248 



SCIENCE 



[N.S. Vol. XXXI. No. 790 



tions are constituted by coincidence of re- 

 gional conditions. 



The periodicity of diastrophism is the 

 fundamental fact of geographic history. 

 It carries with it corresponding periodic 

 effects, both direct and indirect, in erosion, 

 sedimentation, climatic changes, and even 

 in organic evolution. All of these proc- 

 esses depend upon the initiative action of 

 the earth's internal energy and they all are 

 rhythmic because its action is rhythmic. 

 Thus this general principle gives rise to 

 correlative principles, vrhich may be stated 

 independently for each of the processes. 



PEKIODICITT OP EROSION AND SEDIMENTA- 

 TION 



American geologists need no restatement 

 of the phenomena of cycles of deposition 

 and erosion which Newberry^ emphasized 

 and which have led through the work of 

 Powell, Gilbert and Davis to recognition of 

 the principle that epochs of marked relief 

 and vigorous erosion have alternated with 

 periods of base-leveling, and that sedi- 

 ments have alternated correspondingly in 

 character and volume. I may pass the 

 subject of base-level periods and orogenic 

 epochs as related to erosion and sedimen- 

 tation with this reference to it, mentioning 

 only that it is the essential principle in 

 Chamberlin V latest contribution to the 

 philosophy of correlation; but though the 

 principle is accepted there is still occasion 

 to dwell upon the constancy of erosion and 

 the inconstancy of sedimentation, espe- 

 cially since the facts may be the reverse of 

 what is sometimes assumed and since they 

 lie at the foundation of our interpretations 

 of the geographic record. 



' Newberry, J. S., " Circles of Deposition," 

 Amer. Assoc. Adv. Sci., Proceedings, Vol. 22, pt. 

 2, 1874, pp. 185-196. 



" Chamberlin, T. C, " Diastrophism as the Ulti- 

 mate Basis of Correlation," Jour, of Geol., Chi- 

 cago, Vol. 17, 1909, pp. 685-693. 



It is assumed in some instances that 

 erosion on supposed subaerial surfaces has 

 either not occurred or has left no traces, 

 whereas on the other hand the surfaces, if 

 they had been submarine, must invariably 

 have been covered with sediment, which 

 would constitute a record. And the con- 

 elusion is drawn that sections which ex- 

 hibit an incomplete sequence of strata must 

 have been land areas at certain times. It 

 is a hazardous conclusion in the absence of 

 definite evidence of erosion, for subaerial 

 processes never fail to leave some kind of 

 mark, and submarine processes are con- 

 sistent with non-deposition. 



Constancy of Erosion. — The atmosphere 

 is never at rest. Wind, rain and snow; 

 heat and cold ; moisture, carbonic acid and 

 other chemical agents; all these have ever 

 worked unceasingly, according to the cir- 

 cumstances that condition them, upon ex- 

 posed land surfaces. No land has ever 

 been exempt from their attack, which re- 

 sults in decay, denudation or aggradation, 

 as the case may be. 



Decay, denudation and aggradation are 

 processes of erosion which invariably leave 

 chemical or mechanical evidences of their 

 activity. There is to-day no surface of any 

 land, however high or low, under any 

 climate whatever, which does not bear in- 

 dubitable marks of one or the other of 

 these processes. There is abundant evi- 

 dence that they have been at least equally 

 active and effective during past ages and 

 that they have marked ancient lands as 

 they do those of to-day. 



We frequently recognize ancient land 

 surfaces on evidence of soils, wear or sub- 

 aerial deposits. Or, if they have passed 

 through processes, such as marine trans- 

 gression, that destroy the earlier effects, we 

 observe the sequence of changes and reason 

 back to the corresponding conditions. But 

 there are seemingly continuous sections 



