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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.
1. 2. 3.
3 to 4 11 to 12 As large as walnuts; mingled in every case with
some smaller ones.
6 to 7 17 to 19 As large as hazel-nuts.
10 to 11 23 to 25 From three- to four-tenths of an inch in diameter.
12 30 to 40 Two-tenths of an inch.
22 to 150 45 to 65 One-tenth of an inch, to the finest sand.
I particularly attended to the size of the pebbles on the 355 feet Santa
Cruz plain, and I noticed that on the summit-edge of the present sea cliffs
many were as large as half a man's head; and in crossing from these cliffs
to the foot of the next highest escarpment, a distance of six miles, I
could not observe any increase in their size. We shall presently see that
the theory of a slow and almost insensible rise of the land, will explain
all the facts connected with the gravel-capped terraces, better than the
theory of sudden elevations of from one to two hundred feet.
M. d'Orbigny has argued, from the upraised shells at San Blas being
embedded in the positions in which they lived, and from the valves of the
Azara labiata high on the banks of the Parana being united and unrolled,
that the elevation of Northern Patagonia and of La Plata must have been
sudden; for he thinks, if it had been gradual, these shells would all have
been rolled on successive beach-lines. But in PROTECTED bays, such as in
that of Bahia Blanca, wherever the sea is accumulating extensive mud-banks,
or where the winds quietly heap up sand-dunes, beds of shells might
assuredly be preserved buried in the positions in which they had lived,
even whilst the land retained the same level; any, the smallest, amount of
elevation would directly aid in their preservation. I saw a multitude of
spots in Bahia Blanca where this might have been effected; and at Maldonado
it almost certainly has been effected. In speaking of the elevation of the
land having been slow, I do not wish to exclude the small starts which
accompany earthquakes, as on the coast of Chile; and by such movements beds
of shells might easily be uplifted, even in positions exposed to a heavy
surf, without undergoing any attrition: for instance, in 1835, a rocky flat
off the island of Santa Maria was at one blow upheaved above high-water
mark, and was left covered with gaping and putrefying mussel-shells, still
attached to the bed on which they had lived. If M. d'Orbigny had been aware
of the many long parallel lines of sand-hillocks, with infinitely numerous
shells of the Mactra and Venus, at a low level near the Uruguay; if he had
seen at Bahia Blanca the immense sand-dunes, with water-worn pebbles of
pumice, ranging in parallel lines, one behind the other, up a height of at
least 120 feet; if he had seen the sand-dunes, with the countless
Paludestrinas, on the low plain near the Fort at this place, and that long
line on the edge of the cliff, sixty feet higher up; if he had crossed that
long and great belt of parallel sand-dunes, eight miles in width, standing
at the height of from forty to fifty feet above the Colorado, where sand
could not now collect,--I cannot believe he would have thought that the
elevation of this great district had been sudden. Certainly the sand-dunes
(especially when abounding with shells), which stand in ranges at so many
different levels, must all have required long time for their accumulation;
and hence I do not doubt that the last 100 feet of elevation of La Plata
and Northern Patagonia has been exceedingly slow.
If we extend this conclusion to Central and Southern Patagonia, the
inclination of the successively rising gravel-capped plains can be
explained quite as well, as by the more obvious view already given of a few
comparatively great and sudden elevations; in either case we must admit
long periods of rest, during which the sea ate deeply into the land. Let us
suppose the present coast to rise at a nearly equable, slow rate, yet
sufficiently quick to prevent the waves quite removing each part as soon as
brought up; in this case every portion of the present bed of the sea will
successively form a beach-line, and from being exposed to a like action
will be similarly affected. It cannot matter to what height the tides rise,
even if to forty feet as at Santa Cruz, for they will act with equal force
and in like manner on each successive line. Hence there is no difficulty in
the fact of the 355 feet plain at Santa Cruz sloping up 108 feet to the
foot of the next highest escarpment, and yet having no marks of any one
particular beach-line on it; for the whole surface on this view has been a
beach. I cannot pretend to follow out the precise action of the tidal-waves
during a rise of the land, slow, yet sufficiently quick to prevent or check
denudation: but if it be analogous to what takes place on protected parts
of the present coast, where gravel is now accumulating in large quantities,
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.
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 ascendin
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