1835.] EFFECT OF SXOW OX ROCKS. 305 



exception of a few pretty alpine flowers, became exceedingly 

 scanty; and of quadrupeds, birds, or insects, scarcely one could 

 be seen. The lofty mountains, their summits marked with a few- 

 patches of snow, stood well separated from each other, the valleys 

 being filled up with an immense thickness of stratified alluvium. 

 The features in the scenery of the Andes Avhich struck me most, 

 as contrasted with the other mountain chains with which I am 

 acquainted, were, the flat fringes sometimes expanding into 

 narrow plains on each side of the valleys, the bright colours, 

 chiefly red and purple, of the utterly bare and precipitous hills 

 of porphyry, the grand and continuous wall-like dykes, the 

 plainly-divided strata which, where nearly vertical, formed the 

 picturesque and wild central pinnacles, but where less inclined, 

 composed the great massive mountains on the outskirts of the 

 range, and lastly, the smooth conical piles of fine and brightly 

 coloured detritus, which sloped up at a high angle from the baso 

 of the mountains, sometimes to a height of more than 2000 feet. 



I frequently observed, both in Tierra del Fuego and within the 

 Andes, that where the rock was covered during the greater part of 

 the year with snow, it was shivered in a very extraordinary manner 

 into small angular fragments. Scoresby * has observed the same 

 fact in Spitzbergen. The case appears to me rather obscure : for 

 that part of the mountain which is protected by a mantle of snow, 

 must be less subject to repeated and great changes of temperature 

 than any other part. I haA'e sometimes thought, that the earth 

 and fragments of stone on the surface, were perhaps less effectually 

 removed by slowly percolating snow- water f than by rain, and 

 therefore that the appearance of a quicker disintegration of the 

 solid rock under the snow, was deceptive. 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. 



* Scoresby's Arctic Eegions, 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 than 

 when it proceeds from the snow melting in the Welsh mountains. 

 D'Orbigny (torn. 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. 



