Smith.] 



Some Aspects of Erosion. 



165 



adjacent slopes and tributary streams. On the other hand, the 

 amount of material furnished by the slopes will be adjusted to the 

 rate of cutting in the stream (as will be shown later), for the two 

 are interdependent factors. After a stream has acquired a " toler- 

 ably systematic curve of descent," and its cutting has been adjusted 

 to the factors given above, it would seem that it could be called a 

 graded stream. 



Thus the tendency of a stream is always to decrease its grade; 

 and this may be done either by vertical corrasion, or by lateral 

 corrasion, or by the two together. If a stream can not decrease its 

 grade by vertical cutting it may do so by meandering, thus increas- 

 ing the length of its course for a given fall. 



If a stream empties into the ocean (as all streams do, either 

 directly or indirectly, except such as empty into an inland basin 

 without outlet) its vertical cutting is limited at its mouth. If, then, 

 the stream preserves a systematic curve of descent, after it is once 

 acquired, and is continually cutting except at sea-level, the amount 

 of vertical corrasion must progressively decrease in going down 

 stream, until it reaches zero at sea-level. 



The slopes adjacent to a graded stream at any given point are 

 slopes of equal erosion, for the erosion on such mature slopes must 

 be everywhere essentially the same. If this were not the case, but 

 if one part of the slope were more rapidly eroded than another, 

 sooner or later the more rapid erosion would cause a decrease in 

 the angle of the slope at this point, and in consequence of the 

 decreased angle the rate of erosion would be, in turn, decreased, 

 and would, therefore, in time become the same as that over the rest 

 of the slope. Such adjusted slopes are graded slopes. 



Erosion on all sides of a peak or on both sides of a divide, 

 should be uniform ; otherwise the summit of the peak or crest of 

 the divide will shift until uniformity of erosion is attained. In 

 homogeneous rocks, with equal precipitation at all points, the two 

 slopes of a divide are of necessity equal. Hereafter the term slope 

 will be used to refer to one slope of a divide or peak, from its base 

 to, and including, its summit. The angle of a slope, even when 

 fixed, is, of course, not the same from top to bottom. 



The angle of the graded slope is dependent partly on the rate 

 of erosion in the adjacent stream bed, and partly on the character 



