generally been
much more modified by the wearing action of the sea, than those of
Mauritius.
[4] “Voyage à l’Isle de France, par un Officier du Roi,” part i, pp.
192, 200.
Many islands[5] are fringed by reefs quite similar to those of
Mauritius; but on coasts where the sea deepens very suddenly the reefs
are much narrower, and their limited extension seems evidently to
depend on the high inclination of the submarine slope; a relation,
which, as we have seen, does not exist in reefs of the barrier class.
The fringing-reefs on steep coasts are frequently not more than from
fifty to one hundred yards in width; they have a nearly smooth, hard
surface, scarcely uncovered at low water, and without any interior
shoal channel, like that within those fringing-reefs, which lie at a
greater distance from the land. The fragments torn up during gales from
the outer margin are thrown over the reef on the shores of the island.
I may give as instances, Wateeo, where the reef is described by Cook as
being a hundred yards wide; and Mauti and Elizabeth[6] Islands, where
it is only fifty yards in width: the sea round these islands is very
deep.
[5] I may give Cuba, as another instance; Mr. Taylor (_Loudon’s Mag.
of Nat. Hist.,_ vol. ix, p. 449) has described a reef several miles in
length between Gibara and Vjaro, which extends parallel to the shore
at the distance of between half and the third part of a mile, and
encloses a space of shallow water, with a sandy bottom and tufts of
coral. Outside the edge of the reef, which is formed of great
branching corals, the depth is six and seven fathoms. This coast has
been upheaved at no very distant geological period.
[6] Mauti is described by Lord Byron in the voyage of H.M.S. _
Blonde_, and Elizabeth Island by Captain Beechey.
Fringing-reefs, like barrier-reefs, both surround islands, and front
the shores of continents. In the charts of the eastern coast of Africa,
by Captain Owen, many extensive fringing-reefs are laid down; thus, for
a space of nearly forty miles, from latitude 1° 15′ to 1° 45′ S., a
reef fringes the shore at an average distance of rather more than one
mile, and therefore at a greater distance than is usual in reefs of
this class; but as the coast-land is not lofty, and as the bottom
shoals very gradually (the depth being only from eight to fourteen
fathoms at a mile and a half outside the reef), its extension thus far
from the land offers no difficulty. The external margin of this reef is
described, as formed of projecting points, within which there is a
space, from six to twelve feet deep, with patches of living coral on
it. At Mukdeesha (lat. 2° 1′ N.) “the port is formed,” it is said,[7]
“by a long reef extending eastward, four or five miles, within which
there is a narrow channel, with ten to twelve feet of water at low
spring-tides;” it lies at the distance of a quarter of a mile from the
shore. Again, in the plan of Mombas (lat. 4° S.), a reef extends for
thirty-six miles, at the distance of from half a mile to one mile and a
quarter from the shore; within it, there is a channel navigable “for
canoes and small craft,” between six and fifteen feet deep: outside the
reef the depth is about thirty fathoms at the distance of nearly half a
mile. Part of this reef is very symmetrical, and has a uniform breadth
of two hundred yards.
[7] Owen’s “Africa,” vol. i, p. 357, from which work the foregoing
facts are likewise taken.
The coast of Brazil is in many parts fringed by reefs. Of these, some
are not of coral formation; for instance, those near Bahia and in front
of Pernambuco; but a few miles south of this latter city, the reef
follows[8] so closely every turn of the shore, that I can hardly doubt
it is of coral; it runs at the distance of three-quarters of a mile
from the land, and within it the depth is from ten to fifteen feet. I
was assured by an intelligent pilot that at Ports Frances and Maceio,
the outer part of the reef consists of living coral, and the inner of a
white stone, full of large irregular cavities, communicating with the
sea. The bottom of the sea off the coast of Brazil shoals gradually to
between thirty and forty fathoms, at the distance of between nine and
ten leagues from the land.
[8] See Baron Roussin’s “Pilote du Brésil,” and accompanying
hydrographical memoir.
From the description now given, we must conclude that the dimensions
and structure of fringing-reefs depend entirely on the greater or less
inclination of the submarine slope, conjoined with the fact that
reef-building polypifers can exist only at limited depths. It follows
from this, that where the sea is very shallow, as in the Persian Gulf
and in parts of the East Indian Archipelago, the reefs lose their
fringing character, and appear as separate and irregularly scattered
patches, often of considerable area. From the more vigorous growth of
the coral on the outside, and from the conditions being less favourable
in several respects within, such reefs are generally higher and more
perfect in their marginal than in their central parts; hence these
reefs sometimes assume (and this circumstance ought not to be
overlooked) the appearance of atolls; but they differ from atolls in
their central expanse being much less deep, in their form being less
defined, and in being based on a shallow foundation. But when in a deep
sea reefs fringe banks of sediment, which have accumulated beneath the
surface, round either islands or submerged rocks, they are
distinguished with difficulty on the one hand from encircling
barrier-reefs, and on the other from atolls. In the West Indies there
are reefs, which I should probably have arranged under both these
classes, had not the existence of large and level banks, lying a little
beneath the surface, ready to serve as the basis for the attachment of
coral, been occasionally brought into view by the entire or partial
absence of reefs on them, and had not the formation of such banks,
through the accumulation of sediment now in progress, been sufficiently
evident. Fringing-reefs sometimes coat, and thus protect the
foundations of islands, which have been worn down by the surf to the
level of the sea. According to Ehrenberg, this has been extensively the
case with the islands in the Red Sea, which formerly ranged parallel to
the shores of the mainland, with deep water within them: hence the
reefs now coating their bases are situated relatively to the land like
barrier-reefs, although not belonging to that class; but there are, as
I believe, in the Red Sea some true barrier-reefs. The reefs of this
sea and of the West Indies will be described in the Appendix. In some
cases, fringing-reefs appear to be considerably modified in outline by
the course of the prevailing currents. Dr. J. Allan informs me that on
the east coast of Madagascar almost every headland and low point of
sand has a coral-reef extending from it in a S.W. and N.E. line,
parallel
to the currents on that shore. I should think the influence of the
currents chiefly consisted in causing an extension, in a certain
direction, of a proper foundation for the attachment of the coral.
Round many intertropical islands, for instance the Abrolhos on the
coast of Brazil surveyed by Captain Fitzroy, and, as I am informed by
Mr. Cuming, round the Philippines, the bottom of the sea is entirely
coated by irregular masses of coral, which although often of large
size, do not reach the surface and form proper reefs. This must be
owing, either to insufficient growth, or to the absence of those kinds
of corals which can withstand the breaking of the waves.
The three classes, atoll-formed, barrier, and fringing-reefs, together
with the modifications just described of the latter, include all the
most remarkable coral formations anywhere existing. At the commencement
of the last chapter in the volume, where I detail the principles on
which the map (Plate III) is coloured, the exceptional cases will be
enumerated.
Chapter IV ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS
In this chapter I will give all the facts which I have collected,
relating to the distribution of coral-reefs,—to the conditions
favourable to their increase,—to the rate of their growth,—and to the
depth at which they are formed.
These subjects have an important bearing on the theory of the origin of
the different classes of coral-reefs.
_Section I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE
With regard to the limits of latitude, over which coral-reefs extend, I
have nothing new to add. The Bermuda Islands, in 32° 15′ N., is the
point furthest removed from the equator, in which they appear to exist;
and it has been suggested that their extension so far northward in this
instance is owing to the warmth of the Gulf Stream. In the Pacific, the
Loo Choo Islands, in latitude 27° N., have reefs on their shores, and
there is an atoll in 28° 30′, situated N.W. of the Sandwich
Archipelago. In the Red Sea there are coral-reefs in latitude 30°. In
the southern hemisphere coral-reefs do not extend so far from the
equatorial sea. In the Southern Pacific there are only a few reefs
beyond the line of the tropics, but Houtmans Abrolhos, on the western
shores of Australia in latitude 29° S., are of coral formation.
The proximity of volcanic land, owing to the lime generally evolved
from it, has been thought to be favourable to the increase of
coral-reefs.
There is, however, not much foundation for this view; for nowhere are
coral-reefs more extensive than on the shores of New Caledonia, and of
north-eastern Australia, which consist of primary formations; and in
the largest groups of atolls, namely the Maldiva, Chagos, Marshall,
Gilbert, and Low Archipelagoes, there is no volcanic or other kind of
rock, excepting that formed of coral.
The entire absence of coral-reefs in certain large areas within the
tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
during the surveying voyages of the _Beagle_ and her tender on the west
coast of South America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none[1] north of the equator;
Mr. Lloyd, who surveyed the Isthmus of Panama, remarked to me, that
although he had seen corals living in the Bay of Panama, yet he had
never observed any reefs formed by them. I at first attributed this
absence of reefs on the coasts of Peru and of the Galapagos Islands,[2]
to the coldness of the currents from the south, but the Gulf of Panama
is one of the hottest pelagic districts in the world.[3] In the central
parts of the Pacific there are islands entirely free from reefs; in
some few of these cases I have thought that this was owing to recent
volcanic action; but the existence of reefs round the greater part of
Hawaii, one of the Sandwich Islands, shows that recent volcanic action
does not necessarily prevent their growth.
[1] I have been informed that this is the case, by Lieutenant Ryder,
R.N., and others who have had ample opportunities for observation.
[2] The mean temperature of the surface sea from observations made by
the direction of Captain Fitzroy on the shores of the Galapagos
Islands, between the 16th of September and the 20th of October, 1835,
was 68° Fahr. The lowest temperature observed was 58.5° at the
south-west end of Albemarle Island; and on the west coast of this
island, it was several times 62° and 63°. The mean temperature of the
sea in the Low Archipelago of atolls, and near Tahiti, from similar
observations made on board the _Beagle_, was (although further from
the equator) 77.5°, the lowest any day being 76.5°. Therefore we have
here a difference of 9.5° in mean temperature, and 18° in extremes; a
difference doubtless quite sufficient to affect the distribution of
organic beings in the two areas.
[3] Humboldt’s “Personal Narrative,” vol. vii, p. 434.
In the last chapter I stated that the bottom of the sea round some
islands is thickly coated with living corals, which nevertheless do not
form reefs, either from insufficient growth, or from the species not
being adapted to contend with the breaking waves.
I have been assured by several people, that there are no coral-reefs on
the west coast of Africa,[4] or round the islands in the Gulf of
Guinea. This perhaps may be attributed, in part, to the sediment
brought down by the many rivers debouching on that coast, and to the
extensive mud-banks,
which line great part of it. But the islands of St. Helena, Ascension,
the Cape Verdes, St. Paul’s, and Fernando Noronha, are, also, entirely
without reefs, although they lie far out at sea, are composed of the
same ancient volcanic rocks, and have the same general form, with those
islands in the Pacific, the shores of which are surrounded by gigantic
walls of coral-rock. With the exception of Bermuda, there is not a
single coral-reef in the central expanse of the Atlantic Ocean. It
will, perhaps, be suggested that the quantity of carbonate of lime in
different parts of the sea, may regulate the presence of reefs. But
this cannot be the case, for at Ascension, the waves charged to excess
precipitate a thick layer of calcareous matter on the tidal rocks; and
at St. Jago, in the Cape Verdes, carbonate of lime not only is abundant
on the shores, but it forms the chief part of some upraised
post-tertiary strata. The apparently capricious distribution,
therefore, of coral-reefs, cannot be explained by any of these obvious
causes; but as the study of the terrestrial and better known half of
the world must convince every one that no station capable of supporting
life is lost,—nay more, that there is a struggle for each station,
between the different orders of nature,—we may conclude that in those
parts of the intertropical sea, in which there are no coral-reefs,
there are other organic bodies supplying the place of the reef-building
polypifers. It has been shown in the chapter on Keeling atoll that
there are some species of large fish, and the whole tribe of Holothuriæ
which prey on the tenderer parts of the corals. On the other hand, the
polypifers in their turn must prey on some other organic beings; the
decrease of which from any cause would cause a proportionate
destruction of the living coral. The relations, therefore, which
determine the formation of reefs on any shore, by the vigorous growth
of the efficient kinds of coral, must be very complex, and with our
imperfect knowledge quite inexplicable. From these considerations, we
may infer that changes in the condition of the sea, not obvious to our
senses, might destroy all the coral-reefs in one area, and cause them
to appear in another: thus, the Pacific or Indian Ocean might become as
barren of coral-reefs as the Atlantic now is, without our being able to
assign any adequate cause for such a change.
[4] It might be concluded, from a paper by Captain Owen (_Geograph.
Journ._, vol. ii, p. 89), that the reefs off Cape St. Anne and the
Sherboro’ Islands were of coral, although the author states that they
are not purely coralline. But I have been assured by Lieutenant
Holland, R.N., that these reefs are not of coral, or at least that
they do not at all resemble those in the West Indies.
It has been a question with some naturalists, which part of a reef is
most favourable to the growth of coral. The great mounds of living
Porites and of Millepora round Keeling atoll occur exclusively on the
extreme verge of the reef, which is washed by a constant succession of
breakers; and living coral nowhere else forms solid masses. At the
Marshall islands the larger kinds of coral (chiefly species of Astræa,
a genus closely allied to Porites) “which form rocks measuring several
fathoms in thickness,” prefer, according to Chamisso,[5] the most
violent surf. I have stated that the outer margin of the Maldiva atolls
consists of living corals (some of which, if not all, are of the same
species with those at Keeling atoll), and here the surf is so
tremendous, that even large ships have been thrown, by a single heave
of the sea, high and dry on the reef, all on board thus escaping with
their lives.
Ehrenberg[6] remarks, that in the Red Sea the strongest corals live on
the outer reefs, and appear to love the surf; he adds, that the more
branched kinds abound a little way within, but that even these in still
more protected places, become smaller. Many other facts having a
similar tendency might be adduced.[7] It has, however, been doubted by
MM. Quoy and Gaimard, whether any kind of coral can even withstand,
much less flourish in, the breakers of an open sea:[8] they affirm that
the saxigenous lithophytes flourish only where the water is tranquil,
and the heat intense. This statement has passed from one geological
work to another; nevertheless, the protection of the whole reef
undoubtedly is due to those kinds of coral, which cannot exist in the
situations thought by these naturalists to be most favourable to them.
For should the outer and living margin perish, of any one of the many
low coral-islands, round which a line of great breakers is incessantly
foaming, the whole, it is scarcely possible to doubt, would be washed
away and destroyed, in less than half a century. But the vital energies
of the corals conquer the mechanical power of the waves; and the large
fragments of reef torn up by every storm, are replaced by the slow but
steady growth of the innumerable polypifers, which form the living zone
on its outer edge.
[5] Kotzebue’s “First Voyage” (Eng. Trans.), vol. iii, pp. 142, 143,
331.
[6] Ehrenberg, “Über die Natur und Bildung der Corallen Bänke im
rothen Meere,” p. 49.
[7] In the West Indies, as I am informed by Captain Bird Allen, R.N.,
it is the common belief of those, who are best acquainted with the
reefs, that the coral flourishes most, where freely exposed to the
swell of the open sea.
[8] “Annales des Sciences Naturelles,” tome vi, pp. 276, 278.—“Là où
les ondes sont agitées, les Lytophytés ne peuvent travailler, parce
qu’elles détruiraient leurs fragiles édifices,” etc.
From these facts, it is certain, that the strongest and most massive
corals flourish, where most exposed. The less perfect state of the reef
of most atolls on the leeward and less exposed side, compared with its
state to windward; and the analogous case of the greater number of
breaches on the near sides of those atolls in the Maldiva Archipelago,
which afford some protection to each other, are obviously explained by
this circumstance. If the question had been, under what conditions the
greater number of species of coral, not regarding their bulk and
strength, were developed, I should answer,—probably in the situations
described by MM. Quoy and Gaimard, where the water is tranquil and the
heat intense. The total number of species of coral in the
circumtropical seas must be very great: in the Red Sea alone, 120
kinds, according to Ehrenberg,[9] have been observed.
[9] Ehrenberg, “Über die Natur,” etc., p. 46.
The same author has observed that the recoil of the sea from a steep
shore is injurious to the growth of coral, although waves breaking over
a bank are not so. Ehrenberg also states, that where there is much
sediment, placed so as to be liable to be moved by the waves there is
little or no coral; and a collection of living specimens placed by him
on a sandy shore died in the course of a few days.[10] An experiment,
however, will presently be related in which some large masses of living
coral increased rapidly in size, after having been secured by stakes on
a sandbank. That loose sediment should be injurious to the living
polypifers, appears, at first sight, probable; and accordingly, in
sounding off Keeling atoll, and (as will hereafter be shown) off
Mauritius, the arming of the lead invariably came up clean, where the
coral was growing vigorously. This same circumstance has probably given
rise to a strange belief, which, according to Captain Owen,[11] is
general amongst the inhabitants of the Maldiva atolls, namely that
corals have roots, and therefore that if merely broken down to the
surface, they grow up again; but, if rooted out, they are permanently
destroyed. By this means the inhabitants keep their harbours clear; and
thus the French Governor of St. Mary’s in Madagascar, “cleared out and
made a beautiful little port at that place.” For it is probable that
sand would accumulate in the hollows formed by tearing out the corals,
but not on the broken and projecting stumps, and therefore, in the
former case, the fresh growth of the coral might be thus prevented.
[10] _Ibid_., p. 49.
[11] Captain Owen on the Geography of the Maldiva Islands, _Geograph.
Journal_, vol. ii, p. 88.
In the last chapter I remarked that fringing-reefs are almost
universally breached, where streams enter the sea.[12] Most authors
have attributed this fact to the injurious effects of the fresh water,
even where it enters the sea only in small quantity, and during a part
of the year. No doubt brackish water would prevent or retard the growth
of coral; but I believe that the mud and sand which is deposited, even
by rivulets when flooded, is a much more efficient check. The reef on
each side of the channel leading into Port Louis at Mauritius, ends
abruptly in a wall, at the foot of which I sounded and found a bed of
thick mud. This steepness of the sides appears to be a general
character in such breaches. Cook,[13] speaking of one at Raiatea, says,
“like all the rest, it is very steep on both sides.” Now, if it were
the fresh water mingling with the salt which prevented the growth of
coral, the reef certainly would not terminate abruptly, but as the
polypifers nearest the impure stream would grow less vigorously than
those farther off, so would the reef gradually thin away. On the other
hand, the sediment brought down from the land would only prevent the
growth of the coral in the line of its deposition, but would not check
it on the side, so that the reefs might increase till they overhung the
bed of the channel. The breaches are much fewer in number, and front
only the larger valleys in reefs of the encircling barrier class. They
probably are kept open in the same manner as those into the lagoon of
an atoll, namely, by the
force of the currents and the drifting outwards of fine sediment. Their
position in front of valleys, although often separated from the land by
deep water lagoon-channels, which it might be thought would entirely
remove the injurious effects both of the fresh water and the sediment,
will receive a simple explanation when we discuss the origin of
barrier-reefs.
[12] Lieutenant Wellstead and others have remarked that this is the
case in the Red Sea; Dr. Rüppell (“Reise in Abyss.” Band. i, p. 142)
says that there are pear-shaped harbours in the upraised coral-coast,
into which periodical streams enter. From this circumstance, I
presume, we must infer that before the upheaval of the strata now
forming the coast-land, fresh water and sediment entered the sea at
these points; and the coral being thus prevented growing, the
pear-shaped harbours were produced.
[13] Cook’s “First Voyage,” vol. ii, p. 271 (Hawkesworth’s edit.)
In the vegetable kingdom every different station has its peculiar group
of plants, and similar relations appear to prevail with corals. We have
already described the great difference between the corals within the
lagoon of an atoll and those on its outer margin. The corals, also, on
the margin of Keeling Island occurred in zones; thus the _Porites_ and
_Millepora complanata_ grow to a large size only where they are washed
by a heavy sea, and are killed by a short exposure to the air; whereas,
three species of Nullipora also live amidst the breakers, but are able
to survive uncovered for a part of each tide; at greater depths, a
strong Madrepora and _Millepora alcicornis_ are the commonest kinds,
the former appearing to be confined to this part, beneath the zone of
massive corals, minute encrusting corallines and other organic bodies
live. If we compare the external margin of the reef at Keeling atoll
with that on the leeward side of Mauritius, which are very differently
circumstanced, we shall find a corresponding difference in the
appearance of the corals. At the latter place, the genus Madrepora is
preponderant over every other kind, and beneath the zone of massive
corals there are large beds of Seriatopora. There is also a marked
difference, according to Captain Moresby,[14]between the great
branching corals of the Red Sea, and those on the reefs of the Maldiva
atolls.
[14] Captain Moresby on the Northern Maldiva atolls, _Geograph.
Journal_, vol. v, p. 401.
These facts, which in themselves are deserving of notice, bear,
perhaps, not very remotely, on a remarkable circumstance which has been
pointed out to me by Captain Moresby, namely, that with very few
exceptions, none of the coral-knolls within the lagoons of Peros
Banhos, Diego Garcia, and the Great Chagos Bank (all situated in the
Chagos group), rise to the surface of the water; whereas all those,
with equally few exceptions, within Solomon and Egmont atolls in the
same group, and likewise within the large southern Maldiva atolls,
reach the surface. I make these statements, after having examined the
charts of each atoll. In the lagoon of Peros Banhos, which is nearly
twenty miles across, there is only one single reef which rises to the
surface; in Diego Garcia there are seven, but several of these lie
close to the margin of the lagoon, and need scarcely have been
reckoned; in the Great Chagos Bank there is not one. On the other hand,
in the lagoons of some of the great southern Maldiva atolls, although
thickly studded with reefs, every one without exception rises to the
surface; and on an average there are less than two submerged reefs in
each atoll; in the northern atolls, however, the submerged lagoon-reefs
are not quite so rare. The submerged reefs in the Chagos atolls
generally have from one to seven fathoms water on them, but some have
from seven to ten. Most of them are small with
very steep sides;[15] at Peros Banhos they rise from a depth of about
thirty fathoms, and some of them in the Great Chagos Bank from above
forty fathoms; they are covered, Captain Moresby informs me, with
living and healthy coral, two and three feet high, consisting of
several species. Why then have not these lagoon-reefs reached the
surface, like the innumerable ones in the atolls above named? If we
attempt to assign any difference in their external conditions, as the
cause of this diversity, we are at once baffled. The lagoon of Diego
Garcia is not deep, and is almost wholly surrounded by its reef; Peros
Banhos is very deep, much larger, with many wide passages communicating
with the open sea. On the other hand, of those atolls, in which all or
nearly all the lagoon-reefs have reached the surface, some are small,
others large, some shallow, others deep, some well-enclosed, and others
open.
[15] Some of these statements were not communicated to me verbally by
Captain Moresby, but are taken from the MS. account before alluded to,
of the Chagos Group.
Captain Moresby informs me that he has seen a French chart of Diego
Garcia made eighty years before his survey, and apparently very
accurate; and from it he infers, that during this interval there has
not been the smallest change in the depth on any of the knolls within
the lagoon. It is also known that during the last fifty-one years, the
eastern channel into the lagoon has neither become narrower, nor
decreased in depth; and as there are numerous small knolls of living
coral within it, some change might have been anticipated. Moreover, as
the whole reef round the lagoon of this atoll has been converted into
land—an unparalleled case, I believe, in an atoll of such large
size,—and as the strip of land is for considerable spaces more than
half a mile wide—also a very unusual circumstance,—we have the best
possible evidence, that Diego Garcia has remained at its present level
for a very long period. With this fact, and with the knowledge that no
sensible change has taken place during eighty years in the
coral-knolls, and considering that every single reef has reached the
surface in other atolls, which do not present the smallest appearance
of being older than Diego Garcia and Peros Banhos, and which are placed
under the same external conditions with them, one is led to conclude
that these submerged reefs, although covered with luxuriant coral, have
no tendency to grow upwards, and that they would remain at their
present levels for an almost indefinite period.
From the number of these knolls, from their position, size, and form,
many of them being only one or two hundred yards across, with a rounded
outline, and precipitous sides,—it is indisputable that they have been
formed by the growth of coral; and this makes the case much more
remarkable. In Peros Banhos and in the Great Chagos Bank, some of these
almost columnar masses are 200 feet high, and their summits lie only
from two to eight fathoms beneath the surface; therefore, a small
proportional amount more of growth would cause them to attain the
surface, like those numerous knolls, which rise from an equally great
depth within the Maldiva atolls. We can hardly suppose that time has
been wanting for the upward growth of
the coral, whilst in Diego Garcia, the broad annular strip of land,
formed by the continued accumulation of detritus, shows how long this
atoll has remained at its present level. We must look to some other
cause than the rate of growth; and I suspect it will be found in the
reefs being formed of different species of corals, adapted to live at
different depths. The Great Chagos Bank is situated in the centre of
the Chagos Group, and the Pitt and Speaker Banks at its two extreme
points. These banks resemble atolls, except in their external rim being
about eight fathoms submerged, and in being formed of dead rock, with
very little living coral on it: a portion nine miles long of the
annular reef of Peros Banhos atoll is in the same condition. These
facts, as will hereafter be shown, render it very probable that the
whole group at some former period subsided seven or eight fathoms; and
that the coral perished on the outer margin of those atolls which are
now submerged, but that it continued alive, and grew up to the surface
on those which are now perfect. If these atolls did subside, and if
from the suddenness of the movement or from any other cause, those
corals which are better adapted to live at a certain depth than at the
surface, once got possession of the knolls, supplanting the former
occupants, they would exert little or no tendency to grow upwards. To
illustrate this, I may observe, that if the corals of the upper zone on
the outer edge of Keeling atoll were to perish, it is improbable that
those of the lower zone would grow to the surface, and thus become
exposed to conditions for which they do not appear to be adapted. The
conjecture, that the corals on the submerged knolls within the Chagos
atolls have analogous habits with those of the lower zone outside
Keeling atoll, receives some support from a remark by Captain Moresby,
namely, that they have a different appearance from those on the reefs
in the Maldiva atolls, which, as we have seen, all rise to the surface:
he compares the kind of difference to that of the vegetation under
different climates. I have entered at considerable length into this
case, although unable to throw much light on it, in order to show that
an equal tendency to upward growth ought not to be attributed to all
coral-reefs,—to those situated at different depths,—to those forming
the ring of an atoll or those on the knolls within a lagoon,—to those
in one area and those in another. The inference, therefore, that one
reef could not grow up to the surface within a given time, because
another, not known to be covered with the same species of corals, and
not known to be placed under conditions exactly the same, has not
within the same time reached the surface, is unsound.
_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS
The remark made at the close of the last section, naturally leads to
this division of our subject, which has not, I think, hitherto been
considered under a right point of view. Ehrenberg[16] has stated, that
in
the Red Sea, the corals only coat other rocks in a layer from one to
two feet in thickness, or at most to a fathom and a half; and he
disbelieves that, in any case, they form, by their own proper growth,
great masses, stratum over stratum. A nearly similar observation has
been made by MM. Quoy and Gaimard,[17] with respect to the thickness of
some upraised beds of coral, which they examined at Timor and some
other places. Ehrenberg[18] saw certain large massive corals in the Red
Sea, which he imagines to be of such vast antiquity, that they might
have been beheld by Pharaoh; and according to Mr. Lyell[19] there are
certain corals at Bermuda, which are known by tradition, to have been
living for centuries. To show how slowly coral-reefs grow upwards,
Captain Beechey[20] has adduced the case of the Dolphin Reef off
Tahiti, which has remained at the same depth beneath the surface,
namely about two fathoms and a half, for a period of sixty-seven years.
There are reefs in the Red Sea, which certainly do not appear[21] to
have increased in dimensions during the last half-century, and from the
comparison of old charts with recent surveys, probably not during the
last two hundred years. These, and other similar facts, have so
strongly impressed many with the belief of the extreme slowness of the
growth of corals, that they have even doubted the possibility of
islands in the great oceans having been formed by their agency. Others,
again, who have not been overwhelmed by this difficulty, have admitted
that it would require thousands, and tens of thousands of years, to
form a mass, even of inconsiderable thickness; but the subject has not,
I believe, been viewed in the proper light.
[16] Ehrenberg, as before cited, pp. 39, 46, and 50.
[17] “Annales des Sciences Nat.” tom. vi, p. 28.
[18] Ehrenberg, _ut sup._, p. 42.
[19] Lyell’s “Principles of Geology,” book iii, ch. xviii.
[20] Beechey’s “Voyage to the Pacific,” ch. viii.
[21] Ehrenberg, _ut sup._, p. 43.
That masses of considerable thickness have been formed by the growth of
coral, may be inferred with certainty from the following facts. In the
deep lagoons of Peros Banhos and of the Great Chagos Bank, there are,
as already described, small steep-sided knolls covered with living
coral. There are similar knolls in the southern Maldiva atolls, some of
which, as Captain Moresby assures me, are less than a hundred yards in
diameter, and rise to the surface from a depth of between two hundred
and fifty and three hundred feet. Considering their number, form, and
position, it would be preposterous to suppose that they are based on
pinnacles of any rock, not of coral formation; or that sediment could
have been heaped up into such small and steep isolated cones. As no
kind of living coral grows above the height of a few feet, we are
compelled to suppose that these knolls have been formed by the
successive growth and death of many individuals,—first one being broken
off or killed by some accident, and then another, and one set of
species being replaced by another set with different habits, as the
reef rose nearer the surface, or as other changes supervened. The
spaces between the corals would become filled up with fragments and
sand, and such matter would probably soon be consolidated, for we learn
from
Lieutenant Nelson,[22] that at Bermuda a process of this kind takes
place beneath water, without the aid of evaporation. In reefs, also, of
the barrier class, we may feel sure, as I have shown, that masses of
great thickness have been formed by the growth of the coral; in the
case of Vanikoro, judging only from the depth of the moat between the
land and the reef, the wall of coral-rock must be at least three
hundred feet in vertical thickness.
[22] “Geological Transactions,” vol. v, p. 113.
It is unfortunate that the upraised coral-islands in the Pacific have
not been examined by a geologist. The cliffs of Elizabeth Island, in
the Low Archipelago, are eighty feet high, and appear, from Captain
Beechey’s description, to consist of a homogeneous coral-rock. From the
isolated position of this island, we may safely infer that it is an
upraised atoll, and therefore that it has been formed by masses of
coral, grown together. Savage Island seems, from the description of the
younger Forster,[23] to have a similar structure, and its shores are
about forty feet high: some of the Cook Islands also appear[24] to be
similarly composed. Captain Belcher, R.N., in a letter which Captain
Beaufort showed me at the admiralty, speaking of Bow atoll, says, “I
have succeeded in boring forty-five feet through coral-sand, when the
auger became jammed by the falling in of the surrounding _ creamy_
matter.” On one of the Maldiva atolls, Captain Moresby bored to a depth
of twenty-six feet, when his auger also broke: he has had the kindness
to give me the matter brought up; it is perfectly white, and like
finely triturated coral-rock.
[23] Forster’s “Voyage round the World with Cook,” vol. ii, pp. 163,
167.
[24] Williams’s “Narrative of Missionary Enterprise,” p. 30.
In my description of Keeling atoll, I have given some facts, which show
that the reef probably has grown outwards; and I have found, just
within the outer margin, the great mounds of Porites and of Millepora,
with their summits lately killed, and their sides subsequently
thickened by the growth of the coral: a layer, also, of Nullipora had
already coated the dead surface. As the external slope of the reef is
the same round the whole of this atoll, and round many other atolls,
the angle of inclination must result from an adaption between the
growing powers of the coral, and the force of the breakers, and their
action on the loose sediment. The reef, therefore, could not increase
outwards, without a nearly equal addition to every part of the slope,
so that the original inclination might be preserved, and this would
require a large amount of sediment, all derived from the wear of corals
and shells, to be added to the lower part. Moreover, at Keeling atoll,
and probably in many other cases, the different kinds of corals would
have to encroach on each other; thus the Nulliporæ cannot increase
outwards without encroaching on the Porites and _ Millepora
complanata,_ as is now taking place; nor these latter without
encroaching on the strongly branched Madreporet, the _ Millepora
alcicornis,_ and some Astræas; nor these again without a foundation
being formed for them within the requisite depth, by the accumulation
of sediment. How slow, then, must be the ordinary lateral or outward
growth of such reefs. But off
Christmas atoll, where the sea is much more shallow than is usual, we
have good reason to believe that, within a period not very remote, the
reef has increased considerably in width. The land has the
extraordinary breadth of three miles; it consists of parallel ridges of
shells and broken corals, which furnish “an incontestable proof,” as
observed by Cook,[25] “that the island has been produced by accessions
from the sea, and is in a state of increase.” The land is fronted by a
coral-reef, and from the manner in which islets are known to be formed,
we may feel confident that the reef was not three miles wide, when the
first, or most backward ridge, was thrown up; and, therefore, we must
conclude that the reef has grown outwards during the accumulation of
the successive ridges. Here then, a wall of coral-rock of very
considerable breadth has been formed by the outward growth of the
living margin, within a period during which ridges of shells and
corals, lying on the bare surface, have not decayed. There can be
little doubt, from the account given by Captain Beechey, that Matilda
atoll, in the Low Archipelago, has been converted in the space of
thirty-four years, from being, as described by the crew of a wrecked
whaling vessel, a “reef of rocks” into a lagoon-island, fourteen miles
in length, with “one of its sides covered nearly the whole way with
high trees.”[26] The islets, also, on Keeling atoll, it has been shown,
have increased in length, and since the construction of an old chart,
several of them have become united into one long islet; but in this
case, and in that of Matilda atoll, we have no proof, and can only
infer as probable, that the reef, that is the foundation of the islets,
has increased as well as the islets themselves.
[25] Cook’s “Third Voyage,” book III, ch. x.
[26] Beechey’s “Voyage to the Pacific,” ch. vii and viii.
After these considerations, I attach little importance, as indicating
the ordinary and still less the possible rate of _ outward_ growth of
coral-reefs, to the fact that certain reefs in the Red Sea have not
increased during a long interval of time; or to other such cases, as
that of Ouluthy atoll in the Caroline group, where every islet,
described a thousand years before by Cantova was found in the same
state by Lutké,[27]—without it could be shown that, in these cases, the
conditions were favourable to the vigorous and unopposed growth of the
corals living in the different zones of depth, and that a proper basis
for the extent of the reef was present. The former conditions must
depend on many contingencies, and in the deep oceans where coral
formations most abound, a basis within the requisite depth can rarely
be present.
[27] F. Lutké’s “Voyage autour du Monde.” In the group Elato, however,
it appears that what is now the islet Falipi, is called in Cantova’s
Chart, the Banc de Falipi. It is not stated whether this has been
caused by the growth of coral, or by the accumulation of sand.
Nor do I attach any importance to the fact of certain submerged reefs,
as those off Tahiti, or those within Diego Garcia not now being nearer
the surface than they were many years ago, as an indication of the rate
under favourable circumstances of the _upward_ growth of reefs; after
it has been shown, that all the reefs have grown to the surface in some
of the Chagos atolls, but that in neighbouring atolls which appear to
be of equal antiquity and to be exposed to the same external
conditions, every reef remains submerged; for we are almost driven to
attribute this to a difference, not in the rate of growth, but in the
habits of the corals in the two cases.
In an old-standing reef, the corals, which are so different in kind on
different parts of it, are probably all adapted to the stations they
occupy, and hold their places, like other organic beings, by a struggle
one with another, and with external nature; hence we may infer that
their growth would generally be slow, except under peculiarly
favourable circumstances. Almost the only natural condition, allowing a
quick upward growth of the whole surface of a reef, would be a slow
subsidence of the area in which it stood; if, for instance, Keeling
atoll were to subside two or three feet, can we doubt that the
projecting margin of live coral, about half an inch in thickness, which
surrounds the dead upper surfaces of the mounds of Porites, would in
this case form a concentric layer over them, and the reef thus increase
upwards, instead of, as at present, outwards? The Nulliporæ are now
encroaching on the Porites and Millepora, but in this case might we not
confidently expect that the latter would, in their turn, encroach on
the Nulliporæ? After a subsidence of this kind, the sea would gain on
the islets, and the great fields of dead but upright corals in the
lagoon, would be covered by a sheet of clear water; and might we not
then expect that these reefs would rise to the surface, as they
anciently did when the lagoon was less confined by islets, and as they
did within a period of ten years in the schooner-channel, cut by the
inhabitants? In one of the Maldiva atolls, a reef, which within a very
few years existed as an islet bearing cocoa-nut trees, was found by
Lieutenant Prentice “_entirely covered with live coral and Madrepore._”
The natives believe that the islet was washed away by a change in the
currents, but if, instead of this, it had quietly subsided, surely
every part of the island which offered a solid foundation, would in a
like manner have become coated with living coral.
Through steps such as these, any thickness of rock, composed of a
singular intermixture of various kinds of corals, shells, and
calcareous sediment, might be formed; but without subsidence, the
thickness would necessarily be determined by the depth at which the
reef-building polypifers can exist. If it be asked, at what rate in
years I suppose a reef of coral favourably circumstanced could grow up
from a given depth; I should answer, that we have no precise evidence
on this point, and comparatively little concern with it. We see, in
innumerable points over wide areas, that the rate has been sufficient,
either to bring up the reefs from various depths to the surface, or, as
is more probable, to keep them at the surface, during progressive
subsidences; and this is a much more important standard of comparison
than any cycle of years.
It may, however, be inferred from the following facts, that the rate
in years under favourable circumstances would be very far from slow.
Dr. Allan, of Forres, has, in his MS. Thesis deposited in the library
of the Edinburgh University (extracts from which I owe to the kindness
of Dr. Malcolmson), the following account of some experiments, which he
tried during his travels in the years 1830 to 1832 on the east coast of
Madagascar. “To ascertain the rise and progress of the coral-family,
and fix the number of species met with at Foul Point (latitude 17° 40′)
twenty species of coral were taken off the reef and planted apart on a
sand-bank _three feet deep at low water._ Each portion weighed ten
pounds, and was kept in its place by stakes. Similar quantities were
placed in a clump and secured as the rest. This was done in December
1830. In July following, each detached mass was nearly level with the
sea at low water, quite immovable, and several feet long, stretching as
the parent reef, with the coast current from north to south. The masses
accumulated in a clump were found equally increased, but some of the
species in such unequal ratios, as to be growing over each other.” The
loss of Dr. Allan’s magnificent collection by shipwreck, unfortunately
prevents its being known to what genera these corals belonged; but from
the numbers experimented on, it is certain that all the more
conspicuous kinds must have been included. Dr. Allan informs me, in a
letter, that he believes it was a Madrepora, which grew most
vigorously. One may be permitted to suspect that the level of the sea
might possibly have been somewhat different at the two stated periods;
nevertheless, it is quite evident that the growth of the ten-pound
masses, during the six or seven months, at the end of which they were
found immovably fixed[28] and several feet in length, must have been
very great. The fact of the different kinds of coral, when placed in
one clump, having increased in extremely unequal ratios, is very
interesting, as it shows the manner in which a reef, supporting many
species of coral, would probably be affected by a change in the
external conditions favouring one kind more than another. The growth of
the masses of coral in N. and S. lines parallel to the prevailing
currents, whether due to the drifting of sediment or to the simple
movement of the water, is, also, a very interesting circumstance.
[28] It is stated by De la Beche (“Geological Manual,” p. 143), on the
authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that some
specimens of Polypifers, placed by him in a sheltered pool of water,
were found in the course of a few days firmly fixed by the secretion
of a stony matter, to the bottom.
A fact, communicated to me by Lieutenant Wellstead, I.N., in some
degree corroborates the result of Dr. Allan’s experiments: it is, that
in the Persian Gulf a ship had her copper bottom encrusted in the
course of twenty months with a layer of coral, _two feet_ in thickness,
which it required great force to remove, when the vessel was docked: it
was not ascertained to what order this coral belonged. The case of the
schooner-channel choked up with coral in an interval of less than ten
years, in the lagoon of Keeling atoll, should be here borne
in mind. We may also infer, from the trouble which the inhabitants of
the Maldiva atolls take to root out, as they express it, the
coral-knolls from their harbours, that their growth can hardly be very
slow.[29]
[29] Mr. Stutchbury (_West of England Journal_, No. I, p. 50.) has
described a specimen of Agaricia, “weighing 2 lbs. 9 oz., which
surrounds a species of oyster, whose age could not be more than two
years, and yet is completely enveloped by this dense coral.” I presume
that the oyster was living when the specimen was procured; otherwise
the fact tells nothing. Mr. Stutchbury also mentions an anchor, which
had become entirely encrusted with coral in fifty years; other cases,
however, are recorded of anchors which have long remained amidst
coral-reefs without having become coated. The anchor of the _Beagle_,
in 1832, after having been down exactly one month at Rio de Janeiro,
was so thickly coated by two species of Tubularia, that large spaces
of the iron were entirely concealed; the tufts of this horny zoophyte
were between two and three inches in length. It has been attempted to
compute, but I believe erroneously, the rate of growth of a reef, from
the fact mentioned by Captain Beechey, of the _ Chama gigas_ being
embedded in coral-rock. But it should be remembered, that some species
of this genus invariably live, both whilst young and old, in cavities,
which the animal has the power of enlarging with its growth. I saw
many of these shells thus embedded in the outer “flat” of Keeling
atoll, which is composed of dead rock; and therefore the cavities in
this case had no relation whatever with the growth of coral. M.
Lesson, also, speaking of this shell (Partie Zoolog. “Voyage de la
_Coquille_”), has remarked, “que constamment ses valves étaient
engagés complétement dans la masse des Madrepores.”
From the facts given in this section, it may be concluded, first, that
considerable thicknesses of rock have certainly been formed within the
present geological area by the growth of coral and the accumulation of
its detritus; and, secondly, that the increase of individual corals and
of reefs, both outwards or horizontally and upwards or vertically,
under the peculiar conditions favourable to such increase, is not slow,
when referred either to the standard of the average oscillations of
level in the earth’s crust, or to the more precise but less important
one of a cycle of years.
_Section III_—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE
I have already described in detail, which might have appeared trivial,
the nature of the bottom of the sea immediately surrounding Keeling
atoll; and I will now describe with almost equal care the soundings off
the fringing-reefs of Mauritius. I have preferred this arrangement, for
the sake of grouping together facts of a similar nature. I sounded with
the wide bell-shaped lead which Captain Fitzroy used at Keeling Island,
but my examination of the bottom was confined to a few miles of coast
(between Port Louis and Tomb Bay) on the leeward side of the island.
The edge of the reef is formed of great shapeless masses
of branching Madrepores, which chiefly consist of two
species,—apparently _M. corymbosa_ and _ pocillifera_,—mingled with a
few other kinds of coral. These masses are separated from each other by
the most irregular gullies and cavities, into which the lead sinks many
feet. Outside this irregular border of Madrepores, the water deepens
gradually to twenty fathoms, which depth generally is found at the
distance of from half to three-quarters of a mile from the reef. A
little further out the depth is thirty fathoms, and thence the bank
slopes rapidly into the depths of the ocean. This inclination is very
gentle compared with that outside Keeling and other atolls, but
compared with most coasts it is steep. The water was so clear outside
the reef, that I could distinguish every object forming the rugged
bottom. In this part, and to a depth of eight fathoms, I sounded
repeatedly, and at each cast pounded the bottom with the broad lead,
nevertheless the arming invariably came up perfectly clean, but deeply
indented. From eight to fifteen fathoms a little calcareous sand was
occasionally brought up, but more frequently the arming was simply
indented. In all this space the two Madrepores above mentioned, and two
species of Astræa, with rather large[30] stars, seemed the commonest
kinds; and it must be noticed that twice at the depth of fifteen
fathoms, the arming was marked with a clean impression of an Astræa.
Besides these lithophytes, some fragments of the _Millepora
alcicornis,_ which occurs in the same relative position at Keeling
Island, were brought up; and in the deeper parts there were large beds
of a Seriatopora, different from _S. subulata_, but closely allied to
it. On the beach within the reef, the rolled fragments consisted
chiefly of the corals just mentioned, and of a massive Porites, like
that at Keeling atoll, of a Meandrina, _ Pocillopora verrucosa_, and of
numerous fragments of Nullipora. From fifteen to twenty fathoms the
bottom was, with few exceptions, either formed of sand, or thickly
covered with Seriatopora: this delicate coral seems to form at these
depths extensive beds unmingled with any other kind. At twenty fathoms,
one sounding brought up a fragment of Madrepora apparently _M.
pocillifera_, and I believe it is the same species (for I neglected to
bring specimens from both stations) which mainly forms the upper margin
of the reef; if so, it grows in depths varying from
0 to 20 fathoms. Between 20 and 23 fathoms I obtained several
soundings, and they all showed a sandy bottom, with one exception at 30
fathoms, when the arming came up scooped out, as if by the margin of a
large Caryophyllia. Beyond 33 fathoms I sounded only once; and from 86
fathoms, at the distance of one mile and a third from the edge of the
reef, the arming brought up calcareous sand with a pebble of volcanic
rock. The circumstance of the arming having invariably come up quite
clean, when sounding within a certain number of fathoms off the reefs
of Mauritius and Keeling atoll (eight fathoms in the former case, and
twelve in the latter) and of its having always come up (with one
exception) smoothed and covered with sand, when the depth exceeded
twenty fathoms, probably indicates a criterion, by which the limits of
the vigorous growth of coral might in all cases be readily ascertained.
I do not, however, suppose that if a vast number of soundings were
obtained round these islands, the limit above assigned would be found
never to vary, but I conceive the facts are sufficient to show, that
the exceptions would be few. The circumstance of a _gradual_ change, in
the two cases, from a field of clean coral to a smooth sandy bottom, is
far more important in indicating the depth at which the larger kinds of
coral flourish than almost any number of separate observations on the
depth, at which certain species have been dredged up. For we can
understand the gradation, only as a prolonged struggle against
unfavourable conditions. If a person were to find the soil clothed with
turf on the banks of a stream of water, but on going to some distance
on one side of it, he observed the blades of grass growing thinner and
thinner, with intervening patches of sand, until he entered a desert of
sand, he would safely conclude, especially if changes of the same kind
were noticed in other places, that the presence of the water was
absolutely necessary to the formation of a thick bed of turf: so may we
conclude, with the same feeling of certainty, that thick beds of coral
are formed only at small depths beneath the surface of the sea.
[30] Since the preceding pages were printed off, I have received from
Mr. Lyell a very interesting pamphlet, entitled “Remarks upon Coral
Formations,” etc., by J. Couthouy, Boston, United States, 1842. There
is a statement (p. 6), on the authority of the Rev. J. Williams,
corroborating the remarks made by Ehrenberg and Lyell (p. 71 of this
volume), on the antiquity of certain individual corals in the Red Sea
and at Bermuda; namely, that at Upolu, one of the Navigator Islands,
“particular clumps of coral are known to the fishermen by name,
derived from either some particular configuration or tradition
attached to them, and handed down from time immemorial.” With respect
to the thickness of masses of coral-rock, it clearly appears, from the
descriptions given by Mr. Couthouy (pp. 34, 58) that Mangaia and
Aurora Islands are upraised atolls, composed of coral rock: the level
summit of the former is about three hundred feet, and that of Aurora
Island is two hundred feet above the sea-level.
I have endeavoured to collect every fact, which might either invalidate
or corroborate this conclusion. Captain Moresby, whose opportunities
for observation during his survey of the Maldiva and Chagos
Archipelagoes have been unrivalled, informs me, that the upper part or
zone of the steep-sided reefs, on the inner and outer coasts of the
atolls in both groups, invariably consists of coral, and the lower
parts of sand. At seven or eight fathoms depth, the bottom is formed,
as could be seen through the clear water, of great living masses of
coral, which at about ten fathoms generally stand some way apart from
each other, with patches of white sand between them, and at a little
greater depth these patches become united into a smooth steep slope,
without any coral. Captain Moresby, also, informs me in support of his
statement, that he found only decayed coral on the Padua Bank (northern
part of the Laccadive group) which has an average depth between
twenty-five and thirty-five fathoms, but that on some other banks in
the same group with only ten or twelve fathoms water on them (for
instance, the Tillacapeni bank), the coral was living.
With regard to the coral-reefs in the Red Sea, Ehrenberg has the
following passage:—“The living corals do not descend there into great
depths. On the edges of islets and near reefs, where the depth was
small, very many lived; but we found no more even at six fathoms. The
pearl-fishers at Yemen and Massaua asserted that there was no coral
near the pearl-banks at nine fathoms depth, but only sand. We were not
able to institute any more special researches.”[31] I am, however,
assured both by Captain Moresby and Lieutenant Wellstead, that in the
more northern parts of the Red Sea, there are extensive beds of living
coral at a depth of twenty-five fathoms, in which the anchors of their
vessels were frequently entangled. Captain Moresby attributes the less
depth, at which the corals are able to live in the places mentioned by
Ehrenberg, to the greater quantity of sediment there; and the
situations, where they were flourishing at the depth of twenty-five
fathoms, were protected, and the water was extraordinarily limpid. On
the leeward side of Mauritius where I found the coral growing at a
somewhat greater depth than at Keeling atoll, the sea, owing apparently
to its tranquil state, was likewise very clear. Within the lagoons of
some of the Marshall atolls, where the water can be but little
agitated, there are, according to Kotzebue, living beds of coral in
twenty-five fathoms. From these facts, and considering the manner in
which the beds of clean coral off Mauritius, Keeling Island, the
Maldiva and Chagos atolls, graduated into a sandy slope, it appears
very probable that the depth, at which reef-building polypifers can
exist, is partly determined by the extent of inclined surface, which
the currents of the sea and the recoiling waves have the power to keep
free from sediment.
[31] Ehrenberg, “Über die Natur,” etc., p. 50.
MM. Quoy and Gaimard[32] believe that the growth of coral is confined
within very limited depths; and they state that they never found any
fragment of an Astræa (the genus they consider most efficient in
forming reefs) at a depth above twenty-five or thirty feet. But we have
seen that in several places the bottom of the sea is paved with massive
corals at more than twice this depth; and at fifteen fathoms (or twice
this depth) off the reefs of Mauritius, the arming was marked with the
distinct impression of a living Astræa. _Millepora alcicornis_ lives in
from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0
to 20 fathoms. Captain Moresby has given me a specimen of _Sideropora
scabra_ (Porites of Lamarck) brought up alive from 17 fathoms. Mr.
Couthouy[33] states that he has dredged up on the Bahama banks
considerable masses of Meandrina from 16 fathoms, and he has seen this
coral growing in 20 fathoms. A Caryophyllia, half an inch in diameter,
was dredged up alive from 80 fathoms off Juan Fernandez (latitude 33°
S.) by Captain P. P. King:[34] this is the most remarkable fact with
which I am acquainted, showing the depth at which a genus of
corals often found on reefs, can exist.[35] We ought, however, to feel
less
surprise at this fact, as Caryophyllia alone of the lamelliform genera,
ranges far beyond the tropics; it is found in Zetland[36] in Lat. 60°
N. in deep water, and I procured a small species from Tierra del Fuego
in Lat. 53° S. Captain Beechey informs me, that branches of pink and
yellow coral were frequently brought up from between twenty and
twenty-five fathoms off the Low atolls; and Lieutenant Stokes, writing
to me from the N.W. coast of Australia, says that a strongly branched
coral was procured there from thirty fathoms; unfortunately it is not
known to what genera these corals belong.
[32] “Annales des Sci. Nat.” tom. vi.
[33] “Remarks on Coral Formations,” p. 12.
[34] I am indebted to Mr. Stokes for having kindly communicated this
fact to me, together with much other valuable information.
[35] I will record in the form of a note all the facts that I have
been able to collect on the depths, both within and without the
tropics, at which those corals and corallines can live, which there is
no reason to suppose ever materially aid in the construction of a
reef.
Ellis (“Nat. Hist. of Coralline,” p. 96) states that Ombellularia
was procured in latitude 79° N. _ sticking_ to a _line_ from the
depth of 236 fathoms; hence this coral either must have been
floating loose, or was entangled in stray line at the bottom. Off
Keeling atoll a compound Ascidia (Sigillina) was brought up from 39
fathoms, and a piece of sponge, apparently living, from 70, and a
fragment of Nullipora also apparently living from 92 fathoms. At a
greater depth than 90 fathoms off this coral island, the bottom was
thickly strewed with joints of Halimeda and small fragments of
other Nulliporæ, but all dead. Captain B. Allen, R.N., informs me
that in the survey of the West Indies it was noticed that between
the depth of 10 and 200 fathoms, the sounding lead very generally
came up coated with the dead joints of a Halimeda, of which he
showed me specimens. Off Pernambuco, in Brazil, in about twelve
fathoms, the bottom was covered with fragments dead and alive of a
dull red Nullipora, and I infer from Roussin’s chart, that a bottom
of this kind extends over a wide area. On the beach, within the
coral-reefs of Mauritius, vast quantities of fragments of Nulliporæ
were piled up. From these facts it appears, that these simply
organized bodies are amongst the most abundant productions of the
sea.
[36] Fleming’s “British Animals,” genus Caryophyllia.
Name of Zoophyte Depth in
Fathoms Country and
S. Latitude Authority Sertularia 40 Cape Horn 66° (Where
none is given, the observation is my own.) Cellaria Ditto Ditto
Cellaria. A minute scarlet encrusted species, found
living 190 Keeling Atoll 12° Cellaria. An allied, small stony
sub-generic form 48 S. Cruz River 50° A coral allied to
Vincularia, with eight rows of cells 40 Cape Horn Tubulipora,
near to T. patima Ditto Ditto Ditto 94 East Chiloe 43°
Cellepora, several species, and allied sub-generic forms 40 Cape
Horn Ditto 40 and 57 Chonos Arch. 45° Ditto 48 S. Cruz 50°
Eschara 30 Tierra del Fuego 53° Ditto 48 S. Cruz R. 50°
Retepora 40 Cape Horn Ditto 100 C. Good Hope 34° Quoy
and Gaimard, _Ann. Scien. Nat.,_ t. vi, p. 284. Millepora, a strong
coral with cylindrical branches, of a pink colour, abut two inches
high, resembling in the form of its orifices M. aspera of Lamarck 94
and 30 E. Chiloe 43°
Tierra del Fuego 53° Coralium 120 Barbary 33° N. Peyssonel in
paper read to Royal society May 1752. Antipathes 16 Chonos 45°
Gorgonia (or an allied form) 160 Abrolhos on the coast of Brazil
18° Capt. Beechey informed me of this fact in a letter.
Although the limit of depth, at which each particular kind of coral
ceases to exist, is far from being accurately known; yet when we bear
in mind the manner in which the clumps of coral gradually became
infrequent at about the same depth, and wholly disappeared at a greater
depth than twenty fathoms, on the slope round Keeling atoll, on the
leeward side of the Mauritius, and at rather less depth, both without
and within the atolls of the Maldiva and Chagos Archipelagoes; and when
we know that the reefs round these islands do not differ from other
coral formations in their form and structure, we may, I think, conclude
that in ordinary cases, reef- building polypifers do not flourish at
greater depths than between twenty and thirty fathoms.
It has been argued[37] that reefs may possibly rise from very great
depths through the means of small corals, first making a platform for
the growth of the stronger kinds. This, however, is an arbitrary
supposition: it is not always remembered, that in such cases there is
an antagonist power in action, namely, the decay of organic bodies,
when not protected by a covering of sediment, or by their own rapid
growth. We have, moreover, no right to calculate on unlimited time for
the accumulation of small organic bodies into great masses. Every fact
in geology proclaims that neither the land, nor the bed of the sea
retain for indefinite periods the same level. As well might it be
imagined that the British Seas would in time become choked up with beds
of oysters, or that the numerous small corallines off the inhospitable
shores of Tierra del Fuego would in time form a solid and extensive
coral-reef.
[37] _Journal of the Royal Geographical Society,_ 1831, p. 218.
Chapter V THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS
The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment.—Immense areas interspersed with
atolls.—Their subsidence.—The effects of storms and earthquakes on
atolls.—Recent changes in their state.—The origin of barrier-reefs and
of atolls.—Their relative forms.—The step-formed ledges and walls round
the shores of some lagoons.—The ring-formed reefs of the Maldiva
atolls.—The submerged condition of parts or of the whole of some
annular reefs.—The disseverment of large atolls.—The union of atolls by
linear reefs.—The Great Chagos Bank.—Objections from the area and
amount of subsidence required by the theory, considered.—The probable
composition of the lower parts of atolls.
The naturalists who have visited the Pacific, seem to have had their
attention riveted by the lagoon-islands, or atolls,—those singular
rings of coral-land which rise abruptly out of the unfathomable
ocean—and have passed over, almost unnoticed, the scarcely less
wonderful encircling barrier-reefs. The theory most generally received
on the formation of atolls, is that they are based on submarine
craters; but where can we find a crater of the shape of Bow atoll,
which is five times as long as it is broad (Plate I, Fig. 4); or like
that of Menchikoff Island (Plate II, Fig. 3), with its three loops,
together sixty miles in length; or like Rimsky Korsacoff, narrow,
crooked, and fifty-four miles long; or like the northern Maldiva
atolls, made up of numerous ring-formed reefs, placed on the margin of
a disc,—one of which discs is eighty-eight miles in length, and only
from ten to twenty in breadth? It is, also, not a little improbable,
that there should have existed as many craters of immense size crowded
together beneath the sea, as there are now in some parts atolls. But
this theory lies under a greater difficulty, as will be evident, when
we consider on what foundations the atolls of the larger archipelagoes
rest: nevertheless, if the rim of a crater afforded a basis at the
proper depth, I am far from denying that a reef like a perfectly
characterised atoll might not be formed; some such, perhaps, now exist;
but I cannot believe in the possibility of the greater number having
thus originated.
An earlier and better theory was proposed by Chamisso;[1] he supposes
that as the more massive kinds of corals prefer the surf, the outer
portions, in a reef rising from a submarine basis, would first reach
the surface and consequently form a ring. But on this view it must be
assumed, that in every case the basis consists of a flat bank; for if
it were conically formed, like a mountainous mass, we can see no reason
why the coral should spring up from the flanks, instead of from the
central and highest parts: considering the number of the atolls in the
Pacific and Indian Oceans, this assumption is very improbable. As the
lagoons of atolls are sometimes even more than forty fathoms deep, it
must, also, be assumed on this view, that at a depth at which the waves
do not break, the coral grows more vigorously on the edges of a bank
than on its central part; and this is an assumption without any
evidence in support of it. I remarked, in the third chapter, that a
reef, growing on a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a bank, with a
level surface some fathoms submerged, having steep sides and being
situated in a deep sea, a reef not to be distinguished from an atoll,
might be formed: I believe some such exist in the West Indies. But a
difficulty of the same kind with that affecting the crater theory,
runners, as we shall presently see, this view inapplicable to the
greater number of atolls.
[1] Kotzebue’s “First Voyage,” vol. iii, p. 331.
No theory worthy of notice has been advanced to account for those
barrier-reefs, which encircle islands of moderate dimensions. The great
reef which fronts the coast of Australia has been supposed, but without
any special facts, to rest on the edge of a submarine precipice,
extending parallel to the shore. The origin of the third class or of
fringing-reefs presents, I believe, scarcely any difficulty, and is
simply consequent on the polypifers not growing up from great depths,
and their not flourishing close to gently shelving beaches where the
water is often turbid.
What cause, then, has given to atolls and barrier-reefs their
characteristic forms? Let us see whether an important deduction will
not follow from the consideration of these two circumstances, first,
the reef-building corals flourishing only at limited depths; and
secondly, the vastness of the areas interspersed with coral-reefs and
coral-islets, none of which rise to a greater height above the level of
the sea, than that attained by matter thrown up by the waves and winds.
I do not make this latter statement vaguely; I have carefully sought
for descriptions of every island in the intertropical seas; and my task
has been in some degree abridged by a map of the Pacific, corrected in
1834 by MM. D’Urville and Lottin, in which the low islands are
distinguished from the high ones (even from those much less than a
hundred feet in height) by being written without a capital letter; I
have detected a few errors in this map, respecting the height of some
of the islands, which will be noticed in the Appendix, where I treat of
coral formations in geographical order. To the Appendix, also, I must
refer for a more particular account of the data on which the statements
on the next page are grounded. I have ascertained, and chiefly from the
writings of Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and
Lutké, regarding the Pacific; and from Moresby[2] with respect to the
Indian Ocean, that in the following cases the term “low island”
strictly means land of the height commonly attained by matter thrown up
by the winds and the
waves of an open sea. If we draw a line (the plan I have always
adopted) joining the external atolls of that part of the Low
Archipelago in which the islands are numerous, the figure will be a
pointed ellipse (reaching from Hood to Lazaref Island), of which the
longer axis is 840 geographical miles, and the shorter 420 miles; in
this space[3] none of the innumerable islets united into great rings
rise above the stated level. The Gilbert group is very narrow, and 300
miles in length. In a prolonged line from this group, at the distance
of 240 miles, is the Marshall Archipelago, the figure of which is an
irregular square, one end being broader than the other; its length is
520 miles, with an average width of 240; these two groups together are
1,040 miles in length, and all their islets are low. Between the
southern end of the Gilbert and the northern end of Low Archipelago,
the ocean is thinly strewed with islands, all of which, as far as I
have been able to ascertain, are low; so that from nearly the southern
end of the Low Archipelago, to the northern end of the Marshall
Archipelago, there is a narrow band of ocean, more than 4,000 miles in
length, containing a great number of islands, all of which are low. In
the western part of the Caroline Archipelago, there is a space of 480
miles in length, and about 100 broad, thinly interspersed with low
islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas
is 470 miles in length, and 60 in breadth; that of the Laccadives is
150 by 100 miles; as there is a low island between these two groups,
they may be considered as one group of 1,000 miles in length. To this
may be added the Chagos group of low islands, situated 280 miles
distant, in a line prolonged from the southern extremity of the
Maldivas. This group, including the submerged banks, is 170 miles in
length and 80 in breadth. So striking is the uniformity in direction of
these three archipelagoes, all the islands of which are low, that
Captain Moresby, in one of his papers, speaks of them as parts of one
great chain, nearly 1,500 miles long. I am, then, fully justified in
repeating, that enormous spaces, both in the Pacific and Indian Oceans,
are interspersed with islands, of which not one rises above that
height, to which the waves and winds in an open sea can heap up matter.
On what foundations, then, have these reefs and islets of coral been
constructed? A foundation must originally have been present beneath
each atoll at that limited depth, which is indispensable for the first
growth of the reef-building polypifers. A conjecture will perhaps be
hazarded, that the requisite bases might have been afforded by the
accumulation of great banks of sediment, which owing to the action of
superficial currents (aided possibly by the undulatory movement of the
sea) did not quite reach the surface,—as actually appears to have been
the case in some parts of the West Indian Sea. But in the form and
disposition of the groups of atolls, there is nothing to countenance
this notion; and the assumption without any proof, that a number of
immense piles of sediment have been heaped on the floor of the great
Pacific and Indian Oceans, in their central parts far remote from land,
and where the dark blue colour of the limpid water bespeaks its purity,
cannot for one moment be admitted.
[2] See also Captain Owen’s and Lieutenant Wood’s papers in the
_Geographical Journal_, on the Maldiva and Laccadive Archipelagoes.
These officers particularly refer to the lowness of the islets; but I
chiefly ground my assertion respecting these two groups, and the
Chagos group, from information communicated to me by Captain Moresby.
[3] I find from Mr. Couthouy’s pamphlet (p. 58) that Aurora Island is
about two hundred feet in height; it consists of coral-rock, and seems
to have been formed by the elevation of an atoll. It lies north-east
of Tahiti, close without the line bounding the space coloured dark
blue in the map appended to this volume. Honden Island, which is
situated in the extreme north-west part of the Low Archipelago,
according to measurements made on board the _Beagle_, whilst sailing
by, is 114 feet from the _summit of the trees_ to the water’s edge.
This island appeared to resemble the other atolls of the group.
The many widely-scattered atolls must, therefore, rest on rocky bases.
But we cannot believe that the broad summit of a mountain lies buried
at the depth of a few fathoms beneath every atoll, and nevertheless
throughout the immense areas above-named, with not one point of rock
projecting above the level of the sea; for we may judge with some
accuracy of mountains beneath the sea, by those on the land; and where
can we find a single chain several hundred miles in length and of
considerable breadth, much less several such chains, with their many
broad summits attaining the same height, within from 120 to 180 feet?
If the data be thought insufficient, on which I have grounded my
belief, respecting the depth at which the reef-building polypifers can
exist, and it be assumed that they can flourish at a depth of even one
hundred fathoms, yet the weight of the above argument is but little
diminished, for it is almost equally improbable, that as many submarine
mountains, as there are low islands in the several great and widely
separated areas above specified, should all rise within six hundred
feet of the surface of the sea and not one above it, as that they
should be of the same height within the smaller limit of one or two
hundred feet. So highly improbable is this supposition, that we are
compelled to believe, that the bases of the many atolls did never at
any one period all lie submerged within the depth of a few fathoms
beneath the surface, but that they were brought into the requisite
position or level, some at one period and some at another, through
movements in the earth’s crust. But this could not have been effected
by elevation, for the belief that points so numerous and so widely
separated were successively uplifted to a certain level, but that not
one point was raised above that level, is quite as improbable as the
former supposition, and indeed differs little from it. It will probably
occur to those who have read Ehrenberg’s account of the Reefs of the
Red Sea, that many points in these great areas may have been elevated,
but that as soon as raised, the protuberant parts were cut off by the
destroying action of the waves: a moment’s reflection, however, on the
basin-like form of the atolls, will show that this is impossible; for
the upheaval and subsequent abrasion of an island would leave a flat
disc, which might become coated with coral, but not a deeply concave
surface; moreover, we should expect to see, in some parts at least, the
rock of the foundation brought to the surface. If, then, the
foundations of the many atolls were not uplifted into the requisite
position, they must of necessity have subsided into it; and this at
once solves every difficulty,[4]
for we may safely infer, from the facts given in the last chapter, that
during a gradual subsidence the corals would be favourably
circumstanced for building up their solid frame works and reaching the
surface, as island after island slowly disappeared. Thus areas of
immense extent in the central and most profound parts of the great
oceans, might become interspersed with coral-islets, none of which
would rise to a greater height than that attained by detritus heaped up
by the sea, and nevertheless they might all have been formed by corals,
which absolutely required for their growth a solid foundation within a
few fathoms of the surface.
[4] The additional difficulty on the crater hypothesis before alluded
to, will now be evident; for on this view the volcanic action must be
supposed to have formed within the areas specified a vast number of
craters, all rising within a few fathoms of the surface, and not one
above it. The supposition that the craters were at different times
upraised above the surface, and were there abraded by the surf and
subsequently coated by corals, is subject to nearly the same
objections with those given above in this paragraph; but I consider it
superfluous to detail all the arguments opposed to such a notion.
Chamisso’s theory, from assuming the existence of so many banks, all
lying at the proper depth beneath the water, is also vitally
defective. The same observation applies to an hypothesis of Lieutenant
Nelson’s (“Geolog. Trans.” vol. v, p. 122), who supposes that the
ring-formed structure is caused by a greater number of germs of corals
becoming attached to the declivity, than to the central plateau of a
submarine bank: it likewise applies to the notion formerly entertained
(Forster’s “Observ.,” p. 151), that lagoon-islands owe their peculiar
form to the instinctive tendencies of the polypifers. According to
this latter view, the corals on the outer margin of the reef
instinctively expose themselves to the surf in order to afford
protection to corals living in the lagoon, which belong to other
genera, and to other families!
It would be out of place here to do more than allude to the many facts,
showing that the supposition of a gradual subsidence over large areas
is by no means improbable. We have the clearest proof that a movement
of this kind is possible, in the upright trees buried under the strata
many thousand feet in thickness; we have also every reason for
believing that there are now large areas gradually sinking, in the same
manner as others are rising. And when we consider how many parts of the
surface of the globe have been elevated within recent geological
periods, we must admit that there have been subsidences on a
corresponding scale, for otherwise the whole globe would have swollen.
It is very remarkable that Mr. Lyell,[5] even in the first edition of
his “Principles of Geology,” inferred that the amount of subsidence in
the Pacific must have exceeded that of elevation, from the area of land
being very small relatively to the agents there tending to form it,
namely, the growth of coral and volcanic action. But it will be asked,
are there any direct proofs of a subsiding movement in those areas, in
which subsidence will explain a phenomenon otherwise inexplicable?
This, however, can hardly be expected, for it must ever be most
difficult, excepting in countries long civilised, to detect a movement,
the tendency of which is to conceal the part affected. In barbarous and
semi-civilised
nations how long might not a slow movement, even of elevation such as
that now affecting Scandinavia, have escaped attention!
[5] “Principles of Geology,” sixth edition, vol. iii, p. 386.
Mr. Williams[6] insists strongly that the traditions of the natives,
which he has taken much pains in collecting, do not indicate the
appearance of any new islands: but on the theory of a gradual
subsidence, all that would be apparent would be, the water sometimes
encroaching slowly on the land, and the land again recovering by the
accumulation of detritus its former extent, and perhaps sometimes the
conversion of an atoll with coral islets on it, into a bare or into a
sunken annular reef. Such changes would naturally take place at the
periods when the sea rose above its usual limits, during a gale of more
than ordinary strength; and the effects of the two causes would be
hardly distinguishable. In Kotzebue’s “Voyage” there are accounts of
islands, both in the Caroline and Marshall Archipelagoes, which have
been partly washed away during hurricanes; and Kadu, the native who was
on board one of the Russian vessels, said “he saw the sea at Radack
rise to the feet of the cocoa-nut trees; but it was conjured in
time.”[7] A storm lately entirely swept away two of the Caroline
islands, and converted them into shoals; it partly, also, destroyed two
other islands.[8] According to a tradition which was communicated to
Captain Fitzroy, it is believed in the Low Archipelago, that the
arrival of the first ship caused a great inundation, which destroyed
many lives. Mr. Stutchbury relates, that in 1825, the western side of
Chain Atoll, in the same group, was completely devastated by a
hurricane, and not less than 300 lives lost: “in this instance it was
evident, even to the natives, that the hurricane alone was not
sufficient to account for the violent agitation of the ocean.”[9] That
considerable changes have taken place recently in some of the atolls in
the Low Archipelago, appears certain from the case already given of
Matilda Island: with respect to Whitsunday and Gloucester Islands in
this same group, we must either attribute great inaccuracy to their
discoverer, the famous circumnavigator Wallis, or believe that they
have undergone a considerable change in the period of fifty-nine years,
between his voyage and that of Captain Beechey’s. Whitsunday Island is
described by Wallis as “about four miles long, and three wide,” now it
is only one mile and a half long. The appearance of Gloucester Island,
in Captain Beechey’s words,[10] “has been accurately described by its
discoverer, but its present form and extent differ materially.”
Blenheim reef, in the Chagos group, consists of a water-washed annular
reef, thirteen miles in circumference, surrounding a lagoon ten fathoms
deep: on its surface there were a few worn patches of conglomerate
coral-rock, of about the size of hovels; and these Captain Moresby
considered as being, without doubt, the last remnants of islets; so
that here an atoll has been converted into an atoll-formed reef. The
inhabitants of the Maldiva Archipelago, as long ago as 1605, declared,
“that the high tides and violent currents were diminishing the number
of the islands:”[11] and I have already shown, on the authority of
Captain Moresby, that the work of destruction is still in progress; but
that on the other hand the first formation of some islets is known to
the present inhabitants. In such cases, it would be exceedingly
difficult to detect a gradual subsidence of the foundation, on which
these mutable structures rest.
[6] Williams’s “Narrative of Missionary Enterprise,” p. 31.
[7] Kotzebue’s “First Voyage,” vol. iii, p. 168.
[8] M. Desmoulins in “Comptes Rendus,” 1840, p. 837.
[9] _West of England Journal_, No. I, p. 35.
[10] Beechey’s “Voyage to the Pacific,” chap. vii, and Wallis’s
“Voyage in the _Dolphin_,” chap. iv.
[11] See an extract from Pyrard’s Voyage in Captain Owen’s paper on
the Maldiva Archipelago, in the _Geographical Journal_, vol. ii, p.
84.
Some of the archipelagoes of low coral-islands are subject to
earthquakes: Captain Moresby informs me that they are frequent, though
not very strong, in the Chagos group, which occupies a very central
position in the Indian Ocean, and is far from any land not of coral
formation. One of the islands in this group was formerly covered by a
bed of mould, which, after an earthquake, disappeared, and was believed
by the residents to have been washed by the rain through the broken
masses of underlying rock; the island was thus rendered unproductive.
Chamisso[12] states, that earthquakes are felt in the Marshall atolls,
which are far from any high land, and likewise in the islands of the
Caroline Archipelago. On one of the latter, namely Oulleay atoll,
Admiral Lutké, as he had the kindness to inform me, observed several
straight fissures about a foot in width, running for some hundred yards
obliquely across the whole width of the reef. Fissures indicate a
stretching of the earth’s crust, and, therefore, probably changes in
its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably
they have subsided. In the chapter on Keeling atoll, I attempted to
show by direct evidence, that the island underwent a movement of
subsidence, during the earthquakes lately felt there.
[12] See Chamisso, in Kotzebue’s “First Voyage,” vol. iii, p. 182 and
136.
The facts stand thus;—there are many large tracts of ocean, without any
high land, interspersed with reefs and islets, formed by the growth of
those kinds of corals, which cannot live at great depths; and the
existence of these reefs and low islets, in such numbers and at such
distant points, is quite inexplicable, excepting on the theory, that
the bases on which the reefs first became attached, slowly and
successively sank beneath the level of the sea, whilst the corals
continued to grow upwards. No positive facts are opposed to this view,
and some general considerations render it probable. There is evidence
of change in form, whether or not from subsidence, on some of these
coral-islands; and there is evidence of subterranean disturbances
beneath them. Will then the theory, to which we have thus been led,
solve the curious problem,—what has given to each class of reef its
peculiar form?
Let us in imagination place within one of the subsiding areas, an
island surrounded by a “fringing-reef,”—that kind, which alone offers
no difficulty in the explanation of its origin. Let the unbroken lines,
and the oblique shading in the woodcut (No. 4) represent a vertical
section through such an island; and the horizontal shading will
represent the section of the reef. Now, as the island sinks down,
either a few feet at a time or quite insensibly, we may safely infer
from what we know of the conditions favourable to the growth of coral,
that the living masses bathed by the surf on the margin of the reef,
will soon regain the surface. The water, however, will encroach, little
by little, on the shore, the island becoming lower and smaller, and the
space between the edge of the reef and the beach proportionately
broader. A section of the reef and island in this state, after a
subsidence of several hundred feet, is given by the dotted lines:
coral-islets are supposed to have been formed on the new reef, and a
ship is anchored in the lagoon-channel. This section is in every
respect that of an encircling barrier-reef; it is, in fact, a section
taken[13] east and west through the highest point of the encircled
island of Bolabola; of which a plan is given in Plate I, Fig. 5. The
same section is more clearly shown in the following woodcut (No. 5) by
the unbroken lines. The width of the reef, and its slope, both on the
outer and inner side, will have been determined by the growing powers
of the coral, under the conditions (for instance the force of the
breakers and of the currents) to which it has been exposed; and the
lagoon-channel will be deeper or shallower, in proportion to the growth
of the delicately branched corals within the reef, and to the
accumulation of sediment, relatively, also, to the rate of subsidence
and the length of the intervening stationary periods.
[13] The section has been made from the chart given in the “Atlas of
the Voyage of the _Coquille_.” The scale is .57 of an inch to a mile.
The height of the island, according to M. Lesson, is 4,026 feet. The
deepest part of the lagoon-channel is 162 feet; its depth is
exaggerated in the woodcut for the sake of clearness.
[Illustration: Vertical section of an island of Bolabola.]
AA—Outer edge of the reef at the level of the sea.
BB—Shores of the island.
A′A′—Outer edge of the reef, after its upward growth during a period of
subsidence.
CC—The lagoon-channel between the reef and the shores of the now
encircled land.
B′B′—The shores of the encircled island.
N.B.—In this, and the following woodcut, the subsidence of the land
could only be represented by an apparent rise in the level of the sea.
It is evident in this section, that a line drawn perpendicularly down
from the outer edge of the new reef to the foundation of solid rock,
exceeds by as many feet as there have been feet of subsidence, that
small limit of depth at which the effective polypifers can live—the
corals having grown up, as the whole sank down, from a basis formed of
other corals and their consolidated fragments. Thus the difficulty on
this head, which before seemed so great, disappears.
As the space between the reef and the subsiding shore continued to
increase in breadth and depth, and as the injurious effects of the
sediment and fresh water borne down from the land were consequently
lessened, the greater number of the channels, with which the reef in
its fringing state must have been breached, especially those which
fronted the smaller streams, will have become choked up with the growth
of coral: on the windward side of the reef, where the coral grows most
vigorously, the breaches will probably have first been closed. In
barrier-reefs, therefore, the breaches kept open by draining the tidal
waters of the lagoon-channel, will generally be placed on the leeward
side, and they will still face the mouths of the larger streams,
although removed beyond the influence of their sediment and fresh
water;—and this, it has been shown, is commonly the case.
[Illustration: Vertical section of an island of Bolabola.]
A′A′—Outer edges of the barrier-reef at the level of the sea. The
cocoa-nut trees represent coral-islets formed on the reef.
CC—The lagoon-channel.
B′B′—The shores of the island, generally formed of low alluvial land
and of coral detritus from the lagoon-channel.
A″A″—The outer edges of the reef now forming an atoll.
C′—The lagoon of the newly formed atoll. According to the scale, the
depth of the lagoon and of the lagoon-channel is exaggerated.
Referring to the diagram shown above, in which the newly formed
barrier-reef is represented by unbroken lines, instead of by dots as in
the former woodcut, let the work of subsidence go on, and the doubly
pointed hill will form two small islands (or more, according to the
number of the hills) included within one annular reef. Let the island
continue subsiding, and the coral-reef will continue growing up on its
own foundation, whilst the water gains inch by inch on the land, until
the last and highest pinnacle is covered, and there remains a perfect
atoll. A vertical section of this atoll is shown in the woodcut by the
dotted lines;—a ship is anchored in its lagoon, but islets are not
supposed yet to have been formed on the reef. The depth of the lagoon
and the
width and slope of the reef, will depend on the circumstances just
referred to under barrier-reefs. Any further subsidence will produce no
change in the atoll, except perhaps a diminution in its size, from the
reef not growing vertically upwards; but should the currents of the sea
act violently upon it, and should the corals perish on part or on the
whole of its margin, changes would result during subsidence which will
be presently noticed. I may here observe, that a bank either of rock or
of hardened sediment, level with the surface of the sea, and fringed
with living coral, would (if not so small as to allow the central space
to be quickly filled up with detritus) by subsidence be converted
immediately into an atoll, without passing, as in the case of a reef
fringing the shore of an island, through the intermediate form of a
barrier-reef. If such a bank lay a few fathoms submerged, the simple
growth of the coral (as remarked in the third chapter) without the aid
of subsidence, would produce a structure scarcely to be distinguished
from a true atoll; for in all cases the corals on the outer margin of a
reef, from having space and being freely exposed to the open sea, will
grow vigorously and tend to form a continuous ring whilst the growth of
the less massive kinds on the central expanse, will be checked by the
sediment formed there, and by that washed inwards by the breakers; and
as the space becomes shallower, their growth will, also, be checked by
the impurities of the water, and probably by the small amount of food
brought by the enfeebled currents, in proportion to the surface of
living reefs studded with innumerable craving mouths: the subsidence of
a reef based on a bank of this kind, would give depth to its central
expanse or lagoon, steepness to its flanks, and through the free growth
of the coral, symmetry to its outline:—I may here repeat that the
larger groups of atolls in the Pacific and Indian Oceans cannot be
supposed to be founded on banks of this nature.
If, instead of the island in the diagram, the shore of a continent
fringed by a reef had subsided, a great barrier-reef, like that on the
north-east coast of Australia, would have necessarily resulted; and it
would have been separated from the main land by a deep-water channel,
broad in proportion to the amount of subsidence, and to the less or
greater inclination of the neighbouring coast-line. The effect of the
continued subsidence of a great barrier-reef of this kind, and its
probable conversion into a chain of separate atolls, will be noticed,
when we discuss the apparent progressive disseverment of the larger
Maldiva atolls.
We now are able to perceive that the close similarity in form,
dimensions, structure, and relative position (which latter point will
hereafter be more fully noticed) between fringing and encircling
barrier-reefs, and between these latter and atolls, is the necessary
result of the transformation, during subsidence of the one class into
the other. On this view, the three classes of reefs ought to graduate
into each other. Reefs having intermediate character between those of
the fringing and barrier classes do exist; for instance, on the
south-west coast of Madagascar, a reef extends for several miles,
within which there is a broad channel from seven to eight fathoms deep,
but the sea does not deepen abruptly outside the reef. Such cases,
however, are open to
some doubts, for an old fringing-reef, which had extended itself a
little on a basis of its own formation, would hardly be distinguishable
from a barrier-reef, produced by a small amount of subsidence, and with
its lagoon-channel nearly filled up with sediment during a long
stationary period. Between barrier-reefs, encircling either one lofty
island or several small low ones, and atolls including a mere expanse
of water, a striking series can be shown: in proof of this, I need only
refer to the first plate in this volume, which speaks more plainly to
the eye, than any description could to the ear. The authorities from
which the charts have been engraved, together with some remarks on them
and descriptive of the plates, are given above. At New Caledonia (Plate
II, Fig. 5.) the barrier-reefs extend for 150 miles on each side of the
submarine prolongation of the island; and at their northern extremity
they appear broken up and converted into a vast atoll-formed reef,
supporting a few low coral-islets: we may imagine that we here see the
effects of subsidence actually in progress, the water always
encroaching on the northern end of the island, towards which the
mountains slope down, and the reefs steadily building up their massive
fabrics in the lines of their ancient growth.
