CORRASION BY GRAVITY STREAMS. 259 



case the location of greatest strength beiag vertically 

 under the more central portions of the stream. 



The path of the stream however tends to the parabolic 

 form. The greater the stream strength so much more 

 deeply will it dig vertically below the local base level. 

 Nevertheless, all other things being equal, the more swiftly 

 it moves, just so much is there a tendency for the slope of 

 the headward curve of the corrasive cut produced to become 

 increasingly flat. Again, it is evident that as the stream 

 velocity is increased, and as stream depth is increased also 

 at the expense of stream width, so in like manner will the 

 base of the corrasive cut below base level tend to become 

 flattened and to become possessed of vertical walls. Its 

 head will tend to the cirque or amphitheatre form. 



Upon steep channel slopes, especially when such are 

 associated with variable strength of structures, basins or 

 flattish floors are likewise formed by streams when the 

 velocity is increased. Inasmuch, however, as the removal 

 of a declivity by cirque recession involves rapid increase 

 of channel cross-section and thus a pronounced decrease 

 locally of stream velocity, basins below base level will 

 here not be so pronounced, but troughs with flattish floors, 

 and ending upstream in "steps," will rather express their 

 action. In this case the stream energy increases by descent 

 of the "step" as opposed to the loss of stream energy in 

 descending a basin. The tendency is, therefore, to produce 

 a modification of the true "gravitative form" at its head ; 

 that is, the form produced by matter falling freely toward 

 a central gash or cut. For a description of this important 

 form see the Appendix. 



The first paragraph of this summary suggests that all 

 such basins formed below base level and such " steps" — of 

 which latter the cirque appears to be but a variety — must 

 grow by headward recession until such time as the stream 



Q-Nov. 3, 1909. 



