CORRASION BY GRAVITY STREAMS. 213 



crushed and flow may be transferred to the sand and clay 

 fragments composing the bricks. Still greater pressure 

 may destroy the individuality of such fragments and flow- 

 may depend finally on the differential motion between the 

 various molecules of these substances. Yet in each case 

 the stream characteristics are retained, the mass, as a 

 whole, in each case seeks the lines of least resistance, the 

 lines of quickest descent. This implies the necessity of 

 the various stream types to follow the thalweg, 1 when 

 turned into any stream-formed valley, from considerations 

 of vertical pressure. The thalweg in all cases may be seen 

 to be the locus of maximum corrasive energy. 



It will be seen at a glance how this gives us a basis of 

 comparison for the various stream types as regards their 

 ability to transport infra-stream material; and as regards 

 the locations and values of variable stream velocities. 



Certain interesting results are obtained from a consider- 

 ation of such data, and these are discussed in detail in the 

 following pages. Thus to quote two results only out of 

 many: — Stream recession on declivities is seen to result, 

 primarily, in a resolution of the channel bed into a succes- 

 sion of "steps" and "treads," the steps being approxi- 

 mately of cirque-shape with or without basins on the treads. 

 In proportion as the cutting-back of the declivity is 

 accompanied by rapid increase of channel cross-section, so 

 is the feature of the basin formation seen to be less 

 emphasised. The action in the formation of typical cirque 

 or cup-shaped forms is seen to be dual or even triple. The 

 channel basin, if any, at its foot is mainly an expression of 

 stream corrasion at the point where the greatest per- 

 centage of stream strength may be expended. Such point 

 occurs where the slope of the channel bed presents the 

 least angle (as at the foot of a waterfall or a cascade) to 



1 Path taken by a stream down a valley. 



