286 GEOLOGY 



winter than in summer. The total thickness of a glacier should ex- 

 perience this rigidity of winter 'at its ends and edges, where the 

 ice is thin enough to permit the low temperature to affect its 1 >< >t t < >i 1 1 . 

 The motion in these parts during the winter is, therefore, very 

 small. 



In this view, also, may be found an explanation of the move- 

 ment of glaciers for considerable distances up-slope, even when the 

 surface of the ice, as well as its bed, is inclined backwards. So far 

 does this go, that superglacial streams sometimes run for some 

 distance backwards, i. e., toward the heads of the glaciers, while in 

 other places surface waters are collected into ponds and lakelets. 

 Such a slope of the surface of ice is not difficult to understand if 

 the movement is due to thrust from behind, or if it is occasioned 

 by internal crystalline changes acting on a rigid body; but it must 

 be regarded as very remarkable if the movement of the ice is that 

 of a fluid body, no matter how viscous, for the length of the acclivity 

 is sometimes several times the thickness of the ice. Crevassing 

 and other evidences of brittleness and rigidity find a ready eluci- 

 dation under the view that the ice is really a solid body at all times, 

 and that its apparent fluency is due to the momentary fluidity of 

 small portions of its mass assumed in succession as compression 

 demands. 



In addition to the considerations already adduced, it may be 

 urged that a glacier does not flow as a stiff liquid because its gran- 

 ules are not habitually drawn out into elongated forms, as are 

 cavities in lavas, and plastic lumps in viscous bodies. Flowage 

 lines comparable to those in lavas are unknown in glaciers. 



All this is strictly consistent with our primary thesis, that a 

 glacier is crystalline rock of the purest and simplest type, and that 

 it never has other than the crystalline state. This strictly crysta- 

 line character is incompatible with viscous liquidity. 



Other Views of Glacier Motion 



While these views of glacial motion seem to us to best accord 

 with the known facts, they are not to be regarded MS established in 

 scientific opinion, or as the views most commonly held. The mode 

 of glacial motion has long been a mooted question, and is still so 



