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Chapter VII
CENTRAL CHILE:--STRUCTURE OF THE CORDILLERA.
Central Chile.
Basal formations of the Cordillera.
Origin of the porphyritic clay-stone conglomerate.
Andesite.
Volcanic rocks.
Section of the Cordillera by the Peuquenes are Portillo Pass.
Great gypseous formation.
Peuquenes line; thickness of strata, fossils of.
Portillo line.
Conglomerate, orthitic granite, mica-schist, volcanic rocks of.
Concluding remarks on the denudation and elevation of the Portillo line.
Section by the Cumbre, or Uspallata Pass.
Porphyries.
Gypseous strata.
Section near the Puente del Inca; fossils of.
Great subsidence.
Intrusive porphyries.
Plain of Uspallata.
Section of the Uspallata chain.
Structure and nature of the strata.
Silicified vertical trees.
Great subsidence.
Granitic rocks of axis.
Concluding remarks on the Uspallata range; origin subsequent to that of the
main Cordillera; two periods of subsidence; comparison with the Portillo
chain.
The district between the Cordillera and the Pacific, on a rude average, is
from about eighty to one hundred miles in width. It is crossed by many
chains of mountains, of which the principal ones, in the latitude of
Valparaiso and southward of it, range nearly north and south; but in the
more northern parts of the province, they run in almost every possible
direction. Near the Pacific, the mountain-ranges are generally formed of
syenite or granite, and or of an allied euritic porphyry; in the low
country, besides these granitic rocks and greenstone, and much gneiss,
there are, especially northward of Valparaiso, some considerable districts
of true clay-slate with quartz veins, passing into a feldspathic and
porphyritic slate; there is also some grauwacke and quartzose and jaspery
rocks, the latter occasionally assuming the character of the basis of
claystone porphyry: trap-dikes are numerous. Nearer the Cordillera the
ranges (such as those of S. Fernando, the Prado (Meyen "Reise um Erde" th.
1 s. 235.), and Aconcagua) are formed partly of granitic rocks, and partly
of purple porphyritic conglomerates, claystone porphyry, greenstone
porphyry, and other rocks, such as we shall immediately see, form the basal
strata of the main Cordillera. In the more northern parts of Chile, this
porphyritic series extends over large tracts of country far from the
Cordillera; and even in Central Chile such occasionally occur in outlying
positions.
I will describe the Campana of Quillota, which stands only fifteen miles
from the Pacific, as an instance of one of these outlying masses. This hill
is conspicuous from rising to the height of 6,400 feet: its summit shows a
nucleus, uncovered for a height of 800 feet, of fine greenstone, including
epidote and octahedral magnetic iron ore; its flanks are formed of great
strata of porphyritic claystone conglomerate associated with various true
porphyries and amygdaloids, alternating with thick masses of a highly
feldspathic, sometimes porphyritic, pale-coloured slaty rock, with its
cleavage-laminae dipping inwards at a high angle. At the base of the hill
there are syenites, a granular mixture of quartz and feldspar, and harsh
quartzose rocks, all belonging to the basal metamorphic series. I may
observe that at the foot of several hills of this class, where the
porphyries are first seen (as near S. Fernando, the Prado, Las Vacas,
etc.), similar harsh quartzose rocks and granular mixtures of quartz and
feldspar occur, as if the more fusible constituent parts of the granitic
series had been drawn off to form the overlying porphyries.
In Central Chile, the flanks of the main Cordillera, into which I
penetrated by four different valleys, generally consist of distinctly
stratified rocks. The strata are inclined at angles varying from sometimes
even under ten, to twenty degrees, very rarely exceeding forty degrees: in
some, however, of the quite small, exterior, spur-like ridges, the
inclination was not unfrequently greater. The dip of the strata in the main
outer lines was usually outwards or from the Cordillera, but in Northern
Chile frequently inwards,--that is, their basset-edges fronted the Pacific.
Dikes occur in extraordinary numbers. In the great, central, loftiest
ridges, the strata, as we shall presently see, are almost always highly
inclined and often vertical. Before giving a detailed account of my two
sections across the Cordillera, it will, I think, be convenient to describe
the basal strata as seen, often to a thickness of four or five thousand
feet, on the flanks of the outer lines.
BASAL STRATA OF THE CORDILLERA.
The prevailing rock is a purplish or greenish, porphyritic claystone
conglomerate. The embedded fragments vary in size from mere particles to
blocks as much as six or eight inches (rarely more) in diameter; in many
places, where the fragments were minute, the signs of aqueous deposition
were unequivocally distinct; where they were large, such evidence could
rarely be detected. The basis is generally porphyritic with perfect
crystals of feldspar, and resembles that of a true injected claystone
porphyry: often, however, it has a mechanical or sedimentary aspect, and
sometimes (as at Jajuel) is jaspery. The included fragments are either
angular, or partially or quite rounded (Some of the rounded fragments in
the porphyritic conglomerate near the Baths of Cauquenes, were marked with
radii and concentric zones of different shades of colour: any one who did
not know that pebbles, for instance flint pebbles from the chalk, are
sometimes zoned concentrically with their worn and rounded surfaces, might
have been led to infer, that these balls of porphyry were not true pebbles,
but had originated in concretionary action.); in some parts the rounded, in
others the angular fragments prevail, and usually both kinds are mixed
together: hence the word BRECCIA ought strictly to be appended to the term
PORPHYRITIC CONGLOMERATE. The fragments consist of many varieties of
claystone porphyry, usually of nearly the same colour with the surrounding
basis, namely, purplish-reddish, brownish, mottled or bright green;
occasionally fragments of a laminated, pale-coloured, feldspathic rock,
like altered clay-slate are included; as are sometimes grains of quartz,
but only in one instance in Central Chile (namely, at the mines of Jajuel)
a few pebbles of quartz. I nowhere observed mica in this formation, and
rarely hornblende; where the latter mineral did occur, I was generally in
doubt whether the mass really belonged to this formation, or was of
intrusive origin. Calcareous spar occasionally occurs in small cavities;
and nests and layers of epidote are common. In some few places in the
finer-grained varieties (for instance, at Quillota), there were short,
interrupted layers of earthy feldspar, which could be traced, exactly as at
Port Desire, passing into large crystals of feldspar: I doubt, however,
whether in this instance the layers had ever been separately deposited as
tufaceous sediment.
All the varieties of porphyritic conglomerates and breccias pass into each
other, and by innumerable gradations into porphyries no longer retaining
the least trace of mechanical origin: the transition appears to have been
effected much more easily in the finer-grained, than in the coarser-grained
varieties. In one instance, near Cauquenes, I noticed that a porphyritic
conglomerate assumed a spheroidal structure, and tended to become columnar.
