338 CHARLES DARWIN 



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 frag- 

 ments 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 Cordil- 

 lera 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 

 (2ist), 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 



1 1 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 in the Welsh mountains. D'Orbigny (torn. 

 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. 



