William Shakespeare - "As You Like It"

Moliere - "The School for Husbands"

George Bernard Shaw - "The Philanderer"

William Shakespeare - "Much Ado About Nothing"

Geological Observations On South America

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




The basin-like plains at the foot of the Cordillera are in several respects
remarkable; that on which the capital of Chile stands is fifteen miles in
width, in an east and west line, and of much greater length in a north and
south line; it stands 1,750 feet above the sea; its surface appears smooth,
but really falls and rises in wide gentle undulations, the hollows
corresponding with the main valleys of the Cordillera: the striking manner
in which it abruptly comes up to the foot of this great range has been
remarked by every author since the time of Molina. (This plain is partially
separated into two basins by a range of hills; the southern half, according
to Meyen ("Reise um Erde" Th. 1 s. 274), falls in height, by an abrupt
step, of between fifteen and twenty feet.) Near the Cordillera it is
composed of a stratified mass of pebbles of all sizes, occasionally
including rounded boulders: near its western boundary, it consists of
reddish sandy clay, containing some pebbles and numerous fragments of
pumice, and sometimes passes into pure sand or into volcanic ashes. At
Podaguel, on this western side of the plain, beds of sand are capped by a
calcareous tuff, the uppermost layers being generally hard and
substalagmitic, and the lower ones white and friable, both together
precisely resembling the beds at Coquimbo, which contain recent marine
shells. Abrupt, but rounded, hummocks of rock rise out of this plain: those
of Sta. Lucia and S. Cristoval are formed of greenstone-porphyry almost
entirely denuded of its original covering of porphyritic claystone breccia;
on their summits, many fragments of rock (some of them kinds not found in
situ) are coated and united together by a white, friable, calcareous tuff,
like that found at Podaguel. When this matter was deposited on the summit
of S. Cristoval, the water must have stood 946 feet above the surface of
the surrounding plain. (Or 2,690 feet above the sea, as measured
barometrically by Mr. Eck. This tuff appears to the eye nearly pure; but
when placed in acid it leaves a considerable residue of sand and broken
crystals, apparently of feldspar. Dr. Meyen ("Reise" Th. 1 s. 269) says he
found a similar substance on the neighbouring hill of Dominico (and I found
it also on the Cerro Blanco), and he attributes it to the weathering of the
stone. In some places which I examined, its bulk put this view of its
origin quite out of the question; and I should much doubt whether the
decomposition of a porphyry would, in any case, leave a crust chiefly
composed of carbonate of lime. The white crust, which is commonly seen on
weathered feldspathic rocks, does not appear to contain any free carbonate
of lime.)

To the south this basin-like plain contracts, and rising scarcely
perceptibly with a smooth surface, passes through a remarkable level gap in
the mountains, forming a true land-strait, and called the Angostura. It
then immediately expands into a second basin-formed plain: this again to
the south contracts into another land-strait, and expands into a third
basin, which, however, falls suddenly in level about forty feet. This third
basin, to the south, likewise contracts into a strait, and then again opens
into the great plain of San Fernando, stretching so far south that the
snowy peaks of the distant Cordillera are seen rising above its horizon as
above the sea. These plains, near the Cordillera, are generally formed of a
thick stratified mass of shingle (The plain of San Fernando has, according
to MM. Meyen and Gay "Reise" etc. Th. 1 ss. 295 and 298, near the
Cordillera, an upper step-formed plain of clay, on the surface of which
they found numerous blocks of rocks, from two to three feet long, either
lying single or piled in heaps, but all arranged in nearly straight
lines.); in other parts, of a red sandy clay, often with an admixture of
pumiceous matter. Although these basins are connected together like a
necklace, in a north and south line, by smooth land-straits, the streams
which drain them do not all flow north and south, but mostly westward,
through breaches worn in the bounding mountains; and in the case of the
second basin, or that of Rancagua, there are two distinct breaches. Each
basin, moreover, is not drained singly; thus, to give the most striking
instance, but not the only one, in proceeding southward over the plain of
Rancagua, we first find the water flowing northward to and through the
northern land-strait; then, without crossing any marked ridge or watershed,
we see it flowing south-westward towards the northern one of the two
breaches in the western mountainous boundary; and lastly, again without any
ridge, it flows towards the southern breach in these same mountains. Hence
the surface of this one basin-like plain, appearing to the eye so level,
has been modelled with great nicety, so that the drainage, without any
conspicuous watersheds, is directed towards three openings in the
encircling mountains. ((It appears from Captain Herbert's account of the
Diluvium of the Himalaya, "Gleanings of Science" Calcutta volume 2 page
164, that precisely similar remarks apply to the drainage of the plains or
valleys between those great mountains.) The streams flowing from the
southern basin-like plains, after passing through the breaches to the west,
unite and form the river Rapel, which enters the Pacific near Navidad. I
followed the southernmost branch of this river, and found that the basin or
plain of San Fernando is continuously and smoothly united with those
plains, which were described in the Second Chapter, as being worn near the
coast into successive cave-eaten escarpments, and still nearer to the
coast, as being strewed with upraised recent marine remains.

