228 E. C. ANDREWS. 



An objection may be raised here on the ground of stream 

 position. Is it not possible that the stream, powerful as 

 it is, may lose its efficiency at some excessively rapid rate 

 below base level ? This is undoubtedly so, especially if the 

 increased velocity be attended with no increase in volume. 

 But along the same channel, natural streams derive such 

 great increase of velocity by means of enormous increase 

 of volume. So long as the stream can transport the 

 material occupying the basin as a whole, so long can it 

 corrade the entire basin base. In the case under consider- 

 ation such volume would be so great that the lifting power 

 required to drag the stream material from twice the original 

 depth of the basin would be a negligible quantity. This is 

 evident also from considerations of speed apart from volume 

 since eight times the amount of stream material now 

 passes through any given cross-section with eight times 

 the velocity as compared with that of the original stream. 

 Thus the vertical measure of the corrasive efficiency of a 

 stream will be rapidly increasing while velocity is more 

 slowly increasing. The new basin will therefore take on 

 some much larger dimensions such as A"0"B". Let us call 

 such depths as Z O' and Z O", (which indicate the limits of 

 the corrasive power of the streams when vertically con- 

 sidered), the vertical measures of stream efficiency. The 

 apprehension of such principle for streams generally would 

 throw much light on disputed physiographical points, as 

 will be shown more fully later. 



But to return to the discussion of the basins so formed 

 by streams. In homogeneous structures it is easy to 

 ascertain the general shape of this initial depression formed 

 below the main or local base level upon the increase of 

 stream velocity. Its deepest portion will be situated a 

 little in advance of the topographic configuration deter- 

 mining the maximum velocity along the main axis of the 