Besides the porphyritic conglomerates and the perfectly characterised
porphyries, of metamorphic origin, there are other porphyries, which,
though differing not at all or only slightly in composition, certainly have
had a different origin: these consist of pink or purple claystone
porphyries, sometimes including grains of quartz,--of greenstone porphyry,
and of other dusky rocks, all generally porphyritic with fine, large,
tabular, opaque crystals, often placed crosswise, of feldspar cleaving like
albite (judging from several measurements), and often amygdaloidal with
silex, agate, carbonate of lime, green and brown bole. (This bole is a very
common mineral in the amygdaloidal rocks; it is generally of a greenish-
brown colour, with a radiating structure; externally it is black with an
almost metallic lustre, but often coated by a bright green film. It is soft
and can be scratched by a quill; under the blowpipe swells greatly and
becomes scaly, then fuses easily into a black magnetic bead. This substance
is evidently similar to that which often occurs in submarine volcanic
rocks. An examination of some very curious specimens of a fine porphyry
(from Jajuel) leads me to suspect that some of these amygdaloidal balls,
instead of having been deposited in pre-existing air-vesicles, are of
concretionary origin; for in these specimens, some of the pea-shaped little
masses (often externally marked with minute pits) are formed of a mixture
of green earth with stony matter, like the basis of the porphyry, including
minute imperfect crystals of feldspar; and these pea-shaped little masses
are themselves amygdaloidal with minute spheres of the green earth, each
enveloped by a film of white, apparently feldspathic, earthy matter: so
that the porphyry is doubly amygdaloidal. It should not, however, be
overlooked, that all the strata here have undergone metamorphic action,
which may have caused crystals of feldspar to appear, and other changes to
be effected, in the originally simple amygdaloidal balls. Mr. J.D. Dana, in
an excellent paper on Trap-rocks "Edinburgh New Philosophical Journal"
volume 41 page 198, has argued with great force, that all amygdaloidal
minerals have been deposited by aqueous infiltration. I may take this
opportunity of alluding to a curious case, described in my work on
"Volcanic Islands," of an amygdaloid with many of its cells only half
filled up with a mesotypic mineral. M. Rose has described an amygdaloid,
brought by Dr. Meyen "Reise um Erde" Th. 1. s. 316, from Chile, as
consisting of crystallised quartz, with crystals of stilbite within, and
lined externally by green earth.) These several porphyritic and
amygdaloidal varieties never show any signs of passing into masses of
sedimentary origin: they occur both in great and small intrusive masses,
and likewise in strata alternating with those of the porphyritic
conglomerate, and with the planes of junction often quite distinct, yet not
seldom blended together. In some of these intrusive masses, the porphyries
exhibit, more or less plainly, a brecciated structure, like that often seen
in volcanic masses. These brecciated porphyries could generally be
distinguished at once from the metamorphosed, porphyritic breccia-
conglomerates, by all the fragments being angular and being formed of the
same variety, and by the absence of every trace of aqueous deposition. One
of the porphyries above specified, namely, the greenstone porphyry with
large tabular crystals of albite, is particularly abundant, and in some
parts of the Cordillera (as near St. Jago) seemed more common even than the
purplish porphyritic conglomerate. Numerous dikes likewise consist of this
greenstone porphyry; others are formed of various fine-grained trappean
rocks; but very few of claystone porphyry: I saw no true basaltic dikes.
In several places in the lower part of the series, but not everywhere,
thick masses of a highly feldspathic, often porphyritic, slaty rock occur
interstratified with the porphyritic conglomerate; I believe in one or two
cases blackish limestone has been found in a similar position. The
feldspathic rock is of a pale grey or greenish colour; it is easily
fusible; where porphyritic, the crystals of feldspar are generally small
and vitreous: it is distinctly laminated, and sometimes includes parallel
layers of epidote (This mineral is extremely common in all the formations
of Chile; in the gneiss near Valparaiso and in the granitic veins crossing
it, in the injected greenstone crowning the C. of Quillota, in some
granitic porphyries, in the porphyritic conglomerate, and in the
feldspathic clay-slates.); the lamination appears to be distinct from
stratification. Occasionally this rock is somewhat curious; and at one
spot, namely, at the C. of Quillota, it had a brecciated structure. Near
the mines of Jajuel, in a thick stratum of this feldspathic, porphyritic
slate, there was a layer of hard, blackish, siliceous, infusible, compact
clay-slate, such as I saw nowhere else; at the same place I was able to
follow for a considerable distance the junction between the slate and the
conformably underlying porphyritic conglomerate, and they certainly passed
gradually into each other. Wherever these slaty feldspathic rocks abound,
greenstone seems common; at the C. of Quillota a bed of well-crystallised
greenstone lay conformably in the midst of the feldspathic slate, with the
upper and lower junctions passing insensibly into it. From this point, and
from the frequently porphyritic condition of the slate, I should perhaps
have considered this rock as an erupted one (like certain laminated
feldspathic lavas in the trachytic series), had I not seen in Tierra del
Fuego how readily true clay-slate becomes feldspathic and porphyritic, and
had I not seen at Jajuel the included layer of black, siliceous clay-slate,
which no one could have thought of igneous origin. The gentle passage of
the feldspathic slate, at Jajuel, into the porphyritic conglomerate, which
is certainly of aqueous origin, should also be taken in account.
The alternating strata of porphyries and porphyritic conglomerate, and with
the occasionally included beds of feldspathic slate, together make a grand
formation; in several places within the Cordillera, I estimated its
thickness at from six to seven thousand feet. It extends for many hundred
miles, forming the western flank of the Chilean Cordillera; and even at
Iquique in Peru, 850 miles north of the southernmost point examined by me
in Chile, the coast-escarpment which rises to a height of between two and
three thousand feet is thus composed. In several parts of Northern Chile
this formation extends much further towards the Pacific, over the granitic
and metamorphic lower rocks, than it does in Central Chile; but the main
Cordillera may be considered as its central line, and its breadth in an
east and west direction is never great. At first the origin of this thick,
massive, long but narrow formation, appeared to me very anomalous: whence
were derived, and how were dispersed the innumerable fragments, often of
large size, sometimes angular and sometimes rounded, and almost invariably
composed of porphyritic rocks? Seeing that the interstratified porphyries
are never vesicular and often not even amygdaloidal, we must conclude that
the pile was formed in deep water; how then came so many fragments to be
well rounded and so many to remain angular, sometimes the two kinds being
equally mingled, sometimes one and sometimes the other preponderating? That
the claystone, greenstone, and other porphyries and amygdaloids, which lie
CONFORMABLY between the beds of conglomerate, are ancient submarine lavas,
I think there can be no doubt; and I believe we must look to the craters
whence these streams were erupted, as the source of the breccia-
conglomerate; after the great explosion, we may fairly imagine that the
water in the heated and scarcely quiescent crater would remain for a
considerable time sufficiently agitated to triturate and round the loose
fragments, few or many in number, would be shot forth at the next eruption,
associated with few or many angular fragments, according to the strength of
the explosion. (This certainly seems to have taken place in some recent
volcanic archipelagos, as at the Galapagos, where numerous craters are
exclusively formed of tuff and fragments of lava.) The porphyritic
conglomerate being purple or reddish, even when alternating with dusty-
coloured or bright green porphyries and amygdaloids, is probably an
analogous circumstance to the scoriae of the blackish basalts being often
bright red. The ancient submarine orifices whence the porphyries and their
fragments were ejected having been arranged in a band, like most still
active volcanoes, accounts for the thickness, the narrowness, and linear
extension of this formation.
