188 STRUCTURAL GEOLOGY. 



METHODS OF DEGRADATION. 



These are, first, erosion or degradation of the general surface; second, 

 corrasiori or degradation of the stream channels ; and, third, sapping or deg- 

 radation of cliffs. 



EROSION. This is distributed over the general surface; the rocks are 

 disintegrated by climatic agencies and transported into streams by the wash 

 of rains, both by driving and flotation, and flotation is largely promoted by 

 the beating of rains. In this method the rate of degradation depends on the 

 rate of transportation. This is a fact of almost universal observation; for 

 wherever there is soil or loose earth, the amount of such matter is the excess 

 of disintegration over transportation. 



We have already seen in the former analysis of transportation that 

 with a given quantity of water, transportation will depend on declivity, but 

 in the transportation belonging to this method of degradation the quantity 

 of water is a factor of transportation only to a limited extent, for increased 

 rainfall promotes the growth of vegetation which serves as a protection to 

 the soil. Nor is this protection inconsiderable, for it preserves the rocks 

 from the beating of the storms, and prevents the waters from gathering rap- 

 idly into rills and brooks, and strains the water of its earthy sediments. I 

 have many times witnessed the action of a - storm in an arid region where 

 the disintegrated rocks were unprotected by forests, shrubbery, or turf, and 

 as often have I been impressed with the wonderful power of the infrequent 

 storm to gather up and carry away the land, as compared with the frequent 

 storm in the prairie or forest of a land more richly clad. The same contrast 

 may be observed in a region brought under the dominion of man by culti- 

 vation where the surface of a plowed field is swept away by a storm, and 

 the furrows are the channels for floods of mud, while the meadow receives 

 the rain with outstretched arms of verdure, which bear it gently to the earth, 

 where it is gathered into quiet rills, which feed a stream made turbid it is 

 true, but pure when compared witli the stream of mud flowing from the field 

 from which the plowman was driven by the storm. 



Erosion, then, or surface degradation is not greatly promoted by 

 increased rainfall, and it may be that its effect is rather to retard the pro- 



