THE WORK OF SNOW AND ICE 285 



lesser thicknesses of ice. In the lower reaches of Alpine glaciers, 

 where the temperatures are near the melting-point, and where the 

 ice is bathed in water much of the time, movement may take place 

 in the ice which is thin and compact. 



If the views here presented are correct, there is also, at all 

 points below the source, 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 trans- 

 mitted on the hydrostatic principle of equal pressure in all direc- 

 tions. Thrust would be most effective toward the end or edge of 

 a glacier. 



Corroborative Phenomena 



The conception of the glacier and its movement here presented 

 explains some of the anomalies that otherwise seem paradoxical. 

 If the ice is always a rigid body which yields only as its interlock- 

 ing 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 

 intelligible. Stones in the base of a glacier may be held with 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 

 radically 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. 



It is difficult to explain the furrows and grooves cut by glaciers 

 in firm rock if the ice is 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 held firmly 

 while a groove is cut. This is especially to the point since viscous 

 fluids flow by a partially rotary movement. 



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



