16. _Mutual adjustment of river courses_.--In certain structures, chiefly those of mountainous disorder on which the streams are at first high above baselevel, there is a process of adjustment extremely characteristic of quiet river development, by which the down-hill courses that were chosen in early life, and as we may say unadvisedly and with the heedlessness and little foresight of youth, are given up for others better fitted for the work of the mature river system. A change of this kind happens when the young stream taking the lowest line for its guide happens to flow on a hard bed at a considerable height above baselevel, while its branches on one side or the other have opened channels on softer beds: a part of the main channel may then be deserted by the withdrawal of its upper waters to a lower course by way of a side stream. The change to better adjustment also happens when the initial course of the main stream is much longer than a course that may be offered to its upper portion by the backward gnawing of an adjacent stream (Löwl, Penck). Sometimes the lateral cutting or planation that characterizes the main trunk of a mature river gives it possession of an adjacent smaller stream whose bed is at a higher level (Gilbert). A general account of these processes may be found in Phillippson's serviceable "Studien über Wasserscheiden" (Leipzig, 1886). This whole matter is of much importance and deserves deliberate examination. It should be remembered that changes in river courses of the kind now referred to are unconnected with any external disturbance of the river basin, and are purely normal spontaneous acts during advancing development. Two examples, pertinent to our special study, will be considered. [Illustration: FIG. 9.] [Illustration: FIG. 10.]