Smith.] 



Some Aspects of Erosion. 



■57 



mountainous area. A series of ranges, in a given region, under 

 similar conditions as regards general attitude, character of rocks, 

 climate etc., would usually be worn down at the same rate, and 

 might be considered as parts of a larger and complex geographical 

 unit. 



As illustrations of independent ranges, we may take the Sierra 

 Nevadas and Coast Range of California. The latter might be 

 reduced to a peneplain while the former was still far from that 

 condition, since they are independent units, and the erosion of one 

 range does not affect that of the other, except indirectly. It would 

 be possible for both to be reduced to the peneplain condition at the 

 same time, but this would not be on account of any causal connec- 

 tion between the two. If conditions favorable to peneplanation 

 should obtain at the same time in both, the more rapid erosion in 

 the higher range, and the progressive decrease in rate of erosion in 

 the topographically older one, with advance in topographic age, 

 might so far counterbalance the original differences between them 

 that by the time the older range had reached the peneplain form, 

 the other would be not far from the same condition. The proba- 

 bility of such a combination of circumstances is lessened, however, 

 by the possibility of differential movements, and by actual differ- 

 ences in climate, character of the rocks and other conditions, in 

 addition to the differences in average altitude and present degree of 

 topographic development. 



On the other hand, owing to similarity of conditions through- 

 out the range (possible differential movements within the mass 

 excepted), we should expect the whole of the Coast Range to be 

 worn down as a unit. Although long before the general peneplain 

 condition should be reached, partial or local peneplains might be 

 developed along those parts of the stream courses nearest base- 

 level, these could be of but very limited extent, until the range as a 

 whole should begin to assume the same aspect. 



OBJECTIONS TO THE THEORY OF PENEPLANATION. 



Absence of Present Peneplains. — Professor Tarr's first important 

 objection to the peneplain hypothesis is the fact that no such forms 

 are found on the earth to-day, unchanged, as features of the present 