I might have given descriptions of numerous other plains of the same
general form, some at the foot of the Cordillera, some near the coast, and
some halfway between these points. I will allude only to one other, namely,
the plain of Uspallata, lying on the eastern or opposite side of the
Cordillera, between that great range and the parallel lower range of
Uspallata. According to Miers, its surface is 6,000 feet above the level of
the sea: it is from ten to fifteen miles in width, and is said to extend
with an unbroken surface for 180 miles northwards: it is drained by two
rivers passing through breaches in the mountains to the east. On the banks
of the River Mendoza it is seen to be composed of a great accumulation of
stratified shingle, estimated at 400 feet in thickness. In general
appearance, and in numerous points of structure, this plain closely
resembles those of Chile.

The origin and manner of formation of the thick beds of gravel, sandy clay,
volcanic detritus, and calcareous tuff, composing these basin-like plains,
is very important; because, as we shall presently show, they send arms or
fringes far up the main valleys of the Cordillera. Many of the inhabitants
believe that these plains were once occupied by lakes, suddenly drained;
but I conceive that the number of the separate breaches at nearly the same
level in the mountains surrounding them quite precludes this idea. Had not
such distinguished naturalists as MM. Meyen and Gay stated their belief
that these deposits were left by great debacles rushing down from the
Cordillera, I should not have noticed a view, which appears to me from many
reasons improbable in the highest degree--namely, from the vast
accumulation of WELL-ROUNDED PEBBLES--their frequent stratification with
layers of sand--the overlying beds of calcareous tuff--this same substance
coating and uniting the fragments of rock on the hummocks in the plain of
Santiago--and lastly even from the worn, rounded, and much denuded state of
these hummocks, and of the headlands which project from the surrounding
mountains. On the other hand, these several circumstances, as well as the
continuous union of the basins at the foot of the Cordillera, with the
great plain of the Rio Rapel which still retains the marks of sea-action at
various levels, and their general similarity in form and composition with
the many plains near the coast, which are either similarly marked or are
strewed with upraised marine remains, fully convince me that the mountains
bounding these basin-plains were breached, their islet-like projecting
rocks worn, and the loose stratified detritus forming their now level
surfaces deposited, by the sea, as the land slowly emerged. It is hardly
possible to state too strongly the perfect resemblance in outline between
these basin-like, long, and narrow plains of Chile (especially when in the
early morning the mists hanging low represented water), and the creeks and
fiords now intersecting the southern and western shores of the continent.
We can on this view of the sea, when the land stood lower, having long and
tranquilly occupied the spaces between the mountain-ranges, understand how
the boundaries of the separate basins were breached in more than one place;
for we see that this is the general character of the inland bays and
channels of Tierra del Fuego; we there, also, see in the sawing action of
the tides, which flow with great force in the cross channels, a power
sufficient to keep the breaches open as the land emerged. We can further
see that the waves would naturally leave the smooth bottom of each great
bay or channel, as it became slowly converted into land, gently inclined to
as many points as there were mouths, through which the sea finally
retreated, thus forming so many watersheds, without any marked ridges, on a
nearly level surface. The absence of marine remains in these high inland
plains cannot be properly adduced as an objection to their marine origin:
for we may conclude, from shells not being found in the great shingle beds
of Patagonia, though copiously strewed on their surfaces, and from many
other analogous facts, that such deposits are eminently unfavourable for
the embedment of such remains; and with respect to shells not being found
strewed on the surface of these basin-like plains, it was shown in the last
chapter that remains thus exposed in time decay and disappear.

