Smith.] 



Some Aspects of Erosion. 



169 



in these areas, provided, of course, that other conditions are favor- 

 able to its growth.* 



These results refer, of course, to areas of some size, where the 

 stream courses are free to develop. The erosion of small areas of 

 soft rocks in larger areas of hard ones, or vice versa, is more or less 

 affected by the erosion of the surrounding areas. Saddles and 

 valleys tend to develop in the soft rocks, while in the hard rocks 

 bordered by softer ones are formed peaks and ridges which stand 

 above the average altitude of the softer area. Erosion in these 

 smaller areas probably tends more to differentiation of altitude than 

 where the areas are larger. Even in the larger areas there is more 

 or less tendency to differentiation, but this is not usually sufficient 

 to destroy the effect of a generally even sky-line. The result of 

 erosion, as seen in mature forms, is a greater or less differentiation 

 of upland altitudes when viewed in detail, with, however, in ordi- 

 nary cases, an impression of uniformity when taken as a whole. 



In areas well differentiated by erosion, where the drainage is all 

 subsequent, there may be a subequality of summits, due to the fact 

 that the more resistant rocks then form the peaks and ridges, while 

 the less resistant form the valleys. With the tendency to uni- 

 formity in the resistance of the rocks of the uplands, there would 

 be a greater reason for uniformity of altitude. This, however, 

 appears to be a point of minor importance. 



Graded streams cutting in both hard and soft rocks do not cut 

 vertically, on the whole, any more rapidly in one than in the other. 

 For, since the parts are all interdependent, the cutting at one point 

 can not be, on the whole, any more rapid than the cutting at all 

 points in the stream course. There will be differential erosion, but 

 only to a minor extent. The profile of the graded stream, while it 

 shows irregularity in detail, will present simple uniformity when 

 taken as a whole. 



* From the general principles here given the roughly equal spacing of the 

 principal rivers of a topographic unit, in even a mature region (see Shaler, 

 loc. cit. ), will follow as a necessary corollary. The spacingof the minor drain- 

 age lines in areas of rocks of differing degrees of resistance may differ some- 

 what ; the streams will be generally closer together in the areas of harder 

 rocks, other things being equal, but in either the harder or the softer areas, 

 considered alone, the spacing for streams of the same class should be practi- 

 cally uniform 



