Smith.] 



Some Aspects of Erosion. 



159 



two distinct ideas, (1) the instability of the earth's crust, and (2) the 

 slowness of the process of denudation, these are considered only 

 in their relation to each other. The objection can not, therefore, be 

 answered by treating them separately, as Professor Davis has done.* 

 It is not a question of the length of geologic time, nor of the stability 

 of the earth's crust; the question is whether the crust would remain 

 approximately quiet long enough for the completion of a certain 

 very long process. For this reason we" can not disregard the length 

 of the process. The consideration of the rate of erosion, especially 

 with advance in topographic age, is therefore a most important one. 



Examples of the rate of erosion may be drawn from the mountain 

 ranges of California. According to the observations of various 

 writers f (and my own as well), erosion since the Glacial epoch, in 

 the Sierra Nevadas, has been practically nil, but since the elevation 

 of the range at the end of the Pliocene there has been sufficient 

 time for the cutting of the deep gorges which incise the western 

 slopes. The vertical range of cutting is great, reaching a maximum 

 of several thousand feet, though the horizontal range is compara- 

 tively insignificant. The streams are young, and the total amount 

 of material removed is small as compared with the bulk of the 

 range. This cutting represents the results of the erosion of the 

 whole post-Pliocene time — -of which the post-Glacial, judging from 

 this evidence, is such a small proportion. 



Turning to the Coast Range, we have an approximate measure 

 of the post-Pliocene erosion here in San Clemente Island,! the 

 present stream canons of which belong wholly to this period. In 

 the main Coast Range the forms are mature, and quite unlike those 

 of the Sierras or of San Clemente. The Coast Range forms appear 

 to have been developed, in the main, during the post-Miocene 

 elevation of the coast, and modified during the Pliocene depression. 

 Judging, therefore, from these results, the post-Miocene interval of 



*Loc. cit., pp. 223-227. 



fTenth Ann. Rept, U. S. Geol. Survey, Pt. I., p. 28. 

 Seventeenth Ann. Rept., U. S. Geol Survey, Pt. I., p. 710. 

 Geol. Atlas of U. S., Folio No. 39 (Truckee, Calif.), p. 6. 

 jSee Eighteenth Ann. Rept., U. S. Geol. Surv., Pt. I., p. 469, and Plate 

 LXXXVII. 



2 



