326 ADJUSTMENT OF RIVERS 



surface sheet of glacial drift and is now engaged in eroding the 

 ancient crystalline rocks which the drift had covered. When the 

 stream has everywhere cut through the newer rocks, its course 

 will be seen to have no relation to the structure of the older rocks 

 which it is now trenching. If, as frequently has happened, denuda- 

 tion has stripped away almost all the newer strata, the drainage of 

 the country seems to be quite inexplicable and to be arranged with- 

 out any reference to the structure of the rocks across which the 

 streams flow. Such a system of drainage is said to be superimposed, 

 inherited, or epigenetic. 



Subsequent Streams. — As a river system approaches maturity, 

 and as the drainage of the area becomes more complete, it will 

 increase the number of its branches. Those branches which 

 were not at all represented in the youthful stages of the system, 

 and are opened out along lines of yielding rocks, are called sub- 

 sequent, and all streams will develop more or fewer of such 

 branches as they advance to maturity. 



Adjustment of Streams. — However the streams of a district 

 may have been established in the first instance, whether they 

 were consequent, antecedent, or superimposed, they are liable to 

 changes more or less profound and far-reaching. These changes, 

 which belong to the normal development of the drainage system 

 and are not dependent upon diastrophism, are due to adjustment 

 of the streams to the rock structure of the district, the streams 

 searching out the lines of weakness and least resistance, and 

 everywhere taking the easiest path to their destination. The up- 

 stream extension of branches and the shifting of the divides result 

 in the capture of streams, or parts of such, by others more favour- 

 ably situated, one master stream gradually absorbing many smaller 

 ones which had originally been independent. 



A divide, or water-parting, between two streams is gradually 

 shifted by the lengthening of the more favourably situated stream, 

 or of one of its subsequent branches. This more favourable situa- 

 tion may be because it has a shorter course and greater fall, giving 

 a swifter flow, or because it flows at a lower level, giving greater fall 

 to its tributaries, or because its course is through soft and easily 