We have as yet only considered the origin of barrier-reefs and atolls
in their simplest form; but there remain some peculiarities in
structure and some special cases, described in the two first chapters,
to be accounted for by our theory. These consist—in the inclined ledge
terminated by a wall, and sometimes succeeded by a second ledge with a
wall, round the shores of certain lagoons and lagoon-channels; a
structure which cannot, as I endeavoured to show, be explained by the
simple growing powers of the corals,—in the ring or basin-like forms of
the central reefs, as well as of the separate marginal portions of the
northern Maldiva atolls,—in the submerged condition of the whole, or of
parts of certain barrier and atoll-formed reefs; where only a part is
submerged, this being generally to leeward,—in the apparent progressive
disseverment of some of the Maldiva atolls,—in the existence of
irregularly formed atolls, some being tied together by linear reefs,
and others with spurs projecting from them,—and, lastly, in the
structure and origin of the Great Chagos Bank.
_Step-formed ledges round certain lagoons._—If we suppose an atoll to
subside at an extremely slow rate, it is difficult to follow out the
complex results. The living corals would grow up on the outer margin;
and likewise probably in the gullies and deeper parts of the bare
surface of the annular reef; the water would encroach on the islets,
but the accumulation of fresh detritus might possibly prevent their
entire submergence. After a subsidence of this very slow nature, the
surface of the annular reef sloping gently into the lagoon, would
probably become united with the irregular reefs and banks of sand,
which line the shores of most lagoons. Should, however, the atoll be
carried down by a more rapid movement, the whole surface of the annular
reef, where there was a foundation of solid matter, would be favourably
circumstanced for the fresh growth of coral; but as the corals grew
upwards on its exterior margin, and the waves broke heavily on this
part, the increase of the massive polypifers on the inner side would be
checked from the want of water. Consequently, the exterior parts would
first reach the surface, and the new annular reef thus formed on the
old one, would have its summit inclined inwards, and be terminated by a
subaqueous wall, formed by the upward growth of the coral (before being
much checked), from the inner edge of the solid parts of the old reef.
The inner portion of the new reef, from not having grown to the
surface, would be covered by the waters of the lagoon. Should a
subsidence of the same kind be repeated, the corals would again grow up
in a wall, from all the solid parts of the resunken reef, and,
therefore, not from within the sandy shores of the lagoon; and the
inner part of the new annular reef would, from being as before checked
in its upward growth, be of less height than the exterior parts, and
therefore would not reach the surface of the lagoon. In this case the
shores of the lagoon would be surrounded by two inclined ledges, one
beneath the other, and both abruptly terminated by subaqueous
cliffs.[14]
[14] According to Mr. Couthouy (p. 26) the external reef round many
atolls descends by a succession of ledges or terraces. He attempts, I
doubt whether successfully, to explain this structure somewhat in the
same manner as I have attempted, with respect to the internal ledges
round the lagoons of some atolls. More facts are wanted regarding the
nature both of the interior and exterior step-like ledges: are all the
ledges, or only the upper ones, covered with living coral? If they are
all covered, are the kinds different on the ledges according to the
depth? Do the interior and exterior ledges occur together in the same
atolls; if so, what is their total width, and is the intervening
surface-reef narrow, etc.?
_The ring or basin-formed reefs of the northern Maldiva atolls._—I may
first observe, that the reefs within the lagoons of atolls and within
lagoon-channels, would, if favourably circumstanced, grow upwards
during subsidence in the same manner as the annular rim; and,
therefore, we might expect that such lagoon- reefs, when not surrounded
and buried by an accumulation of sediment more rapid than the rate of
subsidence, would rise abruptly from a greater depth than that at which
the efficient polypifers can flourish: we see this well exemplified in
the small abruptly-sided reefs, with which the deep lagoons of the
Chagos and Southern Maldiva atolls are studded. With respect to the
ring or basin-formed reefs of the Northern Maldiva atolls, it is
evident, from the perfectly continuous series which exists that the
marginal rings, although wider than the exterior or bounding reef of
ordinary atolls, are only modified portions of such a reef; it is also
evident that the central rings, although wider than the knolls or reefs
which commonly occur in lagoons, occupy their place. The ring-like
structure has been shown to be contingent on the breaches into the
lagoon being broad and numerous, so that all the reefs which are bathed
by the waters of the lagoon are placed under nearly the same conditions
with the outer coast of an atoll standing in the open sea. Hence the
exterior and living margins of these reefs must have been favourably
circumstanced for growing outwards, and increasing beyond the usual
breadth; and they must likewise have been favourably circumstanced for
growing
vigorously upwards, during the subsiding movements, to which by our
theory the whole archipelago has been subjected; and subsidence with
this upward growth of the margins would convert the central space of
each little reef into a small lagoon. This, however, could only take
place with those reefs, which had increased to a breadth sufficient to
prevent their central spaces from being almost immediately filled up
with the sand and detritus driven inwards from all sides: hence it is
that few reefs, which are less than half a mile in diameter, even in
the atolls where the basin-like structure is most strikingly exhibited,
include lagoons. This remark, I may add, applies to all coral-reefs
wherever found. The basin-formed reefs of the Maldiva Archipelago may,
in fact, be briefly described, as small atolls formed during subsidence
over the separate portions of large and broken atolls, in the same
manner as these latter were formed over the barrier-reefs, which
encircled the islands of a large archipelago now wholly submerged.
_Submerged and dead reefs._—In the second section of the first chapter,
I have shown that there are in the neighbourhood of atolls, some deeply
submerged banks, with level surfaces; that there are others, less
deeply but yet wholly submerged, having all the characters of perfect
atolls, but consisting merely of dead coral-rock; that there are
barrier-reefs and atolls with merely a portion of their reef, generally
on the leeward side, submerged; and that such portions either retain
their perfect outline, or they appear to be quite effaced, their former
place being marked only by a bank, conforming in outline with that part
of the reef which remains perfect. These several cases are, I believe,
intimately related together, and can be explained by the same means.
There, perhaps, exist some submerged reefs, covered with living coral
and growing upwards, but to these I do not here refer. As we see that
in those parts of the ocean, where coral-reefs are most abundant, one
island is fringed and another neighbouring one is not fringed; as we
see in the same archipelago, that all the reefs are more perfect in one
part of it than in another, for instance, in the southern half compared
with the northern half of the Maldiva Archipelago, and likewise on the
outer coasts compared with the inner coasts of the atolls in this same
group, which are placed in a double row; as we know that the existence
of the innumerable polypifers forming a reef, depends on their
sustenance, and that they are preyed on by other organic beings; and,
lastly, as we know that some inorganic causes are highly injurious to
the growth of coral, it cannot be expected that during the round of
change to which earth, air, and water are exposed, the reef-building
polypifers should keep alive for perpetuity in any one place; and still
less can this be expected, during the progressive subsidences, perhaps
at some periods more rapid than at others, to which by our theory these
reefs and islands have been subjected and are liable. It is, then, not
improbable that the corals should sometimes perish either on the whole
or on part of a reef; if on part, the dead portion, after a small
amount of subsidence, would still retain its proper outline and
position beneath the water. After a more prolonged
subsidence, it would probably form, owing to the accumulation of
sediment, only the margin of a flat bank, marking the limits of the
former lagoon. Such dead portions of reef would generally lie on the
leeward side,[15] for the impure water and fine sediment would more
easily flow out from the lagoon over this side of the reef, where the
force of the breakers is less than to windward; and therefore the
corals would be less vigorous on this side, and be less able to resist
any destroying agent. It is likewise owing to this same cause, that
reefs are more frequently breached to leeward by narrow channels,
serving as by ship-channels, than to windward. If the corals perished
entirely, or on the greater part of the circumference of an atoll, an
atoll-shaped bank of dead rock, more or less entirely submerged, would
be produced; and further subsidence, together with the accumulation of
sediment, would often obliterate its atoll-like structure, and leave
only a bank with a level surface.
[15] Mr. Lyell, in the first edition of his “Principles of Geology,”
offered a somewhat different explanation of this structure. He
supposes that there has been subsidence; but he was not aware that the
submerged portions of reef were in most cases, if not in all, dead;
and he attributes the difference in height in the two sides of most
atolls, chiefly to the greater accumulation of detritus to windward
than to leeward. But as matter is accumulated only on the backward
part of the reef, the front part would remain of the same height on
both sides. I may here observe that in most cases (for instance, at
Peros Banhos, the Gambier group and the Great Chagos Bank), and I
suspect in all cases, the dead and submerged portions do not blend or
slope into the living and perfect parts, but are separated from them
by an abrupt line. In some instances small patches of living reef rise
to the surface from the middle of the submerged and dead parts.
In the Chagos group of atolls, within an area of 160 miles by 60, there
are two atoll-formed banks of dead rock (besides another very imperfect
one), entirely submerged; a third, with merely two or three very small
pieces of living reef rising to the surface; and a fourth, namely,
Peros Banhos (Plate I, Fig. 9), with a portion nine miles in length
dead and submerged. As by our theory this area has subsided, and as
there is nothing improbable in the death, either from changes in the
state of the surrounding sea or from the subsidence being great or
sudden, of the corals on the whole, or on portions of some of the
atolls, the case of the Chagos group presents no difficulty. So far
indeed are any of the above-mentioned cases of submerged reefs from
being inexplicable, that their occurrence might have been anticipated
on our theory, and as fresh atolls are supposed to be in progressive
formation by the subsidence of encircling barrier-reefs, a weighty
objection, namely that the number of atolls must be increasing
infinitely, might even have been raised, if proofs of the occasional
destruction and loss of atolls could not have been adduced.
_The disseverment of the larger Maldiva atolls._—The apparent
progressive disseverment in the Maldiva Archipelago of large atolls
into smaller ones, is, in many respects, an important consideration,
and requires an explanation. The graduated series which marks, as I
believe, this process, can be observed only in the northern half of the
group, where the atolls have exceedingly imperfect margins, consisting
of detached basin-formed reefs. The currents of the sea flow across
these atolls, as I am informed by Captain Moresby, with considerable
force, and drift the sediment from side to side during the monsoons,
transporting much of it seaward; yet the currents sweep with greater
force round their flanks. It is historically known that these atolls
have long existed in their present state; and we can believe, that even
during a very slow subsidence they might thus remain, the central
expanse being kept at nearly its original depth by the accumulation of
sediment. But in the action of such nicely balanced forces during a
progressive subsidence (like that, to which by our theory this
archipelago has been subjected), it would be strange if the currents of
the sea should never make a direct passage across some one of the
atolls, through the many wide breaches in their margins. If this were
once effected, a deep-water channel would soon be formed by the removal
of the finer sediment, and the check to its further accumulation; and
the sides of the channel would be worn into a slope like that on the
outer coasts, which are exposed to the same force of the currents. In
fact, a channel precisely like that bifurcating one which divides
Mahlos Mahdoo (Plate II, Fig. 4), would almost necessarily be formed.
The scattered reefs situated near the borders of the new ocean-channel,
from being favourably placed for the growth of coral, would, by their
extension, tend to produce fresh margins to the dissevered portions;
such a tendency is very evident (as may be seen in the large published
chart) in the elongated reefs on the borders of the two channels
intersecting Mahlos Mahdoo. Such channels would become deeper with
continued subsidence, and probably from the reefs not growing up
perpendicularly, somewhat broader. In this case, and more especially if
the channels had been formed originally of considerable breadth, the
dissevered portions would become perfect and distinct atolls, like Ari
and Ross atolls (Plate II, Fig. 6), or like the two Nillandoo atolls,
which must be considered as distinct, although related in form and
position, and separated from each other by channels, which though deep
have been sounded. Further subsidence would render such channels
unfathomable, and the dissevered portions would then resemble Phaleedoo
and Moluque atolls, or Mahlos Mahdoo and Horsburgh atolls (Plate II,
Fig. 4), which are related to each other in no respect except in
proximity and position. Hence, on the theory of subsidence, the
disseverment of large atolls, which have imperfect margins (for
otherwise their disseverment would be scarcely possible), and which are
exposed to strong currents, is far from being an improbable event; and
the several stages, from close relation to entire isolation in the
atolls of the Maldiva Archipelago, are readily explicable.
We might go even further, and assert as not improbable, that the first
formation of the Maldiva Archipelago was due to a barrier-reef, of
nearly the same dimensions with that of New Caledonia (Plate II, Fig.
5), for if, in imagination, we complete the subsidence of that great
island, we might anticipate from the present broken condition of the
northern portion of the reef, and from the almost entire absence of
reefs on the eastern coast, that the barrier-reef after repeated
subsidences, would become during its upward growth separated into
distinct portions; and these portions would tend to assume an
atoll-like structure, from the coral growing with vigour round their
entire circumferences, when freely exposed to an open sea. As we have
some large islands partly submerged with barrier-reefs marking their
former limits, such as New Caledonia, so our theory makes it probable
that there should be other large islands wholly submerged; and these,
we may now infer, would be surmounted, not by one enormous atoll, but
by several large elongated ones, like the atolls in the Maldiva group;
and these again, during long periods of subsidence, would sometimes
become dissevered into smaller atolls. I may add, that both in the
Marshall and Caroline Archipelagoes, there are atolls standing close
together, which have an evident relationship in form: we may suppose,
in such cases, either that two or more encircled islands originally
stood close together, and afforded bases for two or more atolls, or
that one atoll has been dissevered. From the position, as well as form,
of three atolls in the Caroline Archipelago (the Namourrek and Elato
group), which are placed in an irregular circle, I am strongly tempted
to believe that they have originated by the process of
disseverment.[16]
[16] The same remark is, perhaps, applicable to the islands of Ollap,
Fanadik, and Tamatam in the Caroline Archipelago, of which charts are
given in the atlas of Duperrey’s voyage: a line drawn through the
linear reefs and lagoons of these three islands forms a semicircle.
Consult also, the atlas of Lutké’s voyage; and for the Marshall group
that of Kotzebue; for the Gilbert group consult the atlas of
Duperrey’s voyage. Most of the points here referred to may, however,
be seen in Krusenstern’s general Atlas of the Pacific.
_Irregularly formed atolls._—In the Marshall group, Musquillo atoll
consists of two loops united in one point; and Menchikoff atoll is
formed of three loops, two of which (as may be seen in Fig. 3, Plate
II) are connected by a mere ribbon-shaped reef, and the three together
are sixty miles in length. In the Gilbert group some of the atolls have
narrow strips of reef, like spurs, projecting from them. There occur
also in parts of the open sea, a few linear and straight reefs,
standing by themselves; and likewise some few reefs in the form of
crescents, with their extremities more or less curled inwards. Now, the
upward growth of a barrier-reef which fronted only one side of an
island, or one side of an elongated island with its extremities (of
which cases exist), would produce after the complete subsidence of the
land, mere strips or crescent or hook-formed reefs: if the island thus
partially fronted became divided during subsidence into two or more
islands, these islands would be united together by linear reefs; and
from the further growth of the coral along their shores together with
subsidence, reefs of various forms might ultimately be produced, either
atolls united together by linear reefs, or atolls with spurs projecting
from them. Some, however, of the more simple forms above specified,
might, as we have seen, be equally well produced by the coral perishing
during
subsidence on part of the circumference of an atoll, whilst on the
other parts it continued to grow up till it reached the surface.
_The Great Chagos Bank._—I have already shown that the submerged
condition of the Great Chagos Bank (Plate II, Fig. 1, with its section
Fig. 2), and of some other banks in the Chagos group, may in all
probability be attributed to the coral having perished before or during
the movements of subsidence, to which this whole area by our theory has
been subjected. The external rim or upper ledge (shaded in the chart),
consists of dead coral-rock thinly covered with sand; it lies at an
average depth of between five and eight fathoms, and perfectly
resembles in form the annular reef of an atoll. The banks of the second
level, the boundaries of which are marked by dotted lines in the chart,
lie from about fifteen to twenty fathoms beneath the surface; they are
several miles broad, and terminate in a very steep slope round the
central expanse. This central expanse I have already described, as
consisting of a level muddy flat between thirty and forty fathoms deep.
The banks of the second level, might at first sight be thought
analogous to the internal step-like ledge of coral-rock which borders
the lagoons of some atolls, but their much greater width, and their
being formed of sand, are points of essential difference. On the
eastern side of the atoll some of the banks are linear and parallel,
resembling islets in a great river, and pointed directly towards a
great breach on the opposite side of the atoll; these are best seen in
the large published chart. I inferred from this circumstance, that
strong currents sometimes set directly across this vast bank; and I
have since heard from Captain Moresby that this is the case. I
observed, also, that the channels or breaches through the rim, were all
of the same depth as the central lagoon-like space into which they
lead; whereas the channels into the other atolls of the Chagos group,
and as I believe into most other large atolls, are not nearly as deep
as their lagoons: for instance at Peros Banhos, the channels are only
of the same depth, namely between ten and twenty fathoms, as the bottom
of the lagoon for a space about a mile and a half in width round its
shores, whilst the central expanse of the lagoon is from thirty-five to
forty fathoms deep. Now, if an atoll during a gradual subsidence once
became entirely submerged, like the Great Chagos Bank, and therefore no
longer exposed to the surf, very little sediment could be formed from
it; and consequently the channels leading into the lagoon from not
being filled up with drifted sand and coral detritus, would continue
increasing in depth, as the whole sank down. In this case, we might
expect that the currents of the open sea, instead of any longer
sweeping round the submarine flanks, would flow directly through the
breaches across the lagoon, removing in their course the finer
sediment, and preventing its further accumulation. We should then have
the submerged reef forming an external and upper rim of rock, and
beneath this portion of the sandy bottom of the old lagoon, intersected
by deep-water channels or breaches, and thus formed into separate
marginal banks; and these would be cut off by steep slopes, overhanging
the central space, worn down by the passage of the oceanic currents.
By these means, I have scarcely any doubt that the Great Chagos Bank
has originated,—a structure which at first appeared to me far more
anomalous than any I had met with. The process of formation is nearly
the same with that, by which Mahlos Mahdoo had been trisected; but in
the Chagos Bank the channels of the oceanic currents entering at
several different quarters, have united in a central space.
This great atoll-formed bank appears to be in an early stage of
disseverment; should the work of subsidence go on, from the submerged
and dead condition of the whole reef, and the imperfection of the
south-east quarter a mere wreck would probably be left. The Pitt’s
Bank, situated not far southward, appears to be precisely in this
state; it consists of a moderately level, oblong bank of sand, lying
from 10 to 20 fathoms beneath the surface, with two sides protected by
a narrow ledge of rock which is submerged between 5 and 8 fathoms. A
little further south, at about the same distance as the southern rim of
the Great Chagos Bank is from the northern rim, there are two other
small banks with from 10 to 20 fathoms on them; and not far eastward
soundings were struck on a sandy bottom, with between 110 and 145
fathoms. The northern portion with its ledge-like margin, closely
resembles any one segment of the Great Chagos Bank, between two of the
deep-water channels, and the scattered banks, southward appear to be
the last wrecks of less perfect portions.
I have examined with care the charts of the Indian and Pacific Oceans,
and have now brought before the reader all the examples, which I have
met with, of reefs differing from the type of the class to which they
belong; and I think it has been satisfactorily shown, that they are all
included in our theory, modified by occasional accidents which might
have been anticipated as probable. In this course we have seen, that in
the lapse of ages encircling barrier-reefs are occasionally converted
into atolls, the name of atoll being properly applicable, at the moment
when the last pinnacle of encircled land sinks beneath the surface of
the sea. We have, also, seen that large atolls during the progressive
subsidence of the areas in which they stand, sometimes become
dissevered into smaller ones; at other times, the reef-building
polypifers having entirely perished, atolls are converted into
atoll-formed banks of dead rock; and these again through further
subsidence and the accumulation of sediment modified by the force of
the oceanic currents, pass into level banks with scarcely any
distinguishing character. Thus may the history of an atoll be followed
from its first origin, through the occasional accidents of its
existence, to its destruction and final obliteration.
_Objections to the theory of the formation of atolls and
barrier-reefs._—The vast amount of subsidence, both horizontally or in
area, and vertically or in depth, necessary to have submerged every
mountain, even the highest, throughout the immense spaces of ocean
interspersed with atolls, will probably strike most people as a
formidable objection to my theory. But as continents, as large as the
spaces supposed to have subsided, have been raised above the level of
the sea,—as whole regions are now rising, for instance, in Scandinavia
and South America,—and as
no reason can be assigned, why subsidences should not have occurred in
some parts of the earth’s crust on as great a scale both in extent and
amount as those of elevation, objections of this nature strike me as of
little force. The remarkable point is that movements to such an extent
should have taken place within a period, during which the polypifers
have continued adding matter on and above the same reefs. Another and
less obvious objection to the theory will perhaps be advanced from the
circumstance, of the lagoons within atolls and within barrier-reefs
never having become in any one instance during prolonged subsidences of
a greater depth than sixty fathoms, and seldom more than forty fathoms;
but we already admit, if the theory be worth considering, that the rate
of subsidence has not exceeded that of the upward growth of the coral
on the exterior margin; we are, therefore, only further required to
admit, that the subsidence has not exceeded in rate the filling up of
the interior spaces by the growth of the corals living there, and by
the accumulation of sediment. As this filling up must take place very
slowly within barrier-reefs lying far from the land, and within atolls
which are of large dimensions and which have open lagoons with very few
reefs, we are led to conclude that the subsidence thus
counter-balanced, must have been slow in an extraordinary degree; a
conclusion which accords with our only means, namely, with what is
known of the rate and manner of recent elevatory movements, of judging
by analogy what is the probable rate of subsidence.
In this chapter it has, I think, been shown, that the theory of
subsidence, which we were compelled to receive from the necessity of
giving to the corals, in certain large areas, foundations at the
requisite depth, explains both the normal structure and the less
regular forms of those two great classes of reefs, which have justly
excited the astonishment of all persons who have sailed through the
Pacific and Indian Oceans. But further to test the truth of the theory,
a crowd of questions will occur to the reader: Do the different kinds
of reefs, which have been produced by the same kind of movement,
generally lie within the same areas? What is their relation of form and
position,—for instance, do adjoining groups of atolls, and the separate
atolls in these groups, bear the same relation to each other which
islands do in common archipelagoes? Have we reason to believe, that
where there are fringing-reefs, there has not lately been subsidence;
or, for it is almost our only way of ascertaining this point, are there
frequently proofs of recent elevation? Can we by this means account for
the presence of certain classes of reefs in some large areas, and their
entire absence in others? Do the areas which have subsided, as
indicated by the presence of atolls and barrier-reefs, and the areas
which have remained stationary or have been upraised, as shown by
fringing-reefs, bear any determinate relation to each other; and are
the dimensions of these areas such as harmonise with the greatness of
the subterranean changes, which, it must be supposed, have lately taken
place beneath them? Is there any connection between the movements thus
indicated, and recent volcanic action? All these questions ought to
receive answers in accordance with the theory; and if this can be
satisfactorily shown, not only is the theory
confirmed, but as deductions, the answers are in themselves important.
Under this latter point of view, these questions will be chiefly
considered in the following chapter.[17]
[17] I may take this opportunity of briefly considering the
appearances, which would probably be presented by a vertical and deep
section across a coral formation (referring chiefly to an atoll),
formed by the upward growth of coral during successive subsidences.
This is a subject worthy of attention, as a means of comparison with
ancient coral-strata. The circumferential parts would consist of
massive species, in a vertical position, with their interstices filled
up with detritus; but this would be the part most subject to
subsequent denudation and removal. It is useless to speculate how
large a portion of the exterior annular reef would consist of upright
coral, and how much of fragmentary rock, for this would depend on many
contingencies,—such as on the rate of subsidence, occasionally
allowing a fresh growth of coral to cover the whole surface, and on
the breakers having force sufficient to throw fragments over this same
space. The conglomerate which composes the base of the islets, would
(if not removed by denudation together with the exterior reef on which
it rests) be conspicuous from the size of the fragments,—the different
degrees in which they have been rounded,—the presence of fragments of
conglomerate torn up, rounded, and recemented,—and from the oblique
stratification. The corals which lived in the lagoon-reefs at each
successive level, would be preserved upright, and they would consist
of many kinds, generally much branched. In this part, however, a very
large proportion of the rock (and in some cases nearly all of it)
would be formed of sedimentary matter, either in an excessively fine,
or in a moderately coarse state, and with the particles almost blended
together. The conglomerate which was formed of rounded pieces of the
branched corals, on the shores of the lagoon, would differ from that
formed on the islets and derived from the outer coast; yet both might
have accumulated very near each other. I have seen a conglomerate
limestone from Devonshire like a conglomerate now forming on the
shores of the Maldiva atolls. The stratification taken as a whole,
would be horizontal; but the conglomerate beds resting on the exterior
reef, and the beds of sandstone on the shores of the lagoon (and no
doubt on the external flanks) would probably be divided (as at Keeling
atoll and at Mauritius) by numerous layers dipping at considerable
angles in different directions. The calcareous sandstone and
coral-rock would almost necessarily contain innumerable shells,
echini, and the bones of fish, turtle, and perhaps of birds; possibly,
also, the bones of small saurians, as these animals find their way to
the islands far remote from any continent. The large shells of some
species of Tridacna would be found vertically imbedded in the solid
rock, in the position in which they lived. We might expect also to
find a mixture of the remains of pelagic and littoral animals in the
strata formed in the lagoon, for pumice and the seeds of plants are
floated from distant countries into the lagoons of many atolls: on the
outer coast of Keeling atoll, near the mouth of the lagoon, the case
of a pelagic Pteropodous animal was brought up on the arming of the
sounding lead. All the loose blocks of coral on Keeling atoll were
burrowed by vermiform animals; and as every cavity, no doubt,
ultimately becomes filled with spathose limestone, slabs of the rock
taken from a considerable depth, would, if polished, probably exhibit
the excavations of such burrowing animals. The conglomerate and
fine-grained beds of coral-rock would be hard, sonorous, white and
composed of nearly pure calcareous matter; in some few parts, judging
from the specimens at Keeling atoll, they would probably contain a
small quantity of iron. Floating pumice and scoriæ, and occasionally
stones transported in the root of trees (see my “Journal of
Researches,” page 549) appear the only sources, through which foreign
matter is brought to coral-formations standing in the open ocean. The
area over which sediment is transported from coral-reefs must be
considerable: Captain Moresby informs me that during the change of
monsoons the sea is discoloured to a considerable distance off the
Maldiva and Chagos atolls. The sediment of fringing and barrier
coral-reefs must be mingled with the mud, which is brought down from
the land, and is transported seaward through the breaches, which occur
in front of almost every valley. If the atolls of the larger
archipelagoes were upraised, the bed of the ocean being converted into
land, they would form flat-topped mountains, varying in diameter from
a few miles (the smallest atolls being worn away) to sixty miles; and
from being horizontally stratified and of similar composition, they
would, as Mr. Lyell has remarked, falsely appear as if they had
originally been united into one vast continuous mass. Such great
strata of coral-rock would rarely be associated with erupted volcanic
matter, for this could only take place, as may be inferred from what
follows in the next chapter, when the area, in which they were
situated, commenced to rise, or at least ceased to subside. During the
enormous period necessary to effect an elevation of the kind just
alluded to, the surface would necessarily be denuded to a great
thickness; hence it is highly improbable that any fringing-reef, or
even any barrier-reef, at least of those encircling small islands,
would be preserved. From this same cause, the strata which were formed
within the lagoons of atolls and lagoon-channels of barrier-reefs, and
which must consist in a large part of sedimentary matter, would more
often be preserved to future ages, than the exterior solid reef,
composed of massive corals in an upright position; although it is on
this exterior part that the present existence and further growth of
atolls and barrier-reefs entirely depend.
_ Plate III_—Map showing the distribution of coral-reefs and active
volcanoes.
[Illustration: Map showing distribution of coral-reefs and active
volcanoes.]
[Illustration: Map showing distribution of coral-reefs and active
volcanoes.]
[Illustration: Map showing distribution of coral-reefs and active
volcanoes.]
The principles, on which this map was coloured, are explained in the
beginning of Chapter VI; and the authorities for each particular spot
are detailed in the Appendix to _Coral Reefs._ The names not printed in
upper case in the Index refer to the Appendix.
Chapter VI ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION
Description of the coloured map.—Proximity of atolls and
barrier-reefs.—Relation in form and position of atolls with ordinary
islands.—Direct evidence of subsidence difficult to be detected.—Proofs
of recent elevation where fringing-reefs occur.—Oscillations of
level.—Absence of active volcanoes in the areas of
subsidence.—Immensity of the areas which have been elevated and have
subsided.—Their relation to the present distribution of the land.—Areas
of subsidence elongated, their intersection and alternation with those
of elevation.—Amount and slow rate of the subsidence.—Recapitulation.
It will be convenient to give here a short account of the appended map
(Plate III):[1] a fuller one, with the data for colouring each spot, is
reserved
for the Appendix; and every place there referred to may be found in the
Index. A larger chart would have been desirable; but, small as the
adjoined one is, it is the result of many months’ labour. I have
consulted, as far as I was able, every original voyage and map; and the
colours were first laid down on charts on a larger scale. The same blue
colour, with merely a difference in the depth of tint, is used for
atolls or lagoon-islands, and barrier-reefs, for we have seen, that as
far as the actual coral-formation is concerned, they have no
distinguishing character. Fringing-reefs have been coloured red, for
between them on the one hand, and barrier-reefs and atolls on the
other, there is an important distinction with respect to the depth
beneath the surface, at which we are compelled to believe their
foundations lie. The two distinct colours, therefore, mark two great
types of structure.
[1] Inasmuch as the coloured map would have proved too costly to be
given in this series, the indications of colour have been replaced by
numbers referring to the dotted groups of reefs, etc. The author’s
original wording, however, is retained in full, as it will be easy to
refer to the map by the numbers, and thus the flow of the narrative is
undisturbed.
The _dark blue colour_ [represented by (3) in our plate] represents
atolls and submerged annular reefs, with deep water in their centres. I
have coloured as atolls, a few low and small coral-islands, without
lagoons; but this has been done only when it clearly appeared that they
originally contained lagoons, since filled up with sediment: when there
were not good grounds for this belief, they have been left uncoloured.
The _pale blue colour_ [represented by (2)] represents barrier-reefs.
The most obvious character of reefs of this class is the broad and
deep-water moat within the reef: but this, like the lagoons of small
atolls, is liable to become filled up with detritus and with reefs of
delicately branched corals: when, therefore, a reef round the entire
circumference of an island extends very far into a profoundly deep sea,
so that it can hardly be confounded with a fringing-reef which must
rest on a foundation of rock within a small depth, it has been coloured
pale blue, although it does not include a deep-water moat: but this has
only been done rarely, and each case is distinctly mentioned in the
Appendix.
The _red colour_ (4) represents reefs fringing the land quite closely
where the sea is deep, and where the bottom is gently inclined
extending to a moderate distance from it, but not having a deep-water
moat or lagoon-like space parallel to the shore. It must be remembered
that fringing-reefs are frequently _breached_ in front of rivers and
valleys by deepish channels, where mud has been deposited. A space of
thirty miles in width has been coloured round or in front of the reefs
of each class, in order that the colours might be conspicuous on the
appended map, which is reduced to so small a scale.
The _vermillion spots_, and streaks (1) represent volcanoes now in
action, or historically known to have been so. They are chiefly laid
down from Von Buch’s work on the Canary Islands; and my reasons for
making a few alterations are given in the note below.[2]
[2] I have also made considerable use of the geological part of
Berghaus’ “Physical Atlas.” Beginning at the eastern side of the
Pacific, I have added to the number of the volcanoes in the southern
part of the Cordillera, and have coloured Juan Fernandez according to
observations collected during the voyage of the _Beagle_ (“Geol.
Trans.,” vol. v, p. 601). I have added a volcano to Albemarle Island,
one of the Galapagos Archipelago (the author’s “Journal of
Researches,” p. 457). In the Sandwich group there are no active
volcanoes, except at Hawaii; but the Rev. W. Ellis informs me, there
are streams of lava apparently modern on Maui, having a very recent
appearance, which can be traced to the craters whence they flowed. The
same gentleman informs me, that there is no reason to believe that any
active volcano exists in the Society Archipelago; nor are there any
known in the Samoa or Navigator group, although some of the streams of
lava and craters there appear recent. In the Friendly group, the Rev.
J. Williams says (“Narrative of Missionary Enterprise,” p. 29) that
Toofoa and Proby Islands are active volcanoes. I infer from Hamilton’s
“Voyage in the _Pandora_” (p. 95), that Proby Island is synonymous
with Onouafou, but I have not ventured to colour it. There can be no
doubt respecting Toofoa, and Captain Edwards (Von Buch, p. 386) found
the lava of recent eruption at Amargura still smoking. Berghaus marks
four active volcanoes actually within the Friendly group; but I do not
know on what authority: I may mention that Maurelle describes Latte as
having a burnt-up appearance: I have marked only Toofoa and Amargura.
South of the New Hebrides lies Matthews Rock, which is drawn and
described as an active crater in the “Voyage of the _Astrolabe_.”
Between it and the volcano on the eastern side of New Zealand, lies
Brimstone Island, which from the high temperature of the water in the
crater, may be ranked as active (Berghaus “Vorbemerk,” II Lief. S.
56). Malte Brun, vol. xii, p. 231, says that there is a volcano near
port St. Vincent in New Caledonia. I believe this to be an error,
arising from a smoke seen on the _opposite_ coast by Cook (“Second
Voyage,” vol. ii, p. 23) which smoke went out at night. The Mariana
Islands, especially the northern ones, contain many craters (see
Freycinet’s “Hydrog. Descript.”) which are not active. Von Buch,
however, states (p. 462) on the authority of La Peyrouse, that there
are no less than seven volcanoes between these islands and Japan.
Gemelli Creri (Churchill’s “Collect.” vol. iv, p. 458), says there are
two active volcanoes in latitude 23° 30′, and in latitude 24°: but I
have not coloured them. From the statements in Beechey’s “Voyage” (p.
518, 4to edit.) I have coloured one in the northern part of the Bonin
group. M. S. Julien has clearly made out from Chinese manuscripts not
very ancient (“Comptes Rendus,” 1840, p. 832), that there are two
active volcanoes on the eastern side of Formosa. In Torres Straits, on
Cap Island (9° 48′ S., 142° 39′ E.) a volcano was seen burning with
great violence in 1793 by Captain Bampton (see Introduction to
Flinders’ “Voyage,” p. 41). Mr. M’Clelland (Report of Committee for
investigating Coal in India, p. 39) has shown that the volcanic band
passing through Barren Island must be extended northwards. It appears
by an old chart, that Cheduba was once an active volcano (see also _
Silliman’s North American Journal_, vol. xxxviii, p. 385). In
Berghaus’ “Phys. Atlas,” 1840, No. 7 of Geological Part, a volcano on
the coast of Pondicherry is said to have burst forth in 1757.
Ordinaire (“Hist. Nat. des Volcans,” p. 218) says that there is one at
the mouth of the Persian Gulf, but I have not coloured it, as he gives
no particulars. A volcano in Amsterdam, or St. Paul’s, in the southern
part of the Indian Ocean, has been seen (_Naut. Mag._ 1838, p. 842) in
action. Dr. J. Allan, of Forres, informs me in a letter, that when he
was at Joanna, he saw at night flames apparently volcanic, issuing
from the chief Comoro Island, and that the Arabs assured him that they
were volcanic, adding that the volcano burned more during the wet
season. I have marked this as a volcano, though with some hesitation,
on account of the possibility of the flame arising from gaseous
sources.
The uncoloured coasts consist, first and chiefly, of those, where there
are no coral-reefs, or such small portions as to be quite
insignificant. Secondly, of those coasts where there are reefs, but
where the sea is very shallow, for in this case the reefs generally lie
far from the land, and become very irregular, in their forms: where
they have not become irregular, they have been coloured. thirdly, if I
had the means of ascertaining the fact, I should not colour a reef
merely coating the edges of a submarine crater, or of a level submerged
bank; for such superficial
formations differ essentially, even when not in external appearance,
from reefs whose foundations as well as superficies have been wholly
formed by the growth of coral. Fourthly, in the Red Sea, and within
some parts of the East Indian Archipelago (if the imperfect charts of
the latter can be trusted), there are many scattered reefs, of small
size, represented in the chart by mere dots, which rise out of deep
water: these cannot be arranged under either of the three classes: in
the Red Sea, however, some of these little reefs, from their position,
seem once to have formed parts of a continuous barrier. There exist,
also, scattered in the open ocean, some linear and irregularly formed
strips of coral-reef, which, as shown in the last chapter, are probably
allied in their origin to atolls; but as they do not belong to that
class, they have not been coloured; they are very few in number and of
insignificant dimensions. Lastly, some reefs are left uncoloured from
the want of information respecting them, and some because they are of
an intermediate structure between the barrier and fringing classes. The
value of the map is lessened, in proportion to the number of reefs
which I have been obliged to leave uncoloured, although, in a
theoretical point of view, few of them present any great difficulty:
but their number is not very great, as will be found by comparing the
map with the statements in the Appendix. I have experienced more
difficulty in colouring fringing-reefs than in colouring barrier-reefs,
as the former, from their much less dimensions, have less attracted the
attention of navigators. As I have had to seek my information from all
kinds of sources, and often from indirect ones, I do not venture to
hope that the map is free from many errors. Nevertheless, I trust it
will give an approximately correct view of the general distribution of
the coral-reefs over the whole world (with the exception of some
fringing-reefs on the coast of Brazil, not included within the limits
of the map), and of their arrangement into the three great classes,
which, though necessarily very imperfect from the nature of the objects
classified, have been adopted by most voyagers. I may further remark,
that the dark blue colour represents land entirely composed of
coral-rock; the pale blue, land with a wide and thick border of
coral-rock; and the red, a mere narrow fringe of coral-rock.
Looking now at the map under the theoretical point of view indicated in
the last chapter, the two blue tints signify that the foundations of
the reefs thus coloured have subsided to a considerable amount, at a
slower rate than that of the upward growth of the corals, and that
probably in many cases they are still subsiding. The red signifies that
the shores which support fringing-reefs have not subsided (at least to
any considerable
amount, for the effects of a subsidence on a small scale would in no
case be distinguishable); but that they have remained nearly stationary
since the period when they first became fringed by reefs; or that they
are now rising or have been upraised, with new lines of reefs
successively formed on them: these latter alternatives are obviously
implied, as newly formed lines of shore, after elevations of the land,
would be in the same state with respect to the growth of
fringing-reefs, as stationary coasts. If during the prolonged
subsidence of a shore, coral-reefs grew for the first time on it, or if
an old barrier-reef were destroyed and submerged, and new reefs became
attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of
error, any coast has been coloured wrongly with respect to movement
indicated. Well characterised atolls and encircling barrier-reefs,
where several occur in a group, or a single barrier-reef if of large
dimensions, leave scarcely any doubt on the mind respecting the
movement by which they have been produced; and even a small amount of
subsequent elevation is soon betrayed. The evidence from a single atoll
or a single encircling barrier-reef, must be received with some
caution, for the former may possibly be based upon a submerged crater
or bank, and the latter on a submerged margin of sediment, or of
worn-down rock. From these remarks we may with greater certainty infer
that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or
have been upraised.
_On the grouping of the different classes of reefs._—Having made these
preliminary remarks, I will consider first how far the grouping of the
different kinds of coral-islands and reefs is corroborative of the
truth of the theory. A glance at the map shows that the reefs, coloured
blue and red, produced under widely different conditions, are not
indiscriminately mixed together. Atolls and barrier-reefs, on the other
hand, as may be seen by the two blue tints, generally lie near each
other; and this would be the natural result of both having been
produced during the subsidence of the areas in which they stand. Thus,
the largest group of encircled islands is that of the Society
Archipelago; and these islands are surrounded by atolls, and only
separated by a narrow space from the large group of Low atolls. In the
midst of the Caroline atolls, there are three fine encircled islands.
The northern point of the barrier-reef of New Caledonia seems itself,
as before remarked, to form a complete large atoll. The great
Australian barrier is described as including both atolls and small
encircled islands. Captain King[3] mentions many atoll-formed and
encircling coral-reefs, some of which lie within the barrier, and
others may be said (for instance between lat. 16° and 13°) to form part
of it. Flinders[4] has described an atoll-formed reef in lat. 10°,
seven miles long and from one to three broad, resembling a boot in
shape, with apparently very deep water
within. Eight miles westward of this, and forming part of the barrier,
lie the Murray Islands, which are high and are encircled. In the
Corallian Sea, between the two great barriers of Australia and New
Caledonia, there are many low islets and coral-reefs, some of which are
annular, or horse-shoe shaped. Observing the smallness of the scale of
the map, the parallels of latitude being nine hundred miles apart, we
see that none of the large groups of reefs and islands supposed to have
been produced by long-continued subsidence, lie near extensive lines of
coast coloured red, which are supposed to have remained stationary
since the growth of their reefs, or to have been upraised and new lines
of reefs formed on them. Where the red and blue circles do occur near
each other, I am able, in several instances, to show that there have
been oscillations of level, subsidence having preceded the elevation of
the red spots; and elevation having preceded the subsidence of the blue
spots: and in this case the juxtaposition of reefs belonging to the two
great types of structure is little surprising. We may, therefore,
conclude that the proximity in the same areas of the two classes of
reefs, which owe their origin to the subsidence of the earth’s crust,
and their separation from those formed during its stationary or
uprising condition, holds good to the full extent, which might have
been anticipated by our theory.
[3] Sailing directions, appended to vol. ii of his “Surveying Voyage
to Australia.”
[4] “Voyage to Terra Australis,” vol. ii, p. 336.
As groups of atolls have originated in the upward growth, at each fresh
sinking of the land, of those reefs which primarily fringed the shores
of one great island, or of several smaller ones; so we might expect
that these rings of coral-rock, like so many rude outline charts, will
still retain some traces of the general form, or at least general
range, of the land, round which they were first modelled. That this is
the case with the atolls in the Southern Pacific as far as their range
is concerned, seems highly probable, when we observe that the three
principal groups are directed in north-west and south-east lines, and
that nearly all the land in the S. Pacific ranges in this same
direction; namely, N. Western Australia, New Caledonia, the northern
half of New Zealand, the New Hebrides, Saloman, Navigator, Society,
Marquesas, and Austral archipelagoes: in the Northern Pacific, the
Caroline atolls abut against the north-west line of the Marshall
atolls, much in the same manner as the east and west line of islands
from Ceram to New Britain do on New Ireland: in the Indian Ocean the
Laccadive and Maldiva atolls extend nearly parallel to the western and
mountainous coast of India. In most respects, there is a perfect
resemblance with ordinary islands in the grouping of atolls and in
their form: thus the outline of all the larger groups is elongated; and
the greater number of the individual atolls are elongated in the same
direction with the group, in which they stand. The Chagos group is less
elongated than is usual with other groups, and the individual atolls in
it are likewise but little elongated; this is strikingly seen by
comparing them with the neighbouring Maldiva atolls. In the Marshall
and Maldiva archipelagoes, the atolls are ranged in two parallel lines,
like the mountains in a great double mountain-chain. Some of the
atolls, in the larger archipelagoes, stand so near to each other, and
have such an evident relationship in
form, that they compose little sub-groups: in the Caroline Archipelago,
one such sub-group consists of Pouynipete, a lofty island encircled by
a barrier-reef, and separated by a channel only four miles and a half
wide from Andeema atoll, with a second atoll a little further off. In
all these respects an examination of a series of charts will show how
perfectly groups of atolls resemble groups of common islands.
_On the direct evidence of the blue spaces in the map having subsided
during the upward growth of the reefs so coloured, and of the red
spaces having remained stationary, or having been upraised._—With
respect to subsidence, I have shown in the last chapter, that we cannot
expect to obtain in countries inhabited only by semi-civilised races,
demonstrative proofs of a movement, which invariably tends to conceal
its own evidence. But on the coral-islands supposed to have been
produced by subsidence, we have proofs of changes in their external
appearance—of a round of decay and renovation—of the last vestiges of
land on some—of its first commencement on others: we hear of storms
desolating them to the astonishment of their inhabitants: we know by
the great fissures with which some of them are traversed, and by the
earthquakes felt under others, that subterranean disturbances of some
kind are in progress. These facts, if not directly connected with
subsidence, as I believe they are, at least show how difficult it would
be to discover proofs of such movement by ordinary means. At Keeling
atoll, however, I have described some appearances, which seem directly
to show that subsidence did take place there during the late
earthquakes. Vanikoro, according to Chevalier Dillon,[5] is often
violently shaken by earthquakes, and there, the unusual depth of the
channel between the shore and the reef,—the almost entire absence of
islets on the reef,—its wall-like structure on the inner side, and the
small quantity of low alluvial land at the foot of the mountains, all
seem to show that this island has not remained long at its present
level, with the lagoon-channel subjected to the accumulation of
sediment, and the reef to the wear and tear of the breakers. At the
Society Archipelago, on the other hand, where a slight tremor is only
rarely felt, the shoaliness of the lagoon-channels round some of the
islands, the number of islets formed on the reefs of others, and the
broad belt of low land at the foot of the mountains, indicate that,
although there must have been great subsidence to have produced the
barrier-reefs, there has since elapsed a long stationary period.[6]
[5] See Captain Dillon’s “Voyage in search of La Peyrouse.” M. Cordier
in his “Report on the Voyage of the ‘Astrolabe’” (p. cxi, vol. i),
speaking of Vanikoro, says the shores are surrounded by reefs of
madrepore, “qu’on assure etre de formation tout-a-fait moderne.” I
have in vain endeavoured to learn some further particulars about this
remarkable passage. I may here add, that according to our theory, the
island of Pouynipete (Plate I, Fig. 7), in the Caroline Archipelago,
being encircled by a barrier-reef, must have subsided. In the _ New S.
Wales Lit. Advert._ February 1835 (which I have seen through the
favour of Dr. Lloghtsky), there is an account of this island
(subsequently confirmed by Mr. Campbell), in which it is said, “At the
N.E. end, at a place called Tamen, there are ruins of a town, _now
only_ accessible by boats, the waves _reaching to the steps of the
houses._” Judging from this passage, one would be tempted to conclude
that the island must have subsided, since these houses were built. I
may, also, here append a statement in Malte Brun (vol. ix, p. 775,
given without any authority), that the sea gains in an extraordinary
manner on the coast of Cochin China, which lies in front and near the
subsiding coral-reefs in the China Sea: as the coast is granitic, and
not alluvial, it is scarcely possible that the encroachment of the sea
can be owing to the washing away of the land; and if so, it must be
due to subsidence.
[6] Mr. Couthouy states (“Remarks,” p. 44) that at Tahiti and Eimeo
the space between the reef and the shore has been nearly filled up by
the extension of those coral-reefs, which within most barrier-reefs
merely fringe the land. From this circumstance, he arrives at the same
conclusion as I have done, that the Society Islands since their
subsidence, have remained stationary during a long period; but he
further believes that they have recently commenced rising, as well as
the whole area of the Low Archipelago. He does not give any detailed
proofs regarding the elevation of the Society Islands, but I shall
refer to this subject in another part of this chapter. Before making
some further comments, I may observe how satisfactory it is to me, to
find Mr. Couthouy affirming, that “having personally examined a large
number of coral-islands, and also residing eight months among the
volcanic class, having shore and partially encircling reefs, I may be
permitted to state that my own observations have impressed a
conviction of the correctness of the theory of Mr. Darwin.”
This gentleman believes, that subsequently to the subsidence by
which the atolls in the Low Archipelago were produced, the whole
area has been elevated to the amount of a few feet; this would
indeed be a remarkable fact; but as far as I am able to judge, the
grounds of his conclusion are not sufficiently strong. He states
that he found in almost every atoll which he visited, the shores of
the lagoon raised from eighteen to thirty inches above the
sea-level, and containing imbedded Tridacnæ and corals standing as
they grew; some of the corals were dead in their upper parts, but
below a certain line they continued to flourish. In the lagoons,
also, he frequently met with clusters of Madrepore, with their
extremities standing from one inch to a foot above the surface of
the water. Now, these appearances are exactly what I should have
expected, without any subsequent elevation having taken place; and
I think Mr. Couthouy has not borne in mind the indisputable fact,
that corals, when constantly bathed by the surf, can exist at a
higher level than in quite tranquil water, as in a lagoon. As long,
therefore, as the waves continued at low water to break entirely
over parts of the annular reef of an atoll, submerged to a small
depth, the corals and shells attached on these parts might continue
living at a level above the smooth surface of the lagoon, into
which the waves rolled; but as soon as the outer edge of the reef
grew up to its utmost possible height, or if the reef were very
broad nearly to that height, the force of the breakers would be
checked, and the corals and shells on the inner parts near the
lagoon would occasionally be left dry, and thus be partially or
wholly destroyed. Even in atolls, which have not lately subsided,
if the outer margin of the reef continued to increase in breadth
seaward (each fresh zone of corals rising to the same vertical
height as at Keeling atoll), the line where the waves broke most
heavily would advance outwards, and therefore the corals, which
when living near the margin, were washed by the breaking waves
during the whole of each tide, would cease being so, and would
therefore be left on the backward part of the reef standing exposed
and dead. The case of the madrepores in the lagoons with the tops
of their branches exposed, seems to be an analogous fact, to the
great fields of dead but upright corals in the lagoon of Keeling
atoll; a condition of things which I have endeavoured to show, has
resulted from the lagoon having become more and more enclosed and
choked up with reefs, so that during high winds, the rising of the
tide (as observed by the inhabitants) is checked, and the corals,
which had formerly grown to the greatest possible height, are
occasionally exposed, and thus are killed: and this is a condition
of things, towards which almost every atoll in the intervals of its
subsidence must be tending. Or if we look to the state of an atoll
directly after a subsidence of some fathoms, the waves would roll
heavily over the entire circumference of the reef, and the surface
of the lagoon would, like the ocean, never be quite at rest, and
therefore the corals in the lagoon, from being constantly laved by
the rippling water, might extend their branches to a little greater
height than they could, when the lagoon became enclosed and
protected. Christmas atoll (2° N. lat.) which has a very shallow
lagoon, and differs in several respects from most atolls, possibly
may have been elevated recently; but its highest part appears
(Couthouy, p. 46) to be only ten feet above the sea-level. The
facts of a second class, adduced by Mr. Couthouy, in support of the
alleged recent elevation of the Low Archipelago, are not all
(especially those referring to a shelf of rock) quite intelligible
to me; he believes that certain enormous fragments of rock on the
reef, must have been moved into their present position, when the
reef was at a lower level; but here again the force of the breakers
on any inner point of the reef being diminished by its outward
growth without any change in its level, has not, I think, been
borne in mind. We should, also, not overlook the occasional agency
of waves caused by earthquakes and hurricanes. Mr. Couthouy further
argues, that since these great fragments were deposited and fixed
on the reef, they have been elevated; he infers this from the
greatest amount of erosion not being near their bases, where they
are unceasingly washed by the reflux of the tides, but at some
height on their sides, near the line of high-water mark, as shown
in an accompanying diagram. My former remark again applies here,
with this further observation, that as the waves have to roll over
a wide space of reef before they reach the fragments, their force
must be greatly increased with the increasing depth of water as the
tide rises, and therefore I should have expected that the chief
line of present erosion would have coincided with the line of
high-water mark; and if the reef had grown outwards, that there
would have been lines of erosion at greater heights. The
conclusion, to which I am finally led by the interesting
observations of Mr. Couthouy is, that the atolls in the Low
Archipelago have, like the Society Islands, remained at a
stationary level for a long period: and this probably is the
ordinary course of events, subsidence supervening after long
intervals of rest.
Turning now to the red colour; as on our map, the areas which have sunk
slowly downwards to great depths are many and large, we might naturally
have been led to conjecture, that with such great changes of level in
progress, the coasts which have been fringed probably for ages (for we
have no reason to believe that coral-reefs are of short duration),
would not have remained all this time stationary, but would frequently
have undergone movements of elevation. This supposition, we shall
immediately see, holds good to a remarkable extent; and although a
stationary condition of the land can hardly ever be open to proof, from
the evidence being only negative, we are, in some degree, enabled to
ascertain the correctness of the parts coloured red on the map, by the
direct testimony of upraised organic remains of a modern date. Before
going into the details on this head (printed in small type), I may
mention, that when reading a memoir on coral formations by MM. Quoy and
Gaimard[7] I was astonished to find, for I knew that they had crossed
both the Pacific and Indian Oceans, that their descriptions were
applicable only to reefs of the fringing class; but my astonishment
ended satisfactorily, when I discovered that, by a strange chance, all
the islands which these eminent naturalists had visited, though several
in number, namely, the Mauritius, Timor, New Guinea, the Mariana, and
Sandwich Archipelagoes, could be shown by their own statements to have
been elevated within a recent geological era.
[7] “Annales des Sciences Nat.” tom. vi, p. 279, etc.
In the eastern half of the Pacific, the _ Sandwich Islands_ are all
fringed, and almost every naturalist who has visited them, has remarked
on the abundance of elevated corals and shells, apparently identical
with living species. The Rev. W. Ellis informs me, that he has noticed
round several parts of Hawaii, beds of coral-detritus, about twenty
feet above the level of the sea, and where the coast is low they extend
far inland. Upraised coral-rock forms a considerable part of the
borders of Oahu; and at Elizabeth Island[8] it composes three strata,
each about ten feet thick. Nihau, which forms the northern, as Hawaii
does the southern end of the group (350 miles in length), likewise
seems to consist of coral and volcanic rocks. Mr. Couthouy[9] has
lately described with interesting details, several upraised beaches,
ancient reefs with their surfaces perfectly preserved, and beds of
recent shells and corals, at the islands of Maui, Morokai, Oahu, and
Tauai (or Kauai) in this group. Mr. Pierce, an intelligent resident at
Oahu, is convinced, from changes which have taken place within his
memory, during the last sixteen years, “that the elevation is at
present going forward at a very perceptible rate.” The natives at Kauai
state that the land is there gaining rapidly on the sea, and Mr.
Couthouy has no doubt, from the nature of the strata, that this has
been effected by an elevation of the land.
[8] “Zoology of Captain Beechey’s Voyage,” p. 176. See also MM. Quoy
and Gaimard in “Annales de Scien. Nat.” tom. vi.
[9] “Remarks on Coral Formations,” p. 51.
In the southern part of the Low Archipelago, Elizabeth Island is
described by Captain Beechey,[10] as being quite flat, and about eighty
feet in height; it is entirely composed of dead corals, forming a
honeycombed, but compact rock. In cases like this, of an island having
exactly the appearance, which the elevation of any one of the smaller
surrounding atolls with a shallow lagoon would present, one is led to
conclude (with little better reason, however, than the improbability of
such small and low fabrics lasting, for an immense period, exposed to
the many destroying agents of nature), that the elevation has taken
place at an epoch not geologically remote. When merely the surface of
an island of ordinary formation is strewed with marine
bodies, and that continuously, or nearly so, from the beach to a
certain height, and not above that height, it is exceedingly improbable
that such organic remains, although they may not have been specially
examined, should belong to any ancient period. It is necessary to bear
these remarks in mind, in considering the evidence of the elevatory
movements in the Pacific and Indian Oceans, as it does not often rest
on specific determinations, and therefore should be received with
caution. Six of the _Cook and Austral Islands_ (S.W. of the Society
group), are fringed; of these, five were described to me by the Rev. J.
Williams, as formed of coral-rock, associated with some basalt in
Mangaia), and the sixth as lofty and basaltic. Mangaia is nearly three
hundred feet high, with a level summit; and according to Mr. S.
Wilson[11] it is an upraised reef; “and there are in the central
hollow, formerly the bed of the lagoon, many scattered patches of
coral-rock, some of them raised to a height of forty feet.” These
knolls of coral-rock were evidently once separate reefs in the lagoon
of an atoll. Mr. Martens, at Sydney, informed me that this island is
surrounded by a terrace-like plain at about the height of a hundred
feet, which probably marks a pause in its elevation. From these facts
we may infer, perhaps, that the Cook and Austral Islands have been
upheaved at a period probably not very remote.
[10] Beechey’s “Voyage in the Pacific,” p. 46, 4to ed.
[11] Couthouy’s “Remarks,” p. 34.
_Savage Island_ (S.E. of the Friendly group), is about forty feet in
height. Forster[12] describes the plants as already growing out of the
dead, but still upright and spreading trees of coral; and the younger
Forster[13] believes that an ancient lagoon is now represented by a
central plain; here we cannot doubt that the elevatory forces have
recently acted. The same conclusion may be extended, though with
somewhat less certainty, to the islands of the _friendly group_, which
have been well described in the second and third voyages of Cook. The
surface of Tongatabou is low and level, but with some parts a hundred
feet high; the whole consists of coral-rock, “which yet shows the
cavities and irregularities worn into it by the action of the
tides.”[14] On Eoua the same appearances were noticed at an elevation
of between two hundred and three hundred feet. Vavao, also, at the
opposite or northern end of the group, consists, according to the Rev.
J. Williams, of coral-rock. Tongatabou, with its northern extensive
reefs, resembles either an upraised atoll with one half originally
imperfect, or one unequally elevated; and Anamouka, an atoll equally
elevated. This latter island contains[15] in its centre a salt-water
lake, about a mile-and-a-half in diameter, without any communication
with the sea, and around it the land rises gradually like a bank; the
highest part is only between twenty and thirty feet; but on this part,
as well as on the rest of the land (which, as Cook observes, rises
above the height of true lagoon-islands), coral-rock, like that on the
beach, was found. In the _Navigator Archipelago_, Mr. Couthouy[16]
found on Manua many and very large fragments of coral at the height of
eighty feet, “on a steep hill-side, rising half a mile inland from a
low sandy plain abounding in marine remains.” The fragments were
embedded in a mixture of decomposed lava and sand. It is not stated
whether they were accompanied by shells, or whether the corals
resembled recent species; as these
remains were embedded they possibly may belong to a remote epoch; but I
presume this was not the opinion of Mr. Couthouy. Earthquakes are very
frequent in this archipelago.
[12] “Observations made during Voyage round the World,” p. 147.
[13] “Voyage,” vol. ii, p. 163.
[14] Cook’s “Third Voyage” (4to ed.), vol. i, p. 314.
[15] _Ibid_., vol. i, p. 235.
[16] “Remarks on Coral-Formations,” p. 50.
Still proceeding westward we come to the _New Hebrides_; on these
islands, Mr. G. Bennett (author of “Wanderings in New South Wales”),
informs me he found much coral at a great altitude, which he considered
of recent origin. Respecting _Santa Cruz_, and the _Solomon
Archipelago_, I have no information; but at New Ireland, which forms
the northern point of the latter chain, both Labillardiere and Lesson
have described large beds of an apparently very modern madreporitic
rock, with the form of the corals little altered. The latter author[17]
states that this formation composes a newer line of coast, modelled
round an ancient one. There only remains to be described in the
Pacific, that curved line of fringed islands, of which the _ Marianas_
form the main part. Of these Guam, Rota, Tiniam, Saypan, and some
islets farther north, are described by Quoy and Gaimard,[18] and
Chamisso,[19] as chiefly composed of madreporitic limestone, which
attains a considerable elevation, and is in several cases worn into
successively rising cliffs: the two former naturalists seem to have
compared the corals and shells with the existing ones, and state that
they are of recent species. _Fais_, which lies in the prolonged line of
the Marianas, is the only island in this part of the sea which is
fringed; it is ninety feet high, and consists entirely of madreporitic
rock.[20]
[17] “Voyage de la _Coquille_,” Part. Zoolog.
[18] Freycinet’s “Voyage autour du Monde.” See also the
“Hydrographical Memoir,” p. 215.
[19] Kotzebue’s “First Voyage.”
[20] Lutké’s “Voyage,” vol. ii, p. 304.
In the _East Indian Archipelago_, many authors have recorded proofs of
recent elevation. M. Lesson[21] states, that near Port Dory, on the
north coast of New Guinea, the shores are flanked, to the height of 150
feet, by madreporitic strata of modern date. He mentions similar
formations at Waigiou, Amboina, Bourou, Ceram, Sonda, and Timor: at
this latter place, MM. Quoy and Gaimard[22] have likewise described the
primitive rocks, as coated to a considerable height with coral. Some
small islets eastward of Timor are said in Kolff’s “Voyage,”[23] to
resemble small coral islets upraised some feet above the sea. Dr.
Malcolmson informs me that Dr. Hardie found in JAVA an extensive
formation, containing an abundance of shells, of which the greater part
appear to be of existing species. Dr. Jack[24] has described some
upraised shells and corals, apparently recent, on Pulo Nias off
_Sumatra_; and Marsden relates in his history of this great island,
that the names of many promontories, show that they were originally
islands. On part of the west
coast of _Borneo_ and at the _Sooloo Islands_, the form of the land,
the nature of the soil, and the water-washed rocks, present
appearances[25] (although it is doubtful whether such vague evidence is
worthy of mention), of having recently been covered by the sea; and the
inhabitants of the Sooloo Islands believe that this has been the case.
Mr. Cuming, who has lately investigated, with so much success, the
natural history of the _Philippines_, found near Cabagan, in Luzon,
about fifty feet above the level of the R. Cagayan, and seventy miles
from its mouth, a large bed of fossil shells: these, he informs me, are
of the same species with those now existing on the shores of the
neighbouring islands. From the accounts given us by Captain Basil Hall
and Captain Beechey[26] of the lines of inland reefs, and walls of
coral-rock worn into caves, above the present reach of the waves, at
the _Loo Choo_ Islands, there can be little doubt that they have been
upraised at no very remote period.
[21] Partie Zoolog., “Voyage de la _Coquille_.”
[22] “Ann. des Scien. Nat.” tom. vi, p. 281.
[23] Translated by Windsor Earl, chapters vi, vii.
[24] “Geolog. Transact.” 2nd series, vol. i, p. 403. On the Peninsula
of Malacca, in front of Pinang, 5° 30′ N., Dr. Ward collected some
shells, which Dr. Malcolmson informs me, although not compared with
existing species, had a recent appearance. Dr. Ward describes in this
neighbourhood (“Trans. Asiat. Soc.” vol. xviii, part ii, p. 166) a
single water-worn rock, with a conglomerate of sea-shells at its base,
situated six miles inland, which, according to the traditions of the
natives, was once surrounded by the sea. Captain Low has also
described (_Ibid_., part i, p. 131) mounds of shells lying two miles
inland on this line of coast.
[25] “Notices of the East Indian Arch.” Singapore, 1828, p. 6, and
Append., p. 43.
[26] Captain B. Hall, “Voyage to Loo Choo,” Append., pp. xxi and xxv.
Captain Beechey’s “Voyage,” p. 496.
Dr. Davy[27] describes the northern province of _Ceylon_ as being very
low, and consisting of a limestone with shells and corals of very
recent origin; he adds, that it does not admit of a doubt that the sea
has retired from this district even within the memory of man. There is
also some reason for believing that the western shores of India, north
of Ceylon, have been upraised within the recent period.[28] _Mauritius_
has certainly been upraised within the recent period, as I have stated
in the chapter on fringing-reefs. The northern extremity of
_Madagascar_ is described by Captain Owen[29] as formed of madreporitic
rock, as likewise are the shores and outlying islands along an immense
space of _ Eastern Africa_, from a little north of the equator for nine
hundred miles southward. Nothing can be more vague than the expression
“madreporitic rock;” but at the same time it is, I think, scarcely
possible to look at the chart of the linear islets, which rise to a
greater height than can be accounted for by the growth of coral, in
front of the coast, from the equator to 2° S., without feeling
convinced that a line of fringing-reefs has been elevated at a period
so recent, that no great changes have since taken place on the surface
of this part of the globe. Some, also, of the
higher islands of madreporitic rock on this coast, for instance Pemba,
have very singular forms, which seem to show the combined effect of the
growth of coral round submerged banks, and their subsequent upheaval.
Dr. Allan informs me that he never observed any elevated organic
remains on the _Seychelles_, which come under our fringed class.
[27] “Travels in Ceylon,” p. 13. This madreporitic formation is
mentioned by M. Cordier in his report to the Institute (May 4th,
1839), on the voyage of the _Chevrette_, as one of immense extent, and
belonging to the latest tertiary period.
[28] Dr. Benza, in his “Journey through the N. Circars” (the _ Madras
Lit. and Scient. Journ._ vol. v.) has described a formation with
recent fresh-water and marine shells, occurring at the distance of
three or four miles from the present shore. Dr. Benza, in conversation
with me, attributed their position to a rise of the land. Dr.
Malcolmson, however (and there cannot be a higher authority on the
geology of India) informs me that he suspects that these beds may have
been formed by the mere action of the waves and currents accumulating
sediment. From analogy I should much incline to Dr. Benza’s opinion.
[29] Owen’s “Africa,” vol. ii, p. 37, for Madagascar; and for S.
Africa, vol. i, pp. 412 and 426. Lieutenant Boteler’s narrative
contains fuller particulars regarding the coral-rock, vol. i, p. 174,
and vol. ii, pp. 41 and 54. See also Ruschenberger’s “Voyage round the
World,” vol. i, p. 60.
The nature of the formations round the shores of the _Red Sea_, as
described by several authors, shows that the whole of this large area
has been elevated within a very recent tertiary epoch. A part of this
space in the appended map, is coloured blue, indicating the presence of
barrier-reefs: on which circumstance I shall presently make some
remarks. Rüppell[30] states that the tertiary formation, of which he
has examined the organic remains, forms a fringe along the shores with
a uniform height of from thirty and forty feet from the mouth of the
Gulf of Suez to about latitude 26°; but that south of 26°, the beds
attain only the height of from twelve to fifteen feet. This, however,
can hardly be quite accurate; although possibly there may be a decrease
in the elevation of the shores in the middle parts of the Red Sea, for
Dr. Malcolmson (as he informs me) collected from the cliffs of Camaran
Island (lat. 15° 30′ S.) shells and corals, apparently recent, at a
height between thirty and forty feet; and Mr. Salt (“Travels in
Abyssinia”) describes a similar formation a little southward on the
opposite shore at Amphila. Moreover, near the mouth of the Gulf of
Suez, although on the coast opposite to that on which Dr. Rüppell says
that the modern beds attain a height of only thirty to forty feet, Mr.
Burton[31] found a deposit replete with existing species of shells, at
the height of 200 feet. In an admirable series of drawings by Captain
Moresby, I could see how continuously the cliff-bounded low plains of
this formation extended with a nearly equable height, both on the
eastern and western shores. The southern coast of Arabia seems to have
been subjected to the same elevatory movement, for Dr. Malcolmson found
at Sahar low cliffs containing shells and corals, apparently of recent
species.
[30] Rüppell, “Reise in Abyssinien,” Band i., s. 141.
[31] Lyell’s “Principles of Geology,” 5th ed., vol. iv, p. 25.
The _Persian Gulf_ abounds with coral-reefs; but as it is difficult to
distinguish them from sand-banks in this shallow sea, I have coloured
only some near the mouth; towards the head of the gulf Mr.
Ainsworth[32] says that the land is worn into terraces, and that the
beds contain organic remains of existing forms. The _West Indian
Archipelago_ of “fringed” islands, alone remains to be mentioned;
evidence of an elevation within a late tertiary epoch of nearly the
whole of this great area, may be found in the works of almost all the
naturalists who have visited it. I will give some of the principal
references in a note.[33]
[32] Ainsworth’s “Assyria and Babylon,” p. 217.
[33] On Florida and the north shores of the Gulf of Mexico, Rogers’
“Report to Brit. Assoc.” vol. iii, p. 14.—On the shores of Mexico,
Humboldt, “Polit. Essay on New Spain,” vol. i, p. 62. (I have also
some corroborative facts with respect to the shores of
Mexico.)—Honduras and the Antilles, Lyell’s “Principles,” 5th ed.,
vol. iv, p. 22.—Santa Cruz and Barbadoes, Prof. Hovey, “Silliman’s
Journal”, vol. xxxv, p. 74.—St. Domingo, Courrojolles, “Journ de
Phys.” tom. liv., p. 106.—Bahamas, “United Service Journal”, No. lxxi,
pp. 218 and 224. Jamaica, De la Beche, “Geol. Man.” p. 142.—Cuba,
Taylor in “Lond. and Edin. Mag.” vol. xi, p. 17. Dr. Daubeny also, at
a meeting of the Geolog. Soc., orally described some very modern beds
lying on the N.W. parts of Cuba. I might have added many other less
important references.
It is very remarkable on reviewing these details, to observe in how
many instances fringing-reefs round the shores, have coincided with the
existence on the land of upraised organic remains, which seem, from
evidence more or less satisfactory, to belong to a late tertiary
period. It may, however, be objected, that similar proofs of elevation,
perhaps, occur on the coasts coloured blue in our map: but this
certainly is not the case with the few following and doubtful
exceptions.
The entire area of the Red Sea appears to have been upraised within a
modern period; nevertheless I have been compelled (though on
unsatisfactory evidence, as given in the Appendix) to class the reefs
in the middle part, as barrier-reefs; should, however, the statements
prove accurate to the less height of the tertiary bed in this middle
part, compared with the northern and southern districts, we might well
suspect that it had subsided subsequently to the general elevation by
which the whole area has been upraised. Several authors[34] have stated
that they have observed shells and corals high up on the mountains of
the Society Islands,—a group encircled by barrier-reefs, and,
therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found on
the apex of one of the highest mountains, between 5,000 and 7,000 feet
above the level of the sea, “a distinct and regular stratum of
semi-fossil coral.” At Tahiti, however, other naturalists, as well as
myself, have searched in vain at a low level near the coast, for
upraised shells or masses of coral-reef, where if present they could
hardly have been overlooked. From this fact, I concluded that probably
the organic remains strewed high up on the surface of the land, had
originally been embedded in the volcanic strata, and had subsequently
been washed out by the rain. I have since heard from the Rev. W. Ellis,
that the remains which he met with, were (as he believes)
interstratified with an argillaceous tuff; this likewise was the case
with the shells observed by the Rev. D. Tyerman at Huaheine. These
remains have not been specifically examined; they may, therefore, and
especially the stratum observed by Mr. Stutchbury at an immense height,
be contemporaneous with the first formation of the Society Islands, and
be of any degree of antiquity; or they may have been deposited at some
subsequent, but probably not very recent, period of elevation; for if
the period had been recent, the entire surface of the coast land of
these islands, where the reefs are so extensive, would have been coated
with upraised coral,
which certainly is not the case. Two of the Harvey, or Cook Islands,
namely, Aitutaki and Manouai, are encircled by reefs, which extend so
far from the land, that I have coloured them blue, although with much
hesitation, as the space within the reef is shallow, and the outline of
the land is not abrupt. These two islands consist of coral-rock; but I
have no evidence of their recent elevation, besides, the improbability
of Mangaia, a fringed island in the same group (but distant 170 miles),
having retained its nearly perfect atoll-like structure, during any
immense lapse of time after its upheaval. The Red Sea, therefore, is
the only area in which we have clear proofs of the recent elevation of
a district, which, by our theory (although the barrier-reefs are there
not well characterised), has lately subsided. But we have no reason to
be surprised at oscillation, of level of this kind having occasionally
taken place. There can be scarcely any doubt that Savage, Aurora,[35]
and Mangaia Islands, and several of the islands in the Friendly group,
existed originally as atolls, and these have undoubtedly since been
upraised to some height above the level of the sea; so that by our
theory, there has here, also, been an oscillation of level,—elevation
having succeeded subsidence, instead of, as in the middle part of the
Red Sea and at the Harvey Islands, subsidence having probably succeeded
recent elevation.
[34] Ellis, in his “Polynesian Researches,” was the first to call
attention to these remains (vol. i, p. 38), and the tradition of the
natives concerning them. See also Williams, “Nar. of Missionary
Enterprise,” p. 21; also Tyerman and G. Bennett, “Journal of Voyage,”
vol. i, p. 213; also Mr. Couthouy’s “Remarks,” p. 51; but this
principal fact, namely, that there is a mass of upraised coral on the
narrow peninsula of Tiarubu, is from hearsay evidence; also Mr.
Stutchbury, _West of England Journal_, No. i, p. 54. There is a
passage in Von Zach, “Corres. Astronom.” vol. x, p. 266, inferring an
uprising at Tahiti, from a footpath now used, which was formerly
impassable; but I particularly inquired from several native chiefs,
whether they knew of any change of this kind, and they were unanimous
in giving me an answer in the negative.
[35] Aurora Island is described by Mr. Couthouy (“Remarks,” p. 58); it
lies 120 miles north-east of Tahiti; it is not coloured in the
appended map, because it does not appear to be fringed by living
reefs. Mr. Couthouy describes its summit as “presenting a broad
table-land which declines a few feet towards the centre, where we may
suppose the lagoon to have been placed.” It is about two hundred feet
in height, and consists of reef-rock and conglomerate, with existing
species of coral embedded in it. The island has been elevated at two
successive periods; the cliffs being marked halfway up with a
horizontal water-worn line of deep excavations. Aurora Island seems
closely to resemble in structure Elizabeth Island, at the southern end
of the Low Archipelago.
It is an interesting fact, that Fais, which, from its composition,
form, height, and situation at the western end of the Caroline
Archipelago, one is strongly induced to believe existed before its
upheaval as an atoll, lies exactly in the prolongation of the curved
line of the Mariana group, which we know to be a line of recent
elevation. I may add, that Elizabeth Island, in the southern part of
the Low Archipelago, which seems to have had the same kind of origin as
Fais, lies near Pitcairn Island, the only one in this part of the ocean
which is high, and at the same time not surrounded by an encircling
barrier-reef.
_On the absence of active volcanoes in the areas of subsidence, and on
their frequent presence in the areas of elevation._—Before making some
concluding remarks on the relations of the spaces coloured blue and
red, it will be convenient to consider the position on our map of the
volcanoes historically known to have been in action. It is impossible
not to be struck, first with the absence of volcanoes in the great
areas of subsidence tinted pale and dark blue,—namely, in the central
parts of the Indian Ocean, in the China Sea, in the sea between the
barriers
of Australia and New Caledonia, in the Caroline, Marshall, Gilbert, and
Low Archipelagoes; and, secondly, with the coincidence of the principal
volcanic chains with the parts coloured red, which indicates the
presence of fringing-reefs; and, as we have just seen, the presence in
most cases of upraised organic remains of a modern date. I may here
remark that the reefs were all coloured before the volcanoes were added
to the map, or indeed before I knew of the existence of several of
them.
The volcano in Torres Strait, at the northern point of Australia, is
that which lies nearest to a large subsiding area, although situated
125 miles within the outer margin of the actual barrier-reef. The Great
Comoro Island, which probably contains a volcano, is only twenty miles
distant from the barrier-reef of Mohila; Ambil volcano, in the
Philippines, is distant only a little more than sixty miles from the
atoll-formed Appoo reef: and there are two other volcanoes in the map
within ninety miles of circles coloured blue. These few cases, which
thus offer partial exceptions to the rule, of volcanoes being placed
remote from the areas of subsidence, lie either near single and
isolated atolls, or near small groups of encircled islands; and these
by our theory can have, in few instances, subsided to the same amount
in depth or area, as groups of atolls. There is not one active volcano
within several hundred miles of an archipelago, or even a small group
of atolls. It is, therefore, a striking fact that in the Friendly
Archipelago, which owes its origin to the elevation of a group of
atolls, two volcanoes, and, perhaps, others are known to be in action:
on the other hand, on several of the encircled islands in the Pacific,
supposed by our theory to have subsided, there are old craters and
streams of lava, which show the effects of past and ancient eruptions.
In these cases, it would appear as if the volcanoes had come into
action, and had become extinguished on the same spots, according as the
elevating or subsiding movements prevailed.
There are some other coasts on the map, where volcanoes in a state of
action concur with proofs of recent elevation, besides those coloured
red from being fringed by coral-reefs. Thus I hope to show in a future
volume, that nearly the whole line of the west coast of South America,
which forms the greatest volcanic chain in the world, from near the
equator for a space of between 2,000 and 3,000 miles southward, has
undergone an upward movement during a late geological period. The
islands on the north-western shores of the Pacific, which form the
second greatest volcanic chain, are very imperfectly known; but Luzon,
in the Philippines, and the Loo Choo Islands, have been recently
elevated; and at Kamtschatka[36] there are extensive tertiary beds of
modern date. Evidence of the same nature, but not very satisfactory,
may be detected in Northern New Zealand where there are two volcanoes.
The co-existence in other parts of the world of active volcanoes, with
upraised beds of a modern tertiary origin, will occur to
every geologist.[37] Nevertheless, until it could be shown that
volcanoes were inactive, or did not exist in subsiding areas, the
conclusion that their distribution depended on the nature of the
subterranean movements in progress, would have been hazardous. But now,
viewing the appended map, it may, I think, be considered as almost
established, that volcanoes are often (not necessarily always) present
in those areas where the subterranean motive power has lately forced,
or is now forcing outwards, the crust of the earth, but that they are
invariably absent in those, where the surface has lately subsided or is
still subsiding.[38]
[36] At Sedanka, in latitude 58° N. (Von Buch’s “Descrip. des Isles
Canaries,” p. 455). In a forthcoming part, I shall give the evidence
referred to with respect to the elevation of New Zealand.
[37] During the subterranean disturbances which took place in Chile,
in 1835, I have shown (“Geolog. Trans.” 2nd Ser., vol. v, p. 606) that
at the same moment that a large district was upraised, volcanic matter
burst forth at widely separated points, through both new and old
vents.
[38] We may infer from this rule, that in any old deposit, which
contains interstratified beds of erupted matter, there was at the
period, and in the area of its formation, a _tendency_ to an upward
movement in the earth’s surface, and certainly no movement of
subsidence.
_On the relations of the areas of subsidence and elevation._—The
immense surfaces on the map, which, both by our theory and by the plain
evidence of upraised marine remains, have undergone a change of level
either downwards or upwards during a late period, is a most remarkable
fact. The existence of continents shows that the areas have been
immense which at some period have been upraised; in South America we
may feel sure, and on the north-western shores of the Indian Ocean we
may suspect, that this rising is either now actually in progress, or
has taken place quite recently. By our theory, we may conclude that the
areas are likewise immense which have lately subsided, or, judging from
the earthquakes occasionally felt and from other appearances, are now
subsiding. The smallness of the scale of our map should not be
overlooked: each of the squares on it contains (not allowing for the
curvature of the earth) 810,000 square miles. Look at the space of
ocean from near the southern end of the Low Archipelago to the northern
end of the Marshall Archipelago, a length of 4,500 miles, in which, as
far as is known, every island, except Aurora which lies just without
the Low Archipelago, is atoll-formed. The eastern and western
boundaries of our map are continents, and they are rising areas: the
central spaces of the great Indian and Pacific Oceans, are mostly
subsiding; between them, north of Australia, lies the most broken land
on the globe, and there the rising parts are surrounded and penetrated
by areas of subsidence,[39] so that the prevailing movements now in
progress, seem to accord with the actual states of surface of the great
divisions of the world.
[39] I suspect that the Arru and Timor-laut Islands present an
included small area of subsidence, like that of the China Sea, but I
have not ventured to colour them from my imperfect information, as
given in the Appendix.
The blue spaces on the map are nearly all elongated; but it does not
necessarily follow from this (a caution, for which I am indebted to Mr.
Lyell), that the areas of subsidence were likewise elongated; for
the subsidence of a long, narrow space of the bed of the ocean,
including in it a transverse chain of mountains, surmounted by atolls,
would only be marked on the map by a transverse blue band. But where a
chain of atolls and barrier-reefs lies in an elongated area, between
spaces coloured red, which therefore have remained stationary or have
been upraised, this must have resulted either from the area of
subsidence having originally been elongated (owing to some tendency in
the earth’s crust thus to subside), or from the subsiding area having
originally been of an irregular figure, or as broad as long, and having
since been narrowed by the elevation of neighbouring districts. Thus
the areas, which subsided during the formation of the great north and
south lines of atolls in the Indian Ocean,—of the east and west line of
the Caroline atolls,—and of the north-west and south-east line of the
barrier-reefs of New Caledonia and Louisiade, must have originally been
elongated, or if not so, they must have since been made elongated by
elevations, which we know to belong to a recent period.
I infer from Mr. Hopkins’ researches,[40] that for the formation of a
long chain of mountains, with few lateral spurs, an area elongated in
the same direction with the chain, must have been subjected to an
elevatory movement. Mountain-chains, however, when already formed,
although running in very different directions, it seems[41] may be
raised together by a widely-acting force: so, perhaps, mountain-chains
may subside together. Hence, we cannot tell, whether the Caroline and
Marshall Archipelagoes, two groups of atolls running in different
directions and meeting each other, have been formed by the subsidence
of two areas, or of one large area, including two distinct lines of
mountains. We have, however, in the southern prolongation of the
Mariana Islands, probable evidence of a line of recent elevation having
intersected one of recent subsidence. A view of the map will show that,
generally, there is a tendency to alternation in the parallel areas
undergoing opposite kinds of movement; as if the sinking of one area
balanced the rising of another.
[40] “Researches in Physical Geology,” Transact. Cambridge Phil. Soc.,
vol. vi, part i.
[41] For instance in S. America from lat. 34°, for very many degrees
southward there are upraised beds containing recent species of shells,
on both the Atlantic and Pacific side of the continent, and from the
gradual ascent of the land, although with very unequal slopes, on both
sides towards the Cordillera, I think it can hardly be doubted that
the entire width has been upraised in mass within the recent period.
In this case the two W.N.W. and E.S.E. mountain-lines, namely the
Sierra Ventana and the S. Tapalguen, and the great north and south
line of the Cordillera have been together raised. In the West Indies
the N. and S. line of the Eastern Antilles, and the E. and W. line of
Jamaica, appear both to have been upraised within the latest
geological period.
