5/8 



MARIUS R. CAMPBELL 



subsidences in the Appalachian province ; and it is well to con- 

 sider the case in detail so as to familiarize ourselves with the 

 phenomena which it produces. 



Fig. 5. 



Fig. 6. 



Fig. 7. 



Fig. 8. 



Starting with the assumption of two streams, M, N, and Q, 

 P, (Fig. 5) , equally balanced against each other, we will sup- 

 pose a subsidence to occur at the point N, causing it to assume 

 in successive order the position 7V^ N^ and N^. It is obvious 

 that if the depression is more rapid than the corrasion of the 

 stream on either side of the depression, the stream will become 

 ponded at the point A^, and will, in all probability, seek an outlet 

 at right angles to its former course and approximately along the 

 axis of subsidence. If, on the other hand, the movement is so 

 slow that erosion can keep pace with the depression the change 

 will not be so radical, but in the end will result in approximately 

 the same arrangement as sketched for the former case. It will 

 be accomplished about as follows : In the first stage of the 



