264 



based npoD anaound premises. He must not confuse the 

 order of inorganic activities, neither must he confuse the 

 operation of these activities during their most powerful 

 and their decadent phases. 



2. To place the matter clearly before the reader, a valley 

 such as the Yosemite in California is considered, and the 

 various steps in its formation are described. The conditions 

 favouring local increase of glacial velocity are consiih-ivd 

 first, and this leads naturally to the consideration of certain 

 fundamental conceptions in stream mechanics, such as the 

 path of a stream particle, the strengths of streams as com- 

 pared with the channel structures traversed, the kinetic 

 energy of the stream, the energy available for corrasion, 

 the location of maximum and minimum corrasion along a 

 channel, the relation of channel form and stream strength 

 upon declivities and in constrictions, the formation of roches 

 moutonnees, and the profile of the cirque and "step." The 

 relations of cirques to "steps" are also considered. Next 

 the significance of glacial decadence is discussed. This 

 introduces the subject of weathering at the cirque heads 

 after the pronounced shrinkage in volume of the glacier. 

 The effect of decrease in ice volume upon glacial flow in 

 the cirque is also dwelt upon briefly. 



The reader who desires to follow the historical side of 

 the question of glacial corrasion may read with profit the 

 works of Culver, 1 Fairchild, 2 and Hobbs. ! In the work of 

 Hobbs an interesting description of the glacial cycle is given. 

 Reference may also be made here to a recent report by 

 Tarr, 4 in which there is a distinct contribution to the 



; p C«lv,r,.! 



E.."The erosive action of ice," Trans 



Wis. 



Acad. Sci., 



* F.-.ir.-hil.l 



Sor. Am., xvi. 



1 IM,l.s. \\ 



H. Le Roy, " Ice Erosion Theory a fallacy," 

 H., - The Cycle ^Mountain Gladatfon!" G 



Solt 



SX *Tarr°R S 



, " The Yakutat Bay Region, Alaska," 

 sional Paper 64, 1909. 



U. S 



Geological 



