XV GEOLOGY OF THE CORDILLERA 341 



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 ^ 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 mehing 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 liave their source in the Cordillera, where the 

 snow melts. 



