216 BULLETIN OF THE 



contribution of heat poured forth into the ice from the earth's interior. 

 All these actions tend to promote the liquefaction of the lower part 

 of the glacial envelope. 



Although it is easy to perceive the existence of a number of efficient 

 causes tending to bring about melting in the lower portion of a conti- 

 nental glacier, it is difficult to form an adequate conception as to the 

 precise way in which these influences would operate. The facts, how- 

 ever, justify us in supposing that the temperature induced in the lower 

 portion of the ice would to a great extent be retained in the deeper 

 parts of the glacier. It is a well known fact that ice is a poor con- 

 ductor of heat, and therefore we may fairly assume that a considerable 

 increment of warmth would be likely to be brought about in the lower 

 part of the section, while the upper portion remained substantially unaf- 

 fected by the condition of the lower parts of the mass. The hypothesis 

 is, in effect, tha.t at a cei'tain stage in the development in thickness of au 

 ice sheet the portion of the mass next the bottom, while still remaining 

 below the temperature of 32° Fahrenheit, becomes converted into water, 

 or into very much softened ice, which cannot escape vertically or hori- 

 zontally froipa the field in which the melting was developed, but remains 

 as a fluid or semi-fluid sheet interveniug between the solid ice and the 

 surface of the earth. 



The conditions of a mass of water at the base of a glacier, owing its 

 essential fluidity to the combined influence of pressure melting and 

 positive contributions of heat, is so peculiar, that it is necessary for us 

 somewhat carefully to examine into its state. At first sight it may seem 

 likely that such a mass of fluid would inevitably be urged by the pres- 

 sure of the superincumbent ice away from the field in which it was 

 formed, moving in the direction of least resistance, which would gen- 

 erally be towards the margin of the glacial field. It is evident that the 

 motion could not be in an upward direction, for in a deep glacier the 

 yielding nature of the material must prevent the formation of fissures 

 at any great distance below the surfjice. Even if such crevasses should 

 be made, they would quickly be closed by the pressure of molten water, 

 which would instantly freeze when it entered their free spaces. It is 

 more difficult to account for the hindrance to the movement of the 

 fluid toward the margin of the glacier. We must, however, conceive 

 that as soon as such a movement took place, and for the reason that it 

 did take place, the pressure molten water, having attained to a position 

 where less weight was imposed upon it, would quickly refreeze. It 

 would be aided in making a certain excursion towards the marj'in of the 



