Page:The Journal of geology (IA journalofgeology21894univers).pdf/90
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 constant streams, or else they are developed under accidental controls, 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 distinct 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 developed 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 headwater 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