120 E. C. ANDREWS. 



another action of an insidious nature, gradually breaking 

 down the chemical and mineralogical structures so that 

 the action of the descending streams may be hastened. 

 These are the forces of corrasion and of weathering, the 

 former being dependent upon the transporting power of the 

 streams. The attack of the streams themselves on the 

 rock structures (corrasion) may be considered as analogous 

 to a direct muscular assault in the animal kingdom, while 

 the effects of weathering on the rocks may be taken as 

 analogous to the diseases among animals. It will be seen 

 that the corrasive attack is the dominant one in the earlier 

 stages of peneplanation, while the attack by weathering is 

 the dominant one during all but the earlier stages of land 

 reduction. Special attention is directed to these processes. 



Transportation. — Geikie and Gilbert both insist on the 

 geometrical increase of transportation upon simple increase 

 of velocity for the case of ordinary streams. We quote 

 first from Geikie (Geology 1893, p. 380). 



"Mr. David Stevenson's table of power of transportation 

 of different velocities of river currents. 



Just work on fine clay 

 Lift fine sand 



Lift sand coarse as linseed 

 Will sweep along fine gravel 

 Will roll along rounded pebbles one inch 

 in diameter 

 36 2*045 Sweep along slippery angular stones size 

 of egg." 



" The effects of abrasion upon the loose materials on a 

 river bed are but a minor part of the erosive work per- 

 formed by the stream. A Jayer of debris, only the upper 

 portion of which is pushed onward by the normal current, 

 will protect the solid rock of the river channel which it 



Inches per 

 Second. 



Miles per 

 Hour. 



3 



0'170 



6 



0*340 



8 



0*4545 



12 



0*6819 



24 



1*3638 



