NORTHERN CHILE.—CONCLUSION.
A 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.
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 palaeontology 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 palaeontological 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
palaeontological 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 Caribaeus,
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, according to M. d’Orbigny,
now live at the mouth of the estuary. (“Voyage dans l’Amerique Merid.:
Part. Geolog.” page 21.) 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 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. (Ibid page 43.) 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, of
the Azara labiata, lying at about forty feet above the level of the
river, and distant between two and three miles from it. (“Buenos Ayres”
etc. by Sir Woodbine Parish page 168.) 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:—
1. Buccinanops globulosum, d’Orbigny.
2. Olivancillaria auricularia, d’Orbigny.
3. Venus flexuosa, Lam.
4. Cytheraea (imperfect).
5. Mactra Isabellei, d’Orbigny.
6. Ostrea pulchella, d’Orbigny.
Besides these, Sir W. Parish procured (“Buenos Ayres” etc. by Sir W.
Parish page 168.) (as named by Mr. G.B. Sowerby) the following shells:—
7. Voluta colocynthis.
8. Voluta angulata.
9. Buccinum (not spec.?).
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, however, has described groups of sand dunes
scattered over the wide extent of the Pampas southward of Buenos Ayres
(D’Orbigny “Voyage Geolog.” page 44.), which M. d’Orbigny attributes
with much probability to the action of the sea, before the plains were
raised above its level. (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
(latitude 24 degrees 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.”
page 35 and Dr. Meigs in “Transactions of the American Philosophical
Society”), 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 (latitude 13
degrees 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 (latitude 8 degrees 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 latitude 39 degrees 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
scoriae: 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 degrees 40′ S.) a little south of the mouth of the
Colorado, M. d’Orbigny found fourteen species of existing shells (six
of them identical with those from Bahia Blanca), embedded in their
natural positions. (“Voyage” etc. page 54.) 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.
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.
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.
(In the following Diagrams: 1. Baseline is Level of sea. 2. Scale is
1/20 of inch to 100 feet vertical. 3. 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.
DIAGRAM 1. SECTION OF STEP-FORMED PLAINS SOUTH OF NUEVO GULF.
From East (sea level) to West (high): Terrace 1. 80 Est. Terrace 2.
200-220 An. M. Terrace 3. 350 An. M.)
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 1.
(DIAGRAM 2. SECTION OF PLAINS IN THE BAY OF ST. GEORGE.
From East (sea level) to West (high): Terrace 1. 250 An. M. Terrace 2.
330 An. M. Terrace 3. 580 An. M. Terraces 4, 5 and 6 not measured.
Terrace 7. 1,200 Est.)
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 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.
(DIAGRAM 3. SECTION OF PLAINS AT PORT DESIRE.
From East (sea level) to West (high): Terrace 1. 100 Est. Terrace 2.
245-255 Ba. M. Shells on surface. Terrace 3. 330 Ba. M. Shells on
surface. Terrace 4. Not measured.)
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 section Diagram 3.
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.
(DIAGRAM 4. SECTION OF PLAINS AT PORT S. JULIAN.
From East (sea level) to West (high): Terrace 1. Shells on surface. 90
Est. Terrace 2. 430 An. M. Terrace 3. 560 An. M. Terrace 4. 950 An. M.)
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); see Diagram 4.
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.
(DIAGRAM 5. SECTION OF PLAINS AT THE MOUTH OF THE RIO SANTA CRUZ.
From East (sea level) to West (high): Terrace 1. (sloping) 355 Ba. M.
Shells on surface. 463 Ba. M. Terrace 2. 710 An. M. Terrace 3. 840 An.
M.)
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.
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. (I have described this
formation in a paper in the “Geological Transactions” volume 6 page
415.) 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
(longitude 71 degrees 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.
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.
(DIAGRAM 6. NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE
VALLEY OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE.
The height of each terrace, above the level of the river (furthest to
nearest to the river) in feet:
A, north and south: 1,122 B, north and south: 869 C, north and south:
639 D, north: not measured. D, north? (suggest south): 185 E: 20 Bed of
River.
