1835.] GEOLOGY OF THE CORDILLERA. 819 



water* than by rain, and therefore that the appearance of a 

 quicker disintegration of the solid rock under the snow, was decep- 

 tive. 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 covei 

 the snow-drifts in the valleys, thus forming natural ice-houses. 

 We rode over one, the height cf 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 west- 

 ward 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 



* 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 

 (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 then 

 source in the Cordillera, where the snow melts. 



