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Show 388 PASSAGE OF CORDILLERA. March, 1835. The 1r eat ures I. n the scenery of the Andes wh·i ch hs tr· uck m· he most, as contras t e d W'lt h the few other moun• tam c ams •W it which I am acquainted, were,-the fla~ fnnges so~etlmes ex anding into narrow plains on each side the valleys,-the br~ht colours, chiefly red and purple, of ~he utterly b~re an d preC.i pi' to us h'Il ls , -the grand and. contmuous wall-hke dikes,-the strongly-marked strata which: whe~ nearly ver- ti.C a 1, 1r 0rm the most picturesque and wild · pmnachl es,1 but where less inclined, great massive mountams; t e atter occupym· g the outskirts of the range, and the former t· he1 more lofty and central parts,-lastly, .the sm~oth comca piles of fine and brightly-coloured detntu~, whiCh ~lope at a high angle from the flanks of the mountams to their bases, some of the piles having a height of more than two thou-sand feet. I frequently observed both in Tierra del Fuego, ~nd 'tl ·n the Andes that where the rock was covered dunng Will ' · d' the greater part of the year ':"ith snow, it was sh1vere m a very extraordinary manner mto small . angul.ar fragments. Scoresby* has observed the same fact m Spitzbergen: he says, '~The invariably broken state of the rocks appeared to have been the effect of frost. On calcareous rocks, some of which are not impervious to moisture, the effect is su~h as miO'ht have been expected; but how frost can operate m this :ay on quartz is not so easily understood." The whole phenomenon appears to me rather o?scure: for that part of the mountain which is covered durmg many months by a mantle of snow, must be less subject to repeated and great changes of temperature than any other, yet it is the most affected. I have sometimes thought, that the earth and fragments of stone lying on the surface, were pe:haps less effectually removed by means of slowly percolatmg snowwater, t than by the agency of rain, and therefore that the • Scoresby's Arctic Regions, vol. i., p. 122. t I have heard it remarked in Shropshire, that the water, when the Severn is flooded from long-continued rain, is much more turbid, thah when it proceeds from the snow melting on the Welsh mountains. 'The March, 1835. PASSAGE 01<~ CORDILLERA. 389 appearance of a quicker decay of the solid rock may be deceptive. Whatever the cause may be, the quantity of crumbling stone on the Cordillera is very great. Occasionally 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 Y eso. 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 pasture, and amidst the surrounding rocky deserts we had the pleasant sight of a herd of cattle. The valley takes its name of Y eso from a great bed, I should think at least two thousand 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. 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 respectively 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, J 830. |