76 THE JOURNAL OF GEOLOGY. 



frequently appear on consequent streams, but they are rare on 

 subsequent streams. 



It is manifest that the development of subsequent streams 

 will progress to the greatest extent in regions of disordered and 

 complicated structure, in which the attitude of the rocks is 

 varied, and in which contrasts of hardness are well marked. 

 Such is the case in mountainous regions. On the other hand, 

 regions of horizontal structure have no normal subsequent 

 streams. All the branch streams are either perpetuated con- 

 stant streams, or else they are developed under accidental con- 

 trols, of which no definite account can be given. It is to these 

 self-guided streams that McGee applies the term, autogenetic. 



Divides. — The constructional divides waste slowly and become 

 consequent divides. They are well defined in a region of dis- 

 tinct constructional relief ; they are vague or practically absent 

 on the even surface of young plains, where the drainage areas 

 are really undivided. As subsequent streams develop, especially 

 in regions of tilted structure, they frequently split a consequent 

 divide, and make two subsequent divides between which lies the 

 growing subsequent valley. As the subsequent divides are split 

 further and further apart, lateral subsequent streams are devel- 

 oped down the internal slopes of the subsequent valley ; and 

 these are in headwater opposition to the lateral streams on the 

 diminishing slopes of the adjacent consequent valleys. During 

 changes thus produced in the position of divides, they migrate 

 by slow creeping as long as the competing streams are in head- 

 water opposition ; but if, as sometimes happens, the head of an 

 encroaching subsequent stream pushes its divide back until it 

 cuts into the side of a consequent stream, then the divide leaps 

 around the consequent headwaters above the point of capture, 

 and a considerable area that had been tributary to the captured 

 stream is suddenly transferred to the capturing stream. 



A limit of these re-arrangements is gradually approached. 

 The persistent consequent streams and the successful subsequent 

 streams come to an understanding about their drainage areas. 

 The divides as well as the streams are then maturely adjusted to 