Vertical scale 1/20 of inch to 100 feet; but terrace E, being only
twenty feet above the river, has necessarily been raised. The
horizontal distances much contracted; the distance from the edge of A
North to A South being on an average from seven to ten miles.) I have
said that the valley in its whole course is bordered by gravel- capped
plains. The section (Diagram 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 (north) was
869 above the river: very near to where A (north) was measured, C
(north) was 639 above the same level: the terrace D (north) was nowhere
measured: the lowest E (north) 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 (south), however was traced continuously for a
great distance. The terrace B (north), 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 (north) to A (south) being generally nearly
double the width of from B (north) to B (south). Low down the valley,
the summit-plain A (south) is continuous with the 840 feet plain on the
coast, but it is soon lost or unites with the escarpment of B (south).
The corresponding plain A (north), 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 (north),
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 Patellae and Mytili,
scattered on the surface and partially embedded in earth. On the
eastern coast, also, of Tierra del Fuego, in latitude 53 degrees 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 on the surface, that two sea-channels connected
the Strait of Magellan both with Sebastian Bay and with Otway Water.
(“Geological Transactions” volume 6 page 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 degrees 40′ to 53
degrees 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 degrees 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. (Captain King “Voyages of
‘Adventure’ and ‘Beagle’” volume 1 pages 6 and 133.) 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. (See
Mr. Lyell “Proofs of a Gradual Rising in Sweden” in the “Philosophical
Transactions” 1835 page 1. See also Mr. Smith of Jordan Hill in the
“Edinburgh New Philosophical Journal” volume 25 page 393.) 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.
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. (“Voyages of the
‘Adventure’ and ‘Beagle’” volume 2 page 227. And Bougainville’s
“Voyage” tome 1 page 112.) Moreover, we know that in Tierra del Fuego
the boulder formation has been uplifted within the recent period, and a
similar formation occurs on the north-western shores (Byron Sound) of
these islands. (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 (page 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.) 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 (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.), 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 (See Sir W.
Parish’s work on “La Plata” page 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.); 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.
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:—
TABLE 1.
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 Table 2 below, indicate the much greater extension of 780
miles:—
TABLE 2.
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 latitude 54 degrees 30′ to the
mouth of the Rio Negro, in latitude 31 degrees (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.
Table 3 below gives the average result of many soundings off the Santa
Cruz:— TABLE 3.
Under two miles from the shore, many of the pebbles were of large size,
mingled with some small ones.
Column 1. Distance in miles from the shore.
Column 2. Depth in fathoms.
Column 3. Size of Pebbles.
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, an inclined surface, thickly capped by
well-rounded pebbles of about the same size, would be ultimately left.
(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.) 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.
A 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, during which the sea corroded
deeply, as it is still corroding, into the land. (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.) 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 latitude 50 degrees 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.
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
latitude 54 degrees 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. (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.) 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.
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. Table 3 above shows how very uniformly
and gradually the pebbles decrease in size with the gradually seaward
increasing depth and distance. (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” pages 4, 6, 175; also Captain Beechey’s
“Voyage to the Pacific” chapter 18.) 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,
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. (“Soundings in the Channel” pages 4, 166. M. Siau states
(“Edinburgh New Philosophical Journal” volume 31 page 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.”) Groundswells, which are believed to be caused by distant
gales, seem especially to affect the bottom: at such times, according
to Sir R. Schomburgk, 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. (“Journal of Royal
Geographical Society” volume 5 page 25. It appears from Mr. Scott
Russell’s investigations (see Mr. Murchison’s “Anniversary Address
Geological Society” 1843 page 40), that in waves of translation the
motion of the particles of water is nearly as great at the bottom as at
the top.) 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. (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.) 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. (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’” volume 1 page 375 and “Appendix” page
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, 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. (“Philosophical
Transactions” 1834 page 576.) 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.
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.
FORMATION OF CLIFFS.
