THE FORMATION OF GLACIERS. 217 
accumulation of air-bubbles, which, during a 
warm and clear day, may rise from a muddy 
bottom in great numbers. In consequence of 
these occasional collections of air-bubbles, the 
layers differ, not only in density and closeness, 
but also in color, the more compact strata being 
blue and transparent, while those containing a 
greater quantity of air-bubbles are opaque and 
whitish, like water beaten to froth. 
A cake of pond-ice, such as is daily left in 
summer at our doors, if held against the light 
and turned in different directions, will exhibit 
all these phenomena very distinctly, and we may 
learn still more of its structure by watching its 
gradual melting. The process of decomposition 
is as different in fresh-water ice and in land- or 
glacier-ice as that of their formation. Pond-ice, 
in contact with warm air, melts uniformly over 
its whole surface, the mass being thus gradually 
reduced from the exterior till it vanishes com- 
pletely. If the process be slow, the temperature 
of the air-bubbles contained in it may be so raised 
as to form the vertical funnels or tubes alluded 
to above. By the anastomosing of these funnels, 
the whole mass may be reduced to a collection of 
angular pyramids, more or less closely united by 
cross-beams of ice, and it finally falls to pieces 
when the spaces in the interior have become so 
numerous as to render it completely cavernous. 
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