240 E. C. ANDREWS. 



ated as shown at A OB, A'O'B' in Fig. 6 (b). In other 

 words, basins or relatively reduced grades will be formed 

 at such points. But such basins will differ in some respects 

 from those formed below the associated base levels along 

 channel bases of negligible slope. In the case of the basin 

 formed wholly below the general channel slope as shown in 

 Fig. 7, the descent of the stream mass from A to O is 

 attended only with loss of stream energy, since the line 

 A B is horizontal and thus gravity has here no opportunity 

 for increasing the stream energy. 



Fig. 7. 



Rock basin formed wholly below associated local base levels. The 

 profile A B is a section along a local base level also. 



In the case of the excavation on a declivity let A'O'B', 

 Fig. 6 (c) represent an enlargement of A'O'B' in Fig. 6 (b). 

 Let B'ZM represent the slope at which the stream under 

 consideration loses its flowage characteristics, and let M Z' 

 be a horizontal line, Z'B' being vertical. Then the fall 

 A'Z' represents the vertical distance through which the 

 stream gains power while passing from A' to B'. Let O' 

 represent the deepest point at which the stream can move 

 the mass M O'B' as a whole, then Z O' represents the ver- 

 tical measure of the corrasive efficiency of the stream at 

 that point. At B' much of its power is lost now, and it 

 depends for its further momentum on the descent of the 

 slope B'AB Fig. 6 (b). 



There is therefore a double action on the slope A'M O'. 

 From A' to M gravity will express itself in great measure 

 as "free falling" as well as flowage, the stream strength of 



