The Rivers cmd Yalleys of Pennsylvcmia. 203 



the etching of Tertiary valleys in a Cretaceous baselevelled low- 

 land, then we may well conclude with Powell that " mountains 

 cannot remain long as mountains ; they are ephemeral topographic 

 forms."* 



Part Third. General conception of the history of a river. 



15. 21ie complete cycle of river life: youth., adolescence, matu- 

 rity and old age. — The general outline of an ideal river's history 

 may he now considered, preparatory to examining the special 

 history of the rivers of Pennsylvania, as controlled by the geo- 

 logical events just narrated. 



Rivers are so long lived and survive with more or less modifi- 

 cation so many changes in the attitude and even in the structure 

 of the land, that the best way of entering on their discussion 

 seems to be to examine the development of an ideal river of sim- 

 ple history, and from the general features thus discovered, it may 

 then be possible to unravel the comjjlex sequence of events that 

 leads to the present condition of actual rivers of complicated his- 

 tory. 



A river that is established on a new land may be called an ori- 

 ginal river. It must at first be of the kind known as a consequent 

 river, for it has no ancestor from which to be derived. Exam- 

 ples of simple original rivers may be seen in young plains, of 

 which southern New Jersey furnishes a fair illustration. Exam- 

 ples of essentially original rivers may be seen also in regions of 

 recent and rapid displacement, such as the Jura or the broken 

 country of southern Idaho, where the directly consequent charac- 

 ter of the drainage leads us to conclude that, if any rivers occu- 

 pied these regions before their recent deformation, they were so 

 completely extinguished by the newly made slopes that we see 

 nothing of them now. 



Once established, an original river advances through its long 

 life, manifesting certain peculiarities of j^outh, maturity and old 

 age, by which its successive stages of growth may be recognized 

 without much difficulty. For the sake of simplicity, let us sup- 

 pose the land mass, on which an original river has begun its work, 

 stands perfectly still after its first elevation or deformation, and 

 so remains until the river has completed its task of carrying away 

 all the mass of rocks that rise above its baselevel. This lapse of 

 time will be called a cycle in the life of a river. A complete 



* Geol. Uinta Mountains, 1876, 196. 