(DIAGRAM 7.—SECTION OF COAST-CLIFFS AND BOTTOM OF SEA, OFF THE ISLAND
OF ST. HELENA.
Height in feet above sea level.
Depths in fathoms.
Vertical and horizontal scale, two inches to a nautical mile. The point
marked 1,600 feet is at the foot of High Knoll; point marked 510 feet
is on the edge of Ladder Hill. The strata consist of basaltic streams.
A Section left to right:
Height at the foot of High Knoll: 1,600 at top of strata.
Height on the edge of Ladder Hill: 510 at top of strata.
Bottom at coast rocky only to a depth of five or six fathoms.
30 fathoms: bottom mud and sand.
100 fathoms sloping more sharply to 250 fathoms.)
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 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.
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 of
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.
Commencing at the south and proceeding northward, the first place at
which I landed, was at Cape Tres Montes, in latitude 46 degrees 35′.
Here, on the shores of Christmas Cove, I observed in several places a
beach of pebbles with recent shells, about twenty feet above high-water
mark. Southward of Tres Montes (between latitude 47 and 48 degrees),
Byron remarks, “We thought it very strange, that upon the summits of
the highest hills were found beds of shells, a foot or two thick.”
(“Narrative of the Loss of the ‘Wager’.”) In the Chonos Archipelago,
the island of Lemus (latitude 44 degrees 30′) was, according to M.
Coste, suddenly elevated eight feet, during the earthquake of 1829: he
adds, “Des roches jadis toujours couvertes par la mer, restant
aujourd’hui constamment decouvertes.” (“Comptes Rendus” October 1838
page 706.) In other parts of this archipelago, I observed two terraces
of gravel, abutting to the foot of each other: at Lowe’s Harbour (43
degrees 48′), under a great mass of the boulder formation, about three
hundred feet in thickness, I found a layer of sand, with numerous
comminuted fragments of sea-shells, having a fresh aspect, but too
small to be identified.
THE ISLAND OF CHILOE.
The evidence of recent elevation is here more satisfactory. The bay of
San Carlos is in most parts bounded by precipitous cliffs from about
ten to forty feet in height, their bases being separated from the
present line of tidal action by a talus, a few feet in height, covered
with vegetation. In one sheltered creek (west of P. Arena), instead of
a loose talus, there was a bare sloping bank of tertiary mudstone,
perforated, above the line of the highest tides, by numerous shells of
a Pholas now common in the harbour. The upper extremities of these
shells, standing upright in their holes with grass growing out of them,
were abraded about a quarter of an inch, to the same level with the
surrounding worn strata. In other parts, I observed (as at Pudeto) a
great beach, formed of comminuted shells, twenty feet above the present
shore. In other parts again, there were small caves worn into the foot
of the low cliffs, and protected from the waves by the talus with its
vegetation: one such cave, which I examined, had its mouth about twenty
feet, and its bottom, which was filled with sand containing fragments
of shells and legs of crabs, from eight to ten feet above high-water
mark. From these several facts, and from the appearance of the upraised
shells, I inferred that the elevation had been quite recent; and on
inquiring from Mr. Williams, the Portmaster, he told me he was
convinced that the land had risen, or the sea fallen, four feet within
the last four years. During this period, there had been one severe
earthquake, but no particular change of level was then observed; from
the habits of the people who all keep boats in the protected creeks, it
is absolutely impossible that a rise of four feet could have taken
place suddenly and been unperceived. Mr. Williams believes that the
change has been quite gradual. Without the elevatory movement continues
at a quick rate, there can be no doubt that the sea will soon destroy
the talus of earth at the foot of the cliffs round the bay, and will
then reach its former lateral extension, but not of course its former
level: some of the inhabitants assured me that one such talus, with a
footpath on it, was even already sensibly decreasing in width.