This whole great pile of rock has suffered much metamorphic action, as is
very obvious in the gradual formation and appearance of the crystals of
albitic feldspar and of epidote--in the bending together of the fragments--
in the appearance of a laminated structure in the feldspathic slate--and,
lastly, in the disappearance of the planes of stratification, which could
sometimes be seen on the same mountain quite distinct in the upper part,
less and less plain on the flanks, and quite obliterated at the base.
Partly owing to this metamorphic action, and partly to the close
relationship in origin, I have seen fragments of porphyries--taken from a
metamorphosed conglomerate--from a neighbouring stream of lava--from the
nucleus or centre (as it appeared to me) of the whole submarine volcano--
and lastly from an intrusive mass of quite subsequent origin, all of which
were absolutely undistinguishable in external characters.
One other rock, of plutonic origin, and highly important in the history of
the Cordillera, from having been injected in most of the great axes of
elevation, and from having apparently been instrumental in metamorphosing
the superincumbent strata, may be conveniently described in this
preliminary discussion. It has been called by some authors ANDESITE: it
mainly consists of well-crystallised white albite (as determined with the
goniometer in numerous specimens both by Professor Miller and myself), of
less perfectly crystallised green hornblende, often associated with much
mica, with chlorite and epidote, and occasionally with a few grains of
quartz: in one instance in Northern Chile, I found crystals of orthitic or
potash feldspar, mingled with those of albite. (I here, and elsewhere, call
by this name, those feldspathic minerals which cleave like albite: but it
now appears ("Edinburgh New Philosophical Journal" volume 24 page 181) that
Abich has analysed a mineral from the Cordillera, associated with
hornblende and quartz (probably the same rock with that here under
discussion), which cleaves like albite, but which is a new and distinct
kind, called by him ANDESINE. It is allied to leucite, with the greater
proportion of its potash replaced by lime and soda. This mineral seems
scarcely distinguishable from albite, except by analysis.) Where the mica
and quartz are abundant, the rock cannot be distinguished from granite; and
it may be called andesitic granite. Where these two minerals are quite
absent, and when, as often then happens, the crystals of albite are
imperfect and blend together, the rock may be called andesitic porphyry,
which bears nearly the same relation to andesitic granite that euritic
porphyry does to common granite. These andesitic rocks form mountain masses
of a white colour, which, in their general outline and appearance--in their
joints--in their occasionally including dark-coloured, angular fragments,
apparently of some pre-existing rock--and in the great dikes branching from
them into the superincumbent strata, manifest a close and striking
resemblance to masses of common granite and syenite: I never, however, saw
in these andesitic rocks, those granitic veins of segregation which are so
common in true granites. We have seen that andesite occurs in three places
in Tierra del Fuego; in Chile, from S. Fernando to Copiapo, a distance of
450 miles, I found it under most of the axes of elevation; in a collection
of specimens from the Cordillera of Lima in Peru, I immediately recognised
it; and Erman states that it occurs in Eastern Kamtschatka. ("Geographical
Journal" volume 9 page 510.) From its wide range, and from the important
part it has played in the history of the Cordillera, I think this rock has
well deserved its distinct name of Andesite.
The few still active volcanoes in Chile are confined to the central and
loftiest ranges of the Cordillera; and volcanic matter, such as appears to
have been of subaerial eruption, is everywhere rare. According to Meyen,
there is a hill of pumice high up the valley of the Maypu, and likewise a
trachytic formation at Colina, a village situated north of St. Jago.
("Reise um Erde" Th. 1 ss. 338 and 362.) Close to this latter city, there
are two hills formed of a pale feldspathic porphyry, remarkable from being
doubly columnar, great cylindrical columns being subdivided into smaller
four- or five-sided ones; and a third hillock (Cerro Blanco) is formed of a
fragmentary mass of rock, which I believed to be of volcanic origin,
intermediate in character between the above feldspathic porphyry and common
trachyte, and containing needles of hornblende and granular oxide of iron.
Near the Baths of Cauquenes, between two short parallel lines of elevation,
where they are intersected by the valley, there is a small, though distinct
volcanic district; the rock is a dark grey (andesitic) trachyte, which
fuses into a greenish-grey bead, and is formed of long crystals of
fractured glassy albite (judging from one measurement) mingled with well-
formed crystals, often twin, of augite. The whole mass is vesicular, but
the surface is darker coloured and much more vesicular than any other part.
This trachyte forms a cliff-bounded, horizontal, narrow strip on the steep
southern side of the valley, at the height of four or five hundred feet
above the river-bed; judging from an apparently corresponding line of cliff
on the northern side, the valley must once have been filled up to this
height by a field of lava. On the summit of a lofty mountain some leagues
higher up this same valley of the Cachapual, I found columnar pitchstone
porphyritic with feldspar; I do not suppose this rock to be of volcanic
origin, and only mention it here, from its being intersected by masses and
dikes of a VESICULAR rock, approaching in character to trachyte; in no
other part of Chile did I observe vesicular or amygdaloidal dikes, though
these are so common in ordinary volcanic districts.
PASSAGE OF THE ANDES BY THE PORTILLO OR PEQUENES PASS.
