MUSEUxM OF COMPAKATIVE ZOOLOGY. 221 



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 in 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 I'ock. 



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 in a 

 measure at ail 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 

 encovmter are cleared away. 



It seems impossible to explain the motion of a continental glacier on 



