20th. - As We Ascended The Valley, The Vegetation, With
The Exception Of A Few Pretty Alpine Flowers, Became Exceedingly
Scanty, And Of Quadrupeds, Birds, Or Insects, Scarcely
One Could Be Seen.
The lofty mountains, their summits
marked with a few patches of snow, stood well separated
from each other, the valleys being filled up with an immense
thickness of stratified alluvium.
The features in the scenery
of the Andes which struck me most, as contrasted with the
other mountain chains with which I am acquainted, were, -
the flat fringes sometimes expanding into narrow plains on
each side of the valleys, - the bright colours, chiefly red and
purple, of the utterly bare and precipitous hills of porphyry,
the grand and continuous wall-like dykes, - the plainly-
divided strata which, where nearly vertical, formed the
picturesque and wild central pinnacles, but where less inclined,
composed the great massive mountains on the outskirts of the
range, - and lastly, the smooth conical piles of fine and
brightly coloured detritus, which sloped up at a high angle
from the base of the mountains, sometimes to a height of
more than 2000 feet.
I frequently observed, both in Tierra del Fuego and within
the Andes, that where the rock was covered during the greater
part of the year with snow, it was shivered in a very
extraordinary manner into small angular fragments. Scoresby [1]
has observed the same fact in Spitzbergen. The case
appears to me rather obscure: for that part of the mountain
which is protected by a mantle of snow, must be less subject
to repeated and great changes of temperature than any other
part. I have sometimes thought, that the earth and fragments
of stone on the surface, were perhaps less effectually
removed by slowly 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 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.
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