Although I crossed the Cordillera only once by this pass, and only once by
that of the Cumbre or Uspallata (presently to be described), riding slowly
and halting occasionally to ascend the mountains, there are many
circumstances favourable to obtaining a more faithful sketch of their
structure than would at first be thought possible from so short an
examination. The mountains are steep and absolutely bare of vegetation; the
atmosphere is resplendently clear; the stratification distinct; and the
rocks brightly and variously coloured: some of the natural sections might
be truly compared for distinctness to those coloured ones in geological
works. Considering how little is known of the structure of this gigantic
range, to which I particularly attended, most travellers having collected
only specimens of the rocks, I think my sketch-sections, though necessarily
imperfect, possess some interest. Section 1/1 in Plate 1 which I will now
describe in detail, is on a horizontal scale of a third of an inch to a
nautical mile, and on a vertical scale of one inch to a mile (or 6,000
feet). The width of the range (excluding a few outlying hillocks), from the
plain on which St. Jago the capital of Chile stands, to the Pampas, is
sixty miles, as far as I can judge from the maps, which differ from each
other and are all EXCEEDINGLY imperfect. The St. Jago plain at the mouth of
the Maypu, I estimate from adjoining known points at 2,300 feet, and the
Pampas at 3,500 feet, both above the level of the sea. The height of the
Pequenes line, according to Dr. Gillies, is 13,210 feet ("Journal of
Natural and Geographical Science" August 1830.); and that of the Portillo
line (both in the gaps where the road crosses them) is 14,345 feet; the
lowest part of the intermediate valley of Tenuyan is 7,530 feet--all above
the level of the sea.
The Cordillera here, and indeed I believe throughout Chile, consist of
several parallel, anticlinal and uniclinal mountain-lines, ranging north,
or north with a little westing, and south. Some exterior and much lower
ridges often vary considerably from this course, projecting like oblique
spurs from the main ranges: in the district towards the Pacific, the
mountains, as before remarked, extend in various directions, even east and
west. In the main exterior lines, the strata, as also before remarked, are
seldom inclined at a high angle; but in the central lofty ridges they are
almost always highly inclined, broken by many great faults, and often
vertical. As far as I could judge, few of the ranges are of great length:
and in the central parts of the Cordillera, I was frequently able to follow
with my eye a ridge gradually becoming higher and higher, as the
stratification increased in inclination, from one end where its height was
trifling and its strata gently inclined to the other end where vertical
strata formed snow-clad pinnacles. Even outside the main Cordillera, near
the baths of Cauquenes, I observed one such case, where a north and south
ridge had its strata in the valley inclined at 37 degrees, and less than a
mile south of it at 67 degrees: another parallel and similarly inclined
ridge rose at the distance of about five miles, into a lofty mountain with
absolutely vertical strata. Within the Cordillera, the height of the ridges
and the inclination of the strata often became doubled and trebled in much
shorter distances than five miles; this peculiar form of upheaval probably
indicates that the stratified crust was thin, and hence yielded to the
underlying intrusive masses unequally, at certain points on the lines of
fissure.
The valleys, by which the Cordillera are drained, follow the anticlinal or
rarely synclinal troughs, which deviate most from the usual north and south
course; or still more commonly those lines of faults or of unequal
curvature (that is, lines with the strata on both hands dipping in the same
direction, but at a somewhat different angle) which deviate most from a
northerly course. Occasionally the torrents run for some distance in the
north and south valleys, and then recover their eastern or western course
by bursting through the ranges at those points where the strata have been
least inclined and the height consequently is less. Hence the valleys,
along which the roads run, are generally zigzag; and, in drawing an east
and west section, it is necessary to contract greatly that which is
actually seen on the road.
Commencing at the western end of Section 1/1 where the R. Maypu debouches
on the plain of St. Jago, we immediately enter on the porphyritic
conglomerate formation, and in the midst of it find some hummocks [A] of
granite and syenite, which probably (for I neglected to collect specimens)
belong to the andesitic class. These are succeeded by some rugged hills [B]
of dark-green, crystalline, feldspathic and in some parts slaty rocks,
which I believe belong to the altered clay-slate formation. From this
point, great mountains of purplish and greenish, generally thinly
stratified, highly porphyritic conglomerates, including many strata of
amygdaloidal and greenstone porphyries, extend up the valley to the
junction of the rivers Yeso and Volcan. As the valley here runs in a very
southerly course, the width of the porphyritic conglomerate formation is
quite conjectural; and from the same cause, I was unable to make out much
about the stratification. In most of the exterior mountains the dip was
gentle and directed inwards; and at only one spot I observed an inclination
as high as 50 degrees. Near the junction of the R. Colorado with the main
stream, there is a hill of whitish, brecciated, partially decomposed
feldspathic porphyry, having a volcanic aspect but not being really of that
nature: at Tolla, however, in this valley, Dr. Meyen met with a hill of
pumice containing mica. ("Reise um Erde" Th.1 ss. 338, 341.) At the
junction of the Yeso and Volcan [D] there is an extensive mass, in white
conical hillocks, of andesite, containing some mica, and passing either
into andesitic granite, or into a spotted, semi-granular mixture of albitic
(?) feldspar and hornblende: in the midst of this formation Dr. Meyen found
true trachyte. The andesite is covered by strata of dark-coloured,
crystalline, obscurely porphyritic rocks, and above them by the ordinary
porphyritic conglomerates,--the strata all dipping away at a small angle
from the underlying mass. The surrounding lofty mountains appear to be
entirely composed of the porphyritic conglomerate, and I estimated its
thickness here at between six and seven thousand feet.
Beyond the junction of the Yeso and Volcan, the porphyritic strata appear
to dip towards the hillocks of andesite at an angle of 40 degrees; but at
some distant points on the same ridge they are bent up and vertical.
Following the valley of the Yeso, trending N.E. (and therefore still
unfavourable for our transverse section), the same porphyritic conglomerate
formation is prolonged to near the Cuestadel Indio, situated at the western
end of the basin (like a drained lake) of Yeso. Some way before arriving at
this point, distant lofty pinnacles capped by coloured strata belonging to
the great gypseous formation could first be seen. From the summit of the
Cuesta, looking southward, there is a magnificent sectional view of a
mountain-mass, at least 2,000 feet in thickness [E], of fine andesite
granite (containing much black mica, a little chlorite and quartz), which
sends great white dikes far into the superincumbent, dark-coloured,
porphyritic conglomerates. At the line of junction the two formations are
wonderfully interlaced together: in the lower part of the porphyritic
conglomerate, the stratification has been quite obliterated, whilst in the
upper part it is very distinct, the beds composing the crests of the
surrounding mountains being inclined at angles of between 70 and 80
degrees, and some being even vertical. On the northern side of the valley,
there is a great corresponding mass of andesitic granite, which is encased
by porphyritic conglomerate, dipping both on the western and eastern sides,
at about 80 degrees to west, but on the eastern side with the tips of the
strata bent in such a manner, as to render it probable that the whole mass
has been on that side thrown over and inverted.
