246 E. C. ANDREWS. 



basins below the associated local base levels and by great 

 undercutting of channel walls as the stream carries its 

 flattish floor sideways. But in the case of our stream on a 

 declivity, such enlargement of cross-section is possible, 

 and is more easily brought about, by cutting away the mass 

 P P Z P"P" which lies above the local base level P'P [Fig. 

 8 (b), than in the other case of the negligible slope where the 

 stream had the much more difficult task of excavating below 

 the associated base levels. 



Now as the cross-sction is much enlarged the velocity of 

 the stream is correspondingly reduced and has therefore 

 but little opportunity for forming basins below the base 

 level P Z M [Fig. 8 (d)] or P'Z [Fig. 8 (b)], since its power 

 decreases in a high geometrical ratio with the decrease 

 of stream velocity. As the amphitheatre therefore 

 works backwards to P'Z, the whole mass, of which P'Z P" 

 is a longitudinal section, has been removed allowing so 

 much greater cross-section, in which the stream may dis- 

 tribute its velocity. Local corrasion here then, below the 

 line P Z, receives a decided check. In the case then of a 

 high declivity, there would be a tendency for ZO 1 [Fig. 8 

 (d)] to be less emphasised than would have been expected 

 on first thought. That is, valley over deepening may here 

 be brought about without basin formations at all. 



Bat another point of importance here also suggests itself. 

 The stream volume is constant, and now works in a much 

 larger cross-section, at the same time flowing from A to M. 

 This implies that Z O may be of decided value considered 

 from point to point of the recession, but it implies also that 

 the vertical measure of the stream is not now so great at 

 the lowest basin portion as it was at the time when this 



1 Z O it will be noted is used throughout these pages as an indication 

 of the greatest depth of a basin below the plane of the enveloping base 

 level or base levels. It is the maximum measurement of " overdeepening." 



J 



