1835.1 VALLE DEL YESO. 317 



earth and iVagnients of stone on the suriace, 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. Occasion- 

 ally 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 (iisi), 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 

 jiad 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 westward 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 ot 

 the submarine craters. These alternating masses are 



• I have heard it remarked in Shropshire, that the water, when the Severn is 

 flooded from long-continued min, is much more turbid than when it proceeds 

 from the Know melting on the Welsh mountains. D'Orbijfny (toni. i., p. 184), 

 in explaining the cause of the various colours of the rivers ui South Amrrica, 

 remarks that those with blue or clear water have their source in the Corilillera, 

 where the snow melts. 



