described: the section given in Plate V, Section 1/2, [see map page
440] 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 preliminary
discussion, and including fine tabular crystals of albite. The
mountain-range north (often with a little westing) and south. The
stratification, wherever I could clearly distinguish it, was inclined
westward or towards the Pacific, and, except near the Cumbre, seldom at
angles above 25°. Only at one spot on this western side, on a lofty
pinnacle not far from the Cumbre, I saw strata apparently belonging to
the gypseous formation, and conformably capping a pile of stratified
porphyries. Hence, both in composition and in stratification, the
structure of the mountains on this western side of the divortium
aquarum, is far more simple than in the corresponding part of the
Peuquenes section. In the porphyritic claystone conglomerate, the
mechanical structure and the planes of stratification have generally
been much obscured and even quite obliterated towards the base of the
series, whilst in the upper parts, near the summits of the mountains,
both are distinctly displayed. In these upper portions the porphyries
are generally lighter coloured. In three places [X, Y, Z] masses of
andesite are exposed: at [Y], this rock contained some quartz, but the
greater part consisted of andesitic porphyry, with only a few
well-developed crystals of albite, and forming a great white mass,
having the external aspect of granite, capped by much dark unstratified
porphyry. In many parts of the mountains, there are dikes of a green
colour, and other white ones, which latter probably spring from
underlying masses of andesite.
The Cumbre, where the road crosses it, is, according to Mr. Pentland,
12,454 feet above the sea; and the neighbouring peaks, composed of dark
purple and whitish porphyries, some obscurely stratified with a
westerly dip, and others without a trace of stratification, must exceed
13,000 feet in height. Descending the eastern slope of the Cumbre, the
structure becomes very complicated, and generally differs on the two
sides of the east and west line of road and section. First we come to a
great mass [A] of nearly vertical, singularly contorted strata,
composed of highly compact red sandstones, and of often calcareous
conglomerates, and penetrated by green, yellow, and reddish dikes; but
I shall presently have an opportunity of describing in some detail an
analogous pile of strata. These vertical beds are abruptly succeeded by
others [B], of apparently nearly the same nature but more
metamorphosed,
alternating with porphyries and limestones; these dip for a short space
westward, but there has been here an extraordinary dislocation, which,
on the north side of the road, appears to have determined the
excavation of the north and south valley of the R. de las Cuevas. On
this northern side of the road, the strata [B] are prolonged till they
come in close contact with a jagged lofty mountain [D] of
dark-coloured, unstratified, intrusive porphyry, where the beds have
been more highly inclined and still more metamorphosed. This mountain
of porphyry seems to form a short axis of elevation, for south of the
road in its line there is a hill [C] of porphyritic conglomerate with
absolutely vertical strata.
We now come to the gypseous formation: I will first describe the
structure of the several mountains, and then give in one section a
detailed account of the nature of the rocks. On the north side of the
road, which here runs in an east and west valley, the mountain of
porphyry [D] is succeeded by a hill [E] formed of the upper gypseous
strata tilted, at an angle of between 70° and 80° to the west, by a
uniclinal axis of elevation which does not run parallel to the other
neighbouring ranges, and which is of short length; for on the south
side of the valley its prolongation is marked only by a small flexure
in a pile of strata inclined by a quite separate axis. A little further
on the north and south valley of Horcones enters at right angles our
line of section; its western side is bounded by a hill of gypseous
strata [F] dipping westward at about 45°, and its eastern side by a
mountain of similar strata [G] inclined westward at 70°, and
superimposed by an oblique fault on another mass of the same strata
[H], also inclined westward, but at an angle of about 30°: the
complicated relation of these three masses [F, G, H] is explained by
the structure of a great mountain-range lying some way to the north, in
which a regular anticlinal axis (represented in the section by dotted
lines) is seen, with the strata on its eastern side again bending up
and forming a distinct uniclinal axis, of which the beds marked [H]
form the lower part. This great uniclinal line is intersected, near the
Puente del Inca, by the valley along which the road runs, and the
strata composing it will be immediately described. On the south side of
the road, in the space corresponding with the mountains [E, F, and G],
the strata everywhere dip westward generally at an angle of 30°,
occasionally mounting up to 45°, but not in an unbroken line, for there
are several vertical faults, forming separate uniclinal masses, all
dipping in the same direction,—a form of elevation common in the
Cordillera. We thus see that within a narrow space, the gypseous strata
have been upheaved and crushed together by a great uniclinal,
anticlinal, and one lesser uniclinal line [E] of elevation; and that
between these three lines and the Cumbre, in the sandstones,
conglomerates and porphyritic formation, there have been at least two
or three other great elevatory axes.
The uniclinal axis [I] intersected near the Puente del Inca[15] (of
which
the strata at [H] form a part) ranges N. by W. and S. by E., forming a
chain of mountains, apparently little inferior in height to the Cumbre:
the strata, as we have seen, dip at an average angle of 30° to the
west. The flanks of the mountains are here quite bare and steep,
affording an excellent section; so that I was able to inspect the
strata to a thickness of about 4,000 feet, and could clearly
distinguish their general nature for 1,000 feet higher, making a total
thickness of 5,000 feet, to which must be added about 1,000 feet of the
inferior strata seen a little lower down the valley, I will describe
this one section in detail, beginning at the bottom.
[15] At this place, there are some hot and cold springs, the warmest
having a temperature, according to Lieutenant Brand (“Travels,” p.
240), of 91°; they emit much gas. According to Mr. Brande, of the
Royal Institution, ten cubical inches contain forty-five grains of
solid matter, consisting chiefly of salt, gypsum, carbonate of lime,
and oxide of iron. The water is charged with carbonic acid and
sulphuretted hydrogen. These springs deposit much tufa in the form of
spherical balls. They burst forth, as do those of Cauquenes, and
probably those of Villa Vicencio, on a line of elevation.
1st. The lowest mass is the altered clay-slate described in the
preliminary discussion, and which in this line of section was here
first met with. Lower down the valley, at the R. de las Vacas, I had a
better opportunity of examining it; it is there in some parts well
characterised, having a distinct, nearly vertical, tortuous cleavage,
ranging N.W. and S.E., and intersected by quartz veins: in most parts,
however, it is crystalline and feldspathic, and passes into a true
greenstone often including grains of quartz. The clay-slate, in its
upper half, is frequently brecciated, the embedded angular fragments
being of nearly the same nature with the paste.