(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE CHILEAN
CORDILLERA.

From Cordillera (left) through Talus-plain and Level surface, 2,700 feet
above sea, to Gravel terraces (right).)

I observed some appearances on the plains at the eastern and opposite foot
of the Cordillera which are worth notice, as showing that the sea there
long acted at nearly the same level as on the basin-plains of Chile. The
mountains on this eastern side are exceedingly abrupt; they rise out of a
smooth, talus-like, very gentle, slope, from five to ten miles in width (as
represented in Figure 13), entirely composed of perfectly rounded pebbles,
often white-washed with an aluminous substance like decomposed feldspar.
This sloping plain or talus blends into a perfectly flat space a few miles
in width, composed of reddish impure clay, with small calcareous
concretions as in the Pampean deposit,--of fine white sand with small
pebbles in layers,--and of the above-mentioned white aluminous earth, all
interstratified together. This flat space runs as far as Mendoza, thirty
miles northward, and stands probably at about the same height, namely,
2,700 feet (Pentland and Miers) above the sea. To the east it is bounded by
an escarpment, eighty feet in height, running for many miles north and
south, and composed of perfectly round pebbles, and loose, white-washed, or
embedded in the aluminous earth: behind this escarpment there is a second
and similar one of gravel. Northward of Mendoza, these escarpments become
broken and quite obliterated; and it does not appear that they ever
enclosed a lake-like area: I conclude, therefore, that they were formed by
the sea, when it reached the foot of the Cordillera, like the similar
escarpments occurring at so many points on the coasts of Chile and
Patagonia.

The talus-like plain slopes up with a smooth surface into the great dry
valleys of the Cordillera. On each hand of the Portillo valley, the
mountains are formed of red granite, mica-slate, and basalt, which all have
suffered a truly astonishing amount of denudation; the gravel in the
valley, as well as on the talus-like plain in front of it, is composed of
these rocks; but at the mouth of the valley, in the middle (height probably
about three thousand five hundred feet above the sea), a few small isolated
hillocks of several varieties of porphyry project, round which, on all
sides, smooth and often white-washed pebbles of these same porphyries, to
the exclusion of all others, extend to a circumscribed distance. Now, it is
difficult to conceive any other agency, except the quiet and long-continued
action of the sea on these hillocks, which could have rounded and
whitewashed the fragments of porphyry, and caused them to radiate from such
small and quite insignificant centres, in the midst of that vast stream of
stones which has descended from the main Cordillera.

SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA.

(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE CORDILLERA, bordered
by smooth, sloping gravel-fringes (AA), worn along the course of the river
into cliffs.)

All the main valleys on both flanks of the Chilean Cordillera have formerly
had, or still have, their bottoms filled up to a considerable thickness by
a mass of rudely stratified shingle. In Central Chile the greater part of
this mass has been removed by the torrents; cliff-bounded fringes, more or
less continuous, being left at corresponding heights on both sides of the
valleys. These fringes, or as they may be called terraces, have a smooth
surface, and as the valleys rise, they gently rise with them: hence they
are easily irrigated, and afford great facilities for the construction of
the roads. From their uniformity, they give a remarkable character to the
scenery of these grand, wild, broken valleys. In width, the fringes vary
much, sometimes being only broad enough for the roads, and sometimes
expanding into narrow plains. Their surfaces, besides gently rising up the
valley, are slightly inclined towards its centre in such a manner as to
show that the whole bottom must once have been filled up with a smooth and
slightly concave mass, as still are the dry unfurrowed valleys of Northern
Chile. Where two valleys unite into one, these terraces are particularly
well exhibited, as is represented in Figure 14. The thickness of the gravel
forming these fringes, on a rude average, may be said to vary from thirty
to sixty or eighty feet; but near the mouths of the valleys it was in
several places from two to three hundred feet. The amount of matter removed
by the torrents has been immense; yet in the lower parts of the valleys the
terraces have seldom been entirely worn away on either side, nor has the
solid underlying rock been reached: higher up the valleys, the terraces
have frequently been removed on one or the other side, and sometimes on
both sides; but in this latter case they reappear after a short interval on
the line, which they would have held had they been unbroken. Where the
solid rock has been reached, it has been cut into deep and narrow gorges.
Still higher up the valleys, the terraces gradually become more and more
broken, narrower, and less thick, until, at a height of from seven to nine
thousand feet, they become lost, and blended with the piles of fallen
detritus.