The existence in many parts of the world of high table-land, proves
that large surfaces have been upraised in mass to considerable heights
above the level of the ocean; although the highest points in almost
every country consist of upturned strata, or erupted matter: and from
the immense spaces scattered with atolls, which indicate that land
originally existed there, although not one pinnacle now remains above
the level of the sea, we may conclude that wide areas have subsided to
an amount, sufficient to bury not only any formerly existing
table-land, but even the heights formed by fractured strata, and
erupted matter. The effects produced on the land by the later elevatory
movements, namely, successively rising cliffs, lines of erosion, and
beds of literal shells and pebbles, all requiring time for their
production, prove that these movements have been very slow; we can,
however, infer this with safety, only with respect to the few last
hundred feet of rise. But with reference to the whole vast amount of
subsidence, necessary to have produced the many atolls widely scattered
over immense spaces, it has already been shown (and it is, perhaps, the
most interesting conclusion in this volume), that the movements must
either have been uniform and exceedingly slow, or have been effected by
small steps, separated from each other by long intervals of time,
during which the reef-constructing polypifers were able to bring up
their solid frameworks to the surface. We have little means of judging
whether many considerable oscillations of level have generally occurred
during the elevation of large tracts; but we know, from clear
geological evidence, that this has frequently taken place; and we have
seen on our map, that some of the same islands have both subsided and
been upraised. I conclude, however, that most of the large blue spaces,
have subsided without many and great elevatory oscillations, because
only a few upraised atolls have been observed: the supposition that
such elevations have taken place, but that the upraised parts have been
worn down by the surf, and thus have escaped observation, is overruled
by the very considerable depth of the lagoons of all the larger atolls;
for this could not have been the case, if they had suffered repeated
elevations and abrasion. From the comparative observations made in
these latter pages, we may finally conclude, that the subterranean
changes which have caused some large areas to rise, and others to
subside, have acted in a very similar manner.
_Recapitulation._—In the three first chapters, the principal kinds of
coral-reefs were described in detail, and they were found to differ
little, as far as relates to the actual surface of the reef. An atoll
differs from an encircling barrier-reef only in the absence of land
within its central expanse; and a barrier-reef differs from a
fringing-reef, in being placed at a much greater distance from the land
with reference to the probable inclination of its submarine foundation,
and in the presence of a deep-water lagoon-like space or moat within
the reef. In the fourth chapter the growing powers of the
reef-constructing polypifers were discussed; and it was shown, that
they cannot flourish beneath a very limited depth. In accordance with
this limit, there is no difficulty respecting the foundations on which
fringing-reefs are based; whereas, with barrier-reefs and atolls, there
is a great apparent difficulty on this head; in barrier-reefs from the
improbability of the rock of the coast or of banks of sediment
extending, in every instance, so far seaward within the required
depth;—and in atolls, from the immensity of the
spaces over which they are interspersed, and the apparent necessity for
believing that they are all supported on mountain-summits, which
although rising very near to the surface-level of the sea, in no one
instance emerge above it. To escape this latter most improbable
admission, which implies the existence of submarine chains of mountains
of almost the same height, extending over areas of many thousand square
miles, there is but one alternative; namely, the prolonged subsidence
of the foundations, on which the atolls were primarily based, together
with the upward growth of the reef-constructing corals. On this view
every difficulty vanishes; fringing reefs are thus converted into
barrier-reefs; and barrier-reefs, when encircling islands, are thus
converted into atolls, the instant the last pinnacle of land sinks
beneath the surface of the ocean.
Thus the ordinary forms and certain peculiarities in the structure of
atolls and barrier-reefs can be explained;—namely, the wall-like
structure on their inner sides, the basin or ring-like shape both of
the marginal and central reefs in the Maldiva atolls—the union of some
atolls as if by a ribbon—the apparent disseverment of others—and the
occurrence, in atolls as well as in barrier-reefs, of portions of reef,
and of the whole of some reefs, in a dead and submerged state, but
retaining the outline of living reefs. Thus can be explained the
existence of breaches through barrier-reefs in front of valleys, though
separated from them by a wide space of deep water; thus, also, the
ordinary outline of groups of atolls and the relative forms of the
separate atolls one to another; thus can be explained the proximity of
the two kinds of reefs formed during subsidence, and their separation
from the spaces where fringing-reefs abound. On searching for other
evidence of the movements supposed by our theory, we find marks of
change in atolls and in barrier-reefs, and of subterranean disturbances
under them; but from the nature of things, it is scarcely possible to
detect any direct proofs of subsidence, although some appearances are
strongly in favour of it. On the fringed coasts, however, the presence
of upraised marine bodies of a recent epoch, plainly show, that these
coasts, instead of having remained stationary, which is all that can be
directly inferred from our theory, have generally been elevated.
Finally, when the two great types of structure, namely barrier-reefs
and atolls on the one hand, and fringing-reefs on the other, were laid
down in colours on our map, a magnificent and harmonious picture of the
movements, which the crust of the earth has within a late period
undergone, is presented to us. We there see vast areas rising, with
volcanic matter every now and then bursting forth through the vents or
fissures with which they are traversed. We see other wide spaces slowly
sinking without any volcanic outburst, and we may feel sure, that this
sinking must have been immense in amount as well as in area, thus to
have buried over the broad face of the ocean every one of those
mountains, above which atolls now stand like monuments, marking the
place of their former existence. Reflecting how powerful an agent with
respect to denudation, and consequently to the nature and thickness of
the deposits in accumulation, the sea must ever be, when acting
for prolonged periods on the land, during either its slow emergence or
subsidence; reflecting, also, on the final effects of these movements
in the interchange of land and ocean-water on the climate of the earth,
and on the distribution of organic beings, I may be permitted to hope,
that the conclusions derived from the study of coral-formations,
originally attempted merely to explain their peculiar forms, may be
thought worthy of the attention of geologists.
APPENDIX.
CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE
III.
In the beginning of the last chapter I stated the principles on which
the map is coloured. There only remains to be said, that it is an exact
copy of one by M. C. Gressier, published by the Dépôt Général de la
Marine, in 1835. The names have been altered into English, and the
longitude has been reduced to that of Greenwich. The colours were first
laid down on accurate charts, on a large scale. The data, on which the
volcanoes historically known to have been in action, have been marked
with vermillion, were given in a note to the last chapter. I will
commence my description on the eastern side of the map, and will
describe each group of islands consecutively, proceeding westward
across the Pacific and Indian Oceans, but ending with the West Indies.
The WESTERN SHORES OF AMERICA appear to be entirely without
coral-reefs; south of the equator the survey of the _Beagle_, and north
of it, the published charts show that this is the case. Even in the Bay
of _Panama_, where corals flourish, there are no true coral-reefs, as I
have been informed by Mr. Lloyd. There are no coral-reefs in the
_Galapagos_ Archipelago, as I know from personal inspection; and I
believe there are none on the _ Cocos, Revilla-gigedo_, and other
neighbouring islands. _ Clipperton_ rock, 10° N., 109° W., has lately
been surveyed by Captain Belcher; in form it is like the crater of a
volcano. From a drawing appended to the MS. plan in the Admiralty, it
evidently is not an atoll. The eastern parts of the Pacific present an
enormous area, without any islands, except _E_, and _Sala_, and _Gomez_
Islands, which do not appear to be surrounded by reefs.
The LOW ARCHIPELAGO.—This group consists of about eighty atolls: it
will be quite superfluous to refer to descriptions of each. In
D’Urville and Lottin’s chart, one island (_Wolchonsky_) is written with
a capital letter, signifying, as explained in a former chapter, that it
is a high island; but this must be a mistake, as the original chart by
Bellinghausen shows that it is a true atoll. Captain Beechey says of
the thirty-two groups which he examined (of the greater number of which
I have seen beautiful MS. charts in the Admiralty), that twenty-nine
now contain lagoons, and he believes the other three originally did.
Bellinghausen (see an account of his Russian voyage, in the “Biblioth.
des Voyages,” 1834, p. 443) says, that the seventeen islands which he
discovered resembled each other in structure, and he has given charts
on a large scale of all of them. Kotzebue has given plans of several;
Cook and Bligh mention others; a few were seen during the voyage of the
_Beagle_; and notices of other atolls are scattered through several
publications. The _ Actæon_ group in this archipelago has lately been
discovered (_Geograph. Journ._, vol. vii, p. 454); it consists of three
small and low islets, one of which has a lagoon. Another lagoon-island
has been discovered (_Naut. Mag._, 1839, p. 770), in 22° 4′ S., and
136° 20′ W. Towards the S.E. part of the group, there are some islands
of different formation: _ Elizabeth_ Island is described by Beechey (p.
46, 4to ed.) as fringed by reefs, at the distance of between two and
three hundred yards; coloured red. _Pitcairn_ Island, in the immediate
neighbourhood, according to the same authority, has no reefs of any
kind, although numerous pieces of coral are thrown up on the beach; the
sea close to its shore is very deep (see “Zool. of Beechey’s Voyage,”
p. 164); it is left uncoloured. _Gambier_ Islands (see Plate I Fig. 8),
are encircled by a barrier-reef; the greatest depth within is
thirty-eight fathoms; coloured pale blue. _Aurora_ Island, which lies
N.E. of Tahiti close to the large space coloured dark blue in the map,
has been already described in a note (), on the authority of Mr.
Couthouy; it is an upraised atoll, but as it does not appear to be
fringed by living reefs, it is left uncoloured.
The SOCIETY Arch. is separated by a narrow space from the Low
Archipelago; and in their parallel direction they manifest some
relation to each other. I have already described the general character
of the reefs of these fine encircled islands. In the “Atlas of the
_Coquille’s_ Voyage” there is a good general chart of the group, and
separate plans of some of the islands. _ Tahiti_, the largest island in
the group, is almost surrounded, as seen in Cook’s chart, by a reef
from half a mile to a mile and a half from the shore, with from ten to
thirty fathoms within it. Some considerable submerged reefs lying
parallel to the shore, with a broad and deep space within, have lately
been discovered (_Naut. Mag._, 1836, p. 264) on the N.E. coast of the
island, where none are laid down by Cook. At _Eimeo_ the reef “which
like a ring surrounds it, is in some places one or two miles distant
from the shore, in others united to the beach” (Ellis, “Polynesian
Researches,” vol. i, p. 18, 12mo edition). Cook found deep water
(twenty fathoms) in some of the harbours within the reef. Mr. Couthouy,
however, states (“Remarks,” p. 45) that both at Tahiti and Eimeo, the
space between the barrier-reef and the shore, has been almost filled
up,—“a nearly continuous fringing-reef surrounding the island, and
varying from a few yards to rather more than a mile in width, the
lagoons merely forming canals between this and the sea-reef,” that is
the barrier-reef. _Tapamanoa_ is surrounded by a reef at a considerable
distance from the shore; from the island being small it is breached, as
I am informed by the Rev. W.
Ellis, only by a narrow and crooked boat channel. This is the lowest
island in the group, its height probably not exceeding 500 feet. A
little way north of Tahiti, the low coral-islets of _ Teturoa_ are
situated; from the description of them given me by the Rev. J. Williams
(the author of the “Narrative of Missionary Enterprise”), I should have
thought they had formed a small atoll, and likewise from the
description given by the Rev. D. Tyerman and G. Bennett (“Journal of
Voyage and Travels,” vol. i, p. 183), who say that ten low coral-islets
“are comprehended within one general reef, and separated from each
other by interjacent lagoons;” but as Mr. Stutchbury (_West of England
Journal_, vol. i, p. 54) describes it as consisting of a mere narrow
ridge, I have left it uncoloured. _Maitea_, eastward of the group, is
classed by Forster as a high encircled island; but from the account
given by the Rev. D. Tyerman and G. Bennett (vol. i, p. 57) it appears
to be an exceedingly abrupt cone, rising from the sea without any reef;
I have left it uncoloured. It would be superfluous to describe the
northern islands in this group, as they may be well seen in the chart
accompanying the 4to edition of Cook’s “Voyages,” and in the “Atlas of
the _Coquille’s_ Voyage.” _Maurua_ is the only one of the northern
islands, in which the water within the reef is not deep, being only
four and a half fathoms; but the great width of the reef, stretching
three miles and a half southward of the land (which is represented in
the drawing in the “Atlas of the _ Coquille’s_ Voyage” as descending
abruptly to the water) shows, on the principle explained in the
beginning of the last chapter, that it belongs to the barrier class. I
may here mention, from information communicated to me by the Rev. W.
Ellis, that on the N.E. side of _Huaheine_ there is a bank of sand,
about a quarter of a mile wide, extending parallel to the shore, and
separated from it by an extensive and deep lagoon; this bank of sand
rests on coral-rock, and undoubtedly was originally a living reef.
North of Bolabola lies the atoll of _Toubai_ (Motou-iti of the
“_Coquille’s_ Atlas”) which is coloured dark blue; the other islands,
surrounded by barrier-reefs, are pale blue; three of them are
represented in Figs 3, 4, and 5, in Plate I. There are three low
coral-groups lying a little E. of the Society Archipelago, and almost
forming part of it, namely _ Bellinghausen_, which is said by Kotzebue
(“Second Voyage,” vol. ii, p. 255), to be a lagoon-island; _Mopeha_,
which, from Cook’s description (“Second Voyage,” book iii, chap. i), no
doubt is an atoll; and the _Scilly_ Islands, which are said by Wallis
(“Voyage,” chap. ix) to form a _group_ of _low_ islets and shoals, and,
therefore, probably, they compose an atoll: the two former have been
coloured blue, but not the latter.
MENDANA OR MARQUESAS Group.—These islands are entirely without reefs,
as may be seen in Krusenstern’s Atlas, making a remarkable contrast
with the adjacent group of the Society Islands. Mr. F. D. Bennett has
given some account of this group, in the seventh volume of the
_Geograph. Journ._ He informs me that all the islands have the same
general character, and that the water is very deep close to their
shores. He visited three of them, namely, _Dominicana, Christiana,_ and
_Roapoa_; their beaches are strewed with rounded masses of coral, and
although no regular reefs exist, yet the shore is in many places lined
by coral-rock, so that a boat grounds on this formation. Hence these
islands ought probably to come within the class of fringed islands and
be coloured red; but as I am determined to err on the cautious side, I
have left them uncoloured.
COOK or HARVEY and AUSTRAL ISLAND.—_Palmerston_ Island is minutely
described as an atoll by Captain Cook during his voyage in 1774;
coloured blue. _Aitutaki_ was partially surveyed by the _ Beagle_ (see
map accompanying “Voyages of _Adventure_ and _Beagle_”); the land is
hilly, sloping gently to the beach; the highest point is 360 feet; on
the southern side the reef projects five miles from the land: off this
point the _Beagle_ found no bottom with 270 fathoms: the reef is
surmounted by many low coral-islets. Although within the reef the water
is exceedingly shallow, not being more than a few feet deep, as I am
informed by the Rev. J. Williams, nevertheless, from the great
extension of this reef into a profoundly deep ocean, this island
probably belongs, on the principle lately adverted to, to the barrier
class, and I have coloured it pale blue; although with much
hesitation.—_Manouai_ or _Harvey_ Island. The highest point is about
fifty feet: the Rev. J. Williams informs me that the reef here,
although it lies far from the shore, is less distant than at Aitutaki,
but the water within the reef is rather deeper: I have also coloured
this pale blue with many doubts.—Round _Mitiaro_ Island, as I am
informed by Mr. Williams, the reef is attached to the shore; coloured
red.—_Mauki_ or Maouti; the reef round this island (under the name of
Parry Island, in the “Voyage of H.M.S. _ Blonde_,” p. 209) is described
as a coral-flat, only fifty yards wide, and two feet under water. This
statement has been corroborated by Mr. Williams, who calls the reef
attached; coloured red.—_Aitu_, or Wateeo; a moderately elevated hilly
island, like the others of this group. The reef is described in Cook’s
“Voyage,” as attached to the shore, and about one hundred yards wide;
coloured red.—_Fenoua-iti_; Cook describes this island as very low, not
more than six or seven feet high (vol. i, book ii, chap. iii, 1777); in
the chart published in the “_Coquille’s_ Atlas,” a reef is engraved
close to the shore: this island is not mentioned in the list given by
Mr. Williams (page 16) in the “Narrative of Missionary Enterprise;”
nature doubtful. As it is so near Atiu, it has been unavoidably
coloured red.—_Rarotonga_; Mr. Williams informs me that it is a lofty
basaltic island with an attached reef; coloured red.—There are three
islands, _Rourouti, Roxburgh_, and _Hull_, of which I have not been
able to obtain any account, and have left them uncoloured. Hull Island,
in the French chart, is written with small letters as being
low.—_Mangaia_; height about three hundred feet; “the surrounding reef
joins the shore” (Williams, “Narrative,” p. 18); coloured
red.—_Rimetara_; Mr. Williams informs me that the reef is rather close
to the shore; but, from information given me by Mr. Ellis, the reef
does not appear to be quite so closely attached to it as in the
foregoing cases: the island is about three hundred feet high (_Naut.
Mag._, 1839, p. 738); coloured red.—_Rurutu_; Mr. Williams and Mr.
Ellis inform me that this island has an attached reef; coloured red. It
is described by Cook under the name of Oheteroa: he says it is not
surrounded, like the neighbouring islands by a reef; he must have meant
a distant reef.—_Toubouai_; in Cook’s chart (“Second Voyage,” vol. ii,
p. 2) the reef is laid down in part one mile, and in part two miles
from the shore. Mr. Ellis (“Polynes. Res.” vol. iii, p. 381) says the
low land round the base of the island is very extensive; and this
gentleman informs me that the water within the reef appears deep;
coloured blue.—_Raivaivai_, or Vivitao; Mr. Williams informs me that
the reef is here distant: Mr. Ellis, however, says that this is
certainly not the case on one side of the island; and he believes that
the water within the reef is not deep; hence I have left it
uncoloured.—_Lancaster_ Reef, described in _ Naut. Mag._, 1833 (p.
693), as an extensive crescent-formed coral-reef. I have not coloured
it.—_Rapa_, or Oparree; from the accounts given of it by Ellis and
Vancouver, there does not appear to be any reef.—_I. de Bass_ is an
adjoining island, of which I cannot find any account.—_Kemin_ Island;
Krusenstern seems hardly to know its position, and gives no further
particulars.
ISLANDS BETWEEN _the Low and Gilbert Archipelagoes._
_Caroline_ Island (10° S., 150 deg W.) is described by Mr. F. D.
Bennett (_Geograph. Journ._, vol. vii, p. 225) as containing a fine
lagoon; coloured blue.—_Flint_ Island (11° S., 151° W.); Krusenstern
believes that it is the same with Peregrino, which is described by
Quiros (Burney’s “Chron. Hist.,” vol. ii, p. 283) as “a cluster of
small islands connected by a reef, and forming a lagoon in the middle;”
coloured blue.—_Wostock_ is an island a little more than half a mile in
diameter, and apparently quite flat and low, and was discovered by
Bellinghausen; it is situated a little west of Caroline Island, but it
is not placed on the French charts; I have not coloured it, although I
entertain little doubt from the chart of Bellinghausen, that it
originally contained a small lagoon.—_Penrhyn_ Island (9° S., 158° W.);
a plan of it in the “Atlas of the First Voyage” of Kotzebue, shows that
it is an atoll; blue.—_Slarbuck_ Island (5° S., 156° W.) is described
in Byron’s “Voyage in the _Blonde_” (p. 206) as formed of a flat
coral-rock, with no trees; the height not given; not coloured.—_Malden_
Island (4° S., 154° W.); in the same voyage (p. 205) this island is
said to be of coral formation, and no part above forty feet high; I
have not ventured to colour it, although, from being of
coral-formation, it is probably fringed; in which case it should be
red.—_Jarvis_, or _Bunker_ Island (0° 20′ S., 160° W.) is described by
Mr. F. D. Bennett (_Geograph. Journ._, vol. vii, p. 227) as a narrow,
low strip of coral-formation; not coloured.—_Brook_, is a small low
island between the two latter; the position, and perhaps even the
existence of it is doubtful; not coloured.—_Pescado_ and _Humphrey_
Islands; I can find out nothing about these islands, except that the
latter appears to be small and low; not coloured.—_Rearson_, or Grand
Duke Alexander’s (10° S., 161° W.); an atoll, of which a plan is given
by Bellinghausen; blue.—_Souvoroff_ Islands (13° S., 163° W.); Admiral
Krusenstern, in the most obliging manner, obtained for me an account of
these islands from Admiral Lazareff, who discovered them. They consist
of five very low
islands of coral-formation, two of which are connected by a reef, with
deep water close to it. They do not surround a lagoon, but are so
placed that a line drawn through them includes an oval space, part of
which is shallow; these islets, therefore, probably once (as is the
case with some of the islands in the Caroline Archipelago) formed a
single atoll; but I have not coloured them.—_Danger_ Island (10° S.,
166° W.); described as low by Commodore Byron, and more lately surveyed
by Bellinghausen; it is a small atoll with three islets on it;
blue.—_Clarence_ Island (9° S., 172° W.); discovered in the _Pandora_
(G. Hamilton’s “Voyage,” p. 75): it is said, “in running along the
land, we saw several canoes crossing the _lagoons_;” as this island is
in the close vicinity of other low islands, and as it is said, that the
natives make reservoirs of water in old cocoa-nut trees (which shows
the nature of the land), I have no doubt it is an atoll, and have
coloured it blue. _York_ Island (8° S., 172° W.) is described by
Commodore Byron (chap. x of his “Voyage”) as an atoll; blue.—_Sydney_
Island (4° S., 172° W.) is about three miles in diameter, with its
interior occupied by a lagoon (Captain Tromelin, “Annal. Marit.” 1829,
p. 297); blue.—_Phoenix_ Island (4° S., 171° W.) is nearly circular,
low, sandy, not more than two miles in diameter, and very steep outside
(Tromelin, “Annal. Marit.” 1829, p. 297); it may be inferred that this
island originally contained a lagoon, but I have not coloured it.—_New
Nantucket_ (0° 15′ N., 174° W.). From the French chart it must be a low
island; I can find nothing more about it or about _Mary_ Island; both
uncoloured.—_Gardner_ Island (5° S., 174° W.) from its position is
certainly the same as _Kemin_ Island described (Krusenstern, p. 435,
Appen. to Mem., published 1827) as having a lagoon in its centre; blue.
ISLANDS SOUTH _of the Sandwich Archipelago._
_Christmas_ Island (2° N., 157° W.). Captain Cook, in his “Third
Voyage” (vol. ii, chap. x), has given a detailed account of this atoll.
The breadth of the islets on the reef is unusually great, and the sea
near it does not deepen so suddenly as is generally the case. It has
more lately been visited by Mr. F. D. Bennett (_Geograph. Journ._, vol.
vii, p. 226); and he assures me that it is low and of coral-formation:
I particularly mention this, because it is engraved with a capital
letter, signifying a high island, in D’Urville and Lottin’s chart. Mr.
Couthouy, also, has given some account of it (“Remarks,” p. 46) from
the Hawaiian “Spectator”; he believes it has lately undergone a small
elevation, but his evidence does not appear to me satisfactory; the
deepest part of the lagoon is said to be only ten feet; nevertheless, I
have coloured it blue.—_Fanning_ Island (4° N., 158° W.) according to
Captain Tromelin (“Ann. Maritim.,” 1829, p. 283), is an atoll: his
account as observed by Krusenstern, differs from that given in
Fanning’s “Voyage” (p. 224), which, however, is far from clear;
coloured blue.—_Washington_ Island (4° N., 159° W.) is engraved as a
low island in D’Urville’s chart, but is described by Fanning (p. 226)
as having a much greater elevation than Fanning Island, and hence I
presume it is not an atoll; not coloured.—_Palmyra_ Island (6° N., 162°
W.) is an atoll divided into two parts (Krusenstern’s “Mem. Suppl.,” p.
50, also Fanning’s “Voyage,” p. 233); blue.—_Smyth’s_ or Johnston’s
Islands (17° N., 170° W.). Captain Smyth, R.N., has had the kindness to
inform me that they consist of two very low, small islands, with a
dangerous reef off the east end of them. Captain Smyth does not
recollect whether these islets, together with the reef, surrounded a
lagoon; uncoloured.
SANDWICH ARCHIPELAGO.—_Hawaii_; in the chart in Freycinet’s “Atlas,”
small portions of the coast are fringed by reefs; and in the
accompanying “Hydrog. Memoir,” reefs are mentioned in several places,
and the coral is said to injure the cables. On one side of the islet of
Kohaihai there is a bank of sand and coral with five feet water on it,
running parallel to the shore, and leaving a channel of about fifteen
feet deep within. I have coloured this island red, but it is very much
less perfectly fringed than others of the group.—_Maui_; in Freycinet’s
chart of the anchorage of Raheina, two or three miles of coast are seen
to be fringed; and in the “Hydrog. Memoir,” “banks of coral along
shore” are spoken of. Mr. F. D. Bennett informs me that the reefs, on
an average, extend about a quarter of a mile from the beach; the land
is not very steep, and outside the reefs the sea does not become deep
very suddenly; coloured red.—_Morotoi_, I presume, is fringed:
Freycinet speaks of the breakers extending along the shore at a little
distance from it. From the chart, I believe it is fringed; coloured
red.—_Oahu_; Freycinet, in his “Hydrog. Memoir,” mentions some of the
reefs. Mr. F. D. Bennett informs me that the shore is skirted for forty
or fifty miles in length. There is even a harbour for ships formed by
the reefs, but it is at the mouth of a valley; red.—_Atooi_, in La
Peyrouse’s charts, is represented as fringed by a reef, in the same
manner as Oahu and Morotoi; and this, as I have been informed by Mr.
Ellis, on part at least of the shore, is of coral-formation: the reef
does not leave a deep channel within; red.—_Oneehow_; Mr. Ellis
believes that this island is also fringed by a coral-reef: considering
its close proximity to the other islands, I have ventured to colour it
red. I have in vain consulted the works of Cook, Vancouver, La
Peyrouse, and Lisiansky, for any satisfactory account of the small
islands and reefs, which lie scattered in a N.W. line prolonged from
the Sandwich group, and hence have left them uncoloured, with one
exception; for I am indebted to Mr. F. D. Bennett for informing me of
an atoll-formed reef, in latitude 28° 22′, longitude 178° 30′ W., on
which the _ Gledstanes_ was wrecked in 1837. It is apparently of large
size, and extends in a N.W. and S.E. line: very few islets have been
formed on it. The lagoon seems to be shallow; at least, the deepest
part which was surveyed was only three fathoms. Mr. Couthouy
(“Remarks,” p. 38) describes this island under the name of _ Ocean_
island. Considerable doubts should be entertained regarding the nature
of a reef of this kind, with a very shallow lagoon, and standing far
from any other atoll, on account of the possibility of a crater or flat
bank of rock lying at the proper depth beneath the surface of the
water, thus affording a foundation for a ring-formed coral-reef. I
have, however, thought myself compelled, from its large size and
symmetrical outline, to colour it blue.
SAMOA or NAVIGATOR GROUP.—Kotzebue, in his “Second Voyage,”
contrasts the structure of these islands with many others in the
Pacific, in not being furnished with harbours for ships, formed by
distant coral-reefs. The Rev. J. Williams, however, informs me, that
coral-reefs do occur in irregular patches on the shores of these
islands; but that they do not form a continuous band, as round Mangaia,
and other such perfect cases of fringed islands. From the charts
accompanying La Peyrouse’s “Voyage,” it appears that the north shore of
_Savaii, Maouna, Orosenga_, and _ Manua_, are fringed by reefs. La
Peyrouse, speaking of Maouna (p. 126), says that the coral-reef
surrounding its shores, almost touches the beach; and is breached in
front of the little coves and streams, forming passages for canoes, and
probably even for boats. Further on (p. 159), he extends the same
observation to all the islands which he visited. Mr. Williams in his
“Narrative,” speaks of a reef going round a small island attached to
_Oyolava_, and returning again to it: all these islands have been
coloured red.—A chart of _Rose_ Island, at the extreme west end of the
group, is given by Freycinet, from which I should have thought that it
had been an atoll; but according to Mr. Couthouy (“Remarks,” p. 43), it
consists of a reef, only a league in circuit, surmounted by a very few
low islets; the lagoon is very shallow, and is strewed with numerous
large boulders of volcanic rock. This island, therefore, probably
consists of a bank of rock, a few feet submerged, with the outer margin
of its upper surface fringed with reefs; hence it cannot be properly
classed with atolls, in which the foundations are always supposed to
lie at a depth, greater than that at which the reef-constructing
polypifers can live; not coloured.
_Beveridge_ Reef, 20° S., 167° W., is described in the _Naut. Mag._
(May 1833, p. 442) as ten miles long in a N. and S. line, and eight
wide; “in the inside of the reef there appears deep water;” there is a
passage near the S.W. corner: this therefore seems to be a submerged
atoll, and is coloured blue.
_Savage_ Island, 19° S., 170° W., has been described by Cook and
Forster. The younger Forster (vol. ii, p. 163) says it is about forty
feet high: he suspects that it contains a low plain, which formerly was
the lagoon. The Rev. J. Williams informs me that the reef fringing its
shores, resembles that round Mangaia; coloured red.
FRIENDLY ARCHIPELAGO.—_Pylstaart_ Island. Judging from the chart in
Freycinet’s “Atlas,” I should have supposed that it had been regularly
fringed; but as nothing is said in the “Hydrog. Memoir” (or in the
“Voyage” of Tasman, the discoverer) about coral-reefs, I have left it
uncoloured.—_Tongatabou_: In the “Atlas of the Voyage of the
_Astrolabe_,” the whole south side of the island is represented as
narrowly fringed by the same reef which forms an extensive platform on
the northern side. The origin of this latter reef, which might have
been mistaken for a barrier-reef, has already been attempted to be
explained, when giving the proofs of the recent elevation of this
island.—In Cook’s charts the little outlying island also of _Eoaigee_,
is represented as fringed; coloured red.—_Eoua._ I cannot make out from
Captain Cook’s charts and descriptions, that this island has any reef,
although the bottom of the neighbouring sea seems to be corally, and
the island itself is formed of coral-rock.
Forster, however, distinctly (“Observations,” p. 14) classes it with
high islands having reefs, but it certainly is not encircled by a
barrier-reef and the younger Forster (“Voyage,” vol. i, p. 426) says,
that “a bed of coral-rocks surrounded the coast towards the
landing-place.” I have therefore classed it with the fringed islands
and coloured it red. The several islands lying N.W. of Tongatabou,
namely _Anamouka, Komango, Kotou, Lefouga, Foa_, etc., are seen in
Captain Cook’s chart to be fringed by reefs, in several of them are
connected together. From the various statements in the first volume of
Cook’s “Third Voyage,” and especially in the fourth and sixth chapters,
it appears that these reefs are of coral-formation, and certainly do
not belong to the barrier class; coloured red.—_Toufoa and Kao_,
forming the western part of the group, according to Forster have no
reefs; the former is an active volcano.—_Vavao._ There is a chart of
this singularly formed island, by Espinoza: according to Mr. Williams
it consists of coral-rock: the Chevalier Dillon informs me that it is
not fringed; not coloured. Nor are the islands of _Latte_ and
_Amargura_, for I have not seen plans on a large scale of them, and do
not know whether they are fringed.
_Niouha_, 16° S., 174° W., or _Keppel_ Island of Wallis, or _Cocos_
Island. From a view and chart of this island given in Wallis’s “Voyage”
(4to ed.) it is evidently encircled by a reef; coloured blue: it is
however remarkable that _Boscawen_ Island, immediately adjoining, has
no reef of any kind; uncoloured.
_Wallis_ Island, 13° S., 176° W., a chart and view of this island in
Wallis’s “Voyage” (4to ed.) shows that it is encircled. A view of it in
the _Naut. Mag._, July 1833, p. 376, shows the same fact; blue.
_Alloufatou_, or _Horn_ Island, _Onouafu_, or _ Proby_ Island, and
_Hunter_ Islands, lie between the Navigator and Fidji groups. I can
find no distinct accounts of them.
FIDJI or VITI GROUP.—The best chart of the numerous islands of this
group, will be found in the “Atlas of the _ Astrolabe’s_ Voyage.” From
this, and from the description given in the “Hydrog. Memoir,”
accompanying it, it appears that many of these islands are bold and
mountainous, rising to the height of between 3,000 and 4,000 feet. Most
of the islands are surrounded by reefs, lying far from the land, and
outside of which the ocean appears very deep. The _Astrolabe_ sounded
with ninety fathoms in several places about a mile from the reefs, and
found no bottom. Although the depth within the reef is not laid down,
it is evident from several expressions, that Captain D’Urville believes
that ships could anchor within, if passages existed through the outer
barriers. The Chevallier Dillon informs me that this is the case: hence
I have coloured this group blue. In the S.E. part lies _ Batoa_, or
_Turtle_ Island of Cook (“Second Voyage,” vol. ii, p. 23, and chart,
4to ed.) surrounded by a coral-reef, “which in some places extends two
miles from the shore;” within the reef the water appears to be deep,
and outside it is unfathomable; coloured pale blue. At the distance of
a few miles, Captain Cook (_Ibid_., p. 24) found a circular coral-reef,
four or five leagues in circuit, with deep water within; “in short, the
bank wants only a few little islets to make it exactly like one of the
half-drowned isles so
often mentioned,”—namely, atolls. South of Batoa, lies the high island
of _Ono_, which appears in Bellinghausen’s “Atlas” to be encircled; as
do some other small islands to the south; coloured pale blue; near Ono,
there is an annular reef, quite similar to the one just described in
the words of Captain Cook; coloured dark blue.
_Rotoumah_, 13° S., 179° E.—From the chart in Duperrey’s “Atlas,” I
thought this island was encircled, and had coloured it blue, but the
Chevallier Dillon assures me that the reef is only a shore or fringing
one; red.
_Independence_ Island, 10° S., 179° E., is described by Mr. G. Bennett,
(_United Service Journ._, 1831, part ii, p. 197) as a low island of
coral-formation, it is small, and does not appear to contain a lagoon,
although an opening through the reef is referred to. A lagoon probably
once existed, and has since been filled up; left uncoloured.
ELLICE GROUP.—_Oscar, Peyster_, and _Ellice_ Islands are figured in
Arrowsmith’s “Chart of the Pacific” (corrected to 1832) as atolls, and
are said to be very low; blue.—_Nederlandisch_ Island. I am greatly
indebted to the kindness of Admiral Krusenstern, for sending me the
original documents concerning this island. From the plans given by
Captains Eeg and Khremtshenko, and from the detailed account given by
the former, it appears that it is a narrow coral-island, about two
miles long, containing a small lagoon. The sea is very deep close to
the shore, which is fronted by sharp coral-rocks. Captain Eeg compares
the lagoon with that of other coral-islands; and he distinctly says,
the land is “very low.” I have therefore coloured it blue. Admiral
Krusenstern (“Memoir on the Pacific,” Append., 1835) states that its
shores are eighty feet high; this probably arose from the height of the
cocoa-nut trees, with which it is covered, being mistaken for
land.—_Gran Cocal_ is said in Krusenstern’s “Memoir,” to be low, and to
be surrounded by a reef; it is small, and therefore probably once
contained a lagoon; uncoloured.—_St. Augustin._ From a chart and view
of it, given in the “Atlas of the _Coquille’s_ Voyage,” it appears to
be a small atoll, with its lagoon partly filled up; coloured blue.
GILBERT GROUP.—The chart of this group, given in the “Atlas of the
_Coquille’s_ Voyage,” at once shows that it is composed of ten well
characterised atolls. In D’Urville and Lottin’s chart, _Sydenham_ is
written with a capital letter, signifying that it is high; but this
certainly is not the case, for it is a perfectly characterised atoll,
and a sketch, showing how low it is, is given in the “_Coquille’s_
Atlas.” Some narrow strip-like reefs project from the southern side of
_Drummond_ atoll, and render it irregular. The southern island of the
group is called _Chase_ (in some charts, _ Rotches_); of this I can
find no account, but Mr. F. D. Bennett discovered (_Geograph. Journ._,
vol. vii, p. 229), a low extensive island in nearly the same latitude,
about three degrees westward of the longitude assigned to Rotches, but
very probably it is the same island. Mr. Bennett informs me that the
man at the masthead reported an appearance of lagoon-water in the
centre; and, therefore, considering its position, I have coloured it
blue.—_Pitt_ Island, at the extreme northern point of the group, is
left uncoloured, as its exact position and nature
is not known.—_Byron_ Island, which lies a little to the eastward, does
not appear to have been visited since Commodore Byron’s voyage, and it
was then seen only from a distance of eighteen miles; it is said to be
low; uncoloured.
_Ocean, Pleasant_, and _Atlantic_ Islands all lie considerably to the
west of the Gilbert group: I have been unable to find any distinct
account of them. Ocean Island is written with small letters in the
French chart, but in Krusenstern’s “Memoir” it is said to be high.
MARSHALL GROUP.—We are well acquainted with this group from the
excellent charts of the separate islands, made during the two voyages
of Kotzebue: a reduced one of the whole group may be easily seen in
Krusenstern’s “Atlas,” and in Kotzebue’s “Second Voyage.” The group
consists (with the exception of two _little_ islands which probably
have had their lagoon filled up) of a double row of twenty-three large
and well-characterised atolls, from the examination of which Chamisso
has given us his well-known account of coral-formations. I include
_Gaspar Rico_, or _Cornwallis_ Island in this group, which is described
by Chamisso (Kotzebue’s “First Voyage,” vol. iii, p. 179) “as a low
sickle-formed group, with mould only on the windward side.” Gaspard
Island is considered by some geographers as a distinct island lying
N.E. of the group, but it is not entered in the chart by Krusenstern;
left uncoloured. In the S.W. part of this group lies _Baring_ Island,
of which little is known (see Krusenstern’s “Appendix,” 1835, p. 149).
I have left it uncoloured; but _Boston_ Island I have coloured blue, as
it is described (_Ibid_.) as consisting of fourteen small islands,
which, no doubt, enclose a lagoon, as represented in a chart in the
“‘Coquille’s’ Atlas.”—Two islands, _Aur Kawen_ and _Gaspar Rico_, are
written in the French chart with capital letters; but this is an error,
for from the account given by Chamisso in Kotzebue’s “First Voyage,”
they are certainly low. The nature, position, and even existence, of
the shoals and small islands north of the Marshall group, are doubtful.
NEW HEBRIDES.—Any chart, on even a small scale, of these islands, will
show that their shores are almost without reefs, presenting a
remarkable contrast with those of New Caledonia on the one hand, and
the Fidji group on the other. Nevertheless, I have been assured by Mr.
G. Bennett, that coral grows vigorously on their shores; as indeed,
will be further shown in some of the following notices. As, therefore,
these islands are not encircled, and as coral grows vigorously on their
shores, we might almost conclude, without further evidence, that they
were fringed, and hence I have applied the red colour with rather
greater freedom than in other instances.—_Matthew’s Rock_, an active
volcano, some way south of the group (of which a plan is given in the
“Atlas of the _Astrolabe’s_ Voyage”) does not appear to have reefs of
any kind about it.—_Annatom_, the southernmost of the Hebrides; from a
rough woodcut given in the _United Service Journal_ (1831, part iii, p.
190), accompanying a paper by Mr. Bennett, it appears that the shore is
fringed; coloured red.—_Tanna._ Forster, in his “Observations” (p. 22),
says Tanna has on its shores coral-rock and madrepores; and the younger
Forster, in his account (vol. ii, p. 269) speaking of the harbour
says, the whole S.E. side consists of coral-reefs, which are overflowed
at high-water; part of the southern shore in Cook’s chart is
represented as fringed; coloured red.—_Immer_ is described (_United
Service Journal,_ 1831, part iii, p. 192) by Mr. Bennett as being of
moderate elevation, with cliffs appearing like sandstone: coral grows
in patches on its shore, but I have not coloured it; and I mention
these facts, because Immer might have been thought from Forster’s
classification (“Observations,” p. 14), to have been a low island or
even an atoll.—_Erromango_ Island; Cook (“Second Voyage,” vol. ii, p.
45, 4to ed.) speaks of rocks everywhere _lining_ the coast, and the
natives offered to haul his boat over the breakers to the sandy beach:
Mr. Bennett, in a letter to the Editor of the _Singapore Chron._,
alludes to the _reefs_ on its shores. It may, I think, be safely
inferred from these passages that the shore is fringed in parts by
coral-reefs; coloured red.—_Sandwich_ Island. The east coast is said
(Cook’s “Second Voyage,” vol. ii, p. 41) to be low, and to be guarded
by a chain of breakers. In the accompanying chart it is seen to be
fringed by a reef; coloured red.—_Mallicollo._ Forster speaks of the
reef-bounded shore: the reef is about thirty yards wide, and so shallow
that a boat cannot pass over it. Forster also (“Observations,” p. 23)
says, that the rocks of the sea-shore consist of madrepore. In the plan
of Sandwich harbour, the headlands are represented as fringed; coloured
red.—_Aurora_ and _Pentecost_ Islands, according to Bougainville,
apparently have no reefs; nor has the large island of _S. Espiritu_,
nor _Bligh_ Island or _Banks’_ Islands, which latter lie to the N.E. of
the Hebrides. But in none of these cases, have I met with any detailed
account of their shores, or seen plans on a large scale; and it will be
evident, that a fringing-reef of only thirty or even a few hundred
yards in width, is of so little importance to navigation, that it will
seldom be noticed, excepting by chance; and hence I do not doubt that
several of these islands, now left uncoloured, ought to be red.
SANTA CRUZ GROUP.—_Vanikoro_ (Fig. 1, Plate I) offers a striking
example of a barrier- reef: it was first described by the Chevalier
Dillon, in his voyage, and was surveyed in the _Astrolabe_; coloured
pale blue.—_Tikopia_ and _Fataka_ Islands appear, from the descriptions
of Dillon and D’Urville, to have no reefs; _ Anouda_ is a low, flat
island, surrounded by cliffs (“_Astrolabe_ Hydrog.” and Krusenstern,
“Mem.” vol. ii, p. 432); these are uncoloured. _Toupoua_ (_Otooboa_ of
Dillon) is stated by Captain Tromelin (“Annales Marit.” 1829, p. 289)
to be almost entirely included in a reef, lying at the distance of two
miles from the shore. There is a space of three miles without any reef,
which, although indented with bays, offers no anchorage from the
extreme depth of the water close to the shore: Captain Dillon also
speaks of the reefs fronting this island; coloured blue.—_Santa-Cruz._
I have carefully examined the works of Carteret, D’Entrecasteaux,
Wilson, and Tromelin, and I cannot discover any mention of reefs on its
shores; left uncoloured.—_Tinakoro_ is a constantly active volcano
without reefs.—_Mendana Isles_ (mentioned by Dillon under the name of
_Mammee_, etc.); said by Krusenstern to be low, and intertwined with
reefs. I do not believe they include a lagoon; I have left them
uncoloured.—_Duff’s_ Islands compose a small group
directed in a N.W. and S.E. band; they are described by Wilson (p. 296,
“Miss. Voy.” 4to ed.), as formed by bold-peaked land, with the islands
surrounded by coral-reefs, extending about half a mile from the shore;
at a distance of a mile from the reefs he found only seven fathoms. As
I have no reason for supposing there is deep water within these reefs,
I have coloured them red. _Kennedy_ Island, N.E. of Duff’s. I have been
unable to find any account of it.
NEW CALEDONIA.—The great barrier-reefs on the shores of this island
have already been described (Fig. 5, Plate II). They have been visited
by Labillardiere, Cook, and the northern point by D’Urville; this
latter part so closely resembles an atoll that I have coloured it dark
blue. The _Loyalty_ group is situated eastward of this island; from the
chart and description given in the “Voyage of the _Astrolabe_,” they do
not appear to have any reefs; north of this group, there are some
extensive low reefs (called _Astrolabe_ and _Beaupré_,) which do not
seem to be atoll-formed; these are left uncoloured.
AUSTRALIAN BARRIER-REEF.—The limits of this great reef, which has
already been described, have been coloured from the charts of Flinders
and King. In the northern parts, an atoll-formed reef, lying outside
the barrier, has been described by Bligh, and is coloured dark blue. In
the space between Australia and New Caledonia, called by Flinders the
Corallian Sea, there are numerous reefs. Of these, some are represented
in Krusenstern’s “Atlas” as having an atoll-like structure; namely,
_Bampton_ shoal, _Frederic, Vine_ or Horse-shoe, and _ Alert_ reefs;
these have been coloured dark blue.
LOUISIADE.—The dangerous reefs which front and surround the western,
southern, and northern coasts of this so-called peninsula and
archipelago, seem evidently to belong to the barrier class. The land is
lofty, with a low fringe on the coast; the reefs are distant, and the
sea outside them profoundly deep. Nearly all that is known of this
group is derived from the labours of D’Entrecasteaux and Bougainville:
the latter has represented one continuous reef ninety miles long,
parallel to the shore, and in places as much as ten miles from it;
coloured pale blue. A little distance northward we have the _Laughlan_
Islands, the reefs round which are engraved in the “Atlas of the Voyage
of the _Astrolabe_,” in the same manner as in the encircled islands of
the Caroline Archipelago, the reef is, in parts, a mile and a half from
the shore, to which it does not appear to be attached; coloured blue.
At some little distance from the extremity of the Louisiade lies the
_Wells_ reef, described in G. Hamilton’s “Voyage in H.M.S. _Pandora_”
(p. 100): it is said, “We found we had got embayed in a double reef,
which will soon be an island.” As this statement is only intelligible
on the supposition of the reef being crescent or horse-shoe formed,
like so many other submerged annular reefs, I have ventured to colour
it blue.
SOLOMON ARCHIPELAGO.—The chart in Krusenstern’s “Atlas” shows that
these islands are not encircled, and as coral appears from the works of
Surville, Bougainville, and Labillardiere, to grow on their shores,
this circumstance, as in the case of the New Hebrides, is a presumption
that they are fringed. I cannot find out anything from
D’Entrecasteaux’s
“Voyage,” regarding the southern islands of the group, so have left
them uncoloured.—_Malayta_ Island in a rough MS. chart in the Admiralty
has its northern shore fringed.—_Ysabel_ Island, the N.E. part of this
island, in the same chart, is also fringed: Mendana, speaking (Burney,
vol. i, p. 280) of an islet adjoining the northern coast, says it is
surrounded by reefs; the shores, also of Port Praslin appear regularly
fringed.—_Choiseul_ Island. In Bougainville’s “Chart of Choiseul Bay,”
parts of the shores are fringed by coral-reefs.—_Bougainville_ Island.
According to D’Entrecasteaux the western shore abounds with
coral-reefs, and the smaller islands are said to be attached to the
larger ones by reefs; all the before-mentioned islands have been
coloured red.—_Bouka_ Islands. Captain Duperrey has kindly informed me
in a letter that he passed close round the northern side of this island
(of which a plan is given in his “Atlas of the _Coquille’s_ Voyage”),
and that it was “garnie d’une bande de récifs à fleur d’eau adherentes
au rivage;” and he infers, from the abundance of coral on the islands
north and south of Bouka, that the reef probably is of coral; coloured
red.
Off the north coast of the Solomon Archipelago there are several small
groups which are little known; they appear to be low, and of
coral-formation; and some of them probably have an atoll-like
structure; the Chevallier Dillon, however, informs me that this is not
the case with the B. de _Candelaria.—Outong Java_, according to the
Spanish navigator, Maurelle, is thus characterised; but this is the
only one which I have ventured to colour blue.
NEW IRELAND.—The shores of the S.W. point of this island and some
adjoining islets, are fringed by reefs, as may be seen in the “Atlases
of the Voyages of the _Coquille_ and _Astrolabe_.” M. Lesson observes
that the reefs are open in front of each streamlet. The _Duke of
York’s_ Island is also fringed; but with regard to the other parts of
_New Ireland, New Hanover_, and the small islands lying northward, I
have been unable to obtain any information. I will only add that no
part of New Ireland appears to be fronted by distant reefs. I have
coloured red only the above specified portions.
NEW BRITAIN AND THE NORTHERN SHORE OF NEW GUINEA.—From the charts in
the “Voyage of the _Astrolabe_,” and from the “Hydrog. Memoir,” it
appears that these coasts are entirely without reefs, as are the
_Schouten_ Islands, lying close to the northern shore of New Guinea.
The western and south-western parts of New Guinea, will be treated of
when we come to the islands of the East Indian Archipelago.
ADMIRALTY GROUP.—From the accounts by Bougainville, Maurelle,
D’Entrecasteaux, and the scattered notices collected by Horsburgh, it
appears, that some of the many islands composing it, are high, with a
bold outline; and others are very low, small and interlaced with reefs.
All the high islands appear to be fronted by distant reefs rising
abruptly from the sea, and within some of which there is reason to
believe that the water is deep. I have therefore little doubt they are
of the barrier class.—In the southern part of the group we have _
Elizabeth_ Island, which is surrounded by a reef at the distance of a
mile; and two miles eastward of it (Krusenstern, “Append.” 1835, p. 42)
there is a little island
containing a lagoon.—Near here, also lies _ Circular-reef_ (Horsburgh,
“Direct.,” vol. i, p. 691, 4th ed.), “three or four miles in diameter
having deep water inside with an opening at the N.N.W. part, and on the
outside steep to.” I have from these data, coloured the group pale
blue, and _ Circular-reef_ dark blue.—The _Anachorites, Echequier_, and
_Hermites_, consist of innumerable low islands of coral-formation,
which probably have atoll-like forms; but not being able to ascertain
this, I have not coloured them, nor _Durour_ Island, which is described
by Carteret as low.
The CAROLINE ARCHIPELAGO is now well-known, chiefly from the
hydrographical labours of Lutké; it contains about forty groups of
atolls, and three encircled islands, two of which are engraved in Figs
2 and 7, Plate I. Commencing with the eastern part; the encircling reef
round _Ualen_ appears to be only about half a mile from the shore; but
as the land is low and covered with mangroves (“Voyage autour du
Monde,” par F. Lutké, vol. i, p. 339), the real margin has not probably
been ascertained. The extreme depth in one of the harbours within the
reef is thirty-three fathoms (see charts in “Atlas of _Coquille’s_
Voyage”), and outside at half a mile distant from the reef, no bottom
was obtained with two hundred and fifty fathoms. The reef is surmounted
by many islets, and the lagoon-like channel within is mostly shallow,
and appears to have been much encroached on by the low land surrounding
the central mountains; these facts show that time has allowed much
detritus to accumulate; coloured pale blue.—_Pouynipète_, or Seniavine.
In the greater part of the circumference of this island, the reef is
about one mile and three quarters distant; on the north side it is five
miles off the included high islets. The reef is broken in several
places; and just within it, the depth in one place is thirty fathoms,
and in another, twenty-eight, beyond which, to all appearance, there
was “un porte vaste et sur” (Lutké, vol. ii, p. 4); coloured pale
blue.—_Hogoleu_ or _Roug_. This wonderful group contains at least
sixty-two islands, and its reef is one hundred and thirty-five miles in
circuit. Of the islands, only a few, about six or eight (see “Hydrog.
Descrip.” p. 428, of the “Voyage of the _Astrolabe_,” and the large
accompanying chart taken chiefly from that given by Duperrey) are high,
and the rest are all small, low, and formed on the reef. The depth of
the great interior lake has not been ascertained; but Captain D’Urville
appears to have entertained no doubt about the possibility of taking in
a frigate. The reef lies no less than fourteen miles distant from the
northern coasts of the interior high islands, seven from their western
sides, and twenty from the southern; the sea is deep outside. This
island is a likeness on a grand scale to the Gambier group in the Low
Archipelago. Of the groups of low[1] islands forming the chief part of
the Caroline Archipelago, all those of larger size, have the true
atoll-structure (as may be seen in the “Atlas” by Captain Lutké), and
some even of the very small ones, as _ Macaskill_ and _Duperrey_, of
which plans are given in the
“Atlas of the _Coquille’s_ Voyage.” There are, however, some low small
islands of coral-formation, namely _Ollap, Tamatam, Bigali, Satahoual_,
which do not contain lagoons; but it is probable that lagoons
originally existed, but have since filled up: Lutké (vol. ii, p. 304)
seems to have thought that all the low islands, with only one
exception, contained lagoons. From the sketches, and from the manner in
which the margins of these islands are engraved in the “Atlas of the
Voyage of the _Coquille_,” it might have been thought that they were
not low; but by a comparison with the remarks of Lutké (vol. ii, p.
107, regarding Bigali) and of Freycinet (“Hydrog. Memoir _ L’Uranie_
Voyage,” p. 188, regarding Tamatam, Ollap, etc.), it will be seen that
the artist must have represented the land incorrectly. The most
southern island in the group, namely _ Piguiram_, is not coloured,
because I have found no account of it. _Nougouor_, or _Monte Verdison_,
which was not visited by Lutké, is described and figured by Mr. Bennett
(_United Service Journal_, January 1832) as an atoll. All the
above-mentioned islands have been coloured blue.
[1] In D’Urville and Lottin’s chart, Peserare is written with capital
letters; but this evidently is an error, for it is one of the low
islets on the reef of Namonouyto (see Lutké’s charts)—a regular atoll.
WESTERN PART OF THE CAROLINE ARCHIPELAGO.—_Fais_ Island is ninety feet
high, and is surrounded, as I have been informed by Admiral Lutké, by a
narrow reef of living coral, of which the broadest part, as represented
in the charts, is only 150 yards; coloured red.—_Philip_ Island., I
believe, is low; but Hunter, in his “Historical Journal,” gives no
clear account of it; uncoloured.—_Elivi_; from the manner in which the
islets on the reefs are engraved, in the “Atlas of the _Astrolabe’s_
Voyage,” I should have thought they were above the ordinary height, but
Admiral Lutké assures me this is not the case: they form a regular
atoll; coloured blue.—_Gouap_ (_Eap_ of Chamisso), is a high island
with a reef (see chart in “Voyage of the _Astrolabe_”), more than a
mile distant in most parts from the shore, and two miles in one part.
Captain D’Urville thinks that there would be anchorage (“Hydrog.
Descript. _Astrolabe_ Voyage,” p. 436) for ships within the reef, if a
passage could be found; coloured pale blue.—_Goulou_, from the chart in
the “_Astrolabe’s_ Atlas,” appears to be an atoll. D’Urville (“Hydrog.
Descript.” p. 437) speaks of the low islets on the reef; coloured dark
blue.
PELEW ISLANDS.—Krusenstern speaks of some of the islands being
mountainous; the reefs are distant from the shore, and there are spaces
within them, and not opposite valleys, with from ten to fifteen
fathoms. According to a MS. chart of the group by Lieutenant Elmer in
the Admiralty, there is a large space within the reef with deepish
water; although the high land does not hold a central position with
respect to the reefs, as is generally the case, I have little doubt
that the reefs of the Pelew Islands ought to be ranked with the barrier
class, and I have coloured them pale blue. In Lieutenant Elmer’s chart
there is a horseshoe-formed shoal, laid down thirteen miles N.W. of
Pelew, with fifteen fathoms within the reef, and some dry banks on it;
coloured dark blue.—_Spanish, Martires, Sanserot, Pulo Anna_ and
_Mariere_ Islands are not coloured, because I know nothing about them,
excepting that according to Krusenstern, the second, third, and fourth
mentioned, are
low, placed on coral-reefs, and therefore, perhaps, contain lagoons;
but Pulo Mariere is a little higher.
MARIANA ARCHIPELAGO, or LADRONES.—_Guahan._ Almost the whole of this
island is fringed by reefs, which extend in most parts about a third of
a mile from the land. Even where the reefs are most extensive, the
water within them is shallow. In several parts there is a navigable
channel for boats and canoes within the reefs. In Freycinet’s “Hydrog.
Mem.” there is an account of these reefs, and in the “Atlas,” a map on
a large scale; coloured red.—_Rota_. “L’ile est presque entièrement
entourée des récifs” (p. 212, Freycinet’s “Hydrog. Mem.”). These reefs
project about a quarter of a mile from the shore; coloured
red.—_Tinian. The eastern_ coast is precipitous, and is without reefs;
but the western side is fringed like the last island; coloured
red.—_Saypan_. The N.E. coast, and likewise the western shores appear
to be fringed; but there is a great, irregular, horn-like reef
projecting far from this side; coloured red.—_Farallon de Medinilla_,
appears so regularly and closely fringed in Freycinet’s charts, that I
have ventured to colour it red, although nothing is said about reefs in
the “Hydrographical Memoir.” The several islands which form the
northern part of the group are volcanic (with the exception perhaps of
Torres, which resembles in form the madreporitic island of Medinilla),
and appear to be without reefs.—_Mangs_, however, is described (by
Freycinet, p. 219, “Hydrog.”) from some Spanish charts, as formed of
small islands placed “au milieu des nombreux récifs;” and as these
reefs in the general chart of the group do not project so much as a
mile; and as there is no appearance from a double line, of the
existence of deep water within, I have ventured, although with much
hesitation, to colour them red. Respecting _Folger_ and _ Marshall_
Islands which lie some way east of the Marianas, I can find out
nothing, excepting that they are probably low. Krusenstern says this of
Marshall Island; and Folger Island is written with small letters in
D’Urville’s chart; uncoloured.
BONIN OR ARZOBISPO GROUP.—_Peel_ Island has been examined by Captain
Beechey, to whose kindness I am much indebted for giving me information
regarding it: “At Port Lloyd there is a great deal of coral; and the
inner harbour is entirely formed by coral-reefs, which extend outside
the port along the coast.” Captain Beechey, in another part of his
letter to me, alludes to the reefs fringing the island in all
directions; but at the same time it must be observed that the surf
washes the volcanic rocks of the coast in the greater part of its
circumference. I do not know whether the other islands of the
Archipelago are fringed; I have coloured Peel Island red.—_Grampus_
Island to the eastward, does not appear (Meare’s “Voyage,” p. 95) to
have any reefs, nor does _ Rosario_ Island (from Lutké’s chart), which
lies to the westward. Respecting the few other islands in this part of
the sea, namely the _Sulphur_ Islands, with an active volcano, and
those lying between Bonin and Japan (which are situated near the
extreme limit in latitude, at which reefs are formed), I have not been
able to find any clear account.
WEST END OF NEW GUINEA.—_Port Dory._ From the charts in the “Voyage of
the _Coquille_,” it would appear that the coast in this part
is fringed by coral-reefs; M. Lesson, however, remarks that the coral
is sickly; coloured red.—_Waigiou._ A considerable portion of the
northern shores of these islands is seen in the charts (on a large
scale) in Freycinet’s “Atlas” to be fringed by coral-reefs. Forrest (p.
21, “Voyage to New Guinea”) alludes to the coral-reefs lining the heads
of Piapis Bay; and Horsburgh (vol. ii, p. 599, 4th edit.), speaking of
the islands in Dampier Strait, says “sharp coral-rocks line their
shores;” coloured red.—In the sea north of these islands, we have
_Guedes_ (or _ Freewill_, or _St. David’s_), which from the chart given
in the 4to edit. of Carteret’s “Voyage,” must be an atoll. Krusenstern
says the islets are very low; coloured blue.—_Carteret’s Shoals_, in 2°
53′ N., are described as circular, with stony points showing all round,
with deeper water in the middle; coloured blue.—_Aiou_; the plan of
this group, given in the “Atlas of the Voyage of the _Astrolabe_,”
shows that it is an atoll; and, from a chart in Forrest’s “Voyage,” it
appears that there is twelve fathoms within the circular reef; coloured
blue.—The S.W. coast of New Guinea appears to be low, muddy, and devoid
of reefs. The _Arru, Timor-laut_, and _ Tenimber_ groups have lately
been examined by Captain Kolff, the MS. translation of which, by Mr. W.
Earl, I have been permitted to read, through the kindness of Captain
Washington, R.N. These islands are mostly rather low, and are
surrounded by distant reefs (the Ki Islands, however, are lofty, and,
from Mr. Stanley’s survey, appear without reefs); the sea in some parts
is shallow, in others profoundly deep (as near Larrat). From the
imperfection of the published charts, I have been unable to decide to
which class these reefs belong. From the distance to which they extend
from the land, where the sea is very deep, I am strongly inclined to
believe they ought to come within the barrier class, and be coloured
blue; but I have been forced to leave them uncoloured.—The
last-mentioned groups are connected with the east end of Ceram by a
chain of small islands, of which the small groups of _Ceram-laut,
Goram_ and _Keffing_ are surrounded by very extensive reefs, projecting
into deep water, which, as in the last case, I strongly suspect belong
to the barrier class; but I have not coloured them. From the south side
of Keffing, the reefs project five miles (Windsor Earl’s “Sailing
Direct. for the Arafura Sea,” p. 9).
CERAM.—In various charts which I have examined, several parts of the
coast are represented as fringed by reefs.—_Manipa_ Island, between
Ceram and Bourou, in an old MS. chart in the Admiralty, is fringed by a
very irregular reef, partly dry at low water, which I do not doubt is
of coral-formation; both islands coloured red.—_Bourou_; parts of this
island appear fringed by coral-reefs, namely, the eastern coast, as
seen in Freycinet’s chart; and _Cajeli Bay_, which is said by Horsburgh
(vol. ii, p. 630) to be lined by coral-reefs, that stretch out a little
way, and have only a few feet water on them. In several charts,
portions of the islands forming the AMBOINA GROUP are fringed by reefs;
for instance, _Noessa, Harenca_, and _Ucaster_, in Freycinet’s charts.
The above-mentioned islands have been coloured red, although the
evidence is not very satisfactory.—North of Bourou the parallel line of
the _ Xulla_ Isles extends: I have not been able to find out anything
about them, excepting
that Horsburgh (vol. ii, p. 543) says that the northern shore is
surrounded by a reef at the distance of two or three miles;
uncoloured.—_Mysol Group_; the Kanary Islands are said by Forrest
(“Voyage,” p. 130) to be divided from each other by deep straits, and
are lined with coral-rocks; coloured red.—_Guebe_, lying between
Waigiou and Gilolo, is engraved as if fringed; and it is said by
Freycinet, that all the soundings under five fathoms were on coral;
coloured red.—_Gilolo_. In a chart published by Dalrymple, the numerous
islands on the western, southern (_Batchian_ and the _Strait of
Patientia_), and eastern sides appear fringed by narrow reefs; these
reefs, I suppose, are of coral, for it is said in “Malte Brun” (vol.
xii, p. 156), “Sur les côtes (of Batchian) comme _dans les plupart_ des
iles de cet archipel, il y a de rocs de médrepores d’une beauté et
d’une variété infimies.” Forrest, also (p. 50), says Seland, near
Batchian, is a little island with reefs of coral; coloured red.—_Morty_
Island (north of Gilolo). Horsburgh (vol. ii, p. 506) says the northern
coast is lined by reefs, projecting one or two miles, and having no
soundings close to them; I have left it uncoloured, although, as in
some former cases, it ought probably to be pale blue.—_Celebes._ The
western and northern coasts appear in the charts to be bold and without
reefs. Near the extreme northern point, however, an islet in the
_Straits of Limbe_, and parts of the adjoining shore, appear to be
fringed: the east side of the bay of _Manado_, has deep water, and is
fringed by sand and coral (“_Astrol._ Voyage,” Hydrog. Part, pp.
453-4); this extreme point, therefore, I have coloured red.—Of the
islands leading from this point to Magindanao, I have not been able to
find any account, except of _ Serangani_, which appears surrounded by
narrow reefs; and Forrest (“Voyage,” p. 164) speaks of coral on its
shores; I have, therefore, coloured this island red. To the eastward of
this chain lie several islands; of which I cannot find any account,
except of _Karkalang_, which is said by Horsburgh (vol. ii, p. 504) to
be lined by a dangerous reef, projecting several miles from the
northern shore; not coloured.
ISLANDS NEAR TIMOR.—The account of the following islands is taken from
Captain D. Kolff’s “Voyage,” in 1825, translated by Mr. W. Earl, from
the Dutch.—_Lette_ has “reefs extending along shore at the distance of
half a mile from the land.”—_Moa_ has reefs on the S.W. part.—_Lakor_
has a reef lining its shore; these islands are coloured red.—Still more
eastward, _ Luan_ has, differently from the last-mentioned islands, an
extensive reef; it is steep outside, and within there is a depth of
twelve feet; from these facts, it is impossible to decide to which
class this island belongs.—_Kissa_, off the point of Timor, has its
“shore fronted by a reef, steep too on the outer side, over which small
proahs can go at the time of high water;” coloured red.—_Timor_; most
of the points, and some considerable spaces of the northern shore, are
seen in Freycinet’s chart to be fringed by coral-reefs; and mention is
made of them in the accompanying “Hydrog. Memoir;” coloured
red.—_Savu_, S.E. of Timor, appears in Flinders’ chart to be fringed;
but I have not coloured it, as I do not know that the reefs are of
coral.—_Sandalwood_ Island has, according to Horsburgh (vol. ii, p.
607), a reef on its southern shore, four miles distant from the land;
as the neighbouring sea is deep,
and generally bold, this probably is a barrier-reef, but I have not
ventured to colour it.
N.W. COAST OF AUSTRALIA.—It appears, in Captain King’s Sailing
Directions (“Narrative of Survey,” vol. ii, pp. 325-369), that there
are many extensive coral-reefs skirting, often at considerable
distances, the N.W. shores, and encompassing the small adjoining
islets. Deep water, in no instance, is represented in the charts
between these reefs and the land; and, therefore, they probably belong
to the fringing class. But as they extend far into the sea, which is
generally shallow, even in places where the land seems to be somewhat
precipitous; I have not coloured them. Houtman’s Abrolhos (lat. 28° S.
on west coast) have lately been surveyed by Captain Wickham (as
described in _Naut. Mag._ 1841, p. 511): they lie on the edge of a
steeply shelving bank, which extends about thirty miles seaward, along
the whole line of coast. The two southern reefs, or islands, enclose a
lagoon-like space of water, varying in depth from five to fifteen
fathoms, and in one spot with twenty-three fathoms. The greater part of
the island has been formed on their inland sides, by the accumulation
of fragments of coral; the seaward face consisting of nearly bare
ledges of rock. Some of the specimens, brought home by Captain Wickham,
contained fragments of marine shells, but others did not; and these
closely resembled a formation at King George’s Sound, principally due
to the action of the wind on calcareous dust, which I shall describe in
a forthcoming part. From the extreme irregularity of these reefs with
their lagoons, and from their position on a bank, the usual depth of
which is only thirty fathoms, I have not ventured to class them with
atolls, and hence have left them uncoloured.—_Rowley Shoals._ These lie
some way from the N.W. coast of Australia: according to Captain King
(“Narrative of Survey,” vol. i, p. 60), they are of coral-formation.
They rise abruptly from the sea, and Captain King had no bottom with
170 fathoms close to them. Three of them are crescent-shaped; they are
mentioned by Mr. Lyell, on the authority of Captain King, with
reference to the direction of their open sides. “A third oval reef of
the same group is entirely submerged” (“Principles of Geology,” book
iii, chap. xviii); coloured blue.—_Scott’s Reefs_, lying north of
Rowley Shoals, are briefly described by Captain Wickham (_Naut. Mag._
1841, p. 440): they appear to be of great size, of a circular form, and
“with smooth water within, forming probably a lagoon of great extent.”
There is a break on the western side, where there probably is an
entrance: the water is very deep off these reefs; coloured blue.
Proceeding westward along the great volcanic chain of the East Indian
Archipelago, _Solor Strait_ is represented in a chart published by
Dalrymple from a Dutch MS., as fringed; as are parts of _Flores_, of
_Adenara_, and of _Solor._ Horsburgh speaks of coral growing on these
shores; and therefore I have no doubt that the reefs are of coral, and
accordingly have coloured them red. We hear from Horsburgh (vol. ii, p.
602) that a coral-flat bounds the shores of _Sapy_ Bay. From the same
authority it appears (p. 610) that reefs fringe the island of _
Timor-Young_, on the N. shore of Sumbawa; and, likewise (p. 600),
that _Bally_ town in _Lombock_, is fronted by a reef, stretching along
the shore at a distance of a hundred fathoms, with channels through it
for boats; these places, therefore, have been coloured red.—_Bally_
Island. In a Dutch MS. chart on a large scale of Java, which was
brought from that island by Dr. Horsfield, who had the kindness to show
it me at the India House, its western, northern, and southern shores
appear very regularly fringed by a reef (see also Horsburgh, vol. ii,
p. 593); and as coral is found abundantly there, I have not the least
doubt that the reef is of coral, and therefore have coloured it red.
JAVA.—My information regarding the reefs of this great island is
derived from the chart just mentioned. The greater part of _Maduara_ is
represented in it as regularly fringed, and likewise portions of the
coast of Java immediately south of it. Dr. Horsfield informs me that
coral is very abundant near _Sourabaya._ The islets and parts of the N.
coast of Java, west of _Point Buang_, or _Japara_, are fringed by
reefs, said to be of coral. _Lubeck_, or _Bavian_ Islands, lying at
some distance from the shore of Java, are regularly fringed by
coral-reefs. _Carimon Java_ appears equally so, though it is not
directly said that the reefs are of coral; there is a depth between
thirty and forty fathoms round these islands. Parts of the shores of
_Sunda Strait_, where the water is from forty to eighty fathoms deep,
and the islets near _Batavia_ appear in several charts to be fringed.
In the Dutch chart the southern shore, in the narrowest part of the
island, is in two places fringed by reefs of coral. West of _
Segorrowodee_ Bay, and the extreme S.E. and E. portions are likewise
fringed by coral-reefs; all the above-mentioned places coloured red.
_Macassar Strait_; the east coast of Borneo appears, in most parts,
free from reefs, and where they occur, as on the east coast of
_Pamaroong_, the sea is very shallow; hence no part is coloured. In
_Macassar_ Strait itself, in about lat. 2° S., there are many small
islands with coral-shoals projecting far from them. There are also (old
charts by Dalrymple) numerous little flats of coral, not rising to the
surface of the water, and shelving suddenly from five fathoms to no
bottom with fifty fathoms; they do not appear to have a lagoon-like
structure. There are similar coral-shoals a little farther south; and
in lat. 4° 55′ there are two, which are engraved from modern surveys,
in a manner which might represent an annular reef with deep water
inside: Captain Moresby, however, who was formerly in this sea, doubts
this fact, so that I have left them uncoloured: at the same time I may
remark, that these two shoals make a nearer approach to the atoll-like
structure than any other within the E. Indian Archipelago. Southward of
these shoals there are other low islands and irregular coral-reefs; and
in the space of sea, north of the great volcanic chain, from Timor to
Java, we have also other islands, such as the _Postillions, Kalatoa,
Tokan-Bessees_, etc., which are chiefly low, and are surrounded by very
irregular and distant reefs. From the imperfect charts I have seen, I
have not been able to decide whether they belong to the atoll or
barrier-classes, or whether they merely fringe submarine banks, and
gently sloping land. In the Bay of _Bonin_, between the two southern
arms of Celebes, there are numerous coral-reefs;
but none of them seem to have an atoll-like structure. I have,
therefore, not coloured any of the islands in this part of the sea; I
think it, however, exceedingly probable that some of them ought to be
blue. I may add that there is a harbour on the S.E. coast of _Bouton_
which, according to an old chart, is formed by a reef, parallel to the
shore, with deep water within; and in the “Voyage of the _Coquille_,”
some neighbouring islands are represented with reefs a good way
distant, but I do not know whether with deep water within. I have not
thought the evidence sufficient to permit me to colour them.
SUMATRA.—Commencing with the west coast and outlying islands, _Engano
Island_ is represented in the published chart as surrounded by a narrow
reef, and Napier, in his “Sailing Directions,” speaks of the reef being
of coral (also Horsburgh, vol. ii, p. 115); coloured red.—_Rat Island_
(3° 51′ S.) is surrounded by reefs of coral, partly dry at low water,
(Horsburgh, vol. ii, p. 96).—_Trieste Island_ (4° 2′ S.). The shore is
represented in a chart which I saw at the India House, as fringed in
such a manner, that I feel sure the fringe consists of coral; but as
the island is so low, that the sea sometimes flows quite over it
(Dampier, “Voyage,” vol. i, p. 474), I have not coloured it.—_Pulo
Dooa_ (lat. 3°). In an old chart it is said there are chasms in the
reefs round the island, admitting boats to the watering-place, and that
the southern islet consists of a mass of sand and coral.—_Pulo Pisang_;
Horsburgh (vol. ii, p. 86) says that the rocky coral-bank, which
stretches about forty yards from the shore, is steep to all round: in a
chart, also, which I have seen, the island is represented as regularly
fringed.—_Pulo Mintao_ is lined with reefs on its west side (Horsburgh,
vol. ii, p. 107).—_Pulo Baniak_; the same authority (vol. ii, p. 105),
speaking of a part, says it is faced with coral-rocks.—_Minguin_ (3°
36′ N.). A coral-reef fronts this place, and projects into the sea
nearly a quarter of a mile (“Notices of the Indian Arch.” published at
Singapore, p. 105).—_Pulo Brassa_ (5° 46′ N.). A reef surrounds it at a
cable’s length (Horsburgh, vol. ii, p. 60). I have coloured all the
above-specified points red. I may here add, that both Horsburgh and Mr.
Moor (in the “Notices” just alluded to) frequently speak of the
numerous reefs and banks of coral on the west coast of Sumatra; but
these nowhere have the structure of a barrier-reef, and Marsden
(“History of Sumatra”) states, that where the coast is flat, the
fringing-reefs extend furthest from it. The northern and southern
points, and the greater part of the east coast, are low, and faced with
mud banks, and therefore without coral.
NICOBAR ISLANDS.—The chart represents the islands of this group as
fringed by reefs. With regard to _Great Nicobar_, Captain Moresby
informs me, that it is fringed by reefs of coral, extending between two
and three hundred yards from the shore. The _Northern Nicobars_ appear
so regularly fringed in the published charts, that I have no doubt the
reefs are of coral. This group, therefore, is coloured red.
ANDAMAN ISLANDS.—From an examination of the MS. chart, on a large
scale, of this island, by Captain Arch. Blair, in the Admiralty,
several portions of the coast appear fringed; and as Horsburgh speaks
of coral-reefs being numerous in the vicinity of these islands, I
should have
coloured them red, had not some expressions in a paper in the “Asiatic
Researches” (vol. iv, p. 402) led me to doubt the existence of reefs;
uncoloured.
The coast of _Malacca, Tenasserim_ and the coasts northward, appear in
the greater part to be low and muddy: where reefs occur, as in parts of
_Malacca Straits_, and near _ Singapore_, they are of the fringing
kind; but the water is so shoal, that I have not coloured them. In the
sea, however, between Malacca and the west coast of Borneo, where there
is a greater depth from forty to fifty fathoms, I have coloured red
some of the groups, which are regularly fringed. The northern _Natunas_
and the _Anambas_ Islands are represented in the charts on a large
scale, published in the “Atlas of the Voyage of the _ Favourite_,” as
fringed by reefs of coral, with very shoal water within
them.—_Tumbelan_ and _Bunoa_ Islands (1° N.) are represented in the
English charts as surrounded by a very regular fringe.—_St. Barbes_ (0°
15′ N.) is said by Horsburgh (vol. ii, p. 279) to be fronted by a reef,
over which boats can land only at high water.—The shore of _Borneo_ at
_Tunjong Apee_ is also fronted by a reef, extending not far from the
land (Horsburgh, vol. ii, p. 468). These places I have coloured red;
although with some hesitation, as the water is shallow. I might perhaps
have added _Pulo Leat_, in Gaspar Strait, _Lucepara_, and _Carimata_;
but as the sea is confined and shallow, and the reefs not very regular,
I have left them uncoloured.
The water shoals gradually towards the whole west coast of _ Borneo_: I
cannot make out that it has any reefs of coral. The islands, however,
off the northern extremity, and near the S.W. end of _Palawan_, are
fringed by very distant coral-reefs; thus the reefs in the case of
_Balabac_ are no less than five miles from the land; but the sea, in
the whole of this district, is so shallow, that the reefs might be
expected to extend very far from the land. I have not, therefore,
thought myself authorised to colour them. The N.E. point of Borneo,
where the water is very shoal, is connected with Magindanao by a chain
of islands called the _Sooloo Archipelago_, about which I have been
able to obtain very little information; _Pangootaran_, although ten
miles long, entirely consists of a bed of coral-rock (“Notices of E.
Indian Arch.” p. 58): I believe from Horsburgh that the island is low;
not coloured.—_Tahow Bank_, in some old charts, appears like a
submerged atoll; not coloured. Forrest (“Voyage,” p. 21) states that
one of the islands near Sooloo is surrounded by coral-rocks; but there
is no distant reef. Near the S. end of _ Basselan_, some of the islets
in the chart accompanying Forrest’s “Voyage,” appear fringed with
reefs; hence I have coloured, though unwillingly, parts of the Sooloo
group red. The sea between Sooloo and Palawan, near the shoal coast of
Borneo, is interspersed with irregular reefs and shoal patches; not
coloured: but in the northern part of this sea, there are two low
islets, _ Cagayanes_ and _Cavilli_, surrounded by extensive
coral-reefs; the breakers round the latter (Horsburgh, vol. ii, p. 513)
extend five or six miles from a sandbank, which forms the only dry
part; these breakers are steep to outside; there appears to be an
opening through them on one side, with four or five fathoms within:
from this description, I strongly suspect that Cavilli
ought to be considered an atoll; but, as I have not seen any chart of
it, on even a moderately large scale, I have not coloured it. The
islets off the northern end of _Palawan_, are in the same case as those
off the southern end, namely they are fringed by reefs, some way
distant from the shore, but the water is exceedingly shallow;
uncoloured. The western shore of Palawan will be treated of under the
head of China Sea.
PHILIPPINE ARCHIPELAGO.—A chart on a large scale of _Appoo Shoal_,
which lies near the S.E. coast of Mindoro, has been executed by Captain
D. Ross: it appears atoll-formed, but with rather an irregular outline;
its diameter is about ten miles; there are two well-defined passages
leading into the interior lagoon, which appears open; close outside the
reef all round, there is no bottom with seventy fathoms; coloured
blue.—_Mindoro_: the N.W. coast is represented in several charts, as
fringed by a reef, and _Luban Island_ is said, by Horsburgh (vol. ii,
p. 436), to be “lined by a reef.”—_Luzon_: Mr. Cuming, who has lately
investigated with so much success the Natural History of the
Philippines, informs me, that about three miles of the shore north of
Point St. Jago, is fringed by a reef; as are (Horsburgh, vol. ii, p.
437) the Three Friars off Silanguin Bay. Between Point Capones and
Playa Honda, the coast is “lined by a coral-reef, stretching out nearly
a mile in some places,” (Horsburgh); and Mr. Cuming visited some
fringing-reefs on parts of this coast, namely, near Puebla, Iba, and
Mansinglor. In the neighbourhood of Solon-solon Bay, the shore is lined
(Horsburgh, ii, p. 439) by coral-reefs, stretching out a great way:
there are also reefs about the islets off Solamague; and as I am
informed by Mr. Cuming, near St. Catalina, and a little north of it.
The same gentleman informs me there are reefs on the S.E. point of this
island in front of Samar, extending from Malalabon to Bulusan. These
appear to be the principal fringing-reefs on the coasts of Luzon; and
they have all been coloured red. Mr. Cuming informs me that none of
them have deep water within; although it appears from Horsburgh that
some few extend to a considerable distance from the shore. Within the
Philippine Archipelago, the shores of the islands do not appear to be
commonly fringed, with the exception of the S. shore of _ Masbate_, and
nearly the whole of _Bohol_; which are both coloured red. On the S.
shore of _Magindanao_, Bunwoot Island is surrounded (according to
Forrest, “Voyage,” p. 253), by a coral-reef, which in the chart appears
one of the fringing class. With respect to the eastern coasts of the
whole Archipelago, I have not been able to obtain any account.
BABUYAN ISLANDS.—Horsburgh says (vol. ii, p. 442), coral-reefs line the
shores of the harbour in Fuga; and the charts show there are other
reefs about these islands. Camiguin has its shore in parts lined by
coral-rock (Horsburgh, p. 443); about a mile off shore there is between
thirty and thirty-five fathoms. The plan of Port San Pio Quinto shows
that its shores are fringed with coral; coloured red.—BASHEE ISLANDS:
Horsburgh, speaking of the southern part of the group (vol. ii, p. 445)
says the shores of both islands are fortified by a reef, and through
some of the gaps in it, the natives can pass in their
boats in fine weather; the bottom near the land is coral-rock. From the
published charts, it is evident that several of these islands are most
regularly fringed; coloured red. The northern islands are left
uncoloured, as I have been unable to find any account of them.—FORMOSA.
The shores, especially the western one, seem chiefly composed of mud
and sand, and I cannot make out that they are anywhere lined by reefs;
except in a harbour (Horsburgh, vol. ii, p. 449) at the extreme
northern point: hence, of course, the whole of this island is left
uncoloured. The small adjoining islands are in the same case.—PATCHOW,
OR MADJIKO-SIMA GROUPS. _Patchuson_ has been described by Captain
Broughton (“Voy. to the N. Pacific,” p. 191); he says, the boats, with
some difficulty, found a passage through the coral-reefs, which extend
along the coast, nearly half a mile off it. The boats were well
sheltered within the reef; but it does not appear that the water is
deep there. Outside the reef the depth is very irregular, varying from
five to fifty fathoms; the form of the land is not very abrupt;
coloured red.—_Taypin-san_; from the description given (p. 195) by the
same author, it appears that a very irregular reef extends, to the
distance of several miles, from the southern island; but whether it
encircles a space of deep water is not evident; nor, indeed, whether
these outlying reefs are connected with those more immediately
adjoining the land; left uncoloured. I may here just add that the shore
of _Kumi_ (lying west of Patchow), has a narrow reef attached to it in
the plan of it, in La Peyrouse’s “Atlas;” but it does not appear in the
account of the voyage that it is of coral; uncoloured.—LOO CHOO. The
greater part of the coast of this moderately hilly island, is skirted
by reefs, which do not extend far from the shore, and which do not
leave a channel of deep water within them, as may be seen in the charts
accompanying Captain B. Hall’s voyage to Loo Choo (see also remarks in
Appendix, pp. xxi. and xxv.). There are, however, some ports with deep
water, formed by reefs in front of valleys, in the same manner as
happens at Mauritius. Captain Beechey, in a letter to me, compares
these reefs with those encircling the Society Islands; but there
appears to me a marked difference between them, in the less distance at
which the Loo Choo reefs lie from the land with relation to the
probable submarine inclination, and in the absence of an interior deep
water-moat or channel, parallel to the land. Hence, I have classed
these reefs with fringing-reefs, and coloured them red.—PESCADORES
(west of Formosa). Dampier (vol. i, p. 416), has compared the
appearance of the land to the southern parts of England. The islands
are interlaced with coral-reefs; but as the water is very shoal, and as
spits of sand and gravel (Horsburgh, vol. ii, p. 450) extend far out
from them, it is impossible to draw any inferences regarding the nature
of the reefs.
