1835.} GEOLOGY OF THE CORDILLERA. 319 
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 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 (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 
* T 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 
(tom. i. p. 184), in explaining the cause of the various colours of the rivers 
int South America, remarks that those with blue or clear water havo their 
source in the Cordillera, where the’snow melts. 
