162 



GEOLOGY 



it is much more common where they do, for unequal resistance 

 puts one stream at a disadvantage as compared with another. 



Figs. 122-124 Figs. 125-127 



Figs. 122-124. Diagrams illustrating piracy, where the stream which does 

 not flow over rock of superior hardness captures those which do. Fig. 

 123 represents a further development of the drainage shown in Fig. 122, 

 and Fig. 124 represents a still later stage. 



Figs. 125-127. Diagrams to illustrate piracy where the competing streams 

 all cross a hard layer. The diagrams represent successive stages of de- 

 velopment. 



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. 1 Since streams 

 and valleys adjust themselves to other conditioas as well, this 



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



