March, 1835. passage of coruillera. 



389 



appearance of a quicker decay of the sohd rock may be 

 deceptive. Whatever the cause may be, the quantity of 

 crumbhng stone on the Cordillera is very great. Occa- 

 sionally in the spring, masses of such matter slide down 

 the mountains, and cover the snow-drifts in the valleys ; 

 thus forming natural ice-houses. We rode over one, the 

 elevation of which was far below the limit of perpetual 

 congelation. 



As the evening drew to a close, we reached the Valle del 

 Yeso. This is a very singular basin, which must have once 

 been a very deep and large lake : the barrier is formed by a 

 huge mountain of alluvium, on one side of which the river 

 has cut a gorge. The plain is covered by a little dry pas- 

 ture, and amidst the surrounding rocky deserts we had the 

 pleasant sight of a herd of cattle. The valley takes its name 

 of Yeso from a great bed, I should think at least two thou- 

 sand 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. 



March 21st. — We set out early in the morning, and 

 continued to follow the course of the river, which by this 

 time had become 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. The Cordillera in this part consists of 

 two principal ranges ; the passes across which attain re- 

 spectively an elevation of 13,210 and 14,365 feet.* The 

 first great line (consisting of course of many subordinate 

 ones) is called Peuquenes. It divides the waters, and there- 

 floods also, in the former case, are said to be more destructive to the 

 land. D'Orbigny (vol. i., p. 184), in explaining the cause of the various 

 colours of the rivers in South America, remarks that those with blue or 

 clear water, have their source in the Cordillera, where the snow melts. 



* Measurements made by Dr. Gillies ; Edinburgh Journal of Nat. 

 and Geograph. Science, August, 1830. 