CHINA SEA.—Proceeding from north to south, we first meet the _Pratas
Shoal_ (lat. 20° N.) which, according to Horsburgh (vol. ii, p. 335),
is composed of coral, is of a circular form, and has a low islet on it.
The reef is on a level with the water’s edge, and when the sea runs
high, there are breakers mostly all round, “but the water within seems
pretty deep in some places; although steep-to in most parts outside,
there appear to be several parts where a ship might find anchorage
outside the breakers;” coloured blue.—The _ Paracells_ have been
accurately surveyed by Captain D. Ross, and charts on a large scale
published: but few low islets have been formed on these shoals, and
this seems to be a general circumstance in the China Sea; the sea close
outside the reefs is very deep; several of them have a lagoon-like
structure; or separate islets (_Prattle, Robert, Drummond_, etc.) are
so arranged round a moderately shallow space, as to appear as if they
had once formed one large atoll.—_Bombay Shoal_ (one of the Paracells)
has the form of an annular reef, and is “apparently deep within;” it
seems to have an entrance (Horsburgh, vol. ii, p. 332) on its west
side; it is very steep outside.—_Discovery Shoal_, also is of an oval
form, with a lagoon-like space within, and three openings leading into
it, in which there is a depth from two to twenty fathoms. Outside, at
the distance (Horsburgh, vol. ii, p. 333) of only twenty yards from the
reef, soundings could not be obtained. The Paracells are coloured
blue.—_Macclesfield Bank_: this is a coral-bank of great size, lying
east of the Paracells; some parts of the bank are level, with a sandy
bottom, but, generally, the depth is very irregular. It is intersected
by deep cuts or channels. I am not able to perceive in the published
charts (its limits, however, are not very accurately known) whether the
central part is deeper, which I suspect is the case, as in the Great
Chagos Bank, in the Indian Ocean; not coloured.—_Scarborough Shoal_:
this coral-shoal is engraved with a double row of crosses, forming a
circle, as if there was deep water within the reef: close outside there
was no bottom, with a hundred fathoms; coloured blue.—The sea off the
west coast of Palawan and the northern part of Borneo is strewed with
shoals: _Swallow Shoal_, according to Horsburgh (vol. ii, p. 431) “is
formed, _like most_ of the shoals hereabouts, of a belt of coral-rocks,
“with a basin of deep water within.”—_Half-Moon Shoal_ has a similar
structure; Captain D. Ross describes it, as a narrow belt of
coral-rock, with a basin of deep water in the centre,” and deep sea
close outside.—_Bombay Shoal_ appears (Horsburgh, vol. ii, p. 432) “to
be a basin of smooth water surrounded by breakers.” These three shoals
I have coloured blue.—The _Paraquas Shoals_ are of a circular form,
with deep gaps running through them; not coloured.—A bank gradually
shoaling to the depth of thirty fathoms, extends to a distance of about
twenty miles from the northern part of _Borneo_, and to thirty miles
from the northern part of _Palawan._ Near the land this bank appears
tolerably free from danger, but a little further out it is thickly
studded with coral-shoals, which do not generally rise quite to the
surface; some of them are very steep-to, and others have a fringe of
shoal-water round them. I should have thought that these shoals had
level surfaces, had it not been for the statement made by Horsburgh
“that most of the shoals hereabouts are formed of a belt of coral.”
But, perhaps that expression was more particularly applied to the
shoals further in the offing. If these reefs of coral have a
lagoon-like structure, they should have been coloured blue, and they
would have formed an imperfect barrier in front of Palawan and the
northern part of Borneo. But, as the water
is not very deep, these reefs may have grown up from inequalities on
the bank: I have not coloured them.—The coast of _China, Tonquin_, and
_Cochin-China_, forming the western boundary of the China Sea, appear
to be without reefs: with regard to the two last-mentioned coasts, I
speak after examining the charts on a large scale in the “Atlas of the
Voyage of the _ Favourite_.”
INDIAN OCEAN.—_South Keeling_ atoll has been specially described. Nine
miles north of it lies North Keeling, a very small atoll, surveyed by
the _ Beagle_, the lagoon of which is dry at low water.—_Christmas
Island_, lying to the east, is a high island, without, as I have been
informed by a person who passed it, any reefs at all.—CEYLON: a space
about eighty miles in length of the south-western and southern shores
of these islands has been described by Mr. Twynam (_Naut. Mag._ 1836,
pp. 365 and 518); parts of this space appear to be very regularly
fringed by coral-reefs, which extend from a quarter to half a mile from
the shore. These reefs are in places breached, and afford safe
anchorage for the small trading craft. Outside, the sea gradually
deepens; there is forty fathoms about six miles off shore: this part I
have coloured red. In the published charts of Ceylon there appear to be
fringing-reefs in several parts of the south-eastern shores, which I
have also coloured red.—At Venloos Bay the shore is likewise fringed.
North of Trincomalee there are also reefs of the same kind. The sea off
the northern part of Ceylon is exceedingly shallow; and therefore I
have not coloured the reefs which fringe portions of its shores, and
the adjoining islets, as well as the Indian promontory of _Madura._
CHAGOS, MALDIVA, AND LACCADIVE ARCHIPELAGOES.—These three great groups
which have already been often noticed, are now well-known from the
admirable surveys of Captain Moresby and Lieutenant Powell. The
published charts, which are worthy of the most attentive examination,
at once show that the _Chagos_ and _Maldiva_ groups are entirely formed
of great atolls, or lagoon-formed reefs, surmounted by islets. In the
_Laccadive_ group, this structure is less evident; the islets are low,
not exceeding the usual height of coral-formations (see Lieutenant
Wood’s account, _Geograph. Journ._, vol. vi, p. 29), and most of the
reefs are circular, as may be seen in the published charts; and within
several of them, as I am informed by Captain Moresby, there is deepish
water; these, therefore, have been coloured blue. Directly north, and
almost forming part of this group, there is a long, narrow, slightly
curved bank, rising out of the depths of the ocean, composed of sand,
shells, and decayed coral, with from twenty-three to thirty fathoms on
it. I have no doubt that it has had the same origin with the other
Laccadive banks; but as it does not deepen towards the centre I have
not coloured it. I might have referred to other authorities regarding
these three archipelagoes; but after the publication of the charts by
Captain Moresby, to whose personal kindness in giving me much
information I am exceedingly indebted, it would have been superfluous.
_Sahia de Malha_ bank consists of a series of narrow banks, with from
eight to sixteen fathoms on them; they are arranged in a semicircular
manner, round a space about forty fathoms deep, which slopes on the
S.E. quarter to unfathomable depths; they are steep-to on both sides,
but more especially on the ocean-side. Hence this bank closely
resembles in structure, and I may add from Captain Moresby’s
information in composition, the Pitt’s Bank in the Chagos group; and
the Pitt’s Bank, must, after what has been shown of the Great Chagos
Bank, be considered as a sunken, half-destroyed atoll; hence coloured
blue.—_Cargados Carajos Bank._ Its southern portion consists of a
large, curved, coral-shoal, with some low islets on its eastern edge,
and likewise some on the western side, between which there is a depth
of about twelve fathoms. Northward, a great bank extends. I cannot
(probably owing to the want of perfect charts) refer this reef and bank
to any class;—therefore not coloured.—_Ile de Sable_ is a little
island, lying west of C. Carajos, only some toises in height (“Voyage
of the _Favourite_,” vol. i, p. 130); it is surrounded by reefs; but
its structure is unintelligible to me. There are some small banks north
of it, of which I can find no clear account.—_Mauritius._ The reefs
round this island have been described in the chapter on fringing-reefs;
coloured red.—_Rodriguez._ The coral-reefs here are exceedingly
extensive; in one part they project even five miles from the shore. As
far as I can make out, there is no deep-water moat within them; and the
sea outside does not deepen very suddenly. The outline, however, of the
land appears to be (“Life of Sir J. Makintosh,” vol. ii, p. 165) hilly
and rugged. I am unable to decide whether these reefs belong to the
barrier class; as seems probable from their great extension, or to the
fringing class; uncoloured.—_Bourbon._ The greater part of the shores
of this island are without reefs; but Captain Carmichael (Hooker’s
“Bot. Misc.”) states that a portion, fifteen miles in length, on the
S.E. side, is imperfectly fringed with coral reefs: I have not thought
this sufficient to colour the island.
SEYCHELLES.—The rocky islands of primary formation, composing this
group, rise from a very extensive and tolerably level bank, having a
depth between twenty and forty fathoms. In Captain Owen’s chart, and in
that in the “Atlas of the Voyage of the _Favourite_,” it appears that
the east side of _Mahe_ and the adjoining islands of _St. Anne_ and _
Cerf_, are regularly fringed by coral-reefs. A portion of the S.E. part
of _Curieuse Island_, the N., and part of the S.W. shore of _Praslin
Island_, and the whole west side of _Digue Island_, appear fringed.
From a MS. account of these islands by Captain F. Moresby, in the
Admiralty, it appears that _ Silhouette_ is also fringed; he states
that all these islands are formed of granite and quartz, that they rise
abruptly from the sea, and that “coral-reefs have grown round them, and
project for some distance.” Dr. Allan, of Forres, who visited these
islands, informs me that there is no deep water between the reefs and
the shore. The above specified points have been coloured red. _
Amirantes Islands_: The small islands of this neighbouring group,
according to the MS. account of them by Captain F. Moresby, are
situated on an extensive bank; they consist of the debris of corals and
shells; are only about twenty feet in height, and are environed by
reefs, some attached to the shore, and some rather distant from it.—I
have taken great pains to procure plans and
information regarding the several islands lying between S.E. and S.W.
of the Amirantes, and the Seychelles; relying chiefly on Captain F.
Moresby and Dr. Allan, it appears that the greater number,
namely—_Platte, Alphonse, Coetivi, Galega, Providence, St. Pierre,
Astova, Assomption_, and _ Glorioso_, are low, formed of sand or
coral-rock, and irregularly shaped; they are situated on very extensive
banks, and are connected with great coral-reefs. Galega is said by Dr.
Allan, to be rather higher than the other islands; and St. Pierre is
described by Captain F. Moresby, as being cavernous throughout, and as
not consisting of either limestone or granite. These islands, as well
as the Amirantes, certainly are not atoll-formed, and they differ as a
group from every other group with which I am acquainted; I have not
coloured them; but probably the reefs belong to the fringing class.
Their formation is attributed, both by Dr. Allan and Captain F.
Moresby, to the action of the currents, here exceedingly violent, on
banks, which no doubt have had an independent geological origin. They
resemble in many respects some islands and banks in the West Indies,
which owe their origin to a similar agency, in conjunction with an
elevation of the entire area. In close vicinity to the several islands,
there are three others of an apparently different nature: first, _Juan
de Nova_, which appears from some plans and accounts to be an atoll;
but from others does not appear to be so; not coloured. Secondly
_Cosmoledo_; “this group consists of a ring of coral, ten leagues in
circumference, and a quarter of a mile broad in some places, enclosing
a magnificent lagoon, into which there did not appear a single opening”
(Horsburgh, vol. i, p. 151); coloured blue. Thirdly, _Aldabra_; it
consists of three islets, about twenty-five feet in height, with red
cliffs (Horsburgh, vol. i, p. 176) surrounding a very shallow basin or
lagoon. The sea is profoundly deep close to the shore. Viewing this
island in a chart, it would be thought an atoll; but the foregoing
description shows that there is something different in its nature; Dr.
Allan also states that it is cavernous, and that the coral-rock has a
vitrified appearance. Is it an upheaved atoll, or the crater of a
volcano?—uncoloured.
COMORO GROUP.—_Mayotta_, according to Horsburgh (vol. i, p. 216, 4th
ed.), is completely surrounded by a reef, which runs at the distance of
three, four, and in some places even five miles from the land; in an
old chart, published by Dalrymple, a depth in many places of thirty-six
and thirty-eight fathoms is laid down within the reef. In the same
chart, the space of open water within the reef in some parts is even
more than three miles wide: the land is bold and peaked; this island,
therefore, is encircled by a well-characterised barrier-reef, and is
coloured pale blue.—_Johanna_; Horsburgh says (vol. i, p. 217) this
island from the N.W. to the S.W. point, is bounded by a reef, at the
distance of two miles from the shore; in some parts, however, the reef
must be attached, since Lieutenant Boteler (“Narr.” vol. i, p. 161)
describes a passage through it, within which there is room only for a
few boats. Its height, as I am informed by Dr. Allan, is about 3,500
feet; it is very precipitous, and is composed of granite, greenstone,
and quartz; coloured blue.—_Mohilla_; on the S. side of this island
there is
anchorage, in from thirty to forty-five fathoms, between a reef and the
shore (Horsburgh, vol. i, p. 214); in Captain Owen’s chart of
Madagascar, this island is represented as encircled; coloured
blue.—_Great Comoro Island_ is, as I am informed by Dr. Allan, about
8,000 feet high, and apparently volcanic; it is not regularly
encircled; but reefs of various shapes and dimensions, jut out from
every headland on the W., S., and S.E. coasts, inside of which reefs
there are channels, often parallel with the shore, with deep water. On
the north-western coasts the reefs appear attached to the shores. The
land near the coast is in some places bold, but generally speaking it
is flat; Horsburgh says (vol. i, p. 214) the water is profoundly deep
close to the _shore_, from which expression I presume some parts are
without reefs. From this description I apprehend the reef belongs to
the barrier class; but I have not coloured it, as most of the charts
which I have seen, represent the reefs round it as very much less
extensive than round the other islands in the group.
MADAGASCAR.—My information is chiefly derived from the published charts
by Captain Owen, and the accounts given by him and by Lieutenant
Boteler. Commencing at the S.W. extremity of the island; towards the
northern part of the _Star Bank_ (in lat. 25° S.) the coast for ten
miles is fringed by a reef; coloured red. The shore immediately S. of
_St. Augustine’s Bay_ appears fringed; but _Tullear_ Harbour, directly
N. of it, is formed by a narrow reef ten miles long, extending parallel
to the shore, with from four to ten fathoms within it. If this reef had
been more extensive, it must have been classed as a barrier-reef; but
as the line of coast falls inwards here, a submarine bank perhaps
extends parallel to the shore, which has offered a foundation for the
growth of the coral; I have left this part uncoloured. From lat. 22°
16′ to 21° 37′, the shore is fringed by coral-reefs (see Lieutenant
Boteler’s “Narrative,” vol. ii, p. 106), less than a mile in width, and
with shallow water within. There are outlying coral-shoals in several
parts of the offing, with about ten fathoms between them and the shore,
and the depth of the sea one mile and a half seaward, is about thirty
fathoms. The part above specified is engraved on a large scale; and as
in the charts on rather a smaller scale the same fringe of reef extends
as far as lat. 33° 15′; I have coloured the whole of this part of the
coast red. The islands of _Juan de Nova_ (in lat. 17° S.) appear in the
charts on a large scale to be fringed, but I have not been able to
ascertain whether the reefs are of coral; uncoloured. The main part of
the west coast appears to be low, with outlying sandbanks, which,
Lieutenant Boteler (vol. ii, p. 106) says, “are faced on the edge of
deep water by a line of sharp-pointed coral-rocks.” Nevertheless I have
not coloured this part, as I cannot make out by the charts that the
coast itself is fringed. The headlands of _Narrenda_ and _Passandava_
Bays (14° 40′) and the islands in front of _Radama Harbour_ are
represented in the plans as regularly fringed, and have accordingly
been coloured red. With respect to the _East coast of Madagascar_, Dr.
Allan informs me in a letter, that the whole line of coast, from
_Tamatave_, in 18° 12′, to _C. Amber_, at the extreme northern point of
the island, is bordered by coral-reefs. The land is low, uneven,
and gradually rising from the coast. From Captain Owen’s charts, also,
the existence of these reefs, which evidently belong to the fringing
class, on some parts, namely N. of _British Sound_, and near _Ngoncy_,
of the above line of coast might have been inferred. Lieutenant Boteler
(vol. i, p. 155) speaks of “the reef surrounding the island of _St.
Mary’s_ at a small distance from the shore.” In a previous chapter I
have described, from the information of Dr. Allan, the manner in which
the reefs extend in N.E. lines from the headlands on this coast, thus
sometimes forming rather deep channels within them, this seems caused
by the action of the currents, and the reefs spring up from the
submarine prolongations of the sandy headlands. The above specified
portion of the coast is coloured red. The remaining S.E. portions do
not appear in any published chart to possess reefs of any kind; and the
Rev. W. Ellis, whose means of information regarding this side of
Madagascar have been extensive, informs me he believes there are none.
EAST COAST OF AFRICA.—Proceeding from the northern part, the coast
appears, for a considerable space, without reefs. My information, I may
here observe, is derived from the survey by Captain Owen, together with
his narrative; and that by Lieutenant Boteler. At _Mukdeesha_ (10° 1′
N.) there is a coral-reef extending four or five miles along the shore
(Owen’s “Narr.” vol. i, p. 357) which in the chart lies at the distance
of a quarter of a mile from the shore, and has within it from six to
ten feet water: this then is a fringing-reef, and is coloured red. From
_ Juba_, a little S. of the equator, to _Lamoo_ (in 2° 20′ S.) “the
coast and islands are formed of madrepore” (Owen’s “Narrative,” vol. i,
p. 363). The chart of this part (entitled _ Dundas Islands_), presents
an extraordinary appearance; the coast of the mainland is quite
straight and it is fronted at the average distance of two miles, by
exceedingly narrow, straight islets, fringed with reefs. Within the
chain of islets, there are extensive tidal flats and muddy bays, into
which many rivers enter; the depths of these spaces varies from one to
four fathoms—the latter depth not being common, and about twelve feet
the average. Outside the chain of islets, the sea, at the distance of a
mile, varies in depth from eight to fifteen fathoms. Lieutenant Boteler
(“Narr.,” vol. i, p. 369) describes the muddy bay of _Patta_, which
seems to resemble other parts of this coast, as fronted by small,
narrow, level islets formed of decomposing coral, the margin of which
is seldom of greater height than twelve feet, overhanging the rocky
surface from which the islets rise. Knowing that the islets are formed
of coral, it is, I think, scarcely possible to view the coast, and not
at once conclude that we here see a fringing-reef, which has been
upraised a few feet: the unusual depth of from two to four fathoms
within some of these islets, is probably due to muddy rivers having
prevented the growth of coral near the shore. There is, however, one
difficulty on this view, namely, that before the elevation took place,
which converted the reef into a chain of islets, the water must
apparently have been still deeper; on the other hand it may be supposed
that the formation of a nearly perfect barrier in front, of so large an
extent of coast, would cause the currents (especially in front of the
rivers), to deepen their muddy beds. When describing in
the chapter on fringing-reefs, those of Mauritius, I have given my
reasons for believing that the shoal spaces within reefs of this kind,
must, in many instances, have been deepened. However this may be, as
several parts of this line of coast are undoubtedly fringed by living
reefs, I have coloured it red.—_Maleenda_ (3° 20′ S.). In the plan of
the harbour, the south headland appears fringed; and in Owen’s chart on
a larger scale, the reefs are seen to extend nearly thirty miles
southward; coloured red.—_Mombas_ (4° 5′ S.). The island which forms
the harbour, “is surrounded by cliffs of madrepore, capable of being
rendered almost impregnable” (Owen’s “Narr.,” vol. i, p. 412). The
shore of the mainland N. and S. of the harbour, is most regularly
fringed by a coral-reef at a distance from half a mile to one mile and
a quarter from the land; within the reef the depth is from nine to
fifteen feet; outside the reef the depth at rather less than half a
mile is thirty fathoms. From the charts it appears that a space about
thirty-six miles in length, is here fringed; coloured red.—_Pemba_ (5°
S.) is an island of coral-formation, level, and about two hundred feet
in height (Owen’s “Narr.,” vol. i, p. 425); it is thirty-five miles
long, and is separated from the mainland by a deep sea. The outer coast
is represented in the chart as regularly fringed; coloured red. The
mainland in front of Pemba is likewise fringed; but there also appear
to be some outlying reefs with deep water between them and the shore. I
do not understand their structure, either from the charts or the
description, therefore have not coloured them.—_Zanzibar_ resembles
Pemba in most respects; its southern half on the western side and the
neighbouring islets are fringed; coloured red. On the mainland, a
little S. of Zanzibar, there are some banks parallel to the coast,
which I should have thought had been formed of coral, had it not been
said (Boteler’s “Narr.,” vol. ii, p. 39) that they were composed of
sand; not coloured.—_Latham’s Bank_ is a small island, fringed by
coral-reefs; but being only ten feet high, it has not been
coloured.—_Monfeea_ is an island of the same character as Pemba; its
outer shore is fringed, and its southern extremity is connected with
Keelwa Point on the mainland by a chain of islands fringed by reefs;
coloured red. The four last-mentioned islands resemble in many respects
some of the islands in the Red Sea, which will presently be
described.—_Keelwa._ In a plan of the shore, a space of twenty miles N.
and S. of this place is fringed by reefs, apparently of coral: these
reefs are prolonged still further southward in Owen’s general chart.
The coast in the plans of the rivers _Lindy_ and _Monghow_ (9° 59′ and
10° 7′ S.) has the same structure; coloured red.—_Querimba Islands_
(from 10° 40′ to 13° S.). A chart on a large scale is given of these
islands; they are low, and of coral-formation (Boteler’s “Narr.,” vol.
ii, p. 54); and generally have extensive reefs projecting from them
which are dry at low water, and which on the outside rise abruptly from
a deep sea: on their insides they are separated from the continent by a
channel, or rather a succession of bays, with an average depth of ten
fathoms. The small headlands on the continent also have coral-banks
attached to them; and the Querimba islands and banks are placed on the
lines of prolongation of these headlands, and are
separated from them by very shallow channels. It is evident that
whatever cause, whether the drifting of sediment or subterranean
movements, produced the headlands, likewise produced, as might have
been expected, submarine prolongations to them; and these towards their
outer extremities, have since afforded a favourable basis for the
growth of coral-reefs, and subsequently for the formation of islets. As
these reefs clearly belong to the fringing class, the Querimba islands
have been coloured red.—_Monabila_ (13° 32′ S.). In the plan of this
harbour, the headlands outside are fringed by reefs apparently of
coral; coloured red.—_Mozambique_ (150° S.) The outer part of the
island on which the city is built, and the neighbouring islands, are
fringed by coral-reefs; coloured red. From the description given in
Owen’s “Narr.” (vol. i, p. 162), the shore from _ Mozambique_ to
_Delagoa Bay_ appears to be low and sandy; many of the shoals and
islets off this line of coast are of coral-formation; but from their
small size and lowness, it is not possible, from the charts, to know
whether they are truly fringed. Hence this portion of coast is left
uncoloured, as are likewise those parts more northward, of which no
mention has been made in the foregoing pages from the want of
information.
PERSIAN GULF.—From the charts lately published on a large scale by the
East India Company, it appears that several parts, especially the
southern shores of this gulf, are fringed by coral-reefs; but as the
water is very shallow, and as there are numerous sandbanks, which are
difficult to distinguish on the chart from reefs, I have not coloured
the upper part red. Towards the mouth, however, where the water is
rather deeper, the islands of _Ormuz_ and _Larrack_, appear so
regularly fringed, that I have coloured them red. There are certainly
no atolls in the Persian Gulf. The shores of _ Immaum_, and of the
promontory forming the southern headland of the Persian Gulf, seem to
be without reefs. The whole S.W. part (except one or two small patches)
of _Arabia Felix_, and the shores of _Socotra_ appear from the charts
and memoir of Captain Haines (_Geograph. Journ._, 1839, p. 125) to be
without any reefs. I believe there are no extensive coral-reefs on any
part of the coasts of _India_, except on the low promontory of _Madura_
(as already mentioned) in front of Ceylon.
RED SEA.—My information is chiefly derived from the admirable charts
published by the East India Company in 1836, from personal
communication with Captain Moresby, one of the surveyors, and from the
excellent memoir, “Über die Natur der Corallen-Bänken des Rothen
Meeres,” by Ehrenberg. The plains immediately bordering the Red Sea
seem chiefly to consist of a sedimentary formation of the newer
tertiary period. The shore is, with the exception of a few parts,
fringed by coral-reefs. The water is generally profoundly deep close to
the shore; but this fact, which has attracted the attention of most
voyagers, seems to have no necessary connection with the presence of
reefs; for Captain Moresby particularly observed to me, that, in lat.
24° 10′ on the eastern side, there is a piece of coast, with very deep
water close to it, without any reefs, but not differing in other
respects from the usual nature of the coast-line. The most remarkable
feature in the Red Sea
is the chain of submerged banks, reefs, and islands, lying some way
from the shore, chiefly on the eastern side; the space within being
deep enough to admit a safe navigation in small vessels. The banks are
generally of an oval form, and some miles in width; but some of them
are very long in proportion to their width. Captain Moresby informs me
that any one, who had not made actual plans of them, would be apt to
think that they were much more elongated than they really are. Many of
them rise to the surface, but the greater number lie from five to
thirty fathoms beneath it, with irregular soundings on them. They
consist of sand and living coral; coral on most of them, according to
Captain Moresby, covering the greater part of their surface. They
extend parallel to the shore, and they are not unfrequently connected
in their middle parts by short transverse banks with the mainland. The
sea is generally profoundly deep quite close to them, as it is near
most parts of the coast of the mainland; but this is not universally
the case, for between lat. 15° and 17° the water deepens quite
gradually from the banks, both on the eastern and western shores,
towards the middle of the sea. Islands in many parts arise from these
banks; they are low, flat-topped, and consist of the same horizontally
stratified formation with that forming the plain-like margin of the
mainland. Some of the smaller and lower islands consist of mere sand.
Captain Moresby informs me, that small masses of rock, the remnants of
islands, are left on many banks where there is now no dry land.
Ehrenberg also asserts that most of the islets, even the lowest, have a
flat abraded basis, composed of the same tertiary formation: he
believes that as soon as the surf wears down the protuberant parts of a
bank, just beneath the level of the sea, the surface becomes protected
from further abrasion by the growth of coral, and he thus accounts for
the existence of so many banks standing on a level with the surface of
this sea. It appears that most of the islands are certainly decreasing
in size.
The form of the banks and islands is most singular in the part just
referred to, namely, from lat. 15° to 17°, where the sea deepens quite
gradually: the _Dhalac_ group, on the western coast, is surrounded by
an intricate archipelago of islets and shoals; the main island is very
irregularly shaped, and it includes a bay seven miles long, by four
across, in which no bottom was found with 252 feet: there is only one
entrance into this bay, half a mile wide, and with an island in front
of it. The submerged banks on the eastern coast, within the same
latitudes, round _ Farsan Island_, are, likewise, penetrated by many
narrow creeks of deep water; one is twelve miles long, in the form of a
hatchet, in which, close to its broad upper end, soundings were not
struck with 360 feet, and its entrance is only half a mile wide: in
another creek of the same nature, but even with a more irregular
outline, there was no bottom with 480 feet. The island of Farsan,
itself, has as singular a form as any of its surrounding banks. The
bottom of the sea round the Dhalac and Farsan Islands consists chiefly
of sand and agglutinated fragments, but, in the deep and narrow creeks,
it consists of mud; the islands themselves consist of thin,
horizontally stratified, modern tertiary
beds, containing but little broken coral,[2] their shores are fringed
by living coral-reefs.
[2] Rüppell, “Reise in Abyssinie,” Band. i, S. 247.
From the account given by Rüppell[3] of the manner in which Dhalac has
been rent by fissures, the opposite sides of which have been unequally
elevated (in one instance to the amount of fifty feet), it seems
probable that its irregular form, as well as probably that of Farsan,
may have been partly caused by unequal elevations; but, considering the
general form of the banks, and of the deep-water creeks, together with
the composition of the land, I think their configuration is more
probably due in great part to strong currents having drifted sediment
over an uneven bottom: it is almost certain that their form cannot be
attributed to the growth of coral. Whatever may have been the precise
origin of the Dhalac and Farsan Archipelagoes, the greater number of
the banks on the eastern side of the Red Sea seem to have originated
through nearly similar means. I judge of this from their similarity in
configuration (in proof of which I may instance a bank on the east
coast in lat. 22°; and although it is true that the northern banks
generally have a less complicated outline), and from their similarity
in composition, as may be observed in their upraised portions. The
depth within the banks northward of lat. 17°, is usually greater, and
their outer sides shelve more abruptly (circumstances which seem to go
together) than in the Dhalac and Farsan Archipelagoes; but this might
easily have been caused by a difference in the action of the currents
during their formation: moreover, the greater quantity of living coral,
which, according to Captain Moresby, exists on the northern banks,
would tend to give them steeper margins.
[3] _Ibid_., S. 245.
From this account, brief and imperfect as it is, we can see that the
great chain of banks on the eastern coast, and on the western side in
the southern portion, differ greatly from true barrier-reefs wholly
formed by the growth of coral. It is indeed the direct conclusion of
Ehrenberg (“Über die,” etc., pp. 45 and 51), that they are connected in
their origin quite secondarily with the growth of coral; and he remarks
that the islands off the coast of Norway, if worn down level with the
sea, and merely coated with living coral, would present a nearly
similar appearance. I cannot, however, avoid suspecting, from
information given me by Dr. Malcolmson and Captain Moresby, that
Ehrenberg has rather under-rated the influence of corals, in some
places at least, on the formation of the tertiary deposits of the Red
Sea.
_The west coast of the Red Sea between lat. 19° and 22°._—There are, in
this space, reefs, which, if I had known nothing of those in other
parts of the Red Sea, I should unhesitatingly have considered as
barrier-reefs; and, after deliberation, I have come to the same
conclusion. One of these reefs, in 20° 15′, is twenty miles long, less
than a mile in width (but expanding at the northern end into a disc),
slightly sinuous, and extending parallel to the mainland at the
distance of five miles from it, with very deep water within; in one
spot soundings were not obtained with 205 fathoms. Some leagues further
south, there is another linear reef, very narrow, ten miles long, with
other small portions
of reef, north and south, almost connected with it; and within this
line of reefs (as well as outside) the water is profoundly deep. There
are also some small linear and sickle-formed reefs, lying a little way
out at sea. All these reefs are covered, as I am informed by Captain
Moresby, by living corals. Here, then, we have all the characters of
reefs of the barrier class; and in some outlying reefs we have an
approach to the structure of atolls. The source of my doubts about the
classification of these reefs, arises from having observed in the
Dhalac and Farsan groups the narrowness and straightness of several
spits of sand and rock: one of these spits in the Dhalac group is
nearly fifteen miles long, only two broad, and it is bordered on each
side with deep water; so that, if worn down by the surf, and coated
with living corals, it would form a reef nearly similar to those within
the space under consideration. There is, also, in this space (lat. 21°)
a peninsula, bordered by cliffs, with its extremity worn down to the
level of the sea, and its basis fringed with reefs: in the line of
prolongation of this peninsula, there lies the island of _ Macowa_
(formed, according to Captain Moresby, of the usual tertiary deposit),
and some smaller islands, large parts of which likewise appear to have
been worn down, and are now coated with living corals. If the removal
of the strata in these several cases had been more complete, the reefs
thus formed would have nearly resembled those barrier-like ones now
under discussion. Notwithstanding these facts, I cannot persuade myself
that the many very small, isolated, and sickle-formed reefs and others,
long, nearly straight, and very narrow, with the water unfathomably
deep close round them, could possibly have been formed by corals merely
coating banks of sediment, or the abraded surfaces of irregularly
shaped islands. I feel compelled to believe that the foundations of
these reefs have subsided, and that the corals, during their upward
growth, have given to these reefs their present forms: I may remark
that the subsidence of narrow and irregularly-shaped peninsulas and
islands, such as those existing on the coasts of the Red Sea, would
afford the requisite foundations for the reefs in question.
_The west coast from lat. 22° to 24°._—This part of the coast (north of
the space coloured blue on the map) is fronted by an irregularly
shelving bank, from about ten to thirty fathoms deep; numerous little
reefs, some of which have the most singular shapes, rise from this
bank. It may be observed, respecting one of them, in lat. 23° 10′, that
if the promontory in lat. 24° were worn down to the level of the sea,
and coated with corals, a very similar and grotesquely formed reef
would be produced. Many of the reefs on this part of the coast may thus
have originated; but there are some sickle, and almost atoll-formed
reefs lying in deep water off the promontory in lat. 24°, which lead me
to suppose that all these reefs are more probably allied to the barrier
or atoll classes. I have not, however, ventured to colour this portion
of coast. _On the west coast from lat. 19° to 17°_ (south of space
coloured blue on the map), there are many low islets of very small
dimensions, not much elongated, and rising out of great depths at a
distance from the coast; these cannot be classed either with atolls, or
barrier- or fringing-reefs. I may here remark that the outlying reefs
on the west
coast, between lat. 19° and 24°, are the only ones in the Red Sea,
which approach in structure to the true atolls of the Indian and
Pacific Oceans, but they present only imperfect miniature likenesses of
them.
_Eastern coast._—I have felt the greatest doubt about colouring any
portion of this coast, north of the fringing-reefs round the Farsan
Islands in 16° 10′. There are many small outlying coral-reefs along the
whole line of coast; but as the greater number rise from banks not very
deeply submerged (the formation of which has been shown to be only
secondarily connected with the growth of coral), their origin may be
due simply to the growth of knolls of corals, from an irregular
foundation situated within a limited depth. But between lat. 18° and
20°, there are so many linear, elliptic, and extremely small reefs,
rising abruptly out of profound depths, that the same reasons, which
led me to colour blue a portion of the west coast, have induced me to
do the same in this part. There exist some small outlying reefs rising
from deep water, north of lat. 20° (the northern limit coloured blue),
on the east coast; but as they are not very numerous and scarcely any
of them linear, I have thought it right to leave them uncoloured.
In the _southern parts_ of the Red Sea, considerable spaces of the
mainland, and of some of the Dhalac islands, are skirted by reefs,
which, as I am informed by Captain Moresby, are of living coral, and
have all the characters of the fringing class. As in these latitudes,
there are no outlying linear or sickle-formed reefs, rising out of
unfathomable depths, I have coloured these parts of the coast red. On
similar grounds, I have coloured red the _northern parts of the western
coast_ (north of lat. 24° 30′), and likewise the shores of the chief
part of the _Gulf of Suez._ In the _Gulf of Acaba_, as I am informed by
Captain Moresby there are no coral-reefs, and the water is profoundly
deep.
WEST INDIES.—My information regarding the reefs of this area, is
derived from various sources, and from an examination of numerous
charts; especially of those lately executed during the survey under
Captain Owen, R.N. I lay under particular obligation to Captain Bird
Allen, R.N., one of the members of the late survey, for many personal
communications on this subject. As in the case of the Red Sea, it is
necessary to make some preliminary remarks on the submerged banks of
the West Indies, which are in some degree connected with coral-reefs,
and cause considerable doubts in their classification. That large
accumulations of sediment are in progress on the West Indian shores,
will be evident to any one who examines the charts of that sea,
especially of the portion north of a line joining Yucutan and Florida.
The area of deposition seems less intimately connected with the
debouchement of the great rivers, than with the course of the
sea-currents; as is evident from the vast extension of the banks from
the promontories of Yucutan and Mosquito.
Besides the coast-banks, there are many of various dimensions which
stand quite isolated; these closely resemble each other, they lie from
two or three to twenty or thirty fathoms under water, and are composed
of sand, sometimes firmly agglutinated, with little or no coral; their
surfaces are smooth and nearly level, shelving only to the amount of a
few fathoms, very gradually all round towards their edges, where they
plunge abruptly into the unfathomable sea. This steep inclination of
their sides, which is likewise characteristic of the coast-banks, is
very remarkable: I may give as an instance, the Misteriosa Bank, on the
edges of which the soundings change in 250 fathoms horizontal distance,
from 11 to 210 fathoms; off the northern point of the bank of Old
Providence, in 200 fathoms horizontal distance, the change is from 19
to 152 fathoms; off the Great Bahama Bank, in 160 fathoms horizontal
distance, the inclination is in many places from 10 fathoms to no
bottom with 190 fathoms. On coasts in all parts of the world, where
sediment is accumulating, something of this kind may be observed; the
banks shelve very gently far out to sea, and then terminate abruptly.
The form and composition of the banks standing in the middle parts of
the W. Indian Sea, clearly show that their origin must be chiefly
attributed to the accumulation of sediment; and the only obvious
explanation of their isolated position is the presence of a nucleus,
round which the currents have collected fine drift matter. Any one who
will compare the character of the bank surrounding the hilly island of
Old Providence, with those banks in its neighbourhood which stand
isolated, will scarcely doubt that they surround submerged mountains.
We are led to the same conclusion by examining the bank called Thunder
Knoll, which is separated from the Great Mosquito Bank by a channel
only seven miles wide, and 145 fathoms deep. There cannot be any doubt
that the Mosquito Bank has been formed by the accumulation of sediment
round the promontory of the same name; and Thunder Knoll resembles the
Mosquito Bank, in the state of its surface submerged twenty fathoms, in
the inclinations of its sides, in composition, and in every other
respect. I may observe, although the remark is here irrelevant, that
geologists should be cautious in concluding that all the outlyers of
any formation have once been connected together, for we here see that
deposits, doubtless of exactly the same nature, may be deposited with
large valley-like spaces between them.
Linear strips of coral-reefs and small knolls project from many of the
isolated, as well as coast-banks; sometimes they occur quite
irregularly placed, as on the Mosquito Bank, but more generally they
form crescents on the windward side, situated some little distance
within the outer edge of the banks:—thus on the Serranilla Bank they
form an interrupted chain which ranges between two and three miles
within the windward margin: generally they occur, as on Roncador,
Courtown, and Anegada Banks, nearer the line of deep water. Their
occurrence on the windward side is conformable to the general rule, of
the efficient kinds of corals flourishing best where most exposed; but
their position some way within the line of deep water I cannot explain,
without it be, that a depth somewhat less than that close to the outer
margin of the banks, is most favourable to their growth. Where the
corals have formed a nearly continuous rim, close to the windward edge
of a bank some fathoms submerged, the reef closely resembles an atoll;
but if the bank surrounds an island (as in the case of Old Providence),
the reef resembles an encircling barrier-reef. I should undoubtedly
have classed some of these fringed banks as imperfect atolls, or
barrier-reefs, if the sedimentary nature of their foundations had not
been evident from the presence of other neighbouring banks, of similar
forms and of similar composition, but without the crescent-like
marginal reef: in the third chapter, I observed that probably some
atoll-like reefs did exist, which had originated in the manner here
supposed.
Proofs of elevation within recent tertiary periods abound, as referred
to in the sixth chapter, over nearly the whole area of the West Indies.
Hence it is easy to understand the origin of the low land on the
coasts, where sediment is now accumulating; for instance on the
northern part of Yucutan, and on the N.E. part of Mosquito, where the
land is low, and where extensive banks appear to be in progressive
formation. Hence, also, the origin of the Great Bahama Banks, which are
bordered on their western and southern edges by very narrow, long,
singularly shaped islands, formed of sand, shells, and coral-rock, and
some of them about a hundred feet in height, is easily explained by the
elevation of banks fringed on their windward (western and southern)
sides by coral-reefs. On this view, however, we must suppose either
that the chief part of the surfaces of the great Bahama sandbanks were
all originally deeply submerged, and were brought up to their present
level by the same elevatory action, which formed the linear islands; or
that during the elevation of the banks, the superficial currents and
swell of the waves continued wearing them down and keeping them at a
nearly uniform level: the level is not quite uniform; for, in
proceeding from the N.W. end of the Bahama group towards the S.E. end,
the depth of the banks increases, and the area of land decreases, in a
very gradual and remarkable manner. The latter view, namely, that these
banks have been worn down by the currents and swell during their
elevation, seems to me the most probable one. It is, also, I believe,
applicable to many banks, situated in widely distant parts of the West
Indian Sea, which are wholly submerged; for, on any other view, we must
suppose, that the elevatory forces have acted with astonishing
uniformity.
The shores of the Gulf of Mexico, for the space of many hundred miles,
is formed by a chain of lagoons, from one to twenty miles in breadth
(“Columbian Navigator,” p. 178, etc.), containing either fresh or salt
water, and separated from the sea by linear strips of sand. Great
spaces of the shores of Southern Brazil,[4] and of the United States
from Long Island (as observed by Professor Rogers) to Florida have the
same character. Professor Rogers, in his “Report to the British
Association” (vol. iii, p. 13), speculates on the origin of these low,
sandy, linear islets; he states that the layers of which they are
composed are too homogeneous, and contain too large a proportion of
shells, to permit the common supposition of their formation being
simply due to matter thrown up, where it now lies, by the surf: he
considers these islands as upheaved bars or shoals, which were
deposited in lines where opposed currents met. It is evident that these
islands and spits of sand parallel to the coast, and separated from it
by shallow lagoons, have no necessary connection with coral-formations.
But in Southern Florida, from the accounts I have received from persons
who have resided there, the upraised islands seem to be formed of
strata, containing a good deal of coral, and they are extensively
fringed by living reefs; the channels within these islands are in some
places between two and three miles wide, and five or six fathoms deep,
though generally[5] they are less in depth than width. After having
seen how frequently banks of sediment in the West Indian Sea are
fringed by reefs, we can readily conceive that bars of sediment might
be greatly aided in their formation along a line of coast, by the
growth of corals; and such bars would, in that case, have a deceptive
resemblance with true barrier-reefs.
[4] In the _London and Edinburgh Philosophical Journal,_ 1841, p. 257,
I have described a singular bar of sandstone lying parallel to the
coast off Pernambuco in Brazil, which probably is an analogous
formation.
[5] In the ordinary sea-charts, no lagoons appear on the coast of
Florida, north of 26°; but Major Whiting (_Silliman’s Journal_, vol.
xxxv, p. 54) says that many are formed by sand thrown up along the
whole line of coast from St. Augustine’s to Jupiter Inlet.
Having now endeavoured to remove some sources of doubt in classifying
the reefs of the West Indies, I will give my authorities for colouring
such portions of the coast as I have thought myself warranted in doing.
Captain Bird Allen informs me, that most of the islands on the _Bahama
Banks_ are fringed, especially on their windward sides, with living
reefs; and hence I have coloured those, which are thus represented in
Captain Owen’s late chart, red. The same officer informs me, that the
islands along the southern part of _Florida_ are similarly fringed;
coloured red. CUBA: Proceeding along the northern coast, at the
distance of forty miles from the extreme S.E. point, the shores are
fringed by reefs, which extend westward for a space of 160 miles, with
only a few breaks. Parts of these reefs are represented in the plans of
the harbours on this coast by Captain Owen; and an excellent
description is given of them by Mr. Taylor (Loudon’s “Mag. of Nat.
Hist.,” vol. ix, p. 449); he states that they enclosed a space called
the “_baxo_,” from half to three-quarters of a mile in width, with a
sandy bottom, and a little coral. In most parts people can wade, at low
water, to the reef; but in some parts the depth is between two and
three fathoms. Close outside the reef, the depth is between six and
seven fathoms; these well-characterised fringing-reefs are coloured
red. Westward of longitude 77° 30′, on the northern side of Cuba, a
great bank commences, which extends along the coast for nearly four
degrees of longitude. In the place of its commencement, in its
structure, and in the “_cays_,” or low islands on its edge, there is a
marked correspondence (as observed by Humboldt, “Pers. Narr.,” vol.
vii, p. 88) between it and the Great Bahama and Sal Banks, which lie
directly in front. Hence one is led to attribute the same origin to
both these sets of banks; namely, the accumulation of sediment,
conjoined with an elevatory movement, and the growth of
coral on their outward edges; those parts which appear fringed by
living reefs are coloured red. Westward of these banks, there is a
portion of coast apparently without reefs, except in the harbours, the
shores of which seem in the published plans to be fringed. The
_Colorado Shoals_ (see Captain Owen’s charts), and the low land at the
western end of Cuba, correspond as closely in relative position and
structure to the banks at the extreme point of Florida, as the banks
above described on the north side of Cuba, do to the Bahamas, the depth
within the islets and reefs on the outer edge of the _Colorados_, is
generally between two and three fathoms, increasing to twelve fathoms
in the southern part, where the bank becomes nearly open, without
islets or coral-reefs; the portions which are fringed are coloured red.
The southern shore of Cuba is deeply concave, and the included space is
filled up with mud and sandbanks, low islands and coral-reefs. Between
the mountainous _Isle of Pines_ and the southern shore of Cuba, the
general depth is only between two and three fathoms; and in this part
small islands, formed of fragmentary rock and broken madrepores
(Humboldt, “Pers. Narr.,” vol. vii, pp. 51, 86 to 90, 291, 309, 320),
rise abruptly, and just reach the surface of the sea. From some
expressions used in the “Columbian Navigator” (vol. i, pt ii, p. 94),
it appears that considerable spaces along the outer coast of Southern
Cuba are bounded by cliffs of coral-rock, formed probably by the
upheaval of coral-reefs and sandbanks. The charts represent the
southern part of the Isle of Pines as fringed by reefs, which the
“Columb. Navig.” says extend some way from the coast, but have only
from nine to twelve feet water on them; these are coloured red.—I have
not been able to procure any detailed description of the large groups
of banks and “cays” further eastward on the southern side of Cuba;
within them there is a large expanse, with a muddy bottom, from eight
to twelve fathoms deep; although some parts of this line of coast are
represented in the general charts of the West Indies, as fringed, I
have not thought it prudent to colour them. The remaining portion of
the south coast of Cuba appears to be without coral-reefs.
YUCUTAN.—The N.E. part of the promontory appears in Captain Owen’s
charts to be fringed; coloured red. The eastern coast, from 20° to 18°
is fringed. South of lat. 18°, there commences the most remarkable reef
in the West Indies: it is about one hundred and thirty miles in length,
ranging in a N. and S. line, at an average distance of fifteen miles
from the coast. The islets on it are all low, as I have been informed
by Captain B. Allen; the water deepens suddenly on the outside of the
reef, but not more abruptly than off many of the sedimentary banks:
within its southern extremity (off _Honduras_) the depth is twenty-five
fathoms; but in the more northern parts, the depth soon increases to
ten fathoms, and within the northernmost part, for a space of twenty
miles, the depth is only from one to two fathoms. In most of these
respects we have the characteristics of a barrier-reef; nevertheless,
from observing, first, that the channel within the reef is a
continuation of a great irregular bay, which penetrates the mainland to
the depth of fifty miles; and secondly, that considerable spaces of
this barrier-like reef are
described in the charts (for instance, in lat. 16° 45′ and 16° 12′) as
formed of pure sand; and thirdly, from knowing that sediment is
accumulating in many parts of the West Indies in banks parallel to the
shore; I have not ventured to colour this reef as a barrier, without
further evidence that it has really been formed by the growth of
corals, and that it is not merely in parts a spit of sand, and in other
parts a worn down promontory, partially coated and fringed by reefs; I
lean, however, to the probability of its being a barrier-reef, produced
by subsidence. To add to my doubts, immediately on the outside of this
barrier-like reef, _Turneffe, Lighthouse_, and _Glover_ reefs are
situated, and these reefs have so completely the form of atolls, that
if they had occurred in the Pacific, I should not have hesitated about
colouring them blue. _Turneffe Reef_ seems almost entirely filled up
with low mud islets; and the depth within the other two reefs is only
from one to three fathoms. From this circumstance and from their
similarity in form, structure, and relative position, both to the bank
called _Northern Triangles_, on which there is an islet between seventy
and eighty feet, and to _Cozumel_ Island, the level surface of which is
likewise between seventy and eighty feet in height, I consider it more
probable that the three foregoing banks are the worn down bases of
upheaved shoals, fringed with corals, than that they are true atolls,
wholly produced by the growth of coral during subsidence; left
uncoloured.
In front of the eastern _Mosquito_ coast, there are between lat. 12°
and 16° some extensive banks (already mentioned, p. 148), with high
islands rising from their centres; and there are other banks wholly
submerged, both of which kinds of banks are bordered, near their
windward margins, by crescent-shaped coral-reefs. But it can hardly be
doubted, as was observed in the preliminary remarks, that these banks
owe their origin, like the great bank extending from the Mosquito
promontory, almost entirely to the accumulation of sediment, and not to
the growth of corals; hence I have not coloured them.
_Cayman Island:_ this island appears in the charts to be fringed; and
Captain B. Allen informs me that the reefs extend about a mile from the
shore, and have only from five to twelve feet water within them;
coloured red.—_Jamaica:_ judging from the charts, about fifteen miles
of the S.E. extremity, and about twice that length on the S.W.
extremity, and some portions on the S. side near Kingston and Port
Royal, are regularly fringed, and therefore are coloured red. From the
plans of some harbours on the N. side of Jamaica, parts of the coast
appear to be fringed; but as these are not represented in the charts of
the whole island, I have not coloured them.—_St. Domingo:_ I have not
been able to obtain sufficient information, either from plans of the
harbours, or from general charts, to enable me to colour any part of
the coast, except sixty miles from Port de Plata westward, which seems
very regularly fringed; many other parts, however, of the coast are
probably fringed, especially towards the eastern end of the
island.—_Puerto Rico:_ considerable portions of the southern, western,
and eastern coasts, and some parts of the northern coast, appear in the
charts to be fringed; coloured red.—Some miles in length of the
southern side of the Island of _St. Thomas_ is fringed; most of the
_Virgin Gorda_ Islands, as I am informed by Mr. Schomburgk, are
fringed; the shores of _Anegada_, as well as the bank on which it
stands, are likewise fringed; these islands have been coloured red. The
greater part of the southern side of _Santa Cruz_ appears in the Danish
survey to be fringed (see also Prof. Hovey’s account of this island, in
_Silliman’s Journal_, vol. xxxv, p. 74); the reefs extend along the
shore for a considerable space, and project rather more than a mile;
the depth within the reef is three fathoms; coloured red.—The _
Antilles_, as remarked by Von Buch (“Descrip. Iles Canaries,” p. 494),
may be divided into two linear groups, the western row being volcanic,
and the eastern of modern calcareous origin; my information is very
defective on the whole group. Of the eastern islands, _Barbuda_ and the
western coasts of _Antigua_ and _Mariagalante_ appear to be fringed:
this is also the case with _Barbadoes_, as I have been informed by a
resident; these islands are coloured red. On the shores of the Western
Antilles, of volcanic origin, very few coral-reefs appear to exist. The
island of _Martinique_, of which there are beautifully executed French
charts, on a very large scale, alone presents any appearance worthy of
special notice. The south-western, southern, and eastern coasts,
together forming about half the circumference of the island, are
skirted by very irregular banks, projecting generally rather less than
a mile from the shore, and lying from two to five fathoms submerged. In
front of almost every valley, they are breached by narrow, crooked,
steep-sided passages. The French engineers ascertained by boring, that
these submerged banks consisted of madreporitic rocks, which were
covered in many parts by thin layers of mud or sand. From this fact,
and especially from the structure of the narrow breaches, I think there
can be little doubt that these banks once formed living reefs, which
fringed the shores of the island, and like other reefs probably reached
the surface. From some of these submerged banks reefs of living coral
rise abruptly, either in small detached patches, or in lines parallel
to, but some way within the outer edges of the banks on which they are
based. Besides the above banks which skirt the shores of the island,
there is on the eastern side a range of linear banks, similarly
constituted, twenty miles in length, extending parallel to the coast
line, and separated from it by a space between two and four miles in
width, and from five to fifteen fathoms in depth. From this range of
detached banks, some linear reefs of living coral likewise rise
abruptly; and if they had been of greater length (for they do not front
more than a sixth part of the circumference of the island), they would
necessarily from their position have been coloured as barrier-reefs; as
the case stands they are left uncoloured. I suspect that after a small
amount of subsidence, the corals were killed by sand and mud being
deposited on them, and the reefs being thus prevented from growing
upwards, the banks of madreporitic rock were left in their present
submerged condition.
THE BERMUDA ISLANDS have been carefully described by Lieutenant Nelson,
in an excellent Memoir in the “Geological Transactions” (vol. v, part
i, p. 103). In the form of the bank or reef, on one side of which
the islands stand, there is a close general resemblance to an atoll;
but in the following respects there is a considerable
difference,—first, in the margin of the reef not forming (as I have
been informed by Mr. Chaffers, R.N.) a flat, solid surface, laid bare
at low water, and regularly bounding the internal space of shallow
water or lagoon; secondly, in the border of gradually shoaling water,
nearly a mile and a half in width, which surrounds the entire outside
of the reef (as is laid down in Captain Hurd’s chart); and thirdly, in
the size, height, and extraordinary form of the islands, which present
little resemblance to the long, narrow, simple islets, seldom exceeding
half a mile in breadth, which surmount the annular reefs of almost all
the atolls in the Indian and Pacific Oceans. Moreover, there are
evident proofs (Nelson, _ Ibid_., p. 118), that islands similar to the
existing ones, formerly extended over other parts of the reef. It
would, I believe, be difficult to find a true atoll with land exceeding
thirty feet in height; whereas, Mr. Nelson estimates the highest point
of the Bermuda Islands to be 260 feet; if, however, Mr. Nelson’s view,
that the whole of the land consists of sand drifted by the winds, and
agglutinated together, were proved correct, this difference would be
immaterial; but, from his own account (p. 118), there occur in one
place, five or six layers of red earth, interstratified with the
ordinary calcareous rock, and including stones too heavy for the wind
to have moved, without having at the same time utterly dispersed every
grain of the accompanying drifted matter. Mr. Nelson attributes the
origin of these several layers, with their embedded stones, to as many
violent catastrophes; but further investigation in such cases has
generally succeeded in explaining phenomena of this kind by ordinary
and simpler means. Finally, I may remark, that these islands have a
considerable resemblance in shape to Barbuda in the West Indies, and to
Pemba on the eastern coast of Africa, which latter island is about two
hundred feet in height, and consists of coral-rock. I believe that the
Bermuda Islands, from being fringed by living reefs, ought to have been
coloured red; but I have left them uncoloured, on account of their
general resemblance in external form to a lagoon-island or atoll.
GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
CRITICAL INTRODUCTION.
The preparation of the series of works published under the general
title “Geology of the Voyage of the _Beagle_” occupied a great part of
Darwin’s time during the ten years that followed his return to England.
The second volume of the series, entitled “Geological Observations on
Volcanic Islands, with Brief Notices on the Geology of Australia and
the Cape of Good Hope,” made its appearance in 1844. The materials for
this volume were collected in part during the outward voyage, when the
_Beagle_ called at St. Jago in the Cape de Verde Islands, and St.
Paul’s Rocks, and at Fernando Noronha, but mainly during the homeward
cruise; then it was that the Galapagos Islands were surveyed, the Low
Archipelago passed through, and Tahiti visited; after making calls at
the Bay of Islands, in New Zealand, and also at Sydney, Hobart Town and
King George’s Sound in Australia, the _Beagle_ sailed across the Indian
Ocean to the little group of the Keeling or Cocos Islands, which Darwin
has rendered famous by his observations, and thence to Mauritius;
calling at the Cape of Good Hope on her way, the ship then proceeded
successively to St. Helena and Ascension, and revisited the Cape de
Verde Islands before finally reaching England.
Although Darwin was thus able to gratify his curiosity by visits to a
great number of very interesting volcanic districts, the voyage opened
for him with a bitter disappointment. He had been reading Humboldt’s
“Personal Narrative” during his last year’s residence in Cambridge, and
had copied out from it long passages about Teneriffe. He was actually
making inquiries as to the best means of visiting that island, when the
offer was made to him to accompany Captain Fitzroy in the _Beagle._ His
friend Henslow too, on parting with him, had given him the advice to
procure and read the recently published first volume of the
“Principles of Geology,” though he warned him against accepting the
views advocated by its author. During the time the _ Beagle_ was
beating backwards and forwards when the voyage commenced, Darwin,
although hardly ever able to leave his berth, was employing all the
opportunities which the terrible sea-sickness left him, in studying
Humboldt and Lyell. We may therefore form an idea of his feelings when,
on the ship reaching Santa Cruz, and the Peak of Teneriffe making its
appearance among the clouds, they were suddenly informed that an
outbreak of cholera would prevent any landing!
Ample compensation for this disappointment was found, however, when the
ship reached Porta Praya in St. Jago, the largest of the Cape de Verde
Islands. Here he spent three most delightful weeks, and really
commenced his work as a geologist and naturalist. Writing to his father
he says, “Geologising in a volcanic country is most delightful; besides
the interest attached to itself, it leads you into most beautiful and
retired spots. Nobody but a person fond of Natural History can imagine
the pleasure of strolling under cocoa-nuts in a thicket of bananas and
coffee-plants, and an endless number of wild flowers. And this island,
that has given me so much instruction and delight, is reckoned the most
uninteresting place that we perhaps shall touch at during our voyage.
It certainly is generally very barren, but the valleys are more
exquisitely beautiful, from the very contrast. It is utterly useless to
say anything about the scenery; it would be as profitable to explain to
a blind man colours, as to a person who has not been out of Europe, the
total dissimilarity of a tropical view. Whenever I enjoy anything, I
always look forward to writing it down, either in my log-book (which
increases in bulk), or in a letter; so you must excuse raptures, and
those raptures badly expressed. I find my collections are increasing
wonderfully, and from Rio I think I shall be obliged to send a cargo
home.”
The indelible impression made on Darwin’s mind by this first visit to a
volcanic island, is borne witness to by a remarkable passage in the
“Autobiography” written by him in 1876. “The geology of St. Jago is
very striking, yet simple; a stream of lava formerly flowed over the
bed of the sea, formed of triturated recent shells and corals, which it
has baked into a hard white rock. Since then the whole island has been
upheaved. But the line of white rock revealed to me a new and important
fact, namely that there had been afterwards subsidence round the
craters which had
since been in action, and had poured forth lava. It then first dawned
on me that I might perhaps write a book on the geology of the various
countries visited, and this made me thrill with delight. That was a
memorable hour to me, and how distinctly I can call to mind the low
cliff of lava beneath which I rested, with the sun glaring hot, a few
strange desert plants growing near and with living corals in the tidal
pools at my feet.”
Only five years before, when listening to poor Professor Jameson’s
lectures on the effete Wernerianism, which at that time did duty for
geological teaching, Darwin had found them “incredibly dull,” and he
declared that “the sole effect they produced on me was a determination
never so long as I lived to read a book on Geology, or in any way to
study the science.”
What a contrast we find in the expressions which he makes use of in
referring to Geological Science, in his letters written home from the
_Beagle_! After alluding to the delight of collecting and studying
marine animals, he exclaims, “But Geology carries the day!” Writing to
Henslow he says, “I am quite charmed with Geology, but, like the wise
animal between two bundles of hay, I do not know which to like best;
the old crystalline group of rocks, or the softer and more
fossiliferous beds.” And just as the long voyage is about to come to a
close he again writes, “I find in Geology a never-failing interest; as
it has been remarked, it creates the same grand ideas respecting this
world which Astronomy does for the Universe.” In this passage Darwin
doubtless refers to a remark of Sir John Herschel’s in his admirable
“Preliminary Discourse on the Study of Natural Philosophy,”—a book
which exercised a most remarkable and beneficial influence on the mind
of the young naturalist.
If there cannot be any doubt as to the strong predilection in Darwin’s
mind for geological studies, both during and after the memorable
voyage, there is equally little difficulty in perceiving the school of
geological thought which, in spite of the warnings of Sedgwick and
Henslow, had obtained complete ascendancy over his mind. He writes in
1876: “The very first place which I examined, namely St. Jago in the
Cape de Verde Islands, showed me clearly the wonderful superiority of
Lyell’s manner of treating Geology, compared with that of any other
author, whose works I had with me, or ever afterwards read.” And again,
“The science of Geology is enormously indebted to Lyell—more so, as I
believe, than to any other man who ever lived . . . I am proud to
remember that the first place, namely, St. Jago, in the Cape de
Verde Archipelago, in which I geologised, convinced me of the infinite
superiority of Lyell’s views over those advocated in any other work
known to me.”
The passages I have cited will serve to show the spirit in which Darwin
entered upon his geological studies, and the perusal of the following
pages will furnish abundant proofs of the enthusiasm, acumen, and
caution with which his researches were pursued.
Large collections of rocks and minerals were made by Darwin during his
researches, and sent home to Cambridge, to be kept under the care of
his faithful friend Henslow. After visiting his relations and friends,
Darwin’s first care on his return to England was to unpack and examine
these collections. He accordingly, at the end of 1836, took lodgings
for three months in Fitzwilliam Street, Cambridge, so as to be near
Henslow; and in studying and determining his geological specimens
received much valuable aid from the eminent crystallographer and
mineralogist, Professor William Hallows Miller.
The actual writing of the volume upon volcanic islands was not
commenced till 1843, when Darwin had settled in the spot which became
his home for the rest of his life—the famous house at Down, in Kent.
Writing to his friend Mr. Fox, on March 28th, 1843, he says, “I am very
slowly progressing with a volume, or rather pamphlet, on the volcanic
islands which we visited: I manage only a couple of hours per day, and
that not very regularly. It is uphill work writing books, which cost
money in publishing, and which are not read even by geologists.”
The work occupied Darwin during the whole of the year 1843, and was
issued in the spring of the following year, the actual time engaged in
preparing it being recorded in his diary as “from the summer of 1842 to
January 1844;” but the author does not appear to have been by any means
satisfied with the result when the book was finished. He wrote to
Lyell, “You have pleased me much by saying that you intend looking
through my ‘Volcanic Islands;’ it cost me eighteen months!!! and I have
heard of very few who have read it. Now I shall feel, whatever little
(and little it is) there is confirmatory of old work, or new, will work
its effect and not be lost.” To Sir Joseph Hooker he wrote, “I have
just finished a little volume on the volcanic islands which we visited.
I do not know how far you care for dry simple geology, but I hope you
will let me send you a copy.”
Every geologist knows how full of interest and suggestiveness is
this book of Darwin’s on volcanic islands. Probably the scant
satisfaction which its author seemed to find in it may be traced to the
effect of a contrast which he felt between the memory of glowing
delights he had experienced when, hammer in hand, he roamed over new
and interesting scenes, and the slow, laborious, and less congenial
task of re-writing and arranging his notes in book-form.
In 1874, in writing an account of the ancient volcanoes of the
Hebrides, I had frequent occasion to quote Mr. Darwin’s observations on
the Atlantic volcanoes, in illustration of the phenomena exhibited by
the relics of still older volcanoes in our own islands. Darwin, in
writing to his old friend Sir Charles Lyell upon the subject, says, “I
was not a little pleased to see my volcanic book quoted, for I thought
it was completely dead and forgotten.”
Two years later the original publishers of this book and of that on
South America proposed to re-issue them. Darwin at first hesitated, for
he seemed to think there could be little of abiding interest in them;
he consulted me upon the subject in one of the conversations which I
used to have with him at that time, and I strongly urged upon him the
reprint of the works. I was much gratified when he gave way upon the
point, and consented to their appearing just as originally issued. In
his preface he says, “Owing to the great progress which Geology has
made in recent times, my views on some few points may be somewhat
antiquated, but I have thought it best to leave them as they originally
appeared.”
It may be interesting to indicate, as briefly as possible, the chief
geological problem upon which the publication of Darwin’s “Volcanic
Islands” threw new and important light. The merit of the work consisted
in supplying interesting observations, which in some cases have proved
of crucial value in exploding prevalent fallacies; in calling attention
to phenomena and considerations that had been quite overlooked by
geologists, but have since exercised an important influence in moulding
geological speculation; and lastly in showing the importance which
attaches to small and seemingly insignificant causes, some of which
afford a key to the explanation of very curious geological problems.
Visiting as he did the districts in which Von Buch and others had found
what they thought to be evidence of the truth of “Elevation-craters,”
Darwin was able to show that the facts were capable of a totally
different interpretation. The views originally put forward by the old
German geologist and traveller, and almost
universally accepted by his countrymen, had met with much support from
Elie de Beaumont and Dufrenoy, the leaders of geological thought in
France. They were, however, stoutly opposed by Scrope and Lyell in this
country, and by Constant Prevost and Virlet on the other side of the
channel. Darwin, in the work before us, shows how little ground there
is for the assumption that the great ring-craters of the Atlantic
islands have originated in gigantic blisters of the earth’s surface
which, opening at the top, have given origin to the craters. Admitting
the influence of the injection of lava into the structure of the
volcanic cones, in increasing their bulk and elevation, he shows that,
in the main, the volcanoes are built up by repeated ejections causing
an accumulation of materials around the vent.
While, however, agreeing on the whole with Scrope and Lyell, as to the
explosive origin of ordinary volcanic craters, Darwin clearly saw that,
in some cases, great craters might be formed or enlarged, by the
subsidence of the floors after eruptions. The importance of this
agency, to which too little attention has been directed by geologists,
has recently been shown by Professor Dana, in his admirable work on
Kilauea and the other great volcanoes of the Hawaiian Archipelago.
The effects of subsidence at a volcanic centre in producing a downward
dip of the strata around it, was first pointed out by Darwin, as the
result of his earliest work in the Cape de Verde Islands. Striking
illustrations of the same principle have since been pointed out by M.
Robert and others in Iceland, by Mr. Heaphy in New Zealand, and by
myself in the Western Isles of Scotland.
Darwin again and again called attention to the evidence that volcanic
vents exhibit relations to one another which can only be explained by
assuming the existence of lines of fissure in the earth’s crust, along
which the lavas have made their way to the surface. But he, at the same
time, clearly saw that there was no evidence of the occurrence of great
deluges of lava along such fissures; he showed how the most remarkable
plateaux, composed of successive lava sheets, might be built up by
repeated and moderate ejections from numerous isolated vents; and he
expressly insists upon the rapidity with which the cinder-cones around
the orifices of ejection and the evidences of successive outflows of
lava would be obliterated by denudation.
One of the most striking parts of the book is that in which he deals
with the effects of denudation in producing “basal wrecks”
or worn down stumps of volcanoes. He was enabled to examine a series of
cases in which could be traced every gradation, from perfect volcanic
cones down to the solidified plugs which had consolidated in the vents
from which ejections had taken place. Darwin’s observations on these
points have been of the greatest value and assistance to all who have
essayed to study the effects of volcanic action during earlier periods
of the earth’s history. Like Lyell, he was firmly persuaded of the
continuity of geological history, and ever delighted in finding
indications, in the present order of nature, that the phenomena of the
past could be accounted for by means of causes which are still in
operation. Lyell’s last work in the field was carried on about his home
in Forfarshire, and only a few months before his death he wrote to
Darwin: “All the work which I have done has confirmed me in the belief
that the only difference between Palæozoic and recent volcanic rocks is
no more than we must allow for, by the enormous time to which the
products of the oldest volcanoes have been subjected to chemical
changes.”
Darwin was greatly impressed, as the result of his studies of volcanic
phenomena, followed by an examination of the great granite-masses of
the Andes, with the relations between the so-called Plutonic rocks and
those of undoubtedly volcanic origin. It was indeed a fortunate
circumstance, that after studying some excellent examples of recent
volcanic rocks, he proceeded to examine in South America many fine
illustrations of the older igneous rock-masses, and especially of the
most highly crystalline types of the same, and then on his way home had
opportunities of reviving the impression made upon him by the fresh and
unaltered volcanic rocks. Some of the general considerations suggested
by these observations were discussed in a paper read by him before the
Geological Society, on March 7th, 1838, under the title “On the
Connection of Certain Volcanic Phenomena, and On the Formation of
Mountain-chains, and the Effect of Continental Elevations.” The exact
bearing of these two classes of facts upon one another are more fully
discussed in his book on South American geology.
The proofs of recent elevation around many of the volcanic islands led
Darwin to conclude that volcanic areas were, as a rule, regions in
which upward movements were taking place, and he was naturally led to
contrast them with the areas in which, as he showed, the occurrence of
atolls, encircling reefs, and barrier-reefs afford indication of
subsidence. In this way he was able to
map out the oceanic areas in different zones, along which opposite
kinds of movement were taking place. His conclusions on this subject
were full of novelty and suggestiveness.
Very clearly did Darwin recognise the importance of the fact that most
of the oceanic islands appear to be of volcanic origin, though he was
careful to point out the remarkable exceptions which somewhat
invalidate the generalisation. In his “Origin of Species” he has
elaborated the idea and suggested the theory of the permanence of
ocean-basins, a suggestion which has been adopted and pushed farther by
subsequent authors, than we think its originator would have approved.
His caution and fairness of mind on this and similar speculative
questions was well-known to all who were in the habit of discussing
them with him.
Some years before the voyage of the _Beagle,_ Mr. Poulett Scrope had
pointed out the remarkable analogies that exist between certain igneous
rocks of banded structure, as seen in the Ponza Islands, and the
foliated crystalline schists. It does not appear that Darwin was
acquainted with this remarkable memoir, but quite independently he
called attention to the same phenomena when he came to study some very
similar rocks which occur in the island of Ascension. Coming fresh from
the study of the great masses of crystalline schist in the South
American continent, he was struck by the circumstance that in the
undoubtedly igneous rocks of Ascension we find a similar separation of
the constituent minerals along parallel “folia.” These observations led
Darwin to the same conclusion as that arrived at some time before by
Scrope—namely that when crystallisation takes place in rock masses
under the influence of great deforming stresses, a separation and
parallel arrangement of the constituent minerals will result. This is a
process which is now fully recognised as having been a potent factor in
the production of the metamorphic rock, and has been called by more
recent writers “dynamo-metamorphism.”
In this, and in many similar discussions, in which exact mineralogical
knowledge was required, it is remarkable how successful Darwin was in
making out the true facts with regard to the rocks he studied by the
simple aid of a penknife and pocket-lens, supplemented by a few
chemical tests and the constant use of the blowpipe. Since his day, the
method of study of rocks by thin sections under the microscope has been
devised, and has become a most efficient aid in all petrographical
inquiries. During the voyage of H.M.S. _Challenger,_ many of the
islands studied by
Darwin have been revisited and their rocks collected. The results of
their study by one of the greatest masters of the science of
micropetrography—Professor Renard of Brussels—have been recently
published in one of the volumes of “Reports on the _Challenger_
Expedition.” While much that is new and valuable has been contributed
to geological science by these more recent investigations, and many
changes have been made in nomenclature and other points of detail, it
is interesting to find that all the chief facts described by Darwin and
his friend Professor Miller have stood the test of time and further
study, and remain as a monument of the acumen and accuracy in minute
observation of these pioneers in geological research.
JOHN W. JUDD.
Chapter I ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
Rocks of the lowest series.—A calcareous sedimentary deposit, with
recent shells, altered by the contact of superincumbent lava, its
horizontality and extent.—Subsequent volcanic eruptions, associated
with calcareous matter in an earthy and fibrous form, and often
enclosed within the separate cells of the scoriæ.—Ancient and
obliterated orifices of eruption of small size. Difficulty of tracing
over a bare plain recent streams of lava.—Inland hills of more ancient
volcanic rock.—Decomposed olivine in large masses. Feldspathic rocks
beneath the upper crystalline basaltic strata. Uniform structure and
form of the more ancient volcanic hills.—Form of the valleys near the
coast. Conglomerate now forming on the sea beach.
The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty
miles in length by about twelve in breadth. My observations, made
during two visits, were confined to the southern portion within the
distance of a few leagues from Porto Praya. The country, viewed from
the sea, presents a varied outline: smooth conical hills of a reddish
colour (like Red Hill in Fig. 1[1]) and others less regular,
flat-topped, and of a blackish colour (like A, B, C,) rise from
successive, step-formed plains of lava. At a distance, a chain of
mountains, many thousand feet in height, traverses the interior of the
island. There is no active volcano in St. Jago, and only one in the
group, namely at Fogo. The island since being inhabited has not
suffered from destructive earthquakes.
[1] The outline of the coast, the position of the villages,
streamlets, and of most of the hills in this woodcut, are copied from
the chart made on board H.M.S. _Leven._ The square-topped hills (A, B,
C, etc.) are put in merely by eye, to illustrate my description.
The lowest rocks exposed on the coast near Porto Praya, are highly
crystalline and compact; they appear to be of ancient, submarine,
volcanic origin; they are unconformably covered by a thin, irregular,
calcareous deposit, abounding with shells of a late tertiary period;
and this again is capped by a wide sheet of basaltic lava, which has
flowed in successive streams from the interior of the island, between
the square-topped hills marked A, B, C, etc. Still more recent streams
of lava have been erupted from the scattered cones, such as Red and
Signal Post Hills. The upper strata of the square-topped hills are
intimately related in mineralogical composition, and in other respects,
with the lowest series of the coast-rocks, with which they seem to be
continuous.
[Illustration: Part of St. Jago, one of the Cape de Verde islands.]
Part of St. Jago, one of the Cape de Verde islands.
_Mineralogical description of the rocks of the lowest series._—These
rocks possess an extremely varying character; they consist of black,
brown, and grey, compact, basaltic bases, with numerous crystals of
augite, hornblende, olivine, mica, and sometimes glassy feldspar. A
common variety is almost entirely composed of crystals of augite with
olivine. Mica, it is known, seldom occurs where augite abounds; nor
probably does the present case offer a real exception, for the mica (at
least in my best characterised specimen, in which one nodule of this
mineral is nearly half an inch in length) is as perfectly rounded as a
pebble in a conglomerate, and evidently has not been crystallised in
the base, in which it is now enclosed, but has proceeded from the
fusion of some pre-existing rock. These compact lavas alternate with
tuffs, amygdaloids, and wacke, and in some places with coarse
conglomerate. Some of the argillaceous wackes are of a dark green
colour, others, pale yellowish-green, and others nearly white; I was
surprised to find that some of the latter varieties, even where
whitest, fused into a jet black enamel, whilst some of the green
varieties afforded only a pale gray bead. Numerous dikes, consisting
chiefly of highly compact augitic rocks, and of gray amygdaloidal
varieties, intersect the strata, which have in several places been
dislocated with considerable violence, and thrown into highly inclined
positions. One line of disturbance crosses the northern end of Quail
Island (an islet in the Bay of Porto Praya), and can be followed to the
mainland. These disturbances took place before the deposition of the
recent sedimentary bed; and the
surface, also, had previously been denuded to a great extent, as is
shown by many truncated dikes.
_Description of the calcareous deposit overlying the foregoing volcanic
rocks._—This stratum is very conspicuous from its white colour, and
from the extreme regularity with which it ranges in a horizontal line
for some miles along the coast. Its average height above the sea,
measured from the upper line of junction with the superincumbent
basaltic lava, is about sixty feet; and its thickness, although varying
much from the inequalities of the underlying formation, may be
estimated at about twenty feet. It consists of quite white calcareous
matter, partly composed of organic _débris_, and partly of a substance
which may be aptly compared in appearance with mortar. Fragments of
rock and pebbles are scattered throughout this bed, often forming,
especially in the lower part, a conglomerate. Many of the fragments of
rock are whitewashed with a thin coating of calcareous matter. At Quail
Island, the calcareous deposit is replaced in its lowest part by a
soft, brown, earthy tuff, full of Turritellæ; this is covered by a bed
of pebbles, passing into sandstone, and mixed with fragments of echini,
claws of crabs, and shells; the oyster-shells still adhering to the
rock on which they grew. Numerous white balls appearing like pisolitic
concretions, from the size of a walnut to that of an apple, are
embedded in this deposit; they usually have a small pebble in their
centres. Although so like concretions, a close examination convinced me
that they were Nulliporæ, retaining their proper forms, but with their
surfaces slightly abraded: these bodies (plants as they are now
generally considered to be) exhibit under a microscope of ordinary
power, no traces of organisation in their internal structure. Mr.
George R. Sowerby has been so good as to examine the shells which I
collected: there are fourteen species in a sufficiently perfect
condition for their characters to be made out with some degree of
certainty, and four which can be referred only to their genera. Of the
fourteen shells, of which a list is given in the Appendix, eleven are
recent species; one, though undescribed, is perhaps identical with a
species which I found living in the harbour of Porto Praya; the two
remaining species are unknown, and have been described by Mr. Sowerby.
Until the shells of this Archipelago and of the neighbouring coasts are
better known, it would be rash to assert that even these two latter
shells are extinct. The number of species which certainly belong to
existing kinds, although few in number, are sufficient to show that the
deposit belongs to a late tertiary period. From its mineralogical
character, from the number and size of the embedded fragments, and from
the abundance of Patellæ, and other littoral shells, it is evident that
the whole was accumulated in a shallow sea, near an ancient coast-line.
_Effects produced by the flowing of the superincumbent basaltic lava
over the calcareous deposit._—These effects are very curious. The
calcareous matter is altered to the depth of about a foot beneath the
line of junction; and a most perfect gradation can be traced, from
loosely aggregated, small, particles of shells, corallines, and
Nulliporæ, into a rock, in which not a trace of mechanical origin can
be discovered,
even with a microscope. Where the metamorphic change has been greatest,
two varieties occur. The first is a hard, compact, white, fine-grained
rock, striped with a few parallel lines of black volcanic particles,
and resembling a sandstone, but which, upon close examination, is seen
to be crystallised throughout, with the cleavages so perfect that they
can be readily measured by the reflecting goniometer. In specimens,
where the change has been less complete, when moistened and examined
under a strong lens, the most interesting gradation can be traced, some
of the rounded particles retaining their proper forms, and others
insensibly melting into the granulo-crystalline paste. The weathered
surface of this stone, as is so frequently the case with ordinary
limestones, assumes a brick-red colour.
The second metamorphosed variety is likewise a hard rock, but without
any crystalline structure. It consists of a white, opaque, compact,
calcareous stone, thickly mottled with rounded, though regular, spots
of a soft, earthy, ochraceous substance. This earthy matter is of a
pale yellowish-brown colour, and appears to be a mixture of carbonate
of lime with iron; it effervesces with acids, is infusible, but
blackens under the blowpipe, and becomes magnetic. The rounded form of
the minute patches of earthy substance, and the steps in the progress
of their perfect formation, which can be followed in a suit of
specimens, clearly show that they are due either to some power of
aggregation in the earthy particles amongst themselves, or more
probably to a strong attraction between the atoms of the carbonate of
line, and consequently to the segregation of the earthy extraneous
matter. I was much interested by this fact, because I have often seen
quartz rocks (for instance, in the Falkland Islands, and in the lower
Silurian strata of the Stiper-stones in Shropshire), mottled in a
precisely analogous manner, with little spots of a white, earthy
substance (earthy feldspar?); and these rocks, there was good reason to
suppose, had undergone the action of heat,—a view which thus receives
confirmation. This spotted structure may possibly afford some
indication in distinguishing those formations of quartz, which owe
their present structure to igneous action, from those produced by the
agency of water alone; a source of doubt, which I should think from my
own experience, that most geologists, when examining arenaceo-quartzose
districts must have experienced.
The lowest and most scoriaceous part of the lava, in rolling over the
sedimentary deposit at the bottom of the sea, has caught up large
quantities of calcareous matter, which now forms a snow-white, highly
crystalline basis to a breccia, including small pieces of black, glossy
scoriæ. A little above this, where the lime is less abundant, and the
lava more compact, numerous little balls, composed of spicula of
calcareous spar, radiating from common centres, occupy the interstices.
In one part of Quail Island, the lime has thus been crystallised by the
heat of the superincumbent lava, where it is only thirteen feet in
thickness; nor had the lava been originally thicker, and since reduced
by degradation, as could be told from the degree of cellularity of its
surface. I have already observed that the sea must have been shallow
in which the calcareous deposit was accumulated. In this case,
therefore, the carbonic acid gas has been retained under a pressure,
insignificant compared with that (a column of water, 1,708 feet in
height) originally supposed by Sir James Hall to be requisite for this
end: but since his experiments, it has been discovered that pressure
has less to do with the retention of carbonic acid gas, than the nature
of the circumjacent atmosphere; and hence, as is stated to be the case
by Mr. Faraday,[2] masses of limestone are sometimes fused and
crystallised even in common limekilns. Carbonate of lime can be heated
to almost any degree, according to Faraday, in an atmosphere of
carbonic acid gas, without being decomposed; and Gay-Lussac found that
fragments of limestone, placed in a tube and heated to a degree, not
sufficient by itself to cause their decomposition, yet immediately
evolved their carbonic acid, when a stream of common air or steam was
passed over them: Gay-Lussac attributes this to the mechanical
displacement of the nascent carbonic acid gas. The calcareous matter
beneath the lava, and especially that forming the crystalline spicula
between the interstices of the scoriæ, although heated in an atmosphere
probably composed chiefly of steam, could not have been subjected to
the effects of a passing stream; and hence it is, perhaps, that they
have retained their carbonic acid, under a small amount of pressure.
[2] I am much indebted to Mr. E. W. Brayley in having given me the
following references to papers on this subject: Faraday in the
_Edinburgh New Philosophical Journal_, vol. xv, p. 398; Gay-Lussac in
_Annales de Chem. et Phys.,_ tome lxiii, p. 219, translated in the
_London and Edinburgh Philosophical Magazine,_ vol. x, p. 496.
The fragments of scoriæ, embedded in the crystalline calcareous basis,
are of a jet black colour, with a glossy fracture like pitchstone.
Their surfaces, however, are coated with a layer of a reddish-orange,
translucent substance, which can easily be scratched with a knife;
hence they appear as if overlaid by a thin layer of rosin. Some of the
smaller fragments are partially changed throughout into this substance:
a change which appears quite different from ordinary decomposition. At
the Galapagos Archipelago (as will be described in a future chapter),
great beds are formed of volcanic ashes and particles of scoriæ, which
have undergone a closely similar change.