In the valley basin of the Yeso, which I estimated at 7,000 feet above the
level of the sea, we first reach at [F] the gypseous formation. Its
thickness is very great. It consists in most parts of snow-white, hard,
compact gypsum, which breaks with a saccharine fracture, having translucent
edges; under the blowpipe gives out much vapour; it frequently includes
nests and exceedingly thin layers of crystallised, blackish carbonate of
lime. Large, irregularly shaped concretions (externally still exhibiting
lines of aqueous deposition) of blackish-grey, but sometimes white,
coarsely and brilliantly crystallised, hard anhydrite, abound within the
common gypsum. Hillocks, formed of the hardest and purest varieties of the
white gypsum, stand up above the surrounding parts, and have their surfaces
cracked and marked, just like newly baked bread. There is much pale brown,
soft argillaceous gypsum; and there were some intercalated green beds which
I had not time to reach. I saw only one fragment of selenite or transparent
gypsum, and that perhaps may have come from some subsequently formed vein.
From the mineralogical characters here given, it is probable that these
gypseous beds have undergone some metamorphic action. The strata are much
hidden by detritus, but they appeared in most parts to be highly inclined;
and in an adjoining lofty pinnacle they could be distinctly seen bending
up, and becoming vertical, conformably with the underlying porphyritic
conglomerate. In very many parts of the great mountain-face [F], composed
of thin gypseous beds, there were innumerable masses, irregularly shaped
and not like dikes, yet with well-defined edges, of an imperfectly
granular, pale greenish, or yellowish-white rock, essentially composed of
feldspar, with a little chlorite or hornblende, epidote, iron-pyrites, and
ferruginous powder: I believe that these curious trappean masses have been
injected from the not far distant mountain-mass [E] of andesite whilst
still fluid, and that owing to the softness of the gypseous strata they
have not acquired the ordinary forms of dikes. Subsequently to the
injection of these feldspathic rocks, a great dislocation has taken place;
and the much shattered gypseous strata here overlie a hillock [G], composed
of vertical strata of impure limestone and of black highly calcareous shale
including threads of gypsum: these rocks, as we shall presently see, belong
to the upper parts of the gypseous series, and hence must here have been
thrown down by a vast fault.
Proceeding up the valley-basin of the Yeso, and taking our section
sometimes on one hand and sometimes on the other, we come to a great hill
of stratified porphyritic conglomerate [H] dipping at 45 degrees to the
west; and a few hundred yards farther on, we have a bed between three or
four hundred feet thick of gypsum [I] dipping eastward at a very high
angle: here then we have a fault and anticlinal axis. On the opposite side
of the valley, a vertical mass of red conglomerate, conformably underlying
the gypsum, appears gradually to lose its stratification and passes into a
mountain of porphyry. The gypsum [I] is covered by a bed [K], at least
1,000 feet in thickness, of a purplish-red, compact, heavy, fine-grained
sandstone or mudstone, which fuses easily into a white enamel, and is seen
under a lens to contain triturated crystals. This is succeeded by a bed
[L], 1,000 feet thick (I believe I understate the thickness) of gypsum,
exactly like the beds before described; and this again is capped by another
great bed [M] of purplish-red sandstone. All these strata dip eastward; but
the inclination becomes less and less, as we leave the first and almost
vertical bed [I] of gypsum.
Leaving the basin-plain of Yeso, the road rapidly ascends, passing by
mountains composed of the gypseous and associated beds, with their
stratification greatly disturbed and therefore not easily intelligible:
hence this part of the section has been left uncoloured. Shortly before
reaching the great Pequenes ridge, the lowest stratum visible [N] is a red
sandstone or mudstone, capped by a vast thickness of black, compact,
calcareous, shaly rock [O], which has been thrown into four lofty, though
small ridges: looking northward, the strata in these ridges are seen
gradually to rise in inclination, becoming in some distant pinnacles
absolutely vertical.
The ridge of Pequenes, which divides the waters flowing into the Pacific
and Atlantic Oceans, extends in a nearly N.N.W. and S.S.E. line; its strata
dip eastward at an angle of between 30 and 45 degrees, but in the higher
peaks bending up and becoming almost vertical. Where the road crosses this
range, the height is 13,210 feet above the sea-level, and I estimated the
neighbouring pinnacles at from fourteen to fifteen thousand feet. The
lowest stratum visible in this ridge is a red stratified sandstone [P]; on
it are superimposed two great masses [Q and S] of black, hard, compact,
even having a conchoidal fracture, calcareous, more or less laminated
shale, passing into limestone: this rock contains organic remains,
presently to be enumerated. The compacter varieties fuse easily in a white
glass; and this I may add is a very general character with all the
sedimentary beds in the Cordillera: although this rock when broken is
generally quite black, it everywhere weathers into an ash-grey tint.
Between these two great masses [Q and S], a bed [R] of gypsum is
interposed, about three hundred feet in thickness, and having the same
characters as heretofore described. I estimated the total thickness of
these three beds [Q, R, S] at nearly three thousand feet; and to this must
be added, as will be immediately seen, a great overlying mass of red
sandstone.
In descending the eastern slope of this great central range, the strata,
which in the upper part dip eastward at about an angle of 40 degrees,
become more and more curved, till they are nearly vertical; and a little
further onwards there is seen on the further side of a ravine, a thick mass
of strata of bright red sandstone [T], with their upper extremities
slightly curved, showing that they were once conformably prolonged over the
beds [S]: on the southern and opposite side of the road, this red sandstone
and the underlying black shaly rocks stand vertical, and in actual
juxtaposition. Continuing to descend, we come to a synclinal valley filled
with rubbish, beyond which we have the red sandstone [T2] corresponding
with [T], and now dipping, as is seen both north and south of the road, at
45 degrees to the west; and under it, the beds [S2, R2, Q2, and I believe
P2] in corresponding order and of similar composition, with those on the
western flank of the Pequenes range, but dipping westward. Close to the
synclinal valley the dip of these strata is 45 degrees, but at the eastern
or farther end of the series it increases to 60 degrees. Here the great
gypseous formation abruptly terminates, and is succeeded eastward by a pile
of more modern strata. Considering how violently these central ranges have
been dislocated, and how very numerous dikes are in the exterior and lower
parts of the Cordillera, it is remarkable that I did not here notice a
single dike. The prevailing rock in this neighbourhood is the black,
calcareous, compact shale, whilst in the valley-basin of the Yeso the
purplish red sandstone or mudstone predominates,--both being associated
with gypseous strata of exactly the same nature. It would be very difficult
to ascertain the relative superposition of these several masses, for we
shall afterwards see in the Cumbre Pass that the gypseous and intercalated
beds are lens-shaped, and that they thin out, even where very thick, and
disappear in short horizontal distances: it is quite possible that the
black shales and red sandstones may be contemporaneous, but it is more
probable that the former compose the uppermost parts of the series.
