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fore likewise the republics of Chile and Mendoza. To the
eastward, a mountainous and elevated region separates it
from the second range (called the Portillo) overlooking the
Pampas. The streams from the intermediate tract find a
passage a little way to the southward through this second
line.
I will here give a very brief sketch of the geological
structure of these mountains : first, of the Peuquenes, or
western line; for the constitution of the two ranges is
totally different. The lowest stratified rock is a dull red or
purple claystone porphyry, of many varieties, alternating
with conglomerates, and breccia composed of a similar
substance : this formation attains a thickness of more than
a mile. Above it there is a grand mass of gypsum, which
alternates, passes into, and is replaced by, red sandstone,
conglomerates, and black calcareous clay-slate. I hardly
dare venture to guess the thickness of this second division ;
but I have already said some of the beds of gypsum alone
attain a thickness of at least two thousand feet. Even at
the very crest of the Peuquenes, at the height of 13,210
feet, and above it, the black clay-slate contained numerous
marine remains, amongst which a gryphæa is the most abundant,
likewise shells, resembling turritellæ, terebratulæ, and
an ammonite. It is an old story, but not the less wonderful,
to hear of shells, wdiich formerly were crawling about
at the bottom of the sea, being now elevated nearly fourteen
thousand feet above its level. The formation probably
is of the age of the central parts of the secondary series of
Europe.
These great piles of strata have been penetrated, up-
heaved, and overturned, in the most extraordinary manner,
by masses of injected rock, equalling mountains in size. On
the bare sides of the hiUs, complicated dikes, and wedges
of variously-coloured porphyries and other stones, are seen
traversing the strata in every possible form and direction ;
proving also by their intersections, successive periods of
violence. The rock which composes the axis of these great
lines of dislocation, at a distance very closely resembles
granite, but on examination, it is found rarely to contain
any quartz; and instead of ordinary felspar, albite.
The metamorphic action has been very great, as might
have been expected from the close proximity of such grand
masses of rock, which were injected when iu a liquefied
state from heat. When it is known, first, that the stratified
porphyries have flowed as streams of submarine lava under
an enormous pressure, and that the mechanical beds separating
them owe their origin to explosions from the same
submarine craters; secondly, that the whole mass in the
lower part has generally been so completely fused into one
solid rock by metamorphic action, that the lines of division
can only be traced with much difficulty; and thirdly, that
masses of porphyry, undistinguishable by their mineralogical
characters from the two first kinds, hav-e been subsequently
injected;—the extreme complication of the whole will readily
be believed.
We now- come to the second range, which is of even
greater altitude than the first. Its nucleus in the section
seen in crossing the Portillo pass, consists of magnificent
liinnacles of coarsely-crystalhzed red granite. On the eastern
flank, a few patches of mica slate still adhere to the unstratified
mass ; and at the foot a stream of basaltic lava has
burst forth at some remote period,—perhaps when the sea
covered the wide surface of the Pampas. On the western
side of the axis, between the two ranges, laminated fine sandstone
has been penetrated by immense granitic dikes proceeding
from the central mass, and has thus been converted into
granular quartz rock. The sandstone is covered by other
sedimentary deposits, and these again by a coarse conglomerate,
the vast thickness of which I will not attempt even to
estimate. All these coarse mechanical beds dip from the
red granite directly towards the Peuquenes range, as if
they passed beneath it; though such is not the case.
Oil examining the pebbles composing this conglomerate
(which, to my surprise, betrayed no signs of metamorphic
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