I received several accounts of beds of shells, existing at considerable
heights in the inland parts of Chiloe; and to one of these, near
Catiman, I was guided by a countryman. Here, on the south side of the
peninsula of Lacuy, there was an immense bed of the Venus costellata
and of an oyster, lying on the summit-edge of a piece of tableland, 350
feet (by the barometer) above the level of the sea. The shells were
closely packed together, embedded in and covered by a very black, damp,
peaty mould, two or three feet in thickness, out of which a forest of
great trees was growing. Considering the nature and dampness of this
peaty soil, it is surprising that the fine ridges on the outside of the
Venus are perfectly preserved, though all the shells have a blackened
appearance. I did not doubt that the black soil, which when dry, cakes
hard, was entirely of terrestrial origin, but on examining it under the
microscope, I found many very minute rounded fragments of shells,
amongst which I could distinguish bits of Serpulae and mussels. The
Venus costellata, and the Ostrea (O. edulis, according to Captain King)
are now the commonest shells in the adjoining bays. In a bed of shells,
a few feet below the 350 feet bed, I found a horn of the little Cervus
humilis, which now inhabits Chiloe.
The eastern or inland side of Chiloe, with its many adjacent islets,
consists of tertiary and boulder deposits, worn into irregular plains
capped by gravel. Near Castro, and for ten miles southward, and on the
islet of Lemuy, I found the surface of the ground to a height of
between twenty and thirty feet above high-water mark, and in several
places apparently up to fifty feet, thickly coated by much comminuted
shells, chiefly of the Venus costellata and Mytilus Chiloensis; the
species now most abundant on this line of coast. As the inhabitants
carry immense numbers of these shells inland, the continuity of the bed
at the same height was often the only means of recognising its natural
origin. Near Castro, on each side of the creek and rivulet of the
Gamboa, three distinct terraces are seen: the lowest was estimated at
about one hundred and fifty feet in height, and the highest at about
five hundred feet, with the country irregularly rising behind it;
obscure traces, also, of these same terraces could be seen along other
parts of the coast. There can be no doubt that their three escarpments
record pauses in the elevation of the island. I may remark that several
promontories have the word Huapi, which signifies in the Indian tongue,
island, appended to them, such as Huapilinao, Huapilacuy, Caucahuapi,
etc.; and these, according to Indian traditions, once existed as
islands. In the same manner the term Pulo in Sumatra is appended to the
names of promontories, traditionally said to have been islands
(Marsden’s “Sumatra” page 31.); in Sumatra, as in Chiloe, there are
upraised recent shells. The Bay of Carelmapu, on the mainland north of
Chiloe, according to Aguerros, was in 1643 a good harbour (“Descripcion
Hist. de la Provincia de Chiloe” page 78. From the account given by the
old Spanish writers, it would appear that several other harbours,
between this point and Concepcion, were formerly much deeper than they
now are.); it is now quite useless, except for boats.
VALDIVIA.
I did not observe here any distinct proofs of recent elevation; but in
a bed of very soft sandstone, forming a fringe-like plain, about sixty
feet in height, round the hills of mica-slate, there are shells of
Mytilus, Crepidula, Solen, Novaculina, and Cytheraea, too imperfect to
be specifically recognised. At Imperial, seventy miles north of
Valdivia, Aguerros states that there are large beds of shells, at a
considerable distance from the coast, which are burnt for lime. (Ibid
page 25.) The island of Mocha, lying a little north of Imperial, was
uplifted two feet, during the earthquake of 1835. (“Voyages of
‘Adventure’ and ‘Beagle’” volume 2 page 415.)
CONCEPCION.
