320 GEOLOGY. 



the lower reaches of alpine glaciers, where the temperatures are near 

 the melting-point, and the ice is bathed in water, movement may take 

 place in ice which is thin and compact. 



If the views here presented are correct, there is also, near the end 

 or edge of a glacier, the cooperation of rigid thrust from behind with 

 the tendency of the mass to move on its own account. The latter is 

 controlled by gravity, and conforms in its results to laws of liquid flow. 

 The former is a derived factor, and is a mechanical thrust. This thrust 

 is different from the pressure of the upper part of a liquid stream on 

 the lower part, because it is transmitted through a body whose rigidity 

 is effective, while the latter is transmitted on the hydrostatic principle 

 of equal pressure in all directions. 



Corroborative Phenomena, 



The conception of the glacier and its movement here presented 

 explains some of the anomaUes that otherwise seem paradoxical. While 

 a glacier in a sense flows over a surface, it often cuts long, deep furrows 

 in firm rock. It is difficult to explain this if the ice be so yielding as to 

 flow under its own weight on a surface which is almost flat. If the mass 

 is really viscous, its hold on its imbedded debris should also be viscous, 

 and a bowlder in the bottom should be rotated in the yielding mass 

 when its lower point catches on the rock beneath, instead of being firmly 

 held while a deep groove is cut. This is more to the point since viscous 

 fluids flow by a partially rotary movement. If, on the other hand, the 

 ice is always a rigid body which yields only as its interlocking granules 

 change their form by loss and gain, a rigid hold on the imbedded rock 

 at some times, and a yielding hold at others, is inteUigible, for on this 

 view the nature of its hold is dependent on the temperature and dry- 

 ness of the ice. Stones in the base of a glacier may be held with very 

 great rigidity when the ice is dry, scoring the bottom with much force, 

 while they may be rotated with relative ease when the ice is wet. In 

 short, the relation of the ice to the bowlders in its bottom varies radi- 

 cally according to its dryness and temperature. A dry glacier is a rigid 

 glacier. A dry glacier is necessarily cold, and a cold glacier is necessarily 



dry. 



On the view here presented, a glacier should be more rigid in winter 