_The extent and horizontality of the calcareous stratum._—The upper
line of surface of the calcareous stratum, which is so conspicuous from
being quite white and so nearly horizontal, ranges for miles along the
coast, at the height of about sixty feet above the sea. The sheet of
basalt, by which it is capped, is on an average eighty feet in
thickness. Westward of Porto Praya beyond Red Hill, the white stratum
with the superincumbent basalt is covered up by more recent streams.
Northward of Signal Post Hill, I could follow it with my eye, trending
away for several miles along the sea cliffs. The distance thus observed
is about seven miles; but I cannot doubt from its regularity that it
extends much farther. In some ravines at right angles to the coast, it
is seen gently dipping towards the sea, probably with the same
inclination
as when deposited round the ancient shores of the island. I found only
one inland section, namely, at the base of the hill marked A, where, at
the height of some hundred feet, this bed was exposed; it here rested
on the usual compact augitic rock associated with wacke, and was
covered by the widespread sheet of modern basaltic lava. Some
exceptions occur to the horizontality of the white stratum: at Quail
Island, its upper surface is only forty feet above the level of the
sea; here also the capping of lava is only between twelve and fifteen
feet in thickness; on the other hand, at the north-east side of Porto
Praya harbour, the calcareous stratum, as well as the rock on which it
rests, attain a height above the average level: the inequality of level
in these two cases is not, as I believe, owing to unequal elevation,
but to original irregularities at the bottom of the sea. Of this fact,
at Quail Island, there was clear evidence in the calcareous deposit
being in one part of much greater than the average thickness, and in
another part being entirely absent; in this latter case, the modern
basaltic lavas rested directly on those of more ancient origin.
Fig. 2
[Illustration: Signal Post Hill]
SIGNAL POST HILL
A—Ancient volcanic rocks. B—Calcareous stratum. C—Upper balastic lava.
Under Signal Post Hill, the white stratum dips into the sea in a
remarkable manner. This hill is conical, 450 feet in height, and
retains some traces of having had a crateriform structure; it is
composed chiefly of matter erupted posteriorly to the elevation of the
great basaltic plain, but partly of lava of apparently submarine origin
and of considerable antiquity. The surrounding plain, as well as the
eastern flank of this hill, has been worn into steep precipices,
overhanging the sea. In these precipices, the white calcareous stratum
may be seen, at the height of about seventy feet above the beach,
running for some miles both northward and southward of the hill, in a
line appearing to be perfectly horizontal; but for a space of a quarter
of a mile directly under the hill, it dips into the sea and disappears.
On the south side the dip is gradual, on the north side it is more
abrupt, as is shown in Fig. 2. As neither the calcareous stratum, nor
the superincumbent basaltic lava (as far as the latter can be
distinguished from the more modern ejections), appears to thicken as it
dips, I infer that these strata were not originally accumulated in a
trough, the centre of which afterwards became a point of eruption; but
that they have subsequently been disturbed and bent. We may suppose
either that Signal Post Hill subsided after its elevation with the
surrounding country, or that it never was uplifted to the same height
with it. This latter seems to me the most probable alternative, for
during the slow and equable elevation of this portion of the island,
the subterranean motive
power, from expending part of its force in repeatedly erupting volcanic
matter from beneath this point, would, it is likely, have less force to
uplift it. Something of the same kind seems to have occurred near Red
Hill, for when tracing upwards the naked streams of lava from near
Porto Praya towards the interior of the island, I was strongly induced
to suspect, that since the lava had flowed, the slope of the land had
been slightly modified, either by a small subsidence near Red Hill, or
by that portion of the plain having been uplifted to a less height
during the elevation of the whole area.
_The basaltic lava, superincumbent on the calcareous deposit._—This
lava is of a pale grey colour, fusing into a black enamel; its fracture
is rather earthy and concretionary; it contains olivine in small
grains. The central parts of the mass are compact, or at most
crenulated with a few minute cavities, and are often columnar. At Quail
Island this structure was assumed in a striking manner; the lava in one
part being divided into horizontal laminæ, which became in another part
split by vertical fissures into five-sided plates; and these again,
being piled on each other, insensibly became soldered together, forming
fine symmetrical columns. The lower surface of the lava is vesicular,
but sometimes only to the thickness of a few inches; the upper surface,
which is likewise vesicular, is divided into balls, frequently as much
as three feet in diameter, made up of concentric layers. The mass is
composed of more than one stream; its total thickness being, on an
average, about eighty feet: the lower portion has certainly flowed
beneath the sea, and probably likewise the upper portion. The chief
part of this lava has flowed from the central districts, between the
hills marked A, B, C, etc., in the woodcut-map. The surface of the
country, near the coast, is level and barren; towards the interior, the
land rises by successive terraces, of which four, when viewed from a
distance, could be distinctly counted.
_Volcanic eruptions subsequent to the elevation of the coastland; the
ejected matter associated with earthy lime._—These recent lavas have
proceeded from those scattered, conical, reddish-coloured hills, which
rise abruptly from the plain-country near the coast. I ascended some of
them, but will describe only one, namely, _Red Hill_, which may serve
as a type of its class, and is remarkable in some especial respects.
Its height is about six hundred feet; it is composed of bright red,
highly scoriaceous rock of a basaltic nature; on one side of its summit
there is a hollow, probably the last remnant of a crater. Several of
the other hills of this class, judging from their external forms, are
surmounted by much more perfect craters. When sailing along the coast,
it was evident that a considerable body of lava had flowed from Red
Hill, over a line of cliff about one hundred and twenty feet in height,
into the sea: this line of cliff is continuous with that forming the
coast, and bounding the plain on both sides of this hill; these
streams, therefore, were erupted, after the formation of the
coast-cliffs, from Red Hill, when it must have stood, as it now does,
above the level of the sea. This conclusion accords with the highly
scoriaceous condition of all the rock on it, appearing to be of
subaerial formation: and this is
important, as there are some beds of calcareous matter near its summit,
which might, at a hasty glance, have been mistaken for a submarine
deposit. These beds consist of white, earthy, carbonate of lime,
extremely friable so as to be crushed with the least pressure; the most
compact specimens not resisting the strength of the fingers. Some of
the masses are as white as quicklime, and appear absolutely pure; but
on examining them with a lens, minute particles of scoriæ can always be
seen, and I could find none which, when dissolved in acids, did not
leave a residue of this nature. It is, moreover, difficult to find a
particle of the lime which does not change colour under the blowpipe,
most of them even becoming glazed. The scoriaceous fragments and the
calcareous matter are associated in the most irregular manner,
sometimes in obscure beds, but more generally as a confused breccia,
the lime in some parts and the scoriæ in others being most abundant.
Sir H. De la Beche has been so kind as to have some of the purest
specimens analysed, with a view to discover, considering their volcanic
origin, whether they contained much magnesia; but only a small portion
was found, such as is present in most limestones.
Fragments of the scoriæ embedded in the calcareous mass, when broken,
exhibit many of their cells lined and partly filled with a white,
delicate, excessively fragile, moss-like, or rather conferva-like,
reticulation of carbonate of lime. These fibres, examined under a lens
of one-tenth of an inch focal distance, appear cylindrical; they are
rather above one-thousandth of an inch in diameter; they are either
simply branched, or more commonly united into an irregular mass of
network, with the meshes of very unequal sizes and of unequal numbers
of sides. Some of the fibres are thickly covered with extremely minute
spicula, occasionally aggregated into little tuffs; and hence they have
a hairy appearance. These spicula are of the same diameter throughout
their length; they are easily detached, so that the object-glass of the
microscope soon becomes scattered over with them. Within the cells of
many fragments of the scoria, the lime exhibits this fibrous structure,
but generally in a less perfect degree. These cells do not appear to be
connected with one another. There can be no doubt, as will presently be
shown, that the lime was erupted, mingled with the lava in its fluid
state, and therefore I have thought it worth while to describe minutely
this curious fibrous structure, of which I know nothing analogous. From
the earthy condition of the fibres, this structure does not appear to
be related to crystallisation.
Other fragments of the scoriaceous rock from this hill, when broken,
are often seen marked with short and irregular white streaks, which are
owing to a row of separate cells being partly, or quite, filled with
white calcareous powder. This structure immediately reminded me of the
appearance in badly kneaded dough, of balls and drawn-out streaks of
flour, which have remained unmixed with the paste; and I cannot doubt
that small masses of the lime, in the same manner remaining unmixed
with the fluid lava, have been drawn out when the whole was in motion.
I carefully examined, by trituration and solution in acids, pieces of
the scoriæ, taken from within half-an-inch of those
cells which were filled with the calcareous powder, and they did not
contain an atom of free lime. It is obvious that the lava and lime have
on a large scale been very imperfectly mingled; and where small
portions of the lime have been entangled within a piece of the viscid
lava, the cause of their now occupying, in the form of a powder or of a
fibrous reticulation, the vesicular cavities, is, I think, evidently
due to the confined gases having most readily expanded at the points
where the incoherent lime rendered the lava less adhesive.
A mile eastward of the town of Praya, there is a steep-sided gorge,
about one hundred and fifty yards in width, cutting through the
basaltic plain and underlying beds, but since filled up by a stream of
more modern lava. This lava is dark grey, and in most parts compact and
rudely columnar; but at a little distance from the coast, it includes
in an irregular manner a brecciated mass of red scoriæ mingled with a
considerable quantity of white, friable, and in some parts, nearly pure
earthy lime, like that on the summit of Red Hill. This lava, with its
entangled lime, has certainly flowed in the form of a regular stream;
and, judging from the shape of the gorge, towards which the drainage of
the country (feeble though it now be) still is directed, and from the
appearance of the bed of loose water-worn blocks with their interstices
unfilled, like those in the bed of a torrent, on which the lava rests,
we may conclude that the stream was of subaerial origin. I was unable
to trace it to its source, but, from its direction, it seemed to have
come from Signal Post Hill, distant one mile and a quarter, which, like
Red Hill, has been a point of eruption subsequent to the elevation of
the great basaltic plain. It accords with this view, that I found on
Signal Post Hill, a mass of earthy, calcareous matter of the same
nature, mingled with scoriæ. I may here observe that part of the
calcareous matter forming the horizontal sedimentary bed, especially
the finer matter with which the embedded fragments of rock are
whitewashed, has probably been derived from similar volcanic eruptions,
as well as from triturated organic remains: the underlying, ancient,
crystalline rocks, also, are associated with much carbonate of lime,
filling amygdaloidal cavities, and forming irregular masses, the nature
of which latter I was unable to understand.
Considering the abundance of earthy lime near the summit of Red Hill, a
volcanic cone six hundred feet in height, of subaerial
growth,—considering the intimate manner in which minute particles and
large masses of scoriæ are embedded in the masses of nearly pure lime,
and on the other hand, the manner in which small kernels and streaks of
the calcareous powder are included in solid pieces of the
scoriæ,—considering, also, the similar occurrence of lime and scoriæ
within a stream of lava, also supposed, with good reason, to have been
of modern subaerial origin, and to have flowed from a hill, where
earthy lime also occurs: I think, considering these facts, there can be
no doubt that the lime has been erupted, mingled with the molten lava.
I am not aware that any similar case has been described: it appears to
me an interesting one, inasmuch as most geologists must have speculated
on the probable effects of a volcanic focus, bursting through
deep-seated beds
of different mineralogical composition. The great abundance of free
silex in the trachytes of some countries (as described by Beudant in
Hungary, and by P. Scrope in the Panza Islands), perhaps solves the
inquiry with respect to deep-seated beds of quartz; and we probably
here see it answered, where the volcanic action has invaded subjacent
masses of limestone. One is naturally led to conjecture in what state
the now earthy carbonate of lime existed, when ejected with the
intensely heated lava: from the extreme cellularity of the scoriæ on
Red Hill, the pressure cannot have been great, and as most volcanic
eruptions are accompanied by the emission of large quantities of steam
and other gases, we here have the most favourable conditions, according
to the views at present entertained by chemists, for the expulsion of
the carbonic acid.[3] Has the slow re-absorption of this gas, it may be
asked, given to the lime in the cells of the lava, that peculiar
fibrous structure, like that of an efflorescing salt? Finally, I may
remark on the great contrast in appearance between this earthy lime,
which must have been heated in a free atmosphere of steam and other
gases, while the white, crystalline, calcareous spar, produced by a
single thin sheet of lava (as at Quail Island) rolling over similar
earthy lime and the _débris_ of organic remains, at the bottom of a
shallow sea.
[3] Whilst deep beneath the surface, the carbonate of lime was, I
presume, in a fluid state. Hutton, it is known, thought that all
amygdaloids were produced by drops of molten limestone floating in the
trap, like oil in water: this no doubt is erroneous, but if the matter
forming the summit of Red Hill had been cooled under the pressure of a
moderately deep sea, or within the walls of a dike, we should, in all
probability, have had a trap rock associated with large masses of
compact, crystalline, calcareous spar, which, according to the views
entertained by many geologists, would have been wrongly attributed to
subsequent infiltration.
_Signal Post Hill._—This hill has already been several times mentioned,
especially with reference to the remarkable manner in which the white
calcareous stratum, in other parts so horizontal (Fig. 2), dips under
it into the sea. It has a broad summit, with obscure traces of a
crateriform structure, and is composed of basaltic rocks,[4] some
compact, others highly cellular with inclined beds of loose scoriæ, of
which some are associated with earthy lime. Like Red Hill, it has been
the source of eruptions, subsequently to the elevation of the
surrounding basaltic plain; but unlike that hill, it has undergone
considerable denudation, and has been the seat of volcanic action at a
remote period, when beneath the sea. I judge of this latter
circumstance from finding on its inland flank the last remnants of
three small
points of eruption. These points are composed of glossy scoriæ,
cemented by crystalline calcareous spar, exactly like the great
submarine calcareous deposit, where the heated lava has rolled over it:
their demolished state can, I think, be explained only by the denuding
action of the waves of the sea. I was guided to the first orifice by
observing a sheet of lava, about two hundred yards square, with
steepish sides, superimposed on the basaltic plain with no adjoining
hillock, whence it could have been erupted; and the only trace of a
crater which I was able to discover, consisted of some inclined beds of
scoriæ at one of its corners. At the distance of fifty yards from a
second level-topped patch of lava, but of much smaller size, I found an
irregular circular group of masses of cemented, scoriaceous breccia,
about six feet in height, which doubtless had once formed the point of
eruption. The third orifice is now marked only by an irregular circle
of cemented scoriæ, about four yards in diameter, and rising in its
highest point scarcely three feet above the level of the plain, the
surface of which, close all round, exhibits its usual appearance: here
we have a horizontal basal section of a volcanic spiracle, which,
together with all its ejected matter, has been almost totally
obliterated.
[4] Of these, one common variety is remarkable for being full of small
fragments of a dark jasper-red earthy mineral, which, when examined
carefully, shows an indistinct cleavage; the little fragments are
elongated in form, are soft, are magnetic before and after being
heated, and fuse with difficulty into a dull enamel. This mineral is
evidently closely related to the oxides of iron, but I cannot
ascertain what it exactly is. The rock containing this mineral is
crenulated with small angular cavities, which are lined and filled
with yellowish crystals of carbonate of lime.
The stream of lava, which fills the narrow gorge[5] eastward of the
town of Praya, judging from its course, seems, as before remarked, to
have come from Signal Post Hill, and to have flowed over the plain,
after its elevation: the same observation applies to a stream (possibly
part of the same one) capping the sea cliffs, a little eastward of the
gorge. When I endeavoured to follow these streams over the stony level
plain, which is almost destitute of soil and vegetation, I was much
surprised to find, that although composed of hard basaltic matter, and
not having been exposed to marine denudation, all distant traces of
them soon became utterly lost. But I have since observed at the
Galapagos Archipelago, that it is often impossible to follow even great
deluges of quite recent lava across older streams, except by the size
of the bushes growing on them, or by the comparative states of
glossiness of their surfaces,—characters which a short lapse of time
would be sufficient quite to obscure. I may remark, that in a level
country, with a dry climate, and with the wind blowing always in one
direction (as at the Cape de Verde Archipelago), the effects of
atmospheric degradation are probably much greater than would at first
be expected; for soil in this case accumulates only in a few protected
hollows, and being blown in one direction, it is always travelling
towards the sea in the form of the finest dust, leaving the surface of
the rocks bare, and exposed to the full effects of renewed meteoric
action.
[5] The sides of this gorge, where the upper basaltic stratum is
intersected, are almost perpendicular. The lava, which has since
filled it up, is attached to these sides, almost as firmly as a dike
is to its walls. In most cases, where a stream of lava has flowed down
a valley, it is bounded on each side by loose scoriaceous masses.
_Inland hills of more ancient volcanic rocks._—These hills are laid
down by eye, and marked as A, B, C, etc., in Map 1. They are related in
mineralogical composition, and are probably directly
continuous with the lowest rocks exposed on the coast. These hills,
viewed from a distance, appear as if they had once formed part of an
irregular tableland, and from their corresponding structure and
composition this probably has been the case. They have flat, slightly
inclined summits, and are, on an average, about six hundred feet in
height; they present their steepest slope towards the interior of the
island, from which point they radiate outwards, and are separated from
each other by broad and deep valleys, through which the great streams
of lava, forming the coast-plains, have descended. Their inner and
steeper escarpments are ranged in an irregular curve, which rudely
follows the line of the shore, two or three miles inland from it. I
ascended a few of these hills, and from others, which I was able to
examine with a telescope, I obtained specimens, through the kindness of
Mr. Kent, the assistant-surgeon of the _Beagle_; although by these
means I am acquainted with only a part of the range, five or six miles
in length, yet I scarcely hesitate, from their uniform structure, to
affirm that they are parts of one great formation, stretching round
much of the circumference of the island.
The upper and lower strata of these hills differ greatly in
composition. The upper are basaltic, generally compact, but sometimes
scoriaceous and amygdaloidal, with associated masses of wacke: where
the basalt is compact, it is either fine-grained or very coarsely
crystallised; in the latter case it passes into an augitic rock,
containing much olivine; the olivine is either colourless, or of the
usual yellow and dull reddish shades. On some of the hills, beds of
calcareous matter, both in an earthy and in a crystalline form,
including fragments of glossy scoriæ, are associated with the basaltic
strata. These strata differ from the streams of basaltic lava forming
the coast-plains, only in being more compact, and in the crystals of
augite, and in the grains of olivine being of much greater
size;—characters which, together with the appearance of the associated
calcareous beds, induce me to believe that they are of submarine
formation.
Some considerable masses of wacke, which are associated with these
basaltic strata, and which likewise occur in the basal series on the
coast, especially at Quail Island, are curious. They consist of a pale
yellowish-green argillaceous substance, of a crumbling texture when
dry, but unctuous when moist: in its purest form, it is of a beautiful
green tint, with translucent edges, and occasionally with obscure
traces of an original cleavage. Under the blowpipe it fuses very
readily into a dark grey, and sometimes even black bead, which is
slightly magnetic. From these characters, I naturally thought that it
was one of the pale species, decomposed, of the genus augite;—a
conclusion supported by the unaltered rock being full of large separate
crystals of black augite, and of balls and irregular streaks of dark
grey augitic rock. As the basalt ordinarily consists of augite, and of
olivine often tarnished and of a dull red colour, I was led to examine
the stages of decomposition of this latter mineral, and I found, to my
surprise, that I could trace a nearly perfect gradation from unaltered
olivine to the green wacke. Part of the same grain under the blowpipe
would in some instances
behave like olivine, its colour being only slightly changed, and part
would give a black magnetic bead. Hence I can have no doubt that the
greenish wacke originally existed as olivine; but great chemical
changes must have been effected during the act of decomposition thus to
have altered a very hard, transparent, infusible mineral, into a soft,
unctuous, easily melted, argillaceous substance.[6]
[6] D’Aubuisson “Traité de Géognosie” (tome ii, p. 569) mentions, on
the authority of M. Marcel de Serres, masses of green earth near
Montpellier, which are supposed to be due to the decomposition of
olivine. I do not, however, find, that the action of this mineral
under the blowpipe being entirely altered, as it becomes decomposed,
has been noticed; and the knowledge of this fact is important, as at
first it appears highly improbable that a hard, transparent,
refractory mineral should be changed into a soft, easily fused clay,
like this of St. Jago. I shall hereafter describe a green substance,
forming threads within the cells of some vesicular basaltic rocks in
Van Diemen’s Land, which behave under the blowpipe like the green
wacke of St. Jago; but its occurrence in cylindrical threads, shows it
cannot have resulted from the decomposition of olivine, a mineral
always existing in the form of grains or crystals.
The basal strata of these hills, as well as some neighbouring,
separate, bare, rounded hillocks, consist of compact, fine-grained,
non-crystalline (or so slightly as scarcely to be perceptible),
ferruginous, feldspathic rocks, and generally in a state of
semi-decomposition. Their fracture is exceedingly irregular, and
splintery; yet small fragments are often very tough. They contain much
ferruginous matter, either in the form of minute grains with a metallic
lustre, or of brown hair-like threads: the rock in this latter case
assuming a pseudo-brecciated structure. These rocks sometimes contain
mica and veins of agate. Their rusty brown or yellowish colour is
partly due to the oxides of iron, but chiefly to innumerable,
microscopically minute, black specks, which, when a fragment is heated,
are easily fused, and evidently are either hornblende or augite. These
rocks, therefore, although at first appearing like baked clay or some
altered sedimentary deposit, contain all the essential ingredients of
trachyte; from which they differ only in not being harsh, and in not
containing crystals of glassy feldspar. As is so often the case with
trachytic formation, no stratification is here apparent. A person would
not readily believe that these rocks could have flowed as lava; yet at
St. Helena there are well-characterised streams (as will be described
in an ensuing chapter) of nearly similar composition. Amidst the
hillocks composed of these rocks, I found in three places, smooth
conical hills of phonolite, abounding with fine crystals of glassy
feldspar, and with needles of hornblende. These cones of phonolite, I
believe, bear the same relation to the surrounding feldspathic strata
which some masses of coarsely crystallised augitic rock, in another
part of the island, bear to the surrounding basalt, namely, that both
have been injected. The rocks of a feldspathic nature being anterior in
origin to the basaltic strata, which cap them, as well as to the
basaltic streams of the coast-plains, accords with the usual order of
succession of these two grand divisions of the volcanic series.
The strata of most of these hills in the upper part, where alone the
planes of division are distinguishable, are inclined at a small angle
from the interior of the island towards the sea-coast. The inclination
is not the same in each hill; in that marked A it is less than in B, D,
or E; in C the strata are scarcely deflected from a horizontal plane,
and in F (as far as I could judge without ascending it) they are
slightly inclined in a reverse direction, that is, inwards and towards
the centre of the island. Notwithstanding these differences of
inclination, their correspondence in external form, and in the
composition both of their upper and lower parts,—their relative
position in one curved line, with their steepest sides turned
inwards,—all seem to show that they originally formed parts of one
platform; which platform, as before remarked, probably extended round a
considerable portion of the circumference of the island. The upper
strata certainly flowed as lava, and probably beneath the sea, as
perhaps did the lower feldspathic masses: how then come these strata to
hold their present position, and whence were they erupted?
In the centre of the island[7] there are lofty mountains, but they are
separated from the steep inland flanks of these hills by a wide space
of lower country: the interior mountains, moreover, seem to have been
the source of those great streams of basaltic lava which, contracting
as they pass between the bases of the hills in question, expand into
the coast-plains. Round the shores of St. Helena there is a rudely
formed ring of basaltic rocks, and at Mauritius there are remnants of
another such a ring round part, if not round the whole, of the island;
here again the same question immediately occurs, how came these masses
to hold their present position, and whence were they erupted? The same
answer, whatever it may be, probably applies in these three cases; and
in a future chapter we shall recur to this subject.
[7] I saw very little of the inland parts of the island. Near the
village of St. Domingo, there are magnificent cliffs of rather
coarsely crystallised basaltic lava. Following the little stream in
this valley, about a mile above the village, the base of the great
cliff was formed of a compact fine-grained basalt, conformably covered
by a bed of pebbles. Near Fuentes, I met with pap-formed hills of the
compact feldspathic series of rocks.
_Valleys near the coast._—These are broad, very flat, and generally
bounded by low cliff-formed sides. Portions of the basaltic plain are
sometimes nearly or quite isolated by them; of which fact, the space on
which the town of Praya stands offers an instance. The great valley
west of the town has its bottom filled up to a depth of more than
twenty feet by well-rounded pebbles, which in some parts are firmly
cemented together by white calcareous matter. There can be no doubt,
from the form of these valleys, that they were scooped out by the waves
of the sea, during that equable elevation of the land, of which the
horizontal calcareous deposit, with its existing species of marine
remains, gives evidence. Considering how well shells have been
preserved in this stratum, it is singular that I could not find even a
single small fragment of shell in the conglomerate at the bottom of the
valleys. The bed of pebbles in the valley west of the town is
intersected by a second valley joining it as a tributary, but even this
valley appears much too wide and flat-bottomed to have been formed by
the small quantity of water, which falls only during one short wet
season; for at other times of the year these valleys are absolutely
dry.
_Recent conglomerate._—On the shores of Quail Island, I found fragments
of brick, bolts of iron, pebbles, and large fragments of basalt, united
by a scanty base of impure calcareous matter into a firm conglomerate.
To show how exceedingly firm this recent conglomerate is, I may
mention, that I endeavoured with a heavy geological hammer to knock out
a thick bolt of iron, which was embedded a little above low-water mark,
but was quite unable to succeed.
Chapter II FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.
FERNANDO NORONHA.—Precipitous hill of phonolite. TERCEIRA.—Trachytic
rocks: their singular decomposition by steam of high temperature.
TAHITI.—Passage from wacke into trap; singular volcanic rock with the
vesicles half-filled with mesotype. MAURITIUS.—Proofs of its recent
elevation. Structure of its more ancient mountains; similarity with St.
Jago. ST. PAUL’S ROCKS.—Not of volcanic origin. Their singular
mineralogical composition.
_Fernando Noronha._—During our short visit at this and the four
following islands, I observed very little worthy of description.
Fernando Noronha is situated in the Atlantic Ocean, in lat. 3° 50′ S.,
and 230 miles distant from the coast of South America. It consists of
several islets, together nine miles in length by three in breadth. The
whole seems to be of volcanic origin; although there is no appearance
of any crater, or of any one central eminence. The most remarkable
feature is a hill 1,000 feet high, of which the upper 400 feet consist
of a precipitous, singularly shaped pinnacle, formed of columnar
phonolite, containing numerous crystals of glassy feldspar, and a few
needles of hornblende. From the highest accessible point of this hill,
I could distinguish in different parts of the group several other
conical hills, apparently of the same nature. At St. Helena there are
similar, great, conical, protuberant masses of phonolite, nearly one
thousand feet in height, which have been formed by the injection of
fluid feldspathic lava into yielding strata. If this hill has had, as
is probable, a similar origin, denudation has been here effected on an
enormous scale. Near the base of this hill, I observed beds of white
tuff, intersected by numerous dikes, some of amygdaloidal basalt and
others of trachyte; and beds of slaty phonolite with the planes of
cleavage directed N.W. and S.E. Parts of this rock, where the crystals
were scanty, closely resembled common clay-slate, altered by the
contact of a trap-dike. The lamination of rocks, which undoubtedly have
once been fluid, appears to me a subject well deserving attention. On
the beach there were numerous
fragments of compact basalt, of which rock a distant façade of columns
seemed to be formed.
_Terceira in the Azores._—The central parts of this island consist of
irregularly rounded mountains of no great elevation, composed of
trachyte, which closely resembles in general character the trachyte of
Ascension, presently to be described. This formation is in many parts
overlaid, in the usual order of superposition, by streams of basaltic
lava, which near the coast compose nearly the whole surface. The course
which these streams have followed from their craters, can often be
followed by the eye. The town of Angra is overlooked by a crateriform
hill (Mount Brazil), entirely built of thin strata of fine-grained,
harsh, brown-coloured tuff. The upper beds are seen to overlap the
basaltic streams on which the town stands. This hill is almost
identical in structure and composition with numerous crateriformed
hills in the Galapagos Archipelago.
_Effects of steam on the trachytic rocks._—In the central part of the
island there is a spot, where steam is constantly issuing in jets from
the bottom of a small ravine-like hollow, which has no exit, and which
abuts against a range of trachytic mountains. The steam is emitted from
several irregular fissures: it is scentless, soon blackens iron, and is
of much too high temperature to be endured by the hand. The manner in
which the solid trachyte is changed on the borders of these orifices is
curious: first, the base becomes earthy, with red freckles evidently
due to the oxidation of particles of iron; then it becomes soft; and
lastly, even the crystals of glassy feldspar yield to the dissolving
agent. After the mass is converted into clay, the oxide of iron seems
to be entirely removed from some parts, which are left perfectly white,
whilst in other neighbouring parts, which are of the brightest red
colour, it seems to be deposited in greater quantity; some other masses
are marbled with two distinct colours. Portions of the white clay, now
that they are dry, cannot be distinguished by the eye from the finest
prepared chalk; and when placed between the teeth they feel equally
soft-grained; the inhabitants use this substance for white-washing
their houses. The cause of the iron being dissolved in one part, and
close by being again deposited, is obscure; but the fact has been
observed in several other places.[1] In some half-decayed specimens, I
found small, globular aggregations of yellow hyalite, resembling
gum-arabic, which no doubt had been deposited by the steam.
[1] Spallanzani, Dolomieu, and Hoffman have described similar cases in
the Italian volcanic islands. Dolomieu says the iron at the Panza
Islands is redeposited in the form of veins (p. 86 “Mémoire sur les
Isles Ponces”). These authors likewise believe that the steam deposits
silica: it is now experimentally known that vapour of a high
temperature is able to dissolve silica.
As there is no escape for the rain-water, which trickles down the sides
of the ravine-like hollow, whence the steam issues, it must all
percolate downwards through the fissures at its bottom. Some of the
inhabitants informed me that it was on record that flames (some
luminous appearance?) had originally proceeded from these cracks,
and that the flames had been succeeded by the steam; but I was not able
to ascertain how long this was ago, or anything certain on the subject.
When viewing the spot, I imagined that the injection of a large mass of
rock. like the cone of phonolite at Fernando Noronha, in a semi-fluid
state, by arching the surface might have caused a wedge-shaped hollow
with cracks at the bottom, and that the rain-water percolating to the
neighbourhood of the heated mass, would during many succeeding years be
driven back in the form of steam.
_Tahiti (Otaheite)._—I visited only a part of the north-western side of
this island, and this part is entirely composed of volcanic rocks. Near
the coast there are several varieties of basalt, some abounding with
large crystals of augite and tarnished olivine, others compact and
earthy,—some slightly vesicular, and others occasionally amygdaloidal.
These rocks are generally much decomposed, and to my surprise, I found
in several sections that it was impossible to distinguish, even
approximately, the line of separation between the decayed lava and the
alternating beds of tuff. Since the specimens have become dry, it is
rather more easy to distinguish the decomposed igneous rocks from the
sedimentary tuffs. This gradation in character between rocks having
such widely different origins, may I think be explained by the yielding
under pressure of the softened sides of the vesicular cavities, which
in many volcanic rocks occupy a large proportion of their bulk. As the
vesicles generally increase in size and number in the upper parts of a
stream of lava, so would the effects of their compression increase; the
yielding, moreover, of each lower vesicle must tend to disturb all the
softened matter above it. Hence we might expect to trace a perfect
gradation from an unaltered crystalline rock to one in which all the
particles (although originally forming part of the same solid mass) had
undergone mechanical displacement; and such particles could hardly be
distinguished from others of similar composition, which had been
deposited as sediment. As lavas are sometimes laminated in their upper
parts even horizontal lines, appearing like those of aqueous
deposition, could not in all cases be relied on as a criterion of
sedimentary origin. From these considerations it is not surprising that
formerly many geologists believed in real transitions from aqueous
deposits, through wacke, into igneous traps.
In the valley of Tia-auru, the commonest rocks are basalts with much
olivine, and in some cases almost composed of large crystals of augite.
I picked up some specimens, with much glassy feldspar, approaching in
character to trachyte. There were also many large blocks of vesicular
basalt, with the cavities beautifully lined with chabasie (?), and
radiating bundles of mesotype. Some of these specimens presented a
curious appearance, owing to a number of the vesicles being half filled
up with a white, soft, earthy mesotypic mineral, which intumesced under
the blowpipe in a remarkable manner. As the upper surfaces in all the
half-filled cells are exactly parallel, it is evident that this
substance has sunk to the bottom of each cell from its weight.
Sometimes, however, it entirely fills the cells. Other cells are either
quite
filled, or lined, with small crystals, apparently of chabasie; these
crystals, also, frequently line the upper half of the cells partly
filled with the earthy mineral, as well as the upper surface of this
substance itself, in which case the two minerals appear to blend into
each other. I have never seen any other amygdaloid[2] with the cells
half filled in the manner here described; and it is difficult to
imagine the causes which determined the earthy mineral to sink from its
gravity to the bottom of the cells, and the crystalline mineral to
adhere in a coating of equal thickness round the sides of the cells.
[2] MacCulloch, however, has described and given a plate of (“Geolog.
Trans.” 1st series, vol. iv, p. 225) a trap rock, with cavities filled
up horizontally with quartz and chalcedony. The upper halves of these
cavities are often filled by layers, which follow each irregularity of
the surface, and by little depending stalactites of the same siliceous
substances.
The basic strata on the sides of the valley are gently inclined
seaward, and I nowhere observed any sign of disturbance; the strata are
separated from each other by thick, compact beds of conglomerate, in
which the fragments are large, some being rounded, but most angular.
From the character of these beds, from the compact and crystalline
condition of most of the lavas, and from the nature of the infiltrated
minerals, I was led to conjecture that they had originally flowed
beneath the sea. This conclusion agrees with the fact that the Rev. W.
Ellis found marine remains at a considerable height, which he believes
were interstratified with volcanic matter; as is likewise described to
be the case by Messrs. Tyerman and Bennett at Huaheine, an island in
this same archipelago. Mr. Stutchbury also discovered near the summit
of one of the loftiest mountains of Tahiti, at the height of several
thousand feet, a stratum of semi-fossil coral. None of these remains
have been specifically examined. On the coast, where masses of
coral-rock would have afforded the clearest evidence, I looked in vain
for any signs of recent elevation. For references to the above
authorities, and for more detailed reasons for not believing that
Tahiti has been recently elevated, I must refer to the “Structure and
Distribution of Coral-Reefs.”
_Mauritius._—Approaching this island on the northern or north-western
side, a curved chain of bold mountains, surmounted by rugged pinnacles,
is seen to rise from a smooth border of cultivated land, which gently
slopes down to the coast. At the first glance, one is tempted to
believe that the sea lately reached the base of these mountains, and
upon examination, this view, at least with respect to the inferior
parts of the border, is found to be perfectly correct. Several
authors[3] have described masses of upraised coral-rock round the
greater part of the circumference of the island. Between Tamarin Bay
and the Great Black River I observed, in company with Captain Lloyd,
two hillocks of coral-rock, formed in their lower part of hard
calcareous sandstone, and in their upper of great blocks, slightly
aggregated, of Astræa and Madrepora, and of fragments of basalt; they
were divided into beds dipping seaward, in one case at an angle of 8°,
and in the other at 18°; they had a water-worn appearance, and they
rose abruptly from a smooth surface, strewed with rolled débris of
organic remains, to a height of about twenty feet. The Officier du Roi,
in his most interesting tour in 1768 round the island, has described
masses of upraised coral-rocks, still retaining that moat-like
structure (see my “Coral Reefs”) which is characteristic of the living
reefs. On the coast northward of Port Louis, I found the lava concealed
for a considerable space inland by a conglomerate of corals and shells,
like those on the beach, but in parts consolidated by red ferruginous
matter. M. Bory St. Vincent has described similar calcareous beds over
nearly the whole of the plain of Pamplemousses. Near Port Louis, when
turning over some large stones, which lay in the bed of a stream at the
head of a protected creek, and at the height of some yards above the
level of spring tides, I found several shells of serpula still adhering
to their under sides.
[3] Captain Carmichael, in Hooker’s “Bot. Misc.,” vol. ii, p. 301.
Captain Lloyd has lately, in the “Proceedings of the Geological
Society” (vol. iii, p. 317), described carefully some of these masses.
In the “Voyage à l’Isle de France, par un Officier du Roi,” many
interesting facts are given on this subject. Consult also “Voyage aux
Quatre Isles d’Afrique, par M. Bory St. Vincent.”
The jagged mountains near Port Louis rise to a height of between two
and three thousand feet; they consist of strata of basalt, obscurely
separated from each other by firmly aggregated beds of fragmentary
matter; and they are intersected by a few vertical dikes. The basalt in
some parts abounds with large crystals of augite and olivine, and is
generally compact. The interior of the island forms a plain, raised
probably about a thousand feet above the level of the sea, and composed
of streams of lava which have flowed round and between the rugged
basaltic mountains. These more recent lavas are also basaltic, but less
compact, and some of them abound with feldspar, so that they even fuse
into a pale coloured glass. On the banks of the Great River, a section
is exposed nearly five hundred feet deep, worn through numerous thin
sheets of the lava of this series, which are separated from each other
by beds of scoriæ. They seem to have been of subaerial formation, and
to have flowed from several points of eruption on the central platform,
of which the Piton du Milieu is said to be the principal one. There are
also several volcanic cones, apparently of this modern period, round
the circumference of the island, especially at the northern end, where
they form separate islets.
The mountains composed of the more compact and crystalline basalt, form
the main skeleton of the island. M. Bailly[4] states that they all “se
développent autour d’elle comme une ceinture d’immenses remparts,
toutes affectant une pente plus ou moins enclinée vers le rivage de la
mer; tandis, au contraire, que vers le centre de l’ile elles presentent
une coupe abrupte, et souvent taillée à pic. Toutes ces montagnes sont
formées de couches parallèles inclinées du centre de l’ile vers la
mer.” These statements have been disputed, though not in detail, by M.
Quoy, in the voyage of Freycinet. As far as my limited means of
observation went, I found
them perfectly correct.[5] The mountains on the N.W. side of the
island, which I examined, namely, La Pouce, Peter Botts, Corps de
Garde, Les Mamelles, and apparently another farther southward, have
precisely the external shape and stratification described by M. Bailly.
They form about a quarter of his girdle of ramparts. Although these
mountains now stand quite detached, being separated from each other by
breaches, even several miles in width, through which deluges of lava
have flowed from the interior of the island; nevertheless, seeing their
close general similarity, one must feel convinced that they originally
formed parts of one continuous mass. Judging from the beautiful map of
the Mauritius, published by the Admiralty from a French MS., there is a
range of mountains (M. Bamboo) on the opposite side of the island,
which correspond in height, relative position, and external form, with
those just described. Whether the girdle was ever complete may well be
doubted; but from M. Bailly’s statements, and my own observations, it
may be safely concluded that mountains with precipitous inland flanks,
and composed of strata dipping outwards, once extended round a
considerable portion of the circumference of the island. The ring
appears to have been oval and of vast size; its shorter axis, measured
across from the inner sides of the mountains near Port Louis and those
near Grand Port, being no less than thirteen geographical miles in
length. M. Bailly boldly supposes that this enormous gulf, which has
since been filled up to a great extent by streams of modern lava, was
formed by the sinking in of the whole upper part of one great volcano.
[4] “Voyage aux Terres Australes,” tome i, p. 54.
[5] M. Lesson, in his account of this island, in the “Voyage of the
_Coquille_,” seems to follow M. Bailly’s views.
It is singular in how many respects those portions of St. Jago and of
Mauritius which I visited agree in their geological history. At both
islands, mountains of similar external form, stratification, and (at
least in their upper beds) composition, follow in a curved chain the
coast-line. These mountains in each case appear originally to have
formed parts of one continuous mass. The basaltic strata of which they
are composed, from their compact and crystalline structure, seem, when
contrasted with the neighbouring basaltic streams of subaerial
formation, to have flowed beneath the pressure of the sea, and to have
been subsequently elevated. We may suppose that the wide breaches
between the mountains were in both cases worn by the waves, during
their gradual elevation—of which process, within recent times, there is
abundant evidence on the coast-land of both islands. At both, vast
streams of more recent basaltic lavas have flowed from the interior of
the island, round and between the ancient basaltic hills; at both,
moreover, recent cones of eruption are scattered around the
circumference of the island; but at neither have eruptions taken place
within the period of history. As remarked in the last chapter, it is
probable that these ancient basaltic mountains, which resemble (at
least in many respects) the basal and disturbed remnants of two
gigantic volcanoes, owe their present form, structure, and position, to
the action of similar causes.
_St. Paul’s Rocks._—This small island is situated in the Atlantic
Ocean, nearly one degree north of the equator, and 540 miles distant
from South America, in 29° 15′ west longitude. Its highest point is
scarcely fifty feet above the level of the sea; its outline is
irregular, and its entire circumference barely three-quarters of a
mile. This little point of rock rises abruptly out of the ocean; and,
except on its western side, soundings were not obtained, even at the
short distance of a quarter of a mile from its shore. It is not of
volcanic origin; and this circumstance, which is the most remarkable
point in its history (as will hereafter be referred to), properly ought
to exclude it from the present volume. It is composed of rocks, unlike
any which I have met with, and which I cannot characterise by any name,
and must therefore describe.
The simplest, and one of the most abundant kinds, is a very compact,
heavy, greenish-black rock, having an angular, irregular fracture, with
some points just hard enough to scratch glass, and infusible. This
variety passes into others of paler green tints, less hard, but with a
more crystalline fracture, and translucent on their edges; and these
are fusible into a green enamel. Several other varieties are chiefly
characterised by containing innumerable threads of dark-green
serpentine, and by having calcareous matter in their interstices. These
rocks have an obscure, concretionary structure, and are full of
variously coloured angular pseudo fragments. These angular pseudo
fragments consist of the first-described dark green rock, of a brown
softer kind, of serpentine, and of a yellowish harsh stone, which,
perhaps, is related to serpentine rock. There are other vesicular,
calcareo-ferruginous, soft stones. There is no distinct stratification,
but parts are imperfectly laminated; and the whole abounds with
innumerable veins, and vein-like masses, both small and large. Of these
vein-like masses, some calcareous ones, which contain minute fragments
of shells, are clearly of subsequent origin to the others.
_A glossy incrustation._—Extensive portions of these rocks are coated
by a layer of a glossy polished substance, with a pearly lustre and of
a greyish white colour; it follows all the inequalities of the surface,
to which it is firmly attached. When examined with a lens, it is found
to consist of numerous exceedingly thin layers, their aggregate
thickness being about the tenth of an inch. It is considerably harder
than calcareous spar, but can be scratched with a knife; under the
blowpipe it scales off, decrepitates, slightly blackens, emits a fetid
odour, and becomes strongly alkaline: it does not effervesce in
acids.[6] I presume this substance has been deposited by water draining
from the birds’ dung, with which the rocks are covered. At Ascension,
near a cavity in the rocks which was filled with a laminated mass of
infiltrated birds’ dung, I found some irregularly formed, stalactitical
masses of apparently the same nature. These masses, when broken, had an
earthy texture; but on their outsides, and especially at their
extremities, they were formed of a pearly substance, generally in
little globules, like the
enamel of teeth, but more translucent, and so hard as just to scratch
plate-glass. This substance slightly blackens under the blowpipe, emits
a bad smell, then becomes quite white, swelling a little, and fuses
into a dull white enamel; it does not become alkaline; nor does it
effervesce in acids. The whole mass had a collapsed appearance, as if
in the formation of the hard glossy crust the whole had shrunk much. At
the Abrolhos Islands on the coast of Brazil, where also there is much
birds’ dung, I found a great quantity of a brown, arborescent substance
adhering to some trap-rock. In its arborescent form, this substance
singularly resembles some of the branched species of Nullipora. Under
the blowpipe, it behaves like the specimens from Ascension; but it is
less hard and glossy, and the surface has not the shrunk appearance.
[6] In my “Journal” I have described this substance; I then believed
that it was an impure phosphate of lime.
Chapter III ASCENSION.
Basaltic lavas.—Numerous craters truncated on the same side.—Singular
structure of volcanic bombs.—Aeriform explosions.—Ejected granitic
fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
formation.—Concretions in pumiceous tuff.—Calcareous deposits and
frondescent incrustations on the coast.—Remarkable laminated beds,
alternating with, and passing into, obsidian.—Origin of
obsidian.—Lamination of volcanic rocks.
This island is situated in the Atlantic Ocean, in lat. 8° S., long. 14°
W. It has the form of an irregular triangle (see map below), each side
being about six miles in length. Its highest point is 2,870 feet[1]
above the level of the sea. The whole is volcanic, and, from the
absence of proofs to the contrary, I believe of subaerial origin. The
fundamental rock is everywhere of a pale colour, generally compact, and
of a feldspathic nature. In the S.E. portion of the island, where the
highest land is situated, well characterised trachyte, and other
congenerous rocks of that varying here and there a hill or single point
of rock (one of which near the sea-coast, north of the Fort, is only
two or three yards across) of the trachyte still remaining exposed.
[1] _Geographical Journal_, vol. v, p. 243.
Illustration: Island of Ascension PLATE IV.
_Basaltic rocks._—The overlying basaltic lava is in some parts
extremely vesicular, in others little so; it is of a black colour, but
sometimes contains crystals of glassy feldspar, and seldom much
olivine. These streams appear to have possessed singularly little
fluidity; their side walls and lower ends being very steep, and even as
much as between twenty and thirty feet in height. Their surface is
extraordinarily rugged,
and from a short distance appears as if studded with small craters.
These projections consist of broad, irregularly conical, hillocks,
traversed by fissures, and composed of the same unequally scoriaceous
basalt with the surrounding streams, but having an obscure tendency to
a columnar structure; they rise to a height between ten and thirty feet
above the general surface, and have been formed, as I presume, by the
heaping up of the viscid lava at points of greater resistance. At the
base of several of these hillocks, and occasionally likewise on more
level parts, solid ribs, composed of angulo-globular masses of basalt,
resembling in size and outline arched sewers or gutters of brickwork,
but not being hollow, project between two or three feet above the
surface of the streams; what their origin may have been, I do not know.
Many of the superficial fragments from these basaltic streams present
singularly convoluted forms; and some specimens could hardly be
distinguished from logs of dark-coloured wood without their bark.
Many of the basaltic streams can be traced, either to points of
eruption at the base of the great central mass of trachyte, or to
separate, conical, red-coloured hills, which are scattered over the
northern and western borders of the island. Standing on the central
eminence, I counted between twenty and thirty of these cones of
eruption. The greater number of them had their truncated summits cut
off obliquely, and they all sloped towards the S.E., whence the
trade-wind blows.[2] This structure no doubt has been caused by the
ejected fragments and ashes being always blown, during eruptions, in
greater quantity towards one side than towards the other. M. Moreau de
Jonnes has made a similar observation with respect to the volcanic
orifices in the West Indian Islands.
[2] M. Lesson in the “Zoology of the Voyage of the _Coquille_,” p. 490
has observed this fact. Mr. Hennah (“Geolog. Proceedings,” 1835, p.
189) further remarks that the most extensive beds of ashes at
Ascension invariably occur on the leeward side of the island.
_Volcanic bombs._—These occur in great numbers strewed on the ground,
and some of them lie at considerable distances from any points of
eruption. They vary in size from that of an apple to that of a man’s
body; they are either spherical or pear-shaped, or with the hinder part
(corresponding to the tail of a comet) irregular, studded with
projecting points, and even concave. Their surfaces are rough, and
fissured with branching cracks; their internal structure is either
irregularly scoriaceous and compact, or it presents a symmetrical and
very curious appearance. An irregular segment of a bomb of this latter
kind, of which I found several, is accurately represented in figure No.
3. Its size was about that of a man’s head. The whole interior is
coarsely cellular; the cells averaging in diameter about the tenth of
an inch; but nearer the outside they gradually decrease in size. This
part is succeeded by a well-defined shell of compact lava, having a
nearly uniform thickness of about the third of an inch; and the shell
is overlaid by a somewhat thicker coating of finely cellular lava (the
cells varying from the fiftieth to the hundredth of an inch in
diameter), which forms the external surface: the line separating the
shell of compact
lava from the outer scoriaceous crust is distinctly defined. This
structure is very simply explained, if we suppose a mass of viscid,
scoriaceous matter, to be projected with a rapid, rotatory motion
through the air; for whilst the external crust, from cooling, became
solidified (in the state we now see it), the centrifugal force, by
relieving the pressure in the interior parts of the bomb, would allow
the heated vapours to expand their cells; but these being driven by the
same force against the already-hardened crust, would become, the nearer
they were to this part, smaller and smaller or less expanded, until
they became packed into a solid, concentric shell. As we know that
chips from a grindstone[3] can be flirted off, when made to revolve
with sufficient velocity, we need not doubt that the centrifugal force
would have power to modify the structure of a softened bomb, in the
manner here supposed. Geologists have remarked, that the external form
of a bomb at once bespeaks the history of its aerial course, and few
now see that the internal structure can speak, with almost equal
plainness, of its rotatory movement.
[3] Nichol’s “Architecture of the Heavens.”
No. 3
[Illustration: Fragment of a spherical volcanic bomb.]
Fragment of a spherical volcanic bomb, with the inferior parts coarsely
cellular, coated by a concentric layer of compact lava, and this again
by a crust of finely cellular rock.
No. 4
[Illustration: Volcanic bomb of obsidian from Australia.]
Volcanic bomb of obsidian from Australia. The figure at left gives a
front view; the figure at right a side view of the same object.
M. Bory St. Vincent[4] has described some balls of lava from the Isle
of Bourbon, which have a closely similar structure. His explanation,
however (if I understand it rightly), is very different from that which
I have given; for he supposes that they have rolled, like snowballs,
down the sides of the crater. M. Beudant,[5] also, has described some
singular little balls of obsidian, never more than six or eight inches
in diameter, which he found strewed on the surface of the ground: their
form is always oval; sometimes they are much swollen in the middle, and
even spindle-shaped: their surface is regularly marked with concentric
ridges and furrows, all of which on the same ball are at right angles
to one axis: their interior is compact and glassy. M. Beudant supposes
that masses of lava, when soft, were shot into the air, with a rotatory
movement round the same axis, and that the form and superficial ridges
of the bombs were thus produced. Sir Thomas Mitchell has given me what
at first appears to be the half of a much flattened oval ball of
obsidian; it has a singular artificial-like appearance, which is well
represented (of the natural size) in figure No. 4. It was found in its
present state, on a great sandy plain between the rivers Darling and
Murray, in Australia, and at the distance of several hundred miles from
any known volcanic region. It seems to have been embedded in some
reddish tufaceous matter; and may have been transported either by the
aborigines or by natural means. The external saucer consists of compact
obsidian, of a bottle-green colour, and is filled with finely cellular
black lava, much less transparent and glassy than the obsidian. The
external surface is marked with four or five not quite perfect ridges,
which are represented rather too distinctly in figure No. 4. Here,
then, we have the external structure described by M. Beudant, and the
internal cellular condition of the bombs from Ascension. The lip of the
saucer is slightly concave, exactly like the margin of a soup-plate,
and its inner edge overlaps a little the central cellular lava. This
structure is so symmetrical round the entire circumference, that one is
forced to suppose that the bomb burst during its rotatory course,
before being quite solidified, and that the lip and edges were thus
slightly modified and turned inwards. It may be remarked that the
superficial ridges are in planes, at right angles to an axis,
transverse to the longer axis of the flattened oval: to explain this
circumstance, we may suppose that when the bomb burst, the axis of
rotation changed.
[4] “Voyage aux Quatre Isles d’Afrique” tome i, p. 222.
[5] “Voyage en Hongrie,” tome ii, p. 214.
_Aeriform explosions._—The flanks of Green Mountain and the surrounding
country are covered by a great mass, some hundred feet in thickness, of
loose fragments. The lower beds generally consist of fine-grained,
slightly consolidated tuffs,[6] and the upper beds of great
loose fragments, with alternating finer beds.[7] One white ribbon-like
layer of decomposed, pumiceous breccia, was curiously bent into deep
unbroken curves, beneath each of the large fragments in the
superincumbent stratum. From the relative position of these beds, I
presume that a narrow-mouthed crater, standing nearly in the position
of Green Mountain, like a great air-gun, shot forth, before its final
extinction, this vast accumulation of loose matter. Subsequently to
this event, considerable dislocations have taken place, and an oval
circus has been formed by subsidence. This sunken space lies at the
north-eastern foot of Green Mountain, and is well represented in Map 2.
Its longer axis, which is connected with a N.E. and S.W. line of
fissure, is three-fifths of a nautical mile in length; its sides are
nearly perpendicular, except in one spot, and about four hundred feet
in height; they consist, in the lower part, of a pale basalt with
feldspar, and in the upper part, of the tuff and loose ejected
fragments; the bottom is smooth and level, and under almost any other
climate a deep lake would have been formed here. From the thickness of
the bed of loose fragments, with which the surrounding country is
covered, the amount of aeriform matter necessary for their projection
must have been enormous; hence we may suppose it probable that after
the explosions vast subterranean caverns were left, and that the
falling in of the roof of one of these produced the hollow here
described. At the Galapagos Archipelago, pits of a similar character,
but of a much smaller size, frequently occur at the bases of small
cones of eruption.
[6] Some of this peperino, or tuff, is sufficiently hard not to be
broken by the greatest force of the fingers.
[7] On the northern side of the Green Mountain a thin seam, about an
inch in thickness, of compact oxide of iron, extends over a
considerable area; it lies conformably in the lower part of the
stratified mass of ashes and fragments. This substance is of a
reddish-brown colour, with an almost metallic lustre; it is not
magnetic, but becomes so after having been heated under the blowpipe,
by which it is blackened and partly fused. This seam of compact stone,
by intercepting the little rain-water which falls on the island, gives
rise to a small dripping spring, first discovered by Dampier. It is
the only fresh water on the island, so that the possibility of its
being inhabited has entirely depended on the occurrence of this
ferruginous layer.
_Ejected granitic fragments._—In the neighbourhood of Green Mountain,
fragments of extraneous rock are not unfrequently found embedded in the
midst of masses of scoriæ. Lieutenant Evans, to whose kindness I am
indebted for much information, gave me several specimens, and I found
others myself. They nearly all have a granitic structure, are brittle,
harsh to the touch, and apparently of altered colours. _First_, a white
syenite, streaked and mottled with red; it consists of
well-crystallised feldspar, numerous grains of quartz, and brilliant,
though small, crystals of hornblende. The feldspar and hornblende in
this and the succeeding cases have been determined by the reflecting
goniometer, and the quartz by its action under the blowpipe. The
feldspar in these ejected fragments, like the glassy kind in the
trachyte, is from its cleavage a potash-feldspar. _Secondly_, a
brick-red mass of feldspar, quartz, and small dark patches of a decayed
mineral; one minute particle of which I was able to ascertain, by its
cleavage, to be hornblende.
_Thirdly_, a mass of confusedly crystallised white feldspar, with
little nests of a dark-coloured mineral, often carious, externally
rounded, having a glossy fracture, but no distinct cleavage: from
comparison with the second specimen, I have no doubt that it is fused
hornblende. _Fourthly_, a rock, which at first appears a simple
aggregation of distinct and large-sized crystals of dusty-coloured
Labrador feldspar;[8] but in their interstices there is some white
granular feldspar, abundant scales of mica, a little altered
hornblende, and, as I believe, no quartz. I have described these
fragments in detail, because it is rare[9] to find granitic rocks
ejected from volcanoes with their _minerals unchanged_, as is the case
with the first specimen, and partially with the second. One other large
fragment, found in another spot, is deserving of notice; it is a
conglomerate, containing small fragments of granitic, cellular, and
jaspery rocks, and of hornstone porphyries, embedded in a base of
wacke, threaded by numerous thin layers of a concretionary pitchstone
passing into obsidian. These layers are parallel, slightly tortuous,
and short; they thin out at their ends, and resemble in form the layers
of quartz in gneiss. It is probable that these small embedded fragments
were not separately ejected, but were entangled in a fluid volcanic
rock, allied to obsidian; and we shall presently see that several
varieties of this latter series of rock assume a laminated structure.
[8] Professor Miller has been so kind as to examine this mineral. He
obtained two good cleavages of 86° 30′ and 86° 50′. The mean of
several, which I made, was 86° 30′. Professor Miller states that these
crystals, when reduced to a fine powder, are soluble in hydrochloric
acid, leaving some undissolved silex behind; the addition of oxalate
of ammonia gives a copious precipitate of lime. He further remarks,
that according to Von Kobell, anorthite (a mineral occurring in the
ejected fragments at Mount Somma) is always white and transparent, so
that if this be the case, these crystals from Ascension must be
considered as Labrador feldspar. Professor Miller adds, that he has
seen an account, in Erdmann’s “Journal für tecnische Chemie,” of a
mineral ejected from a volcano which had the external characters of
Labrador feldspar, but differed in the analysis from that given by
mineralogists of this mineral: the author attributed this difference
to an error in the analysis of Labrador feldspar, which is very old.
[9] Daubeny, in his work on Volcanoes (p. 386), remarks that this is
the case; and Humboldt, in his “Personal Narrative,” vol. i, p. 236,
says “In general, the masses of known primitive rocks, I mean those
which perfectly resemble our granites, gneiss, and mica-slate, are
very rare in lavas: the substances we generally denote by the name of
granite, thrown out by Vesuvius, are mixtures of nepheline, mica, and
pyroxene.”
_Trachytic series of rocks._—Those occupy the more elevated and
central, and likewise the south-eastern, parts of the island. The
trachyte is generally of a pale brown colour, stained with small darker
patches; it contains broken and bent crystals of glassy feldspar,
grains of specular iron, and black microscopical points, which latter,
from being easily fused, and then becoming magnetic, I presume are
hornblende. The greater number of the hills, however, are composed of a
quite white, friable stone, appearing like a trachytic tuff. Obsidian,
hornstone, and several kinds of laminated feldspathic rocks, are
associated with the trachyte. There is no distinct stratification; nor
could I distinguish a crateriform structure in any of the hills of this
series. Considerable dislocations have taken place; and many fissures
in these rocks are yet left open, or are only partially filled with
loose fragments. Within the space,[10] mainly formed of trachyte, some
basaltic streams have burst forth; and not far from the summit of Green
Mountain, there is one stream of quite black, vesicular basalt,
containing minute crystals of glassy feldspar, which have a rounded
appearance.
[10] This space is nearly included by a line sweeping round Green
Mountain, and joining the hills, called the Weather Port Signal,
Holyhead, and that denominated (improperly in a geological sense) “the
Crater of an old volcano.”
The soft white stone above mentioned is remarkable from its singular
resemblance, when viewed in mass, to a sedimentary tuff: it was long
before I could persuade myself that such was not its origin; and other
geologists have been perplexed by closely similar formations in
trachytic regions. In two cases, this white earthy stone formed
isolated hills; in a third, it was associated with columnar and
laminated trachyte; but I was unable to trace an actual junction. It
contains numerous crystals of glassy feldspar and black microscopical
specks, and is marked with small darker patches, exactly as in the
surrounding trachyte. Its basis, however, when viewed under the
microscope, is generally quite earthy; but sometimes it exhibits a
decidedly crystalline structure. On the hill marked “Crater of an old
volcano,” it passes into a pale greenish-grey variety, differing only
in its colour, and in not being so earthy; the passage was in one case
effected insensibly; in another, it was formed by numerous, rounded and
angular, masses of the greenish variety, being embedded in the white
variety;—in this latter case, the appearance was very much like that of
a sedimentary deposit, torn up and abraded during the deposition of a
subsequent stratum. Both these varieties are traversed by innumerable
tortuous veins (presently to be described), which are totally unlike
injected dikes, or indeed any other veins which I have ever seen. Both
varieties include a few scattered fragments, large and small, of
dark-coloured scoriaceous rocks, the cells of some of which are
partially filled with the white earthy stone; they likewise include
some huge blocks of a cellular porphyry.[11] These fragments project
from the weathered surface, and perfectly resemble fragments embedded
in a true sedimentary tuff. But as it is known that extraneous
fragments of cellular rock are sometimes included in columnar trachyte,
in phonolite,[12] and in other compact lavas, this circumstance is not
any real argument for the sedimentary origin of the white earthy
stone.[13] The insensible passage of the greenish variety
into the white one, and likewise the more abrupt passage by fragments
of the former being embedded in the latter, might result from slight
differences in the composition of the same mass of molten stone, and
from the abrading action of one such part still fluid on another part
already solidified. The curiously formed veins have, I believe, been
formed by siliceous matter being subsequently segregated. But my chief
reason for believing that these soft earthy stones, with their
extraneous fragments, are not of sedimentary origin, is the extreme
improbability of crystals of feldspar, black microscopical specks, and
small stains of a darker colour occurring in the same proportional
numbers in an aqueous deposit, and in masses of solid trachyte.
Moreover, as I have remarked, the microscope occasionally reveals a
crystalline structure in the apparently earthy basis. On the other
hand, the partial decomposition of such great masses of trachyte,
forming whole mountains, is undoubtedly a circumstance of not easy
explanation.
[11] The porphyry is dark coloured; it contains numerous, often
fractured, crystals of white opaque feldspar, also decomposing
crystals of oxide of iron; its vesicles include masses of delicate,
hair-like, crystals, apparently of analcime.
[12] D’Aubuisson, “Traité de Géognosie,” tome ii, p. 548.
[13] Dr. Daubeny (on Volcanoes, p. 180) seems to have been led to
believe that certain trachytic formations of Ischia and of the Puy de
Dôme, which closely resemble these of Ascension, were of sedimentary
origin, chiefly from the frequent presence in them “of scoriform
portions, different in colour from the matrix.” Dr. Daubeny adds, that
on the other hand, Brocchi, and other eminent geologists, have
considered these beds as earthy varieties of trachyte; he considers
the subject deserving of further attention.
_Veins in the earthy trachytic masses._—These veins are extraordinarily
numerous, intersecting in the most complicated manner both coloured
varieties of the earthy trachyte: they are best seen on the flanks of
the “Crater of the old volcano.” They contain crystals of glassy
feldspar, black microscopical specks and little dark stains, precisely
as in the surrounding rock; but the basis is very different, being
exceedingly hard, compact, somewhat brittle, and of rather less easy
fusibility. The veins vary much, and suddenly, from the tenth of an
inch to one inch in thickness; they often thin out, not only on their
edges, but in their central parts, thus leaving round, irregular
apertures; their surfaces are rugged. They are inclined at every
possible angle with the horizon, or are horizontal; they are generally
curvilinear, and often interbranch one with another. From their
hardness they withstand weathering, and projecting two or three feet
above the ground, they occasionally extend some yards in length; these
plate-like veins, when struck, emit a sound, almost like that of a
drum, and they may be distinctly seen to vibrate; their fragments,
which are strewed on the ground, clatter like pieces of iron when
knocked against each other. They often assume the most singular forms;
I saw a pedestal of the earthy trachyte, covered by a hemispherical
portion of a vein, like a great umbrella, sufficiently large to shelter
two persons. I have never met with, or seen described, any veins like
these; but in form they resemble the ferruginous seams, due to some
process of segregation, occurring not uncommonly in sandstones,—for
instance, in the New Red sandstone of England. Numerous veins of jasper
and of siliceous sinter, occurring on the summit of this same hill,
show that there has been some abundant source of silica, and as these
plate-like veins differ from the trachyte
only in their greater hardness, brittleness, and less easy fusibility,
it appears probable that their origin is due to the segregation or
infiltration of siliceous matter, in the same manner as happens with
the oxides of iron in many sedimentary rocks.
_Siliceous sinter and jasper._—The siliceous sinter is either quite
white, of little specific gravity, and with a somewhat pearly fracture,
passing into pinkish pearl quartz; or it is yellowish white, with a
harsh fracture, and it then contains an earthy powder in small
cavities. Both varieties occur, either in large irregular masses in the
altered trachyte, or in seams included in broad, vertical, tortuous,
irregular veins of a compact, harsh stone of a dull red colour,
appearing like a sandstone. This stone, however, is only altered
trachyte; and a nearly similar variety, but often honeycombed,
sometimes adheres to the projecting plate-like veins, described in the
last paragraph. The jasper is of an ochre yellow or red colour; it
occurs in large irregular masses, and sometimes in veins, both in the
altered trachyte and in an associated mass of scoriaceous basalt. The
cells of the scoriaceous basalt are lined or filled with fine,
concentric layers of chalcedony, coated and studded with bright-red
oxide of iron. In this rock, especially in the rather more compact
parts, irregular angular patches of the red jasper are included, the
edges of which insensibly blend into the surrounding mass; other
patches occur having an intermediate character between perfect jasper
and the ferruginous, decomposed, basaltic base. In these patches, and
likewise in the large vein-like masses of jasper, there occur little
rounded cavities, of exactly the same size and form with the air-cells,
which in the scoriaceous basalt are filled and lined with layers of
chalcedony. Small fragments of the jasper, examined under the
microscope, seem to resemble the chalcedony with its colouring matter
not separated into layers, but mingled in the siliceous paste, together
with some impurities. I can understand these facts,—namely, the
blending of the jasper into the semi-decomposed basalt,—its occurrence
in angular patches, which clearly do not occupy pre-existing hollows in
the rock,—and its containing little vesicles filled with chalcedony,
like those in the scoriaceous lava,—only on the supposition that a
fluid, probably the same fluid which deposited the chalcedony in the
air-cells, removed in those parts where there were no cavities, the
ingredients of the basaltic rock, and left in their place silica and
iron, and thus produced the jasper. In some specimens of silicified
wood, I have observed, that in the same manner as in the basalt, the
solid parts were converted into a dark-coloured homogeneous stone,
whereas the cavities formed by the larger sap-vessels (which may be
compared with the air-vesicles in the basaltic lava) and other
irregular hollows, apparently produced by decay, were filled with
concentric layers of chalcedony; in this case, there can be little
doubt that the same fluid deposited the homogeneous base and the
chalcedonic layers. After these considerations, I cannot doubt but that
the jasper of Ascension may be viewed as a volcanic rock silicified, in
precisely the same sense as this term is applied to wood, when
silicified; we are equally ignorant of the means by which every atom of
wood, whilst in a perfect state, is
removed and replaced by atoms of silica, as we are of the means by
which the constituent parts of a volcanic rock could be thus acted
on.[14] I was led to the careful examination of these rocks, and to the
conclusion here given, from having heard the Rev. Professor Henslow
express a similar opinion, regarding the origin in trap-rocks of many
chalcedonies and agates. Siliceous deposits seem to be very general, if
not of universal occurrence, in partially decomposed trachytic
tuffs;[15] and as these hills, according to the view above given,
consist of trachyte softened and altered in situ, the presence of free
silica in this case may be added as one more instance to the list.
[14] Beudant (“Voyage en Hongrie,” tome iii, pp. 502, 504) describes
kidney-shaped masses of jasper-opal, which either blend into the
surrounding trachytic conglomerate, or are embedded in it like
chalk-flints; and he compares them with the fragments of opalised
wood, which are abundant in this same formation. Beudant, however,
appears to have viewed the process of their formation rather as one of
simple infiltration than of molecular exchange; but the presence of a
concretion, wholly different from the surrounding matter, if not
formed in a pre-existing hollow, clearly seems to me to require,
either a molecular or mechanical displacement of the atoms, which
occupied the space afterwards filled by it. The jasper-opal of Hungary
passes into chalcedony, and therefore in this case, as in that of
Ascension, jasper seems to be intimately related in origin with
chalcedony.
[15] Beudant (“Voyage Min.,” tome iii, p. 507) enumerates cases in
Hungary, Germany, Central France, Italy, Greece, and Mexico.
_Concretions in pumiceous tuff._—The hill, marked in Map 2 “Crater of
an old volcano,” has no claims to this appellation, which I could
discover, except in being surmounted by a circular, very shallow,
saucer-like summit, nearly half a mile in diameter. This hollow has
been nearly filled up with many successive sheets of ashes and scoriæ,
of different colours, and slightly consolidated. Each successive
saucer-shaped layer crops out all round the margin, forming so many
rings of various colours, and giving to the hill a fantastic
appearance. The outer ring is broad, and of a white colour; hence it
resembles a course round which horses have been exercised, and has
received the name of the Devil’s Riding School, by which it is most
generally known. These successive layers of ashes must have fallen over
the whole surrounding country, but they have all been blown away except
in this one hollow, in which probably moisture accumulated, either
during an extraordinary year when rain fell, or during the storms often
accompanying volcanic eruptions. One of the layers of a pinkish colour,
and chiefly derived from small, decomposed fragments of pumice, is
remarkable, from containing numerous concretions. These are generally
spherical, from half an inch to three inches in diameter; but they are
occasionally cylindrical, like those of iron-pyrites in the chalk of
Europe. They consist of a very tough, compact, pale-brown stone, with a
smooth and even fracture. They are divided into concentric layers by
thin white partitions, resembling the external superficies; six or
eight of such layers are distinctly defined near the outside; but those
towards the inside generally become indistinct, and blend into a
homogeneous
mass. I presume that these concentric layers were formed by the
shrinking of the concretion, as it became compact. The interior part is
generally fissured by minute cracks or septaria, which are lined, both
by black, metallic, and by other white and crystalline specks, the
nature of which I was unable to ascertain. Some of the larger
concretions consist of a mere spherical shell, filled with slightly
consolidated ashes. The concretions contain a small proportion of
carbonate of lime: a fragment placed under the blowpipe decrepitates,
then whitens and fuses into a blebby enamel, but does not become
caustic. The surrounding ashes do not contain any carbonate of lime;
hence the concretions have probably been formed, as is so often the
case, by the aggregation of this substance. I have not met with any
account of similar concretions; and considering their great toughness
and compactness, their occurrence in a bed, which probably has been
subjected only to atmospheric moisture, is remarkable.
_Formation of calcareous rocks on the sea-coast._—On several of the
sea-beaches, there are immense accumulations of small, well-rounded
particles of shells and corals, of white, yellowish, and pink colours,
interspersed with a few volcanic particles. At the depth of a few feet,
these are found cemented together into stone, of which the softer
varieties are used for building; there are other varieties, both coarse
and fine-grained, too hard for this purpose: and I saw one mass divided
into even layers half an inch in thickness, which were so compact that
when struck with a hammer they rang like flint. It is believed by the
inhabitants, that the particles become united in the course of a single
year. The union is effected by calcareous matter; and in the most
compact varieties, each rounded particle of shell and volcanic rock can
be distinctly seen to be enveloped in a husk of pellucid carbonate of
lime. Extremely few perfect shells are embedded in these agglutinated
masses; and I have examined even a large fragment under a microscope,
without being able to discover the least vestige of striæ or other
marks of external form: this shows how long each particle must have
been rolled about, before its turn came to be embedded and
cemented.[16] One of the most compact varieties, when placed in acid,
was entirely dissolved, with the exception of some flocculent animal
matter; its specific gravity was 2·63. The specific gravity of ordinary
limestone varies from 2·6 to 2·75; pure Carrara marble was found by Sir
H. De la Beche[17] to be 2·7. It is remarkable that these rocks of
Ascension, formed close to the surface, should be nearly as compact as
marble, which has undergone the action of heat and pressure in the
plutonic regions.
[16] The eggs of the turtle being buried by the parent, sometimes
become enclosed in the solid rock. Mr. Lyell has given a figure
(“Principles of Geology,” book iii, ch. 17) of some eggs, containing
the bones of young turtles, found thus entombed.
[17] “Researches in Theoretical Geology,” p. 12.
The great accumulation of loose calcareous particles, lying on the
beach near the Settlement, commences in the month of October, moving
towards the S.W., which, as I was informed by Lieutenant
Evans, is caused by a change in the prevailing direction of the
currents. At this period the tidal rocks, at the S.W. end of the beach,
where the calcareous sand is accumulating, and round which the currents
sweep, become gradually coated with a calcareous incrustation, half an
inch in thickness. It is quite white, compact, with some parts slightly
spathose, and is firmly attached to the rock. After a short time it
gradually disappears, being either redissolved, when the water is less
charged with lime, or more probably is mechanically abraded. Lieutenant
Evans has observed these facts, during the six years he has resided at
Ascension. The incrustation varies in thickness in different years: in
1831 it was unusually thick. When I was there in July, there was no
remnant of the incrustation; but on a point of basalt, from which the
quarrymen had lately removed a mass of the calcareous freestone, the
incrustation was perfectly preserved. Considering the position of the
tidal-rocks, and the period at which they become coated, there can be
no doubt that the movement and disturbance of the vast accumulation of
calcareous particles, many of them being partially agglutinated
together, cause the waves of the sea to be so highly charged with
carbonate of lime, that they deposit it on the first objects against
which they impinge. I have been informed by Lieutenant Holland, R.N.,
that this incrustation is formed on many parts of the coast, on most of
which, I believe, there are likewise great masses of comminuted shells.
_A frondescent calcareous incrustation._—In many respects this is a
singular deposit; it coats throughout the year the tidal volcanic
rocks, that project from the beaches composed of broken shells. Its
general appearance is well represented in Figure 5; but the fronds or
discs, of which it is composed, are generally so closely crowded
together as to touch. These fronds have their sinuous edges finely
crenulated, and they project over their pedestals or supports; their
upper surfaces are either slightly concave, or slightly convex; they
are highly polished, and of a dark grey or jet black colour; their form
is irregular, generally circular, and from the tenth of an inch to one
inch and a half in diameter; their thickness, or amount of their
projection from the rock on which they stand, varies much, about a
quarter of an inch being perhaps most usual. The fronds occasionally
become more and more convex, until they pass into botryoidal masses
with their summits fissured; when in this state, they are glossy and of
an intense black, so as to resemble some fused metallic substance. I
have shown the incrustation, both in this latter and in its ordinary
state to several geologists, but not one could conjecture its origin,
except that perhaps it was of volcanic nature!
No. 5
[Illustration: An incrustration of calcareous and animal matter.]
An incrustation of calcareous and animal matter, coating the
tidal-rocks at Ascension.
The substance forming the fronds has a very compact and often almost
crystalline fracture; the edges being translucent, and hard enough
easily to scratch calcareous spar. Under the blowpipe it immediately
becomes white, and emits a strong animal odour, like that from fresh
shells. It is chiefly composed of carbonate of lime; when placed in
muriatic acid it froths much, leaving a residue of sulphate of lime,
and of an oxide of iron, together with a black powder, which is not
soluble in heated acids. This latter substance seems to be
carbonaceous,
and is evidently the colouring matter. The sulphate of lime is
extraneous, and occurs in distinct, excessively minute, lamellar
plates, studded on the surface of the fronds, and embedded between the
fine layers of which they are composed; when a fragment is heated in
the blowpipe, these lamellæ are immediately rendered visible. The
original outline of the fronds may often be traced, either to a minute
particle of shell fixed in a crevice of the rock, or to several
cemented together; these first become deeply corroded, by the
dissolving power of the waves, into sharp ridges, and then are coated
with successive layers of the glossy, grey, calcareous incrustation.
The inequalities of the primary support affect the outline of every
successive layer, in the same manner as may often be seen in
bezoar-stones, when an object like a nail forms the centre of
aggregation. The crenulated edges, however, of the frond appear to be
due to the corroding power of the surf on its own deposit, alternating
with fresh depositions. On some smooth basaltic rocks on the coast of
St. Jago, I found an exceedingly thin layer of brown calcareous matter,
which under a lens presented a miniature likeness of the crenulated and
polished fronds of Ascension; in this case a basis was not afforded by
any projecting extraneous particles. Although the incrustation at
Ascension is persistent throughout the year; yet from the abraded
appearance of some parts, and from the fresh appearance of other parts,
the whole seems to undergo a round of decay and renovation, due
probably to changes in the form of the shifting beach, and consequently
in the action of the breakers: hence probably it is, that the
incrustation never acquires a great thickness. Considering the position
of the encrusted rocks in the midst of the calcareous beach, together
with its composition, I think there can be no doubt that its origin is
due to the dissolution and subsequent deposition of the matter
composing the rounded particles of shells and corals.[18] From this
source
it derives its animal matter, which is evidently the colouring
principle. The nature of the deposit, in its incipient stage, can often
be well seen upon a fragment of white shell, when jammed between two of
the fronds; it then appears exactly like the thinnest wash of a pale
grey varnish. Its darkness varies a little, but the jet blackness of
some of the fronds and of the botryoidal masses seems due to the
translucency of the successive grey layers. There is, however, this
singular circumstance, that when deposited on the under side of ledges
of rock or in fissures, it appears always to be of a pale, pearly grey
colour, even when of considerable thickness: hence one is led to
suppose, that an abundance of light is necessary to the development of
the dark colour, in the same manner as seems to be the case with the
upper and exposed surfaces of the shells of living mollusca, which are
always dark, compared with their under surfaces and with the parts
habitually covered by the mantle of the animal. In this
circumstance,—in the immediate loss of colour and in the odour emitted
under the blowpipe,—in the degree of hardness and translucency of the
edges,—and in the beautiful polish of the surface,[19] rivalling when
in a fresh state that of the finest Oliva, there is a striking analogy
between this inorganic incrustation and the shells of living molluscous
animals.[20] This appears to me to be an interesting physiological
fact.[21]
[18] The selenite, as I have remarked is extraneous, and must have
been derived from the sea-water. It is an interesting circumstance
thus to find the waves of the ocean, sufficiently charged with
sulphate of lime, to deposit it on the rocks, against which they dash
every tide. Dr. Webster has described (“Voyage of the _Chanticleer,_”
vol. ii, p. 319) beds of gypsum and salt, as much as two feet in
thickness, left by the evaporation of the spray on the rocks on the
windward coast. Beautiful stalactites of selenite, resembling in form
those of carbonate of lime, are formed near these beds. Amorphous
masses of gypsum, also, occur in caverns in the interior of the
island; and at Cross Hill (an old crater) I saw a considerable
quantity of salt oozing from a pile of scoriæ. In these latter cases,
the salt and gypsum appear to be volcanic products.)
[19] From the fact described in my “Journal of Researches” of a
coating of oxide of iron, deposited by a streamlet on the rocks in its
bed (like a nearly similar coating at the great cataracts of the
Orinoco and Nile), becoming finely polished where the surf acts, I
presume that the surf in this instance, also, is the polishing agent.)
[20] In the section descriptive of St. Paul’s Rocks, I have described
a glossy, pearly substance, which coats the rocks, and an allied
stalactitical incrustation from Ascension, the crust of which
resembles the enamel of teeth, but is hard enough to scratch
plate-glass. Both these substances contain animal matter, and seem to
have been derived from water in filtering through birds’ dung.
[21] Mr. Horner and Sir David Brewster have described (“Philosophical
Transactions,” 1836, p. 65) a singular “artificial substance,
resembling shell.” It is deposited in fine, transparent, highly
polished, brown-coloured laminæ, possessing peculiar optical
properties, on the inside of a vessel, in which cloth, first prepared
with glue and then with lime, is made to revolve rapidly in water. It
is much softer, more transparent, and contains more animal matter,
than the natural incrustation at Ascension; but we here again see the
strong tendency which carbonate of lime and animal matter evince to
form a solid substance allied to shell.
_Singular laminated beds alternating with and passing into
obsidian._—These beds occur within the trachytic district, at the
western base of Green Mountain, under which they dip at a high
inclination. They are only partially exposed, being covered up by
modern ejections; from this cause, I was unable to trace their junction
with the trachyte, or to discover whether they had flowed as a stream
of lava, or had been injected amidst the overlying strata. There are
three principal beds of obsidian, of which the thickest forms the base
of the section. The alternating stony layers appear to me eminently
curious, and shall be first described, and afterwards their passage
into the obsidian. They have an extremely diversified appearance; five
principal varieties may be noticed, but these insensibly blend into
each other by endless gradations.
First.—A pale grey, irregularly and coarsely laminated,[22]
harsh-feeling rock, resembling clay-slate which has been in contact
with a trap-dike, and with a fracture of about the same degree of
crystalline structure. This rock, as well as the following varieties,
easily fuses into a pale glass. The greater part is honeycombed with
irregular, angular, cavities, so that the whole has a curious
appearance, and some fragments resemble in a remarkable manner
silicified logs of decayed wood. This variety, especially where more
compact, is often marked with thin whitish streaks, which are either
straight or wrap round, one behind the other, the elongated carious
hollows.
[22] This term is open to some misinterpretation, as it may be applied
both to rocks divided into laminæ of exactly the same composition, and
to layers firmly attached to each other, with no fissile tendency, but
composed of different minerals, or of different shades of colour. The
term “laminated,” in this chapter, is applied in these latter senses;
where a homogeneous rock splits, as in the former sense, in a given
direction, like clay-slate, I have used the term “fissile.”
Secondly.—A bluish grey or pale brown, compact, heavy, homogeneous
stone, with an angular, uneven, earthy fracture; viewed, however, under
a lens of high power, the fracture is seen to be distinctly
crystalline, and even separate minerals can be distinguished.
Thirdly.—A stone of the same kind with the last, but streaked with
numerous, parallel, slightly tortuous, white lines of the thickness of
hairs. These white lines are more crystalline than the parts between
them; and the stone splits along them: they frequently expand into
exceedingly thin cavities, which are often only just perceptible with a
lens. The matter forming the white lines becomes better crystallised in
these cavities, and Professor Miller was fortunate enough, after
several trials, to ascertain that the white crystals, which are the
largest, were of quartz,[23] and that the minute green transparent
needles were augite, or, as they would more generally be called,
diopside: besides these crystals, there are some minute, dark specks
without a trace of
crystalline, and some fine, white, granular, crystalline matter which
is probably feldspar. Minute fragments of this rock are easily fusible.
[23] Professor Miller informs me that the crystals which he measured
had the faces _P, z, m_ of the figure (147) given by Haidinger in his
Translation of Mohs; and he adds, that it is remarkable, that none of
them had the slightest trace of faces _r_ of the regular six-sided
prism.
