250 CHAMBERLIN— THE AGE OF THE EARTH. 



radio-activity greatly exceed this. Astronomical opinion has recently 

 been trending toward the view that long periods are necessary for 

 certain typical phases of celestial evolution. Perhaps I may over- 

 step my proper limits far enough to say that I have recently tried to 

 form some notion of the time required for the gathering of planetesi- 

 mals from what seemed a probable distribution into the collecting 

 planetary nuclei, and found a period of the order of two or three 

 billion years the most probable.® These current views in the collateral 

 fields warrant me in assuming that there is a wide discrepancy be- 

 tween the geological estimates just cited and the present estimates in 

 the related fields. In view of this I can perhaps serve you best by 

 inquiring whether the recent additions to geologic evidence and the 

 newer modes of interpretation mitigate this discrepancy in any ap- 

 preciable measure. Let us consider first what the newer evidence 

 relative to the sediments has to say, and turn later to the solutions. 



The Testimony of the Sediments. 



In considering possible modifications of the foregoing estimates 

 five questions arise : ( i ) How far do recent investigations tend to 

 lengthen or to shorten the older estimates? (2) To what extent has 

 human action made the present rate of wash and deposit faster than 

 the mean pre-human rate? (3) How far does the present state of 

 elevation make the present rate faster or slower than the mean rate 

 of the past? (4) How does the present area of erosion compare 

 with the mean area? And, finally, (5) does the lower end of the 

 geologic column give us the point from which the accumulation of 

 the sediments began ? I can try to answer these questions only very 

 briefly and inadequately. 



I. The Effect of Intensive Studies on Earlier Time Estimates. — 

 A strictly accurate chronology reaching back from the present for 

 several thousands of years is now being worked out by De Geer,'^ 



^ Diatrophism and the Formative Processes, XIII., The Time over which 

 the Ingathering of the Planetesimals was Spread, T. C. Chamberlin, Jour. 

 GeoL, Vol. XXVIII. (1920), pp. 675-81. 



'^ Gerald De Geer, "A Geochronology of the Last 12,000 Years," Proc. Int. 

 Congr., Stockholm, 1910, p. 241 ; " Kontinentale Niveauveranderungen im 

 Norden Europas," ibid., p. 849 ; Spitzbergen, ibid., p. 1205 ; " Phenomenes 

 Quaternaires de Stockholm," ibid., p. 1290; "Quaternary Phenomena in the 

 Southern Part of Sweden," ibid., p. 1339. 



