COEKASION BY GRAVITY STREAMS. 229 



channel constriction. The depth will be a measure of the 

 mass and velocity of the stream, and in a measure also, 

 that of the time occupied by stream cutting. 



Two laws of stream corrasion in homogeneous structures 

 deserve mention here : 



(1) All other things being equal the greater the stream 

 velocity the more will the headward profiles of the cut 

 formed below base level by corrasion be inclined to the 

 vertical. 



(2) The more freely does the force of gravity act, the 

 more nearly will the basin head approach a vertical form. 



These follow from a consideration of the path of a stream 

 particle and the constancy of the time factor involved. 

 From the deepest point of the cut so determined the slope 

 of the basin can now be determined geometrically. In 

 Fig. 3 (c) let A"0" represent the head of the cut, and O" 

 its deepest point. 



Let us first consider the profile A"0". Although A"0" is 

 a descent, nevertheless, it lies wholly below the associated 

 local base level because the corrasive cut has been made 

 locally in a horizontal channel base and must therefore 

 pass into a reversed grade downstream. From mechanical 

 considerations every measurement of descent made by the 

 stream into this basin thus represents loss of velocity. For 

 the stream derived its velocity from a point higher 

 upstream and in order to maintain this basin bottom as the 

 new channel grade for the transportation of debris, the 

 stream has to lift the dead weight of the mass filling the 

 basin, and notwithstanding this, it must still maintain its 

 general surface slope down stream over the basin itself. 

 This implies pronounced loss of velocity locally. But 

 this increasing loss of velocity is accompanied by a decrease 

 of corrasive power in a high geometrical ratio. Therefore, 

 from point to point along A"0" the power of the stream is 



