352 THE JOURNAL OF GEOLOGY. 



energy available in generating movement — may be denominated 

 the dow?i-stream impulse of the glacier. Such impulse, in com- 

 bination with the simple weight of ice at any point, constitutes 

 the inteTisity of glacial action at that point. 



But, ceteris paribus, the measure of rock-grinding is the friction 

 between the glacier and its bed. Now such friction is a complex 

 function of the weight and down-stream impulse, and varies with, 

 but probably less rapidly than, their product. The general law of 

 friction, applicable under wide ranges of pressure and velocity, 

 has never, indeed, been clearly formulated ; and where the con- 

 tiguous surfaces are so unlike as rock and ice the friction is 

 scarcely known even in the simplest case.^ In case of such 

 substances, too, if detached rock-fragments intervene, they will 

 project into the more yielding material and thereby increase 

 the frictional surface; when the slip may either (i) occur in 

 part on each side of the fragments {i. e., the ice may flow over 

 the fragments, while they themselves move at a slower rate 

 •over the valley-bottom, as has, indeed, been observed by 

 Niles), or (2) may be confined to the inosculating rock-surfaces. 

 Also, if a continuous sheet of comminuted debris intervene, 

 the movement may be divided between its upper and lower sur- 

 faces ; and if the intercalated sheet be thick, several planes of slip 

 may exist within it and its own motion become differential. 

 Again, if fragments of large angles and not greatly different diam- 

 eters project into the ice or lie within a differentially-moving 

 ground moraine, the unequal flow will most rapidly carry for- 

 ward their summits, initiate rolling, and thus diminish friction 

 (and at the same time, perhaps, produce "fluxion-structure"). 

 It follows that the friction in any given case cannot be even 

 approximately evaluated ; and its expression must, therefore, 

 include an indeterminate factor of considerable moment. 



But, again, the disposition to attack the glacier-bed is 



^ Tylor found that with a pressure of two pounds to the square inch the co-efficient 

 of friction of ice upon ice was between o.i and 0.2, and concluded that glacier motion 

 would be impossible without water to lubricate the bottom. Geol. Mag., Dec. II., 

 Vol., II., 1875, P- 280. 



