310 



BULLETIN: museum of comparative zoology. 



energy to lateral cutting. The stream would wander back and forth 

 across its valley floor, unimpeded save by its flood-plain bank, until it 

 came against a lateral terrace ; and the terrace would be worn back 

 to the limit of the belt of wandering. Sooner or later the stream 

 would consume all the high-level and intermediate terraces, pushing 

 back their united scarps into a single scarp by which the highest 

 terrace plain would then descend at once to the flood plain. Stepping 

 terraces would no longer characterize this stage of valley development; 

 but a few low terraces might remain not yet consumed here and 

 there, as in Figure 2. 



Fig. 22. 



Miller seems to suggest that an increase in the height of terrace scarps 

 is in itself a cause for a decrease in interscarp space. " The restraint 

 exercised by rock upon the modern rivers strengthens their natural 

 tendency, of which sufficient account has not been made, to occupy nar- 

 rower portions of valleys the more they deepen them. It has been too 

 hastily concluded, because rivers now occupy narrowed valleys flanked 

 by terraces comparatively broad, that therefore they have vastly shrunk, 

 — from dimensions, in fact, proportional to the greater breadth " (299). 

 He then goes on with the statement already quoted regarding the 

 " banks nowadays eight or ten times as high" as formerly. It thus 

 seems to be implied that it is " natural " for the interscarp space of a 

 stream of constant volume to lose width ; and that not only rock ledges 

 restrain the breadth of the belt of wandering, but that the increase in 

 height of the enclosing scarps also has a share in determining this feature 

 of valley form. A similar opinion is expressed by Gilbert (133). This 