I carefully examined in many places the state of the gravel, and almost
everywhere found the pebbles equally and perfectly rounded, occasionally
with great blocks of rock, and generally distinctly stratified, often with
parting seams of sand. The pebbles were sometimes coated with a white
aluminous, and less frequently with a calcareous, crust. At great heights
up the valleys the pebbles become less rounded; and as the terraces become
obliterated, the whole mass passes into the nature of ordinary detritus. I
was repeatedly struck with the great difference between this detritus high
up the valleys, and the gravel of the terraces low down, namely, in the
greater number of the quite angular fragments in the detritus,--in the
unequal degree to which the other fragments have been rounded,--in the
quantity of associated earth,--in the absence of stratification,--and in
the irregularity of the upper surfaces. This difference was likewise well
shown at points low down the valleys, where precipitous ravines, cutting
through mountains of highly coloured rock, have thrown down wide, fan-
shaped accumulations of detritus on the terraces: in such cases, the line
of separation between the detritus and the terrace could be pointed out to
within an inch or two; the detritus consisting entirely of angular and only
partially rounded fragments of the adjoining coloured rocks; the stratified
shingle (as I ascertained by close inspection, especially in one case, in
the valley of the River Mendoza) containing only a small proportion of
these fragments, and those few well rounded.

I particularly attended to the appearance of the terraces where the valleys
made abrupt and considerable bends, but I could perceive no difference in
their structure: they followed the bends with their usual nearly equable
inclination. I observed, also, in several valleys, that wherever large
blocks of any rock became numerous, either on the surface of the terrace or
embedded in it, this rock soon appeared higher up in situ: thus I have
noticed blocks of porphyry, of andesitic syenite, of porphyry and of
syenite, alternately becoming numerous, and in each case succeeded by
mountains thus constituted. There is, however, one remarkable exception to
this rule; for along the valley of the Cachapual, M. Gay found numerous
large blocks of white granite, which does not occur in the neighbourhood. I
observed these blocks, as well as others of andesitic syenite (not
occurring here in situ), near the baths of Cauquenes at a height of between
two and three hundred feet above the river, and therefore quite above the
terrace or fringe which borders that river; some miles up the valleys there
were other blocks at about the same height. I also noticed, at a less
height, just above the terrace, blocks of porphyries (apparently not found
in the immediately impending mountains), arranged in rude lines, as on a
sea-beach. All these blocks were rounded, and though large, not gigantic,
like the true erratic boulders of Patagonia and Fuegia. M. Gay states that
the granite does not occur in situ within a distance of twenty leagues
("Annales des Science Nat. " 1 series tome 28. M. Gay, as I was informed,
penetrated the Cordillera by the great oblique valley of Los Cupressos, and
not by the most direct line.); I suspect, for several reasons, that it will
ultimately be found at a much less distance, though certainly not in the
immediate neighbourhood. The boulders found by MM. Meyen and Gay on the
upper plain of San Fernando (mentioned in a previous note) probably belong
to this same class of phenomena.

