COKRASION BY GRAVITY STREAMS. 231 



(b) Over a plane surface trenched by a narrow canon 

 500 feet deep. Here velocity and pressure are unifom over 

 the upland surface, and no differential corrasion here occurs; 

 along the canon however the pressure, and hence velocity 

 [Fig. 2 (a)], is of much greater value than on the uplands, 

 and corrasion is here very pronounced since corrasion 

 increases in a geometrical ratio with the increase of velocity. 



So marked would be the variable corrasion along upland 

 and valley that a casual observer would fail to see the 

 signs of upland corrasion in his astonishment at the evidence 

 of relatively strong corrasion along the canon. 



(c) Over a plane upland surface as before, the upland 

 now being trenched with a narrow canon 5,000 feet deep. 

 The action here is similar to that described under (b) with 

 this exception, that the basin formations, and general 

 valley over deepening, of the profound canon are now still 

 more emphasised. 



(d) Over a plane surface as before with a narrow canon 

 5,000 feet deep as in (c) but with a channel base possessed 

 of a steep slope. The action is much the same as that 

 shown for (c) but still more emphasised. 



In other words a stream of whatsoever material composed 

 and 10,000 feet deep might apparently corrade the prairies 

 of the United States but slightly; it might act more or 

 less as a weak plane over New York, but might still scour 

 most powerfully along the Hudson Channel; it might 

 scour the valleys of the Scotch or English Highlands 

 strongly yet abrade the uplands themselves in a minor 

 degree only, while in the case of Norway it could scour the 

 canon bases to great depths below sea level, even if, at the 

 same time it could only weakly abrade the uplands. 



There appears to be no escape from the action of this 

 deep underlying principle namely, rapid rise of energy with 



