CORRASION BY GRAVITY STREAMS. 223 



under the influence of enormous pressure, then by greatly 

 increasing its volume, the friction between its textural 

 units may be overcome and a stream formed of the mass. 

 The momentum in this case will probably be so great that 

 the very force of its impact upon the channel bed and sides 

 — especially when it is armed with a rock load — may over- 

 come the coherence of the ordinary rock structures 1 and 

 allow it to quarry huge masses along, or across, the main 

 joint planes. At the same time it will be able to strongly 

 abrade the fresh surfaces thus exposed to its influence by 

 the quarrying action. 



Phases of stream action considered. — It will be advis- 

 able to consider : — 



(a) the initial form produced in each of these cases by 

 stream action ; then to consider : — 



(b) the subsequent history of such corrasive forms 

 when the stream volume is considered as still 

 constant ; and 



(c) to mention briefly the significance of a reduction 



of stream volume. 



Geological Assumptions.— We will suppose — unles other- 

 wise stated — that the rock structures acted upon are homo- 

 geneous, having joints well developed and not set very 

 widely apart, and that the rocks, moreover, are homogene- 

 ous both in texture and in mineralogical composition. From 

 this simple case the other more complicated ones may be 

 appreciated quite easily. 



A. Initial Corrasive forms. 



(1) Along channel bottoms of negligible slope, and at 

 locations of channel constrictions. 



1 The plasticity of the mass also under such great stress will help it to 

 exercise great quarrying action by moulding itself closely to the channel 

 bed. 



