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Geological Observations On South America

C >> Charles Darwin >> Geological Observations On South America




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.

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