Smith.] 



Some Aspects of Erosion. 



been most advanced."* What the connection may be between the 

 increase of uniformity near the sea and advance in development, is 

 not made clear. By "near the sea" is evidently meant, in the 

 lower stretches of the important stream courses; for where the 

 main divides and stream courses are parallel to the coast-line, the 

 statement as made would not necessarily hold. Apparently Pro- 

 fessor Tarr's idea is that " beveling " begins at the lower end of 

 the main stream courses, and since " in the coastal region there 

 may well have been an approach to the condition of a local pene- 

 plain,"! ^ would follow that the uniformity in this region would be 

 most marked. 



According to Professor Tarr's view, it is only the " peaks whose 

 rocks were approximately the same in power of resistance " which 

 "would in time approach each other in altitude," J and the explana- 

 tion will not account for the rough uniformity of many crests in 

 mature regions where the rocks are quite different in power of 

 resistance. 



CONSIDERATIONS MODIFYING USE OF TERM. 



It seems probable that Professor Tarr is correct in his main idea, 

 that there is a certain beveling effect in the forces of erosion acting 

 under ordinary conditions; and, further, that this will explain many 

 cases of supposed peneplanation. But if it is true, the explanation 

 given is inadequate to account for it, and some of the premises are 

 wrongly assumed. 



To state briefly the writer's view of the case : there is and 

 should be, in all wetl-matured regions, a roughly even sky-line, or 

 subequality in the altitudes of the ridges, due to subaerial denuda- 

 tion, as will be shown. (In a region which for any reason already 

 presents in infancy an even sky-line, due, for instance, to penepla- 

 nation or marine abrasion, an approximate evenness of sky-line will 

 probably be persistent from youth to old age.) In a region of 

 mature drainage, neighboring streams of the same class, at corre- 

 sponding points, will have their beds roughly at the same altitude, 



* Loc. cit., p. 366. 



t Loc. cit. 



+ Loc. cit., p. 368. 



