CHAMBERLIN— THE AGE OF THE EARTH. 249 



cally there are serious difficulties. In the first dozen years of this 

 century four notable estimates were made in this way by able geolo- 

 gists : two American, Clarke^ and Becker ;- two British, Joly^ and 

 Sollas.'* These estimates form an admirable point of departure for 

 this discussion. They represent the mode of geologic interpretation 

 that has been most current until recently ; they typify opinions widely 

 held by the conservative school of geologists ; they stand out in con- 

 trast to the views of the new school.^ The mean of the four esti- 

 mates, on the basis of the sediments, was 90,000,000 years, roundly ; 

 on the basis of the ocean, 95,000,000 years. The highest individual 

 estimate was 150,000,000 years; the lowest 70,000,000 years. I shall 

 not deal with the individual estimates, but merely with their mean 

 value, and with that only as representative. 



My discussion can not be specific and concrete without some refer- 

 ence to views in other fields. My colleagues in this Symposium will 

 give you the last word from their viewpoints, and if I could follow 

 them I would gladly take their estimates as specifically representative 

 in their several lines. In lieu of this, I can only use such general 

 views as are current. It has long been known to be the view of many 

 biologists that the evolution of life required much more than 100,- 

 000,000 years. It is also well known that most estimates based on 



1 F. W. Clarke, " A Preliminary Study of Chemical Denudation," Smiths. 

 Misc. Coll., LVL, No. 5 (1910) ; "The Data of Geochemistry." 



2 George F. Becker, " The Age of the Earth," Smiths. Misc. Coll., LVL, 

 No. 6 (1910). 



^ J- Joly, " An Estimate of the Geological Age of the Earth," Trans. Roy. 

 Soc. Dublin, VIL (1899), pp. 23-66; "The Age of the Earth," Phil. Mag., 6th 

 sen, Vol. XXn. (191 1), pp. 359-80. 



* W. J. Sollas, " Presidential Address," Quart. Jour. Geol. Soc, Vol. 65 

 (May, 1909), Proc. Geol. Soc. of London., Sess. 1908-9, pp. I-cxxii. 



^ It is not practicable to summarize the time-estimates of the newer school 

 consistently with the division of labor adopted in this Symposium since they 

 are composite, embracing organic, astronomic, and radioactive factors, with 

 some emphasis on the last. The following papers of this class may be 

 taken as representative : J. Joly, " Radioactivity and Geology," Van Nostrand, 

 New York (1909), pp. 233-51; Arthur Holmes, "The Age of the Earth," 

 Harper Bros., London and New York (1913), pp. 1-196; J. Barrell, "Rhythm 

 and the Measurement of Geological Time," Bull. Geol. Soc. Am., Vol. 28 

 (1917), pp. 745-904; T. C. Chamberlin, " Diastrophism and the Formative 

 Processes," XHL, " The Time over which the Ingathering of Planetesimals 

 was Spread," Jour. Geol., Vol. XXVIII. (1920), pp. 675-81. 



