36o THE JOURNAL OF GEOLOGY. 



at the canon-mouth be not relatively increased in a ^considerable 

 degree. 



It follows that the second feature of the typical glacial 

 canons may naturally result from temporary occupation of water- 

 cut canons by ice, and that it does not necessarily argue profound 

 glacial erosion. 



IV. 



In obedience to the law of varigradation,^ all and particularly 

 smaller streams tend to depart in a minor degree from uniform 

 gradient, and to develop in their channels a longitudinal profile 



Fig. 3. 



of slightly variable declivity ; this law finding expression in the 

 alternating pools and rapids of mountain brooks and in the 

 always perceptible and often conspicuous alternations of greater 

 and less declivity in the courses of water-cut caiions. 



If now an otherwise uniform V canon of irregular gradient 

 become occupied by a glacier, the flow, varying as it does with 

 the declivity, will become unequal and the ice will tend to accu- 

 mulate on the planes of low declivity until it approaches a uni- 

 form surface slope ; when the weight of ice at different points in 

 the medial or other longitudinal plane of the glacier will become 

 variable, and will reach a maximum over the greatest depression 

 (fig. 4). With such increased weight will go [a) direct increase 

 of intensity with the augmentation of its principal factor, {Ji) 

 indirect increase of intensity in virtue of the office of weight as a 

 function of the down-stream impulse, and [c') direct diminution 



' Op. cit, p. 295. 



I 



