INTRODUCTION AND SUMMARY 747 



should be looked on only as sujpp lying a beginning for investigation; 

 the establishment of a datum plane against which may be sought out 

 and measured the amplitudes of oscillations of all factors which have 

 found a direct or indirect record in the crust. This, of course, is not 

 easy, since the causes are often obscure and the degree of variation in 

 period and in amplitude of each factor must be detemiined and tested 

 by comparison with unrelated factors. 



Although, in a later pai't of this article, a review is given of the evi- 

 dence furnished by radioactivity, the viewpoint of the paper is geological, 

 the first tliree parts consisting in a study of the variations to which the 

 rates of geologic processes luive been subject. In the assemblage of 

 material and the manner in which the suhject is treated here, it is es- 

 sentially new, although Blytt, Gilbert, F. B. Taylor, E. Huntington, and 

 others have dwelt on one aspect or another of this subject. 



In Part I of the paper it is pointed out that erosion is essentially a 

 pulsatory process, as Davis has made most clear and the students of 

 physiography are now well aware. A single rhythm is the erosion cycle ; 

 and small partial cycles are superimposed on larger. How much has 

 this pulsatory nature led to variations in the ]'ate of regional and con- 

 tinental denudation in. tiie past, and what is its significance in regard 

 to the sedimentary measures of geologic time? l^he answer is reaiched 

 from several points of view in this article that, because of the present 

 great elevation of the continents, because of the magiiitude of recent 

 orogenic movements, and because of the pulsatory nature of the Pliocene- 

 Pleistocene uplifts, forming an accelerated series, a concurrence of factors 

 has taken place each of which makes for a high rate of denudation. 

 Their combination must give a rate very much greater than tbe mean 

 for geologic time. 



Passing in Part II to the next step in the argument: in all strati- 

 gU'aphic measures of geologic time, so far as the writer is aware, tlie 

 rate of deposition of a sedimentary series has been previously I'egarded 

 as dependent on the type of sediment, whether sandstone, shale, or lime- 

 stone, combined with the present rate of supply of such sediment to 

 regions of deposition. Here is developed an opposite view: that the 

 deposition of nearly all sediments occurs just below the local baselevel. 

 represented by wa\'e base or river flood level, and is dependent on upward 

 oscillations of basele\e1 or downward oscillations of the bottom, either 

 of which makes rcxmi for sediments below baselevel. According to this 

 control, the rate of vertical tbix-kening is something less than the rate 

 of supply, and tbe balances is carried farther by the agents of trausporta- 