2nd. Several strata of purplish porphyritic conglomerate, of no very
great thickness, rest conformably upon the feldspathic slate. A thick
bed of fine, purple, claystone porphyry, obscurely brecciated (but not
of metamorphosed sedimentary origin), and capped by porphyritic
conglomerate, was the lowest bed actually examined in this section at
the Puente del Inca.
3rd. A stratum, eighty feet thick, of hard and very compact impure
whitish limestone, weathering bright red, with included layers
brecciated and recemented. Obscure marks of shell are distinguishable
in it.
4th. A red, quartzose, fine-grained conglomerate, with grains of
quartz, and with patches of white earthy feldspar, apparently due to
some process of concretionary crystalline action; this bed is more
compact and metamorphosed than any of the overlying conglomerates.
5th. A whitish cherty limestone, with nodules of bluish argillaceous
limestone.
6th. A white conglomerate, with many particles of quartz, almost
blending into the paste.
7th. Highly siliceous, fine-grained white sandstone.
8th and 9th. Red and white beds not examined.
10th. Yellow, fine-grained, thinly stratified, magnesian (judging from
its slow dissolution in acids) limestone: it includes some white quartz
pebbles, and little cavities, lined with calcareous spar, some
retaining the form of shells.
11th. A bed between twenty and thirty feet thick, quite conformable
with the underlying ones, composed of a hard basis, tinged lilac-grey
porphyritic with _numerous_ crystals of whitish feldspar, with black
mica and little spots of soft ferruginous matter: evidently a submarine
lava.
12th. Yellow magnesian limestone, as before, part-stained purple.
13th. A most singular rock; basis purplish grey, obscurely crystalline,
easily fusible into a dark green glass, not hard, thickly speckled with
crystals more or less perfect of white carbonate of lime, of red
hydrous oxide of iron, of a white and transparent mineral like
analcime, and of a green opaque mineral like soap-stone; the basis is
moreover amygdaloidal with many spherical balls of white crystallised
carbonate of lime, of which some are coated with the red oxide of iron.
I have no doubt, from the examination of a superincumbent stratum (19),
that this is a submarine lava; though in Northern Chile, some of the
metamorphosed sedimentary beds are almost as crystalline, and of as
varied composition.
14th. Red sandstone, passing in the upper part into a coarse, hard, red
conglomerate, 300 feet thick, having a calcareous cement, and including
grains of quartz and broken crystals of feldspar; basis infusible; the
pebbles consist of dull purplish porphyries, with some of quartz, from
the size of a nut to a man’s head. This is the coarsest conglomerate in
this part of the Cordillera: in the middle there was a white layer not
examined.
15th. Grand thick bed, of a very hard, yellowish-white rock, with a
crystalline feldspathic base, including large crystals of white
feldspar, many little cavities mostly full of soft ferruginous matter,
and numerous hexagonal plates of black mica. The upper part of this
great bed is slightly cellular; the lower part compact: the thickness
varied a little in different parts. Manifestly a submarine lava; and is
allied to bed 11.
16th and 17th. Dull purplish, calcareous, fine-grained, compact
sandstones, which pass into coarse white conglomerates with numerous
particles of quartz.
18th. Several alternations of red conglomerate, purplish sandstone, and
submarine lava, like that singular rock forming bed 13.
19th. A very heavy, compact, greenish-black stone, with a fine-grained
obviously crystalline basis, containing a few specks of white
calcareous spar, many specks of the crystallised hydrous red oxide of
iron, and some specks of a green mineral; there are veins and nests
filled with epidote: certainly a submarine lava.
20th. Many thin strata of compact, fine-grained, pale purple sandstone.
21st. Gypsum in a nearly pure state, about three hundred feet in
thickness: this bed, in its concretions of anhydrite and layers of
small blackish crystals of carbonate of lime, exactly resembles the
great gypseous beds in the Peuquenes range.
22nd. Pale purple and reddish sandstone, as in bed 20: about three
hundred feet in thickness.
23rd. A thick mass composed of layers, often as thin as paper and
convoluted, of pure gypsum with others very impure, of a purplish
colour.
24th. Pure gypsum, thick mass.
25th. Red sandstones, of great thickness.
26th. Pure gypsum, of great thickness.
27th. Alternating layers of pure and impure gypsum, of great thickness.
I was not able to ascend to these few last great strata, which compose
the neighbouring loftiest pinnacles. The thickness, from the lowest to
the uppermost bed of gypsum, cannot be less than 2,000 feet: the beds
beneath I estimated at 3,000 feet, and this does not include either the
lower parts of the porphyritic conglomerate, or the altered clay-slate;
I conceive the total thickness must be about six thousand feet. I
distinctly observed that not only the gypsum, but the alternating
sandstones and conglomerates were lens-shaped, and repeatedly thinned
out and replaced each other: thus in the distance of about a mile, a
bed 300 feet thick of sandstone between two beds of gypsum, thinned out
to nothing and disappeared. The lower part of this section differs
remarkably,—in the much greater diversity of its mineralogical
composition,—in the abundance of calcareous matter,—in the greater
coarseness of some of the conglomerates,—and in the numerous particles
and well-rounded pebbles, sometimes of large size, of quartz,— from any
other section hitherto described in Chile. From these peculiarities and
from the lens-form of the strata, it is probable that this great pile
of strata was accumulated on a shallow and very uneven bottom, near
some pre-existing land formed of various porphyries and quartz-rock.
The formation of porphyritic claystone conglomerate does not in this
section attain nearly its ordinary thickness; this may be PARTLY
attributed to the metamorphic action having been here much less
energetic than usual, though the lower beds have been affected to a
certain degree. If it had been as energetic as in most other parts of
Chile, many of the beds of sandstone and conglomerate, containing
rounded masses of porphyry, would doubtless have been converted into
porphyritic conglomerate; and these would have alternated with, and
even blended into, crystalline and porphyritic strata without a trace
of mechanical structure,—namely, into those which, in the present state
of the section, we see are unquestionably submarine lavas.
The beds of gypsum, together with the red alternating sandstones and
conglomerates, present so perfect and curious a resemblance with those
seen in our former section in the basin-valley of Yeso, that I cannot
doubt the identity of the two formations: I may add, that a little
westward of the P. del Inca, a mass of gypsum passed into a
fine-grained, hard, brown sandstone, which contained some layers of
black, calcareous, compact, shaly rock, precisely like that seen in
such vast masses on the Peuquenes range.
