110 GEOLOGY. 



considerations enter into the problem of floods. The presence of forests 

 and other forms of vegetation on the slopes retards the flow of water 

 into the valleys, and so tends to prevent floods, or at any rate to make 

 them less severe. Porous soil and subsoil, or in their absence porous 

 rock, absorb the rainfall, and prevent its prompt descent into the 

 valleys and so tends to prevent or diminish floods. 



The acreage of arable land within a given area stands in some rela- 

 tion to its drainage development. At an early stage in its erosion 

 history, before an upland has been dissected by valleys, nearly all of 

 it may be arable. I.ater, when drainage is at its maturity, and when 

 hillsides and ridge slopes constitute a large part of the area, there 

 is probably the least acreage of arable land. This is especially true 

 if the slopes are so steep as to allow the soil to be readily washed away. 

 At a still later stage, when the valley bottoms have become wide 

 and the slopes of the ridges and hills so reduced as to be available, 

 the area of cultivable land is again increased. 



Marshes, ponds, and lakes have some bearing on the resources and 

 industries of a region, and they stand in a more or less definite rela- 

 tion to the stage of erosion in which a region finds itself. In its youth 

 ponds and lakes may occupy much of the surface; in its maturity 

 they will have been largely drained. 



These suggestions are sufficient to show that the topography of 

 a region, even in so far as shaped by erosion, touches human interests 

 at many points. 



ANALYSIS OF EROSION.' 



Erosion is the term applied to all the processes by which earthy 

 matter or rock is loosened and removed from one place to another. 

 It consists of three sub-processes,, namely, weathering, transportation, 

 and corrasion. 



Weathering. 



The term weathering is applied to nearly all those natural pro- 

 cesses which tend to loosen or change the exposed surfaces of rock. 

 The lettering of inscriptions on exposed marble becomes fainter and 

 fainter as time goes by, and finally disappears, because the rock in 



* An excellent discussion of this subject is given by Gilbert in The Henry Moun- 

 tains, pp. 99 et seq., and more briefly in the Am. Jour. Sci., Vol. XII, p. 85 et seq. 

 1876. 



