232 PORTILLO PASS. [CHAP. xv. 



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 surround- 

 ing rocky deserts. The valley takes its name of Yeso from a great bed, 

 I should think at least 2,000 feet thick, of white, and in some parts 

 quite pure, gypsum. "We slept with a party of men who were em- 

 ployed 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 briet 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 ot 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 of the submarine craters. These alternating masses are 

 covered in the central parts, by a great thickness of red sandstone, 

 conglomerate, and calcareous clay-slate, associated with, and passing 

 into, prodigious beds of gypsum. In these upper beds shells are 

 tolerably frequent ; and they belong to about the period of the lower 

 chalk of Europe. It is an old story, but not Hie less wonderful, to hear 

 of shells which were once crawling on the bottom of the sea, now 

 standing nearly 14,000 feet above its level. The lower beds in this 

 great pile of strata, have been dislocated, baked, crystallized and 

 almost blended together, throush the agency of mountain masses of a 

 peculiar white soaa-granitic rock. 



The other main line, namely, that of the Portillo, is of a totally 

 different formation : it consists chiefly of grand bare pinnacles of a red 

 potash-granite, which low down on the western flank are covered by a 

 sandstone, converted by the former heat into a quartz-rock. On the 

 quartz, there rest beds of a conglomerate several thousand feet in 

 thickness, which have been upheaved by the red granite, and dip at an 



in South America, remarks that those with blue or clear water have their 

 source in the Cordillera, where the snow melts. 



