CORKASION BY GRAVITY STREAMS. 327 



or open at one end. The slower the headward retreat of 

 the cut the greater is the tendency of the plan of the 

 amphitheatre to be a blunt curve. [See also Davis (d) p. 

 178 - 181] Its vertical profiles tend to become parabolas. 

 The study of this "gravitative" form is especially helpful 

 in the study of the cirque. 



Fig. 12. 



A = Gravitative cu?ve or slope induced in material falling 



freely towards a central gash. 

 B = Reversed gravitative curve caused by rapidly dissipating 



energy of stream. 

 A P - Reversed gravitative curve caused by gravitative curve 



passing upward from weaker to stronger structures. 



Appendix III. — Stream Energy. 



In Part I of this series the corrasive strength of streams 

 has been shown to be at least proportional to the square 

 of the stream velocity. The following note suggests that 

 stream corrasion varies as the third power of the velocity. 

 [For the accompanying proof the writer is indebted to 

 Mr. L. A. Cotton of Sydney University.] 



Let m be the mass of a substance which is forced to flow 

 past a point A in unit time under the action of gravity. 