The fossils above alluded to in the black calcareous shales are few in
number, and are in an imperfect condition; they consist, as named for me by
M. d'Orbigny, of:--
1. Ammonite, indeterminable, near to A. recticostatus, d'Orbigny, "Pal.
Franc." (Neocomian formation).
2. Gryphaea, near to G. Couloni (Neocomian formations of France and
Neufchatel).
3. Natica, indeterminable.
4. Cyprina rostrata, d'Orbigny, "Pal. Franc." (Neocomian formation).
5. Rostellaria angulosa (?), d'Orbigny, "Pal. de l'Amer. Mer."
6. Terebratula (?).
Some of the fragments of Ammonites were as thick as a man's arm: the
Gryphaea is much the most abundant shell. These fossils M. d'Orbigny
considers as belonging to the Neocomian stage of the Cretaceous system. Dr.
Meyen, who ascended the valley of the Rio Volcan, a branch of the Yeso,
found a nearly similar, but apparently more calcareous formation, with much
gypsum, and no doubt the equivalent of that here described ("Reise um Erde"
etc. Th. 1 s. 355.): the beds were vertical, and were prolonged up to the
limits of perpetual snow; at the height of 9,000 feet above the sea, they
abounded with fossils, consisting, according to Von Buch ("Descript. Phys.
des Iles Canaries" page 471.), of:--
1. Exogyra (Gryphaea) Couloni, absolutely identical with specimens from the
Jura and South of France.
2. Trigonia costata, identical with those found in the upper Jurassic beds
at Hildesheim.
3. Pecten striatus, identical with those found in the upper Jurassic beds
at Hildesheim.
4. Cucullaea, corresponding in form to C. longirostris, so frequent in the
upper Jurassic beds of Westphalia.
5. Ammonites resembling A. biplex.
Von Buch concludes that this formation is intermediate between the
limestone of the Jura and the chalk, and that it is analogous with the
uppermost Jurassic beds forming the plains of Switzerland. Hence M.
D'Orbigny and Von Buch, under different terms, compare these fossils to
those from the same late stage in the secondary formations of Europe.
Some of the fossils which I collected were found a good way down the
western slope of the main ridge, and hence must originally have been
covered up by a great thickness of the black shaly rock, independently of
the now denuded, thick, overlying masses of red sandstone. I neglected at
the time to estimate how many hundred or rather thousand feet thick the
superincumbent strata must have been: and I will not now attempt to do so.
This, however, would have been a highly interesting point, as indicative of
a great amount of subsidence, of which we shall hereafter find in other
parts of the Cordillera analogous evidence during this same period. The
altitude of the Peuquenes Range, considering its not great antiquity, is
very remarkable; many of the fossils were embedded at the height of 13,210
feet, and the same beds are prolonged up to at least from fourteen to
fifteen thousand feet above the level of the sea.
THE PORTILLO OR EASTERN CHAIN.
The valley of Tenuyan, separating the Peuquenes and Portillo lines, is, as
estimated by Dr. Gillies and myself, about twenty miles in width; the
lowest part, where the road crosses the river, being 7,500 feet above the
sea-level. The pass on the Portillo line is 14,365 feet high (1,100 feet
higher than that on the Peuquenes), and the neighbouring pinnacles must, I
conceive, rise to nearly 16,000 feet above the sea. The river draining the
intermediate valley of Tenuyan, passes through the Portillo line. To return
to our section:--shortly after leaving the lower beds [P2] of the gypseous
formation, we come to grand masses of a coarse, red conglomerate [V],
totally unlike any strata hitherto seen in the Cordillera. This
conglomerate is distinctly stratified, some of the beds being well defined
by the greater size of the pebbles: the cement is calcareous and sometimes
crystalline, though the mass shows no signs of having been metamorphosed.
The included pebbles are either perfectly or only partially rounded: they
consist of purplish sandstones, of various porphyries, of brownish
limestone, of black calcareous, compact shale precisely like that in situ
in the Peuquenes range, and CONTAINING SOME OF THE SAME FOSSIL SHELLS; also
very many pebbles of quartz, some of micaceous schist, and numerous,
broken, rounded crystals of a reddish orthitic or potash feldspar (as
determined by Professor Miller), and these from their size must have been
derived from a coarse-grained rock, probably granite. From this feldspar
being orthitic, and even from its external appearance, I venture positively
to affirm that it has not been derived from the rocks of the western
ranges; but, on the other hand, it may well have come, together with the
quartz and metamorphic schists, from the eastern or Portillo line, for this
line mainly consists of coarse orthitic granite. The pebbles of the
fossiliferous slate and of the purple sandstone, certainly have been
derived from the Peuquenes or western ranges.
The road crosses the valley of Tenuyan in a nearly east and west line, and
for several miles we have on both hands the conglomerate, everywhere
dipping west and forming separate great mountains. The strata, where first
met with, after leaving the gypseous formation, are inclined westward at an
angle of only 20 degrees, which further on increases to about 45 degrees.
The gypseous strata, as we have seen, are also inclined westward: hence,
when looking from the eastern side of the valley towards the Peuquenes
range, a most deceptive appearance is presented, as if the newer beds of
conglomerate dipped directly under the much older beds of the gypseous
formation. In the middle of the valley, a bold mountain of unstratified
lilac-coloured porphyry (with crystals of hornblende) projects; and further
on, a little south of the road, there is another mountain, with its strata
inclined at a small angle eastwards, which in its general aspect and
colour, resembles the porphyritic conglomerate formation, so rare on this
side of the Peuquenes line and so grandly developed throughout the western
ranges.
The conglomerate is of great thickness: I do not suppose that the strata
forming the separate mountain-masses [V,V,V] have ever been prolonged over
each other, but that one mass has been broken up by several, distinct,
parallel, uniclinal lines of elevation. Judging therefore of the thickness
of the conglomerate, as seen in the separate mountain-masses, I estimated
it at least from one thousand five hundred to two thousand feet. The lower
beds rest conformably on some singularly coloured, soft strata [W], which I
could not reach to examine; and these again rest conformably on a thick
mass of micaceous, thinly laminated, siliceous sandstone [X], associated
with a little black clay-slate. These lower beds are traversed by several
dikes of decomposing porphyry. The laminated sandstone is directly
superimposed on the vast masses of granite [Y,Y] which mainly compose the
Portillo range. The line of junction between this latter rock, which is of
a bright red colour, and the whitish sandstone was beautifully distinct;
the sandstone being penetrated by numerous, great, tortuous dikes branching
from the granite, and having been converted into a granular quartz rock
(singularly like that of the Falkland Islands), containing specks of an
ochrey powder, and black crystalline atoms, apparently of imperfect mica.
