68 I'UKTJLLU PASS. 



by rain, and therefore that the appearance of a 

 quicker disintegi-ation 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 mount- 

 ains, 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 hei-d of cattle amidst the sur- 

 rounding rocky deserts. The valley takes its name 

 of Yeso from a gi-eat bed, I should think at least 2000 

 feet thick, of white, and, in some paits, 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 arx-ived 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 Men- 

 doza. 



I will here give a very brief sketch of the geol- 

 ogy of the several parallel lines forming the Cor- 

 dillera. 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 Por- 

 tillo ridge, on the Mendoza side, which is 14,305 

 feet. The lower beds of the Peuquenes ridge, and 