I cannot add anything to the excellent account by Captain Fitzroy of
the elevation of the land at this place, which accompanied the
earthquake of 1835. (Ibid volume 2 page 412 et seq. In volume 5 page
601 of the “Geological Transactions” I have given an account of the
remarkable volcanic phenomena, which accompanied this earthquake. These
phenomena appear to me to prove that the action, by which large tracts
of land are uplifted, and by which volcanic eruptions are produced, is
in every respect identical.) I will only recall to the recollection of
geologists, that the southern end of the island of St. Mary was
uplifted eight feet, the central part nine, and the northern end ten
feet; and the whole island more than the surrounding districts. Great
beds of mussels, patellae, and chitons still adhering to the rocks were
upraised above high-water mark; and some acres of a rocky flat, which
was formerly always covered by the sea, was left standing dry, and
exhaled an offensive smell, from the many attached and putrefying
shells. It appears from the researches of Captain Fitzroy that both the
island of St. Mary and Concepcion (which was uplifted only four or five
feet) in the course of some weeks subsided, and lost part of their
first elevation. I will only add as a lesson of caution, that round the
sandy shores of the great Bay of Concepcion, it was most difficult,
owing to the obliterating effects of the great accompanying wave, to
recognise any distinct evidence of this considerable upheaval; one spot
must be excepted, where there was a detached rock which before the
earthquake had always been covered by the sea, but afterwards was left
uncovered.
On the island of Quiriquina (in the Bay of Concepcion), I found, at an
estimated height of four hundred feet, extensive layers of shells,
mostly comminuted, but some perfectly preserved and closely packed in
black vegetable mould; they consisted of Concholepas, Fissurella,
Mytilus, Trochus, and Balanus. Some of these layers of shells rested on
a thick bed of bright-red, dry, friable earth, capping the surface of
the tertiary sandstone, and extending, as I observed whilst sailing
along the coast, for 150 miles southward: at Valparaiso, we shall
presently see that a similar red earthy mass, though quite like
terrestrial mould, is really in chief part of recent marine origin. On
the flanks of this island of Quiriquina, at a less height than the 400
feet, there were spaces several feet square, thickly strewed with
fragments of similar shells. During a subsequent visit of the “Beagle”
to Concepcion, Mr. Kent, the assistant-surgeon, was so kind as to make
for me some measurements with the barometer: he found many marine
remains along the shores of the whole bay, at a height of about twenty
feet; and from the hill of Sentinella behind Talcahuano, at the height
of 160 feet, he collected numerous shells, packed together close
beneath the surface in black earth, consisting of two species of
Mytilus, two of Crepidula, one of Concholepas, of Fissurella, Venus,
Mactra, Turbo, Monoceros, and the Balanus psittacus. These shells were
bleached, and within some of the Balani other Balani were growing,
showing that they must have long lain dead in the sea. The above
species I compared with living ones from the bay, and found them
identical; but having since lost the specimens, I cannot give their
names: this is of little importance, as Mr. Broderip has examined a
similar collection, made during Captain Beechey’s expedition, and
ascertained that they consisted of ten recent species, associated with
fragments of Echini, crabs, and Flustrae; some of these remains were
estimated by Lieutenant Belcher to lie at the height of nearly a
thousand feet above the level of the sea. (“Zoology of Captain
Beechey’s Voyage” page 162.) In some places round the bay, Mr. Kent
observed that there were beds formed exclusively of the Mytilus
Chiloensis: this species now lives in parts never uncovered by the
tides. At considerable heights, Mr. Kent found only a few shells; but
from the summit of one hill, 625 feet high, he brought me specimens of
the Concholepas, Mytilus Chiloensis, and a Turbo. These shells were
softer and more brittle than those from the height of 164 feet; and
these latter had obviously a much more ancient appearance than the same
species from the height of only twenty feet.
COAST NORTH OF CONCEPCION.
The first point examined was at the mouth of the Rapel (160 miles north
of Concepcion and sixty miles south of Valparaiso), where I observed a
few shells at the height of 100 feet, and some barnacles adhering to
the rocks three or four feet above the highest tides: M. Gay found here
recent shells at the distance of two leagues from the shore. (“Annales
des Scienc. Nat.” Avril 1833.) Inland there are some wide,
gravel-capped plains, intersected by many broad, flat-bottomed valleys
(now carrying insignificant streamlets), with their sides cut into
successive wall-like escarpments, rising one above another, and in many
places, according to M. Gay, worn into caves. The one cave (C. del
Obispo) which I examined, resembled those formed on many sea-coasts,
with its bottom filled with shingle. These inland plains, instead of
sloping towards the coast, are inclined in an opposite direction
towards the Cordillera, like the successively rising terraces on the
inland or eastern side of Chiloe: some points of granite, which project
through the plains near the coast, no doubt once formed a chain of
outlying islands, on the inland shores of which the plains were
accumulated. At Bucalemu, a few miles northward of the Rapel, I
observed at the foot, and on the summit-edge of a plain, ten miles from
the coast, many recent shells, mostly comminuted, but some perfect.