Near the Puente del Inca, numerous fragments of limestone, containing
some fossil remains, were scattered on the ground: these fragments so
perfectly resemble the limestone of bed No. 3, in which I saw
impressions of shells, that I have no doubt they have fallen from it.
The yellow magnesian limestone of bed No. 10, which also includes
traces of shells, has a different appearance. These fossils (as named
by M. d’Orbigny) consist of:—
Gryphæa, near to _G. Couloni_ (Neocomian formation).
Arca, perhaps _A. Gabrielis,_ d’Orbigny, “Pal. Franc.” (Neocomian
formation).
Mr. Pentland made a collection of shells from this same spot, and Von
Buch[16] considers them as consisting of:—
Trigonia, resembling in form _T. costata._
Pholadomya, like one found by M. Dufresnoy near Alencon.
Isocardi excentrica, Voltz., identical with that from the Jura.
[16] “Description Phys. des Iles Can.,” p. 472.
Two of these shells, namely, the Gryphæa and Trigonia, appear to be
identical with species collected by Meyen and myself on the Peuquenes
range; and in the opinion of Von Buch and M. d’Orbigny, the two
formations belong to the same age. I must here add, that Professor E.
Forbes, who has examined my specimens from this place and from the
Peuquenes range, has likewise a strong impression that they indicate
the Cretaceous period, and probably an early epoch in it: so that all
the palæontologists who have seen these fossils nearly coincide in
opinion regarding their age. The limestone, however, with these fossils
here lies at the very base of the formation, just above the porphyritic
conglomerate, and certainly several thousand feet lower in the series,
than the equivalent, fossiliferous, black, shaly rocks high up on the
Peuquenes range.
It is well worthy of remark that these shells, or at least those of
which I saw impressions in the limestone (bed No. 3), must have been
covered up, on the _least_ computation, by 4,000 feet of strata: now we
know from Professor E. Forbes’s researches, that the sea at greater
depths than 600 feet becomes exceedingly barren of organic beings,—a
result quite in accordance with what little I have seen of deep-sea
soundings. Hence, after this limestone with its shells was deposited,
the bottom of the sea where the main line of the Cordillera now stands,
must have subsided some thousand feet to allow of the deposition of the
superincumbent submarine strata. Without supposing a movement of this
kind, it would, moreover, be impossible to understand the accumulation
of the several lower strata of _coarse,_ well-rounded conglomerates,
which it is scarcely possible to believe were spread out in profoundly
deep water, and which, especially those containing pebbles of quartz,
could hardly have been rounded in submarine craters and afterwards
ejected from them, as I believe to have been the case with much of the
porphyritic conglomerate formation. I may add that, in Professor
Forbes’s opinion, the above-enumerated species of mollusca probably did
not live at a much greater depth than twenty fathoms, that is only 120
feet.
To return to our section down the valley; standing on the great N. by
W. and S. by E. uniclinal axis of the Puente del Inca, of which a
section has just been given, and looking north-east, greater tabular
masses of gypseous formation (KK) could be seen in the distance, very
slightly inclined towards the east. Lower down the valley, the
mountains are almost exclusively composed of porphyries, many of them
of intrusive origin and non-stratified, others stratified, but with the
stratification seldom distinguishable except in the upper parts.
Disregarding local disturbances, the beds are either horizontal or
inclined at a small angle eastwards: hence, when standing on the plain
of Uspallata and looking to the west or backwards, the Cordillera
appear composed of huge, square, nearly horizontal, tabular masses: so
wide a space, with such lofty mountains so equably elevated, is rarely
met with within the Cordillera. In this line of section, the interval
between the Puente del Inca and the neighbourhood of the Cumbre,
includes all the chief axes of dislocation.
The altered clay-slate formation, already described, is seen in several
parts of the valley as far down as Las Vacas, underlying the
porphyritic conglomerate. At the Casa de Pujios [L], there is a hummock
of (andesitic?) granite; and the stratification of the surrounding
mountains here changes from W. by S. to S.W. Again, near the R. Vacas
there is a larger formation of (andesitic?) granite [M], which sends a
meshwork of veins into the superincumbent clay-slate, and which locally
throws off the strata, on one side to N.W. and on the other to S.E. but
not at a high angle: at the junction, the clay-slate is altered into
fine-grained greenstone. This granitic axis is intersected by a green
dike, which I mention, because I do not remember having elsewhere seen
dikes in this lowest and latest intrusive rock. From the R. Vacas to
the plain of Uspallata, the valley runs N.E., so that I have had to
contract my section; it runs exclusively through porphyritic rocks. As
far as the Pass of Jaula, the claystone conglomerate formation, in most
parts highly porphyritic, and crossed by numerous dikes of greenstone
porphyry, attains a great thickness: there is also much intrusive
porphyry. From the Jaula to the plain, the stratification has been in
most places obliterated, except near the tops of some of the mountains;
and the metamorphic action has been extremely great. In this space, the
number and bulk of the intrusive masses of differently coloured
porphyries, injected one into another and intersected by dikes, is
truly extraordinary. I saw one mountain of whitish porphyry, from which
two huge dikes, thinning out, branched _downwards_ into an adjoining
blackish porphyry. Another hill of white porphyry, which had burst
through dark-coloured strata, was itself injected by a purple,
brecciated, and recemented porphyry, both being crossed by a green
dike, and both having been upheaved and injected by a granitic dome.
One brick-red porphyry, which above the Jaula forms an isolated mass in
the midst of the porphyritic conglomerate formation, and lower down the
valley a magnificent group of peaked mountains, differs remarkably from
all the other porphyries. It consists of a red feldspathic base,
including some rather large crystals of red feldspar, numerous large
angular grains of quartz, and little bits of a soft green mineral
answering in most of its characters to soapstone. The crystals of red
feldspar resemble in external appearance those of orthite, though, from
being
partially decomposed, I was unable to measure them; and they certainly
are quite unlike the variety, so abundantly met with in almost all the
other rocks of this line of section, and which, wherever I tried it,
cleaved like albite. This brick-red porphyry appears to have burst
through all the other porphyries, and numerous red dikes traversing the
neighbouring mountains have proceeded from it: in some few places,
however, it was intersected by white dikes. From this posteriority of
intrusive origin,—from the close general resemblance between this red
porphyry and the red granite of the Portillo line, the only difference
being that the feldspar here is less perfectly granular, and that
soapstone replaces the mica, which is there imperfect and passes into
chlorite,—and from the Portillo line a little southward of this point
appearing to blend (according to Dr. Gillies) into the western
ranges,—I am strongly urged to believe (as formerly remarked) that the
grand mountain-masses composed of this brick-red porphyry belong to the
same axis of injection with the granite of the Portillo line; if so,
the injection of this porphyry probably took place, as long
subsequently to the several axes of elevation in the gypseous formation
near the Cumbre, as the injection of the Portillo granite has been
shown to have been subsequent to the elevation of the gypseous strata
composing the Peuquenes range; and this interval, we have seen, must
have been a very long one.
