CORRASION BY GRAVITY STREAMS. 215 



elusions already arrived at in other ways by Davis, Gilbert, 

 W. D. Johnson, Penck, Tarr and others ; conclusions perhaps 

 stated the most convincingly by Davis. 1 W. D. Johnson 

 also was one of the Sierran Party and to him the writer is 

 indebted for a discussion of glacial problems during the trip. 

 To Professors Davis, Ohamberliu, Tarr, Daly, J. W. Gregory, 

 Harker and Marr the writer desires also to record his 

 thanks for reading and criticising the report in part, or in 

 whole, while in manuscript; and also to Professors Tarr, 

 Marr and J. W. Gregory, and to Mr. Von Englen of Cornell 

 for their kindness in showing him numerous localities of 

 great glacial interest. For incisive statements as to the 

 mechanics of glaciers the reader should consult: — W. J. 

 McGee, Glacial Canyons, Journ. Geology ii, 1894, pp. 350- 

 364 ; H. P. Reid, The Mechanics of Glaciers, Journ. Geol., 

 iv, 1896, pp. 712-728. 



Some Characteristics of Streams.— The volume of a 

 stream and the topography traversed determine the relative 

 ease or difficulty with which the stream shall reach the 

 local or main base-level. The variable motion induced by 

 gravity between the textural units of the mass constitutes 

 flow. Great freedom of motion between the various tex- 

 tural units gives mobility. The action of gravity is normal 

 to the earth's general surface. Therefore, if we confine 

 our attention to the considerations only of increase or 

 decrease of velocity and the lines of direction taken by 

 various stream masses, it is evident that whatever may be 

 the stream composition, each will respond in a similar way 

 to gravity (but of course not quantitatively) when travers- 

 ing similar topographic profiles, for a fixed vertical force 

 at any spot has also a fixed component along a slope 

 possessing a fixed angular value. 



Surface of Stream. — Each stream, considered as a whole, 

 must have a surface declining in the direction of flow. 



1 Proc. Boston Soc. Nat. Hist., xxix, 1900, pp. 277 - 322, also Davis (c). 



