222 BULLETIN OF THE 
the supposition that the ice throughout the field rests upon the bed rock. 
Under these conditions it appears necessary for its surface to have a 
slope towards the margin of some degrees of declivity in order that the 
sheet may be impelled downward with sufficient energy to overcome the 
great resistance due to its friction on the bed rock. A slope sufficient to 
accomplish this purpose would require an inconceivable thickness of ice 
in the central part of the North American glacier. The hypothesis of 
pressure melting shows us a way out of this difficulty. We have only 
to conceive the central parts of the area of the glacier to be freed from 
the basal friction, to avoid the need of hypothecating a considerable 
slope of the surface except near the margin of the ice. In this view, 
the element of friction on the bed rock is substantially reduced to a belt 
of limited width into which the ice is fed from the areas where pressure 
melting occurs. 
The sudden advances and recessions in the position of the glacial 
front can be better accounted for on this hypothesis than in any other 
way. A slight increase in the pressure in the central portions of the 
field, such as might be brought about by an increased snowfall extending 
over a term of a few years, would probably lead to the discharge of 
water rendered more or less fluid by compression into the marginal 
portions of the area. This would naturally be attended by a sudden 
outward march of the ice. In this way we may explain the prevailingly 
wide fringe of territory in the Mississippi Valley which lies to the south- 
ward of the southernmost distinct moraine, and which appears to have 
been temporarily occupied by the ice sheet. This district is covered 
by a layer of glacial waste, but at its outer margin we find none of 
those accumulations of detritus which indicate the permanent occupation 
of a line by a glacial front. 
It appears to me that we may by the hypothesis of pressure melting 
explain the formation of those very thick deposits of till which occur in 
certain parts of the glaciated area, and this in the following manner. 
Until a glacial sheet has accumulated to such a depth as to bring 
about pressure melting, the combined erosion of the bed rock and the 
irregular movement of the ice near the surface over which it moves 
bring about the admixture of rocky material with the frozen water to 
the depth, it may be, of some hundred feet above the earth. If now 
pressure melting begins, the débris will gradually drop upon the surface, 
and this action will continue until perhaps all the detritus previously 
intermingled with the ice has become separated from it. If from time 
to time the glacier became so far thinned that its solid parts again 