The Plain of Uspallata has been briefly described in Chapter 3; it
resembles the basin-plains of Chile; it is ten or fifteen miles wide,
and is said to extend for 180 miles northward; its surface is nearly
six thousand feet above the sea; it is composed, to a thickness of some
hundred feet of loosely aggregated, stratified shingle, which is
prolonged with a gently sloping surface up the valleys in the mountains
on both sides. One section in this plain [Z] is interesting, from the
unusual[17] circumstance of alternating layers of almost loose red and
white sand with lines of pebbles (from the size of a nut to that of an
apple), and beds of gravel, being inclined at an angle of 45°, and in
some spots even at a higher angle. These beds are dislocated by small
faults: and are capped by a thick mass of horizontally stratified
gravel, evidently of subaqueous origin. Having been accustomed to
observe the irregularities of beds accumulated under currents, I feel
sure that the inclination here has not been thus produced. The pebbles
consist chiefly of the brick-red porphyry just described and of white
granite, both probably derived from the ranges to the west, and of
altered clay-slate and of certain porphyries, apparently belonging to
the rocks of the Uspallata chain. This plain corresponds geographically
with the valley of Tenuyan between the Portillo and Peuquenes ranges;
but in that valley the shingle, which likewise has been derived both
from the eastern and western ranges, has been cemented into a hard
conglomerate, and has been throughout tilted at a considerable
inclination; the gravel there apparently attains a much greater
thickness, and is probably of higher antiquity.
[17] I find that Mr. Smith of Jordan Hill has described (_Edinburgh
New Phil. Journ.,_ vol. xxv, p. 392) beds of sand and gravel, near
Edinburgh, tilted at an angle of 60°, and dislocated by miniature
faults.
_The Uspallata range._—The road by the Villa Vicencio Pass does not
strike directly across the range, but runs for some leagues northward
along its western base: and I must briefly describe the rocks here
seen, before continuing with the coloured east and west section. At the
mouth of the valley of Canota, and at several points northwards, there
is an extensive formation of a glossy and harsh, and of a feldspathic
clay-slate, including strata of grauwacke, and having a tortuous,
nearly vertical cleavage, traversed by numerous metalliferous veins and
others of quartz. The clay-slate is in many parts capped by a thick
mass of fragments of the same rock, firmly recemented; and both
together have been injected and broken up by very numerous hillocks,
ranging north and south, of lilac, white, dark and salmon-coloured
porphyries: one steep, now denuded, hillock of porphyry had its face as
distinctly impressed with the angles of a fragmentary mass of the
slate, with some of the points still remaining embedded, as sealing-wax
could be by a seal. At the mouth of this same valley of Canota, in a
fine escarpment having the strata dipping from 50° to 60° to the
N.E.,[18] the clay-slate formation is seen to be covered by—(1st) a
purple, claystone porphyry resting unconformably in some parts on the
solid slate, and in others on a thick fragmentary mass; (2nd), a
conformable stratum of compact blackish rock, having a spheroidal
structure, full of minute acicular crystals of glassy feldspar, with
red spots of oxide of iron; (3rd), a great stratum of purplish-red
claystone porphyry, abounding with crystals of opaque feldspar, and
laminated with thin, parallel, often short, layers, and likewise with
great irregular patches of white, earthy, semi-crystalline feldspar;
this rock (which I noticed in other neighbouring places) perfectly
resembles a curious variety described at Port Desire, and occasionally
occurs in the great porphyritic conglomerate formation of Chile; (4th),
a thin stratum of greenish white, indurated tuff, fusible and
containing broken crystals and particles of porphyries; (5th), a grand
mass, imperfectly columnar and divided into three parallel and closely
joined strata, of cream-coloured claystone porphyry; (6th), a thick
stratum of lilac-coloured porphyry, which I could see was capped by
another bed of the cream-coloured variety; I was unable to examine the
still higher parts of the escarpment. These conformably stratified
porphyries, though none are either vesicular are amygdaloidal, have
evidently flowed as submarine lavas: some of them are separated from
each other by seams of indurated tuff, which, however, are quite
insignificant in thickness compared with the porphyries. This whole
pile resembles, but not very closely, some of the less brecciated parts
of the great porphyritic conglomerate formation of Chile; but it does
not probably belong to the same age, as the porphyries here rest
unconformably on the altered feldspathic clay-slate, whereas the
porphyritic conglomerate formation alternates with
and rests conformably on it. These porphyries, moreover, with the
exception of the one blackish stratum, and of the one indurated, white
tufaceous bed, differ from the beds composing the Uspallata range in
the line of the Villa Vicencio Pass.
[18] Nearly opposite to this escarpment, there is another
corresponding one, with the strata dipping not to the exactly opposite
point, or S.W., but to S.S.W.: consequently the two escarpments trend
towards each other, and some miles southward they become actually
united: this is a form of elevation which I have not elsewhere seen.
I will now give, first, a sketch of the structure of the range, as
represented in the section, and will then describe its composition and
interesting history. At its western foot, a hillock [N] is seen to rise
out of the plain, with its strata dipping at 70° to the west, fronted
by strata [O] inclined at 45° to the east, thus forming a little north
and south anticlinal axis. Some other little hillocks of similar
composition, with their strata highly inclined, range N.E. and S.W.,
obliquely to the main Uspallata line. The cause of these dislocations,
which, though on a small scale, have been violent and complicated, is
seen to lie in hummocks of lilac, purple and red porphyries, which have
been injected in a liquified state through and into the underlying
clay-slate formation. Several dykes were exposed here, but in no other
part, that I saw of this range. As the strata consist of black, white,
greenish and brown-coloured rocks, and as the intrusive porphyries are
so brightly tinted, a most extraordinary view was presented, like a
coloured geological drawing. On the gently inclined main western slope
[PP], above the little anticlinal ridges just mentioned, the strata dip
at an average angle of 25° to the west; the inclination in some places
being only 19°, in some few others as much as 45°. The masses having
these different inclinations, are separated from each other by parallel
vertical faults [as represented at Pa], often giving rise to separate,
parallel, uniclinal ridges. The summit of the main range is broad and
undulatory, with the stratification undulatory and irregular: in a few
places granitic and porphyritic masses [Q] protrude, which, from the
small effect they have locally produced in deranging the strata,
probably form the upper points of a regular, great underlying dome.
