CORRASION BY GRAVITY STREAMS. 257 



large obstacle the surface slope of the stream is suddenly 

 and markedly steepened, because here the flowing mass 

 is presented in its entirety with an opportunity to rapidly 

 take up a position much nearer the main base level, and 

 the case is then one of accelerated velocity, in which reduc- 

 tion of the stream cross-section must follow as a necessity 

 (from the fact that time is a constantly flowing quantity). 

 The action of gravity is now to produce a constantly 

 increasing stream velocity, and it works partly to produce 

 free falling action and partly to produce flowage phenomena. 

 In the extreme case where the downstream aspect is a 

 perpendicular or overhanging wall, gravity has free play, 

 the mass then in great measure falls over the wall, and 

 flowage is resumed lower down. But even in this general 

 case the constant acceleration of gravity operates all along 

 the declivity and the heaviest action is therefore delivered 

 at or near the foot of the declivity. 



Masses which are traversed by streams and which also 

 are very large as compared with the stream volume suffer 

 immense loss by the corrasive action of the stream, but 

 give added life to the stream as a whole by causing it to 

 traverse their down stream slopes with increased velocity. 

 On the one hand they rob the stream of power through 

 being corraded by it ; on the other hand they force it rapidly 

 to occupy a position nearer the base level, and thus its 

 power is increased. The algebraic sum of the activities is 

 a net gain to the stream strength. Certain qualifying con- 

 ditions should be borne in mind in this connection : — 



(i) With increasing stream velocity comes greater ability 

 to shoot over the obstacle slope before the vertical acceler- 

 ant of gravity at any point has time to operate fully. 

 (Fig. 3.) 



(ii) With increasing slope the same law operates. 



