PROGRESSION OF GLACIERS. 
239 
some measure explain this retardation, at least 
of the lower part of the glacier. This agency, 
like that of the compression of the snow by its 
own weight and the pressure from behind, is 
most effective where the accumulation is largest. 
In the centre, where the body of the mass is 
greatest, it will imbibe the most moisture. But 
here a modifying influence comes in, not suffi- 
ciently considered by the investigators of glacial 
structure. We have already seen that snow and 
ice, at different degrees of compactness, are not 
equally permeable to moisture. Above the line 
at which the annual winter snow melts, there 
is, of course, little moisture ; but below that 
point, as soon as the temperature rises in summer 
sufficiently to melt the surface, the water easily 
penetrates the mass, passing through it more 
readily where the snow is lightest and least com- 
pact, — in short, where it has not begun its 
transformation into ice. A summer’s day sends 
countless rills of water trickling through such a 
mass of snow. If the snow be loose and porous 
throughout, the water will pass through its whole 
thickness, accumulating at the bottom, so that 
the lower portion of the mass will be damper, 
more completely soaked with water, than the 
upper part ; if, on the contrary, in consequence 
of the process previously described, alternate 
melting and freezing combined with pressure, 
