n where, because of the thinning of the
glacier, it would enter a field where pressure melting did not occur.
It would then resume the solid state, and thence journey to the margin
of the ice in the ordinary manner. We thus can imagine how such a
glacier as occupied the northern part of this continent could have
moved from the central parts toward its periphery, as we can not do if
we assume that the glacier everywhere lay upon the bed rock. There is
no slope from Lake Erie to the Ohio River at Cincinnati. Knowing that
the ice moved down this line, there are but two methods of accounting
for its motion: either the slope of the upper surface to the northward
was so steep that the mass would have been thus urged down, the upper
parts dragging the bottom along with them, or the ice sheet for the
greater part of its extent rested upon pressure-molten water, or
sludge ice, which was easily squeezed out toward the front. The first
supposition appears inadmissible, for the reason that the ice would
have to be many miles deep at Hudson Bay in order that its upper
surface should have slope enough to overcome the rigidity of the
material and bring about the movement. We know that any such depth is
not supposable.

The recent studies in Greenland supply us with strong corroborative
evidence for the support of the view which is here urged. The wide
central field of that area, where the ice has an exceeding slight
declivity, and is unruptured by crevices, can not be explained except
on the supposition that it rests on pressure-molten water. The thinner
section next the shore, where the glacier is broken up by those
irregular movements which its wrestle with the bottom inevitably
induces, shows that there it is in contact with the bed rock, for it
behaves exactly as do the valley glaciers of like thickness.

The view above suggested as to the condition of continental glaciers
enables us to explain not only their movements, but the relatively
slight amount of wearing which they brought about on the lands they
occupied. Beginning to develop in mountain regions, or near the poles
on the lowlands, these sheets, as soon as they attained the thickness
where the ice at their bottom became molten, would rapidly advance for
great distances until they attained districts where the melting
exceeded the supply of frozen material. In this excursion only the
marginal portion of the glacier would do erosive work. This would
evidently be continued