The quartzose strata in one spot were folded into a regular dome.
The granite which composes the magnificent bare pinnacles and the steep
western flank of the Portillo chain, is of a brick-red colour, coarsely
crystallised, and composed of orthitic or potash feldspar, quartz, and
imperfect mica in small quantity, sometimes passing into chlorite. These
minerals occasionally assume a laminar or foliated arrangement. The fact of
the feldspar being orthitic in this range, is very remarkable, considering
how rare, or rather, as I believe, entirely absent, this mineral is
throughout the western ranges, in which soda-feldspar, or at least a
variety cleaving like albite, is so extremely abundant. In one spot on the
western flank, and on the eastern flank near Los Manantiales and near the
crest, I noticed some great masses of a whitish granite, parts of it fine-
grained, and parts containing large crystals of feldspar; I neglected to
collect specimens, so I do not know whether this feldspar is also orthitic,
though I am inclined to think so from its general appearance. I saw also
some syenite and one mass which resembled andesite, but of which I likewise
neglected to collect specimens. From the manner in which the whitish
granites formed separate mountain-masses in the midst of the brick-red
variety, and from one such mass near the crest being traversed by numerous
veins of flesh-coloured and greenish eurite (into which I occasionally
observed the brick-red granite insensibly passing), I conclude that the
white granites probably belong to an older formation, almost overwhelmed
and penetrated by the red granite.
On the crest I saw also, at a short distance, some coloured stratified
beds, apparently like those [W] at the western base, but was prevented
examining them by a snowstorm: Mr. Caldcleugh, however, collected here
specimens of ribboned jasper, magnesian limestone, and other minerals.
("Travels" etc. volume 1 page 308.) A little way down the eastern slope a
few fragments of quartz and mica-slate are met with; but the great
formation of this latter rock [Z], which covers up much of the eastern
flank and base of the Portillo range, cannot be conveniently examined until
much lower down at a place called Mal Paso. The mica-schist here consists
of thick layers of quartz, with intervening folia of finely-scaly mica,
often passing into a substance like black glossy clay-slate: in one spot,
the layers of the quartz having disappeared, the whole mass became
converted into glossy clay-slate. Where the folia were best defined, they
were inclined at a high angle westward, that is, towards the range. The
line of junction between the dark mica-slate and the coarse red granite was
most clearly distinguishable from a vast distance: the granite sent many
small veins into the mica-slate, and included some angular fragments of it.
As the sandstone on the western base has been converted by the red granite
into a granular quartz-rock, so this great formation of mica-schist may
possibly have been metamorphosed at the same time and by the same means;
but I think it more probable, considering its more perfect metamorphic
character and its well-pronounced foliation, that it belongs to an anterior
epoch, connected with the white granites: I am the more inclined to this
view, from having found at the foot of the range the mica-schist
surrounding a hummock [Y2], exclusively composed of white granite. Near Los
Arenales, the mountains on all sides are composed of the mica-slate; and
looking backwards from this point up to the bare gigantic peaks above, the
view was eminently interesting. The colours of the red granite and the
black mica-slate are so distinct, that with a bright light these rocks
could be readily distinguished even from the Pampas, at a level of at least
9,000 feet below. The red granite, from being divided by parallel joints,
has weathered into sharp pinnacles, on some of which, even on some of the
loftiest, little caps of mica-schist could be clearly seen: here and there
isolated patches of this rock adhered to the mountain-flanks, and these
often corresponded in height and position on the opposite sides of the
immense valleys. Lower down the schist prevailed more and more, with only a
few quite small points of granite projecting through. Looking at the entire
eastern face of the Portillo range, the red colour far exceeds in area the
black; yet it was scarcely possible to doubt that the granite had once been
almost wholly encased by the mica-schist.
At Los Arenales, low down on the eastern flank, the mica-slate is traversed
by several closely adjoining, broad dikes, parallel to each other and to
the foliation of the schist. The dikes are formed of three different
varieties of rock, of which a pale brown feldspathic porphyry with grains
of quartz was much the most abundant. These dikes with their granules of
quartz, as well as the mica-schist itself, strikingly resemble the rocks of
the Chonos Archipelago. At a height of about twelve hundred feet above the
dikes, and perhaps connected with them, there is a range of cliffs formed
of successive lava-streams [AA], between three and four hundred feet in
thickness, and in places finely columnar. The lava consists of dark-
greyish, harsh rocks, intermediate in character between trachyte and
basalt, containing glassy feldspar, olivine, and a little mica, and
sometimes amygdaloidal with zeolite: the basis is either quite compact, or
crenulated with air-vesicles arranged in laminae. The streams are separated
from each other by beds of fragmentary brown scoriae, firmly cemented
together, and including a few well-rounded pebbles of lava. From their
general appearance, I suspect that these lava-streams flowed at an ancient
period under the pressure of the sea, when the Atlantic covered the Pampas
and washed the eastern foot of the Cordillera. (This conclusion might,
perhaps, even have been anticipated, from the general rarity of volcanic
action, except near the sea or large bodies of water. Conformably with this
rule, at the present day, there are no active volcanoes on this eastern
side of the Cordillera; nor are severe earthquakes experienced here.) On
the opposite and northern side of the valley there is another line of lava-
cliffs at a corresponding height; the valley between being of considerable
breadth, and as nearly as I could estimate 1,500 feet in depth. This field
of lava is confined on both sides by the mountains of mica-schist, and
slopes down rapidly but irregularly to the edge of the Pampas, where,
having a thickness of about two hundred feet, it terminates against a
little range of claystone porphyry. The valley in this lower part expands
into a bay-like, gentle slope, bordered by the cliffs of lava, which must
certainly once have extended across this wide expanse. The inclination of
the streams from Los Arenales to the mouth of the valley is so great, that
at the time (though ignorant of M. Elie de Beaumont's researches on the
extremely small slope over which lava can flow, and yet retain a compact
structure and considerable thickness) I concluded that they must
subsequently to their flowing have been upheaved and tilted from the
mountains; of this conclusion I can now entertain not the smallest doubt.
At the mouth of the valley, within the cliffs of the above lava-field,
there are remnants, in the form of separate small hillocks and of lines of
low cliffs, of a considerable deposit of compact white tuff (quarried for
filtering-stones), composed of broken pumice, volcanic crystals, scales of
mica, and fragments of lava. This mass has suffered much denudation; and
the hard mica-schist has been deeply worn, since the period of its
deposition; and this period must have been subsequent to the denudation of
the basaltic lava-streams, as attested by their encircling cliffs standing
at a higher level. At the present day, under the existing arid climate,
ages might roll past without a square yard of rock of any kind being
denuded, except perhaps in the rarely moistened drainage-channel of the
valley. Must we then look back to that ancient period, when the waves of
the sea beat against the eastern foot of the Cordillera, for a power
sufficient to denude extensively, though superficially, this tufaceous
deposit, soft although it be?
