MUSEUM OF COMPARATIVE ZOOLOGY. 217 
field by the fact that it was urged forward by the pressure of the over- 
lying 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 
