THE WORK OF SNOW AND ICE 281 



of greatest stress, the result being accomplished not so much by 

 the lowering of the melting-point as by the development of heat 

 by pressure. 



This conception of glacial movement involves the momentary 

 liquefaction of minute portions of the ice, while the mass as a whole 

 remains rigid, as its crystalline nature requires. Instead of assign- 

 ing a slow viscous fluidity like that of asphalt to the whole mass, 

 which seems inconsistent with its crystalline character, it assigns 

 a free fluidity, momentarily, to a succession of particles that form 

 only a minute fraction of the whole at any instant. 



This conception is consistent with the retention of the granular 

 condition of the ice, with the heterogeneous (in the main) orientation 

 of the crystals, with the rigidity and brittleness of the ice, and with 

 its strictly crystalline character, a character which a viscous liquid 

 does not possess, however much its high viscosity may make it 

 resemble a rigid body. 



Accumulated motion in the terminal part of a glacier. However 

 slight the relative motion of one granule on its neighbor, the gran- 

 ules in any part of a glacier partake in the accumulated motion of 

 all parts nearer the source, and hence all except those at the head 

 are thrust forward. Herein appears to lie the distinctive nature 

 of glacial movement. Each part of a stream of water feels (1) the 

 hydrostatic pressure of neighboring parts (theoretically equal in 

 all directions), and (2) the momentum of motion, but not the thrust 

 of the water up stream. This is probably one of the fundamental 

 differences between water flow and glacier motion. 



Lava streams are good examples of viscous fluids flowing in 

 asses comparable to those of glaciers, on similar slopes, and, in 

 e later stages of motion, at similar rates; but their special modes 

 flow and their effects on the sides and bottoms of their paths are 



ically different from those of glaciers. Forceful abrasion, and 

 particularly the rigid holding of imbedded stones which score and 

 groove the rock beneath, is unknown in lava streams, and is scarcely 

 conceivable. There is, so far as we know, no experimental or nat- 

 ural evidence that any viscous fluid, in the ordinary sense of that 

 term, detaches and picks up fragments and holds them firmly as 

 graving tools in its base so as to cut deep, long, straight grooves 



