percolating snow-water [2] than by rain, and therefore that the
appearance of a quicker disintegration of the solid rock under the
snow, was deceptive. Whatever the cause may be, the quantity of
crumbling stone on the Cordillera is very great. Occasionally in the
spring, great masses of this detritus slide down the mountains, and
cover the snow-drifts in the valleys, thus forming natural ice-houses.
We rode over one, the height of which was far below the limit of
perpetual snow.
As the evening drew to a close, we reached a singular basin-like plain,
called the Valle del Yeso. It was covered by a little dry pasture, and
we had the pleasant sight of a herd of cattle amidst the surrounding
rocky deserts. The valley takes its name of Yeso from a great bed, I
should think at least 2000 feet thick, of white, and in some parts
quite pure, gypsum. We slept with a party of men, who were employed in
loading mules with this substance, which is used in the manufacture of
wine. We set out early in the morning (21st), and continued to follow
the course of the river, which had become very small, till we arrived
at the foot of the ridge, that separates the waters flowing into the
Pacific and Atlantic Oceans. The road, which as yet had been good with
a steady but very gradual ascent, now changed into a steep zigzag track
up the great range, dividing the republics of Chile and Mendoza.
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
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