220 BULLETIN OF THE 
thickened, thereby increasing the amount of pressure molten water, a 
portion of the fluid would be squeezed under the ice of the peripheral 
zoue, thereby augmenting the thickness of its section, and at the same 
time thinning the ice in the central area. In this manner we can con- 
ceive the creation of a balance in the impulses and resistances affecting 
the movement of the ice which was softened by pressure melting, so 
that the drift of the material toward the margin of the continental 
glacier would be slow and uniform. 
The conditions of our hypothesis require us to suppose that the effects 
of pressure melting would first be felt in the deeper parts‘of the glacier, 
those portions of its mass which lay in the valleys, and that the soften- 
ing of the ice might there be completely effected while the frozen water 
was still in contact with the earth at the higher levels of the surface. 
It thus might well happen that the considerable elevations of the coun- 
try, those hard parts which had survived under the conditions of ordi- 
nary land erosion, would be much more effectively worn down than the 
rock beneath the river valleys. We can thus account for the destruction 
of such a prominence as Iron Hill, which was probably a sharp peak of 
considerable altitude when the glacier began its work, while the neigh- 
boring valleys were but little worn by the action of the glacier. 
So long as a glacier is receiving a considerable annual contribution of 
snow which is built into its mass at a low temperature, it may well be 
that the accumulations of heat due to the work done near the base of the 
ice would not affect any considerable portion of the central section of the 
mass. If now for a time the annual snowfall diminished to the point 
where, by the thinning of the glacier, pressure melting ceased to take 
place, the whole section of the ice might gradually acquire a relatively 
high temperature, so that any sudden increase in pressure might bring 
about very extensive melting. If in this condition of the deposit the 
amount of snowfall should, for a number of years, be greatly increased, 
the result might be a great development of pressure molten water, which 
would be pushed forward towards the margin of the glacier to the point 
where, owing to the diminution in the thickness of the ice, it could be- 
come refrozen. In this way we may perhaps account for those sudden 
and temporary advances in the margin of the glacier which are so clearly 
indicated at various points in this country. 
No direct verification of the hypothesis above deduced is to be ob- 
tained by observation or experiment. The only approach to proof which 
we can hope to secure is by an inspection of the facts exhibited in the 
records of glacial action with a view to ascertaining how far they may 
