244 E. C. ANDREWS. 



In the stereogram Fig. 8 (a), let P P'Q'Q'Q'Q represent 

 a channel with a slope P P'P"P"'. Let P'Z represent the 

 angular value of the gravity stream surface slope when at 

 rest, then the wedge P P'Z P "P" represents the rock struc- 

 tures above the local base level at P P'. 



In the case of a mobile stream, such as water, the height 

 P"Z will not have quite the same significance as for that 

 of a crystalline solid forced to flow only under enormous 

 pressure. This is so because the limiting angular values 

 of such stream slopes at rest are widely different. 



Suppose POP', Fig. 8 (b) to represent the transverse 

 section of the basin which has originated on such a slope 

 as illustrated by Fig. 6 (b) and (e). Then for reasons which 

 have been already set out, when discussing basin formation 

 or valley overdeepening along channel grades of negligible 

 slope, the basin head recedes along the declivity say in 

 some such fashion as P P'M N, Fig. 8 (b) and (c). The 

 stream velocity is increasing (due allowance of course being 

 made for friction) as it descends from P" to P O P'. A basin 

 such as P Z M O [Fig. 8 (d)] may thus originate. 



Z O which marks the deepest portion of the basin then 

 represents the vertical measure of the corrasive efficiency 

 of the stream at the most recent points of headward 

 recession. 



The points now arise : — " Will the amount of such vertical 

 corrasion vary ? and will the basin depths below the associ- 

 ated local base levels become more pronounced, or will 

 they decrease? And moreover, what will be the stream 

 history at P'P [Fig. 8 (b) and (c)], while A M O is progres- 

 sively receding, when the stream volume is supposed to be 

 constant? " 



These questions admit of ready answers. The reader 

 will remember that the natural stream has only one oppor- 



