PROGRESSION OF GLACIERS 
245 
they have given explanations of their appearance 
which I hold to be quite untenable. Indeed, to 
consider these successive lines of dirt on the gla- 
cier as limited only to its surface, and to explain 
them from that point of view, is much as if a 
geologist were to consider the lines presented by 
the strata on a cut through a sedimentary mass 
of rock as representing their whole extent, and 
to explain them as a superficial deposit due to 
external causes. 
A few more details may help to make this state- 
ment clearer to my readers. Let us imagine that 
a fresh layer of snow has fallen in these moun- 
tain regions, and that a deposit of dirt has been 
scattered over its surface, which, if any moisture 
arises from the melting of the snow or from the 
falling of rain or mist, will become more closely 
compacted with it. The next snow-storm depos- 
its a fresh bed of snow, separated from the one 
below it by the sheet of dust just described, and 
this bed may, in its turn, receive a like deposit. 
For greater ease and simplicity of explanation, I 
speak here as if each successive snow-layer were 
thus indicated ; of course this is not literally true, 
because snow-storms in the winter may follow 
each other so fast that there is no time for such 
a collection of foreign materials upon each newly 
formed surface. But whenever such a fresh snow- 
bed, or accumulation of beds, remains with its 
