CHAPTER II.—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC
CHAPTER III.—ASCENSION
CHAPTER IV.—ST. HELENA
CHAPTER V.—GALAPAGOS ARCHIPELAGO
CHAPTER VI.—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES
CHAPTER VII.—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE
INDEX TO VOLCANIC ISLANDS
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.
VOLCANIC ISLANDS.
DETAILED TABLE OF CONTENTS
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 scoriae.—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.
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.
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.
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.
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.
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.
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.—Palaeozoic 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.
INDEX.
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 Palaeozoic 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 scoriae. 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.
(FIGURE 1: MAP 1: PART OF ST. JAGO, ONE OF THE CAPE DE VERDE ISLANDS.)
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 Figure 1 (Map 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.)), 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.
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.
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 debris, 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 Turritellae; 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 Nulliporae, 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 Patellae, 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 Nulliporae, 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
scoriae. 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,
masses of limestone are sometimes fused and crystallised even in common
limekilns. (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” volume 15 page 398; Gay-Lussac in
“Annales de Chem. et Phys.” tome 63 page 219 translated in the “London
and Edinburgh Philosophical Magazine” volume 10 page 496.) 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 scoriae,
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.
The fragments of scoriae, 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 scoriae, which
have undergone a closely similar change.
THE EXTENT AND HORIZONTALITY OF THE CALCAREOUS STRATUM.
(FIGURE 2: SIGNAL POST HILL. (Section with A low and C high.)
A.—Ancient volcanic rocks.
B.—Calcareous stratum.
C.—Upper basaltic lava.)
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.
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 Figure 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 laminae, 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 scoriae 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 scoriae 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 scoriae 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 scoriae, 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 scoriae 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 scoriae. 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 scoriae 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
scoriae,—considering, also, the similar occurrence of lime and scoriae
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 scoriae 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. (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.) 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 debris of organic
remains, at the bottom of a shallow sea.
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 (Figure 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 (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.), some compact, others highly cellular with inclined
beds of loose scoriae, 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
remains of three small points of eruption. These points are composed of
glossy scoriae, 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 scoriae 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 scoriae, 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.
The stream of lava, which fills the narrow gorge 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 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.): 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.
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 scoriae, 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. (D’Aubuisson “Traite de Geognosie” tome 2 page 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 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. (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.) 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.
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.