Th,e Riiiers and Valleys of Pennsylvania. 217 



dent that the possibility of any given transverse stream being 

 antecedent must be regarded only as a suggestion, until some inde- 

 pendent evidence is introduced in its favor. This may be difficult 

 to find, but it certainly must be searched for ; if not then forth- 

 coming, the best conclusion may be to leave the case open until 

 the evidence appears. Certainly, if we find a river course that is 

 accordant in its location with the complicated results of other 

 methods of origin, then the burden of proof may be said to lie 

 with those who would maintain that an antecedent origin would 

 locate the river in so specialized a manner. Even if a river per- 

 sist for a time in an antecedent course, this may not prevent its 

 being afterwai-ds affected by the various adjustments and revi- 

 vals that have been explained above : rivers so distinctly ante- 

 cedent as the Green and the Sutlej may hereafter be more or less 

 affected by processes of adjustment, which they are not yet old 

 enough to experience. Hence in mountains as old as the Appala- 

 chians the courses of the present rivers need not coincide with 

 the location of the pre-Permian rivers, even if the latter per- 

 sisted in their courses through the growth of the Permian fold- 

 ing ; subsequent elevations and adjustments to hard beds, at first 

 buried and unseen, may have greatly displaced them, in accord- 

 ance with Lowl's principle. 



When the deeper channelling of a stream discovers an uncon- 

 formable subjacent terrane, the streams persist at least for a time 

 in the courses that were determined in the overlying mass ; they 

 are then called superimposed (Powell), inherited (Shaler), or 

 epigenetic (Richthofen). Such streams are particularly liable to 

 readjustment by transfer of channels from courses that lead them 

 over hard beds to others on which the hard beds are avoided ; 

 for the first choice of channels, when the unconformable cover 

 was still present, was made without any knowledge of the buried 

 rock structure or of the difficulties in which the streams would 

 be involved when they encountered it. The examples of falls 

 produced when streams terrace their flood-plains and run on 

 buried spurs has already been referred to as superimposed ; and 

 the rivers of Minnesota now disclosing half-buried ledges here 

 and there may be instanced as illustrating the transition stage 

 between simple consequent courses, determined by the form of 

 the drift sheet on which their flow began, and the fully inconse- 

 quent courses that will be developed there in the future. 



22. Shnple, compound^ composite and complex rivers. — We 



