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.
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