There were, also, many at the bottom of the great valley of the Maypu.
At San Antonio, shells are said to be collected and burnt for lime. At
the bottom of a great ravine (Quebrada Onda, on the road to Casa
Blanca), at the distance of several miles from the coast, I noticed a
considerable bed, composed exclusively of Mesodesma donaciforme, Desh.,
lying on a bed of muddy sand: this shell now lives associated together
in great numbers, on tidal-flats on the coast of Chile.
VALPARAISO.
During two successive years I carefully examined, part of the time in
company with Mr. Alison, into all the facts connected with the recent
elevation of this neighbourhood. In very many parts a beach of broken
shells, about fourteen or fifteen feet above high-water mark, may be
observed; and at this level the coast-rocks, where precipitous, are
corroded in a band. At one spot, Mr. Alison, by removing some birds’
dung, found at this same level barnacles adhering to the rocks. For
several miles southward of the bay, almost every flat little headland,
between the heights of 60 and 230 feet (measured by the barometer), is
smoothly coated by a thick mass of comminuted shells, of the same
species, and apparently in the same proportional numbers with those
existing in the adjoining sea. The Concholepas is much the most
abundant, and the best preserved shell; but I extracted perfectly
preserved specimens of the Fissurella biradiata, a Trochus and Balanus
(both well-known, but according to Mr. Sowerby yet unnamed) and parts
of the Mytilus Chiloensis. Most of these shells, as well as an
encrusting Nullipora, partially retain their colour; but they are
brittle, and often stained red from the underlying brecciated mass of
primary rocks; some are packed together, either in black or reddish
moulds; some lie loose on the bare rocky surfaces. The total number of
these shells is immense; they are less numerous, though still far from
rare, up a height of 1,000 feet above the sea. On the summit of a hill,
measured 557 feet, there was a small horizontal band of comminuted
shells, of which MANY consisted (and likewise from lesser heights) of
very young and small specimens of the still living Concholepas,
Trochus, Patellae, Crepidulae, and of Mytilus Magellanicus (?) (Mr.
Cuming informs me that he does not think this species identical with,
though closely resembling, the true M. Magellanicus of the southern and
eastern coast of South America; it lives abundantly on the coast of
Chile.): several of these shells were under a quarter of an inch in
their greatest diameter. My attention was called to this circumstance
by a native fisherman, whom I took to look at these shell-beds; and he
ridiculed the notion of such small shells having been brought up for
food; nor could some of the species have adhered when alive to other
larger shells. On another hill, some miles distant, and 648 feet high,
I found shells of the Concholepas and Trochus, perfect, though very
old, with fragments of Mytilus Chiloensis, all embedded in
reddish-brown mould: I also found these same species, with fragments of
an Echinus and of Balanus psittacus, on a hill 1,000 feet high. Above
this height, shells became very rare, though on a hill 1,300 feet high
(Measured by the barometer: the highest point in the range behind
Valparaiso I found to be 1,626 feet above the level of the sea.), I
collected the Concholepas, Trochus, Fissurella, and a Patella. At these
greater heights the shells are almost invariably embedded in mould, and
sometimes are exposed only by tearing up bushes. These shells obviously
had a very much more ancient appearance than those from the lesser
heights; the apices of the Trochi were often worn down; the little
holes made by burrowing animals were greatly enlarged; and the
Concholepas was often perforated quite through, owing to the inner
plates of shell having scaled off.