MUSEUM OF COMPARATIVE ZOOLOGY. 217 



field by the fact tluat it was urged forward by the pressure of the over- 

 lyiug ice, and this energy of movement would, to a certain extent, be 

 converted into heat through the frictions which the liquid encountered 

 on its journey. 



The migration of pressure molten water towards the margin of the 

 ice would doubtless be somewhat restrained by the plastic condition 

 which occurs in ice when pressure melting begins. In the familiar 

 experiment which is made by subjecting a column of ice to com- 

 pression, we observe that the melting does not occur simultaneously 

 throughout the mass, but it begins along the planes of junction of its 

 crystalline or fragmental elements, films of water developing along these 

 planes, and gradually extending in width until the whole mass becomes 

 softened to the point where it loses its rigidity without becoming 

 generally fluid. It seems reasonable to conceive that the passage from 

 a sub-glacial area, where the water was melted at a temperature below 

 32° Fahrenheit, to a thinner part of the glacier, where the solid ice 

 rested on the ground, would be through a belt where the ice was in a 

 semi-fluid condition, which would serve through the frictions which 

 would there be engendered somewhat to restrain the flow. With these 

 preliminary suggestions as to the probable state of the bottom of a 

 deep glacier, we may now proceed to examine into certain corollaries 

 which may fairly be drawn from the main propositions. 



As long as a glacier rests upon the bed rock in the form of ice, its 

 foundation seems necessarily subjected to intense erosive action, but as 

 soon as the ice next the bed rock is converted into pressure molten 

 ■water, this wearing must cease, and the area would probably at once 

 become more perfectly insured from any form of erosion than any other 

 portion of the earth's surface. This exemption from change would 

 continue until, by a process of thinning of the glacier, its base was 

 permitted to return to the frozen state. It therefore seems possible 

 that where a deep glacier is developed upon any area we are likely 

 to have at first active erosion ; then a state in which wearing rather 

 suddenly ceases, because the ice thickens and becomes warmed, and 

 therefore melts in the manner before described ; and, last of all, with 

 the passing away of the ice, the thinned sheet may come again to 

 move over the bottom, and for a time to repeat the erosive work which 

 was discontinued while the ice retained a great depth. 



In case pressure molten water were extensively developed at the 

 base of a great glacier, such as occupied the northern part of this con- 

 tinent, we should have to conceive the bottom of the ice, as regards its 