Fourthly.—A compact crystalline rock, banded in straight lines with
innumerable layers of white and grey shades of colour, varying in width
from the thirtieth to the two-hundredth of an inch; these layers seem
to be composed chiefly of feldspar, and they contain numerous perfect
crystals of glassy feldspar, which are placed lengthways; they are also
thickly studded with microscopically minute, amorphous, black specks,
which are placed in rows, either standing separately, or more
frequently united, two or three or several together, into black lines,
thinner than a hair. When a small fragment is heated in the blowpipe,
the black specks are easily fused into black brilliant beads, which
become magnetic,—characters that apply to no common mineral except
hornblende or augite. With the black specks there are mingled some
others of a red colour, which are magnetic before being heated, and no
doubt are oxide of iron. Round two little cavities, in a specimen of
this variety, I found the black specks aggregated into minute crystals,
appearing like those of augite or hornblende, but too dull and small to
be measured by the goniometer; in the specimen, also, I could
distinguish amidst the crystalline feldspar, grains, which had the
aspect of quartz. By trying with a parallel ruler, I found that the
thin grey layers and the black hair-like lines were absolutely straight
and parallel to each other. It is impossible to trace the gradation
from the homogeneous grey rocks to these striped varieties, or indeed
the character of the different layers in the same specimen, without
feeling convinced that the more or less perfect whiteness of the
crystalline feldspathic matter depends on the more or less perfect
aggregation of diffused matter, into the black and red specks of
hornblende and oxide of iron.
Fifthly.—A compact heavy rock, not laminated, with an irregular,
angular, highly crystalline, fracture; it abounds with distinct
crystals of glassy feldspar, and the crystalline feldspathic base is
mottled with a black mineral, which on the weathered surface is seen to
be aggregated into small crystals, some perfect, but the greater number
imperfect. I showed this specimen to an experienced geologist, and
asked him what it was; he answered, as I think every one else would
have done, that it was a primitive greenstone. The weathered surface,
also, of the banded variety in figure No. 4, strikingly resembles a
worn fragment of finely laminated gneiss.
These five varieties, with many intermediate ones, pass and repass into
each other. As the compact varieties are quite subordinate to the
others, the whole may be considered as laminated or striped. The
laminæ, to sum up their characteristics, are either quite straight, or
slightly tortuous, or convoluted; they are all parallel to each other,
and to the intercalating strata of obsidian; they are generally of
extreme thinness; they consist either of an apparently homogeneous,
compact rock, striped with different shades of grey and brown colours,
or of crystalline feldspathic layers in a more or less perfect state of
purity, and of different thicknesses, with distinct crystals of glassy
feldspar
placed lengthways, or of very thin layers chiefly composed of minute
crystals of quartz and augite, or composed of black and red specks of
an augitic mineral and of an oxide of iron, either not crystallised or
imperfectly so. After having fully described the obsidian, I shall
return to the subject of the lamination of rocks of the trachytic
series.
The passage of the foregoing beds into the strata of glassy obsidian is
effected in several ways: first, angulo-modular masses of obsidian,
both large and small, abruptly appear disseminated in a slaty, or in an
amorphous, pale-coloured, feldspathic rock, with a somewhat pearly
fracture. Secondly, small irregular nodules of the obsidian, either
standing separately, or united into thin layers, seldom more than the
tenth of an inch in thickness, alternate repeatedly with very thin
layers of a feldspathic rock, which is striped with the finest parallel
zones of colour, like an agate, and which sometimes passes into the
nature of pitchstone; the interstices between the nodules of obsidian
are generally filled by soft white matter, resembling pumiceous ashes.
Thirdly, the whole substance of the bounding rock suddenly passes into
an angulo-concretionary mass of obsidian. Such masses (as well as the
small nodules) of obsidian are of a pale green colour, and are
generally streaked with different shades of colour, parallel to the
laminæ of the surrounding rock; they likewise generally contain minute
white sphærulites, of which half is sometimes embedded in a zone of one
shade of colour, and half in a zone of another shade. The obsidian
assumes its jet black colour and perfectly conchoidal fracture, only
when in large masses; but even in these, on careful examination and on
holding the specimens in different lights, I could generally
distinguish parallel streaks of different shades of darkness.
No. 6
[Illustration: Opaque brown sphærulites, drawn on an enlarged scale.]
Opaque brown sphærulites, drawn on an enlarged scale. The upper ones
are externally marked with parallel ridges. The internal radiating
structure of the lower ones, is much too plainly represented.
No. 7
[Illustration: A layer, formed by the union of minute brown
sphærulites.
A layer, formed by the union of minute brown sphærulites, intersecting
two other similar layers: the whole represented of nearly the natural
size.
One of the commonest transitional rocks deserves in several respects a
further description. It is of a very complicated nature, and consists
of numerous thin, slightly tortuous layers of a pale-coloured
feldspathic stone, often passing into an imperfect pitchstone,
alternating with layers formed of numberless little globules of two
varieties of obsidian, and of two kinds of sphærulites, embedded in a
soft or in a hard pearly base. The sphærulites are either white and
translucent, or dark brown and opaque; the former are quite spherical,
of small size, and distinctly radiated from their centre. The dark
brown sphærulites are less perfectly round, and vary in diameter from
the twentieth to the thirtieth of an inch; when broken they exhibit
towards their centres, which are whitish, an obscure radiating
structure; two of them when united sometimes have only one central
point of radiation; there is occasionally a trace of or a hollow
crevice in their centres. They stand either separately, or are united
two or three or many together into irregular groups, or more commonly
into layers, parallel to the stratification of the mass. This union in
many cases is so perfect, that the two sides of the layer thus formed,
are quite even; and these layers, as they become less brown and opaque,
cannot be distinguished from the alternating layers of the
pale-coloured feldspathic stone. The sphærulites, when not united, are
generally compressed in the plane of the lamination of the mass; and in
this same plane, they are often marked internally, by zones of
different
shades of colour, and externally by small ridges and furrows. In the
upper part of figure No. 6, the sphærulites with the parallel ridges
and furrows are represented on an enlarged scale, but they are not well
executed; and in the lower part, their usual manner of grouping is
shown. In another specimen, a thin layer formed of the brown
sphærulites closely united together, intersects, as represented in
figure No. 7, a layer of similar composition; and after running for a
short space in a slightly curved line, again intersects it, and
likewise a second layer lying a little way beneath that first
intersected. The small nodules also of obsidian are sometimes
externally marked with ridges and furrows, parallel to the lamination
of the mass, but always less plainly than the sphærulites. These
obsidian nodules are generally angular, with their edges blunted: they
are often impressed with the form of the adjoining sphærulites, than
which they are always larger; the separate nodules seldom appear to
have drawn each other out by exerting a mutually attractive force. Had
I not found in some cases, a distinct centre of attraction in these
nodules of obsidian, I should have
been led to have considered them as residuary matter, left during the
formation of the pearlstone, in which they are embedded, and of the
sphærulitic globules.
The sphærulites and the little nodules of obsidian in these rocks so
closely resemble, in general form and structure, concretions in
sedimentary deposits, that one is at once tempted to attribute to them
an analogous origin. They resemble ordinary concretions in the
following respects: in their external form,—in the union of two or
three, or of several, into an irregular mass, or into an even-sided
layer,—in the occasional intersection of one such layer by another, as
in the case of chalk-flints,—in the presence of two or three kinds of
nodules, often close together, in the same basis,—in their fibrous,
radiating structure, with occasional hollows in their centres,—in the
co-existence of a laminary, concretionary, and radiating structure, as
is so well developed in the concretions of magnesian limestone,
described by Professor Sedgwick.[24] Concretions in sedimentary
deposits, it is known, are due to the separation from the surrounding
mass of the whole or part of some mineral substance, and its
aggregation round certain points of attraction. Guided by this fact, I
have endeavoured to discover whether obsidian and the sphærulites (to
which may be added marekanite and pearlstone, both of them occurring in
nodular concretions in the trachytic series) differ in their
constituent parts, from the minerals generally composing trachytic
rocks. It appears from three analyses, that obsidian contains on an
average 76 per cent of silica; from one analysis, that sphærulites
contain 79·12; from two, that marekanite contains 79·25; and from two
other analyses, that pearlstone contains 75·62 of silica.[25] Now, the
constituent parts of trachyte, as far as they can be distinguished
consist of feldspar, containing 65·21 of silica; or of albite,
containing 69·09; of hornblende, containing 55·27,[26] and of oxide of
iron: so that the foregoing glassy concretionary substances all contain
a larger proportion of silica than that occurring in ordinary
feldspathic or trachytic rocks. D’Aubuisson,[27] also, has remarked on
the large proportion of silica compared with alumina, in six analyses
of obsidian and pearlstone given in Brongniart’s “Mineralogy.” Hence I
conclude, that the foregoing concretions have been formed by a process
of aggregation, strictly analogous to that which takes place in aqueous
deposits, acting chiefly on the silica, but likewise on some of the
other elements of the surrounding mass, and thus producing the
different concretionary varieties. From the well-known effects of rapid
cooling[28] in giving glassiness of
texture, it is probably necessary that the entire mass, in cases like
that of Ascension, should have cooled at a certain rate; but
considering the repeated and complicated alterations of nodules and
thin layers of a glassy texture with other layers quite stony or
crystalline, all within the space of a few feet or even inches, it is
hardly possible that they could have cooled at different rates, and
thus have acquired their different textures.
[24] “Geological Transactions,” vol. 3, part i, p. 37.
[25] The foregoing analyses are taken from Beudant “Traité de
Minéralogie,” tome ii, p. 113; and one analysis of obsidian from
Phillips’s “Mineralogy.”
[26] These analyses are taken from Von Kobell’s “Grundzüge der
Mineralogie,” 1838.
[27] “Traité de Géogn.,” tome ii, p. 535.
[28] This is seen in the manufacture of common glass, and in Gregory
Watts’s experiments on molten trap; also on the natural surfaces of
lava-streams, and on the side-walls of dikes.
The natural sphærulites in these rocks[29] very closely resemble those
produced in glass, when slowly cooled. In some fine specimens of
partially devitrified glass, in the possession of Mr. Stokes, the
sphærulites are united into straight layers with even sides, parallel
to each other, and to one of the outer surfaces, exactly as in the
obsidian. These layers sometimes interbranch and form loops; but I did
not see any case of actual intersection. They form the passage from the
perfectly glassy portions, to those nearly homogeneous and stony, with
only an obscure concretionary structure. In the same specimen, also,
sphærulites differing slightly in colour and in structure, occur
embedded close together. Considering these facts, it is some
confirmation of the view above given of the concretionary origin of the
obsidian and natural sphærulites, to find that M. Dartigues,[30] in his
curious paper on this subject, attributes the production of sphærulites
in glass, to the different ingredients obeying their own laws of
attraction and becoming aggregated. He is led to believe that this
takes place, from the difficulty in remelting sphærulitic glass,
without the whole be first thoroughly pounded and mixed together; and
likewise from the fact, that the change takes place most readily in
glass composed of many ingredients. In confirmation of M. Dartigues’
view, I may remark, that M. Fleuriau de Bellevue[31] found that the
sphærulitic portions of devitrified glass were acted on both by nitric
acid and under the blowpipe, in a different manner from the compact
paste in which they were embedded.
[29] I do not know whether it is generally known, that bodies having
exactly the same appearance as sphærulites, sometimes occur in agates.
Mr. Robert Brown showed me in an agate, formed within a cavity in a
piece of silicified wood, some little specks, which were only just
visible to the naked eye: these specks, when placed by him under a
lens of high power, presented a beautiful appearance: they were
perfectly circular, and consisted of the finest fibres of a brown
colour, radiating with great exactness from a common centre. These
little radiating stars are occasionally intersected, and portions are
quite cut off by the fine, ribbon-like zones of colour in the agate.
In the obsidian of Ascension, the halves of a sphærulite often lie in
different zones of colour, but they are not cut off by them, as in the
agate.
[30] _Journal de Physique,_ tome 59 (1804), pp. 10, 12.
[31] _Idem,_ tome 60 (1805), p. 418.
_Comparison of the obsidian beds and alternating strata of ascension,
with those of other countries._—I have been struck with much surprise,
how closely the excellent description of the obsidian rocks of Hungary,
given by Beudant,[32] and that by Humboldt, of the same formation in
Mexico and Peru,[33] and likewise the descriptions given by several
authors[34] of the trachytic regions in the Italian islands, agree with
my observations at Ascension. Many passages might have been transferred
without alteration from the works of the above authors, and would have
been applicable to this island. They all agree in the laminated and
stratified character of the whole series; and Humboldt speaks of some
of the beds of obsidian being ribboned like jasper.[35] They all agree
in the nodular or concretionary character of the obsidian, and of the
passage of these nodules into layers. They all refer to the repeated
alterations, often in undulatory planes, of glassy, pearly, stony, and
crystalline layers: the crystalline layers, however, seem to be much
more perfectly developed at Ascension, than in the above-named
countries. Humboldt compares some of the stony beds, when viewed from a
distance, to strata of a schistose sandstone. Sphærulites are described
as occurring abundantly in all cases; and they everywhere seem to mark
the passage, from the perfectly glassy to the stony and crystalline
beds. Beudant’s account[36] of his “perlite lithoide globulaire” in
every, even the most trifling particular, might have been written for
the little brown sphærulitic globules of the rocks of Ascension.
[32] “Voyage en Hongrie,” tome i, p. 330; tome ii, pp. 221 and 315;
tome iii, pp. 369, 371, 377, 381.
[33] “Essai Géognostique,” pp. 176, 326, 328.
[34] P. Scrope, in “Geological Transactions,” vol. ii (second series)
p. 195. Consult, also, Dolomieu’s “Voyage aux Isles Lipari,” and
D’Aubuisson, “Traité de Géogn.,” tome ii, p. 534.
[35] In Mr. Stokes’ fine collection of obsidians from Mexico, I
observe that the sphærulites are generally much larger than those of
Ascension; they are generally white, opaque, and are united into
distinct layers: there are many singular varieties, different from any
at Ascension. The obsidians are finely zoned, in quite straight or
curved lines, with exceedingly slight differences of tint, of
cellularity, and of more or less perfect degrees of glassiness.
Tracing some of the less perfectly glassy zones, they are seen to
become studded with minute white sphærulites, which become more and
more numerous, until at last they unite and form a distinct layer: on
the other hand, at Ascension, only the brown sphærulites unite and
form layers; the white ones always being irregularly disseminated.
Some specimens at the Geological Society, said to belong to an
obsidian formation from Mexico, have an earthy fracture, and are
divided in the finest parallel laminæ, by specks of a black mineral,
like the augitic or hornblendic specks in the rocks at Ascension.
[36] Beudant’s “Voyage,” tome iii, p. 373.
From the close similarity in so many respects, between the obsidian
formations of Hungary, Mexico, Peru, and of some of the Italian
islands, with that of Ascension, I can hardly doubt that in all these
cases, the obsidian and the sphærulites owe their origin to a
concretionary aggregation of the silica, and of some of the other
constituent elements, taking place whilst the liquified mass cooled at
a certain required rate. It is, however, well-known, that in several
places, obsidian has flowed in streams like lava; for instance, at
Teneriffe, at the Lipari Islands, and at Iceland.[37] In these cases,
the superficial parts are the most
perfectly glassy, the obsidian passing at the depth of a few feet into
an opaque stone. In an analysis by Vauquelin of a specimen of obsidian
from Hecla, which probably flowed as lava, the proportion of silica is
nearly the same as in the nodular or concretionary obsidian from
Mexico. It would be interesting to ascertain, whether the opaque
interior portions and the superficial glassy coating contained the same
proportional constituent parts: we know from M. Dufrénoy[38] that the
exterior and interior parts of the same stream of lava sometimes differ
considerably in their composition. Even should the whole body of the
stream of obsidian turn out to be similarly composed with nodular
obsidian, it would only be necessary, in accordance with the foregoing
facts, to suppose that lava in these instances had been erupted with
its ingredients mixed in the same proportion, as in the concretionary
obsidian.
[37] For Teneriffe, see von Buch, “Descript. des Isles Canaries,” pp.
184 and 190; for the Lipari Islands, see Dolomieu’s “Voyage,” p. 34;
for Iceland, see Mackenzie’s “Travels,” p. 369.
[38] “Mémoires pour servir a une Descript. Géolog. de la France,” tome
iv, p. 371.
_Lamination of volcanic rocks of the trachytic series._
We have seen that, in several and widely distant countries, the strata
alternating with beds of obsidian, are highly laminated. The nodules,
also, both large and small, of the obsidian, are zoned with different
shades of colour; and I have seen a specimen from Mexico in Mr. Stokes’
collection, with its external surface weathered[39] into ridges and
furrows, corresponding with the zones of different degrees of
glassiness: Humboldt,[40] moreover, found on the Peak of Teneriffe, a
stream of obsidian divided by very thin, alternating, layers of pumice.
Many other lavas of the feldspathic series are laminated; thus, masses
of common trachyte at Ascension are divided by fine earthy lines, along
which the rock splits, separating thin layers of slightly different
shades of colour; the greater number, also, of the embedded crystals of
glassy feldspar are placed lengthways in the same direction. Mr. P.
Scrope[41] has described a remarkable columnar trachyte in the Panza
Islands, which seems to have been injected into an overlying mass of
trachytic conglomerate: it is striped with zones, often of extreme
tenuity, of different textures and colours; the harder and darker zones
appearing to contain a larger proportion of silica. In another part of
the island, there are layers of pearlstone and pitchstone, which in
many respects resemble those of Ascension. The zones in the columnar
trachyte are generally contorted; they extend uninterruptedly for a
great length in a vertical direction, and apparently parallel to the
walls of the dike-like mass. Von Buch[42] has described at Teneriffe, a
stream of lava
containing innumerable thin, plate-like crystals of feldspar, which are
arranged like white threads, one behind the other, and which mostly
follow the same direction. Dolomieu[43] also states, that the grey
lavas of the modern cone of Vulcano, which have a vitreous texture, are
streaked with parallel white lines: he further describes a solid
pumice-stone which possesses a fissile structure, like that of certain
micaceous schists. Phonolite, which I may observe is often, if not
always, an injected rock, also, often has a fissile structure; this is
generally due to the parallel position of the embedded crystals of
feldspar, but sometimes, as at Fernando Noronha, seems to be nearly
independent of their presence.[44] From these facts we see, that
various rocks of the feldspathic series have either a laminated or
fissile structure, and that it occurs both in masses which have
injected into overlying strata, and in others which have flowed as
streams of lava.
[39] MacCulloch states (“Classification of Rocks,” p. 531), that the
exposed surfaces of the pitchstone dikes in Arran are furrowed “with
undulating lines, resembling certain varieties of marbled paper, and
which evidently result from some corresponding difference of laminar
structure.”
[40] “Personal Narrative,” vol. i, p. 222.
[41] “Geological Transactions,” vol. ii (second series), p. 195.
[42] “Description des Iles Canaries,” p. 184.
[43] “Voyage aux Isles de Lipari,” pp. 35 and 85.
[44] In this case, and in that of the fissile pumice-stone, the
structure is very different from that in the foregoing cases, where
the laminæ consist of alternate layers of different composition or
texture. In some sedimentary formations, however, which apparently are
homogeneous and fissile, as in glossy clay-slate, there is reason to
believe, according to D’Aubuisson, that the laminæ are really due to
excessively thin, alternating, layers of mica.
The laminæ of the beds, alternating with the obsidian at Ascension, dip
at a high angle under the mountain, at the base of which they are
situated; and they do not appear as if they had been inclined by
violence. A high inclination is common to these beds in Mexico, Peru,
and in some of the Italian islands:[45] on the other hand, in Hungary,
the layers are horizontal; the laminæ, also, of some of the
lava-streams above referred to, as far as I can understand the
descriptions given of them, appear to be highly inclined or vertical. I
doubt whether in any of these cases, the laminæ have been tilted into
their present position; and in some instances, as in that of the
trachyte described by Mr. Scrope, it is almost certain that they have
been originally formed with a high inclination. In many of these cases,
there is evidence that the mass of liquified rock has moved in the
direction of the laminæ. At Ascension, many of the air-cells have a
drawn out appearance, and are crossed by coarse semi-glassy fibres, in
the direction of the laminæ; and some of the layers, separating the
sphærulitic globules, have a scored appearance, as if produced by the
grating of the globules. I have seen a specimen of zoned obsidian from
Mexico, in Mr. Stokes’ collection, with the surfaces of the
best-defined layers streaked or furrowed with parallel lines; and these
lines or streaks precisely resembled those, produced on the surface of
a mass of artificial glass by its having been poured out of a vessel.
Humboldt, also, has described little cavities, which he compares to the
tails of comets, behind sphærulites in laminated obsidian rocks from
Mexico, and Mr. Scrope has
described other cavities behind fragments embedded in his laminated
trachyte, and which he supposes to have been produced during the
movement of the mass.[46] From such facts, most authors have attributed
the lamination of these volcanic rocks to their movement whilst
liquified. Although it is easy to perceive, why each separate air-cell,
or each fibre in pumice-stone,[47] should be drawn out in the direction
of the moving mass; it is by no means at first obvious why such
air-cells and fibres should be arranged by the movement, in the same
planes, in laminæ absolutely straight and parallel to each other, and
often of extreme tenuity; and still less obvious is it, why such layers
should come to be of slightly different composition and of different
textures.
[45] See Phillips’ “Mineralogy,” for the Italian Islands, p. 136. For
Mexico and Peru, see Humboldt’s “Essai Géognostique.” Mr. Edwards also
describes the high inclination of the obsidian rocks of the Cerro del
Navaja in Mexico in the _Proc. of the Geolog. Soc._ June 1838.
[46] “Geological Transactions,” vol. ii (second series), p. 200 etc.
These embedded fragments, in some instances, consist of the laminated
trachyte broken off and “enveloped in those parts, which still
remained liquid.” Beudant, also, frequently refers in his great work
on “Hungary” (tome iii, p. 386), to trachytic rocks, irregularly
spotted with fragments of the same varieties, which in other parts
form the parallel ribbons. In these cases, we must suppose, that after
part of the molten mass had assumed a laminated structure, a fresh
irruption of lava broke up the mass, and involved fragments, and that
subsequently the whole became relaminated.
[47] Dolomieu’s “Voyage,” p. 64.
In endeavouring to make out the cause of the lamination of these
igneous feldspathic rocks, let us return to the facts so minutely
described at Ascension. We there see, that some of the thinnest layers
are chiefly formed by numerous, exceedingly minute, though perfect,
crystals of different minerals; that other layers are formed by the
union of different kinds of concretionary globules, and that the layers
thus formed, often cannot be distinguished from the ordinary
feldspathic and pitchstone layers, composing a large portion of the
entire mass. The fibrous radiating structure of the sphærulites seems,
judging from many analogous cases, to connect the concretionary and
crystalline forces: the separate crystals, also, of feldspar all lie in
the same parallel planes.[48] These allied forces, therefore, have
played an important part in the lamination of the mass, but they cannot
be considered the primary force; for the several kinds of nodules, both
the smallest and largest, are internally zoned with excessively fine
shades of colour, parallel to the lamination of the whole; and many of
them are, also, externally marked in the same direction with parallel
ridges and furrows, which have not been produced by weathering.
[48] The formation, indeed, of a large crystal of any mineral in a
rock of mixed composition implies an aggregation of the requisite
atoms, allied to concretionary action. The cause of the crystals of
feldspar in these rocks of Ascension, being all placed lengthways, is
probably the same with that which elongates and flattens all the brown
sphærulitic globules (which behave like feldspar under the blowpipe)
in this same direction.
Some of the finest streaks of colour in the stony layers, alternating
with the obsidian, can be distinctly seen to be due to an incipient
crystallisation of the constituent minerals. The extent to which the
minerals have crystallised can, also, be distinctly seen to be
connected
with the greater or less size, and with the number, of the minute,
flattened, crenulated air-cavities or fissures. Numerous facts, as in
the case of geodes, and of cavities in silicified wood, in primary
rocks, and in veins, show that crystallisation is much favoured by
space. Hence, I conclude, that, if in a mass of cooling volcanic rock,
any cause produced in parallel planes a number of minute fissures or
zones of less tension (which from the pent-up vapours would often be
expanded into crenulated air-cavities), the crystallisation of the
constituent parts, and probably the formation of concretions, would be
superinduced or much favoured in such planes; and thus, a laminated
structure of the kind we are here considering would be generated.
That some cause does produce parallel zones of less tension in volcanic
rocks, during their consolidation, we must admit in the case of the
thin alternate layers of obsidian and pumice described by Humboldt, and
of the small, flattened, crenulated air-cells in the laminated rocks of
Ascension; for on no other principle can we conceive why the confined
vapours should through their expansion form air-cells or fibres in
separate, parallel planes, instead of irregularly throughout the mass.
In Mr. Stokes’ collection, I have seen a beautiful example of this
structure, in a specimen of obsidian from Mexico, which is shaded and
zoned, like the finest agate, with numerous, straight, parallel layers,
more or less opaque and white, or almost perfectly glassy; the degree
of opacity and glassiness depending on the number of microscopically
minute, flattened air-cells; in this case, it is scarcely possible to
doubt but that the mass, to which the fragment belonged, must have been
subjected to some, probably prolonged, action, causing the tension
slightly to vary in the successive planes.
Several causes appear capable of producing zones of different tension,
in masses semi-liquified by heat. In a fragment of devitrified glass, I
have observed layers of sphærulites which appeared, from the manner in
which they were abruptly bent, to have been produced by the simple
contraction of the mass in the vessel, in which it cooled. In certain
dikes on Mount Etna, described by M. Elie de Beaumont,[49] as bordered
by alternating bands of scoriaceous and compact rock, one is led to
suppose that the stretching movement of the surrounding strata, which
originally produced the fissures, continued whilst the injected rock
remained fluid. Guided, however, by Professor Forbes’[50] clear
description of the zoned structure of glacier-ice, far the most
probable explanation of the laminated structure of these feldspathic
rocks appears to be, that they have been stretched whilst slowly
flowing onwards in a pasty condition,[51] in precisely the same manner
as Professor Forbes believes, that the ice of moving glaciers is
stretched and fissured. In both cases,
the zones may be compared to those in the finest agates; in both, they
extend in the direction in which the mass has flowed, and those exposed
on the surface are generally vertical: in the ice, the porous laminæ
are rendered distinct by the subsequent congelation of infiltrated
water, in the stony feldspathic lavas, by subsequent crystalline and
concretionary action. The fragment of glassy obsidian in Mr. Stokes’
collection, which is zoned with minute air-cells must strikingly
resemble, judging from Professor Forbes’ descriptions, a fragment of
the zoned ice; and if the rate of cooling and nature of the mass had
been favourable to its crystallisation or to concretionary action, we
should here have had the finest parallel zones of different composition
and texture. In glaciers, the lines of porous ice and of minute
crevices seem to be due to an incipient stretching, caused by the
central parts of the frozen stream moving faster than the sides and
bottom, which are retarded by friction: hence in glaciers of certain
forms and towards the lower end of most glaciers, the zones become
horizontal. May we venture to suppose that in the feldspathic lavas
with horizontal laminæ, we see an analogous case? All geologists, who
have examined trachytic regions, have come to the conclusion, that the
lavas of this series have possessed an exceedingly imperfect fluidity;
and as it is evident that only matter thus characterised would be
subject to become fissured and to be formed into zones of different
tensions, in the manner here supposed, we probably see the reason why
augitic lavas, which appear generally to have possessed a high degree
of fluidity, are not,[52] like the feldspathic lavas, divided into
laminæ of different composition and texture. Moreover, in the augitic
series, there never appears to be any tendency to concretionary action,
which we have seen plays an important part in the lamination of rocks,
of the trachytic series, or at least in rendering that structure
apparent.
[49] “Mém. pour servir,” etc., tome iv, p. 131.
[50] _Edinburgh New Phil. Journal,_ 1842, p. 350.
[51] I presume that this is nearly the same explanation which Mr.
Scrope had in his mind, when he speaks (“Geolog. Transact.,” vol. ii,
second series, p. 228) of the ribboned structure of his trachytic
rocks, having arisen, from “a linear extension of the mass, while in a
state of imperfect liquidity, coupled with a concretionary process.”
[52] Basaltic lavas, and many other rocks, are not unfrequently
divided into thick laminæ or plates, of the same composition, which
are either straight or curved; these being crossed by vertical lines
of fissure, sometimes become united into columns. This structure seems
related, in its origin, to that by which many rocks, both igneous and
sedimentary, become traversed by parallel systems of fissures.
Whatever may be thought of the explanation here advanced of the
laminated structure of the rocks of the trachytic series, I venture to
call the attention of geologists to the simple fact, that in a body of
rock at Ascension, undoubtedly of volcanic origin, layers often of
extreme tenuity, quite straight, and parallel to each other, have been
produced;—some composed of distinct crystals of quartz and diopside,
mingled with amorphous augitic specks and granular feldspar,—others
entirely composed of these black augitic specks, with granules of oxide
of iron,—and lastly, others formed of crystalline feldspar, in a more
or less perfect state of purity, together with numerous crystals of
feldspar, placed lengthways. At this island, there is reason to
believe, and in some analogous cases, it is certainly known, that the
laminæ have originally been formed with their present high inclination.
Facts of this nature are manifestly of importance, with relation to the
structural
origin of that grand series of plutonic rocks, which like the volcanic
have undergone the action of heat, and which consist of alternate
layers of quartz, feldspar, mica and other minerals.
Chapter IV ST. HELENA
Lavas of the feldspathic, basaltic, and submarine series.—Section of
Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous
Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge
and a parapet. Cones of phonolite. Superficial beds of calcareous
sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the
land.—Denudation.—Craters of elevation.
The whole island is of volcanic origin; its circumference, according to
Beatson,[1] is about twenty-eight miles. The central and largest part
consists of rocks of a feldspathic nature, generally decomposed to an
extraordinary degree; and when in this state, presenting a singular
assemblage of alternating, red, purple, brown, yellow, and white, soft,
argillaceous beds. From the shortness of our visit, I did not examine
these beds with care; some of them, especially those of the white,
yellow, and brown shades, originally existed as streams of lava, but
the greater number were probably ejected in the form of scoriæ and
ashes: other beds of a purple tint, porphyritic with crystal-shaped
patches of a white, soft substance, which are now unctuous, and yield,
like wax, a polished streak to the nail, seem once to have existed as
solid claystone-porphyries: the red argillaceous beds generally have a
brecciated structure, and no doubt have been formed by the
decomposition of scoriæ. Several extensive streams, however, belonging
to this series, retain their stony character; these are either of a
blackish-green colour, with minute acicular crystals of feldspar, or of
a very pale tint, and almost composed of minute, often scaly, crystals
of feldspar, abounding with microscopical black specks; they are
generally compact and laminated; others, however, of similar
composition, are cellular and somewhat decomposed. None of these rocks
contain large crystals of feldspar, or have the harsh fracture peculiar
to trachyte. These feldspathic lavas and tuffs are the uppermost or
those last erupted; innumerable dikes, however, and great masses of
molten rock, have subsequently been injected into them. They converge,
as they rise, towards the central curved ridge, of which one point
attains the elevation of 2,700 feet. This ridge is the highest land in
the island; and it once formed the northern rim of a great crater,
whence the lavas of this series flowed: from its ruined condition, from
the southern half having been removed, and from the violent dislocation
which the whole island has undergone, its structure is rendered very
obscure.
[1] Governor Beatson’s “Account of St. Helena.”
_Basaltic series._—The margin of the island is formed by a rude circle
of great, black, stratified, ramparts of basalt, dipping seaward, and
worn into cliffs, which are often nearly perpendicular, and vary in
height from a few hundred feet to two thousand. This circle, or rather
horse-shoe shaped ring, is open to the south, and is breached by
several other wide spaces. Its rim or summit generally projects little
above the level of the adjoining inland country; and the more recent
feldspathic lavas, sloping down from the central heights, generally
abut against and overlap its inner margin; on the north-western side of
the island, however, they appear (judging from a distance) to have
flowed over and concealed portions of it. In some parts, where the
basaltic ring has been breached, and the black ramparts stand detached,
the feldspathic lavas have passed between them, and now overhang the
sea-coast in lofty cliffs. The basaltic rocks are of a black colour and
thinly stratified; they are generally highly vesicular, but
occasionally compact; some of them contain numerous crystals of glassy
feldspar and octahedrons of titaniferous iron; others abound with
crystals of augite and grains of olivine. The vesicles are frequently
lined with minute crystals (of chabasie?) and even become amygdaloidal
with them. The streams are separated from each other by cindery matter,
or by a bright red, friable, saliferous tuff, which is marked by
successive lines like those of aqueous deposition; and sometimes it has
an obscure, concretionary structure. The rocks of this basaltic series
occur nowhere except near the coast. In most volcanic districts the
trachytic lavas are of anterior origin to the basaltic; but here we
see, that a great pile of rock, closely related in composition to the
trachytic family, has been erupted subsequently to the basaltic strata:
the number, however, of dikes, abounding with large crystals of augite,
with which the feldspathic lavas have been injected, shows perhaps some
tendency to a return to the more usual order of superposition.
_Basal submarine lavas._—The lavas of this basal series lie immediately
beneath both the basaltic and feldspathic rocks. According to Mr.
Seale,[2] they may be seen at intervals on the sea-beach round the
entire island. In the sections which I examined, their nature varied
much; some of the strata abound with crystals of augite; others are of
a brown colour, either laminated or in a rubbly condition; and many
parts are highly amygdaloidal with calcareous matter. The successive
sheets are either closely united together, or are separated from each
other by beds of scoriaceous rock and of laminated tuff, frequently
containing well-rounded fragments. The interstices of these beds are
filled with gypsum and salt; the gypsum also sometimes occurring in
thin layers. From the large quantity of these two substances, from the
presence of rounded pebbles in the tuffs, and from the abundant
amygdaloids, I cannot doubt that these basal volcanic strata flowed
beneath the sea. This remark ought perhaps to be extended to a part of
the superincumbent basaltic rocks; but on this point, I was not able to
obtain clear evidence. The
strata of the basal series, whenever I examined them, were intersected
by an extraordinary number of dikes.
[2] “Geognosy of the Island of St. Helena.” Mr. Seale has constructed
a gigantic model of St. Helena, well worth visiting, which is now
deposited at Addiscombe College, in Surrey.
_Flagstaff Hill and the Barn._—I will now describe some of the more
remarkable sections, and will commence with these two hills, which form
the principal external feature on the north-eastern side of the island.
The square, angular outline, and black colour of the Barn, at once show
that it belongs to the basaltic series; whilst the smooth, conical
figure, and the varied bright tints of Flagstaff Hill, render it
equally clear, that it is composed of the softened, feldspathic rocks.
These two lofty hills are connected (as is shown in figure No. 8) by a
sharp ridge, which is composed of the rubbly lavas of the basal series.
The strata of this ridge dip westward, the inclination becoming less
and less towards the Flagstaff; and the upper feldspathic strata of
this hill can be seen, though with some difficulty, to dip conformably
to the W.S.W. Close to the Barn, the strata of the ridge are nearly
vertical, but are much obscured by innumerable dikes; under this hill,
they probably change from being vertical into being inclined into an
opposite direction; for the upper or basaltic strata, which are about
eight hundred or one thousand feet in thickness, are inclined
north-eastward, at an angle between thirty and forty degrees.
No. 8
[Illustration: Flagstaff Hill and the Barn.]
The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata; the dikes
are shaded transversely.
This ridge, and likewise the Barn and Flagstaff Hills, are interlaced
by dikes, many of which preserve a remarkable parallelism in a N.N.W.
and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic
with large crystals of augite; others are formed of a fine-grained and
brown-coloured trap. Most of these dikes are coated by a glossy
layer,[3] from one to two-tenths of an inch in thickness, which, unlike
true pitchstone, fuses into a black enamel; this layer is evidently
analogous to the glossy superficial coating of many lava streams. The
dikes can often be followed for great lengths both horizontally and
vertically, and
they seem to preserve a nearly uniform thickness:[4] Mr. Seale states,
that one near the Barn, in a height of 1,260 feet, decreases in width
only four inches,—from nine feet at the bottom, to eight feet and eight
inches at the top. On the ridge, the dikes appear to have been guided
in their course, to a considerable degree, by the alternating soft and
hard strata: they are often firmly united to the harder strata, and
they preserve their parallelism for such great lengths, that in very
many instances it was impossible to conjecture, which of the beds were
dikes, and which streams of lava. The dikes, though so numerous on this
ridge, are even more numerous in the valleys a little south of it, and
to a degree I never saw equalled anywhere else: in these valleys they
extend in less regular lines, covering the ground with a network, like
a spider’s web, and with some parts of the surface even appearing to
consist wholly of dikes, interlaced by other dikes.
[3] This circumstance has been observed (Lyell, “Principles of
Geology,” vol. iv, chap. x, p. 9) in the dikes of the Atrio del
Cavallo, but apparently it is not of very common occurrence. Sir G.
Mackenzie, however, states (p. 372, “Travels in Iceland”) that all the
veins in Iceland have a “black vitreous coating on their sides.”
Captain Carmichael, speaking of the dikes in Tristan d’Acunha, a
volcanic island in the Southern Atlantic, says (“Linnæan
Transactions,” vol. xii, p. 485) that their sides, “where they come in
contact with the rocks, are invariably in a semi-vitrified state.”
[4] “Geognosy of the Island of St. Helena,” plate 5.
From the complexity produced by the dikes, from the high inclination
and anticlinal dip of the strata of the basal series, which are
overlaid, at the opposite ends of the short ridge, by two great masses
of different ages and of different composition, I am not surprised that
this singular section has been misunderstood. It has even been supposed
to form part of a crater; but so far is this from having been the case,
that the summit of Flagstaff Hill once formed the lower extremity of a
sheet of lava and ashes, which were erupted from the central,
crateriform ridge. Judging from the slope of the contemporaneous
streams in an adjoining and undisturbed part of the island, the strata
of the Flagstaff Hill must have been upturned at least twelve hundred
feet, and probably much more, for the great truncated dikes on its
summit show that it has been largely denuded. The summit of this hill
now nearly equals in height the crateriform ridge; and before having
been denuded, it was probably higher than this ridge, from which it is
separated by a broad and much lower tract of country; we here,
therefore, see that the lower extremities of a set of lava-streams have
been tilted up to as great a height as, or perhaps greater height than,
the crater, down the flanks of which they originally flowed. I believe
that dislocations on so grand a scale are extremely rare[5] in volcanic
districts. The formation of such numbers of dikes in this part of the
island shows that the surface must here have been stretched to a quite
extraordinary degree: this stretching, on the ridge between Flagstaff
and Barn Hills, probably took place subsequently (though perhaps
immediately so) to the strata being tilted; for had the strata at that
time extended horizontally, they would in all probability have been
fissured and injected transversely, instead of in the planes of their
stratification. Although the space between the Barn and Flagstaff Hill
presents a distinct anticlinal line extending north and south, and
though most of the dikes range with much regularity in the same line,
nevertheless, at only a mile due south of the ridge the strata lie
undisturbed. Hence the disturbing force seems to have acted under
a point, rather than along a line. The manner in which it has acted, is
probably explained by the structure of Little Stony-top, a mountain
2,000 feet high, situated a few miles southward of the Barn; we there
see, even from a distance, a dark-coloured, sharp, wedge of compact
columnar rock, with the bright-coloured feldspathic strata, sloping
away on each side from its uncovered apex. This wedge, from which it
derives its name of Stony-top, consists of a body of rock, which has
been injected whilst liquified into the overlying strata; and if we may
suppose that a similar body of rock lies injected, beneath the ridge
connecting the Barn and Flagstaff, the structure there exhibited would
be explained.
[5] M. Constant Prevost (“Mém. de la Soc. Géolog.,” tome ii) observes
that “les produits volcaniques n’ont que localement et rarement même
dérangé le sol, à travers lequel ils se sont fait jour.”
No. 9
[Illustration: Prosperous Hill and The Barn.]
The double lines represent the basaltic strata; the single, the basal
submarine strata; the dotted, the upper feldspathic strata.
_Turk’s Cap and Prosperous Bays._—Prosperous Hill is a great, black,
precipitous mountain, situated two miles and a half south of the Barn,
and composed, like it, of basaltic strata. These rest, in one part, on
the brown-coloured, porphyritic beds of the basal series, and in
another part, on a fissured mass of highly scoriaceous and amygdaloidal
rock, which seems to have formed a small point of eruption beneath the
sea, contemporaneously with the basal series. Prosperous Hill, like the
Barn, is traversed by many dikes, of which the greater number range
north and south, and its strata dip, at an angle of about 20 degrees,
rather obliquely from the island towards the sea. The space between
Prosperous Hill and the Barn, as represented in figure No. 9, consists
of lofty cliffs, composed of the lavas of the upper or feldspathic
series, which rest, though unconformably, on the basal submarine
strata, as we have seen that they do at Flagstaff Hill. Differently,
however, from in that hill, these upper strata are nearly horizontal,
gently rising towards the interior of the island; and they are composed
of greenish-black, or more commonly, pale brown, compact lavas, instead
of softened and highly coloured matter. These brown-coloured, compact
lavas, consist almost entirely of small glimmering scales, or of minute
acicular crystals, of feldspar, placed close by the side of each other,
and abounding with minute black specks, apparently of hornblende. The
basaltic strata of Prosperous Hill project only a little above the
level of the gently-sloping, feldspathic streams, which wind round and
abut against their upturned edges. The inclination of the basaltic
strata seems to be too great to have been caused by their having flowed
down a slope, and they must have been tilted into their present
position before the eruption of the feldspathic streams.
_Basaltic ring._—Proceeding round the Island, the lavas of the upper
series, southward of Prosperous Hill, overhang the sea in lofty
precipices. Further on, the headland, called Great Stony-top, is
composed, as I
believe, of basalt; as is Long Range Point, on the inland side of which
the coloured beds abut. On the southern side of the island, we see the
basaltic strata of the South Barn, dipping obliquely seaward at a
considerable angle; this headland, also, stands a little above the
level of the more modern, feldspathic lavas. Further on, a large space
of coast, on each side of Sandy Bay, has been much denuded, and there
seems to be left only the basal wreck of the great, central crater. The
basaltic strata reappear, with their seaward dip, at the foot of the
hill, called Man-and-Horse; and thence they are continued along the
whole north-western coast to Sugar-Loaf Hill, situated near to the
Flagstaff; and they everywhere have the same seaward inclination, and
rest, in some parts at least, on the lavas of the basal series. We thus
see that the circumference of the island is formed by a much-broken
ring, or rather, a horse-shoe, of basalt, open to the south, and
interrupted on the eastern side by many wide breaches. The breadth of
this marginal fringe on the north-western side, where alone it is at
all perfect, appears to vary from a mile to a mile and a half. The
basaltic strata, as well as those of the subjacent basal series, dip,
with a moderate inclination, where they have not been subsequently
disturbed, towards the sea. The more broken state of the basaltic ring
round the eastern half, compared with the western half of the island,
is evidently due to the much greater denuding power of the waves on the
eastern or windward side, as is shown by the greater height of the
cliffs on that side, than to leeward. Whether the margin of basalt was
breached, before or after the eruption of the lavas of the upper
series, is doubtful; but as separate portions of the basaltic ring
appear to have been tilted before that event, and from other reasons,
it is more probable, that some at least of the breaches were first
formed. Reconstructing in imagination, as far as is possible, the ring
of basalt, the internal space or hollow, which has since been filled up
with the matter erupted from the great central crater, appears to have
been of an oval figure, eight or nine miles in length by about four
miles in breadth, and with its axis directed in a N.E. and S.W. line,
coincident with the present longest axis of the island.
_The central curved ridge._—This ridge consists, as before remarked, of
grey feldspathic lavas, and of red, brecciated, argillaceous tuffs,
like the beds of the upper coloured series. The grey lavas contain
numerous, minute, black, easily fusible specks; and but very few large
crystals of feldspar. They are generally much softened; with the
exception of this character, and of being in many parts highly
cellular, they are quite similar to those great sheets of lava which
overhang the coast at Prosperous Bay. Considerable intervals of time
appear to have elapsed, judging from the marks of denudation, between
the formation of the successive beds, of which this ridge is composed.
On the steep northern slope, I observed in several sections a much worn
undulating surface of red tuff, covered by grey, decomposed,
feldspathic lavas, with only a thin earthy layer interposed between
them. In an adjoining part, I noticed a trap-dike, four feet wide, cut
off and covered up by the feldspathic lava, as is represented in figure
No. 9. The ridge ends on the eastern side in a hook, which is not
represented clearly enough in any
map which I have seen; towards the western end, it gradually slopes
down and divides into several subordinate ridges. The best defined
portion between Diana’s Peak and Nest Lodge, which supports the highest
pinnacles in the island varying from 2,000 to 2,700 feet, is rather
less than three miles long in a straight line. Throughout this space
the ridge has a uniform appearance and structure; its curvature
resembles that of the coast-line of a great bay, being made up of many
smaller curves, all open to the south. The northern and outer side is
supported by narrow ridges or buttresses, which slope down to the
adjoining country. The inside is much steeper, and is almost
precipitous; it is formed of the basset edges of the strata, which
gently decline outwards. Along some parts of the inner side, a little
way beneath the summit, a flat ledge extends, which imitates in outline
the smaller curvatures of the crest. Ledges of this kind occur not
unfrequently within volcanic craters, and their formation seems to be
due to the sinking down of a level sheet of hardened lava, the edges of
which remain (like the ice round a pool, from which the water has been
drained) adhering to the sides.[6]
[6] A most remarkable instance of this structure is described in Ellis
“Polynesian Researches” (second edition), where an admirable drawing
is given of the successive ledges or terraces, on the borders of the
immense crater at Hawaii, in the Sandwich Islands.
No. 10
[Illustration: Dike]
1—Grey feldspathic lava.
2—A layer, one inch in thickness, of a reddish earthy matter.
3—Brecciated, red, argillaceous tuff.
In some parts, the ridge is surmounted by a wall or parapet,
perpendicular on both sides. Near Diana’s Peak this wall is extremely
narrow. At the Galapagos Archipelago I observed parapets, having a
quite similar structure and appearance, surmounting several of the
craters; one, which I more particularly examined, was composed of
glossy, red scoriæ firmly cemented together; being externally
perpendicular, and extending round nearly the whole circumference of
the crater, it rendered it almost inaccessible. The Peak of Teneriffe
and Cotopaxi, according to Humboldt, are similarly constructed; he
states[7] that “at their summits a circular wall surrounds the crater,
which wall, at a distance, has the appearance of a small cylinder
placed on a truncated cone. On Cotopaxi[8] this peculiar structure is
visible to the naked eye at more than two thousand toises’ distance;
and no person has ever reached its crater. On the Peak of Teneriffe,
the parapet is so high, that it would be impossible to reach the
caldera, if on the eastern side there did not exist a breach.” The
origin of these circular parapets is probably due to the heat or
vapours from the crater, penetrating and hardening the sides to a
nearly equal depth, and afterwards to the mountain being slowly acted
on by the weather, which would leave the hardened part, projecting in
the form of a cylinder or circular parapet.
[7] “Personal Narrative,” vol. i, p. 171.
[8] Humboldt’s “Picturesque Atlas,” folio, pl. 10.
From the points of structure in the central ridge, now
enumerated,—namely, from the convergence towards it of the beds of the
upper series,—from the lavas there becoming highly cellular,—from the
flat ledge, extending along its inner and precipitous side, like that
within some still active craters,—from the parapet-like wall on its
summit,—and lastly, from its peculiar curvature, unlike that of any
common line of elevation, I cannot doubt that this curved ridge forms
the last remnant of a great crater. In endeavouring, however, to trace
its former outline, one is soon baffled; its western extremity
gradually slopes down, and, branching into other ridges, extends to the
sea-coast; the eastern end is more curved, but it is only a little
better defined. Some appearances lead me to suppose that the southern
wall of the crater joined the present ridge near Nest Lodge; in this
case the crater must have been nearly three miles long, and about a
mile and a half in breadth. Had the denudation of the ridge and the
decomposition of its constituent rocks proceeded a few steps further,
and had this ridge, like several other parts of the island, been broken
up by great dikes and masses of injected matter, we should in vain have
endeavoured to discover its true nature. Even now we have seen that at
Flagstaff Hill the lower extremity and most distant portion of one
sheet of the erupted matter has been upheaved to as great a height as
the crater down which it flowed, and probably even to a greater height.
It is interesting thus to trace the steps by which the structure of a
volcanic district becomes obscured, and finally obliterated: so near to
this last stage is St. Helena, that I believe no one has hitherto
suspected that the central ridge or axis of the island is the last
wreck of the crater, whence the most modern volcanic streams were
poured forth.
The great hollow space or valley southward of the central curved ridge,
across which the half of the crater must once have extended, is formed
of bare, water-worn hillocks and ridges of red, yellow, and brown
rocks, mingled together in chaos-like confusion, interlaced by dikes,
and without any regular stratification. The chief part consists of red
decomposing scoriæ, associated with various kinds of tuff and yellow
argillaceous beds, full of broken crystals, those of augite being
particularly large. Here and there masses of highly cellular and
amygdaloidal lavas protrude. From one of the ridges in the midst of the
valley, a conical precipitous hill, called Lot, boldly stands up, and
forms a most singular and conspicuous object. It is composed of
phonolite, divided in one part into great curved laminæ, in another,
into angular concretionary balls, and in a third part into outwardly
radiating columns. At its base the strata of lava, tuff, and scoriæ,
dip away on all sides;[9] the uncovered portion is 197 feet[10] in
height, and its horizontal section gives an oval figure. The phonolite
is of a greenish-grey
colour, and is full of minute acicular crystals of feldspar; in most
parts it has a conchoidal fracture, and is sonorous, yet it is
crenulated with minute air-cavities. In a S.W. direction from Lot,
there are some other remarkable columnar pinnacles, but of a less
regular shape, namely, Lot’s Wife, and the Asses’ Ears, composed of
allied kinds of rock. From their flattened shape, and their relative
position to each other, they are evidently connected on the same line
of fissure. It is, moreover, remarkable that this same N.E. and S.W.
line, joining Lot and Lot’s Wife, if prolonged would intersect
Flagstaff Hill, which, as before stated, is crossed by numerous dikes
running in this direction, and which has a disturbed structure,
rendering it probable that a great body of once fluid rock lies
injected beneath it.
[9] Abich in his “Views of Vesuvius” (plate vi), has shown the manner
in which beds, under nearly similar circumstances, are tilted up. The
upper beds are more turned up than the lower; and he accounts for
this, by showing that the lava insinuates itself horizontally between
the lower beds.
[10] This height is given by Mr. Seale in his Geognosy of the island.
The height of the summit above the level of the sea is said to be
1,444 feet.
In this same great valley there are several other conical masses of
injected rock (one, I observed, was composed of compact greenstone),
some of which are not connected, as far as is apparent, with any line
of dike; whilst others are obviously thus connected. Of these dikes,
three or four great lines stretch across the valley in a N.E. and S.W.
direction, parallel to that one connecting the Asses’ Ears, Lot’s Wife,
and probably Lot. The number of these masses of injected rock is a
remarkable feature in the geology of St. Helena. Besides those just
mentioned, and the hypothetical one beneath Flagstaff Hill, there is
Little Stony-top and others, as I have reason to believe, at the
Man-and-Horse, and at High Hill. Most of these masses, if not all of
them, have been injected subsequently to the last volcanic eruptions
from the central crater. The formation of conical bosses of rock on
lines of fissure, the walls of which are in most cases parallel, may
probably be attributed to inequalities in the tension, causing small
transverse fissures, and at these points of intersection the edges of
the strata would naturally yield, and be easily turned upwards.
Finally, I may remark, that hills of phonolite everywhere are apt[11]
to assume singular and even grotesque shapes, like that of Lot: the
peak at Fernando Noronha offers an instance; at St. Jago, however, the
cones of phonolite, though tapering, have a regular form. Supposing, as
seems probable, that all such hillocks or obelisks have originally been
injected, whilst liquified, into a mould formed by yielding strata, as
certainly has been the case with Lot, how are we to account for the
frequent abruptness and singularity of their outlines, compared with
similarly injected masses of greenstone and basalt? Can it be due to a
less perfect degree of fluidity, which is generally supposed to be
characteristic of the allied trachytic lavas?
[11] D’Aubuisson, in his “Traité de Géognosie” (tome ii, p. 540)
particularly remarks that this is the case.
_Superficial deposits._—Soft calcareous sandstone occurs in extensive,
though thin, superficial beds, both on the northern and southern shores
of the island. It consists of very minute, equal-sized, rounded
particles of shells, and other organic bodies, which partially retain
their yellow, brown, and pink colours, and occasionally, though very
rarely, present an obscure trace of their original external forms. I in
vain endeavoured to find a single unrolled fragment of a shell. The
colour of the particles
is the most obvious character by which their origin can be recognised,
the tints being affected (and an odour produced) by a moderate heat, in
the same manner as in fresh shells. The particles are cemented
together, and are mingled with some earthy matter: the purest masses,
according to Beatson, contain 70 per cent of carbonate of lime. The
beds, varying in thickness from two or three feet to fifteen feet, coat
the surface of the ground; they generally lie on that side of the
valley which is protected from the wind, and they occur at the height
of several hundred feet above the level of the sea. Their position is
the same which sand, if now drifted by the trade-wind, would occupy;
and no doubt they thus originated, which explains the equal size and
minuteness of the particles, and likewise the entire absence of whole
shells, or even of moderately-sized fragments. It is remarkable that at
the present day there are no shelly beaches on any part of the coast,
whence calcareous dust could be drifted and winnowed; we must,
therefore, look back to a former period when before the land was worn
into the present great precipices, a shelving coast, like that of
Ascension, was favourable to the accumulation of shelly detritus. Some
of the beds of this limestone are between six hundred and seven hundred
feet above the sea; but part of this height may possibly be due to an
elevation of the land, subsequent to the accumulation of the calcareous
sand.
The percolation of rain-water has consolidated parts of these beds into
a solid rock, and has formed masses of dark brown, stalagmitic
limestone. At the Sugar-Loaf quarry, fragments of rock on the adjoining
slopes[12] have been thickly coated by successive fine layers of
calcareous matter. It is singular, that many of these pebbles have
their entire surfaces coated, without any point of contact having been
left uncovered; hence, these pebbles must have been lifted up by the
slow deposition between them of the successive films of carbonate of
lime. Masses of white, finely oolitic rock are attached to the outside
of some of these coated pebbles. Von Buch has described a compact
limestone at Lanzarote, which seems perfectly to resemble the
stalagmitic deposition just mentioned: it coats pebbles, and in parts
is finely oolitic: it forms a far-extended layer, from one inch to two
or three feet in thickness, and it occurs at the height of 800 feet
above the sea, but only on that side of the island exposed to the
violent north-western winds. Von Buch remarks,[13] that it is not found
in hollows, but only on the unbroken and inclined surfaces of the
mountain. He believes, that it has been deposited by the spray which is
borne over the whole island by these violent winds. It appears,
however, to me much more probable that it has been formed, as at St.
Helena, by the percolation of water through finely comminuted shells:
for when sand is blown on
a much-exposed coast, it always tends to accumulate on broad, even
surfaces, which offer a uniform resistance to the winds. At the
neighbouring island, moreover, of Feurteventura,[14] there is an earthy
limestone, which, according to Von Buch, is quite similar to specimens
which he has seen from St. Helena, and which he believes to have been
formed by the drifting of shelly detritus.
[12] In the earthy detritus on several parts of this hill, irregular
masses of very impure, crystallised sulphate of lime occur. As this
substance is now being abundantly deposited by the surf at Ascension,
it is possible that these masses may thus have originated; but if so,
it must have been at a period when the land stood at a much lower
level. This earthy selenite is now found at a height of between six
hundred and seven hundred feet.
[13] “Description des Isles Canaries,” p. 293.
[14] _Idem,_ pp. 314 and 374.
The upper beds of the limestone, at the above-mentioned quarry on the
Sugar-Loaf Hill, are softer, finer-grained and less pure, than the
lower beds. They abound with fragments of land-shells, and with some
perfect ones; they contain, also, the bones of birds, and the large
eggs,[15] apparently of water-fowl. It is probable that these upper
beds remained long in an unconsolidated form, during which time, these
terrestrial productions were embedded. Mr. G. R. Sowerby has kindly
examined three species of land-shells, which I procured from this bed,
and has described them in detail. One of them is a Succinea, identical
with a species now living abundantly on the island; the two others,
namely, _ Cochlogena fossilis_ and _Helix biplicata_, are not known in
a recent state: the latter species was also found in another and
different locality, associated with a species of Cochlogena which is
undoubtedly extinct.
[15] Colonel Wilkes, in a catalogue presented with some specimens to
the Geological Society, states that as many as ten eggs were found by
one person. Dr. Buckland has remarked (“Geolog. Trans.,” vol. v, p.
474) on these eggs.
_Beds of extinct land-shells._—Land-shells, all of which appear to be
species now extinct, occur embedded in earth, in several parts of the
island. The greater number have been found at a considerable height on
Flagstaff Hill. On the N.W. side of this hill, a rain-channel exposes a
section of about twenty feet in thickness, of which the upper part
consists of black vegetable mould, evidently washed down from the
heights above, and the lower part of less black earth, abounding with
young and old shells, and with their fragments: part of this earth is
slightly consolidated by calcareous matter, apparently due to the
partial decomposition of some of the shells. Mr. Seale, an intelligent
resident, who first called attention to these shells, gave me a large
collection from another locality, where the shells appear to have been
embedded in very black earth. Mr. G. R. Sowerby has examined these
shells, and has described them. There are seven species, namely, one
Cochlogena, two species of the genus Cochlicopa, and four of Helix;
none of these are known in a recent state, or have been found in any
other country. The smaller species were picked out of the inside of the
large shells of the _Cochlogena aurisvulpina._ This last-mentioned
species is in many respects a very singular one; it was classed, even
by Lamarck, in a marine genus, and having thus been mistaken for a
sea-shell, and the smaller accompanying species having been overlooked,
the exact localities where it was found have been measured, and the
elevation of this island thus deduced! It is very remarkable that all
the shells of this species found by me in one spot, form a distinct
variety, as described by Mr. Sowerby, from those
procured from another locality by Mr. Seale. As this Cochlogena is a
large and conspicuous shell, I particularly inquired from several
intelligent countrymen whether they had ever seen it alive; they all
assured me that they had not, and they would not even believe that it
was a land animal: Mr. Seale, moreover, who was a collector of shells
all his life at St. Helena, never met with it alive. Possibly some of
the smaller species may turn out to be yet living kinds; but, on the
other hand, the two land-shells which are now living on the island in
great numbers, do not occur embedded, as far as is yet known, with the
extinct species. I have shown in my “Journal,”[16] that the extinction
of these land-shells possibly may not be an ancient event; as a great
change took place in the state of the island about one hundred and
twenty years ago, when the old trees died, and were not replaced by
young ones, these being destroyed by the goats and hogs, which had run
wild in numbers, from the year 1502. Mr. Seale states, that on
Flagstaff Hill, where we have seen that the embedded land-shells are
especially numerous, traces are everywhere discoverable, which plainly
indicate that it was once thickly clothed with trees; at present not
even a bush grows there. The thick bed of black vegetable mould which
covers the shell-bed, on the flanks of this hill, was probably washed
down from the upper part, as soon as the trees perished, and the
shelter afforded by them was lost.
[16] “Journal of Researches,” p. 582.
_Elevation of the land._—Seeing that the lavas of the basal series,
which are of submarine origin, are raised above the level of the sea,
and at some places to the height of many hundred feet, I looked out for
superficial signs of the elevation of the land. The bottoms of some of
the gorges, which descend to the coast, are filled up to the depth of
about a hundred feet, by rudely divided layers of sand, muddy clay, and
fragmentary masses; in these beds, Mr. Seale has found the bones of the
tropic-bird and of the albatross; the former now rarely, and the latter
never visiting the island. From the difference between these layers,
and the sloping piles of detritus which rest on them, I suspect that
they were deposited, when the gorges stood beneath the sea. Mr. Seale,
moreover, has shown that some of the fissure-like gorges[17] become,
with a concave outline, gradually rather wider at the bottom than at
the top; and this peculiar structure was probably caused by the wearing
action of the sea, when it entered the lower part of these gorges. At
greater heights, the evidence of the rise of the land is even less
clear: nevertheless, in a bay-like depression on the table-land behind
Prosperous Bay, at the height of about a thousand feet, there are
flat-topped masses of rock, which it is scarcely conceivable, could
have been insulated from the surrounding and similar strata, by any
other agency than the denuding action of a sea-beach. Much denudation,
indeed, has been effected at great elevations, which it would not be
easy to explain by any other means: thus, the flat summit of the Barn,
which is 2,000 feet high, presents, according to Mr. Seale, a perfect
network of truncated dikes; on hills like the Flagstaff, formed of soft
rock, we might suppose that the dikes had been worn down and cut off by
meteoric agency, but we can hardly suppose this possible with the hard,
basaltic strata of the Barn.
[17] A fissure-like gorge, near Stony-top, is said by Mr. Seale to be
840 feet deep, and only 115 feet in width.
_Coast denudation._—The enormous cliffs, in many parts between one and
two thousand feet in height, with which this prison-like island is
surrounded, with the exception of only a few places, where narrow
valleys descend to the coast, is the most striking feature in its
scenery. We have seen that portions of the basaltic ring, two or three
miles in length by one or two miles in breadth, and from one to two
thousand feet in height, have been wholly removed. There are, also,
ledges and banks of rock, rising out of profoundly deep water, and
distant from the present coast between three and four miles, which,
according to Mr. Seale, can be traced to the shore, and are found to be
the continuations of certain well-known great dikes. The swell of the
Atlantic Ocean has obviously been the active power in forming these
cliffs; and it is interesting to observe that the lesser, though still
great, height of the cliffs on the leeward and partially protected side
of the island (extending from the Sugar-Loaf Hill to South West Point),
corresponds with the lesser degree of exposure. When reflecting on the
comparatively low coasts of many volcanic islands, which also stand
exposed in the open ocean, and are apparently of considerable
antiquity, the mind recoils from an attempt to grasp the number of
centuries of exposure, necessary to have ground into mud and to have
dispersed the enormous cubic mass of hard rock which has been pared off
the circumference of this island. The contrast in the superficial state
of St. Helena, compared with the nearest island, namely, Ascension, is
very striking. At Ascension, the surfaces of the lava-streams are
glossy, as if just poured forth, their boundaries are well defined, and
they can often be traced to perfect craters, whence they were erupted;
in the course of many long walks, I did not observe a single dike; and
the coast round nearly the entire circumference is low, and has been
eaten back (though too much stress must not be placed on this fact, as
the island may have been subsiding) into a little wall only from ten to
thirty feet high. Yet during the 340 years, since Ascension has been
known, not even the feeblest signs of volcanic action have been
recorded.[18] On the other hand, at St. Helena, the course of no one
stream of lava can be traced, either by the state of its boundaries or
of its superficies; the mere wreck of one great crater is left; not the
valleys only, but the surfaces of some of the highest hills, are
interlaced by worn-down dikes, and, in many
places, the denuded summits of great cones of injected rock stand
exposed and naked; lastly, as we have seen, the entire circuit of the
island has been deeply worn back into the grandest precipices.
[18] In the _Nautical Magazine_ for 1835, p. 642, and for 1838, p.
361, and in the “Comptes Rendus,” April 1838, accounts are given of a
series of volcanic phenomena—earthquakes—troubled water—floating
scoriæ and columns of smoke—which have been observed at intervals
since the middle of the last century, in a space of open sea between
longitudes 20° and 22° west, about half a degree south of the equator.
These facts seem to show, that an island or an archipelago is in
process of formation in the middle of the Atlantic: a line joining St.
Helena and Ascension, prolonged, intersects this slowly nascent focus
of volcanic action.
_Craters of Elevation._
There is much resemblance in structure and in geological history
between St. Helena, St. Jago, and Mauritius. All three islands are
bounded (at least in the parts which I was able to examine) by a ring
of basaltic mountains, now much broken, but evidently once continuous.
These mountains have, or apparently once had, their escarpments steep
towards the interior of the island, and their strata dip outwards. I
was able to ascertain, only in a few cases, the inclination of the
beds; nor was this easy, for the stratification was generally obscure,
except when viewed from a distance. I feel, however, little doubt that,
according to the researches of M. Elie de Beaumont, their average
inclination is greater than that which they could have acquired,
considering their thickness and compactness, by flowing down a sloping
surface. At St. Helena, and at St. Jago, the basaltic strata rest on
older and probably submarine beds of different composition. At all
three islands, deluges of more recent lavas have flowed from the centre
of the island, towards and between the basaltic mountains; and at St.
Helena the central platform has been filled up by them. All three
islands have been raised in mass. At Mauritius the sea, within a late
geological period, must have reached to the foot of the basaltic
mountains, as it now does at St. Helena; and at St. Jago it is cutting
back the intermediate plain towards them. In these three islands, but
especially at St. Jago and at Mauritius, when, standing on the summit
of one of the old basaltic mountains, one looks in vain towards the
centre of the island,—the point towards which the strata beneath one’s
feet, and of the mountains on each side, rudely converge,—for a source
whence these strata could have been erupted; but one sees only a vast
hollow platform stretched beneath, or piles of matter of more recent
origin.
These basaltic mountains come, I presume, into the class of Craters of
elevation: it is immaterial whether the rings were ever completely
formed, for the portions which now exist have so uniform a structure,
that, if they do not form fragments of true craters, they cannot be
classed with ordinary lines of elevation. With respect to their origin,
after having read the works of Mr. Lyell,[19] and of MM. C. Prevost and
Virlet, I cannot believe that the great central hollows have been
formed by a simple dome-shaped elevation, and the consequent arching of
the strata. On the other hand, I have very great difficulty in
admitting that these basaltic mountains are merely the basal fragments
of great volcanoes, of which the summits have either been blown off, or
more probably swallowed up by subsidence. These rings are, in some
instances, so immense, as at St. Jago and at Mauritius, and their
occurrence is so frequent, that I can hardly persuade myself to adopt
this explanation. Moreover, I suspect that the following circumstances,
from their frequent concurrence, are someway connected together,—a
connection not implied in either of the above views: namely, first, the
broken state of the ring; showing that the now detached portions have
been exposed to great denudation, and in some cases, perhaps, rendering
it probable that the ring never was entire; secondly, the great amount
of matter erupted from the central area after or during the formation
of the ring; and thirdly, the elevation of the district in mass. As far
as relates to the inclination of the strata being greater than that
which the basal fragments of ordinary volcanoes would naturally
possess, I can readily believe that this inclination might have been
slowly acquired by that amount of elevation, of which, according to M.
Elie de Beaumont, the numerous upfilled fissures or dikes are the
evidence and the measure,—a view equally novel and important, which we
owe to the researches of that geologist on Mount Etna.
[19] “Principles of Geology” (fifth edit.), vol. ii, p. 171.
A conjecture, including the above circumstances, occurred to me, when,—
with my mind fully convinced, from the phenomena of 1835 in South
America,[20] that the forces which eject matter from volcanic orifices
and raise continents in mass are identical,—I viewed that part of the
coast of St. Jago, where the horizontally upraised, calcareous stratum
dips into the sea, directly beneath a cone of subsequently erupted
lava. The conjecture is that, during the slow elevation of a volcanic
district or island, in the centre of which one or more orifices
continue open, and thus relieve the subterranean forces, the borders
are elevated more than the central area; and that the portions thus
upraised do not slope gently into the central, less elevated area, as
does the calcareous stratum under the cone at St. Jago, and as does a
large part of the circumference of Iceland,[21] but that they are
separated from it by curved faults.
We might expect, from what we see along ordinary faults, that the
strata on the upraised side, already dipping outwards from their
original formation as lava-streams, would be tilted from the line of
fault, and thus have their inclination increased. According to this
hypothesis, which I am tempted to extend only to some few cases, it is
not probable that the ring would ever be formed quite perfect; and from
the elevation being slow, the upraised portions would generally be
exposed to much denudation, and hence the ring become broken; we might
also expect to find occasional inequalities in the dip of the upraised
masses, as is the case at St. Jago. By this hypothesis the elevation of
the districts in mass, and the flowing of deluges of lava from the
central platforms, are likewise connected together. On this view the
marginal basaltic mountains of the three foregoing islands might still
be considered as forming “Craters of elevation;” the kind of elevation
implied having been slow, and the central hollow or platform having
been formed, not by the arching of the surface, but simply by that part
having been upraised to a less height.
[20] I have given a detailed account of these phenomena, in a paper
read before the Geological Society in March 1838. At the instant of
time, when an immense area was convulsed and a large tract elevated,
the districts immediately surrounding several of the great vents in
the Cordillera remained quiescent; the subterranean forces being
apparently relieved by the eruptions, which then recommenced with
great violence. An event of somewhat the same kind, but on an
infinitely smaller scale, appears to have taken place, according to
Abich (“Views of Vesuvius,” plates i and ix), within the great crater
of Vesuvius, where a platform on one side of a fissure was raised in
mass twenty feet, whilst on the other side, a train of small volcanoes
burst forth in eruption.
[21] It appears, from information communicated to me in the most
obliging manner by M. E. Robert, that the circumferential parts of
Iceland, which are composed of ancient basaltic strata alternating
with tuff, dip inland, thus forming a gigantic saucer. M. Robert found
that this was the case, with a few and quite local exceptions, for a
space of coast several hundred miles in length. I find this statement
corroborated, as far as regards one place, by Mackenzie in his
“Travels” (p. 377), and in another place by some MS. notes kindly lent
me by Dr. Holland. The coast is deeply indented by creeks, at the head
of which the land is generally low. M. Robert informs me, that the
inwardly dipping strata appear to extend as far as this line, and that
their inclination usually corresponds with the slope of the surface,
from the high coast-mountains to the low land at the head of these
creeks. In the section described by Sir G. Mackenzie, the dip is 120.
The interior parts of the island chiefly consist, as far as is known,
of recently erupted matter. The great size, however, of Iceland,
equalling the bulkiest part of England, ought perhaps to exclude it
from the class of islands we have been considering; but I cannot avoid
suspecting that if the coast-mountains, instead of gently sloping into
the less elevated central area, had been separated from it by
irregularly curved faults, the strata would have been tilted seaward,
and a “Crater of elevation,” like that of St. Jago or that of
Mauritius, but of much vaster dimensions, would have been formed. I
will only further remark, that the frequent occurrence of extensive
lakes at the foot of large volcanoes, and the frequent association of
volcanic and fresh-water strata, seem to indicate that the areas
around volcanoes are apt to be depressed beneath the level of the
adjoining country, either from having been less elevated, or from the
effects of subsidence.
Chapter V GALAPAGOS ARCHIPELAGO.
Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
basaltic craters, with hollows at their bases.—Albemarle Island, fluid
lavas, their composition.—Craters of tuff, inclination of their
exterior diverging strata, and structure of their interior converging
strata.—James Island, segment of a small basaltic crater; fluidity and
composition of its lava-streams, and of its ejected
fragments.—Concluding remarks on the craters of tuff, and on the
breached condition of their southern sides.—Mineralogical composition
of the rocks of the archipelago.—Elevation of the land. Direction of
the fissures of eruption.
This archipelago is situated under the equator, at a distance of
between five and six hundred miles from the west coast of South
America. It consists of five principal islands, and of several small
ones, which together are equal in area,[1] but not in extent of land,
to Sicily, conjointly with the Ionian Islands. They are all volcanic:
on two, craters have been seen in eruption, and on several of the other
islands, streams of lava have a recent appearance. The larger islands
are chiefly composed of solid rock, and they rise with a tame outline
to a height of between one and four thousand feet. They are sometimes,
but not generally, surmounted by one principal orifice. The craters
vary in size from mere spiracles to huge caldrons several miles in
circumference; they are extraordinarily numerous, so that I should
think, if enumerated, they would be found to exceed two thousand; they
are formed either of scoriæ and lava, or of a brown-coloured tuff; and
these latter craters are in several respects remarkable. The whole
group was surveyed by the officers of the _Beagle._ I visited myself
four of the principal islands, and received specimens from all the
others. Under the head of the different islands I will describe only
that which appears to me deserving of attention.
[1] I exclude from this measurement, the small volcanic islands of
Culpepper and Wenman, lying seventy miles northward of the group.
Craters were visible on all the islands of the group, except on Towers
Island, which is one of the lowest; this island is, however, formed of
volcanic rocks.
No. 11
[Illustration: Galapagos Archipelago.]
Galapagos Archipelago
CHATHAM ISLAND. _Craters composed of a singular kind of tuff._—Towards
the eastern end of this island there occur two craters composed of two
kinds of tuff; one kind being friable, like slightly consolidated
ashes; and the other compact, and of a different nature from anything
which I have met with described. This latter substance, where it is
best characterised, is of a yellowish-brown colour, translucent, and
with a lustre somewhat resembling resin; it is brittle, with an
angular, rough, and very irregular fracture, sometimes, however, being
slightly granular, and even obscurely crystalline: it can readily be
scratched with a knife, yet some points are hard enough just to mark
common glass; it fuses with ease into a blackish-green glass. The mass
contains numerous broken crystals of olivine and augite, and small
particles of black and brown scoriæ; it is often traversed by thin
seams of calcareous matter. It generally affects a nodular or
concretionary structure. In a hand specimen, this substance would
certainly be mistaken for a pale and peculiar variety of pitchstone;
but when seen in mass its stratification, and the numerous layers of
fragments of basalt, both angular and rounded, at once render its
subaqueous origin evident. An examination of a series of specimens
shows that this resin-like substance results from a chemical change on
small particles of pale and dark-coloured scoriaceous rocks; and this
change could be distinctly traced in different stages round the edges
of even the same particle. The position near the coast of all the
craters composed of this kind of tuff or peperino, and their breached
condition, renders it probable that they were all formed when standing
immersed in the sea; considering this circumstance, together with the
remarkable absence of large beds of ashes in the whole archipelago, I
think it highly probable that much the greater part of the tuff has
originated from the trituration of fragments of the grey, basaltic
lavas in the mouths of craters standing in the sea. It may be asked
whether the heated water within these craters has produced this
singular change in the small scoriaceous particles and given to them
their translucent, resin-like fracture. Or has the associated lime
played any part in this change? I ask these questions from having found
at St. Jago, in the Cape de Verde Islands, that where a great stream of
molten lava has flowed over a calcareous bottom into the sea, the
outermost film, which in other parts resembles pitchstone, is changed,
apparently by its contact with the carbonate of lime, into a resin-like
substance, precisely like the best characterised specimens of the tuff
from this archipelago.[2]
[2] The concretions containing lime, which I have described at
Ascension, as formed in a bed of ashes, present some degree of
resemblance to this substance, but they have not a resinous fracture.
At St. Helena, also, I found veins of a somewhat similar, compact, but
non-resinous substance, occurring in a bed of pumiceous ashes,
apparently free from calcareous matter: in neither of these cases
could heat have acted.
To return to the two craters: one of them stands at the distance of a
league from the coast, the intervening tract consisting of a calcareous
tuff, apparently of submarine origin. This crater consists of a circle
of hills some of which stand quite detached, but all have a very
regular,
quâ-quâ versal dip, at an inclination of between thirty and forty
degrees. The lower beds, to the thickness of several hundred feet,
consist of the resin-like stone, with embedded fragments of lava. The
upper beds, which are between thirty and forty feet in thickness, are
composed of a thinly stratified, fine-grained, harsh, friable,
brown-coloured tuff, or peperino.[3] A central mass without any
stratification, which must formerly have occupied the hollow of the
crater, but is now attached only to a few of the circumferential hills,
consists of a tuff, intermediate in character between that with a
resin-like, and that with an earthy fracture. This mass contains white
calcareous matter in small patches. The second crater (520 feet in
height) must have existed until the eruption of a recent, great stream
of lava, as a separate islet; a fine section, worn by the sea, shows a
grand funnel-shaped mass of basalt, surrounded by steep, sloping flanks
of tuff, having in parts an earthy, and in others a semi-resinous
fracture. The tuff is traversed by several broad, vertical dikes, with
smooth and parallel sides, which I did not doubt were formed of basalt,
until I actually broke off fragments. These dikes, however, consist of
tuff like that of the surrounding strata, but more compact, and with a
smoother fracture; hence we must conclude, that fissures were formed
and filled up with the finer mud or tuff from the crater, before its
interior was occupied, as it now is, by a solidified pool of basalt.
Other fissures have been subsequently formed, parallel to these
singular dikes, and are merely filled with loose rubbish. The change
from ordinary scoriaceous particles to the substance with a
semi-resinous fracture, could be clearly followed in portions of the
compact tuff of these dikes.
[3] Those geologists who restrict the term of “tuff” to ashes of a
white colour, resulting from the attrition of feldspathic lavas, would
call these brown-coloured strata “peperino.”
No. 12
[Illustration: The Kicker Rock.]
The Kicker Rock, 400 feet high.
At the distance of a few miles from these two craters, stands the
Kicker Rock, or islet, remarkable from its singular form. It is
unstratified, and is composed of compact tuff, in parts having the
resin-like fracture. It is probable that this amorphous mass, like that
similar mass in the case first described, once filled up the central
hollow of a crater, and that its flanks, or sloping walls, have since
been worn quite away by the sea, in which it stands exposed.
_Small basaltic craters._—A bare, undulating tract, at the eastern end
of Chatham Island, is remarkable from the number, proximity, and form
of the small basaltic craters with which it is studded. They consist,
either
of a mere conical pile, or, but less commonly, of a circle, of black
and red, glossy scoriæ, partially cemented together. They vary in
diameter from thirty to one hundred and fifty yards, and rise from
about fifty to one hundred feet above the level of the surrounding
plain. From one small eminence, I counted sixty of these craters, all
of which were within a third of a mile from each other, and many were
much closer. I measured the distance between two very small craters,
and found that it was only thirty yards from the summit-rim of one to
the rim of the other. Small streams of black, basaltic lava, containing
olivine and much glassy feldspar, have flowed from many, but not from
all of these craters. The surfaces of the more recent streams were
exceedingly rugged, and were crossed by great fissures; the older
streams were only a little less rugged; and they were all blended and
mingled together in complete confusion. The different growth, however,
of the trees on the streams, often plainly marked their different ages.
Had it not been for this latter character, the streams could in few
cases have been distinguished; and, consequently, this wide undulatory
tract might have (as probably many tracts have) been erroneously
considered as formed by one great deluge of lava, instead of by a
multitude of small streams, erupted from many small orifices.
In several parts of this tract, and especially at the base of the small
craters, there are circular pits, with perpendicular sides, from twenty
to forty feet deep. At the foot of one small crater, there were three
of these pits. They have probably been formed, by the falling in of the
roofs of small caverns.[4] In other parts, there are mammiform
hillocks, which resemble great bubbles of lava, with their summits
fissured by irregular cracks, which appeared, upon entering them, to be
very deep; lava has not flowed from these hillocks. There are, also,
other very regular, mammiform hillocks, composed of stratified lava,
and surmounted by circular, steep-sided hollows, which, I suppose have
been formed by a body of gas, first, arching the strata into one of the
bubble-like hillocks, and then, blowing off its summit. These several
kinds of hillocks and pits, as well as the numerous, small, scoriaceous
craters, all show that this tract has been penetrated, almost like a
sieve, by the passage of heated vapours. The more regular hillocks
could only have been heaved up, whilst the lava was in a softened
state.[5]
[4] (M. Elie de Beaumont has described (“Mém. pour servir,” etc., tome
iv, p. 113) many “petits cirques d’eboulement” on Etna, of some of
which the origin is historically known.
[5] Sir G. Mackenzie (“Travels in Iceland,” pp. 389 to 392) has
described a plain of lava at the foot of Hecla, everywhere heaved up
into great bubbles or blisters. Sir George states that this cavernous
lava composes the uppermost stratum; and the same fact is affirmed by
Von Buch (“Descript. des Isles Canaries,” p. 159), with respect to the
basaltic stream near Rialejo, in Teneriffe. It appears singular that
it should be the upper streams that are chiefly cavernous, for one
sees no reason why the upper and lower should not have been equally
affected at different times;—have the inferior streams flowed beneath
the pressure of the sea, and thus been flattened, after the passage
through them, of bodies of gas?
ALBEMARLE ISLAND.—This island consists of five, great, flat-topped
craters, which, together with the one on the adjoining island of
Narborough, singularly resemble each other, in form and height. The
southern one is 4,700 feet high, two others are 3,720 feet, a third
only 50 feet higher, and the remaining ones apparently of nearly the
same height. Three of these are situated on one line, and their craters
appear elongated in nearly the same direction. The northern crater,
which is not the largest, was found by the triangulation to measure,
externally, no less than three miles and one-eighth of a mile in
diameter. Over the lips of these great, broad caldrons, and from little
orifices near their summits, deluges of black lava have flowed down
their naked sides.
