THE WORK OF RUNNING WATER. 147 



Streams which do not cross hard layers therefore have an advantage 

 over those which do, and the tributaries to such streams, since they 

 join deeper mains, have an advantage over the tributaries to the others. 

 The valleys of the former may lengthen until their heads reach the latter, 

 and capture their streams. This sequence of events is illustrated in the 

 accompanying diagrams (Figs. 125-27). Even where several streams 

 cross the same resistant bed, piracy is hkely to take place among them, 

 for some are sure to deepen their valleys faster than others, because 

 of inequalities of volume, load, or hardness. This is illustrated by 

 Figs. 128-30. An actual case is sho\vn in Figs. 131, 132. Though 

 piracy may take place when streams do not flow over rock of unequal 

 hardness (p. 103), it is much more common where unequal resistance 

 of the rock puts one stream at a disadvantage as compared with an- 

 other. 



The changes in the courses of streams, by means of which they come 

 to sustain definite and stable relations to the rock structure beneath, 

 are known as processes of adjustment.'^ Since streams and valleys ad- 

 just themselves to other conditions as well, this phase of adjustment 

 may be called structural adjustment. Structural adjustment is not 

 uncommon among rivers flowing over strata which are vertical or 

 highly inclined, since in these positions the hard and soft strata are 

 most likely to come to the surface in frequent alternation. The smaller 

 streams suffer capture and adjustment first, since, as a rule, they have 

 shallower valleys. It often happens that main streams, because of 

 their deeper valleys, hold courses not in adjustment with structure (the 

 Delaware, the Susquehanna, etc.), while tributary streams are cap- 

 tured, diverted, and adjusted. The capture of a tributary, however, 

 leads both to the diminution of its main and to the increase of its captor, 

 and the weakened stream may ultimately fall a prey to the one which 

 is strengthened. 



The processes of adjustment go on until the streams fiow as much 

 as possible on the weaker beds, and as little as possible on the stronger, 

 when adjustment is complete. This amounts to the same thing as 

 saying that the outcrops of the hard layers tend to become divides. 

 In many cases an area is so situated that there is no escape for its drain- 

 age except across resistant rock. In this case its drainage is completely 

 adjusted when as few streams as possible cross the resistant rock, 

 and these by the shortest routes. 



^See Campbell, Jour. Geol., Vol. IV, pp. 567, 657. 



