GLACIAL CANONS. 357 



influence in any direction must be slight, and its effect may be 

 disregarded). 



Combining the several antagonistic factors, it appears uncer- 

 tain whether the general tendency of the friction element is to 

 widen or deepen the gorge, but certain that it is to develop con- 

 cavity of the valley sides and the U form of cafion. 



Since the third and fourth factors in down- stream impulse 

 (available potential energy and friction) are indeterminate, the 

 problem as to the declivity required to render such impulse equal 

 to the weight at any point in a given glacier, or even as to 

 whether such equality ever obtains in nature, cannot be analyt- 

 ically solved ; and very few observations showing the relative 

 value of these components have ever been made. Niles,^ how- 

 ever, found that in the Great Aletsch glacier the ice usually rides 

 upon projecting rugosities and seldom fills the intervening 

 depressions of its bed, and that a bowlder (itself slowly moving) 

 three feet high had formed an inverted trough thirty feet long 

 in the base of the incumbent ice ; whence the down-stream 

 impulse must have exceeded ten times the weight. Bonney,^ 

 also, in the Glacier des Bois and the Glacier d' Argentiere, found 

 all broad and gentle depressions in the glacier beds filled with im- 

 pressed ice, the narrower depressions not quite filled, the lee of 

 projecting knobs protected for a distance equal to their height, and 

 bowlders lying i?i situ beyond the present terminus of the ice gla- 

 ciated above and below (showing that here also motion took 

 place along the two planes), all of which phenomena indicate 

 that, in these glaciers, the down-stream impulse is in excess of 

 weight, but in a less degree than in the Great Aletsch. The 

 several observations then demonstrate (i), that down-stream 

 impulse may greatly exceed weight, and (2), that the relation 

 is variable. All were in the upper portions of the valleys where 

 the declivity is great (15° to 20° in the examples described by 

 Bonney), and where the office of the glaciers is preeminently one 



'Proc. Boston Soc. Nat. Hist., XIX., 1878, 330 ; Am. Jour. Sci., XVI., 1878, 366. 

 *Geol. Mag., Dec. II., Vol. III., 1876, 197. 



