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THE JOURNAL OF GEOLOGY.

energy available in generating movement—may be denominated the down-stream impulse of the glacier. Such impulse, in combination with the simple weight of ice at any point, constitutes the intensity 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 contiguous surfaces are so unlike as rock and ice the friction is scarcely known even in the simplest case.[1] 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 (1) 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 surfaces; 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 diameters project into the ice or lie within a differentially-moving ground moraine, the unequal flow will most rapidly carry forward their summits, initial 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

  1. ↑ Tylor found that with a pressure of two pounds to the square inch the co-efficient of friction of ice upon ice was between 0.1 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.
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