These denuded granitic points, I estimated at about nine thousand feet
in height above the sea. On the eastern slope, the strata in the upper
part are regularly inclined at about 25° to the east, so that the
summit of this chain, neglecting small irregularities, forms a broad
anticlinal axis. Lower down, however, near Los Hornillos [R], there is
a well-marked synclinal axis, beyond which the strata are inclined at
nearly the same angle, namely from 20° to 30°, inwards or westward.
Owing to the amount of denudation which this chain has suffered, the
outline of the gently inclined eastern flank scarcely offers the
slightest indication of this synclinal axis. The stratified beds, which
we have hitherto followed across the range, a little further down are
seen to lie, I believe unconformably, on a broad mountainous band of
clay-slate and grauwacke. The strata and laminæ of this latter
formation, on the extreme eastern flank, are generally nearly vertical;
further inwards they become inclined from 45° to 80° to the west: near
Villa Vicencio [S] there is apparently an anticlinal axis, but the
structure of this outer part of the clay-slate formation is so obscure,
that I have not marked the planes of stratification in the section. On
the margin of the Pampas, some low, much dislocated spurs of this same
formation,
project in a north-easterly line, in the same oblique manner as do the
ridges on the western foot, and as is so frequently the case with those
at the base of the main Cordillera.
I will now describe the nature of the beds, beginning at the base on
the eastern side. First, for the clay-slate formation: the slate is
generally hard and bluish, with the laminæ coated by minute micaceous
scales; it alternates many times with a coarse-grained, greenish
grauwacke, containing rounded fragments of quartz and bits of slate in
a slightly calcareous basis. The slate in the upper part generally
becomes purplish, and the cleavage so irregular that the whole consists
of mere splinters. Transverse veins of quartz are numerous. At the
Calera, some leagues distant, there is a dark crystalline limestone,
apparently included in this formation. With the exception of the
grauwacke being here more abundant, and the clay-slate less altered,
this formation closely resembles that unconformably underlying the
porphyries at the western foot of this same range; and likewise that
alternating with the porphyritic conglomerate in the main Cordillera.
This formation is a considerable one, and extends several leagues
southward to near Mendoza: the mountains composed of it rise to a
height of about two thousand feet above the edge of the Pampas, or
about seven thousand feet above the sea.[19]
[19] I infer this from the height of V. Vicencio, which was
ascertained by Mr. Miers to be 5,328 feet above the sea.
Secondly: the most usual bed on the clay-slate is a coarse, white,
slightly calcareous conglomerate, of no great thickness, including
broken crystals of feldspar, grains of quartz, and numerous pebbles of
brecciated claystone porphyry, but without any pebbles of the
underlying clay-slate. I nowhere saw the actual junction between this
bed and the clay-slate, though I spent a whole day in endeavouring to
discover their relations. In some places I distinctly saw the white
conglomerate and overlying beds inclined at from 25° to 30° to the
west, and at the bottom of the same mountain, the clay-slate and
grauwacke inclined to the same point, but at an angle from 70° to 80°:
in one instance, the clay-slate dipped not only at a different angle,
but to a different point from the overlying formation. In these cases
the two formations certainly appeared quite unconformable: moreover, I
found in the clay-slate one great, vertical, dike-like fissure, filled
up with an indurated whitish tuff, quite similar to some of the upper
beds presently to be described; and this shows that the clay-slate must
have been consolidated and dislocated before their deposition. On the
other hand, the stratification of the slate and grauwacke,[20] in some
cases gradually and entirely disappeared in approaching the overlying
white conglomerate;
in other cases the stratification of the two formations became strictly
conformable; and again in other cases, there was some tolerably well
characterised clay-slate lying above the conglomerate. The most
probable conclusion appears to be, that after the clay-slate formation
had been dislocated and tilted, but whilst under the sea, a fresh and
more recent deposition of clay-slate took place, on which the white
conglomerate was conformably deposited, with here and there a thin
intercalated bed of clay-slate. On this view the white conglomerates
and the presently to be described tuffs and lavas are really
unconformable to the main part of the clay-slate; and this, as we have
seen, certainly is the case with the clay-stone lavas in the valley of
Canota, at the western and opposite base of the range.
[20] The coarse, mechanical structure of many grauwackes has always
appeared to me a difficulty; for the texture of the associated
clay-slate and the nature of the embedded organic remains where
present, indicate that the whole has been a deep-water deposit. Whence
have the sometimes included angular fragments of clay-slate, and the
rounded masses of quartz and other rocks, been derived? Many
deep-water limestones, it is well known, have been brecciated, and
then firmly recemented.
Thirdly: on the white conglomerate, strata several hundred feet in
thickness are superimposed, varying much in nature in short distances:
the commonest variety is a white, much indurated tuff, sometimes
slightly calcareous, with ferruginous spots and water-lines, often
passing into whitish or purplish compact, fine-grained grit or
sandstones; other varieties become semi-porcellanic, and tinted faint
green or blue; others pass into an indurated shale: most of these
varieties are easily fusible.
Fourthly: a bed, about one hundred feet thick of a compact, partially
columnar, pale-grey, feldspathic lava, stained with iron, including
very numerous crystals of opaque feldspar, and with some crystallised
and disseminated calcareous matter. The tufaceous stratum on which this
feldspathic lava rests is much hardened, stained purple, and has a
spherico-concretionary structure; it here contains a good many pebbles
of claystone porphyry.
Fifthly: thin beds, 400 feet in thickness, varying much in nature,
consisting of white and ferruginous tuffs, in some parts having a
concretionary structure, in others containing rounded grains and a few
pebbles of quartz; also passing into hard gritstones and into greenish
mudstones: there is, also, much of a bluish-grey and green
semi-porcellanic stone.
