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,[1] 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.
[1] Meyen “Reise um Erde” Th. I, s. 235.
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-laminæ 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;[2] 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.
[2] 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.)
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.[3] 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.
[3] 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 (_Edin. New Phil. Journ.,_ vol. xli, p.
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. I, s. 316) from Chile, as consisting of crystallised
quartz, with crystals of stilbite within, and lined externally by
green earth.
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;[4] 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.
[4] 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 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[5] 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. 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 scoriæ 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.
[5] 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.
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[6] (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. 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[7] states that
it occurs in Eastern Kamtschatka. 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.
[6] I here, and elsewhere, call by this name, those feldspathic
minerals which cleave like albite: but it now appears (_Edin. New
Phil. Journal.,_ vol. xxiv, p. 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.
[7] _Geograph. Journal,_ vol. ix, p. 510.
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,[8] 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. 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.
[8] “Reise um Erde,” Th. I, ss. 338 and 362.
_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.
Plate V sections (between and 441) 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,[9] is 13,210
feet; 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.
[9] _Journal of Nat. and Geograph. Science,_ August 1830.
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°, and less than a mile south of it at 67°:
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 our section [Plate V] 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°. 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[10] met with
a hill
of pumice containing mica. 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.
[10] “Reise um Erde,” Th. I, ss. 338, 341.)
Beyond the junction of the Yeso and Volcan, the porphyritic strata
appear to dip towards the hillocks of andesite at an angle of 40°; 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° 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° 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°, 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°,
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° 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°, but at the eastern or farther end of the series it increases to
60°. 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:—
Ammonite, indeterminable, near to _A. recticostatus,_ d’Orbigny, “Pal.
Franc.” (Neocomian formation).
Gryphæa, near to _G. Couloni_ (Neocomian formations of France and
Neufchâtel).
Natica, indeterminable.
Cyprina rostrata, d’Orbigny, “Pal. Franc.” (Neocomian formation).
Rostellaria angulosa (?), d’Orbigny, “Pal. de l’Amér. Mer.”
Terebratula (?).
Some of the fragments of Ammonites were as thick as a man’s arm: the
Gryphæa is much the most abundant shell. These fossils M. d’Orbigny
considers as belonging to the Neocomian stage of the Cretaceous system.
Dr. Meyen,[11] 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: 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,[12] of:—
Exogyra (Gryphæa) Couloni, absolutely identical with specimens from the
Jura and South of France.
Trigonia costata, identical with those found in the upper Jurassic beds
at Hildesheim.
Pecten striatus, identical with those found in the upper Jurassic beds
at Hildesheim.
Cucullæa, corresponding in form to _C. longirostris,_ so frequent in
the upper Jurassic beds of Westphalia.
Ammonites resembling _A. biplex._
[11] “Reise um Erde,” etc., Th. I, s. 355.
[12] “Descript. Phys. des Iles Canaries,” p. 471.
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
hich 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°, which further on increases
to about 45°. 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,[13] however, collected
here specimens of ribboned jasper, magnesian limestone, and other
minerals. 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.
[13] “Travels,” etc., vol. i, p. 308.
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 laminæ. The streams are separated from each other by beds
of fragmentary brown scoriæ, 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.[14] 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.
[14] 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.
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° 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._