GEOLOGY OF THE CORDILLERA. [CHAP. xv. 



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 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 the 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, through the agency of mountain masses of a peculiar 

 white soda-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 aro 

 covered by a sandstone, converted by the former heat into a quartz- 



