316 



GEOLOGY. 



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 granules in any 

 part of a glacier partake in the accumulated motion of all parts nearer 

 the source, and hence all are thrust forward. Herein appears to lie 

 the distinctive nature of glacial movement. Each part of a stream 

 of water feels the hydrostatic pressure of neighboring parts (theoretically 



Fig. 293. — Shearing plane well defined. A Spitzbergen glacier. (Hamberg.) 



equal in all directions) and the momentum of motion, but not the rigid 

 thrust of the mass behind. Lava streams are good types of viscous 

 fluids flowing in masses comparable to those of glaciers and on similar 

 slopes, and, in their last stages, at similar rates, but their special modes 

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

 cally different from those of glaciers. Forceful abrasion, and particu- 

 larly the rigid holding of imbedded stones while they score and groove 

 the rock beneath, is unknown in lava streams and is scarcely conceivable. 



