240 
INTERNAL STRUCTURE AND 
the mass has assumed the character of icy snow, 
it does not admit moisture so readily, and still 
farther down, where the snow is actually trans- 
formed into pure compact ice, the amount of 
surface-water admitted into its structure will, 
of course, be greatly diminished. There may, 
however, be conditions under which even the 
looser snow is comparatively impervious to water ; 
as, for instance, when rain falls upon a snow-field 
which has been long under a low temperature, 
and an ice-crust is formed upon its surface, pre- 
venting the water from penetrating below. Ad- 
mitting, as I believe we must, that the water 
thus introduced into the snow and ice is one 
of the most powerful agents to which its motion 
is due, we must suppose that it has a twofold 
influence, since its action when fluid and when 
frozen would be different. When fluid, it would 
contribute to the advance of the mass in propor- 
tion to its quantity ; but when frozen, its expan- 
sion would produce a displacement corresponding 
to the greater volume of ice as compared with 
water ; add to this that while trickling through 
the mass it will loosen and displace the particles 
of already consolidated ice. I have already said 
that I did not intend to trespass on the ground 
of the physicist, and I will not enter here upon 
any discussion as to the probable action of the 
laws of hydrostatic pressure and dilatation in 