Sixthly: a volcanic stratum, 250 feet in thickness, of so varying a
nature that I do not believe a score of specimens would show all the
varieties; much is highly amygdaloidal, much compact; there are
greenish, blackish, purplish, and grey varieties, rarely including
crystals of green augite and minute acicular ones of feldspar, but
often crystals and amygdaloidal masses of white, red, and black
carbonate of lime. Some of the blackish varieties of this rock have a
conchoidal fracture and resemble basalt; others have an irregular
fracture. Some of the grey and purplish varieties are thickly speckled
with green earth and with white crystalline carbonate of lime; others
are largely amygdaloidal with green earth and calcareous spar. Again,
other earthy varieties, of greenish, purplish and grey tints, contain
much iron, and are almost half composed of amygdaloidal balls of dark
brown bole, of a whitish indurated feldspathic matter, of bright green
earth, of agate, and of black and white crystallised carbonate of lime.
All these varieties are easily fusible. Viewed from a distance, the
line of junction with the underlying semi-porcellanic strata was
distinct; but when examined
closely, it was impossible to point out within a foot where the lava
ended and where the sedimentary mass began: the rock at the time of
junction was in most places hard, of a bright green colour, and
abounded with irregular amygdaloidal masses of ferruginous and pure
calcareous spar, and of agate.
Seventhly: strata, eighty feet in thickness, of various indurated
tuffs, as before; many of the varieties have a fine basis including
rather coarse extraneous particles; some of them are compact and
semi-porcellanic, and include vegetable impressions.
Eighthly: a bed, about fifty feet thick, of greenish-grey, compact,
feldspathic lava, with numerous small crystals of opaque feldspar,
black augite, and oxide of iron. The junction with the bed on which it
rested, was ill defined; balls and masses of the feldspathic rock being
enclosed in much altered tuff.
Ninthly: indurated tuffs, as before.
Tenthly: a conformable layer, less than two feet in thickness, of
pitchstone, generally brecciated, and traversed by veins of agate and
of carbonate of lime: parts are composed of apparently concretionary
fragments of a more perfect variety, arranged in horizontal lines in a
less perfectly characterised variety. I have much difficulty in
believing that this thin layer of pitchstone flowed as lava.
Eleventhly: sedimentary and tufaceous beds as before, passing into
sandstone, including some conglomerate: the pebbles in the latter are
of claystone porphyry, well rounded, and some as large as
cricket-balls.
Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of
bed No. 8, divided by numerous joints into large angular blocks.
Thirteenthly: sedimentary beds as before.
Fourteenthly: a thick bed of greenish or greyish black, compact basalt
(fusing into a black enamel), with small crystals, occasionally
distinguishable, of feldspar and augite: the junction with the
underlying sedimentary bed, differently from that in most of the
foregoing streams, here was quite distinct:—the lava and tufaceous
matter preserving their perfect characters within two inches of each
other. This rock closely resembles certain parts of that varied and
singular lava-stream No. 6; it likewise resembles, as we shall
immediately see, many of the great upper beds on the western flank and
on the summit of this range.
The pile of strata here described attains a great thickness; and above
the last-mentioned volcanic stratum, there were several other great
tufaceous beds alternating with submarine lavas, which I had not time
to examine; but a corresponding series, several thousand feet in
thickness, is well exhibited on the crest and western flank of the
range. Most of the lava-streams on the western side are of a jet-black
colour and basaltic nature; they are either compact and fine-grained,
including minute crystals of augite and feldspar, or they are
coarse-grained and abound with rather large coppery-brown crystals of
an augitic mineral.[21] Another variety was of a dull-red colour,
having a claystone brecciated basis, including specks of oxide of iron
and of calcareous spar, and
amygdaloidal with green earth: there were apparently several other
varieties. These submarine lavas often exhibit a spheroidal, and
sometimes an imperfect columnar structure: their upper junctions are
much more clearly defined than their lower junctions; but the latter
are not so much blended into the underlying sedimentary beds as is the
case in the eastern flank. On the crest and western flank of the range,
the streams, viewed as a whole, are mostly basaltic; whilst those on
the eastern side, which stand lower in the series, are, as we have
seen, mostly feldspathic.
[21] Very easily fusible into a jet-black bead, attracted by the
magnet: the crystals are too much tarnished to be measured by the
goniometer.
The sedimentary strata alternating with the lavas on the crest and
western side, are of an almost infinitely varying nature; but a large
proportion of them closely resemble those already described on the
eastern flank: there are white and brown, indurated, easily fusible
tuffs,—some passing into pale blue and green semi-porcellanic
rocks,—others into brownish and purplish sandstones and gritstones,
often including grains of quartz,—others into mudstone containing
broken crystals and particles of rock, and occasionally single large
pebbles. There was one stratum of a bright red, coarse, volcanic
gritstone; another of conglomerate; another of a black, indurated,
carbonaceous shale marked with imperfect vegetable impressions; this
latter bed, which was thin, rested on a submarine lava, and followed
all the considerable inequalities of its upper surface. Mr. Miers
states that coal has been found in this range. Lastly, there was a bed
(like No. 10 on the eastern flank) evidently of sedimentary origin, and
remarkable from closely approaching in character to an imperfect
pitchstone, and from including extremely thin layers of perfect
pitchstone, as well as nodules and irregular fragments (but not
resembling extraneous fragments) of this same rock arranged in
horizontal lines: I conceive that this bed, which is only a few feet in
thickness, must have assumed its present state through metamorphic and
concretionary action. Most of these sedimentary strata are much
indurated, and no doubt have been partially metamorphosed: many of them
are extraordinarily heavy and compact; others have agate and
crystalline carbonate of lime disseminated throughout them. Some of the
beds exhibit a singular concretionary arrangement, with the curves
determined by the lines of fissure. There are many veins of agate and
calcareous spar, and innumerable ones of iron and other metals, which
have blackened and curiously affected the strata to considerable
distances on both sides.
Many of these tufaceous beds resemble, with the exception of being more
indurated, the upper beds of the Great Patagonian tertiary formation,
especially those variously coloured layers high up the River Santa
Cruz, and in a remarkable degree the tufaceous formation at the
northern end of Chiloe. I was so much struck with this resemblance,
that I particularly looked out for silicified wood, and found it under
the following extraordinary circumstances. High up on this western
flank,[22]
at a height estimated at 7,000 feet above the sea, in a broken
escarpment of thin strata, composed of compact green gritstone passing
into a fine mudstone, and alternating with layers of coarser, brownish,
very heavy mudstone, including broken crystals and particles of rock
almost blended together, I counted the stumps of fifty-two trees. They
projected between two and five feet above the ground, and stood at
exactly right angles to the strata, which were here inclined at an
angle of about 25° to the west. Eleven of these trees were silicified
and well preserved; Mr. R. Brown has been so kind as to examine the
wood when sliced and polished; he says it is coniferous, partaking of
the characters of the Araucarian tribe, with some curious points of
affinity with the Yew. The bark round the trunks must have been
circularly furrowed with irregular lines, for the mudstone round them
is thus plainly marked. One cast consisted of dark argillaceous
limestone; and forty of them of coarsely crystallised carbonate of
lime, with cavities lined by quartz crystals: these latter white
calcareous columns do not retain any internal structure, but their
external form plainly shows their origin. All the stumps have nearly
the same diameter, varying from one foot to eighteen inches; some of
them stand within a yard of each other; they are grouped in a clump
within a space of about sixty yards across, with a few scattered round
at the distance of 150 yards. They all stand at about the same level.
