192 J, D. Dana—Note on Erosion. 
Art. XXIV.—Note on Erosion; by James D. DANA, 
In Professor Gilbert's very valuable paper on “The Colorado 
Plateau Province,”* the author speaks of the process of erosiont 
as including the three general divisions, ‘‘(1) weathering, (2) 
transportation, and (8) corrasion ;” and states that “ corrasion is 
rtance, since it determines, not only the speed, at, a 
great extent, the size of the pestles which grind the rocks.” He 
and 
sides of a stream generally causes, besides a dimination of 
; merair } on, at 
of other sources of resistance to displacement, in the a at 
acted upon. The operation is exhibited, on a small scale, 
rather firmly consolidated gravel bank, and, in an incredib 
short time, levels the thick deposits over a | 
arge a 
imvi ” and yet it erodes oe. 
water that impinges has “ no load of detritus, 
with tremendous efficiency. The flooded rivers that teat eo : 
from their foundations, and break off or uproot t traps 
other examples; for the work is, first, rending, and then, un 
portation, The sudden rise of several feet or yards in 4 0 98 
tain stream, or along a western cafion, sometimes occurring Z 
* This volume, pages 16, 85. + Ibid., p. 89. 
