GLACIAL CANONS. 353 



(^ceteris paribus) measured by the ratio between weight and down- 

 stream impulse ; for manifestly, if the weight be in excess, the 

 predominant tendency must ever be to fix and retain in their 

 places all bowlders, pebbles, sandgrains, and smaller particles ; 

 when the weight and impulse are as iv and v in the diagram 

 (fig. i) their resultant will tend to retain rather than remove 



Fig I. 



such fragments, and transportation will be limited to that due 

 to friction and sub-glacial water ; when the factors are equal, as 

 are w' and v' , their resultant will tend equall}^ to retain and to 

 remove particles, and the effects of friction and flowing water 

 will be counteracted by the greater specific gravity of rock than 

 ice; and when the ratio is as zv" to v" , the disposition will be 

 to overturn and sweep forward all fragments. Also, the weight 

 of ice tends to produce crushing of the rock in a degree probably 

 increasing increasingly with its value. Finally, with increased 

 weight will go increased pressure-liquefaction of the ice, and 

 from this will result the antagonistic effects of reduced fric- 

 tion and augmented transportation. The last two agencies are 

 variable, only very roughly determinate in the ordinary case, and 

 generally of inconsiderable value. They may be thrown together 

 as an unknown factor which, in conjunction with the predominant 

 first agency, constitutes the effectivefiess of glacial erosion at any 

 point. 



The three elements of intensity, friction, and effective7iess, there- 

 fore, determine the rate of glacial erosion. To more succintly 

 express their relations, let — 



w^weight of ice at any point ; 



2/= down- stream impluse at any point. 



5= rock-surface in contact with any vertical prism of ice ; 



jir=unknown factor in friction term ; and 



^■=unknown factor in effectiveness term. 



