PORTILLO PASS. [CHAP. XT. 



partly to the novelty of an unusual degree of fatigue arising from 

 a little exertion, habit being thus opposed to the evidence of 

 the senses. I am sure that this extreme clearness of the air gives 

 a peculiar character to the landscape, all objects appearing to be 

 brought nearly into one plane, as in a drawing or panorama. 

 The transparency is, I presume, owing to the equable and high 

 state of atmospheric dryness. This diyness was shown by the 

 manner in which woodwork shrank (as I soon found by the 

 trouble my geological hammer gave me) ; by articles of food, 

 such as bread and sugar, becoming extremely hard ; and by the 

 preservation of the skin and parts of the flesh of the beasts, which 

 had perished on the road. To the same cause we must attribute 

 the singular facility with which electricity is excited. My flan- 

 nel-waistcoat, when rubbed in the dark, appeared as if it had been 

 washed with phosphorus ; every hair on a dog's back crackled ; 

 even the linen sheets, and leathern straps of the saddle, when 

 handled, emitted sparks. 



March 23rd. The descent on the eastern side of the Cordil- 

 lera, is much shorter or steeper than on the Pacific side ; in other 

 words, the mountains rise more abruptly from the plains than 

 from the alpine country of Chile. A level and brilliantly white 

 sea of clouds was stretched out beneath our feet, shutting out the 

 view of the equally level Pampas. We soon entered the band of 

 clouds, and did not again emerge from it that day. About 

 noon, finding pasture for the animals and bushes for firewood at 

 Los Arenales, we stopped for the night. This was near the up- 

 permost limit of bushes, and the elevation, I suppose, was between 

 seven and eight thousand feet. 



I was much struck with the marked difference between the 

 vegetation of these eastern valleys and those on the Chilian side: 

 yet the climate, as well as the kind of soil, is nearly the same, 

 and the difference of longitude very trifling. The same remark 

 holds good with the quadrupeds, and in a lesser degree with the 

 birds and insects. I may instance the mice, of which I obtained 

 thirteen species on the shores of the Atlantic, and five on the 

 Pacific, and not one of them is identical. We must except all 

 those species, which habitually or occasionally frequent elevated 

 mountains ; and certain birds, which range as far south as the 

 Strait of Magellan. This fact is in perfect accordance with tho 



