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, crystallised and almost blended
together, through the agency of mountain masses of a peculiar white
soda-granitic rock.
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