MOTIONS OF GLACIER ICE 



143 



sun- of neighboring parts (theoretically equal in till directions), and (2) the mo- 

 nu-ntum 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. 



l.ava streams are good examples of viscous fluids flowing in masses comparable 

 to those of glaciers, on similar slopes, and, in the last stages of motion, at similar 

 ratrs; but their special modes of flow and their effects on the sides and bottoms of 

 their paths are radically 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 



Fig. 143. A well defined shearing plane in a Spitsbergen glacier. (Hamberg.) 



far as we know, no experimental or natural evidence that any viscous fluid, in the 

 ordinary sense of that term, detaches and picks up fragments and holds them t'irmlv 

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

 bottom over which it flows. It would seem that competency on the part of a 

 viscous body to do this peculiar class of work so distinctive of glaciers should be 

 demonstrated before the viscous theory of glacial movement is accepted as even a 

 good working hypothesis. In contrast with viscous movement, it is conceived that 

 a glacier is thrust forward rigidly by internal elongation, and that it is sheared 

 forcibly over its sides and bottoms, leaving its distinctive marks upon them. 



Slimrin^. In the terminal part of a glacier, where the thrusts are greatest, 

 where the granules are fewest and their interlocking most intimate, shearing takes 

 plan- within the ice. This is illustrated by Figs. 143 and 144. The shearing re- 

 sults in the foliation of the ice, and in the dragging of debris along the planes of 

 shear. Shearing is observed chiefly where the ice below the plane of shearing is 

 pr. 'tected more or less from the force of the thrust, as in the lee of a hill or mass of 