The longest stump stood seven feet out of the ground: the roots, if
they are still preserved, are buried and concealed. No one layer of the
mudstone appeared much darker than the others, as if it had formerly
existed as soil, nor could this be expected, for the same agents which
replaced with silex and lime the wood of the trees, would naturally
have removed all vegetable matter from the soil. Besides the fifty-two
upright trees, there were a few fragments, like broken branches,
horizontally embedded. The surrounding strata are crossed by veins of
carbonate of lime, agate, and oxide of iron; and a poor gold vein has
been worked not far from the trees.
[22] For the information of any future traveller, I will describe the
spot in detail. Proceeding eastward from the Agua del Zorro, and
afterwards leaving on the north side of the road a rancho attached to
some old goldmines, you pass through a gully with low but steep rocks
on each hand: the road then bends, and the ascent becomes steeper. A
few hundred yards farther on, a stone’s throw on the south side of the
road, the white calcareous stumps may be seen. The spot is about half
a mile east of the Agua del Zorro.
The green and brown mudstone beds including the trees, are conformably
covered by much indurated, compact, white or ferruginous tuffs, which
pass upwards into a fine-grained, purplish sedimentary rock: these
strata, which, together, are from four to five hundred feet in
thickness, rest on a thick bed of submarine lava, and are conformably
covered by another great mass of fine-grained basalt,[23] which I
estimated at 1,000 feet in thickness, and which probably has been
formed by more than one stream. Above this mass I could clearly
distinguish five conformable alternations, each several hundred feet in
thickness,
of stratified sedimentary rocks and lavas, such as have been previously
described. Certainly the upright trees have been buried under several
thousand feet in thickness of matter, accumulated under the sea. As the
trees obviously must once have grown on dry land, what an enormous
amount of subsidence is thus indicated! Nevertheless, had it not been
for the trees there was no appearance which would have led any one even
to have conjectured that these strata had subsided. As the land,
moreover, on which the trees grew, is formed of subaqueous deposits, of
nearly if not quite equal thickness with the superincumbent strata, and
as these deposits are regularly stratified and fine-grained, not like
the matter thrown up on a sea-beach, a previous upward movement, aided
no doubt by the great accumulation of lavas and sediment, is also
indicated.[24]
[23] This rock is quite black, and fuses into a black bead, attracted
strongly by the magnet; it breaks with a conchoidal fracture; the
included crystals of augite are distinguishable by the naked eye, but
are not perfect enough to be measured: there are many minute acicular
crystals of glassy feldspar.
[24] At first I imagined, that the strata with the trees might have
been accumulated in a lake: but this seems highly improbable; for,
first, a very deep lake was necessary to receive the matter below the
trees, then it must have been drained for their growth, and afterwards
re-formed and made profoundly deep, so as to receive a subsequent
accumulation of matter _several thousand_ feet in thickness. And all
this must have taken place necessarily before the formation of the
Uspallata range, and therefore on the margin of the wide level expanse
of the Pampas! Hence I conclude, that it is infinitely more probable
that the strata were accumulated under the sea: the vast amount of
denudation, moreover, which this range has suffered, as shown by the
wide valleys, by the exposure of the very trees and by other
appearances, could have been effected, I conceive, only by the
long-continued action of the sea; and this shows that the range was
either upheaved from under the sea, or subsequently let down into it.
From the natural manner in which the stumps (fifty-two in number) are
_grouped in a clump_, and from their all standing vertically to the
strata, it is superfluous to speculate on the chance of the trees
having been drifted from adjoining land, and deposited upright: I may,
however, mention that the late Dr. Malcolmson assured me, that he once
met in the Indian Ocean, fifty miles from land, several cocoa-nut
trees floating upright, owing to their roots being loaded with earth.
In nearly the middle of the range, there are some hills [Q], before
alluded to, formed of a kind of granite externally resembling andesite,
and consisting of a white, imperfectly granular, feldspathic basis,
including some perfect crystals apparently of albite (but I was unable
to measure them), much black mica, epidote in veins, and very little or
no quartz. Numerous small veins branch from this rock into the
surrounding strata; and it is a singular fact that these veins, though
composed of the same kind of feldspar and small scales of mica as in
the solid rock, abound with innumerable minute _rounded_ grains of
quartz: in the veins or dikes also, branching from the great granitic
axis in the peninsula of Tres Montes, I observed that quartz was more
abundant in them than in the main rock: I have heard of other analogous
cases: can we account for this fact, by the long-continued vicinity of
quartz[25] when cooling, and by its having been thus more easily
sucked into fissures than the other constituent minerals of granite?
The strata encasing the flanks of these granitic or andesite masses,
and forming a thick cap on one of their summits, appear originally to
have been of the same tufaceous nature with the beds already described,
but they are now changed into porcellanic, jaspery, and crystalline
rocks, and into others of a white colour with a harsh texture, and
having a siliceous aspect, though really of a feldspathic nature and
fusible. Both the granitic intrusive masses and the encasing strata are
penetrated by innumerable metallic veins, mostly ferruginous and
auriferous, but some containing copper-pyrites and a few silver: near
the veins, the rocks are blackened as if blasted by gunpowder. The
strata are only slightly dislocated close round these hills, and hence,
perhaps, it may be inferred that the granitic masses form only the
projecting points of a broad continuous axis-dome, which has given to
the upper parts of this range its anticlinal structure.
[25] See a paper by M. Elie de Beaumont, “Soc. Philomath.,” May 1839
(“L’Institut.,” 1839, p. 161.)
