KI-IYTHMS IN DENUDATION 775 



periods, to seek out the trends through longer measures of duration, we 

 see an eon-long rise and ebb of sealevel with respect to the continents. It 

 pulsates with the close of eras, falling and then slowly rising again. But 

 from our present elevated continents and lofty mountains, as we look back 

 through geologic time, the eye passes over one S5^stem after another, more 

 and more remote, and only dimly on the far horizon do we see in the Pre- 

 cambrian ages a similar relation in elevation of land to sea. This marks 

 the most far-reaching rhythm of geologic time. 



But superimposed on this greater rhythm are the crescendoes and di- 

 minuendoes of diastrophisms which separate the smaller divisions of time. 

 The Pleistocene represents one of the crescendoes. It has been marked 

 by an acceleration in crustal uplift and oscillation which has raised hio-h 

 the rate of total denudation. Compared to the rate for the whole of the 

 Cenozoic era of reA^olution, it may be twice the mean. The concurrence 

 of the longer rhythm in sealevel, giving wide and high continents, with 

 the rising diastrophisni of a period of revolution may, however, make the 

 present rate of continental denudation ten or fifteen, or even twent}^, times 

 the mean for all of earth history, our knowledge of the Precambrian being 

 especially vague. Estimates of geologic time based on measurements of 

 the present rate of denudation and coupled to the assumption that this is 

 the mean for all the past are likely to err correspondingly. 



This may seem an abandonment of the principle of uniformitarianism 

 on which the science of geology was founded. Davis, however, saw in the 

 existence of cycles of erosion the necessity of giving a more elastic con- 

 ception to this basic principle. He has stated the problem well, as follows : 



"Uniformitarianism, reasonably understood, is not a rigid limitation of pnst 

 processes to the rates of present processes, but a rational association of ob- 

 served effects with competent causes. Events may have progressed both faster 

 and slower in the past than during the brief interval which we call the present, 

 but the past and present events differ in degree and not in kind. This ratlier 

 elastic understanding of uniformitarianism seems to me comparatively safe 

 from the objections that have been urged against the more rigid conception 

 that some authors regard as necessarily intended in the writings of Hutton. 

 Playfair, and Lyell, especially safe if the very remote hypothetical past of 

 unrecorded time is not considered." "^ 



The conclusions reached in this part are so startling that they should 

 be reexamined before acceptance, in order to see if the present can not be 

 regarded as not so exceptional in geologic time. It is clear that the pres- 

 ent rate of continental denudation is very high, owing to a concurrence of 



-^ W. M. Davis : Bearing of physiogvapliy on uniformitarianism. Bull. Geol. See. Am., 

 vol. 7, 1896, pp. 8, 9. 



LVIII — Bull. Geol. Soc. Am., Vol. 28, 191G 