_Fluidity of different lavas._—Near Tagus or Banks’ Cove, I examined
one of these great streams of lava, which is remarkable from the
evidence of its former high degree of fluidity, especially when its
composition is considered. Near the sea-coast this stream is several
miles in width. It consists of a black, compact base, easily fusible
into a black bead, with angular and not very numerous air-cells, and
thickly studded with large, fractured crystals of glassy albite,[6]
varying from the tenth of an inch to half an inch in diameter. This
lava, although at first sight appearing eminently porphyritic, cannot
properly be considered so, for the crystals have evidently been
enveloped, rounded, and penetrated by the lava, like fragments of
foreign rock in a trap-dike. This was very clear in some specimens of a
similar lava, from Abingdon Island, in which the only difference was,
that the vesicles were spherical and more numerous. The albite in these
lavas is in a similar condition with the leucite of Vesuvius, and with
the olivine, described by Von Buch,[7] as projecting in great balls
from the basalt of Lanzarote. Besides the albite, this lava contains
scattered grains of a green mineral, with no distinct cleavage, and
closely resembling olivine;[8] but as it fuses easily into a green
glass, it belongs probably to the augitic family: at James Island,
however, a similar lava contained true olivine. I obtained specimens
from the actual
surface, and from a depth of four feet, but they differed in no
respect. The high degree of fluidity of this lava-stream was at once
evident, from its smooth and gently sloping surface, from the manner in
which the main stream was divided by small inequalities into little
rills, and especially from the manner in which its edges, far below its
source, and where it must have been in some degree cooled, thinned out
to almost nothing; the actual margin consisting of loose fragments, few
of which were larger than a man’s head. The contrast between this
margin, and the steep walls, above twenty feet high, bounding many of
the basaltic streams at Ascension, is very remarkable. It has generally
been supposed that lavas abounding with large crystals, and including
angular vesicles,[9] have possessed little fluidity; but we see that
the case has been very different at Albemarle Island. The degree of
fluidity in different lavas, does not seem to correspond with any
_apparent_ corresponding amount of difference in their composition: at
Chatham Island, some streams, containing much glassy albite and some
olivine, are so rugged, that they may be compared to a sea frozen
during a storm; whilst the great stream at Albemarle Island is almost
as smooth as a lake when ruffled by a breeze. At James Island, black
basaltic lava, abounding with small grains of olivine, presents an
intermediate degree of roughness; its surface being glossy, and the
detached fragments resembling, in a very singular manner, folds of
drapery, cables, and pieces of the bark of trees.[10]
[6] In the Cordillera of Chile, I have seen lava very closely
resembling this variety at the Galapagos Archipelago. It contained,
however, besides the albite, well-formed crystals of augite, and the
base (perhaps in consequence of the aggregation of the augitic
particles) was a shade lighter in colour. I may here remark, that in
all these cases, I call the feldspathic crystals, _albite_, from their
cleavage-planes (as measured by the reflecting goniometer)
corresponding with those of that mineral. As, however, other species
of this genus have lately been discovered to cleave in nearly the same
planes with albite, this determination must be considered as only
provisional. I examined the crystals in the lavas of many different
parts of the Galapagos group, and I found that none of them, with the
exception of some crystals from one part of James Island, cleaved in
the direction of orthite or potash-feldspar.
[7] “Description des Isles Canaries,” p. 295.
[8] Humboldt mentions that he mistook a green augitic mineral,
occurring in the volcanic rocks of the Cordillera of Quito, for
olivine.
[9] The irregular and angular form of the vesicles is probably caused
by the unequal yielding of a mass composed, in almost equal
proportion, of solid crystals and of a viscid base. It certainly seems
a general circumstance, as might have been expected, that in lava,
which has possessed a high degree of fluidity, _as well as an
even-sized grain_, the vesicles are internally smooth and spherical.
[10] A specimen of basaltic lava, with a few small broken crystals of
albite, given me by one of the officers, is perhaps worthy of
description. It consists of cylindrical ramifications, some of which
are only the twentieth of an inch in diameter, and are drawn out into
the sharpest points. The mass has not been formed like a stalactite,
for the points terminate both upwards and downwards. Globules, only
the fortieth of an inch in diameter, have dropped from some of the
points, and adhere to the adjoining branches. The lava is vesicular,
but the vesicles never reach the surface of the branches, which are
smooth and glossy. As it is generally supposed that vesicles are
always elongated in the direction of the movement of the fluid mass, I
may observe, that in these cylindrical branches, which vary from a
quarter to only the twentieth of an inch in diameter, every air-cell
is spherical.
_Craters of tuff._—About a mile southward of Banks’ Cove, there is a
fine elliptic crater, about five hundred feet in depth, and
three-quarters of a mile in diameter. Its bottom is occupied by a lake
of brine, out of which some little crateriform hills of tuff rise. The
lower beds are formed of compact tuff, appearing like a subaqueous
deposit; whilst the upper beds, round the entire circumference, consist
of a harsh, friable tuff, of little specific gravity, but often
containing fragments of rock in layers. This upper tuff contains
numerous pisolitic balls, about the size of small bullets, which differ
from the surrounding matter, only in being slightly
harder and finer grained. The beds dip away very regularly on all
sides, at angles varying, as I found by measurement, from twenty-five
to thirty degrees. The external surface of the crater slopes at a
nearly similar inclination, and is formed by slightly convex ribs, like
those on the shell of a pecten or scallop, which become broader as they
extend from the mouth of the crater to its base. These ribs are
generally from eight to twenty feet in breadth, but sometimes they are
as much as forty feet broad; and they resemble old, plastered, much
flattened vaults, with the plaster scaling off in plates: they are
separated from each other by gullies, deepened by alluvial action. At
their upper and narrow ends, near the mouth of the crater, these ribs
often consist of real hollow passages, like, but rather smaller than,
those often formed by the cooling of the crust of a lava-stream, whilst
the inner parts have flowed onward;—of which structure I saw many
examples at Chatham Island. There can be no doubt but that these hollow
ribs or vaults have been formed in a similar manner, namely, by the
setting or hardening of a superficial crust on streams of mud, which
have flowed down from the upper part of the crater. In another part of
this same crater, I saw open concave gutters between one and two feet
wide, which appear to have been formed by the hardening of the lower
surface of a mud stream, instead of, as in the former case, of the
upper surface. From these facts I think it is certain that the tuff
must have flowed as mud.[11] This mud may have been formed either
within the crater, or from ashes deposited on its upper parts, and
afterwards washed down by torrents of rain. The former method, in most
of the cases, appears the more probable one; at James Island, however,
some beds of the friable kind of tuff extend so continuously over an
uneven surface, that probably they were formed by the falling of
showers of ashes.
[11] This conclusion is of some interest, because M. Dufrenoy (“Mém.
pour servir,” tome iv, p. 274) has argued from strata of tuff,
apparently of similar composition with that here described, being
inclined at angles between 18° and 20°, that Monte Nuevo and some
other craters of Southern Italy have been formed by upheaval. From the
facts given above, of the vaulted character of the separate rills, and
from the tuff not extending in horizontal sheets round these
crateriform hills, no one will suppose that the strata have here been
produced by elevation; and yet we see that their inclination is above
20°, and often as much as 30°. The consolidated strata also, of the
internal talus, as will be immediately seen, dips at an angle of above
30°.
Within this same crater, strata of coarse tuff, chiefly composed of
fragments of lava, abut, like a consolidated talus, against the inside
walls. They rise to a height of between one hundred and one hundred and
fifty feet above the surface of the internal brine-lake; they dip
inwards, and are inclined at an angle varying from thirty to thirty-six
degrees. They appear to have been formed beneath water, probably at a
period when the sea occupied the hollow of the crater. I was surprised
to observe that beds having this great inclination did not, as far as
they could be followed, thicken towards their lower extremities.
_Banks’ Cove._—This harbour occupies part of the interior of a
shattered crater of tuff larger than that last described. All the tuff
is
compact, and includes numerous fragments of lava; it appears like a
subaqueous deposit. The most remarkable feature in this crater is the
great development of strata converging inwards, as in the last case, at
a considerable inclination, and often deposited in irregular curved
layers. These interior converging beds, as well as the proper,
diverging crateriform strata, are represented in figure No. 13, a rude,
sectional sketch of the headlands, forming this Cove. The internal and
external strata differ little in composition, and the former have
evidently resulted from the wear and tear, and redeposition of the
matter forming the external crateriform strata. From the great
development of these inner beds, a person walking round the rim of this
crater might fancy himself on a circular anticlinal ridge of stratified
sandstone and conglomerate. The sea is wearing away the inner and outer
strata, and especially the latter; so that the inwardly converging
strata will, perhaps, in some future age, be left standing alone—a case
which might at first perplex a geologist.[12]
[12] I believe that this case actually occurs in the Azores, where Dr.
Webster (“Description,” p. 185) has described a basin-formed, little
island, composed of _strata of tuff_, dipping inwards and bounded
externally by steep sea-worn cliffs. Dr. Daubeny supposes (on
Volcanoes, p. 266), that this cavity must have been formed by a
circular subsidence. It appears to me far more probable, that we here
have strata which were originally deposited within the hollow of a
crater, of which the exterior walls have since been removed by the
sea.
No. 13
[Illustration: Sectional sketch of headlands forming Banks’ Cove.]
A sectional sketch of the headlands forming Banks’ Cove, showing the
diverging craterform strata, and the converging stratified talus. The
highest point of these hills is 817 feet above the sea.
JAMES ISLAND.—Two craters of tuff on this island are the only remaining
ones which require any notice. One of them lies a mile and a half
inland from Puerto Grande: it is circular, about the third of a mile in
diameter, and 400 feet in depth. It differs from all the other
tuff-craters which I examined, in having the lower part of its cavity,
to the height of between one hundred and one hundred and fifty feet,
formed by a precipitous wall of basalt, giving to the crater the
appearance of having burst through a solid sheet of rock. The upper
part of this crater consists of strata of the altered tuff, with a
semi-resinous fracture. Its bottom is
occupied by a shallow lake of brine, covering layers of salt, which
rest on deep black mud. The other crater lies at the distance of a few
miles, and is only remarkable from its size and perfect condition. Its
summit is 1,200 feet above the level of the sea, and the interior
hollow is 600 feet deep. Its external sloping surface presented a
curious appearance from the smoothness of the wide layers of tuff,
which resembled a vast plastered floor. Brattle Island is, I believe,
the largest crater in the Archipelago composed of tuff; its interior
diameter is nearly a nautical mile. At present it is in a ruined
condition, consisting of little more than half a circle open to the
south; its great size is probably due, in part, to internal
degradation, from the action of the sea.
No. 14
[Illustration: Segment of very small orifice of eruption.]
Segment of a very small orifice of eruption, on the beach of
Fresh-water Bay.
_Segment of a basaltic crater._—One side of Fresh-water Bay, in James
Island, is bounded by a promontory, which forms the last wreck of a
great crater. On the beach of this promontory, a quadrant-shaped
segment of a small subordinate point of eruption stands exposed. It
consists of nine separate little streams of lava piled upon each other;
and of an irregular pinnacle, about fifteen feet high, of
reddish-brown, vesicular basalt, abounding with large crystals of
glassy albite, and with fused augite. This pinnacle, and some adjoining
paps of rock on the beach, represent the axis of the crater. The
streams of lava can be followed up a little ravine, at right angles to
the coast, for between ten and fifteen yards, where they are hidden by
detritus: along the beach they are visible for nearly eighty yards, and
I do not believe that they extend much further. The three lower streams
are united to the pinnacle; and at the point of junction (as shown in
figure No. 14, a rude sketch made on the spot), they are slightly
arched, as if in the act of flowing over the lip of the crater. The six
upper streams no doubt were originally united to this same column
before it was worn down by the sea. The lava of these streams is of
similar composition with that of the pinnacle, excepting that the
crystals of albite appear to be more comminuted, and the grains of
fused augite are absent. Each stream is separated from the one above it
by a few inches, or at most by one or two feet in thickness, of loose
fragmentary scoriæ,
apparently derived from the abrasion of the streams in passing over
each other. All these streams are very remarkable from their thinness.
I carefully measured several of them; one was eight inches thick, but
was firmly coated with three inches above, and three inches below, of
red scoriaceous rock (which is the case with all the streams), making
altogether a thickness of fourteen inches: this thickness was preserved
quite uniformly along the entire length of the section. A second stream
was only eight inches thick, including both the upper and lower
scoriaceous surfaces. Until examining this section, I had not thought
it possible that lava could have flowed in such uniformly thin sheets
over a surface far from smooth. These little streams closely resemble
in composition that great deluge of lava at Albemarle Island, which
likewise must have possessed a high degree of fluidity.
_Pseudo-extraneous, ejected fragments._—In the lava and in the scoriæ
of this little crater, I found several fragments, which, from their
angular form, their granular structure, their freedom from air-cells,
their brittle and burnt condition, closely resembled those fragments of
primary rocks which are occasionally ejected, as at Ascension, from
volcanoes. These fragments consist of glassy albite, much mackled, and
with very imperfect cleavages, mingled with semi-rounded grains, having
tarnished, glossy surfaces, of a steel-blue mineral. The crystals of
albite are coated by a red oxide of iron, appearing like a residual
substance; and their cleavage-planes also are sometimes separated by
excessively fine layers of this oxide, giving to the crystals the
appearance of being ruled like a glass micrometer. There was no quartz.
The steel-blue mineral, which is abundant in the pinnacle, but which
disappears in the streams derived from the pinnacle, has a fused
appearance, and rarely presents even a trace of cleavage; I obtained,
however, one measurement, which proved that it was augite; and in one
other fragment, which differed from the others, in being slightly
cellular, and in gradually blending into the surrounding matrix the
small grains of this mineral were tolerably well crystallised. Although
there is so wide a difference in appearance between the lava of the
little streams, and especially of their red scoriaceous crusts, and one
of these angular ejected fragments, which at first sight might readily
be mistaken for syenite, yet I believe that the lava has originated
from the melting and movement of a mass of rock of absolutely similar
composition with the fragments. Besides the specimen above alluded to,
in which we see a fragment becoming slightly cellular, and blending
into the surrounding matrix, some of the grains of the steel-blue
augite also have their surfaces becoming very finely vesicular, and
passing into the nature of the surrounding paste; other grains are
throughout, in an intermediate condition. The paste seems to consist of
the augite more perfectly fused, or, more probably, merely disturbed in
its softened state by the movement of the mass, and mingled with the
oxide of iron and with finely comminuted, glassy albite. Hence probably
it is that the fused albite, which is abundant in the pinnacle,
disappears in the streams. The albite is in exactly the same state,
with the exception of most of the crystals being smaller in the lava
and in the embedded fragments; but
in the fragments they appear to be less abundant: this, however, would
naturally happen from the intumescence of the augitic base, and its
consequent apparent increase in bulk. It is interesting thus to trace
the steps by which a compact granular rock becomes converted into a
vesicular, pseudo-porphyritic lava, and finally into red scoriæ. The
structure and composition of the embedded fragments show that they are
parts either of a mass of primary rock which has undergone considerable
change from volcanic action, or more probably of the crust of a body of
cooled and crystallised lava, which has afterwards been broken up and
re-liquified; the crust being less acted on by the renewed heat and
movement.
_Concluding remarks on the tuff-craters._—These craters, from the
peculiarity of the resin-like substance which enters largely into their
composition, from their structure, their size and number, present the
most striking feature in the geology of this Archipelago. The majority
of them form either separate islets, or promontories attached to the
larger islands; and those which now stand at some little distance from
the coast are worn and breached, as if by the action of the sea. From
this general circumstance of their position, and from the small
quantity of ejected ashes in any part of the Archipelago, I am led to
conclude, that the tuff has been chiefly produced, by the grinding
together of fragments of lava within active craters, communicating with
the sea. In the origin and composition of the tuff, and in the frequent
presence of a central lake of brine and of layers of salt, these
craters resemble, though on a gigantic scale, the “salses,” or hillocks
of mud, which are common in some parts of Italy and in other
countries.[13] Their closer connection, however, in this Archipelago,
with ordinary volcanic action, is shown by the pools of solidified
basalt, with which they are sometimes filled up.
[13] D’Aubuisson’s “Traité de Géognosie,” tome i, p. 189. I may
remark, that I saw at Terceira, in the Azores, a crater of tuff or
peperino, very similar to these of the Galapagos Archipelago. From the
description given in Freycinet “Voyage,” similar ones occur at the
Sandwich Islands; and probably they are present in many other places.
It at first appears very singular, that all the craters formed of tuff
have their southern sides, either quite broken down and wholly removed,
or much lower than the other sides. I saw and received accounts of
twenty-eight of these craters; of these, twelve form separate
islets,[14] and now exist as mere crescents quite open to the south,
with occasionally a few points of rock marking their former
circumference: of the remaining sixteen, some form promontories, and
others stand at a little distance inland from the shore; but all have
their southern sides either the lowest, or quite broken down. Two,
however, of the sixteen had
their northern sides also low, whilst their eastern and western sides
were perfect. I did not see, or hear of, a single exception to the
rule, of these craters being broken down or low on the side, which
faces a point of the horizon between S.E. and S.W. This rule does not
apply to craters composed of lava and scoriæ. The explanation is
simple: at this Archipelago, the waves from the trade-wind, and the
swell propagated from the distant parts of the open ocean, coincide in
direction (which is not the case in many parts of the Pacific), and
with their united forces attack the southern sides of all the islands;
and consequently the southern slope, even when entirely formed of hard
basaltic rock, is invariably steeper than the northern slope. As the
tuff-craters are composed of a soft material, and as probably all, or
nearly all, have at some period stood immersed in the sea, we need not
wonder that they should invariably exhibit on their exposed sides the
effects of this great denuding power. Judging from the worn condition
of many of these craters, it is probable that some have been entirely
washed away. As there is no reason to suppose, that the craters formed
of scoriæ and lava were erupted whilst standing in the sea, we can see
why the rule does not apply to them. At Ascension, it was shown that
the mouths of the craters, which are there all of terrestrial origin,
have been affected by the trade-wind; and this same power might here,
also, aid in making the windward and exposed sides of some of the
craters originally the lowest.
[14] These consist of the three Crossman Islets, the largest of which
is 600 feet in height; Enchanted Island; Gardner Island (760 feet
high); Champion Island (331 feet high); Enderby Island; Brattle
Island; two islets near Indefatigable Island; and one near James
Island. A second crater near James Island (with a salt lake in its
centre) has its southern side only about twenty feet high, whilst the
other parts of the circumference are about three hundred feet in
height.
_Mineralogical composition of the rocks._—In the northern islands, the
basaltic lavas seem generally to contain more albite than they do in
the southern half of the Archipelago; but almost all the streams
contain some. The albite is not unfrequently associated with olivine. I
did not observe in any specimen distinguishable crystals of hornblende
or augite; I except the fused grains in the ejected fragments, and in
the pinnacle of the little crater, above described. I did not meet with
a single specimen of true trachyte; though some of the paler lavas,
when abounding with large crystals of the harsh and glassy albite,
resemble in some degree this rock; but in every case the basis fuses
into a black enamel. Beds of ashes and far-ejected scoriæ, as
previously stated, are almost absent; nor did I see a fragment of
obsidian or of pumice. Von Buch[15] believes that the absence of pumice
on Mount Etna is consequent on the feldspar being of the Labrador
variety; if the presence of pumice depends on the constitution of the
feldspar, it is remarkable, that it should be absent in this
archipelago, and abundant in the Cordillera of South America, in both
of which regions the feldspar is of the albitic variety. Owing to the
absence of ashes, and the general indecomposable character of the lava
in this Archipelago, the islands are slowly clothed with a poor
vegetation, and the scenery has a desolate and frightful aspect.
[15] “Description des Isles Canaries,” p. 328.
_Elevation of the land._—Proofs of the rising of the land are scanty
and imperfect. At Chatham Island, I noticed some great blocks of lava,
cemented by calcareous matter, containing recent shells; but they
occurred at the height of only a few feet above high-water mark. One
of the officers gave me some fragments of shells, which he found
embedded several hundred feet above the sea, in the tuff of two
craters, distant from each other. It is possible, that these fragments
may have been carried up to their present height in an eruption of mud;
but as, in one instance, they were associated with broken
oyster-shells, almost forming a layer, it is more probable that the
tuff was uplifted with the shells in mass. The specimens are so
imperfect that they can be recognised only as belonging to recent
marine genera. On Charles Island, I observed a line of great rounded
blocks, piled on the summit of a vertical cliff, at the height of
fifteen feet above the line, where the sea now acts during the heaviest
gales. This appeared, at first, good evidence in favour of the
elevation of the land; but it was quite deceptive, for I afterwards saw
on an adjoining part of this same coast, and heard from eye-witnesses,
that wherever a recent stream of lava forms a smooth inclined plane,
entering the sea, the waves during gales have the power of _rolling up
rounded_ blocks to a great height, above the line of their ordinary
action. As the little cliff in the foregoing case is formed by a stream
of lava, which, before being worn back, must have entered the sea with
a gently sloping surface, it is possible or rather it is probable, that
the rounded boulders, now lying on its summit, are merely the remnants
of those which had been _rolled up_ during storms to their present
height.
_Direction of the fissures of eruption._—The volcanic orifices in this
group cannot be considered as indiscriminately scattered. Three great
craters on Albermarle Island form a well-marked line, extending N.W. by
N. and S.E. by S. Narborough Island, and the great crater on the
rectangular projection of Albemarle Island, form a second parallel
line. To the east, Hood’s Island, and the islands and rocks between it
and James Island, form another nearly parallel line, which, when
prolonged, includes Culpepper and Wenman Islands, lying seventy miles
to the north. The other islands lying further eastward, form a less
regular fourth line. Several of these islands, and the vents on
Albemarle Island, are so placed, that they likewise fall on a set of
rudely parallel lines, intersecting the former lines at right angles;
so that the principal craters appear to lie on the points where two
sets of fissures cross each other. The islands themselves, with the
exception of Albemarle Island, are not elongated in the same direction
with the lines on which they stand. The direction of these islands is
nearly the same with that which prevails in so remarkable a manner in
the numerous archipelagoes of the great Pacific Ocean. Finally, I may
remark, that amongst the Galapagos Islands there is no one dominant
vent much higher than all the others, as may be observed in many
volcanic archipelagoes: the highest is the great mound on the
south-western extremity of Albemarle Island, which exceeds by barely a
thousand feet several other neighbouring craters.
Chapter VI TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.
The sinking of crystals in fluid lava.—Specific gravity of the
constituent parts of trachyte and of basalt, and their consequent
separation.—Obsidian.—Apparent non-separation of the elements of
plutonic rocks.—Origin of trap-dikes in the plutonic
series.—Distribution of volcanic islands; their prevalence in the great
oceans.—They are generally arranged in lines.—The central volcanoes of
Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
volcanic islands, and their elevation in mass.—Eruptions on parallel
lines of fissure within the same geological period.
_On the separation of the constituent minerals of lava, according to
their specific gravities._—One side of Fresh-water Bay, in James
Island, is formed by the wreck of a large crater, mentioned in the last
chapter, of which the interior has been filled up by a pool of basalt,
about two hundred feet in thickness. This basalt is of a grey colour,
and contains many crystals of glassy albite, which become much more
numerous in the lower, scoriaceous part. This is contrary to what might
have been expected, for if the crystals had been originally
disseminated in equal numbers, the greater intumescence of this lower
scoriaceous part would have made them appear fewer in number. Von
Buch[1] has described a stream of obsidian on the Peak of Teneriffe, in
which the crystals of feldspar become more and more numerous, as the
depth or thickness increases, so that near the lower surface of the
stream the lava even resembles a primary rock. Von Buch further states,
that M. Dree, in his experiments in melting lava, found that the
crystals of feldspar always tended to precipitate themselves to the
bottom of the crucible. In these cases, I presume there can be no
doubt[2] that the crystals sink from their weight. The specific gravity
of feldspar varies[3] from 2·4 to 2·58, whilst obsidian seems commonly
to be from 2·3 to 2·4; and in a fluidified state its specific gravity
would probably be less, which would
facilitate the sinking of the crystals of feldspar. At James Island,
the crystals of albite, though no doubt of less weight than the grey
basalt, in the parts where compact, might easily be of greater specific
gravity than the scoriaceous mass, formed of melted lava and bubbles of
heated gas.
[1] “Description des Isles Canaries,” pp. 190 and 191.
[2] In a mass of molten iron, it is found (_Edinburgh New
Philosophical Journal_, vol. xxiv, p. 66) that the substances, which
have a closer affinity for oxygen than iron has, rise from the
interior of the mass to the surface. But a similar cause can hardly
apply to the separation of the crystals of these lava-streams. The
cooling of the surface of lava seems, in some cases, to have affected
its composition; for Dufrenoy (“Mém. pour servir,” tome iv, p. 271)
found that the interior parts of a stream near Naples contained
two-thirds of a mineral which was acted on by acids, whilst the
surface consisted chiefly of a mineral unattackable by acids.
[3] I have taken the specific gravities of the simple minerals from
Von Kobell, one of the latest and best authorities, and of the rocks
from various authorities. Obsidian, according to Phillips, is 2·35;
and Jameson says it never exceeds 2·4; but a specimen from Ascension,
weighed by myself, was 2·42.
The sinking of crystals through a viscid substance like molten rock, as
is unequivocally shown to have been the case in the experiments of M.
Drée, is worthy of further consideration, as throwing light on the
separation of the trachytic and basaltic series of lavas. Mr. P. Scrope
has speculated on this subject; but he does not seem to have been aware
of any positive facts, such as those above given; and he has overlooked
one very necessary element, as it appears to me, in the
phenomenon—namely, the existence of either the lighter or heavier
mineral in globules or in crystals. In a substance of imperfect
fluidity, like molten rock, it is hardly credible, that the separate,
infinitely small atoms, whether of feldspar, augite, or of any other
mineral, would have power from their slightly different gravities to
overcome the friction caused by their movement; but if the atoms of any
one of these minerals became, whilst the others remained fluid, united
into crystals or granules, it is easy to perceive that from the
lessened friction, their sinking or floating power would be greatly
increased. On the other hand, if all the minerals became granulated at
the same time, it is scarcely possible, from their mutual resistance,
that any separation could take place. A valuable, practical discovery,
illustrating the effect of the granulation of one element in a fluid
mass, in aiding its separation, has lately been made: when lead
containing a small proportion of silver, is constantly stirred whilst
cooling, it becomes granulated, and the grains of imperfect crystals of
nearly pure lead sink to the bottom, leaving a residue of melted metal
much richer in silver; whereas if the mixture be left undisturbed,
although kept fluid for a length of time, the two metals show no signs
of separating.[4] The sole use of the stirring seems to be, the
formation of detached granules. The specific gravity of silver is 10·4,
and of lead 11·35: the granulated lead, which sinks, is never
absolutely pure, and the residual fluid metal contains, when richest,
only 1/119 part of silver. As the difference in specific gravity,
caused by the different proportions of the two metals, is so
exceedingly small, the separation is probably aided in a great degree
by the difference in gravity between the lead, when granular though
still hot, and when fluid.
[4] A full and interesting account of this discovery, by Mr.
Pattinson, was read before the British Association in September 1838.
In some alloys, according to Turner (“Chemistry,” p. 210), the
heaviest metal sinks, and it appears that this takes place whilst both
metals are fluid. Where there is a considerable difference in gravity,
as between iron and the slag formed during the fusion of the ore, we
need not be surprised at the atoms separating, without either
substance being granulated.
In a body of liquified volcanic rock, left for some time without any
violent disturbance, we might expect, in accordance with the above
facts, that if one of the constituent minerals became aggregated into
crystals or granules, or had been enveloped in this state from some
previously existing mass, such crystals or granules would rise or sink,
according to their specific gravity. Now we have plain evidence of
crystals being embedded in many lavas, whilst the paste or basis has
continued fluid. I need only refer, as instances, to the several,
great, pseudo-porphyritic streams at the Galapagos Islands, and to the
trachytic streams in many parts of the world, in which we find crystals
of feldspar bent and broken by the movement of the surrounding,
semi-fluid matter. Lavas are chiefly composed of three varieties of
feldspar, varying in specific gravity from 2·4 to 2·74; of hornblende
and augite, varying from 3·0 to 3·4; of olivine, varying from 3·3 to
3·4; and lastly, of oxides of iron, with specific gravities from 4·8 to
5·2. Hence crystals of feldspar, enveloped in a mass of liquified, but
not highly vesicular lava, would tend to rise to the upper parts; and
crystals or granules of the other minerals, thus enveloped, would tend
to sink. We ought not, however, to expect any perfect degree of
separation in such viscid materials. Trachyte, which consists chiefly
of feldspar, with some hornblende and oxide of iron, has a specific
gravity of about 2·45;[5] whilst basalt, composed chiefly of augite and
feldspar, often with much iron and olivine, has a gravity of about 3·0.
Accordingly we find, that where both trachytic and basaltic streams
have proceeded from the same orifice, the trachytic streams have
generally been first erupted owing, as we must suppose, to the molten
lava of this series having accumulated in the upper parts of the
volcanic focus. This order of eruption has been observed by Beudant,
Scrope, and by other authors; three instances, also, have been given in
this volume. As the later eruptions, however, from most volcanic
mountains, burst through their basal parts, owing to the increased
height and weight of the internal column of molten rock, we see why, in
most cases, only the lower flanks of the central, trachytic masses, are
enveloped by basaltic streams. The separation of the ingredients of a
mass of lava, would, perhaps, sometimes take place within the body of a
volcanic mountain, if lofty and of great dimensions, instead of within
the underground focus; in which case, trachytic streams might be poured
forth, almost contemporaneously, or at short recurrent intervals, from
its summit, and basaltic streams from its base: this seems to have
taken place at Teneriffe.[6] I need only further remark, that from
violent disturbances the separation of the two series, even under
otherwise favourable conditions, would naturally often be prevented,
and likewise their usual order of eruption be inverted. From the high
degree of fluidity of most basaltic lavas, these perhaps, alone, would
in many cases reach the surface.
[5] Trachyte from Java was found by Von Buch to be 2·47; from
Auvergne, by De la Beche, it was 2·42; from Ascension, by myself, it
was 2·42. Jameson and other authors give to basalt a specific gravity
of 3·0; but specimens from Auvergne were found, by De la Beche, to be
only 2·78; and from the Giant’s Causeway, to be 2·91.
[6] Consult Von Buch’s well-known and admirable “Description Physique”
of this island, which might serve as a model of descriptive geology.
As we have seen that crystals of feldspar, in the instance described by
Von Buch, sink in obsidian, in accordance with their known greater
specific gravity, we might expect to find in every trachytic district,
where obsidian has flowed as lava, that it had proceeded from the upper
or highest orifices. This, according to Von Buch, holds good in a
remarkable manner both at the Lipari Islands and on the Peak of
Teneriffe; at this latter place obsidian has never flowed from a less
height than 9,200 feet. Obsidian, also, appears to have been erupted
from the loftiest peaks of the Peruvian Cordillera. I will only further
observe, that the specific gravity of quartz varies from 2·6 to 2·8;
and therefore, that when present in a volcanic focus, it would not tend
to sink with the basaltic bases; and this, perhaps, explains the
frequent presence, and the abundance of this mineral, in the lavas of
the trachytic series, as observed in previous parts of this volume.
An objection to the foregoing theory will, perhaps, be drawn from the
plutonic rocks not being separated into two evidently distinct series,
of different specific gravities; although, like the volcanic, they have
been liquified. In answer, it may first be remarked, that we have no
evidence of the atoms of any one of the constituent minerals in the
plutonic series having been aggregated, whilst the others remained
fluid, which we have endeavoured to show is an almost necessary
condition of their separation; on the contrary, the crystals have
generally impressed each other with their forms.[7]
[7] The crystalline paste of phonolite is frequently penetrated by
long needles of hornblende; from which it appears that the hornblende,
though the more fusible mineral, has crystallised before, or at the
same time with a more refractory substance. Phonolite, as far as my
observations serve, in every instance appears to be an injected rock,
like those of the plutonic series; hence probably, like these latter,
it has generally been cooled without repeated and violent
disturbances. Those geologists who have doubted whether granite could
have been formed by igneous liquefaction, because minerals of
different degrees of fusibility impress each other with their forms,
could not have been aware of the fact of crystallised hornblende
penetrating phonolite, a rock undoubtedly of igneous origin. The
viscidity, which it is now known, that both feldspar and quartz retain
at a temperature much below their points of fusion, easily explains
their mutual impressment. Consult on this subject Mr. Horner’s paper
on Bonn, “Geolog. Transact.,” vol. iv, p. 439; and “L’Institut,” with
respect to quartz, 1839, p. 161.
In the second place, the perfect tranquillity, under which it is
probable that the plutonic masses, buried at profound depths, have
cooled, would, most likely, be highly unfavourable to the separation of
their constituent minerals; for, if the attractive force, which during
the progressive cooling draws together the molecules of the different
minerals, has power sufficient to keep them together, the friction
between such half-formed crystals or pasty globules would effectually
prevent the heavier ones from sinking, or the lighter ones from rising.
On the other hand, a small amount of disturbance, which would probably
occur in most volcanic foci, and which we have seen does not prevent
the separation of granules of lead from a mixture of molten lead and
silver, or crystals of feldspar from streams of lava, by breaking
and dissolving the less perfectly formed globules, would permit the
more perfect and therefore unbroken crystals, to sink or rise,
according to their specific gravity.
Although in plutonic rocks two distinct species, corresponding to the
trachytic and basaltic series, do not exist, I much suspect that a
certain amount of separation of their constituent parts has often taken
place. I suspect this from having observed how frequently dikes of
greenstone and basalt intersect widely extended formations of granite
and the allied metamorphic rocks. I have never examined a district in
an extensive granitic region without discovering dikes; I may instance
the numerous trap-dikes, in several districts of Brazil, Chile, and
Australia, and at the Cape of Good Hope: many dikes likewise occur in
the great granitic tracts of India, in the north of Europe, and in
other countries. Whence, then, has the greenstone and basalt, forming
these dikes, come? Are we to suppose, like some of the elder
geologists, that a zone of trap is uniformly spread out beneath the
granitic series, which composes, as far as we know, the foundations of
the earth’s crust? Is it not more probable, that these dikes have been
formed by fissures penetrating into partially cooled rocks of the
granitic and metamorphic series, and by their more fluid parts,
consisting chiefly of hornblende, oozing out, and being sucked into
such fissures? At Bahia, in Brazil, in a district composed of gneiss
and primitive greenstone, I saw many dikes, of a dark augitic (for one
crystal certainly was of this mineral) or hornblendic rock, which, as
several appearances clearly proved, either had been formed before the
surrounding mass had become solid, or had together with it been
afterwards thoroughly softened.[8] On both sides of one of these dikes,
the gneiss was penetrated, to the distance of several yards, by
numerous, curvilinear threads or streaks of dark matter, which
resembled in form clouds of the class called cirrhi-comæ; some few of
these threads could be traced to their junction with the dike. When
examining them, I doubted whether such hair-like and curvilinear veins
could have been injected, and I now suspect, that instead of having
been injected from the dike, they were its feeders. If the foregoing
views of the origin of trap-dikes in widely extended granitic regions
far from rocks of any other formation, be admitted as probable, we may
further admit, in the case of a great body of plutonic rock, being
impelled by repeated movements into the axis of a mountain-chain, that
its more liquid constituent parts might drain into deep and unseen
abysses; afterwards, perhaps, to be brought to the surface under the
form, either of injected masses of greenstone and augitic porphyry,[9]
or of basaltic eruptions. Much of
the difficulty which geologists have experienced when they have
compared the composition of volcanic with plutonic formations, will, I
think, be removed, if we may believe that most plutonic masses have
been, to a certain extent, drained of those comparatively weighty and
easily liquified elements, which compose the trappean and basaltic
series of rocks.
[8] Portions of these dikes have been broken off, and are now
surrounded by the primary rocks, with their laminæ conformably winding
round them. Dr. Hubbard also (_Silliman’s Journal,_ vol. xxxiv, p.
119), has described an interlacement of trap-veins in the granite of
the White Mountains, which he thinks must have been formed when both
rocks were soft.
[9] Mr. Phillips (“Lardner’s Encyclop.,” vol. ii, p. 115) quotes Von
Buch’s statement, that augitic porphyry ranges parallel to, and is
found constantly at the base of, great chains of mountains. Humboldt,
also, has remarked the frequent occurrence of trap-rock, in a similar
position; of which fact I have observed many examples at the foot of
the Chilian Cordillera. The existence of granite in the axes of great
mountain chains is always probable, and I am tempted to suppose, that
the laterally injected masses of augitic porphyry and of trap, bear
nearly the same relation to the granitic axes which basaltic lavas
bear to the central trachytic masses, round the flanks of which they
have so frequently been erupted.
_On the distribution of volcanic islands._—During my investigations on
coral-reefs, I had occasion to consult the works of many voyagers, and
I was invariably struck with the fact, that with rare exceptions, the
innumerable islands scattered throughout the Pacific, Indian, and
Atlantic Oceans, were composed either of volcanic, or of modern
coral-rocks. It would be tedious to give a long catalogue of all the
volcanic islands; but the exceptions which I have found are easily
enumerated: in the Atlantic, we have St. Paul’s Rock, described in this
volume, and the Falkland Islands, composed of quartz and clay-slate;
but these latter islands are of considerable size, and lie not very far
from the South American coast:[10] in the Indian Ocean, the Seychelles
(situated in a line prolonged from Madagascar) consist of granite and
quartz: in the Pacific Ocean, New Caledonia, an island of large size,
belongs (as far as is known) to the primary class. New Zealand, which
contains much volcanic rock and some active volcanoes, from its size
cannot be classed with the small islands, which we are now considering.
The presence of a small quantity of non-volcanic rock, as of clay-slate
on three of the Azores,[11] or of tertiary limestone at Madeira, or of
clay-slate at Chatham Island in the Pacific, or of lignite at Kerguelen
Land, ought not to exclude such islands or archipelagoes, if formed
chiefly of erupted matter, from the volcanic class.
[10] Judging from Forster’s imperfect observation, perhaps Georgia is
not volcanic. Dr. Allan is my informant with regard to the Seychelles.
I do not know of what formation Rodriguez, in the Indian Ocean, is
composed.
[11] This is stated on the authority of Count V. de Bedemar, with
respect to Flores and Graciosa (Charlsworth, “Magazine of Nat. Hist.,”
vol. i, p. 557). St. Maria has no volcanic rock, according to Captain
Boyd (Von Buch “Descript.,” p. 365). Chatham Island has been described
by Dr. Dieffenbach in the “Geographical Journal,” 1841, p. 201. As yet
we have received only imperfect notices on Kerguelen Land, from the
Antarctic Expedition.
The composition of the numerous islands scattered through the great
oceans being with such rare exceptions volcanic, is evidently an
extension of that law, and the effect of those same causes, whether
chemical or mechanical, from which it results, that a vast majority of
the volcanoes now in action stand either as islands in the sea, or near
its shores. This fact of the ocean-islands being so generally volcanic
is also interesting in relation to the nature of the mountain-chains on
our
continents, which are comparatively seldom volcanic; and yet we are led
to suppose that where our continents now stand an ocean once extended.
Do volcanic eruptions, we may ask, reach the surface more readily
through fissures formed during the first stages of the conversion of
the bed of the ocean into a tract of land?
Looking at the charts of the numerous volcanic archipelagoes, we see
that the islands are generally arranged either in single, double, or
triple rows, in lines which are frequently curved in a slight
degree.[12] Each separate island is either rounded, or more generally
elongated in the same direction with the group in which it stands, but
sometimes transversely to it. Some of the groups which are not much
elongated present little symmetry in their forms; M. Virlet[13] states
that this is the case with the Grecian Archipelago: in such groups I
suspect (for I am aware how easy it is to deceive oneself on these
points), that the vents are generally arranged on one line, or on a set
of short parallel lines, intersecting at nearly right angles another
line, or set of lines. The Galapagos Archipelago offers an example of
this structure, for most of the islands and the chief orifices on the
largest island are so grouped as to fall on a set of lines ranging
about N.W. by N., and on another set ranging about W.S.W.: in the
Canary Archipelago we have a simpler structure of the same kind: in the
Cape de Verde group, which appears to be the least symmetrical of any
oceanic volcanic archipelago, a N.W. and S.E. line formed by several
islands, if prolonged, would intersect at right angles a curved line,
on which the remaining islands are placed. Von Buch[14] has classed all
volcanoes under two heads, namely, _central volcanoes_, round which
numerous eruptions have taken place on all sides, in a manner almost
regular, and _volcanic chains._ In the examples given of the first
class, as far as position is concerned, I can see no grounds for their
being called “central;” and the evidence of any difference in
mineralogical nature between _central volcanoes_ and _volcanic chains_
appears slight. No doubt some one island in most small volcanic
archipelagoes is apt to be considerably higher than the others; and in
a similar manner, whatever the cause may be, that on the same island
one vent is generally higher than all the others. Von Buch does not
include in his class of volcanic chains small archipelagoes, in which
the islands are admitted by him, as at the Azores, to be arranged in
lines; but when viewing on a map of the world how perfect a series
exists from a few volcanic islands placed in a row to a train of linear
archipelagoes following each other in a straight line, and so on to a
great wall like the Cordillera of America, it is difficult to believe
that there exists any essential difference between short and long
volcanic chains. Von Buch[15] states that his volcanic chains surmount,
or are closely connected with, mountain-ranges of primary formation:
but if trains of linear archipelagoes are, in the course of time, by
the long-continued action of the elevatory and volcanic forces,
converted into mountain-ranges, it would naturally result that the
inferior primary rocks would often be uplifted and brought into view.
[12] Professors William and Henry Darwin Rogers have lately insisted
much, in a memoir read before the American Association, on the
regularly curved lines of elevation in parts of the Appalachian range.
[13] “Bulletin de la Soc. Géolog.,” tome iii, p. 110.
[14] “Description des Isles Canaries,” p. 324.
[15] _Idem,_ p. 393.
Some authors have remarked that volcanic islands occur scattered,
though at very unequal distances, along the shores of the great
continents, as if in some measure connected with them. In the case of
Juan Fernandez, situated 330 miles from the coast of Chile, there was
undoubtedly a connection between the volcanic forces acting under this
island and under the continent, as was shown during the earthquake of
1835. The islands, moreover, of some of the small volcanic groups which
thus border continents, are placed in lines, related to those along
which the adjoining shores of the continents trend; I may instance the
lines of intersection at the Galapagos, and at the Cape de Verde
Archipelagoes, and the best marked line of the Canary Islands. If these
facts be not merely accidental, we see that many scattered volcanic
islands and small groups are related not only by proximity, but in the
direction of the fissures of eruption to the neighbouring continents—a
relation, which Von Buch considers, characteristic of his great
volcanic chains.
In volcanic archipelagoes, the orifices are seldom in activity on more
than one island at a time; and the greater eruptions usually recur only
after long intervals. Observing the number of craters, that are usually
found on each island of a group, and the vast amount of matter which
has been erupted from them, one is led to attribute a high antiquity
even to those groups, which appear, like the Galapagos, to be of
comparatively recent origin. This conclusion accords with the
prodigious amount of degradation, by the slow action of the sea, which
their originally sloping coasts must have suffered, when they are worn
back, as is so often the case, into grand precipices. We ought not,
however, to suppose, in hardly any instance, that the whole body of
matter, forming a volcanic island, has been erupted at the level, on
which it now stands: the number of dikes, which seem invariably to
intersect the interior parts of every volcano, show, on the principles
explained by M. Elie de Beaumont, that the whole mass has been uplifted
and fissured. A connection, moreover, between volcanic eruptions and
contemporaneous elevations in mass[16] has, I think, been shown to
exist in my work on Coral-Reefs, both from the frequent presence of
upraised organic remains, and from the structure of the accompanying
coral-reefs. Finally, I may remark, that in the same Archipelago,
eruptions have taken place within the historical period on more than
one of the parallel lines of fissure: thus, at the Galapagos
Archipelago, eruptions have taken place from a vent on Narborough
Island, and from one on Albemarle Island, which vents do not fall on
the same line; at the Canary Islands, eruptions have taken place in
Teneriffe and Lanzarote; and at
the Azores, on the three parallel lines of Pico, St. Jorge, and
Terceira. Believing that a mountain-axis differs essentially from a
volcano, only in plutonic rocks having been injected, instead of
volcanic matter having been ejected, this appears to me an interesting
circumstance; for we may infer from it as probable, that in the
elevation of a mountain-chain, two or more of the parallel lines
forming it may be upraised and injected within the same geological
period.
[16] A similar conclusion is forced on us, by the phenomena, which
accompanied the earthquake of 1835, at Concepcion, and which are
detailed in my paper (vol. v, p. 601) in the “Geological
Transactions.”
Chapter VII AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.
New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
Land.—Palæozoic formation.—Newer formation with volcanic
rocks.—Travertin with leaves of extinct plants.—Elevation of the
land.—New Zealand.—King George’s Sound.—Superficial ferruginous
beds.—Superficial calcareous deposits, with casts of branches.—Their
origin from drifted particles of shells and corals.—Their extent.—Cape
of Good Hope.—Junction of the granite and clay-slate.—Sandstone
formation.
The _Beagle_, in her homeward voyage, touched at New Zealand,
Australia, Van Diemen’s Land, and the Cape of Good Hope. In order to
confine the Third Part of these Geological Observations to South
America, I will here briefly describe all that I observed at these
places worthy of the attention of geologists.
_New South Wales._—My opportunities of observation consisted of a ride
of ninety geographical miles to Bathurst, in a W.N.W. direction from
Sydney. The first thirty miles from the coast passes over a sandstone
country, broken up in many places by trap-rocks, and separated by a
bold escarpment overhanging the river Nepean, from the great sandstone
platform of the Blue Mountains. This upper platform is 1,000 feet high
at the edge of the escarpment, and rises in a distance of twenty-five
miles to between three and four thousand feet above the level of the
sea. At this distance the road descends to a country rather less
elevated, and composed in chief part of primary rocks. There is much
granite, in one part passing into a red porphyry with octagonal
crystals of quartz, and intersected in some places by trap-dikes. Near
the Downs of Bathurst I passed over much pale-brown, glossy clay-slate,
with the shattered laminæ running north and south; I mention this fact,
because Captain King informs me that, in the country a hundred miles
southward, near Lake George, the mica-slate ranges so invariably north
and south that the inhabitants take advantage of it in finding their
way through the forests.
The sandstone of the Blue Mountains is at least 1,200 feet thick, and
in some parts is apparently of greater thickness; it consists of
small grains of quartz, cemented by white earthy matter, and it abounds
with ferruginous veins. The lower beds sometimes alternate with shales
and coal: at Wolgan I found in carbonaceous shale leaves of the
_Glossopteris Brownii_, a fern which so frequently accompanies the coal
of Australia. The sandstone contains pebbles of quartz; and these
generally increase in number and size (seldom, however, exceeding an
inch or two in diameter) in the upper beds: I observed a similar
circumstance in the grand sandstone formation at the Cape of Good Hope.
On the South American coast, where tertiary and supra-tertiary beds
have been extensively elevated, I repeatedly noticed that the uppermost
beds were formed of coarser materials than the lower: this appears to
indicate that, as the sea became shallower, the force of the waves or
currents increased. On the lower platform, however, between the Blue
Mountains and the coast, I observed that the upper beds of the
sandstone frequently passed into argillaceous shale,—the effect,
probably, of this lower space having been protected from strong
currents during its elevation. The sandstone of the Blue Mountains
evidently having been of mechanical origin, and not having suffered any
metamorphic action, I was surprised at observing that, in some
specimens, nearly all the grains of quartz were so perfectly
crystallised with brilliant facets that they evidently had not in their
_present_ form been aggregated in any previously existing rock.[1] It
is difficult to imagine how these crystals could have been formed; one
can hardly believe that they were separately precipitated in their
present crystallised state. Is it possible that rounded grains of
quartz may have been acted on by a fluid corroding their surfaces, and
depositing on them fresh silica? I may remark that, in the sandstone
formation of the Cape of Good Hope, it is evident that silica has been
profusely deposited from aqueous solution.
[1] I have lately seen, in a paper by Smith (the father of English
geologists), in the _Magazine of Natural History_, that the grains of
quartz in the millstone grit of England are often crystallised. Sir
David Brewster, in a paper read before the British Association, 1840,
states, that in old decomposed glass, the silex and metals separate
into concentric rings, and that the silex regains its crystalline
structure, as is shown by its action on light.
In several parts of the sandstone I noticed patches of shale which
might at the first glance have been mistaken for extraneous fragments;
their horizontal laminæ, however, being parallel with those of the
sandstone, showed that they were the remnants of thin, continuous beds.
One such fragment (probably the section of a long narrow strip) seen in
the face of a cliff, was of greater vertical thickness than breadth,
which proves that this bed of shale must have been in some slight
degree consolidated, after having been deposited, and before being worn
away by the currents. Each patch of the shale shows, also, how slowly
many of the successive layers of sandstone were deposited. These
pseudo-fragments of shale will perhaps explain, in some cases, the
origin of apparently extraneous fragments in crystalline metamorphic
rocks. I mention this, because I found near Rio de Janeiro a
well-defined angular fragment, seven yards long by two yards in
breadth, of gneiss
containing garnets and mica in layers, enclosed in the ordinary,
stratified, porphyritic gneiss of the country. The laminæ of the
fragment and of the surrounding matrix ran in exactly the same
direction, but they dipped at different angles. I do not wish to affirm
that this singular fragment (a solitary case, as far as I know) was
originally deposited in a layer, like the shale in the Blue Mountains,
between the strata of the porphyritic gneiss, before they were
metamorphosed; but there is sufficient analogy between the two cases to
render such an explanation possible.
_Stratification of the escarpment._—The strata of the Blue Mountains
appear to the eye horizontal; but they probably have a similar
inclination with the surface of the platform, which slopes from the
west towards the escarpment over the Nepean, at an angle of one degree,
or of one hundred feet in a mile.[2] The strata of the escarpment dip
almost conformably with its steeply inclined face, and with so much
regularity, that they appear as if thrown into their present position;
but on a more careful examination, they are seen to thicken and to thin
out, and in the upper part to be succeeded and almost capped by
horizontal beds. These appearances render it probable, that we here see
an original escarpment, not formed by the sea having eaten back into
the strata, but by the strata having originally extended only thus far.
Those who have been in the habit of examining accurate charts of
sea-coasts, where sediment is accumulating, will be aware, that the
surfaces of the banks thus formed, generally slope from the coast very
gently towards a certain line in the offing, beyond which the depth in
most cases suddenly becomes great. I may instance the great banks of
sediment within the West Indian Archipelago,[3] which terminate in
submarine slopes, inclined at angles of between thirty and forty
degrees, and sometimes even at more than forty degrees: every one knows
how steep such a slope would appear on the land. Banks of this nature,
if uplifted, would probably have nearly the same external form as the
platform of the Blue Mountains, where it abruptly terminates over the
Nepean.
[2] This is stated on the authority of Sir T. Mitchell, in his
“Travels,” vol. ii, p. 357.
[3] I have described these very curious banks in the Appendix to my
volume on the structure of Coral-Reefs. I have ascertained the
inclination of the edges of the banks, from information given me by
Captain B. Allen, one of the surveyors, and by carefully measuring the
horizontal distances between the last sounding on the bank and the
first in the deep water. Widely extended banks in all parts of the
West Indies have the same general form of surface.
_Current-cleavage._—The strata of sandstone in the low coast country,
and likewise on the Blue Mountains, are often divided by cross or
current laminæ, which dip in different directions, and frequently at an
angle of forty-five degrees. Most authors have attributed these cross
layers to successive small accumulations on an inclined surface; but
from a careful examination in some parts of the New Red Sandstone of
England, I believe that such layers generally form parts of a series of
curves, like gigantic tidal ripples, the tops of which have since been
cut off, either by nearly horizontal layers, or by another set of great
ripples, the folds of which do not exactly coincide with those below
them. It is well-known to surveyors that mud and sand are disturbed
during storms at considerable depths, at least from three hundred to
four hundred and fifty feet,[4] so that the nature of the bottom even
becomes temporarily changed; the bottom, also, at a depth between sixty
and seventy feet, has been observed[5] to be broadly rippled. One may,
therefore, be allowed to suspect, from the appearance just mentioned in
the New Red Sandstone, that at greater depths, the bed of the ocean is
heaped up during gales into great ripple-like furrows and depressions,
which are afterwards cut off by the currents during more tranquil
weather, and again furrowed during gales.
[4] See Martin White, on “Soundings in the British Channel,” pp. 4 and
166.
[5] M. Siau on the “Action of Waves,” _Edin. New Phil. Journ.,_ vol.
xxxi, p. 245.
_Valleys in the sandstone platforms._—The grand valleys, by which the
Blue Mountains and the other sandstone platforms of this part of
Australia are penetrated, and which long offered an insuperable
obstacle to the attempts of the most enterprising colonist to reach the
interior country, form the most striking feature in the geology of New
South Wales. They are of grand dimensions, and are bordered by
continuous links of lofty cliffs. It is not easy to conceive a more
magnificent spectacle, than is presented to a person walking on the
summit-plains, when without any notice he arrives at the brink of one
of these cliffs, which are so perpendicular, that he can strike with a
stone (as I have tried) the trees growing, at the depth of between one
thousand and one thousand five hundred feet below him; on both hands he
sees headland beyond headland of the receding line of cliff, and on the
opposite side of the valley, often at the distance of several miles, he
beholds another line rising up to the same height with that on which he
stands, and formed of the same horizontal strata of pale sandstone. The
bottoms of these valleys are moderately level, and the fall of the
rivers flowing in them, according to Sir T. Mitchell, is gentle. The
main valleys often send into the platform great baylike arms, which
expand at their upper ends; and on the other hand, the platform often
sends promontories into the valley, and even leaves in them great,
almost insulated, masses. So continuous are the bounding lines of
cliff, that to descend into some of these valleys, it is necessary to
go round twenty miles; and into others, the surveyors have only lately
penetrated, and the colonists have not yet been able to drive in their
cattle. But the most remarkable point of structure in these valleys,
is, that although several miles wide in their upper parts, they
generally contract towards their mouths to such a degree as to become
impassable. The Surveyor-General, Sir T. Mitchell,[6] in vain
endeavoured, first on foot and then by crawling between the great
fallen
fragments of sandstone, to ascend through the gorge by which the river
Grose joins the Nepean; yet the valley of the Grose in its upper part,
as I saw, forms a magnificent basin some miles in width, and is on all
sides surrounded by cliffs, the summits of which are believed to be
nowhere less than 3,000 feet above the level of the sea. When cattle
are driven into the valley of the Wolgan by a path (which I descended)
partly cut by the colonists, they cannot escape; for this valley is in
every other part surrounded by perpendicular cliffs, and eight miles
lower down, it contracts, from an average width of half a mile, to a
mere chasm impassable to man or beast. Sir T. Mitchell[7] states, that
the great valley of the Cox river with all its branches contracts,
where it unites with the Nepean, into a gorge 2,200 yards wide, and
about one thousand feet in depth. Other similar cases might have been
added.
[6] “Travels in Australia,” vol. i, p. 154.—I must express my
obligation to Sir T. Mitchell for several interesting personal
communications on the subject of these great valleys of New South
Wales.
[7] _Idem_, vol. ii, p. 358.
The first impression, from seeing the correspondence of the horizontal
strata, on each side of these valleys and great amphitheatre-like
depressions, is that they have been in chief part hollowed out, like
other valleys, by aqueous erosion; but when one reflects on the
enormous amount of stone, which on this view must have been removed, in
most of the above cases through mere gorges or chasms, one is led to
ask whether these spaces may not have subsided. But considering the
form of the irregularly branching valleys, and of the narrow
promontories, projecting into them from the platforms, we are compelled
to abandon this notion. To attribute these hollows to alluvial action,
would be preposterous; nor does the drainage from the summit-level
always fall, as I remarked near the Weatherboard, into the head of
these valleys, but into one side of their bay-like recesses. Some of
the inhabitants remarked to me, that they never viewed one of these
baylike recesses, with the headlands receding on both hands, without
being struck with their resemblance to a bold sea-coast. This is
certainly the case; moreover, the numerous fine harbours, with their
widely branching arms, on the present coast of New South Wales, which
are generally connected with the sea by a narrow mouth, from one mile
to a quarter of a mile in width, passing through the sandstone
coast-cliffs, present a likeness, though on a miniature scale, to the
great valleys of the interior. But then immediately occurs the
startling difficulty, why has the sea worn out these great, though
circumscribed, depressions on a wide platform, and left mere gorges,
through which the whole vast amount of triturated matter must have been
carried away? The only light I can throw on this enigma, is by showing
that banks appear to be forming in some seas of the most irregular
forms, and that the sides of such banks are so steep (as before stated)
that a comparatively small amount of subsequent erosion would form them
into cliffs: that the waves have power to form high and precipitous
cliffs, even in landlocked harbours, I have observed in many parts of
South America. In the Red Sea, banks with an extremely irregular
outline and composed of sediment, are penetrated by the most singularly
shaped creeks with narrow mouths: this is likewise the case, though on
a larger scale,
with the Bahama Banks. Such banks, I have been led to suppose,[8] have
been formed by currents heaping sediment on an irregular bottom. That
in some cases, the sea, instead of spreading out sediment in a uniform
sheet, heaps it round submarine rocks and islands, it is hardly
possible to doubt, after having examined the charts of the West Indies.
To apply these ideas to the sandstone platforms of New South Wales, I
imagine that the strata might have been heaped on an irregular bottom
by the action of strong currents, and of the undulations of an open
sea; and that the valley-like spaces thus left unfilled might, during a
slow elevation of the land, have had their steeply sloping flanks worn
into cliffs; the worn-down sandstone being removed, either at the time
when the narrow gorges were cut by the retreating sea, or subsequently
by alluvial action.
[8] See the “Appendix” to the Part on Coral-Reefs. The fact of the sea
heaping up mud round a submarine nucleus, is worthy of the notice of
geologists: for outlyers of the same composition with the coast banks
are thus formed; and these, if upheaved and worn into cliffs, would
naturally be thought to have been once connected together.
_Van Diemen’s Land._
The southern part of this island is mainly formed of mountains of
greenstone, which often assumes a syenitic character, and contains much
hypersthene. These mountains, in their lower half, are generally
encased by strata containing numerous small corals and some shells.
These shells have been examined by Mr. G. B. Sowerby, and have been
described by him: they consist of two species of Producta, and of six
of Spirifera; two of these, namely, _P. rugata_ and _S. rotundata_,
resemble, as far as their imperfect condition allows of comparison,
British mountain-limestone shells. Mr. Lonsdale has had the kindness to
examine the corals; they consist of six undescribed species, belonging
to three genera. Species of these genera occur in the Silurian,
Devonian, and Carboniferous strata of Europe. Mr. Lonsdale remarks,
that all these fossils have undoubtedly a Palæozoic character, and that
probably they correspond in age to a division of the system above the
Silurian formations.
The strata containing these remains are singular from the extreme
variability of their mineralogical composition. Every intermediate form
is present, between flinty-slate, clay-slate passing into grey wacke,
pure limestone, sandstone, and porcellanic rock; and some of the beds
can only be described as composed of a siliceo-calcareo-clay-slate. The
formation, as far as I could judge, is at least a thousand feet in
thickness: the upper few hundred feet usually consist of a siliceous
sandstone, containing pebbles and no organic remains; the inferior
strata, of which a pale flinty slate is perhaps the most abundant, are
the most variable; and these chiefly abound with the remains. Between
two beds of hard crystalline limestone, near Newtown, a layer of white
soft calcareous matter is quarried, and is used for whitewashing
houses. From information given to me by Mr. Frankland, the
Surveyor-General,
it appears that this Palæozoic formation is found in different parts of
the whole island; from the same authority, I may add, that on the
north-eastern coast and in Bass’ Straits primary rocks extensively
occur.
The shores of Storm Bay are skirted, to the height of a few hundred
feet, by strata of sandstone, containing pebbles of the formation just
described, with its characteristic fossils, and therefore belonging to
a subsequent age. These strata of sandstone often pass into shale, and
alternate with layers of impure coal; they have in many places been
violently disturbed. Near Hobart Town, I observed one dike, nearly a
hundred yards in width, on one side of which the strata were tilted at
an angle of 60 degrees, and on the other they were in some parts
vertical, and had been altered by the effects of the heat. On the west
side of Storm Bay, I found these strata capped by streams of basaltic
lava with olivine; and close by there was a mass of brecciated scoriæ,
containing pebbles of lava, which probably marks the place of an
ancient submarine crater. Two of these streams of basalt were separated
from each other by a layer of argillaceous wacke, which could be traced
passing into partially altered scoriæ. The wacke contained numerous
rounded grains of a soft, grass-green mineral, with a waxy lustre, and
translucent on its edges: under the blowpipe it instantly blackened,
and the points fused into a strongly magnetic, black enamel. In these
characters, it resembles those masses of decomposed olivine, described
at St. Jago in the Cape de Verde group; and I should have thought that
it had thus originated, had I not found a similar substance, in
cylindrical threads, within the cells of the vesicular basalt,—a state
under which olivine never appears; this substance,[9] I believe, would
be classed as bole by mineralogists.
[9] Chlorophæite, described by Dr. MacCulloch (“Western Islands,” vol.
i, p. 504) as occurring in a basaltic amygdaloid, differs from this
substance, in remaining unchanged before the blowpipe, and in
blackening from exposure to the air. May we suppose that olivine, in
undergoing the remarkable change described at St. Jago, passes through
several states?
_Travertin with extinct plants._—Behind Hobart Town there is a small
quarry of a hard travertin, the lower strata of which abound with
distinct impressions of leaves. Mr. Robert Brown has had the kindness
to look at my specimens, and he informed me that there are four or five
kinds, none of which he recognises as belonging to existing species.
The most remarkable leaf is palmate, like that of a fan-palm, and no
plant having leaves of this structure has hitherto been discovered in
Van Diemen’s Land. The other leaves do not resemble the most usual form
of the Eucalyptus (of which tribe the existing forests are chiefly
composed), nor do they resemble that class of exceptions to the common
form of the leaves of the Eucalyptus, which occur in this island. The
travertin containing this remnant of a lost vegetation, is of a pale
yellow colour, hard, and in parts even crystalline; but not compact,
and is everywhere penetrated by minute, tortuous, cylindrical pores. It
contains a very few pebbles of quartz, and occasionally layers of
chalcedonic nodules, like those of chert in our Greensand. From
the pureness of this calcareous rock, it has been searched for in other
places, but has never been found. From this circumstance, and from the
character of the deposit, it was probably formed by a calcareous spring
entering a small pool or narrow creek. The strata have subsequently
been tilted and fissured; and the surface has been covered by a
singular mass, with which, also, a large fissure has been filled up,
formed of balls of trap embedded in a mixture of wacke and a white,
earthy, alumino-calcareous substance. Hence it would appear, as if a
volcanic eruption had taken place on the borders of the pool, in which
the calcareous matter was depositing, and had broken it up and drained
it.
_Elevation of the land._—Both the eastern and western shores of the
bay, in the neighbourhood of Hobart Town, are in most parts covered to
the height of thirty feet above the level of high-water mark, with
broken shells, mingled with pebbles. The colonists attribute these
shells to the aborigines having carried them up for food: undoubtedly,
there are many large mounds, as was pointed out to me by Mr. Frankland,
which have been thus formed; but I think from the numbers of the
shells, from their frequent small size, from the manner in which they
are thinly scattered, and from some appearances in the form of the
land, that we must attribute the presence of the greater number to a
small elevation of the land. On the shore of Ralph Bay (opening into
Storm Bay) I observed a continuous beach about fifteen feet above
high-water mark, clothed with vegetation, and by digging into it,
pebbles encrusted with Serpulæ were found: along the banks, also, of
the river Derwent, I found a bed of broken sea-shells above the surface
of the river, and at a point where the water is now much too fresh for
sea-shells to live; but in both these cases, it is just possible, that
before certain spits of sand and banks of mud in Storm Bay were
accumulated, the tides might have risen to the height where we now find
the shells.[10]
[10] It would appear that some changes are now in progress in Ralph
Bay, for I was assured by an intelligent farmer, that oysters were
formerly abundant in it, but that about the year 1834 they had,
without any apparent cause, disappeared. In the “Transactions of the
Maryland Academy” (vol. i, part i, p. 28) there is an account by Mr.
Ducatel of vast beds of oysters and clams having been destroyed by the
gradual filling up of the shallow lagoons and channels, on the shores
of the southern United States. At Chiloe, in South America, I heard of
a similar loss, sustained by the inhabitants, in the disappearance
from one part of the coast of an edible species of Ascidia.
Evidence more or less distinct of a change of level between the land
and water, has been detected on almost all the land on this side of the
globe. Captain Grey, and other travellers, have found in Southern
Australia upraised shells, belonging either to the recent, or to a late
tertiary period. The French naturalists in Baudin’s expedition, found
shells similarly circumstanced on the S.W. coast of Australia. The Rev.
W. B. Clarke[11] finds proofs of the elevation of the land, to the
amount of 400 feet, at the Cape of Good Hope. In the neighbourhood
of the Bay of Islands in New Zealand,[12] I observed that the shores
were scattered to some height, as at Van Diemen’s Land, with
sea-shells, which the colonists attribute to the natives. Whatever may
have been the origin of these shells, I cannot doubt, after having seen
a section of the valley of the Thames River (37 degrees S.), drawn by
the Rev. W. Williams, that the land has been there elevated: on the
opposite sides of this great valley, three step-like terraces, composed
of an enormous accumulation of rounded pebbles, exactly correspond with
each other: the escarpment of each terrace is about fifty feet in
height. No one after having examined the terraces in the valleys on the
western shores of South America, which are strewed with sea-shells, and
have been formed during intervals of rest in the slow elevation of the
land, could doubt that the New Zealand terraces have been similarly
formed. I may add, that Dr. Dieffenbach, in his description of the
Chatham Islands[13] (S.W. of New Zealand), states that it is manifest
“that the sea has left many places bare which were once covered by its
waters.”
[11] “Proceedings of the Geological Society,” vol. iii, p. 420.
[12] I will here give a catalogue of the rocks which I met with near
the Bay of Islands, in New Zealand:—1st, Much basaltic lava, and
scoriform rocks, forming distinct craters;—2nd, A castellated hill of
horizontal strata of flesh-coloured limestone, showing when fractured
distinct crystalline facets: the rain has acted on this rock in a
remarkable manner, corroding its surface into a miniature model of an
Alpine country: I observed here layers of chert and clay ironstone;
and in the bed of a stream, pebbles of clay-slate;—3rd, The shores of
the Bay of Islands are formed of a feldspathic rock, of a bluish-grey
colour, often much decomposed, with an angular fracture, and crossed
by numerous ferruginous seams, but without any distinct stratification
or cleavage. Some varieties are highly crystalline, and would at once
be pronounced to be trap; others strikingly resembled clay-slate,
slightly altered by heat: I was unable to form any decided opinion on
this formation.
[13] _Geographical Journal_, vol. xi, pp. 202, 205.
_King George’s Sound._
This settlement is situated at the south-western angle of the
Australian continent: the whole country is granitic, with the
constituent minerals sometimes obscurely arranged in straight or curved
laminæ. In these cases, the rock would be called by Humboldt,
gneiss-granite, and it is remarkable that the form of the bare conical
hills, appearing to be composed of great folding layers, strikingly
resembles, on a small scale, those composed of gneiss-granite at Rio de
Janeiro, and those described by Humboldt at Venezuela. These plutonic
rocks are, in many places, intersected by trappean-dikes; in one place,
I found ten parallel dikes ranging in an E. and W. line; and not far
off another set of eight dikes, composed of a different variety of
trap, ranging at right angles to the former ones. I have observed in
several primary districts, the occurrence of systems of dikes parallel
and close to each other.
_Superficial ferruginous beds._—The lower parts of the country are
everywhere covered by a bed, following the inequalities of the surface,
of a honeycombed sandstone, abounding with oxides of iron. Beds of
nearly similar composition are common, I believe, along the whole
western coast of Australia, and on many of the East Indian islands. At
the Cape of Good Hope, at the base of the mountains formed of granite
and capped with sandstone, the ground is everywhere coated either by a
fine-grained, rubbly, ochraceous mass, like that at King George’s
Sound, or by a coarser sandstone with fragments of quartz, and rendered
hard and heavy by an abundance of the hydrate of iron, which presents,
when freshly broken, a metallic lustre. Both these varieties have a
very irregular texture, including spaces either rounded or angular,
full of loose sand: from this cause the surface is always honeycombed.
The oxide of iron is most abundant on the edges of the cavities, where
alone it affords a metallic fracture. In these formations, as well as
in many true sedimentary deposits, it is evident that iron tends to
become aggregated, either in the form of a shell, or of a network. The
origin of these superficial beds, though sufficiently obscure, seems to
be due to alluvial action on detritus abounding with iron.
_ Superficial calcareous deposit._—A calcareous deposit on the summit
of Bald Head, containing branched bodies, supposed by some authors to
have been corals, has been celebrated by the descriptions of many
distinguished voyagers.[14] It folds round and conceals irregular
hummocks of granite, at the height of 600 feet above the level of the
sea. It varies much in thickness; where stratified, the beds are often
inclined at high angles, even as much as at thirty degrees, and they
dip in all directions. These beds are sometimes crossed by oblique and
even-sided laminæ. The deposit consists either of a fine, white
calcareous powder, in which not a trace of structure can be discovered,
or of exceedingly minute, rounded grains, of brown, yellowish, and
purplish colours; both varieties being generally, but not always, mixed
with small particles of quartz, and being cemented into a more or less
perfect stone. The rounded calcareous grains, when heated in a slight
degree, instantly lose their colours; in this and in every other
respect, closely resembling those minute, equal-sized particles of
shells and corals, which at St. Helena have been drifted up the side of
the mountains, and have thus been winnowed of all coarser fragments. I
cannot doubt that the coloured calcareous particles here have had a
similar origin. The impalpable powder has probably been derived from
the decay of the rounded particles; this certainly is possible, for on
the coast of Peru, I have traced _large unbroken_ shells gradually
falling into a substance as fine as powdered chalk. Both of the
above-mentioned varieties of calcareous sandstone frequently alternate
with, and blend into, thin layers of a hard substalagmitic[15] rock,
which, even
when the stone on each side contains particles of quartz, is entirely
free from them: hence we must suppose that these layers, as well as
certain vein-like masses, have been formed by rain dissolving the
calcareous matter and re-precipitating it, as has happened at St.
Helena. Each layer probably marks a fresh surface, when the, now firmly
cemented, particles existed as loose sand. These layers are sometimes
brecciated and re-cemented, as if they had been broken by the slipping
of the sand when soft. I did not find a single fragment of a sea-shell;
but bleached shells of the _Helix melo_, an existing land species,
abound in all the strata; and I likewise found another Helix, and the
case of an Oniscus.
[14] I visited this hill, in company with Captain Fitzroy, and we came
to a similar conclusion regarding these branching bodies.
[15] I adopt this term from Lieutenant Nelson’s excellent paper on the
Bermuda Islands (“Geolog. Trans.,” vol. v, p. 106), for the hard,
compact, cream- or brown-coloured stone, without any crystalline
structure, which so often accompanies superficial calcareous
accumulations. I have observed such superficial beds, coated with
substalagmitic rock, at the Cape of Good Hope, in several parts of
Chile, and over wide spaces in La Plata and Patagonia. Some of these
beds have been formed from decayed shells, but the origin of the
greater number is sufficiently obscure. The causes which determine
water to dissolve lime, and then soon to redeposit it, are not, I
think, known. The surface of the substalagmitic layers appears always
to be corroded by the rain-water. As all the above-mentioned countries
have a long dry season, compared with the rainy one, I should have
thought that the presence of the substalagmitic was connected with the
climate, had not Lieutenant Nelson found this substance forming under
sea-water. Disintegrated shell seems to be extremely soluble; of which
I found good evidence, in a curious rock at Coquimbo in Chile, which
consisted of small, pellucid, empty husks, cemented together. A series
of specimens clearly showed that these husks had originally contained
small rounded particles of shells, which had been enveloped and
cemented together by calcareous matter (as often happens on
sea-beaches), and which subsequently had decayed, and been dissolved
by water, that must have penetrated through the calcareous husks,
without corroding them,—of which processes every stage could be seen.
The branches are absolutely undistinguishable in shape from the broken
and upright stumps of a thicket; their roots are often uncovered, and
are seen to diverge on all sides; here and there a branch lies
prostrate. The branches generally consist of the sandstone, rather
firmer than the surrounding matter, with the central parts filled,
either with friable, calcareous matter, or with a substalagmitic
variety; this central part is also frequently penetrated by linear
crevices, sometimes, though rarely, containing a trace of woody matter.
These calcareous, branching bodies, appear to have been formed by fine
calcareous matter being washed into the casts or cavities, left by the
decay of branches and roots of thickets, buried under drifted sand. The
whole surface of the hill is now undergoing disintegration, and hence
the casts, which are compact and hard, are left projecting. In
calcareous sand at the Cape of Good Hope, I found the casts, described
by Abel, quite similar to these at Bald Head; but their centres are
often filled with black carbonaceous matter not yet removed. It is not
surprising, that the woody matter should have been almost entirely
removed from the casts on Bald Head; for it is certain, that many
centuries must have elapsed since the thickets were buried; at present,
owing to the form and height of the narrow promontory, no sand is
drifted up, and the whole surface, as I have remarked, is wearing away.
We must, therefore,
look back to a period when the land stood lower, of which the French
naturalists[16] found evidence in upraised shells of recent species,
for the drifting on Bald Head of the calcareous and quartzose sand, and
the consequent embedment of the vegetable remains. There was only one
appearance which at first made me doubt concerning the origin of the
cast,—namely, that the finer roots from different stems sometimes
became united together into upright plates or veins; but when the
manner is borne in mind in which fine roots often fill up cracks in
hard earth, and that these roots would decay and leave hollows, as well
as the stems, there is no real difficulty in this case. Besides the
calcareous branches from the Cape of Good Hope, I have seen casts, of
exactly the same forms, from Madeira[17] and from Bermuda; at this
latter place, the surrounding calcareous rocks, judging from the
specimens collected by Lieutenant Nelson, are likewise similar, as is
their subaerial formation. Reflecting on the stratification of the
deposit on Bald Head,—on the irregularly alternating layers of
substalagmitic rock,—on the uniformly sized, and rounded particles,
apparently of sea-shells and corals,—on the abundance of land-shells
throughout the mass,—and finally, on the absolute resemblance of the
calcareous casts, to the stumps, roots, and branches of that kind of
vegetation, which would grow on sand-hillocks, I think there can be no
reasonable doubt, notwithstanding the different opinion of some
authors, that a true view of their origin has been here given.
[16] See M. Péron’s “Voyage,” tome i, p. 204.
[17] Dr. J. Macaulay has fully described (_Edinb. New Phil. Journ.,_
vol. xxix, p. 350) the casts from Madeira. He considers (differently
from Mr. Smith of Jordan Hill) these bodies to be corals, and the
calcareous deposit to be of subaqueous origin. His arguments chiefly
rest (for his remarks on their structure are vague) on the great
quantity of the calcareous matter, and on the casts containing animal
matter, as shown by their evolving ammonia. Had Dr. Macaulay seen the
enormous masses of rolled particles of shells and corals on the beach
of Ascension, and especially on coral-reefs; and had he reflected on
the effects of long-continued, gentle winds, in drifting up the finer
particles, he would hardly have advanced the argument of quantity,
which is seldom trustworthy in geology. If the calcareous matter has
originated from disintegrated shells and corals, the presence of
animal matter is what might have been expected. Mr. Anderson analysed
for Dr. Macaulay part of a cast, and he found it composed of—
Carbonate of lime 73·15 Silica 11·90 Phosphate of lime 8·81
Animal matter 4·25 Sulphate of lime a trace ——— 98·11
Calcareous deposits, like these of King George’s Sound, are of vast
extent on the Australian shores. Dr. Fitton remarks, that “recent
calcareous breccia (by which term all these deposits are included) was
found during Baudin’s voyage, over a space of no less than twenty-five
degrees
of latitude and an equal extent of longitude, on the southern, western,
and north-western coasts.”[18] It appears also from M. Peron, with
whose observations and opinions on the origin of the calcareous matter
and branching casts mine entirely accord, that the deposit is generally
much more continuous than near King George’s Sound. At Swan River,
Archdeacon Scott[19] states that in one part it extends ten miles
inland. Captain Wickham, moreover, informs me that during his late
survey of the western coast, the bottom of the sea, wherever the vessel
anchored, was ascertained, by crowbars being let down, to consist of
white calcareous matter. Hence it seems that along this coast, as at
Bermuda and at Keeling Atoll, submarine and subaerial deposits are
contemporaneously in process of formation, from the disintegration of
marine organic bodies. The extent of these deposits, considering their
origin, is very striking; and they can be compared in this respect only
with the great coral-reefs of the Indian and Pacific Oceans. In other
parts of the world, for instance in South America, there are
_superficial_ calcareous deposits of great extent, in which not a trace
of organic structure is discoverable; these observations would lead to
the inquiry, whether such deposits may not, also, have been formed from
disintegrated shells and corals.
[18] For ample details on this formation consult Dr. Fitton’s
“Appendix to Captain King’s Voyage.” Dr. Fitton is inclined to
attribute a concretionary origin to the branching bodies: I may
remark, that I have seen in beds of sand in La Plata cylindrical stems
which no doubt thus originated; but they differed much in appearance
from these at Bald Head, and the other places above specified.
[19] “Proceedings of the Geolog. Soc.,” vol. i, p. 320.
_Cape of Good Hope._
After the accounts given by Barrow, Carmichael, Basil Hall, and W. B.
Clarke of the geology of this district, I shall confine myself to a few
observations on the junction of the three principal formations. The
fundamental rock is granite,[20] overlaid by clay-slate: the latter is
generally hard, and glossy from containing minute scales of mica; it
alternates with, and passes into, beds of slightly crystalline,
feldspathic, slaty rock. This clay-slate is remarkable from being in
some places (as on the Lion’s Rump) decomposed, even to the depth of
twenty feet, into a pale-coloured, sandstone-like rock, which has been
mistaken, I believe, by some observers, for a separate formation. I was
guided by Dr. Andrew Smith to a fine junction at Green Point between
the granite and clay-slate: the latter at the distance of a quarter of
a mile
from the spot, where the granite appears on the beach (though,
probably, the granite is much nearer underground), becomes slightly
more compact and crystalline. At a less distance, some of the beds of
clay-slate are of a homogeneous texture, and obscurely striped with
different zones of colour, whilst others are obscurely spotted. Within
a hundred yards of the first vein of granite, the clay-slate consists
of several varieties; some compact with a tinge of purple, others
glistening with numerous minute scales of mica and imperfectly
crystallised feldspar; some obscurely granular, others porphyritic with
small, elongated spots of a soft white mineral, which being easily
corroded, gives to this variety a vesicular appearance. Close to the
granite, the clay-slate is changed into a dark-coloured, laminated
rock, having a granular fracture, which is due to imperfect crystals of
feldspar, coated by minute, brilliant scales of mica.
[20] In several places I observed in the granite, small dark-coloured
balls, composed of minute scales of black mica in a tough basis. In
another place, I found crystals of black schorl radiating from a
common centre. Dr. Andrew Smith found, in the interior parts of the
country, some beautiful specimens of granite, with silvery mica
radiating or rather branching, like moss, from central points. At the
Geological Society, there are specimens of granite with crystallised
feldspar branching and radiating in like manner.
The actual junction between the granitic and clay-slate districts
extends over a width of about two hundred yards, and consists of
irregular masses and of numerous dikes of granite, entangled and
surrounded by the clay-slate: most of the dikes range in a N.W. and
S.E. line, parallel to the cleavage of the slate. As we leave the
junction, thin beds, and lastly, mere films of the altered clay-slate
are seen, quite isolated, as if floating, in the coarsely crystallised
granite; but although completely detached, they all retain traces of
the uniform N.W. and S.E. cleavage. This fact has been observed in
other similar cases, and has been advanced by some eminent
geologists,[21] as a great difficulty on the ordinary theory, of
granite having been injected whilst liquified; but if we reflect on the
probable state of the lower surface of a laminated mass, like
clay-slate, after having been violently arched by a body of molten
granite, we may conclude that it would be full of fissures parallel to
the planes of cleavage; and that these would be filled with granite, so
that wherever the fissures were close to each other, mere parting
layers or wedges of the slate would depend into the granite. Should,
therefore, the whole body of rock afterwards become worn down and
denuded, the lower ends of these dependent masses or wedges of slate
would be left quite isolated in the granite; yet they would retain
their proper lines of cleavage, from having been united, whilst the
granite was fluid, with a continuous covering of clay-slate.
[21] See M. Keilhau’s “Theory on Granite” translated in the _Edinburgh
New Philosophical Journal,_ vol.xxiv, p. 402.
Following, in company with Dr. A. Smith, the line of junction between
the granite and the slate, as it stretched inland, in a S.E. direction,
we came to a place, where the slate was converted into a fine-grained,
perfectly characterised gneiss, composed of yellow-brown granular
feldspar, of abundant black brilliant mica, and of few and thin laminæ
of quartz. From the abundance of the mica in this gneiss, compared with
the small quantity and excessively minute scales, in which it exists in
the glossy clay-slate, we must conclude, that it has been here formed
by the metamorphic action—a circumstance doubted, under nearly similar
circumstances, by some authors. The laminæ of the clay-slate are
straight; and it was interesting to observe, that as they assumed
the character of gneiss, they became undulatory with some of the
smaller flexures angular, like the laminæ of many true metamorphic
schists.
_Sandstone formation._—This formation makes the most imposing feature
in the geology of Southern Africa. The strata are in many parts
horizontal, and attain a thickness of about two thousand feet. The
sandstone varies in character; it contains little earthy matter, but is
often stained with iron; some of the beds are very fine-grained and
quite white; others are as compact and homogeneous as quartz rock. In
some places I observed a breccia of quartz, with the fragments almost
dissolved in a siliceous paste. Broad veins of quartz, often including
large and perfect crystals, are very numerous; and it is evident in
nearly all the strata, that silica has been deposited from solution in
remarkable quantity. Many of the varieties of quartzite appeared quite
like metamorphic rocks; but from the upper strata being as siliceous as
the lower, and from the undisturbed junctions with the granite, which
in many places can be examined, I can hardly believe that these
sandstone-strata have been exposed to heat.[22] On the lines of
junction between these two great formations, I found in several places
the granite decayed to the depth of a few inches, and succeeded, either
by a thin layer of ferruginous shale, or by four or five inches in
thickness of the re-cemented crystals of the granite, on which the
great pile of sandstone immediately rested.
