THE FORMATION OF GLACIERS. 
229 
in the upper regions, where on account of the 
extreme cold there is little melting, they perma- 
nently preserve about the same thickness, being 
yearly reduced by their downward motion in a 
proportion equal to their annual increase by fresh 
additions of snow. Indeed, these reservoirs of 
snow maintain themselves at the same level, much 
as a stream, into which many rivulets empty, 
remains within its usual limits in consequence of 
the drainage of the average supply. Of course, 
rery heavy rains or sudden thaws at certain 
seasons or in particular years may cause an oc- 
casional overflow of such a stream ; and irregu- 
larities of the same kind are observed during 
certain years or at different periods of the same 
year in the accumulations of snow, in conse- 
quence of which the successive strata may vary 
in thickness. But in ordinary times layers from 
six to eight feet deep are regularly added annu- 
ally to the accumulation of snow in the higher 
regions, — not taking into account, of course, the 
heavy drifts heaped up in particular localities, 
but estimating the uniform average increase over 
wide fields. This snow is gradually transformed 
into more or less compact ice, passing through 
an intermediate condition analogous to the slosh 
of our roads, and in that condition chiefly occu- 
pies the upper part of the extensive troughs 
into which these masses descend from the loftier 
