254 E. C. ANDREWS. 



are to be considered as the main expression of corrasion in 

 the locality under consideration. 



(ii) Great reduction in stream volume. 



Fig. 10 (c) explains this stage. The slope A P B repre- 

 sented the older channel grade. A heavy stream volume 

 succeeded and made the basin AOB. At a later period 

 the stream volume was wonderfully reduced and Z O [Fig. 

 10 (a)] the vertical measure of corrasive strength as a 

 result shrunk until it became a vanishing quantity. The 

 stream at this stage had no strength to corrade below the 

 profile A P B. It therefore gradually filled the basin AOB 

 in delta fashion and dropped the heavier burden between A 

 and O and silted up the whole portion between A and B. 



Similar shrinking action occurred at the embayments of 

 the cutting curves where the material was dumped on the 

 outside curve, and was crowded towards the central stream 

 portions, for the stream at any time has its lateral and 

 longitudinal as well as its vertical measures of corrasive 

 strength. In brief, along the " basins," the constrictions, 

 and in the channel "broads" alike, such a stream will 

 stagnate and fail to accomplish work for a considerable 

 period following on the reduction of the stream volume. 



D. Action at "broads" and around obstacles. 



Having inquired into the corrasive action of streams 

 along channel constrictipns, at channel confluences, and on 

 structures of variable strength, let us consider briefly the 

 action of a stream upon leaving a canon "narrow" and 

 entering a "broad." 



The cross-section of the channel is here much greater 

 and the stream velocity is thus much decreased. But the 

 power of corrasion suffers immense decrease thereby not 

 being related in a simple ratio to the decrease in stream 

 velocity. The load borne along by the stream when 

 traversing the "narrow" is now too heavy to handle in its 