[22] The Rev. W. B. Clarke, however, states, to my surprise (“Geolog.
Proceedings,” vol. iii, p. 422), that the sandstone in some parts is
penetrated by granitic dikes: such dikes must belong to an epoch
altogether subsequent to that when the molten granite acted on the
clay-slate.
Mr. Schomburgk has described[23] a great sandstone formation in
Northern Brazil, resting on granite, and resembling to a remarkable
degree, in composition and in the external form of the land, this
formation of the Cape of Good Hope. The sandstones of the great
platforms of Eastern Australia, which also rest on granite, differ in
containing more earthy and less siliceous matter. No fossil remains
have been discovered in these three vast deposits. Finally, I may add
that I did not see any boulders of far-transported rocks at the Cape of
Good Hope, or on the eastern and western shores of Australia, or at Van
Diemen’s Land. In the northern island of New Zealand, I noticed some
large blocks of greenstone, but whether their parent rock was far
distant, I had no opportunity of determining.
[23] _Geographical Journal,_ vol. x, p. 246.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.
CRITICAL INTRODUCTION.
Of the remarkable “trilogy” constituted by Darwin’s writings which deal
with the geology of the _Beagle_, the member which has perhaps
attracted least attention, up to the present time is that which treats
of the geology of South America. The actual writing of this book
appears to have occupied Darwin a shorter period than either of the
other volumes of the series; his diary records that the work was
accomplished within ten months, namely, between July 1844 and April
1845; but the book was not actually issued till late in the year
following, the preface bearing the date “September 1846.” Altogether,
as Darwin informs us in his “Autobiography,” the geological books
“consumed four and a half years’ steady work,” most of the remainder of
the ten years that elapsed between the return of the _Beagle_, and the
completion of his geological books being, it is sad to relate, “lost
through illness!”
Concerning the “Geological Observations on South America,” Darwin wrote
to his friend Lyell, as follows:—“My volume will be about 240 pages,
dreadfully dull, yet much condensed. I think whenever you have time to
look through it, you will think the collection of facts on the
elevation of the land and on the formation of terraces pretty good.”
“Much condensed” is the verdict that everyone must endorse, on rising
from the perusal of this remarkable book; but by no means “dull.” The
three and a half years from April 1832 to September 1835, were spent by
Darwin in South America, and were devoted to continuous scientific
work; the problems he dealt with were either purely geological or those
which constitute the borderland between the geological and biological
sciences. It is impossible to read the journal which he kept during
this time without being impressed by the conviction
that it contains all the germs of thought which afterwards developed
into the “Origin of Species.” But it is equally evident that after his
return to England, biological speculations gradually began to exercise
a more exclusive sway over Darwin’s mind, and tended to dispossess
geology, which during the actual period of the voyage certainly
engrossed most of his time and attention. The wonderful series of
observations made during those three and a half years in South America
could scarcely be done justice to, in the 240 pages devoted to their
exposition. That he executed the work of preparing the book on South
America in somewhat the manner of a task, is shown by many references
in his letters. Writing to Sir Joseph Hooker in 1845, he says, “I hope
this next summer to finish my South American Geology, then to get out a
little Zoology, and _hurrah for my species work!_”
It would seem that the feeling of disappointment, which Darwin so often
experienced in comparing a book when completed, with the observations
and speculations which had inspired it, was more keenly felt in the
case of his volume on South America than any other. To one friend he
writes, “I have of late been slaving extra hard, to the great
discomfiture of wretched digestive organs, at South America, and thank
all the fates, I have done three-fourths of it. Writing plain English
grows with me more and more difficult, and never attainable. As for
your pretending that you will read anything so dull as my pure
geological descriptions, lay not such a flattering unction on my soul,
for it is incredible.” To another friend he writes, “You do not know
what you threaten when you propose to read it—it is purely geological.
I said to my brother, ‘You will of course read it,’ and his answer was,
‘Upon my life, I would sooner even buy it.’”
In spite of these disparaging remarks, however, we are strongly
inclined to believe that this book, despised by its author, and
neglected by his contemporaries, will in the end be admitted to be one
of Darwin’s chief titles to fame. It is, perhaps, an unfortunate
circumstance that the great success which he attained in biology by the
publication of the “Origin of Species” has, to some extent,
overshadowed the fact that Darwin’s claims as a geologist, are of the
very highest order. It is not too much to say that, had Darwin not been
a geologist, the “Origin of Species” could never have been written by
him. But apart from those geological questions, which have an important
bearing on biological thought and speculation, such as the proofs of
imperfection in the geological record, the relations of the later
tertiary faunas to the recent ones in the same areas, and the apparent
intermingling of types belonging to distant geological epochs, when we
study the palæontology of remote districts,—there are other purely
geological problems, upon which the contributions made by Darwin are of
the very highest value. I believe that the verdict of the historians of
science will be that if Darwin had not taken a foremost place among the
biologists of this century, his position as a geologist would have been
an almost equally commanding one.
But in the case of Darwin’s principal geological work—that relating to
the origin of the crystalline schists,—geologists were not at the time
prepared to receive his revolutionary teachings. The influence of
powerful authority was long exercised, indeed, to stifle his teaching,
and only now, when this unfortunate opposition has disappeared, is the
true nature and importance of Darwin’s purely geological work beginning
to be recognised.
The two first chapters of the “Geological Observations on South
America,” deal with the proofs which exist of great, but frequently
interrupted, movements of elevation during very recent geological
times. In connection with this subject, Darwin’s particular attention
was directed to the relations between the great earthquakes of South
America—of some of which he had impressive experience—and the permanent
changes of elevation which were taking place. He was much struck by the
rapidity with which the evidence of such great earth movements is
frequently obliterated; and especially with the remarkable way in which
the action of rain-water, percolating through deposits on the earth’s
surface, removes all traces of shells and other calcareous organisms.
It was these considerations which were the parents of the
generalisation that a palæontological record can only be preserved
during those periods in which long-continued slow subsidence is going
on. This in turn, led to the still wider and more suggestive conclusion
that the geological record as a whole is, and never can be more than, a
series of more or less isolated fragments. The recognition of this
important fact constitutes the keystone to any theory of evolution
which seeks to find a basis in the actual study of the types of life
that have formerly inhabited our globe.
In his third chapter, Darwin gives a number of interesting facts,
collected during his visits to the plains and valleys of Chili, which
bear on the question of the origin of saliferous deposits—the
accumulation of salt, gypsum, and nitrate of soda. This is a
problem that has excited much discussion among geologists, and which,
in spite of many valuable observations, still remains to a great extent
very obscure. Among the important considerations insisted upon by
Darwin is that relating to the absence of marine shells in beds
associated with such deposits. He justly argues that if the strata were
formed in shallow waters, and then exposed by upheaval to subaerial
action, all shells and other calcareous organisms would be removed by
solution.
Following Lyell’s method, Darwin proceeds from the study of deposits
now being accumulated on the earth’s surface, to those which have been
formed during the more recent periods of the geological history.
His account of the great Pampean formation, with its wonderful
mammalian remains—_Mastodon, Toxodon, Scelidotherium, Macrauchenia,
Megatherium, Megalonyx, Mylodon,_ and _ Glyptodon_—this full of
interest. His discovery of the remains of a true _Equus_ afforded a
remarkable confirmation of the fact—already made out in North
America—that species of horse had existed and become extinct in the New
World, before their introduction by the Spaniards in the sixteenth
century. Fully perceiving the importance of the microscope in studying
the nature and origin of such deposits as those of the Pampas, Darwin
submitted many of his specimens both to Dr. Carpenter in this country,
and to Professor Ehrenberg in Berlin. Many very important notes on the
microscopic organisms contained in the formation will be found
scattered through the chapter.
Darwin’s study of the older tertiary formations, with their abundant
shells, and their relics of vegetable life buried under great sheets of
basalt, led him to consider carefully the question of climate during
these earlier periods. In opposition to prevalent views on this
subject, Darwin points out that his observations are opposed to the
conclusion that a higher temperature prevailed universally over the
globe during early geological periods. He argues that “the causes which
gave to the older tertiary productions of the quite temperate zones of
Europe a tropical character, _were of a local character and did not
affect the whole globe._” In this, as in many similar instances, we see
the beneficial influence of extensive travel in freeing Darwin’s mind
from prevailing prejudices. It was this widening of experience which
rendered him so especially qualified to deal with the great problem of
the origin of species, and in doing so to emancipate himself from ideas
which were received with unquestioning faith by
geologists whose studies had been circumscribed within the limits of
Western Europe.
In the Cordilleras of Northern and Central Chili, Darwin, when studying
still older formations, clearly recognised that they contain an
admixture of the forms of life, which in Europe are distinctive of the
Cretaceous and Jurassic periods respectively. He was thus led to
conclude that the classification of geological periods, which fairly
well expresses the facts that had been discovered in the areas where
the science was first studied, is no longer capable of being applied
when we come to the study of widely distant regions. This important
conclusion led up to the further generalisation that each great
geological period has exhibited a geographical distribution of the
forms of animal and vegetable life, comparable to that which prevails
in the existing fauna and flora. To those who are familiar with the
extent to which the doctrine of universal formations has affected
geological thought and speculation, both long before and since the time
that Darwin wrote, the importance of this new standpoint to which he
was able to attain will be sufficiently apparent. Like the idea of the
extreme imperfection of the Geological Record, the doctrine of _ local_
geological formations is found permeating and moulding all the
palæontological reasonings of his great work.
In one of Darwin’s letters, written while he was in South America,
there is a passage we have already quoted, in which he expresses his
inability to decide between the rival claims upon his attention of “the
old crystalline group of rocks,” and “the softer fossiliferous beds”
respectively. The sixth chapter of the work before us, entitled
“Plutonic and Metamorphic Rocks—Cleavage and Foliation,” contains a
brief summary of a series of observations and reasonings upon these
crystalline rocks, which are, we believe, calculated to effect a
revolution in geological science, and—though their value and importance
have long been overlooked—are likely to entitle Darwin in the future to
a position among geologists, scarcely, if at all, inferior to that
which he already occupies among biologists.
Darwin’s studies of the great rock-masses of the Andes convinced him of
the close relations between the granitic or Plutonic rocks, and those
which were undoubtedly poured forth as lavas. Upon his return, he set
to work, with the aid of Professor Miller, to make a careful study of
the minerals composing the granites and those which occur in the lavas,
and he was able to show that in all essential respects they are
identical. He was further able to
prove that there is a complete gradation between the highly crystalline
or granitic rock-masses, and those containing more or less glassy
matter between their crystals, which constitute ordinary lavas. The
importance of this conclusion will be realised when we remember that it
was then the common creed of geologists—and still continues to be so on
the Continent—that all highly crystalline rocks are of great geological
antiquity, and that the igneous ejections which have taken place since
the beginning of the tertiary periods differ essentially, in their
composition, their structure, and their mode of occurrence, from those
which have made their appearance at earlier periods of the world’s
history.
Very completely have the conclusions of Darwin upon these subjects been
justified by recent researches. In England, the United States, and
Italy, examples of the gradual passage of rocks of truly granitic
structure into ordinary lavas have been described, and the reality of
the transition has been demonstrated by the most careful studies with
the microscope. Recent researches carried on in South America by
Professor Stelzner, have also shown the existence of a class of highly
crystalline rocks—the “Andengranites”—which combine in themselves many
of the characteristics which were once thought to be distinctive of the
so-called Plutonic and volcanic rocks. No one familiar with recent
geological literature—even in Germany and France, where the old views
concerning the distinction of igneous products of different ages have
been most stoutly maintained—can fail to recognise the fact that the
principles contended for by Darwin bid fair at no distant period to win
universal acceptance among geologists all over the globe.
Still more important are the conclusions at which Darwin arrived with
respect to the origin of the schists and gneisses which cover so large
an area in South America.
Carefully noting, by the aid of his compass and clinometer, at every
point which he visited, the direction and amount of inclination of the
parallel divisions in these rocks, he was led to a very important
generalisation—namely, that over very wide areas the direction (strike)
of the planes of cleavage in slates, and of foliation in schists and
gneisses, remained constant, though the amount of their inclination
(dip) often varied within wide limits. Further than this it appeared
that there was always a close correspondence between the strike of the
cleavage and foliation and the direction of the great axes along which
elevation had taken place in the district.
In Tierra del Fuego, Darwin found striking evidence that the cleavage
intersecting great masses of slate-rocks was quite independent of their
original stratification, and could often, indeed, be seen cutting
across it at right angles. He was also able to verify Sedgwick’s
observation that, in some slates, glossy surfaces on the planes of
cleavage arise from the development of new minerals, chlorite, epidote
or mica, and that in this way a complete graduation from slates to true
schists may be traced.
Darwin further showed that in highly schistose rocks, the folia bend
around and encircle any foreign bodies in the mass, and that in some
cases they exhibit the most tortuous forms and complicated puckerings.
He clearly saw that in all cases the forces by which these striking
phenomena must have been produced were persistent over wide areas, and
were connected with the great movements by which the rocks had been
upheaved and folded.
That the distinct folia of quartz, feldspar, mica, and other minerals
composing the metamorphic schists could not have been separately
deposited as sediment was strongly insisted upon by Darwin; and in
doing so he opposed the view generally prevalent among geologists at
that time. He was thus driven to the conclusion that foliation, like
cleavage, is not an original, but a superinduced structure in
rock-masses, and that it is the result of re-crystallisation, under the
controlling influence of great pressure, of the materials of which the
rock was composed.
In studying the lavas of Ascension, as we have already seen, Darwin was
led to recognise the circumstance that, when igneous rocks are
subjected to great differential movements during the period of their
consolidation, they acquire a foliated structure, closely analogous to
that of the crystalline schists. Like his predecessor in this field of
inquiry, Mr. Poulett Scrope, Charles Darwin seems to have been greatly
impressed by these facts, and he argued from them that the rocks
exhibiting the foliated structure must have been in a state of
plasticity, like that of a cooling mass of lava. At that time the
suggestive experiments of Tresca, Daubree, and others, showing that
solid masses under the influence of enormous pressure become actually
plastic, had not been published. Had Darwin been aware of these facts
he would have seen that it was not necessary to assume a state of
imperfect solidity in rock-masses in order to account for their having
yielded to pressure and tension, and, in doing so, acquiring the new
characters which distinguish the crystalline schists.
The views put forward by Darwin on the origin of the crystalline
schists found an able advocate in Mr. Daniel Sharpe, who in 1852 and
1854 published two papers, dealing with the geology of the Scottish
Highlands and of the Alps respectively, in which he showed that the
principles arrived at by Darwin when studying the South American rocks
afford a complete explanation of the structure of the two districts in
question.
But, on the other hand, the conclusions of Darwin and Sharpe were met
with the strongest opposition by Sir Roderick Murchison and Dr. A.
Geikie, who in 1861 read a paper before the Geological Society “On the
Coincidence between Stratification and Foliation in the Crystalline
Rocks of the Scottish Highlands,” in which they insisted that their
observations in Scotland tended to entirely disprove the conclusions of
Darwin that foliation in rocks is a secondary structure, and entirely
independent of the original stratification of the rock-masses.
Now it is a most significant circumstance that, no sooner did the
officers of the Geological Survey commence the careful and detailed
study of the Scottish Highlands than they found themselves compelled to
make a formal retraction of the views which had been put forward by
Murchison and Geikie in opposition to the conclusions of Darwin. The
officers of the Geological Survey have completely abandoned the view
that the foliation of the Highland rocks has been determined by their
original stratification, and admit that the structure is the result of
the profound movements to which the rocks have been subjected. The same
conclusions have recently been supported by observations made in many
different districts—among which we may especially refer to those of Dr.
H. Reusch in Norway, and those of Dr. J. Lehmann in Saxony. At the
present time the arguments so clearly stated by Darwin in the work
before us, have, after enduring opposition or neglect for a whole
generation, begun to “triumph all along the line,” and we may look
forward confidently to the near future, when his claim to be regarded
as one of the greatest of geological discoverers shall be fully
vindicated.
JOHN W. JUDD.
Chapter I ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.
Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.—Step-formed plains of Patagonia, with upraised
Shells.—Terrace-bounded Valley of Santa Cruz, formerly a
Sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of
the elevated area.—Equability of the movements, as shown by the similar
heights of the plains.—Slowness of the elevatory process.—Mode of
formation of the step-formed plains.—Summary.—Great Shingle Formation
of Patagonia; its extent, origin, and distribution.—Formation of
sea-cliffs.
In the following Volume, which treats of the geology of South America,
and almost exclusively of the parts southward of the Tropic of
Capricorn, I have arranged the chapters according to the age of the
deposits, occasionally departing from this order, for the sake of
geographical simplicity.
The elevation of the land within the recent period, and the
modifications of its surface through the action of the sea (to which
subjects I paid particular attention) will be first discussed; I will
then pass on to the tertiary deposits, and afterwards to the older
rocks. Only those districts and sections will be described in detail
which appear to me to deserve some particular attention; and I will, at
the end of each chapter, give a summary of the results. We will
commence with the proofs of the upheaval of the eastern coast of the
continent, from the Rio Plata southward; and, in the Second Chapter,
follow up the same subject along the shores of Chile and Peru.
On the northern bank of the great estuary of the Rio Plata, near
Maldonado, I found at the head of a lake, sometimes brackish but
generally containing fresh water, a bed of muddy clay, six feet in
thickness, with numerous shells of species still existing in the Plata,
namely, the _Azara labiata_, d’Orbigny, fragments of _Mytilus
eduliformis_, d’Orbigny, _Paludestrina Isabellei_, d’Orbigny, and the
_Solen Caribæus_, Lam., which last was embedded vertically in the
position in which it had lived. These shells lie at the height of only
two feet above the lake, nor would they have been worth mentioning,
except in connection with analogous facts.
At Monte Video, I noticed near the town, and along the base of the
mount, beds of a living Mytilus, raised some feet above the surface of
the Plata: in a similar bed, at a height from thirteen to sixteen feet,
M. Isabelle collected eight species, which,[1] according to M.
d’Orbigny, now live at the mouth of the estuary. At Colonia del
Sacramiento, further westward, I observed at the height of about
fifteen feet above the river, there of quite fresh water, a small bed
of the same Mytilus, which lives in brackish water at Monte Video. Near
the mouth of Uruguay, and for at least thirty-five miles northward,
there are at intervals large sandy tracts, extending several miles from
the banks of the river, but not raised much above its level, abounding
with small bivalves, which occur in such numbers that at the Agraciado
they are sifted and burnt for lime. Those which I examined near the A.
S. Juan were much worn: they consisted of _Mactra Isabellei_,
d’Orbigny, mingled with few of _Venus sinuosa_, Lam., both inhabiting,
as I am informed by M. d’Orbigny, brackish water at the mouth of the
Plata, nearly or quite as salt as the open sea. The loose sand, in
which these shells are packed, is heaped into low, straight, long lines
of dunes, like those left by the sea at the head of many bays. M.
d’Orbigny has described[2] an analogous phenomenon on a greater scale,
near San Pedro on the river Parana, where he found widely extended beds
and hillocks of sand, with vast numbers of the _Azara labiata_, at the
height of nearly 100 feet (English) above the surface of that river.
The Azara inhabits brackish water, and is not known to be found nearer
to San Pedro than Buenos Ayres, distant above a hundred miles in a
straight line. Nearer Buenos Ayres, on the road from that place to San
Isidro, there are extensive beds, as I am informed by Sir Woodbine
Parish,[3] of the _Azara labiata_, lying at about forty feet above the
level of the river, and distant between two and three miles from it.
These shells are always found on the highest banks in the district:
they are embedded in a stratified earthy mass, precisely like that of
the great Pampean deposit hereafter to be described. In one collection
of these shells, there were some valves of the _Venus sinuosa_, Lam.,
the same species found with the Mactra on the banks of the Uruguay.
South of Buenos Ayres, near Ensenada, there are other beds of the
Azara, some of which seem to have been embedded in yellowish,
calcareous, semi-crystalline matter; and Sir W. Parish has given me
from the banks of the Arroyo del Tristan, situated in this same
neighbourhood, at the distance of about a league from the Plata, a
specimen of a pale-reddish, calcereo-argillaceous stone (precisely like
parts of the Pampean deposit the importance of which fact will be
referred to in a succeeding chapter), abounding with shells of an
Azara, much worn, but which in general form and appearance closely
resemble, and are probably identical with, the _A. labiata._ Besides
these shells, cellular, highly crystalline rock, formed of the casts of
small bivalves, is found near Ensenada; and likewise beds of
sea-shells, which from their appearance
appear to have lain on the surface. Sir W. Parish has given me some of
these shells, and M. d’Orbigny pronounces them to be:—
Buccinanops globulosum, d’Orbigny.
Olivancillaria auricularia, d’Orbigny.
Venus flexuosa, Lam.
Cytheræa (imperfect).
Mactra Isabellei, d’Orbigny.
Ostrea pulchella, d’Orbigny.
Besides these, Sir W. Parish procured[4] (as named by Mr. G. B.
Sowerby) the following shells:—
Voluta colocynthis.
Voluta angulata.
Buccinum (not spec.?).
[1] “Voyage dans l’Amérique Mérid.: Part. Géolog.,” p. 21.
[2] _Ibid._, p. 43.
[3] “Buenos Ayres,” etc., by Sir Woodbine Parish, p. 168.
[4] “Buenos Ayres,” etc., by Sir W. Parish, p. 168.
All these species (with, perhaps, the exception of the last) are
recent, and live on the South American coast. These shell-beds extend
from one league to six leagues from the Plata, and must lie many feet
above its level. I heard, also, of beds of shells on the Somborombon,
and on the Rio Salado, at which latter place, as M. d’Orbigny informs
me, the _Mactra Isabellei_ and _Venus sinuosa_ are found.
During the elevation of the Provinces of La Plata, the waters of the
ancient estuary have but little affected (with the exception of the
sand-hills on the banks of the Parana and Uruguay) the outline of the
land. M. Parchappe,[5] however, has described groups of sand dunes
scattered over the wide extent of the Pampas southward of Buenos Ayres,
which M. d’Orbigny attributes with much probability to the action of
the sea, before the plains were raised above its level.[6]
[5] D’Orbigny’s “Voyage Géolog.,” p. 44.
[6] Before proceeding to the districts southward of La Plata, it may
be worth while just to state, that there is some evidence that the
coast of Brazil has participated in a small amount of elevation. Mr.
Burchell informs me, that he collected at Santos (lat. 24° S.)
oyster-shells, apparently recent, some miles from the shore, and quite
above the tidal action. Westward of Rio de Janeiro, Captain Elliot is
asserted (see Harlan, “Med. and Phys. Res.,” p. 35, and Dr. Meigs, in
“Trans. Amer. Phil. Soc.”), to have found human bones, encrusted with
sea-shells, between fifteen and twenty feet above the level of the
sea. Between Rio de Janeiro and Cape Frio I crossed sandy tracts
abounding with sea-shells, at a distance of a league from the coast;
but whether these tracts have been formed by upheaval, or through the
mere accumulation of drift sand, I am not prepared to assert. At Bahia
(lat. 13° S.), in some parts near the coast, there are traces of
sea-action at the height of about twenty feet above its present level;
there are also, in many parts, remnants of beds of sandstone and
conglomerate with numerous recent shells, raised a little above the
sea-level. I may add, that at the head of Bahia Bay there is a
formation, about forty feet in thickness, containing tertiary shells
apparently of fresh-water origin, now washed by the sea and encrusted
with Balini; this appears to indicate a small amount of subsidence
subsequent to its deposition. At Pernambuco (lat. 8° S.), in the
alluvial or tertiary cliffs, surrounding the low land on which the
city stands, I looked in vain for organic remains, or other evidence
of changes in level.
_Southward of the Plata._—The coast as far as Bahia Blanca (in lat. 39°
S.) is formed either of a horizontal range of cliffs, or of immense
accumulations of sand-dunes. Within Bahia Blanca, a small piece of
tableland, about twenty feet above high-water mark, called Punta Alta,
is formed of strata of cemented gravel and of red earthy mud, abounding
with shells (with others lying loose on the surface), and the bones of
extinct mammifers. These shells, twenty in number, together with a
Balanus and two corals, are all recent species, still inhabiting the
neighbouring seas. They will be enumerated in the Fourth Chapter, when
describing the Pampean formation; five of them are identical with the
upraised ones from near Buenos Ayres. The northern shore of Bahia
Blanca is, in main part, formed of immense sand-dunes, resting on
gravel with recent shells, and ranging in lines parallel to the shore.
These ranges are separated from each other by flat spaces, composed of
stiff impure red clay, in which, at the distance of about two miles
from the coast, I found by digging a few minute fragments of
sea-shells. The sand-dunes extend several miles inland, and stand on a
plain, which slopes up to a height of between one hundred and two
hundred feet. Numerous, small, well-rounded pebbles of pumice lie
scattered both on the plain and sand-hillocks: at Monte Hermoso, on the
flat summit of a cliff, I found many of them at a height of 120 feet
(angular measurement) above the level of the sea. These pumice pebbles,
no doubt, were originally brought down from the Cordillera by the
rivers which cross the continent, in the same way as the river Negro
anciently brought down, and still brings down, pumice, and as the river
Chupat brings down scoriæ: when once delivered at the mouth of a river,
they would naturally have travelled along the coasts, and been cast up
during the elevation of the land, at different heights. The origin of
the argillaceous flats, which separate the parallel ranges of
sand-dunes, seems due to the tides here having a tendency (as I believe
they have on most shoal, protected coasts) to throw up a bar parallel
to the shore, and at some distance from it; this bar gradually becomes
larger, affording a base for the accumulation of sand-dunes, and the
shallow space within then becomes silted up with mud. The repetition of
this process, without any elevation of the land, would form a level
plain traversed by parallel lines of sand-hillocks; during a slow
elevation of the land, the hillocks would rest on a gently inclined
surface, like that on the northern shore of Bahia Blanca. I did not
observe any shells in this neighbourhood at a greater height than
twenty feet; and therefore the age of the sea-drifted pebbles of
pumice, now standing at the height of 120 feet, must remain uncertain.
The main plain surrounding Bahia Blanca I estimated at from two hundred
to three hundred feet; it insensibly rises towards the distant Sierra
Ventana. There are in this neighbourhood some other and lower plains,
but they do not abut one at the foot of the other, in the manner
hereafter to be described, so characteristic of Patagonia. The plain on
which the settlement stands is crossed by many low sand-dunes,
abounding with the minute shells of the _ Paludestrina australis_,
d’Orbigny, which now lives in the bay. This low plain is bounded to the
south, at
the Cabeza del Buey, by the cliff-formed margin of a wide plain of the
Pampean formation, which I estimated at sixty feet in height. On the
summit of this cliff there is a range of high sand-dunes extending
several miles in an east and west line.
Southward of Bahia Blanca, the river Colorado flows between two plains,
apparently from thirty to forty feet in height. Of these plains, the
southern one slopes up to the foot of the great sandstone plateau of
the Rio Negro; and the northern one against an escarpment of the
Pampean deposit; so that the Colorado flows in a valley fifty miles in
width, between the upper escarpments. I state this, because on the low
plain at the foot of the northern escarpment, I crossed an immense
accumulation of high sand-dunes, estimated by the Gauchos at no less
than eight miles in breadth. These dunes range westward from the coast,
which is twenty miles distant, to far inland, in lines parallel to the
valley; they are separated from each other by argillaceous flats,
precisely like those on the northern shore of Bahia Blanca. At present
there is no source whence this immense accumulation of sand could
proceed; but if, as I believe, the upper escarpments once formed the
shores of an estuary, in that case the sandstone formation of the river
Negro would have afforded an inexhaustible supply of sand, which would
naturally have accumulated on the northern shore, as on every part of
the coast open to the south winds between Bahia Blanca and Buenos
Ayres.
At San Blas (40° 40′ S.) a little south of the mouth of the Colorado,
M. d’Orbigny[7] found fourteen species of existing shells (six of them
identical with those from Bahia Blanca), embedded in their natural
positions. From the zone of depth which these shells are known to
inhabit, they must have been uplifted thirty-two feet. He also found,
at from fifteen to twenty feet above this bed, the remains of an
ancient beach.
[7] “Voyage,” etc., p. 54.
Ten miles southward, but 120 miles to the west, at Port S. Antonio, the
Officers employed on the Survey assured me that they saw many old
sea-shells strewed on the surface of the ground, similar to those found
on other parts of the coast of Patagonia. At San Josef, ninety miles
south in nearly the same longitude, I found, above the gravel, which
caps an old tertiary formation, an irregular bed and hillock of sand,
several feet in thickness, abounding with shells of _Patella deaurita,
Mytilus Magellanicus,_ the latter retaining much of its colour; _Fusus
Magellanicus_ (and a variety of the same), and a large Balanus
(probably _B. Tulipa_), all now found on this coast: I estimated this
bed at from eighty to one hundred feet above the level of the sea. To
the westward of this bay, there is a plain estimated at between two
hundred and three hundred feet in height: this plain seems, from many
measurements, to be a continuation of the sandstone platform of the
river Negro. The next place southward, where I landed, was at Port
Desire, 340 miles distant; but from the intermediate districts I
received, through the kindness of the Officers of the Survey,
especially from Lieutenant Stokes and Mr. King, many specimens and
sketches, quite
sufficient to show the general uniformity of the whole line of coast. I
may here state, that the whole of Patagonia consists of a tertiary
formation, resting on and sometimes surrounding hills of porphyry and
quartz: the surface is worn into many wide valleys and into level
step-formed plains, rising one above another, all capped by irregular
beds of gravel, chiefly composed of porphyritic rocks. This gravel
formation will be separately described at the end of the chapter.
In the following diagrams:
Baseline is Level of sea.
Scale is 1/20 of inch to 100 feet vertical.
Height is shown in feet thus:
An. M. always stands for angular or trigonometrical measurement.
Ba. M. always stands for barometrical measurement.
Est. always stands for estimation by the Officers of the Survey.
No. 1
Section of step-formed plains south of Nuevo Gulf.
[Illustration: Section of step-formed plains south of Nuevo Gulf.]
My object in giving the following measurements of the plains, as taken
by the Officers of the Survey, is, as will hereafter be seen, to show
the remarkable equability of the recent elevatory movements. Round the
southern parts of Nuevo Gulf, as far as the River Chupat (seventy miles
southward of San Josef), there appear to be several plains, of which
the best defined are here represented.
The upper plain is here well defined (called Table Hills); its edge
forms a cliff or line of escarpment many miles in length, projecting
over a lower plain. The lowest plain corresponds with that at San Josef
with the recent shells on its surface. Between this lowest and the
uppermost plain, there is probably more than one step-formed terrace:
several measurements show the existence of the intermediate one of the
height given in diagram No. 1.
No. 2
Section of plains in the Bay of St. George.
[Illustration: Section of plains in the Bay of St. George.]
Near the north headland of the great Bay of St. George (100 miles south
of the Chupat), two well-marked plains of 250 and 330 feet were
measured: these are said to sweep round a great part of the Bay. At its
south headland, 120 miles distant from the north headland, the 250 feet
plain was again measured. In the middle of the bay, a higher plain was
found at two neighbouring places (Tilli Roads and C. Marques) to be 580
feet in height. Above this plain, towards the interior, Mr. Stokes
informs me that there were several other step-formed plains, the
highest of which was estimated at 1,200 feet, and was seen ranging at
apparently the same height for 150 miles northward. All these plains
have been worn into great valleys and much denuded. The section in
diagram No. 3 is illustrative of the general structure of the great Bay
of St. George. At the south headland of the Bay of St. George (near C.
Three Points) the 250 plain is very extensive. At Port Desire (forty
miles southward) I made several measurements with the barometer of a
plain, which extends along the north side of the port and along the
open coast, and which varies from 245 to 255 feet in height: this plain
abuts against the foot of a higher plain of 330 feet, which extends
also far northward along the coast, and likewise into the interior. In
the distance a higher inland platform was seen, of which I do not know
the height. In three separate places, I observed the cliff of the
245-255 feet plain, fringed by a terrace or narrow plain estimated at
about one hundred feet in height. These plains are represented in the
following section:—
No. 3
Section of plains at Port Desire.
[Illustration: Section of plains at Port Desire.]
In many places, even at the distance of three and four miles from the
coast, I found on the gravel-capped surface of the 245-255 feet, and of
the 330 feet plain, shells of _Mytilus Magellanicus, M. edulis, Patella
deaurita_, and another Patella, too much worn to be identified, but
apparently similar to one found abundantly adhering to the leaves of
the kelp. These species are the commonest now living on this coast. The
shells all appeared very old; the blue of the mussels was much faded;
and only traces of colour could be perceived in the Patellas, of which
the outer surfaces were scaling off. They lay scattered on the smooth
surface of the gravel, but abounded most in certain patches, especially
at the heads of the smaller valleys: they generally contained sand in
their insides; and I presume that they have been washed by alluvial
action out of thin sandy layers, traces of which may sometimes be seen
covering the gravel. The several plains have very level surfaces; but
all are scooped out by numerous broad, winding, flat-bottomed valleys,
in which, judging from the bushes, streams never flow. These remarks on
the state of the shells, and on the
nature of the plains, apply to the following cases, so need not be
repeated.
Southward of Port Desire, the plains have been greatly denuded, with
only small pieces of tableland marking their former extension. But
opposite Bird Island, two considerable step-formed plains were
measured, and found respectively to be 350 and 590 feet in height. This
latter plain extends along the coast close to Port St. Julian (110
miles south of Port Desire); where we have the following section:—
No. 4
Section of plains at Port St. Julian.
[Illustration: Section of plains at Port St. Julian.]
The lowest plain was estimated at ninety feet: it is remarkable from
the usual gravel-bed being deeply worn into hollows, which are filled
up with, as well as the general surface covered by, sandy and reddish
earthy matter: in one of the hollows thus filled up, the skeleton of
the Macrauchenia Patachonica, as will hereafter be described, was
embedded. On the surface and in the upper parts of this earthy mass,
there were numerous shells of Mytilus Magellanicus and M. edulis,
Patella deaurita, and fragments of other species. This plain is
tolerably level, but not extensive; it forms a promontory seven or
eight miles long, and three or four wide. The upper plains in Diagram 4
were measured by the Officers of the Survey; they were all capped by
thick beds of gravel, and were all more or less denuded; the 950 plain
consists merely of separate, truncated, gravel-capped hills, two of
which, by measurement, were found to differ only three feet. The 430
feet plain extends, apparently with hardly a break, to near the
northern entrance of the Rio Santa Cruz (fifty miles to the south); but
it was there found to be only 330 feet in height.
On the southern side of the mouth of the Santa Cruz we have Diagram 5,
which I am able to give with more detail than in the foregoing cases:—
No. 5
Section of plains at the mouth of the Rio Santa Cruz.
[Illustration: Section of plains at the mouth of the Rio Santa Cruz.]
The plain marked 355 feet (as ascertained by the barometer and by
angular measurement) is a continuation of the above-mentioned 330
feet plain: it extends in a N.W. direction along the southern shores of
the estuary. It is capped by gravel, which in most parts is covered by
a thin bed of sandy earth, and is scooped out by many flat-bottomed
valleys. It appears to the eye quite level, but in proceeding in a
S.S.W. course, towards an escarpment distant about six miles, and
likewise ranging across the country in a N.W. line, it was found to
rise at first insensibly, and then for the last half-mile, sensibly,
close up to the base of the escarpment: at this point it was 463 feet
in height, showing a rise of 108 feet in the six miles. On this 355-463
feet plain, I found several shells of _Mytilus Magellanicus_ and of a
Mytilus, which Mr. Sowerby informs me is yet unnamed, though well-known
as recent on this coast; _Patella deaurita_; _Fusus_, I believe, _
Magellanicus_, but the specimen has been lost; and at the distance of
four miles from the coast, at the height of about four hundred feet,
there were fragments of the same Patella and of a Voluta (apparently
_V. ancilla_) partially embedded in the superficial sandy earth. All
these shells had the same ancient appearance with those from the
foregoing localities. As the tides along this part of the coast rise at
the Syzygal period forty feet, and therefore form a well-marked
beach-line, I particularly looked out for ridges in crossing this
plain, which, as we have seen, rises 108 feet in about six miles, but I
could not see any traces of such. The next highest plain is 710 feet
above the sea; it is very narrow, but level, and is capped with gravel;
it abuts to the foot of the 840 feet plain. This summit-plain extends
as far as the eye can range, both inland along the southern side of the
valley of the Santa Cruz, and southward along the Atlantic.
_The Valley of the R. Santa Cruz._—This valley runs in an east and west
direction to the Cordillera, a distance of about one hundred and sixty
miles. It cuts through the great Patagonian tertiary formation,
including, in the upper half of the valley, immense streams of basaltic
lava, which as well as the softer beds, are capped by gravel; and this
gravel, high up the river, is associated with a vast boulder
formation.[8] In ascending the valley, the plain which at the mouth on
the southern side is 355 feet high, is seen to trend towards the
corresponding plain on the northern side, so that their escarpments
appear like the shores of a former estuary, larger than the existing
one: the escarpments, also, of the 840 feet summit-plain (with a
corresponding northern one, which is met with some way up the valley),
appear like the shores of a still larger estuary. Farther up the
valley, the sides are bounded throughout its entire length by level,
gravel-capped terraces, rising above each other in steps. The width
between the upper escarpments is on an average between seven and ten
miles; in one spot, however, where cutting through the basaltic lava,
it was only one mile and a half. Between the escarpments of the second
highest terrace the average width is about four or five miles. The
bottom of the valley, at the distance of 110 miles from its mouth,
begins sensibly
to expand, and soon forms a considerable plain, 440 feet above the
level of the sea, through which the river flows in a gut from twenty to
forty feet in depth. I here found, at a point 140 miles from the
Atlantic, and seventy miles from the nearest creek of the Pacific, at
the height of 410 feet, a very old and worn shell of _Patella
deaurita._ Lower down the valley, 105 miles from the Atlantic (long.
71° W.), and at an elevation of about 300 feet, I also found, in the
bed of the river, two much worn and broken shells of the _Voluta
ancilla_, still retaining traces of their colours; and one of the
_Patella deaurita._ It appeared that these shells had been washed from
the banks into the river; considering the distance from the sea, the
desert and absolutely unfrequented character of the country, and the
very ancient appearance of the shells (exactly like those found on the
plains nearer the coast), there is, I think, no cause to suspect that
they could have been brought here by Indians.
[8] I have described this formation in a paper in the “Geological
Transactions,” vol. vi, p. 415.
The plain at the head of the valley is tolerably level, but water-worn,
and with many sand-dunes on it like those on a sea-coast. At the
highest point to which we ascended, it was sixteen miles wide in a
north and south line; and forty-five miles in length in an east and
west line. It is bordered by the escarpments, one above the other, of
two plains, which diverge as they approach the Cordillera, and
consequently resemble, at two levels, the shores of great bays facing
the mountains; and these mountains are breached in front of the lower
plain by a remarkable gap. The valley, therefore, of the Santa Cruz
consists of a straight broad cut, about ninety miles in length,
bordered by gravel-capped terraces and plains, the escarpments of which
at both ends diverge or expand, one over the other, after the manner of
the shores of great bays. Bearing in mind this peculiar form of the
land—the sand-dunes on the plain at the head of the valley—the gap in
the Cordillera, in front of it—the presence in two places of very
ancient shells of existing species—and lastly, the circumstance of the
355-453 feet plain, with the numerous marine remains on its surface,
sweeping from the Atlantic coast, far up the valley, I think we must
admit, that within the recent period, the course of the Santa Cruz
formed a sea-strait intersecting the continent. At this period, the
southern part of South America consisted of an archipelago of islands
360 miles in a north and south line. We shall presently see, that two
other straits also, since closed, then cut through Tierra del Fuego; I
may add, that one of them must at that time have expanded at the foot
of the Cordillera into a great bay (now Otway Water) like that which
formerly covered the 440 feet plain at the head of the Santa Cruz.
I have said that the valley in its whole course is bordered by
gravel-capped plains. The section (diagram No. 6), supposed to be drawn
in a north and south line across the valley, can scarcely be considered
as more than illustrative; for during our hurried ascent it was
impossible to measure all the plains at any one place. At a point
nearly midway between the Cordillera and the Atlantic, I found the
plain (A north) 1,122 feet above the river; all the lower plains on
this side were here united into one great broken cliff: at a point
sixteen miles lower down
the stream, I found by measurement and estimation that B (_n_) was 869
above the river: very near to where A (_n_) was measured, C (_n_) was
639 above the same level: the terrace D (_n_) was nowhere measured: the
lowest E (_n_) was in many places about twenty feet above the river.
These plains or terraces were best developed where the valley was
widest; the whole five, like gigantic steps, occurred together only at
a few points. The lower terraces are less continuous than the higher
ones, and appear to be entirely lost in the upper third of the valley.
Terrace C (_s_), however was traced continuously for a great distance.
The terrace B (_n_), at a point fifty-five miles from the mouth of the
river, was four miles in width; higher up the valley this terrace (or
at least the second highest one, for I could not always trace it
continuously) was about eight miles wide. This second plain was
generally wider than the lower ones—as indeed follows from the valley
from A (_n_) to A (_s_) being generally nearly double the width of from
B (_n_) to B (_s_).
No. 6
North and South Section across the terraces bounding the valley of the
River Santa Cruz, high up its course.
[Illustration: North and South Section across the terraces bounding the
River Santa Cruz.]
Low down the valley, the summit-plain A (_s_) is continuous with the
840 feet plain on the coast, but it is soon lost or unites with the
escarpment of B (_s_). The corresponding plain A (_n_), on the north
side of the valley, appears to range continuously from the Cordillera
to the head of the present estuary of the Santa Cruz, where it trends
northward towards Port St. Julian. Near the Cordillera the summit-plain
on both sides of the valley is between 3,200 and 3,300 feet in height;
at 100 miles from the Atlantic, it is 1,416 feet, and on the coast 840
feet, all above the sea-beach; so that in a distance of 100 miles the
plain rises 576 feet, and much more rapidly near to the Cordillera. The
lower terraces B and C also appear to rise as they run up the valley;
thus D (_n_), measured at two points twenty-four miles apart, was found
to have risen 185 feet. From several reasons I suspect, that this
gradual inclination of the plains up the valley, has been chiefly
caused by the elevation of the continent in mass, having been the
greater the nearer to the Cordillera.
All the terraces are capped with well-rounded gravel, which rests
either on the denuded and sometimes furrowed surface of the soft
tertiary deposits, or on the basaltic lava. The difference in height
between some of the lower steps or terraces seems to be entirely owing
to a difference in the thickness of the capping gravel. Furrows and
inequalities in the gravel, where such occur, are filled up and
smoothed over with sandy earth. The pebbles, especially on the higher
plains, are often whitewashed, and even cemented together by a white
aluminous substance, and I occasionally found this to be the case with
the gravel on the terrace D. I could not perceive any trace of a
similar deposition on the pebbles now thrown up by the river, and
therefore I do not think that terrace D was river-formed. As the
terrace E generally stands about twenty feet above the bed of the
river, my first impression was to doubt whether even this lowest one
could have been so formed; but it should always be borne in mind, that
the horizontal upheaval of a district, by increasing the total descent
of the streams, will always tend to increase, first near the sea-coast
and then further and further up the valley, their corroding and
deepening powers: so that an alluvial plain, formed almost on a level
with a stream, will, after an elevation of this kind, in time be cut
through, and left standing at a height never again to be reached by the
water. With respect to the three upper terraces of the Santa Cruz, I
think there can be no doubt, that they were modelled by the sea, when
the valley was occupied by a strait, in the same manner (hereafter to
be discussed) as the greater step-formed, shell-strewed plains along
the coast of Patagonia.
To return to the shores of the Atlantic: the 840 feet plain, at the
mouth of the Santa Cruz, is seen extending horizontally far to the
south; and I am informed by the Officers of the Survey, that bending
round the head of Coy Inlet (sixty-five miles southward), it trends
inland. Outliers of apparently the same height are seen forty miles
farther south, inland of the river Gallegos; and a plain comes down to
Cape Gregory (thirty-five miles southward), in the Strait of Magellan,
which was estimated at between eight hundred and one thousand feet in
height, and which, rising towards the interior, is capped by the
boulder formation. South of the Strait of Magellan, there are large
outlying masses of apparently the same great tableland, extending at
intervals along the eastern coast of Tierra del Fuego: at two places
here, 110 miles a part, this plain was found to be 950 and 970 feet in
height.
From Coy Inlet, where the high summit-plain trends inland, a plain
estimated at 350 feet in height, extends for forty miles to the river
Gallegos. From this point to the Strait of Magellan, and on each side
of that Strait, the country has been much denuded and is less level. It
consists chiefly of the boulder formation, which rises to a height of
between one hundred and fifty and two hundred and fifty feet, and is
often capped by beds of gravel. At N.S. Gracia, on the north side of
the Inner Narrows of the Strait of Magellan, I found on the summit of a
cliff, 160 feet in height, shells of existing Patellæ and Mytili,
scattered on the surface and partially embedded in earth. On the
eastern coast, also, of Tierra del Fuego, in latitude 53° 20′ south, I
found many Mytili on some level land, estimated at 200 feet in height.
Anterior to the elevation attested by these shells, it is evident by
the present form of the land, and by the distribution of the great
erratic boulders[9] on the surface, that two sea-channels connected the
Strait of Magellan both with Sebastian Bay and with Otway Water.
[9] “Geolog. Transactions,” vol. vi, p. 419.
_Concluding remarks on the recent elevation of the south-eastern coasts
of America, and on the action of the sea on the land._—Upraised shells
of species, still existing as the commonest kinds in the adjoining sea,
occur, as we have seen, at heights of between a few feet and 410 feet,
at intervals from latitude 33° 40′ to 53° 20′ south. This is a distance
of 1,180 geographical miles—about equal from London to the North Cape
of Sweden. As the boulder formation extends with nearly the same height
150 miles south of 53° 20′, the most southern point where I landed and
found upraised shells; and as the level Pampas ranges many hundred
miles northward of the point, where M. d’Orbigny found at the height of
100 feet beds of the Azara, the space in a north and south line, which
has been uplifted within the recent period, must have been much above
the 1,180 miles. By the term “recent,” I refer only to that period
within which the now living mollusca were called into existence; for it
will be seen in the Fourth Chapter, that both at Bahia Blanca and P. S.
Julian, the mammiferous quadrupeds which co-existed with these shells
belong to extinct species. I have said that the upraised shells were
found only at intervals on this line of coast, but this in all
probability may be attributed to my not having landed at the
intermediate points; for wherever I did land, with the exception of the
river Negro, shells were found: moreover, the shells are strewed on
plains or terraces, which, as we shall immediately see, extend for
great distances with a uniform height. I ascended the higher plains
only in a few places, owing to the distance at which their escarpments
generally range from the coast, so that I am far from knowing that 410
feet is the maximum of elevation of these upraised remains. The shells
are those now most abundant in a living state in the adjoining sea.[10]
All of them have an ancient appearance; but some, especially the
mussels, although lying fully exposed to the weather, retain to a
considerable extent their colours: this circumstance appears at first
surprising, but it is now known that the colouring principle of the
Mytilus is so enduring, that it is preserved when the shell itself is
completely disintegrated.[11] Most of the shells are broken; I nowhere
found two valves united; the fragments are not rounded, at least in
none of the specimens which I brought home.
[10] Captain King, “Voyages of _Adventure_ and _Beagle_,” vol. i, 1
pp. 6 and 133.
[11] See Mr. Lyell “Proofs of a Gradual Rising in Sweden,” in the
“Philosoph. Transact.,” 1835, p. 1. See also Mr. Smith of Jordan Hill
in the _Edin. New Phil. Journal_, vol. xxv, p. 393.
With respect to the breadth of the upraised area in an east and west
line, we know from the shells found at the Inner Narrows of the
Strait of Magellan, that the entire width of the plain, although there
very narrow, has been elevated. It is probable that in this
southernmost part of the continent, the movement has extended under the
sea far eastward; for at the Falkland Islands, though I could not find
any shells, the bones of whales have been noticed by several competent
observers, lying on the land at a considerable distance from the sea,
and at the height of some hundred feet above it.[12] Moreover, we know
that in Tierra del Fuego the boulder formation has been uplifted within
the recent period, and a similar formation occurs[13] on the
north-western shores (Byron Sound) of these islands. The distance from
this point to the Cordillera of Tierra del Fuego, is 360 miles, which
we may take as the probable width of the recently upraised area. In the
latitude of the R. Santa Cruz, we know from the shells found at the
mouth and head, and in the middle of the valley, that the entire width
(about 160 miles) of the surface eastward of the Cordillera has been
upraised. From the slope of the plains, as shown by the course of the
rivers, for several degrees northward of the Santa Cruz, it is probable
that the elevation attested by the shells on the coast has likewise
extended to the Cordillera. When, however, we look as far northward as
the provinces of La Plata, this conclusion would be very hazardous; not
only is the distance from Maldonado (where I found upraised shells) to
the Cordillera great, namely, 760 miles, but at the head of the estuary
of the Plata, a N.N.E. and S.S.W. range of tertiary volcanic rocks has
been observed,[14] which may well indicate an axis of elevation quite
distinct from that of the Andes. Moreover, in the centre of the Pampas
in the chain of Cordova, severe earthquakes have been felt;[15] whereas
at Mendoza, at the eastern foot of the Cordillera, only gentle
oscillations, transmitted from the shores of the Pacific, have ever
been experienced. Hence the elevation of the Pampas may be due to
several distinct axes of movement; and we cannot judge, from the
upraised shells round the estuary of the Plata, of the breadth of the
area uplifted within the recent period.
[12] “Voyages of the _Adventure_ and _ Beagle_,” vol. ii, p. 227. And
Bougainville’s “Voyage,” tome i, p. 112.
[13] I owe this fact to the kindness of Captain Sulivan, R.N., a
highly competent observer. I mention it more especially, as in my
Paper (p. 427) on the Boulder Formation, I have, after having examined
the northern and middle parts of the eastern island, said that the
formation was here wholly absent.
[14] This volcanic formation will be described in Chapter IV. It is
not improbable that the height of the upraised shells at the head of
the estuary of the Plata, being greater than at Bahia Blanca or at San
Blas, may be owing to the upheaval of these latter places having been
connected with the distant line of the Cordillera, whilst that of the
provinces of La Plata was in connection with the adjoining tertiary
volcanic axis.
[15] See Sir W. Parish’s work on “La Plata,” p. 242. For a notice of
an earthquake which drained a lake near Cordova, see also Temple’s
“Travels in Peru.” Sir W. Parish informs me, that a town between Salta
and Tucuman (north of Cordova) was formerly utterly overthrown by an
earthquake.
Not only has the above specified long range of coast been elevated
within the recent period, but I think it may be safely inferred from
the similarity in height of the gravel-capped plains at distant points,
that there has been a remarkable degree of equability in the elevatory
process. I may premise, that when I measured the plains, it was simply
to ascertain the heights at which shells occurred; afterwards,
comparing these measurements with some of those made during the Survey,
I was struck with their uniformity, and accordingly tabulated all those
which represented the summit-edges of plains. The extension of the 330
to 355 feet plain is very striking, being found over a space of 500
geographical miles in a north and south line. A table (Table 1) of the
measurements is given below. The angular measurements and all the
estimations (in feet) are by the Officers of the Survey; the
barometrical ones by myself:—
Feet Gallegos River to Coy Inlet (partly angular partly
estimation) 350 South Side of Santa Cruz (angular and
barometric) 355 North Side of Santa Cruz (angular and
barometric) 330 Bird Island, plain opposite to (angular) 350
Port Desire, plain extending far along coast (barometric) 330
St. George’s Bay, north promontory (angular) 330 Table Land,
south of New Bay (angular) 350
A plain, varying from 245 to 255 feet, seems to extend with much
uniformity from Port Desire to the north of St. George’s Bay, a
distance of 170 miles; and some approximate measurements (in feet),
also given in the table below, indicate the much greater extension of
780 miles:—
Feet Coy Inlet, south of (partly angular and partly
estimation) 200 to 300 Port Desire (barometric) 245 to 255
C. Blanco (angular) 250 North Promontory of St. George’s Bay
(angular) 250 South of New Bay (angular) 200 to 220 North of
S. Josef (estimation) 200 to 300 Plain of Rio Negro
(angular) 200 to 220 Bahia Blanca (estimation) 200 to 300
The extension, moreover, of the 560 to 580, and of the 80 to 100 feet,
plains is remarkable, though somewhat less obvious than in the former
cases. Bearing in mind that I have not picked these measurements out of
a series, but have used all those which represented the edges of
plains, I think it scarcely possible that these coincidences in height
should be accidental. We must therefore conclude that the action,
whatever it may have been, by which these plains have been modelled
into their present forms, has been singularly uniform.
These plains or great terraces, of which three and four often rise like
steps one behind the other, are formed by the denudation of the old
Patagonian tertiary beds, and by the deposition on their surfaces of a
mass of well-rounded gravel, varying, near the coast, from ten to
thirty-five
feet in thickness, but increasing in thickness towards the interior.
The gravel is often capped by a thin irregular bed of sandy earth. The
plains slope up, though seldom sensibly to the eye, from the summit
edge of one escarpment to the foot of the next highest one. Within a
distance of 150 miles, between Santa Cruz to Port Desire, where the
plains are particularly well developed, there are at least seven stages
or steps, one above the other. On the three lower ones, namely, those
of 100 feet, 250 feet, and 350 feet in height, existing littoral shells
are abundantly strewed, either on the surface, or partially embedded in
the superficial sandy earth. By whatever action these three lower
plains have been modelled, so undoubtedly have all the higher ones, up
to a height of 950 feet at S. Julian, and of 1,200 feet (by estimation)
along St. George’s Bay. I think it will not be disputed, considering
the presence of the upraised marine shells, that the sea has been the
active power during stages of some kind in the elevatory process.
We will now briefly consider this subject: if we look at the existing
coast-line, the evidence of the great denuding power of the sea is very
distinct; for, from Cape St. Diego, in lat. 54° 30′ to the mouth of the
Rio Negro, in lat. 31° (a length of more than eight hundred miles), the
shore is formed, with singularly few exceptions, of bold and naked
cliffs: in many places the cliffs are high; thus, south of the Santa
Cruz, they are between eight and nine hundred feet in height, with
their horizontal strata abruptly cut off, showing the immense mass of
matter which has been removed. Nearly this whole line of coast consists
of a series of greater or lesser curves, the horns of which, and
likewise certain straight projecting portions, are formed of hard
rocks; hence the concave parts are evidently the effect and the measure
of the denuding action on the softer strata. At the foot of all the
cliffs, the sea shoals very gradually far outwards; and the bottom, for
a space of some miles, everywhere consists of gravel. I carefully
examined the bed of the sea off the Santa Cruz, and found that its
inclination was exactly the same, both in amount and in its peculiar
curvature, with that of the 355 feet plain at this same place. If,
therefore, the coast, with the bed of the adjoining sea, were now
suddenly elevated one or two hundred feet, an inland line of cliffs,
that is an escarpment, would be formed, with a gravel-capped plain at
its foot gently sloping to the sea, and having an inclination like that
of the existing 355 feet plain. From the denuding tendency of the sea,
this newly formed plain would in time be eaten back into a cliff: and
repetitions of this elevatory and denuding process would produce a
series of gravel-capped sloping terraces, rising one above another,
like those fronting the shores of Patagonia.
The chief difficulty (for there are other inconsiderable ones) on this
view, is the fact,—as far as I can trust two continuous lines of
soundings carefully taken between Santa Cruz and the Falkland Islands,
and several scattered observations on this and other coasts,—that the
pebbles at the bottom of the sea _quickly_ and _regularly_ decrease in
size with the increasing depth and distance from the shore, whereas in
the gravel on the sloping plains, no such decrease in size was
perceptible.
The following table gives the average result of many soundings off the
Santa Cruz:—
Under two miles from the shore, many of the pebbles were of large size,
mingled with some small ones.
Distance in miles from shore Depth in fathoms Size of Pebbles 3
to 4 11 to 12 As large as walnuts; mingled in every case with
some smaller ones. 6 to 7 17 to 19 As large as hazel-nuts. 10 to
11 23 to 25 From three- to four-tenths of an inch in diameter.
12 30 to 40 Two-tenths of an inch. 22 to 150 45 to
65 One-tenth of an inch, to the finest sand.
I particularly attended to the size of the pebbles on the 355 feet
Santa Cruz plain, and I noticed that on the summit-edge of the present
sea cliffs many were as large as half a man’s head; and in crossing
from these cliffs to the foot of the next highest escarpment, a
distance of six miles, I could not observe any increase in their size.
We shall presently see that the theory of a slow and almost insensible
rise of the land, will explain all the facts connected with the
gravel-capped terraces, better than the theory of sudden elevations of
from one to two hundred feet.
M. d’Orbigny has argued, from the upraised shells at San Blas being
embedded in the positions in which they lived, and from the valves of
the _Azara labiata_ high on the banks of the Parana being united and
unrolled, that the elevation of Northern Patagonia and of La Plata must
have been sudden; for he thinks, if it had been gradual, these shells
would all have been rolled on successive beach-lines. But in
_protected_ bays, such as in that of Bahia Blanca, wherever the sea is
accumulating extensive mud-banks, or where the winds quietly heap up
sand-dunes, beds of shells might assuredly be preserved buried in the
positions in which they had lived, even whilst the land retained the
same level; any, the smallest, amount of elevation would directly aid
in their preservation. I saw a multitude of spots in Bahia Blanca where
this might have been effected; and at Maldonado it almost certainly has
been effected. In speaking of the elevation of the land having been
slow, I do not wish to exclude the small starts which accompany
earthquakes, as on the coast of Chile; and by such movements beds of
shells might easily be uplifted, even in positions exposed to a heavy
surf, without undergoing any attrition: for instance, in 1835, a rocky
flat off the island of Santa Maria was at one blow upheaved above
high-water mark, and was left covered with gaping and putrefying
mussel-shells, still attached to the bed on which they had lived. If M.
d’Orbigny had been aware of the many long parallel lines of
sand-hillocks, with infinitely numerous shells of the Mactra and Venus,
at
a low level near the Uruguay; if he had seen at Bahia Blanca the
immense sand-dunes, with water-worn pebbles of pumice, ranging in
parallel lines, one behind the other, up a height of at least 120 feet;
if he had seen the sand-dunes, with the countless Paludestrinas, on the
low plain near the Fort at this place, and that long line on the edge
of the cliff, sixty feet higher up; if he had crossed that long and
great belt of parallel sand-dunes, eight miles in width, standing at
the height of from forty to fifty feet above the Colorado, where sand
could not now collect,—I cannot believe he would have thought that the
elevation of this great district had been sudden. Certainly the
sand-dunes (especially when abounding with shells), which stand in
ranges at so many different levels, must all have required long time
for their accumulation; and hence I do not doubt that the last 100 feet
of elevation of La Plata and Northern Patagonia has been exceedingly
slow.
If we extend this conclusion to Central and Southern Patagonia, the
inclination of the successively rising gravel-capped plains can be
explained quite as well, as by the more obvious view already given of a
few comparatively great and sudden elevations; in either case we must
admit long periods of rest, during which the sea ate deeply into the
land. Let us suppose the present coast to rise at a nearly equable,
slow rate, yet sufficiently quick to prevent the waves quite removing
each part as soon as brought up; in this case every portion of the
present bed of the sea will successively form a beach-line, and from
being exposed to a like action will be similarly affected. It cannot
matter to what height the tides rise, even if to forty feet as at Santa
Cruz, for they will act with equal force and in like manner on each
successive line. Hence there is no difficulty in the fact of the 355
feet plain at Santa Cruz sloping up 108 feet to the foot of the next
highest escarpment, and yet having no marks of any one particular
beach-line on it; for the whole surface on this view has been a beach.
I cannot pretend to follow out the precise action of the tidal-waves
during a rise of the land, slow, yet sufficiently quick to prevent or
check denudation: but if it be analogous to what takes place on
protected parts of the present coast, where gravel is now accumulating
in large quantities,[16] an inclined surface, thickly capped by
well-rounded pebbles of about the same size, would be ultimately left.
On the gravel now accumulating, the waves, aided by the wind, sometimes
throw up a thin covering of sand, together with the common
coast-shells. Shells thus cast up by gales, would, during an elevatory
period, never again be touched by the sea. Hence, on this view of a
slow and gradual rising of the land, interrupted by periods of rest and
denudation, we can understand the pebbles being of about the same size
over the entire width of the step-like plains,—the occasional thin
covering of sandy earth,—and the presence of broken, unrolled fragments
of those shells, which now live exclusively near the coast.
[16] On the eastern side of Chiloe, which island we shall see in the
next chapter is now rising, I observed that all the beaches and
extensive tidal-flats were formed of shingle.
_Summary of results._—It may be concluded that the coast on this side
of the continent, for a space of at least 1,180 miles, has been
elevated to a height of 100 feet in La Plata, and of 400 feet in
Southern Patagonia, within the period of existing shells, but not of
existing mammifers. That in La Plata the elevation has been very slowly
effected: that in Patagonia the movement may have been by considerable
starts, but much more probably slow and quiet. In either case, there
have been long intervening periods of comparative rest,[17] during
which the sea corroded deeply, as it is still corroding, into the land.
That the periods of denudation and elevation were contemporaneous and
equable over great spaces of coast, as shown by the equable heights of
the plains; that there have been at least eight periods of denudation,
and that the land, up to a height of from 950 to 1,200 feet, has been
similarly modelled and affected: that the area elevated, in the
southernmost part of the continent, extended in breadth to the
Cordillera, and probably seaward to the Falkland Islands; that
northward, in La Plata, the breadth is unknown, there having been
probably more than one axis of elevation; and finally, that, anterior
to the elevation attested by these upraised shells, the land was
divided by a Strait where the River Santa Cruz now flows, and that
further southward there were other sea-straits, since closed. I may
add, that at Santa Cruz, in lat. 50° S., the plains have been uplifted
at least 1,400 feet, since the period when gigantic boulders were
transported between sixty and seventy miles from their parent rock, on
floating icebergs.
[17] I say _comparative_ and not _ absolute_ rest, because the sea
acts, as we have seen, with great denuding power on this whole line of
coast; and therefore, during an elevation of the land, if excessively
slow (and of course during a subsidence of the land), it is quite
possible that lines of cliff might be formed.)
Lastly, considering the great upward movements which this long line of
coast has undergone, and the proximity of its southern half to the
volcanic axis of the Cordillera, it is highly remarkable that in the
many fine sections exposed in the Pampean, Patagonian tertiary, and
Boulder formations, I nowhere observed the smallest fault or abrupt
curvature in the strata.
_Gravel Formation of Patagonia._
I will here describe in more detail than has been as yet incidentally
done, the nature, origin, and extent of the great shingle covering of
Patagonia: but I do not mean to affirm that all of this shingle,
especially that on the higher plains, belongs to the recent period. A
thin bed of sandy earth, with small pebbles of various porphyries and
of quartz, covering a low plain on the north side of the Rio Colorado,
is the extreme northern limit of this formation. These little pebbles
have probably been derived from the denudation of a more regular bed of
gravel, capping the old tertiary sandstone plateau of the Rio Negro.
The gravel-bed near the Rio Negro is, on an average, about ten or
twelve feet in thickness; and the pebbles are larger than on the
northern side of the Colorado, being from one or two inches in
diameter, and composed chiefly of rather dark-tinted porphyries.
Amongst them I here first noticed a variety often to be referred to,
namely, a peculiar gallstone-yellow siliceous porphyry, frequently, but
not invariably, containing grains of quartz. The pebbles are embedded
in a white, gritty, calcareous matrix, very like mortar, sometimes
merely coating with a whitewash the separate stones, and sometimes
forming the greater part of the mass. In one place I saw in the gravel
concretionary nodules (not rounded) of crystallised gypsum, some as
large as a man’s head. I traced this bed for forty-five miles inland,
and was assured that it extended far into the interior. As the surface
of the calcareo-argillaceous plain of Pampean formation, on the
northern side of the wide valley of the Colorado, stands at about the
same height with the mortar-like cemented gravel capping the sandstone
on the southern side, it is probable, considering the apparent
equability of the subterranean movements along this side of America,
that this gravel of the Rio Negro and the upper beds of the Pampean
formation northward of the Colorado, are of nearly contemporaneous
origin, and that the calcareous matter has been derived from the same
source.
Southward of the Rio Negro, the cliffs along the great bay of S.
Antonio are capped with gravel: at San Josef, I found that the pebbles
closely resembled those on the plain of the Rio Negro, but that they
were not cemented by calcareous matter. Between San Josef and Port
Desire, I was assured by the Officers of the Survey that the whole face
of the country is coated with gravel. At Port Desire and over a space
of twenty-five miles inland, on the three step-formed plains and in the
valleys, I everywhere passed over gravel which, where thickest, was
between thirty and forty feet. Here, as in other parts of Patagonia,
the gravel, or its sandy covering, was, as we have seen, often strewed
with recent marine shells. The sandy covering sometimes fills up
furrows in the gravel, as does the gravel in the underlying tertiary
formations. The pebbles are frequently whitewashed and even cemented
together by a peculiar, white, friable, aluminous, fusible substance,
which I believe is decomposed feldspar. At Port Desire, the gravel
rested sometimes on the basal formation of porphyry, and sometimes on
the upper or the lower denuded tertiary strata. It is remarkable that
most of the porphyritic pebbles differ from those varieties of porphyry
which occur here abundantly _in situ._ The peculiar gallstone-yellow
variety was common, but less numerous than at Port S. Julian, where it
formed nearly one-third of the mass of the gravel; the remaining part
there consisting of pale grey and greenish porphyries with many
crystals of feldspar. At Port S. Julian, I ascended one of the
flat-topped hills, the denuded remnant of the highest plain, and found
it, at the height of 950 feet, capped with the usual bed of gravel.
Near the mouth of the Santa Cruz, the bed of gravel on the 355 feet
plain is from twenty to about thirty-five feet in thickness. The
pebbles vary from minute ones to the size of a hen’s egg, and even to
that of half a man’s head; they consist of paler varieties of porphyry
than those found further northward, and there are fewer of the
gallstone-yellow kind; pebbles of compact black clay-slate were here
first observed. The gravel, as we have seen, covers the step-formed
plains at the mouth, head, and on the sides of the great valley of the
Santa Cruz. At a distance of 110 miles from the coast, the plain has
risen to the height of 1,416 feet above the sea; and the gravel, with
the associated great boulder formation, has attained a thickness of 212
feet. The plain, apparently with its usual gravel covering, slopes up
to the foot of the Cordillera to the height of between 3,200 and 3,300
feet. In ascending the valley, the gravel gradually becomes entirely
altered in character: high up, we have pebbles of crystalline
feldspathic rocks, compact clay-slate, quartzose schists, and
pale-coloured porphyries; these rocks, judging both from the gigantic
boulders in the surface and from some small pebbles embedded beneath
700 feet in thickness of the old tertiary strata, are the prevailing
kinds in this part of the Cordillera; pebbles of basalt from the
neighbouring streams of basaltic lava are also numerous; there are few
or none of the reddish or of the gallstone-yellow porphyries so common
near the coast. Hence the pebbles on the 350 feet plain at the mouth of
the Santa Cruz cannot have been derived (with the exception of those of
compact clay-slate, which, however, may equally well have come from the
south) from the Cordillera in this latitude; but probably, in chief
part, from farther north.
Southward of the Santa Cruz, the gravel may be seen continuously
capping the great 840 feet plain: at the Rio Gallegos, where this plain
is succeeded by a lower one, there is, as I am informed by Captain
Sulivan, an irregular covering of gravel from ten to twelve feet in
thickness over the whole country. The district on each side of the
Strait of Magellan is covered up either with gravel or the boulder
formation: it was interesting to observe the marked difference between
the perfectly rounded state of the pebbles in the great shingle
formation of Patagonia, and the more or less angular fragments in the
boulder formation. The pebbles and fragments near the Strait of
Magellan nearly all belong to rocks known to occur in Fuegia. I was
therefore much surprised in dredging south of the Strait to find, in
lat. 54° 10′ south, many pebbles of the gallstone-yellow siliceous
porphyry; I procured others from a great depth off Staten Island, and
others were brought me from the western extremity of the Falkland
Islands.[18] The distribution of the pebbles of this peculiar porphyry,
which I venture to affirm is not found _in situ_ either in Fuegia, the
Falkland Islands, or on the coast of Patagonia, is very remarkable, for
they are
found over a space of 840 miles in a north and south line, and at the
Falklands, 300 miles eastward of the coast of Patagonia. Their
occurrence in Fuegia and the Falklands may, however, perhaps be due to
the same ice-agency by which the boulders have been there transported.
[18] At my request, Mr. Kent collected for me a bag of pebbles from
the beach of White Rock harbour, in the northern part of the sound,
between the two Falkland Islands. Out of these well-rounded pebbles,
varying in size from a walnut to a hen’s egg, with some larger,
thirty-eight evidently belonged to the rocks of these islands;
twenty-six were similar to the pebbles of porphyry found on the
Patagonian plains, which rocks do not exist _in situ_ in the
Falklands; one pebble belonged to the peculiar yellow siliceous
porphyry; thirty were of doubtful origin.
We have seen that porphyritic pebbles of a small size are first met
with on the northern side of the Rio Colorado, the bed becoming well
developed near the Rio Negro: from this latter point I have every
reason to believe that the gravel extends uninterruptedly over the
plains and valleys of Patagonia for at least 630 nautical miles
southward to the Rio Gallegos. From the slope of the plains, from the
nature of the pebbles, from their extension at the Rio Negro far into
the interior, and at the Santa Cruz close up to the Cordillera, I think
it highly probable that the whole breadth of Patagonia is thus covered.
If so, the average width of the bed must be about two hundred miles.
Near the coast the gravel is generally from ten to thirty feet in
thickness; and as in the valley of Santa Cruz it attains, at some
distance from the Cordillera, a thickness of 214 feet, we may, I think,
safely assume its average thickness over the whole area of 630 by 200
miles, at fifty feet!
The transportal and origin of this vast bed of pebbles is an
interesting problem. From the manner in which they cap the step-formed
plains, worn by the sea within the period of existing shells, their
deposition, at least on the plains up to a height of 400 feet, must
have been a recent geological event. From the form of the continent, we
may feel sure that they have come from the westward, probably, in chief
part from the Cordillera, but, perhaps, partly from unknown rocky
ridges in the central districts of Patagonia. That the pebbles have not
been transported by rivers, from the interior towards the coast, we may
conclude from the fewness and smallness of the streams of Patagonia:
moreover, in the case of the one great and rapid river of Santa Cruz,
we have good evidence that its transporting power is very trifling.
This river is from two to three hundred yards in width, about seventeen
feet deep in its middle, and runs with a singular degree of uniformity
five knots an hour, with no lakes and scarcely any still reaches:
nevertheless, to give one instance of its small transporting power,
upon careful examination, pebbles of compact basalt could not be found
in the bed of the river at a greater distance than ten miles below the
point where the stream rushes over the debris of the great basaltic
cliffs forming its shore: fragments of the _ cellular_ varieties have
been washed down twice or thrice as far. That the pebbles in Central
and Northern Patagonia have not been transported by ice-agency, as
seems to have been the case to a considerable extent farther south, and
likewise in the northern hemisphere, we may conclude, from the absence
of all angular fragments in the gravel, and from the complete contrast
in many other respects between the shingle and neighbouring boulder
formation.
Looking to the gravel on any one of the step-formed plains, I cannot
doubt, from the several reasons assigned in this chapter, that it has
been spread out and leveled by the long-continued action of the sea,
probably during the slow rise of the land. The smooth and perfectly
rounded
condition of the innumerable pebbles alone would prove long-continued
action. But how the whole mass of shingle on the coast-plains has been
transported from the mountains of the interior, is another and more
difficult question. The following considerations, however, show that
the sea by its ordinary action has considerable power in distributing
pebbles. A table has already been given, showing how very uniformly and
gradually[19] the pebbles decrease in size with the gradually seaward
increasing depth and distance. A series of this kind irresistibly leads
to the conclusion, that the sea has the power of sifting and
distributing the loose matter on its bottom. According to Martin
White,[20] the bed of the British Channel is disturbed during gales at
depths of sixty-three and sixty-seven fathoms, and at thirty fathoms,
shingle and fragments of shells are often deposited, afterwards to be
carried away again. Groundswells, which are believed to be caused by
distant gales, seem especially to affect the bottom: at such times,
according to Sir R. Schomburgk,[21] the sea to a great distance round
the West Indian Islands, at depths from five to fifteen fathoms,
becomes discoloured, and even the anchors of vessels have been moved.
There are, however, some difficulties in understanding how the sea can
transport pebbles lying at the bottom, for, from experiments instituted
on the power of running water, it would appear that the currents of the
sea have not sufficient velocity to move stones of even moderate size:
moreover, I have repeatedly found in the most exposed situations that
the pebbles which lie at the bottom are encrusted with full-grown
living corallines, furnished with the most delicate, yet unbroken
spines: for instance, in ten fathoms water off the mouth of the Santa
Cruz, many pebbles, under half an inch in diameter, were thus coated
with Flustracean zoophytes.[22] Hence we must conclude
that these pebbles are not often violently disturbed: it should,
however, be borne in mind that the growth of corallines is rapid. The
view, propounded by Professor Playfair, will, I believe, explain this
apparent difficulty,—namely, that from the undulations of the sea
_tending_ to lift up and down pebbles or other loose bodies at the
bottom, such are liable, when thus quite or partially raised, to be
moved even by a very small force, a little onwards. We can thus
understand how oceanic or tidal currents of no great strength, or that
recoil movement of the bottom-water near the land, called by sailors
the “undertow” (which I presume must extend out seaward as far as the
_breaking_ waves impel the surface-water towards the beach), may gain
the power during storms of sifting and distributing pebbles even of
considerable size, and yet without so violently disturbing them as to
injure the encrusting corallines.[23]
[19] I may mention, that at the distance of 150 miles from the
Patagonian shore I carefully examined the minute rounded particles in
the sand, and found them to be fusible like the porphyries of the
great shingle bed. I could even distinguish particles of the
gallstone-yellow porphyry. It was interesting to notice how gradually
the particles of white quartz increased, as we approached the Falkland
Islands, which are thus constituted. In the whole line of soundings
between these islands and the coast of Patagonia dead or living
organic remains were most rare. On the relations between the depth of
water and the nature of the bottom, see Martin White on “Soundings in
the Channel,” pp. 4, 6, 175; also Captain Beechey’s “Voyage to the
Pacific,” chap. xviii.
[20] “Soundings in the Channel,” pp. 4, 166. M. Siau states (_Edin.
New Phil. Jour._, vol. xxxi, p. 246), that he found the sediment, at a
depth of 188 metres, arranged in ripples of different degrees of
fineness. There are some excellent discussions on this and allied
subjects in Sir H. De la Beche’s “Theoretical Researches.”
[21] _Journal of Royal Geograph. Soc.,_ vol. v, p. 25. It appears from
Mr. Scott Russell’s investigations (see Mr. Murchison’s “Anniver.
Address Geolog. Soc.,” 1843, p. 40), that in waves of translation the
motion of the particles of water is nearly as great at the bottom as
at the top.
[22] (A pebble, one and a half inch square and half an inch thick, was
given me, dredged up from twenty-seven fathoms depth off the western
end of the Falkland Islands, where the sea is remarkably stormy, and
subject to violent tides. This pebble was encrusted on all sides by a
delicate living coralline. I have seen many pebbles from depths
between forty and seventy fathoms thus encrusted; one from the latter
depth off Cape Horn.
[23] I may take this opportunity of remarking on a singular, but very
common character in the form of the bottom, in the creeks which deeply
penetrate the western shores of Tierra del Fuego; namely, that they
are almost invariably much shallower close to the open sea at their
mouths than inland. Thus, Cook, in entering Christmas Sound, first had
soundings in thirty-seven fathoms, then in fifty, then in sixty, and a
little farther in no bottom with 170 fathoms. The sealers are so
familiar with this fact, that they always look out for anchorage near
the entrances of the creeks. See, also, on this subject, the “Voyages
of the _ Adventure_ and _Beagle_,” vol. i, p. 375 and “Appendix,” p.
313. This Shoalness of the sea-channels near their entrances probably
results from the quantity of sediment formed by the wear and tear of
the outer rocks exposed to the full force of the open sea. I have no
doubt that many lakes, for instance in Scotland, which are very deep
within, and are separated from the sea apparently only by a tract of
detritus, were originally sea-channels with banks of this nature near
their mouths, which have since been upheaved.
The sea acts in another and distinct manner in the distribution of
pebbles, namely by the waves on the beach. Mr. Palmer,[24] in his
excellent memoir on this subject, has shown that vast masses of shingle
travel with surprising quickness along lines of coast, according to the
direction with which the waves break on the beach and that this is
determined by the prevailing direction of the winds. This agency must
be powerful in mingling together and disseminating pebbles derived from
different sources: we may, perhaps, thus understand the wide
distribution of the gallstone-yellow porphyry; and likewise, perhaps,
the great difference in the nature of the pebbles at the mouth of the
Santa Cruz from those in the same latitude at the head of the valley.
[24] “Philosophical Transactions,” 1834, p. 576.
I will not pretend to assign to these several and complicated agencies
their shares in the distribution of the Patagonian shingle: but from
the several considerations given in this chapter, and I may add, from
the frequency of a capping of gravel on tertiary deposits in all parts
of the world, as I have myself observed and seen stated in the works of
various authors, I cannot doubt that the power of widely dispersing
gravel is an ordinary contingent on the action of the sea; and that
even in the case of the great Patagonian shingle-bed we have no
occasion to call in the aid of debacles. I at one time imagined that
perhaps an immense accumulation of shingle had originally been
collected at the foot of the Cordillera; and that this accumulation,
when upraised above the level of the sea, had been eaten into and
partially spread out (as off the present line of coast); and that the
newly-spread out bed had in its turn been upraised, eaten into, and
re-spread out; and so onwards, until the shingle, which was first
accumulated in great thickness at the foot of the Cordillera, had
reached in thinner beds its present extension. By whatever means the
gravel formation of Patagonia may have been distributed, the vastness
of its area, its thickness, its superficial position, its recent
origin, and the great degree of similarity in the nature of its
pebbles, all appear to me well deserving the attention of geologists,
in relation to the origin of the widely-spread beds of conglomerate
belonging to past epochs.
No. 7
Section of coast-cliffs and bottom of sea, off the island of St.
Helena.
[Illustration: Section of clast-cliffs and bottom of sea, off the
island of St. Helena.]
_Formation of Cliffs._—When viewing the sea-worn cliffs of Patagonia,
in some parts between eight hundred and nine hundred feet in height,
and formed of horizontal tertiary strata, which must once have extended
far seaward—or again, when viewing the lofty cliffs round many volcanic
islands, in which the gentle inclination of the lava-streams indicates
the former extension of the land, a difficulty often occurred to me,
namely, how the strata could possibly have been removed by the action
of the sea at a considerable depth beneath its surface. The section in
diagram No. 7, which represents the general form of the land on the
northern and leeward side of St. Helena (taken from Mr. Seale’s large
model and various measurements), and of the bottom of the adjoining sea
(taken chiefly from Captain Austin’s survey and some old charts), will
show the nature of this difficulty.
If, as seems probable, the basaltic streams were originally prolonged
with nearly their present inclination, they must, as shown by the
dotted line in the section, once have extended at least to a point, now
covered
by the sea to a depth of nearly thirty fathoms: but I have every reason
to believe they extended considerably further, for the inclination of
the streams is less near the coast than further inland. It should also
be observed, that other sections on the coast of this island would have
given far more striking results, but I had not the exact measurements;
thus, on the windward side, the cliffs are about two thousand feet in
height and the cut-off lava streams very gently inclined, and the
bottom of the sea has nearly a similar slope all round the island. How,
then, has all the hard basaltic rock, which once extended beneath the
surface of the sea, been worn away? According to Captain Austin, the
bottom is uneven and rocky only to that very small distance from the
beach within which the depth is from five to six fathoms; outside this
line, to a depth of about one hundred fathoms, the bottom is smooth,
gently inclined, and formed of mud and sand; outside the one hundred
fathoms, it plunges suddenly into unfathomable depths, as is so very
commonly the case on all coasts where sediment is accumulating. At
greater depths than the five or six fathoms, it seems impossible, under
existing circumstances, that the sea can both have worn away hard rock,
in parts to a thickness of at least 150 feet, and have deposited a
smooth bed of fine sediment. Now, if we had any reason to suppose that
St. Helena had, during a long period, gone on slowly subsiding, every
difficulty would be removed: for looking at the diagram, and imagining
a fresh amount of subsidence, we can see that the waves would then act
on the coast-cliffs with fresh and unimpaired vigour, whilst the rocky
ledge near the beach would be carried down to that depth, at which sand
and mud would be deposited on its bare and uneven surface: after the
formation near the shore of a new rocky shoal, fresh subsidence would
carry it down and allow it to be smoothly covered up. But in the case
of the many cliff-bounded islands, for instance in some of the Canary
Islands and of Madeira, round which the inclination of the strata shows
that the land once extended far into the depths of the sea, where there
is no apparent means of hard rock being worn away—are we to suppose
that all these islands have slowly subsided? Madeira, I may remark,
has, according to Mr. Smith of Jordan Hill, subsided. Are we to extend
this conclusion to the high, cliff-bound, horizontally stratified
shores of Patagonia, off which, though the water is not deep even at
the distance of several miles, yet the smooth bottom of pebbles
gradually decreasing in size with the increasing depth, and derived
from a foreign source, seem to declare that the sea is now a depositing
and not a corroding agent? I am much inclined to suspect, that we shall
hereafter find in all such cases, that the land with the adjoining bed
of the sea has in truth subsided: the time will, I believe, come, when
geologists will consider it as improbable, that the land should have
retained the same level during a whole geological period, as that the
atmosphere should have remained absolutely calm during an entire
season.