_Concluding remarks on the Uspallata range._—I will not attempt to
estimate the total thickness of the pile of strata forming this range,
but it must amount to many thousand feet. The sedimentary and tufaceous
beds have throughout a general similarity, though with infinite
variations. The submarine lavas in the lower part of the series are
mostly feldspathic, whilst in the upper part, on the summit and western
flank, they are mostly basaltic. We are thus reminded of the relative
position in most recent volcanic districts of the trachytic and
basaltic lavas,—the latter from their greater weight having sunk to a
lower level in the earth’s crust, and having consequently been erupted
at a later period over the lighter and upper lavas of the trachytic
series.[26] Both the basaltic and feldspathic submarine streams are
very compact; none being vesicular, and only a few amygdaloidal: the
effects which some of them, especially those low in the series, have
produced on the tufaceous beds over which they have flowed is highly
curious. Independently of this local metamorphic action, all the strata
undoubtedly display an indurated and altered character; and all the
rocks of this range—the lavas, the alternating sediments, the intrusive
granite and porphyries, and the underlying clay-slate—are intersected
by metalliferous veins. The lava-strata can often be seen extending for
great distances, conformably with the under and overlying beds; and it
was obvious that they thickened towards the west. Hence the points of
eruption must have been situated westward of the present range, in the
direction of the main Cordillera: as, however, the flanks of the
Cordillera are entirely composed of various porphyries, chiefly
claystone and greenstone, some intrusive, and others belonging to the
porphyritic conglomerate formation, but all quite unlike these
submarine lava-streams, we must in all probability look to the plain of
Uspallata for the now deeply buried points of eruption.
[26] See on this subject, “Volcanic Islands,” etc., by the Author.
Comparing our section of the Uspallata range with that of the Cumbre,
we see, with the exception of the underlying clay-slate, and perhaps of
the intrusive rocks of the axes, a striking dissimilarity in the strata
composing them. The great porphyritic conglomerate formation
has not extended as far as this range; nor have we here any of the
gypseous strata, the magnesian and other limestones, the red
sandstones, the siliceous beds with pebbles of quartz, and
comparatively little of the conglomerates, all of which form such vast
masses over the basal series in the main Cordillera. On the other hand,
in the Cordillera, we do not find those endless varieties of indurated
tuffs, with their numerous veins and concretionary arrangement, and
those grit and mud stones, and singular semi-porcellanic rocks, so
abundant in the Uspallata range. The submarine lavas, also, differ
considerably; the feldspathic streams of the Cordillera contain much
mica, which is absent in those of the Uspallata range: in this latter
range we have seen on how grand a scale, basaltic lava has been poured
forth, of which there is not a trace in the Cordillera. This
dissimilarity is the more striking, considering that these two parallel
chains are separated by a plain only between ten and fifteen miles in
width; and that the Uspallata lavas, as well as no doubt the
alternating tufaceous beds, have proceeded from the west, from points
apparently between the two ranges. To imagine that these two piles of
strata were contemporaneously deposited in two closely adjoining, very
deep, submarine areas, separated from each other by a lofty ridge,
where a plain now extends, would be a gratuitous hypothesis. And had
they been contemporaneously deposited, without any such dividing ridge,
surely some of the gypseous and other sedimentary matter forming such
immensely thick masses in the Cordillera, would have extended this
short distance eastwards; and surely some of the Uspallata tuffs and
basalts also accumulated to so great a thickness, would have extended a
little westward. Hence I conclude, that it is far from probable that
these two series are not contemporaneous; but that the strata of one of
the chains were deposited, and even the chain itself uplifted, before
the formation of the other:—which chain, then, is the oldest?
Considering that in the Uspallata range the lowest strata on the
western flank lie unconformably on the clay-slate, as probably is the
case with those on the eastern flank, whereas in the Cordillera all the
overlying strata lie conformably on this formation:—considering that in
the Uspallata range some of the beds, both low down and high up in the
series, are marked with vegetable impressions, showing the continued
existence of neighbouring land;—considering the close general
resemblance between the deposits of this range and those of tertiary
origin in several parts of the continent;—and lastly, even considering
the lesser height and outlying position of the Uspallata range,—I
conclude that the strata composing it are in all probability of
subsequent origin, and that they were accumulated at a period when a
deep sea studded with submarine volcanoes washed the eastern base of
the already partially elevated Cordillera.
This conclusion is of much importance, for we have seen that in the
Cordillera, during the deposition of the Neocomian strata, the bed of
the sea must have subsided many thousand feet: we now learn that at a
later period an adjoining area first received a great accumulation of
strata, and was upheaved into land on which coniferous trees grew, and
that this area then subsided several thousand feet to receive the
superincumbent
submarine strata, afterwards being broken up, denuded, and elevated in
mass to its present height. I am strengthened in this conclusion of
there having been two distinct, great periods of subsidence, by
reflecting on the thick mass of coarse stratified conglomerate in the
valley of Tenuyan, between the Peuquenes and Portillo lines; for the
accumulation of this mass seems to me, as previously remarked, almost
necessarily to have required a prolonged subsidence; and this
subsidence, from the pebbles in the conglomerate having been to a great
extent derived from the gypseous or Neocomian strata of the Peuquenes
line, we know must have been quite distinct from, and subsequent to,
that sinking movement which probably accompanied the deposition of the
Peuquenes strata, and which certainly accompanied the deposition of the
equivalent beds near the Puente del Inca, in this line of section.
The Uspallata chain corresponds in geographical position, though on a
small scale, with the Portillo line; and its clay-slate formation is
probably the equivalent of the mica-schist of the Portillo, there
metamorphosed by the old white granites and syenites. The coloured beds
under the conglomerate in the valley of Tenuyan, of which traces are
seen on the crest of the Portillo, and even the conglomerate itself,
may perhaps be synchronous with the tufaceous beds and submarine lavas
of the Uspallata range; an open sea and volcanic action in the latter
case, and a confined channel between two bordering chains of islets in
the former case, having been sufficient to account for the
mineralogical dissimilarity of the two series. From this correspondence
between the Uspallata and Portillo ranges, perhaps in age and certainly
in geographical position, one is tempted to consider the one range as
the prolongation of the other; but their axes are formed of totally
different intrusive rocks; and we have traced the apparent continuation
of the red granite of the Portillo in the red porphyries diverging into
the main Cordillera. Whether the axis of the Uspallata range was
injected before, or as perhaps is more probable, after that of the
Portillo line, I will not pretend to decide; but it is well to remember
that the highly inclined lava-streams on the eastern flank of the
Portillo line, prove that its angular upheavement was not a single and
sudden event; and therefore that the anticlinal elevation of the
Uspallata range may have been contemporaneous with some of the later
angular movements by which the gigantic Portillo range gained its
present height above the adjoining plain.