CORRASIUN BY GRAVITY STREAMS. 249 



may be seen at once if we consider the "mechanics " of the 

 question. For we have supposed the former stage to be 

 discontinued and the latter half or quarter volume to be 

 the measure of the utmost stream strength during the later 

 stage. The old stream made a basin or series of basins, 

 and generally overdeepened the channel of the weaker 

 stream which it had superseded ; it widened its course 

 pronouncedly here and there ; it cut large long embayments 

 into the wall along side the cutting curves; it made pro- 

 digious "steps" on its channel slopes; it cut the mouths 

 of its weaker tributary valleys so far back that the channel 

 bases of main and tributary valleys were separated by high 

 "steps"; it carried large volumes of debris into the "broads"; 

 and the depth of the basins, the widths, heights and depths 

 of the embayments along the cutting curves, as also other 

 features of interest, expressed the absolute mechanical 

 strength of the stream flood acting throughout a consider- , 

 able period of time. Such stream volume alone was enabled 

 to utilise the channel basin " deeps" or channel "broads" 

 of its own making as bridges for the transport of heavy 

 material ; to urge the whole stream mass filling the basins 

 over this bridge as a unit ; and it alone could enlarge the 

 embayments formed along the cutting curves without 

 destroying the harmony and integrity of curvature of the 

 channel profiles. A stream differing however slightly in 

 volume could not make use of this grade for general cor- 

 rasive purposes. Even if alteration in stream volume were 

 related in a simple ratio to energy or work this would be 

 a mechanical impossibility; how much more so then, when 

 decreasing volume is associated with work suffering 

 decrease in geometrical progression. 



This is the point that may cause misapprehension unless 

 careful note be taken of stream mechanics. For the 

 greater the local variations in velocity are, all other things 



