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. (15/1. Scoresby's "Arctic Regions" volume 1 page
122.) 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 have sometimes thought that the earth and fragments of
stone on the surface were perhaps less effectually removed by
slowly percolating snow-water than by rain, and therefore that the
appearance of a quicker disintegration of the solid rock under the
snow was deceptive. (15/2. 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 on the Welsh mountains. D'Orbigny tome 1 page 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.
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