SCENliRY OF TilK ANUHii. 67 



from each other, the valleys being filled up with 

 an immense thickness of stratified alluvium. The 

 features in the scenery of the Andes which struck 

 me most, as contrasted with the other luountain 

 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 con- 

 tinuous wall-like dikes ; the plainly-divided strata, 

 which, where nearly vertical, formed the pictu- 

 resque 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 froin the base of 

 the luountains, 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 man- 

 ner 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 chan- 

 ges of temperature than any other part. I have 

 sometimes thought that the earth and fragments of 

 stone on the surface were perhaps less effectually 

 removed by slowly percolating snow-watert than 



* 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 tur- 

 bid than when it proceeds from the snow melting on the Welsh 

 mountains. D'Orbigny (torn, i., p. 184), in explaming 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 Cor- 

 dillera, where the snow melts. , .-- 



