MUSEUM OF COMPARATIVE ZOOLOGY. 220 
be explained by the suggestions which we have been considering. Be- 
ginning this comparison with the Greenland glacier, the only field where 
we can find conditions approaching those which existed in the greater 
ice field of the American continent, we note the following facts. In 
the marginal portions of the Greenland ice field the slope of the surface 
towards the sea is tolerably steep, and is rent by numerous crevasses. 
Gradually, as we pass from the frontal portions of the ice to the interior 
of the field, these crevasses disappear, and the slope of the glacier be- 
comes slight and unbroken. On the crest there is a wide field where 
the glacier has the character of a great plain with a slope so slight that 
we cannot well conceive the movement towards the margin as taking 
place over the surface of the underlying rocks in the manner iu which it 
occurs near the borders of the sea. These conditions are reconcilable 
with the assumption that the central part of this great glacier rests upon 
ice which has been softened by pressure to the point where it no longer 
behaves with its normal rigidity, but acts substantially as a fluid, while 
in the peripheral section, that which is beset with crevasses, we have to 
suppose that the glacier rests upon the bed rock. 
Turning now to the conditions of the area on the mainland of this 
continent, so far as they were effected by the ice of the last Glacial 
Epoch, we may briefly review the features which are explicable by the 
hypothesis which we are considering. We note at the outset the fact, 
to which the reader’s attention has already been directed, that the ero- 
sion accomplished by the ice in the interior of the glaciated field is often 
very small. We may now extend this statement by saying that the 
wearing which has occurred in the central portions of the area occupied 
by the ice bears no kind of proportion to the depth to which the sheet 
evidently attained, or to the length of time which it must have remained 
on the surface. If space permitted, it would be possible to bring up an 
extended array of instances, such as that cited from the region north of 
Kingston, Ontario, where in districts in which the glacier must have 
been very deep and long enduring the erosive work was less than in the 
marginal parts of the field. These facts do not seem to be explicable on 
the supposition that the glacier wore the surface over which it lay ina 
measure at all proportionate to its depth or the continuity of its action. 
If, however, we suppose that only the marginal zone of the ice prevail- 
ingly rested on the surface of the earth, and that a great part of the field 
lay upon a fluid or semi-fluid stratum of water, the difficulties which we 
encounter are cleared away. 
it seems impossible to explain the motion of a continental glacier on 
