I Will Here Give A Very Brief Sketch Of The Geology Of The
Several Parallel Lines Forming The Cordillera.
Of these lines,
there are two considerably higher than the others; namely,
on the Chilian side, the Peuquenes ridge, which, where the
road crosses it, is 13,210 feet above the sea; and the Portillo
ridge, on the Mendoza side, which is 14,305 feet.
The lower
beds of the Peuquenes ridge, and of the several great lines
to the westward of it, are composed of a vast pile, many
thousand feet in thickness, of porphyries which have flowed as
submarine lavas, alternating with angular and rounded fragments
of the same rocks, thrown out of the submarine craters.
These alternating masses are covered in the central parts,
by a great thickness of red sandstone, conglomerate, and
calcareous clay-slate, associated with, and passing into,
prodigious beds of gypsum. In these upper beds shells are
tolerably frequent; and they belong to about the period of the
lower chalk of Europe. It is an old story, but not the less
wonderful, to hear of shells which were once crawling on the
bottom of the sea, now standing nearly 14,000 feet above its
level. The lower beds in this great pile of strata, have been
dislocated, baked, crystallized and almost blended together,
through the agency of mountain masses of a peculiar white
soda-granitic rock.
The other main line, namely, that of the Portillo, is of a
totally different formation: it consists chiefly of grand bare
pinnacles of a red potash-granite, which low down on the
western flank are covered by a sandstone, converted by the
former heat into a quartz-rock. On the quartz, there rest
beds of a conglomerate several thousand feet in thickness,
which have been upheaved by the red granite, and dip at an
angle of 45 degs. towards the Peuquenes line. I was astonished
to find that this conglomerate was partly composed of pebbles,
derived from the rocks, with their fossil shells, of the
Peuquenes range; and partly of red potash-granite, like that
of the Portillo. Hence we must conclude, that both the Peuquenes
and Portillo ranges were partially upheaved and exposed
to wear and tear, when the conglomerate was forming;
but as the beds of the conglomerate have been thrown off at
an angle of 45 degs. by the red Portillo granite (with the
underlying sandstone baked by it), we may feel sure, that the
greater part of the injection and upheaval of the already
partially formed Portillo line, took place after the
accumulation of the conglomerate, and long after the elevation
of the Peuquenes ridge. So that the Portillo, the loftiest line
in this part of the Cordillera, is not so old as the less lofty
line of the Peuquenes. Evidence derived from an inclined stream
of lava at the eastern base of the Portillo, might be adduced
to show, that it owes part of its great height to elevations of
a still later date.
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