These fringes of stratified gravel occur along all the great valleys of the
Cordillera, as well as along their main branches; they are strikingly
developed in the valleys of the Maypu, Mendoza, Aconcagua, Cachapual, and
according to Meyen, in the Tinguirica. ("Reise" etc. Th. 1 s. 302.) In the
valleys, however, of Northern Chile, and in some on the eastern flank of
the Cordillera, as in the Portillo Valley, where streams have never flowed,
or are quite insignificant in volume, the presence of a mass of stratified
gravel can be inferred only from the smooth slightly concave form of the
bottom. One naturally seeks for some explanation of so general and striking
a phenomenon; that the matter forming the fringes along the valleys, or
still filling up their entire beds, has not fallen from the adjoining
mountains like common detritus, is evident from the complete contrast in
every respect between the gravel and the piles of detritus, whether seen
high up the valleys on their sides, or low down in front of the more
precipitous ravines; that the matter has not been deposited by debacles,
even if we could believe in debacles having rushed down EVERY valley, and
all their branches, eastward and westward from the central pinnacles of the
Cordillera, we must admit from the following reasons,--from the distinct
stratification of the mass,--its smooth upper surface,--the well-rounded
and sometimes encrusted state of the pebbles, so different from the loose
debris on the mountains,--and especially from the terraces preserving their
uniform inclination round the most abrupt bends. To suppose that as the
land now stands, the rivers deposited the shingle along the course of every
valley, and all their main branches, appears to me preposterous, seeing
that these same rivers not only are now removing and have removed much of
this deposit, but are everywhere tending to cut deep and narrow gorges in
the hard underlying rocks.

I have stated that these fringes of gravel, the origin of which are
inexplicable on the notion of debacles or of ordinary alluvial action, are
directly continuous with the similarly-composed basin-like plains at the
foot of the Cordillera, which, from the several reasons before assigned, I
cannot doubt were modelled by the agency of the sea. Now if we suppose that
the sea formerly occupied the valleys of the Chilean Cordillera, in
precisely the same manner as it now does in the more southern parts of the
continent, where deep winding creeks penetrate into the very heart of, and
in the case of Obstruction Sound quite through, this great range; and if we
suppose that the mountains were upraised in the same slow manner as the
eastern and western coasts have been upraised within the recent period,
then the origin and formation of these sloping, terrace-like fringes of
gravel can be simply explained. For every part of the bottom of each valley
will, on this view, have long stood at the head of a sea creek, into which
the then existing torrents will have delivered fragments of rocks, where,
by the action of the tides, they will have been rolled, sometimes
encrusted, rudely stratified, and the whole surface levelled by the
blending together of the successive beach lines. (Sloping terraces of
precisely similar structure have been described by me "Philosophical
Transactions" 1839 page 58, in the valleys of Lochaber in Scotland, where,
at higher levels, the parallel roads of Glen Roy show the marks of the long
and quiet residence of the sea. I have no doubt that these sloping terraces
would have been present in the valleys of most of the European ranges, had
not every trace of them, and all wrecks of sea-action, been swept away by
the glaciers which have since occupied them. I have shown that this is the
case with the mountains ("London and Edinburgh Philosophical Journal"
volume 21 page 187) of North Wales.) As the land rose, the torrents in
every valley will have tended to have removed the matter which just before
had been arrested on, or near, the beach-lines; the torrents, also, having
continued to gain in force by the continued elevation increasing their
total descent from their sources to the sea. This slow rising of the
Cordillera, which explains so well the otherwise inexplicable origin and
structure of the terraces, judging from all known analogies, will probably
have been interrupted by many periods of rest; but we ought not to expect
to find any evidence of these periods in the structure of the gravel-
terraces: for, as the waves at the heads of deep creeks have little erosive
power, so the only effect of the sea having long remained at the same level
will be that the upper parts of the creeks will have become filled up at
such periods to the level of the water with gravel and sand; and that
afterwards the rivers will have thrown down on the filled-up parts a talus
of similar matter, of which the inclination (as at the head of a partially
filled-up lake) will have been determined by the supply of detritus, and
the force of the stream. (I have attempted to explain this process in a
more detailed manner, in a letter to Mr. Maclaren, published in the
"Edinburgh New Philosophical Journal" volume 35 page 288.) Hence, after the
final conversion of the creeks into valleys, almost the only difference in
the terraces at those points at which the sea stood long, will be a
somewhat more gentle inclination, with river-worn instead of sea-worn
detritus on the surface.

