INTERNAL STRUCTURE AND 
24(3 
surface exposed for some time, such a deposit of 
dirt will inevitably be found upon it. This pro- 
cess may go on till we have a number of succes- 
sive snow-layers divided from each other by thin 
sheets of dust. Of course, such seams, marking 
the stratification of snow, are as permanent and 
indelible as the seams of coarser materials al- 
ternating with the finest mud in a sedimentary 
rock. 
The gradual progress of a glacier, which, 
though more rapid in summer than in winter, is 
never intermitted, changes the relation of these 
beds to each other. Their lower edge is annu- 
ally cut off at a certain level, because the snow 
deposited every winter melts with the coming 
summer, up to a certain line, determined by the 
local climate of the place. But although the 
snow does not melt above this line, we have seen, 
in the preceding article, that it is prevented from 
accumulating indefinitely in the higher regions 
by its own tendency to move down to the lower 
valleys, and crowding itself between their walls, 
thus to force its way toward the outlet below. 
Now, as this movement is very gradual, it is evi- 
dent that there must be a perceptible difference 
in the progress of the successive layers, the lower 
and older ones getting the advance of the upper 
and more recent ones: that is, when the snow 
that has covered the face of the country during 
