GLACIAL CANONS. 



361 



of intensity in consequence of the absolutely reduced down-stream 

 impulse ; also [d) material increase of friction with the augmen- 

 tation of its principal factor, and (^) less material diminution of 

 friction in consequence of the reduced impulse; and finally (/), 

 direct diminution of effectiveness with the absolute decrease of 

 impulse, (^) indirect diminution of effectiveness in consequence 

 of the relative decrease of the same factor, and (/z) direct but 

 slight increase of effectiveness in virtue of the operation of the 

 obscure factor of rock-crushing and pressure-liquefaction ; or, 

 summarily, increase in intensity, slight increase in friction, and 

 decrease in effectiveness. 



Now, in view of the obscure and antagonistic though inter- 



Fig. 4. 



dependent relations involved, it is evident that without exhaust- 

 ive quantitative investigation (impossible in the present absence 

 of knowledge concerning friction between ice and other substan- 

 ces) it cannot be determined in the ordinary case whether the 

 disposition will be to erode the more rapidly where weight 

 increases at the expense of declivity, or where the reverse occurs ; 

 but it appears quite certain that where the surface declivity 

 materially exceeds that at the base, and where, accordingly, the 

 impulse is not reduced proportionally to the declivity of the 

 channel, erosion must progressively increase with the weight. If 

 so, the tendency of glaciers must be to cumulatively intensify 

 the irregularities in gradient normal to water-cut canons. 



But corrasion and transportation in any part of a glacier-bed 

 are limited directly by flow of ice and indirectly by coincident 



