190 STRUCTURAL GEOLOGY. 



sufficient importance to be considered in this connection is size of particles, 

 for the utilization of the particles as instruments is dependent, cceteris paribus, 

 on their size. If the particles be too large they cannot be transported, arid 

 thus cannot be used as instruments; and within the limits of size transported, 

 a greater amount of instrument will fae used as the size of the particles 

 diminish; that is, witli a given stream, the instrument is increased with com- 

 minution of instrument, the instrument being the load, and load being 

 increased by comminution as has been seen. 



We have next to consider the beds to be corraded, and so far as their 

 constitution is a factor in corrasion, it may be brought into the simple 

 expression that heterogeneity promotes corrasion. But a part of the instru- 

 ment of corrasion is derived from the rocks corraded; and as heterogeneity 

 of these rocks promotes disintegration, and to the same extent promotes corra- 

 sion, it further augments the instrument of corrasion in the channel below. 

 But it has been seen that in transportation the utilization of the water power 

 in promoting flotation as a function of heterogeneity of flow is a function of 

 declivity cceteris paribus. Hence, declivity not only increases the force, but 

 it also utilizes the force, and, hence, multiplies itself as a factor of corrasion. 

 A part of the instrument in corrasion is the load of the stream derived from 

 erosion; and we have already seen that the amount of this load depends 

 chiefly upon declivity $ hence, the amount of instrument from this source 

 depends on declivity of erosion, and the amount of instrument derived from 

 corrasion depends on declivity; and the power of corrasion, that is, rock 

 power plus water power, depends on declivity. Hence, with a given quan- 

 tity of water and a given character of bed, rate of corrasion depends on 

 declivity. Again, rapid corrasion increases the declivity of erosion,^: and 

 hence increases erosion; and this increased erosion augments the instru- 

 ment of corrasion, hence, increases corrasion; and declivity by the two 

 methods of degradation enters the general problem of degradation as a factor 

 with a rapidly increasing value. 



Corrasion is a very important method of degradation, although its results 

 are of much less magnitude than those of erosion. The evidence of this can 

 be seen, and the results often strike the student of physical geography with 

 great force. The deep channels in which the rivers run are grand topo- 



