DIASTROPHISM AND THE FORMATIVE PROCESSES 523 



endeavor to establish a mean rate serviceable for computing the 

 age of the earth. The more recent representative treatments deal- 

 ing with specific data may be found in the papers of Walcott/ 

 SoUas,^ Dole and Stabler,^ Joly,'* Becker, s Clarke,^ Arthur Holmes/ 

 in which may also be found references to earlier studies. The 

 data of the United States Geological Survey, as organized and tabu- 

 lated by Dole and Stabler, and discussed by Clarke, furnish the 

 most definite and best determined material available, as well as 

 that most suited to our purpose. It is not the rate of the complete 

 denudation of the land which we wish to use in this discussion but 

 only that part of it which found lodgment on the continental ter- 

 races. This embraced chiefly materials of certain degrees of coarse- 

 ness or gravity and a certain portion of the dissolved material. 



Clarke has pointed out the wide differences in the data obtained 

 from different regions and has assigned reasons for these. It ap- 

 pears in particular that the average denudation of the basins of the 

 Amazon and Uruguay rivers is but a trifle over one-half that of the 

 Mississippi and St. Lawrence basins, though the precipitation 

 on the tropical basins is nearly double that on the temperate basins. 

 The difference is assigned to the forest clothing of the tropical 

 basins. It appears from such data as Clarke found available that 

 the mean denudation of the river basins of Europe is 100 tons per 

 square mile, of Asia 84 tons, of North America 79 tons, of South 



' C. D. Walcott, "Geologic Time, as Indicated by the Sedimentary Rocks of 

 North America," Jour. GeoL, I (1893), 639-76. 



2 Sollas, Brit. Assoc. Rept., Address to Sect. C, 1900, quoted by Joly in Radio- 

 activity aitd Geology (1909), p. 246; Presidential Address, Qiiar. Jour. Geol. Soc, May, 

 1909. 



3 R. B. Dole and H. Stabler, "Denudation," U.S. Water Supply Paper 234 (1909) , 

 pp. 78-93. 



4 J. Joly, Radioactivity and Geology (1909), chap, xi; Phil. Mag., 6th ser., XXII 

 (1911), 358; Trans. R. S. Dublin, VII (1899), 23. 



5 George F. Becker, "The Age of the Earth," Smith. Misc. Coll., LVI, No. 6 

 (1910). "Halley on the Age of the Ocean," Science, N.S., XXXI (1910), No. 795, 

 pp. 459-61 • 



* F. W. Clarke, "A Preliminary Study of Chemical Denudation," Smith Misc. 

 Coll., LVI, No. 5 (1910); "The Data of Geochemistry," U.S. Geol. Surv. Bull. 491. 

 2d ed. (1911), p. 60 f., 137-42, 466-67. 



7 Arthur Holmes, The Age of the Earth (19 13), chaps, iv-vi. 



