THE WORK OF RUNNING WATER. 161 



It is not to be understood that this sequence of events will take 

 place in the degradation of every anticline, but the principles here 

 set forth will always be operative. The result specified will be ac- 

 complished wherever hard and soft layers have the relations indicated 

 in the diagrams; that is, where the stream in the syncHne finds itself 

 on a resistant layer as it approaches base-level, while at the same time 

 the (original) tributary streams are working in softer beds. It is not 

 to be understood, therefore, that streams migrate from syncHnes to 

 anticlines for the sake of getting out of the former positions into the 

 latter. If they shift their courses it is to find easier ones. 



That these changes are not fanciful is shown by the fact that the 

 adjustment described corresponds with that shown in many parts of 

 the Appalachian Mountains, and in other mountains of similar structure. 



If in a later stage of its history, the new main stream, fh, were 

 to cut its bed down to a lower hard layer, while the original stream, 

 ah, reached a softer bed beneath the hard one above, the latter would 

 again have an advantage, and a new series of adjustments would be 

 inaugurated which might result in re-establishing the main stream 

 in its original synclinal position. 



EFFECT OF CHANGES OF LEVEL. 



Rise. — If after being base-leveled, or notably reduced by erosion, 

 a region is uphfted so as to increase the gradients and therefore the 

 velocities of the streams which drain it, the streams are said to be 

 rejuvenated, and a new cycle of erosion is begun. If the rise of the 



Fig. 152. — Cross-section of a wide valley, ab, in the bottom of which a younger valley, 

 cd, has been excavated as the result of uphft. 



,area were equal everywhere, while the coast fine remained constant in 

 position, there would be an immediate increase in velocity only at 

 the debouchures of the streams flowing directly into the sea. At the 

 debouchures of such streams there would be rapids or falls. Each 