There remains only to mention some little water-worn hillocks [BB], a few
hundred feet in height, and mere mole-hills compared with the gigantic
mountains behind them, which rise out of the sloping, shingle-covered
margin of the Pampas. The first little range is composed of a brecciated
purple porphyritic claystone, with obscurely marked strata dipping at 70
degrees to the S.W.; the other ranges consist of--a pale-coloured
feldspathic porphyry,--a purple claystone porphyry with grains of quartz,--
and a rock almost exclusively composed of brick-red crystals of feldspar.
These outermost small lines of elevation extend in a N.W. by W. and S.E. by
S. direction.
CONCLUDING REMARKS ON THE PORTILLO RANGE.
When on the Pampas and looking southward, and whilst travelling northward,
I could see for very many leagues the red granite and dark mica-schist
forming the crest and eastern flank of the Portillo line. This great range,
according to Dr. Gillies, can be traced with little interruption for 140
miles southward to the R. Diamante, where it unites with the western
ranges: northward, according to this same author, it terminates where the
R. Mendoza debouches from the mountains; but a little further north in the
eastern part of the Cumbre section, there are, as we shall hereafter see,
some mountain-masses of a brick-red porphyry, the last injected amidst many
other porphyries, and having so close an analogy with the coarse red
granite of the Portillo line, that I am tempted to believe that they belong
to the same axis of injection; if so, the Portillo line is at least 200
miles in length. Its height, even in the lowest gap in the road, is 14,365
feet, and some of the pinnacles apparently attain an elevation of about
16,000 feet above the sea. The geological history of this grand chain
appears to me eminently interesting. We may safely conclude, that at a
former period the valley of Tenuyan existed as an arm of the sea, about
twenty-miles in width, bordered on one hand by a ridge or chain of islets
of the black calcareous shales and purple sandstones of the gypseous
formation; and on the other hand, by a ridge or chain of islets composed of
mica-slate, white granite, and perhaps to a partial extent of red granite.
These two chains, whilst thus bordering the old sea-channel, must have been
exposed for a vast lapse of time to alluvial and littoral action, during
which the rocks were shattered, the fragments rounded, and the strata of
conglomerate accumulated to a thickness of at least fifteen hundred or two
thousand feet. The red orthitic granite now forms, as we have seen, the
main part of the Portillo chain: it is injected in dikes not only into the
mica-schist and white granites, but into the laminated sandstone, which it
has metamorphosed, and which it has thrown off, together with the
conformably overlying coloured beds and stratified conglomerate, at an
angle of forty-five degrees. To have thrown off so vast a pile of strata at
this angle, is a proof that the main part of the red granite (whether or
not portions, as perhaps is probable, previously existed) was injected in a
liquified state after the accumulation both of the laminated sandstone and
of the conglomerate; this conglomerate, we know, was accumulated, not only
after the deposition of the fossiliferous strata of the Peuquenes line, but
after their elevation and long-continued denudation: and these
fossiliferous strata belong to the early part of the Cretaceous system.
Late, therefore, in a geological sense, as must be the age of the main part
of the red granite, I can conceive nothing more impressive than the eastern
view of this great range, as forcing the mind to grapple with the idea of
the thousands of thousands of years requisite for the denudation of the
strata which originally encased it,--for that the fluidified granite was
once encased, its mineralogical composition and structure, and the bold
conical shape of the mountain-masses, yield sufficient evidence. Of the
encasing strata we see the last vestiges in the coloured beds on the crest,
in the little caps of mica-schist on some of the loftiest pinnacles, and in
the isolated patches of this same rock at corresponding heights on the now
bare and steep flanks.
The lava-streams at the eastern foot of the Portillo are interesting, not
so much from the great denudation which they have suffered at a
comparatively late period as from the evidence they afford by their
inclination taken conjointly with their thickness and compactness, that
after the great range had assumed its present general outline, it continued
to rise as an axis of elevation. The plains extending from the base of the
Cordillera to the Atlantic show that the continent has been upraised in
mass to a height of 3,500 feet, and probably to a much greater height, for
the smooth shingle-covered margin of the Pampas is prolonged in a gentle
unbroken slope far up many of the great valleys. Nor let it be assumed that
the Peuquenes and Portillo ranges have undergone only movements of
elevation; for we shall hereafter see, that the bottom of the sea subsided
several thousand feet during the deposition of strata, occupying the same
relative place in the Cordillera, with those of the Peuquenes ridge;
moreover, we shall see from the unequivocal evidence of buried upright
trees, that at a somewhat later period, during the formation of the
Uspallata chain, which corresponds geographically with that of the
Portillo, there was another subsidence of many thousand feet: here, indeed,
in the valley of Tenuyan, the accumulation of the coarse stratified
conglomerate to a thickness of fifteen hundred or two thousand feet, offers
strong presumptive evidence of subsidence; for all existing analogies lead
to the belief that large pebbles can be transported only in shallow water,
liable to be affected by currents and movements of undulation--and if so,
the shallow bed of the sea on which the pebbles were first deposited must
necessarily have sunk to allow of the accumulation of the superincumbent
strata. What a history of changes of level, and of wear and tear, all since
the age of the latter secondary formations of Europe, does the structure of
this one great mountain-chain reveal!
PASSAGE OF THE ANDES BY THE CUMBRE OR USPALLATA PASS.
This Pass crosses the Andes about sixty miles north of that just described:
the section given in Plate 1, Section 1/2, is on the same scale as before,
namely, at one-third of an inch to a mile in distance, and one inch to a
mile (or 6,000 feet) in height. Like the last section, it is a mere sketch,
and cannot pretend to accuracy, though made under favourable circumstances.
We will commence as before, with the western half, of which the main range
bears the name of the Cumbre (that is the Ridge), and corresponds to the
Peuquenes line in the former section; as does the Uspallata range, though
on a much smaller scale, to that of the Portillo. Near the point where the
river Aconcagua debouches on the basin plain of the same name, at a height
of about two thousand three hundred feet above the sea, we meet with the
usual purple and greenish porphyritic claystone conglomerate. Beds of this
nature, alternating with numerous compact and amygdaloidal porphyries,
which have flowed as submarine lavas, and associated with great mountain-
masses of various, injected, non-stratified porphyries, are prolonged the
whole distance up to the Cumbre or central ridge. One of the commonest
stratified porphyries is of a green colour, highly amygdaloidal with the
various minerals described in the pre
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