The Rivers and Yalleys of Pennsylvania. 



215 



by the diverting stream adds to its ability for further encroach- 

 ments, for with increase of vohime the channel is deepened and a 



Fig. 19. 



Fig. 20. 



flatter slope is assumed, and the whole process of pushing away 

 the divides is thereby accelerated. In general it may be said 

 that the larger the stream and the less its elevation above base- 

 level, the less likely is it to be diverted, for with large volume 

 and small elevation it will early cut down its channel so close to 

 baselevel that no other stream can offer it a better course to the 

 sea ; it may also be said that, as a rule, of two equal streams, the 

 headwaters of the one having a longer or a harder course will be 

 diverted by a branch of the stream on the shorter or easier 

 course. Every case must therefore be examined for itself before 

 the kind of re-arrangement that may be expected or that may 

 have already taken place can be discovered. 



21. Antecedent and superimposed rivers. — It not infrequently 



happens that the surface, on which a drainage system is more or 



less fully developed, suffers deformation by tilting, folding or 



faulting. Then, in accordance with the rate of disturbance, and 



17 



