XV 
GEOLOGY OF THE CORDILLERA 
34i 
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 1 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 
1 I have heard it remarked in Shropshire, that the water, when the Severn is 
flooded from long-continued rain, is much more turbid than when it proceeds from 
the snow melting on the Welsh mountains. D’Orbigny (tom. i. p. 184), in 
explaining the cause of the various colours of the rivers in South America, remarks 
that those with blue or clear water have their source in the Cordillera, where the 
snow melts. 
