i6o 



University of California. 



[Vol. 2. 



erosion was greatly longer than the post- Pliocene has been thus 

 far. Differences in climate, altitude and other factors may have 

 caused a more rapid erosion in the former, thus shortening some- 

 what the time which it is necessary to postulate; but even then the 

 difference between the length of the post-Miocene interval and that 

 of the post- Pliocene must remain very great. 



The Coast Ranges are still far from the peneplain condition. 

 What proportion of the original range has been already removed 

 could not be estimated without knowing approximately the mass 

 of the range at the beginning of post-Miocene times, beside taking 

 account of the considerable amount removed during the Pliocene 

 depression of the coast. That the total amount of erosion has been 

 large can not be doubted; a guess might be hazarded at fifty per cent 

 of the original mass of the range. 



Even on the basis of such a rate of erosion as is thus shown, 

 the formation of a peneplain could be brought within the range of 

 possibility only by the assumption of the most favorable circum- 

 stances. That is, if it could be shown in any way that the Miocene 

 time was as much longer than the post-Miocene as this appears to 

 have been longer than the post-Pliocene, and if, as Dutton* has 

 shown for the Colorado region, the climate of the Miocene was 

 especially favorable to rapid erosion, it might be assumed that there 

 would have been time for the formation of a peneplain during this 

 epoch, provided that the land stood close to the same level during 

 all that time. But if all these conditions were fulfilled, the difficulty 

 still remains that the rate of erosion is not constant, but decreases 

 progressively with advancing topographic age, and thus the time 

 demanded for the reduction to a peneplain is much increased. 

 Thus when we take into account not only the slowness of the 

 process of erosion in general, but its increasing slowness with the 

 wearing down of the land, the improbability that the earth's crust 

 would remain sufficiently stable during such immense periods as 

 would be required for the production of a peneplain under ordinary 

 circumstances, becomes one of the most important objections to the 

 hypothesis as an explanation of commonly existing forms. 



* Monograph II., U. S. Geol. Surv., pp. 189-191. 



