CORBASION BY GRAVITY STREAMS. 221 



of elevation or depression) during the period under con- 

 sideration. 



If now the stream moves the mass of debris whose height 

 is M B bodily over the channel base ABCD then corrasion 

 of the base A BOD must ensue. If, however, the stream 

 has power merely to roll the upper mass, say of height 

 MM', over the lower stream debris (of height BM') then 

 the stream cannot corrade the channel structures at that 

 point — that is to say it does not accomplish "work" at 

 that spot, for in order to accomplish land dissection or 

 peneplanation a stream must have a vertical measure of 

 strength sufficient to allow it to remove the debris and 

 to get at the channel base proper. 



Principles common to stream corrasion. — We have 

 ascertained already that each type of stream will tend to 

 experience increase or decrease of velocity under certain 

 similar conditions ; that each will take practically the same 

 route to base level (allowance being made for the magni- 

 tudes and velocities of the streams considered), each taking 

 the line of least resistance. This indicates that the cor- 

 rasive action by streams is somewhat analogous in general 

 aspect. 1 



It will now be advisable to ascertain the existence of the 

 relations between increase of velocity and the energy of 

 streams. As an example, suppose that the velocity of any 

 stream be doubled. Viewed from the standpoint of stream 

 impact, the doubled velocity is at least four times as effec- 

 tive as the initial one, for now, in any given time, twice as 

 much stream material acts upon a given point with twice 

 the velocity as compared with that of the point acted on 

 by the stream when possessing the original velocity. If 

 the velocity be trebled, then three times the amount of 

 stream material passes a given point in any given time with 



1 See also Appendix. 



