102 



GEOLOGY. 



soon drop out of the race. Between the stronger streams competition 

 still goes on. If a' and f develop faster than c' its prospective drainage 

 territory will be preempted by its rivals (compare Figs. 87 and 88). 

 Thus as the result of the unequal rate at which valleys are lengthened, 

 the larger number of those which come into existence are arrested in 

 their development. As a result of growth in the manner indicated, 



S e f 9 h 



Figs. 



e- J- r 7i- 



86-88. — Diagrams to illustrate successive stages in the struggle for existence 



and dominion among streams. 



the basins of even the large streams remain narrow at their lower ends 

 while they expand above. This is the usual form of a drainage basin 

 the development of which has been normal. 



