230 E. C. ANDKEWS. 



rapidly decreasing while the time factor remains constant. 

 That is, the weaker stream portion has the same time only 

 in which to accomplish its task as the stronger portion. 

 The curve A'O" has a tendency therefore td be steeper at 

 its head than at points lower down, and at a certain point 

 below the head determined by the volume of the stream, 

 the curve will rapidly flatten and at the deepest point will 

 assume tangency with a horizontal plane. (See Appendix). 

 Thence downstream with ever decreasing stream force the 

 curve is reversed, until it assumes tangency once more with 

 a horizontal plane or the plane of the associated local base 

 level. (See Appendix). 



We are still confronted with the problem of the general 

 appearance of the excavation. Before discussing this, 

 however, it will be advisable to inquire a little more fully 

 into the general conditions determining the cut or excava- 

 tion below the associated base level. 



All other things being equal, a stream which flows along 

 a channel (excavated by itself or by another stream type) 

 depends upon increased volume to produce an increase of 

 its velocity along the same channel. Similarly, greatly 

 increased volume is necessary also to produce great increase 

 of velocity here. Therefore if great local irregularities of 

 channel grade occur (in valleys reasonably supposed to be 

 stream developed) the inference is that here a stream acted 

 under conditions of great local volume and velocity com- 

 bined. And this causes us to ask what are the conditions 

 in nature which could induce such a state of increase of 

 stream velocity locally? 



Consider the action of a stream, say 10,000 feet deep : — 



(a) Over the plane surface of a solid composed of homo- 

 geneous structures. Here it will appear to act somewhat as 

 a plane, and no differential corrasion will be accomplished. 



