The Theory of Glacier motion. 151 



If ice were contained in a basin, like water in a lake, or 

 spread out on a level plain, it would neither crack nor move 

 unless thrust out by external pressure, and such pressure in 

 nature can, so far as we see, only be derived from gravi- 

 tation. 



As Mr. Trotter puts it : " The fuller consideration of the 

 physical properties of glacier ice leads to essentially the 

 same conclusions as those to which Forbes was led 41 

 years ago, by the study of the larger phenomena of glacier 

 motion, that is, that the motion is that of a slightly viscous 

 mass, partly sliding upon its bed, partly shearing upon 

 itself under the influence of gravity " {Proc. Roy. Soc, 

 XXXVIII. 107). 



This conclusion is a very important one. It displaces 

 a great deal of ingenious and in some cases transcendental 

 reasoning on the nature and phases of ice, with which the 

 writings of very distinguished men have been sophisticated 

 in the last quarter of a century ; and it effectually disposes 

 of the theories of great ice sheets which the current school 

 of glacial geologists has imposed on the credulity of men 

 of science. 



