The Rivers and Valleys of Pennsylvania. 207 



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 gnaw- 

 ing of an adjacent stream (Lowl, 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 pro- 

 cesses may be found in Phillippson's serviceable " Studien tiber 

 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 re- 

 ferred to are unconnected with any external disturbance of the 

 river basin, and are purely normal spontaneous acts during ad- 

 vancing development. Two examples, pertinent to our special 

 study, will be considered. 



Let AB, fig. 9, be a stream whose initial consequent course led 

 it down the gently sloping axial trough of a syncline. The con- 

 structional surface of the syncline is shown by contours. Let the 

 succession of beds to be discovered by erosion be indicated in a 

 section, laid in proper position on the several diagrams, but 

 revolved into the horizontal plane, the harder beds being dotted 

 and the baselevel standing at 00. Small side streams will soon 

 be developed on the slopes of the syncline, in positions determined 

 by cross-fractures or more often by what we call accident ; the 

 action of streams in similar synclines on the outside of the- 

 enclosing anticlines will be omitted for the sake of simplicity. 

 In time, the side streams will cut through the harder upper bed M 

 and enter the softer bed N, on which longitudinal channels, indi- 

 cated by hachures, will be extended along the strike, fig. 10 (La 

 Noe and Margerie). Let these be called "subsequent" streams. 

 Consider two side streams of this kind, C and D, heading against 

 each other at E, one joining the main stream lower down the 

 axis of the syncline than the other. The headwaters of C will 

 rob the headwaters of D, because the deepening of the channel 



