EXTERNAL APPEARANCE OF GLACIERS. 287 
heated through by the sun in summer. Such a 
heated pebble will of course melt the ice below 
it, and so wear a hole for itself into which it 
sinks. This process will continue as long as the 
sun reaches the pebble with force enough to heat 
it. Numbers of such deep, round holes, like 
organ-pipes, varying in size from the diameter 
of a minute pebble or a grain of coarse sand to 
that of an ordinary stone, are found on the gla- 
cier, and at the bottom of each is the pebble by 
which it was bored. The ice formed by the freez- 
ing of water collecting in such holes and in the 
fissures of the surface is a pure crystallized ice, 
very different in color from the ice of the great 
mass of the glacier produced by snow ; and some- 
times, after a rain and frost, the surface of a 
glacier looks like a mosaic-work, in consequence 
of such veins and cylinders or spots of clear ice 
with which it is inlaid. 
Indeed, the aspect of the glacier changes con- 
stantly with the different conditions of the tem- 
perature. We may see it, when, during a long 
dry season, it has collected upon its surface all 
sorts of light floating materials, as dust, sand, 
and the like, so that it looks dull and soiled, — 
or when a heavy rain has washed the surface 
clean from all impurities and left it bright and 
fresh We may see it when the heat and other 
disintegrating influences have acted upon the ice 
