The Rivers and Yalleys of PennsyT/oania. 213 



of a river's life. It progresses no faster than the weathering 

 away of the slopes of a divide, and here as a rule weathering is 

 deliberate to say the least, unless accelerated by a fortunate com- 

 bination of favoring conditions. Among these conditions, great 

 altitude of the mass exposed to erosion stands first, and deep 

 channeling of streams below the surface — that is, the adolescent 

 stage of drainage development — stands second. The opportunity 

 for the lateral migration of a divide will depend on the inequality 

 of the slopes on its two sides, and here the most important fac- 

 tors are length of the two opposite stream courses from the water 

 parting to the common baselevel of the two, and inequality of 

 structure by which one stream may have an easy course and the 

 other a hard one. It is manifest that all these conditions for 

 active shifting of divides are best united in young and high 

 mountain ranges, and hence it is that river adjustments have 

 been found and studied more in the Alps than elsewhere. 



19. Revival of rivers by elevation and drowning by depression. 

 — I make no contention that any river in the world ever passed 

 through a simple uninterrupted cycle of the orderly kind here 

 described. But by examining many rivers, some young and 

 some old, I do not doubt that this portrayal of the ideal would 

 be found to be fairly correct if opportunity were offered for its 

 development. The intention of the sketch is simply to prepare 

 the way for the better understanding of our actual rivers of more 

 complicated history. 



At the close or at any time during the passage of an initial 

 cycle such as the one just considered, the drainage area of a river 

 system may be bodily elevated. The river is then turned back 

 to a new youth and enters a new cycle of development. This is 

 an extremely common occurrence with rivers, whose life is so long 

 that they commonly outlive the duration of a quiescent stage in 

 the history of the land. Such rivers may be called revived. 

 Examples may be given in which streams are now in their second 

 or third period of revival, the elevations that separate their cycles 

 following so soon that but little work was accomplished in the 

 quiescent intervals. 



The antithesis of this is the effect of depression, by which the 

 lower course may be drowned, flooded or f jorded. This change 

 is, if slow, favorable to the. development of flood-plains in the 

 lower course ; but it is not essential to their production. If the 

 change is more rapid, open estuaries are formed, to be trans- 

 formed to delta-lowlands later on. 



