PROGRESSION OF GLACIERS. 243 
Hied character of metamorphic rocks to the skil- 
ful geologist, even though they may be readily 
mistaken for plutonic masses by the common ob- 
server. Indeed, even those secondary features, 
as the dirt-bands, for instance, which we shall 
see to be intimately connected with snow-strata, 
and which eventually become so prominent as to 
be mistaken for the cause of the lines of stratifi- 
cation, do nevertheless tend, when properly un- 
derstood, to make the evidence of stratification 
more permanent, and to point out its primitive 
lines. 
On the plains, in our latitude, we rarely have 
the accumulated layers of several successive snow- 
storms preserved one above another. We can, 
therefore, hardly imagine with what distinctness 
the sequence of such beds is marked in the upper 
Alpine regions. The first cause of this distinc- 
tion between the layers is the quality of the snow 
when it falls, then the immediate changes it 
undergoes after its deposit, then the falling of 
mist or rain upon it, and lastly and most efficient 
of all, the accumulation of dust upon its surface. 
One who has not felt the violence of a storm in 
the high mountains, and seen the clouds of dust 
and sand carried along with the gusts of wind 
passing over a mountain-ridge and sweeping 
through the valley beyond, can hardly conceive 
that not only the superficial aspect of a glacier, 