I know of only one difficulty on the foregoing view, namely, the far-
transported blocks of rock high on the sides of the valley of the
Cachapual: I will not attempt any explanation of this phenomenon, but I may
state my belief that a mountain-ridge near the Baths of Cauquenes has been
upraised long subsequently to all the other ranges in the neighbourhood,
and that when this was effected the whole face of the country must have
been greatly altered. In the course of ages, moreover, in this and other
valleys, events may have occurred like, but even on a grander scale than,
that described by Molina, when a slip during the earthquake of 1762 banked
up for ten days the great River Lontue, which then bursting its barrier
"inundated the whole country," and doubtless transported many great
fragments of rock. ("Compendio de la Hist." etc. etc. tome 1 page 30. M.
Brongniart, in his report on M. Gay's labours "Annales des Sciences" 1833,
considers that the boulders in the Cachapual belong to the same class with
the erratic boulders of Europe. As the blocks which I saw are not gigantic,
and especially as they are not angular, and as they have not been
transported fairly across low spaces or wide valleys, I am unwilling to
class them with those which, both in the northern and southern hemisphere
"Geological Transactions" volume 6 page 415, have been transported by ice.
It is to be hoped that when M. Gay's long-continued and admirable labours
in Chile are published, more light will be thrown on this subject. However,
the boulders may have been primarily transported; the final position of
those of porphyry, which have been described as arranged at the foot of the
mountain in rude lines, I cannot doubt, has been due to the action of waves
on a beach. The valley of the Cachapual, in the part where the boulders
occur, bursts through the high ridge of Cauquenes, which runs parallel to,
but at some distance from, the Cordillera. This ridge has been subjected to
excessive violence; trachytic lava has burst from it, and hot springs yet
flow at its base. Seeing the enormous amount of denudation of solid rock in
the upper and much broader parts of this valley where it enters the
Cordillera, and seeing to what extent the ridge of Cauquenes now protects
the great range, I could not help believing (as alluded to in the text)
that this ridge with its trachytic eruptions had been thrown up at a much
later period than the Cordillera. If this has been the case, the boulders,
after having been transported to a low level by the torrents (which exhibit
in every valley proofs of their power of moving great fragments), may have
been raised up to their present height, with the land on which they
rested.) Finally, notwithstanding this one case of difficulty, I cannot
entertain any doubt, that these terrace-like fringes, which are
continuously united with the basin-shaped plains at the foot of the
Cordillera, have been formed by the arrestment of river-borne detritus at
successive levels, in the same manner as we see now taking place at the
heads of all those many, deep, winding fiords intersecting the southern
coasts. To my mind, this has been one of the most important conclusions to
which my observations on the geology of South America have led me; for we
thus learn that one of the grandest and most symmetrical mountain-chains in
the world, with its several parallel lines, has been together uplifted in
mass between seven and nine thousand feet, in the same gradual manner as
have the eastern and western coasts within the recent period. (I do not
wish to affirm that all the lines have been uplifted quite equally; slight
differences in the elevation would leave no perceptible effect on the
terraces. It may, however, be inferred, perhaps with one exception, that
since the period when the sea occupied these valleys, the several ranges
have not been dislocated by GREAT and ABRUPT faults or upheavals; for if
such had occurred, the terraces of gravel at these points would not have
been continuous. The one exception is at the lower end of a plain in the
Valle del Yeso (a branch of the Maypu), where, at a great height, the
terraces and valley appear to have been broken through by a line of
upheaval, of which the evidence is plain in the adjoining mountains; this
dislocation, perhaps, occurred AFTER THE ELEVATION of this part of the
valley above the level of the sea. The valley here is almost blocked up by
a pile about one thousand feet in thickness, formed, as far as I could
judge, from three sides, entirely, or at least in chief part, of gravel and
detritus. On the south side, the river has cut quite through this mass; on
the northern side, and on the very summit, deep ravines, parallel to the
line of the valley, are worn, as if the drainage from the valley above had
passed by these two lines before following its present course.)

Copyright (c) 2007. topbookz.net. All rights reserved.