1894. 



FAIRCHILD — THE LENGTH OF GEOLOGIC TIME. 



265 



the sedimentary rocks of Paleozoic time in western America as a basis 

 for comparison and computation, and modifying the time ratios of 

 Haughton and Dana, he concludes that post-Archean time is between 

 25 to 30 million years as a minimum, and 60 to 70 million years as a 

 maximum. More definitely he gives 27,650,000 years for the fossil- 

 iferous rocks and 55 million years for geologic time (since the begin- 

 ning of the Archean). 



Following is a table of estimates of various writers, the physicists 

 placed last. The first column gives those estimates which cover 

 only the time of the fossil-bearing or unaltered sediments, that is, 

 since the beginning of Cambrian time. The second column gives 

 those estimates which include all of " geologic time," that is, since 

 the beginning of the Archean, or since the present agencies began 

 their work. This time would be covered by the existence of the ocean. 

 The third class of estimates are those which cover all the duration of 

 the earth since a state of extreme heat. The estimates of the 

 physicists fall into this third class. 



For fossiliferous 

 sediments. 



') Sir Charles Lyell... 240 million years. 



") Dr. Samuel Haughton 133 " " 



') Dr. James Croll 60 " " 



^) Dr. Charles Darwin 



') Sir Alfred Wallace 



") Sir Archibald Geikie 100 



') Mr. T. Mellard Reade... 95 



') Prof. J. D. Dana 48 



^) Prof. Joseph LeConte 30 



'") N'r. Warren Upham 48 " " 100 



") Mr. C. D. Walcott 28 " " 55 



'') Mr. W. J. McGee 2400 



BIBLIOGRAPHIC REFERENCES. 



(i) Lyell : -"Principles of Geology," loth Ed., 1867, Vol. i, p. 301. 



(2) Haughton: — "Six Lectures on Physical Geography," 1880, p. 04; Philos. Mag., XXVI, 

 Dec, 1877, p. 5t5. 



(5) Croll: — " Climate and Time," 1875, p. ^42. 



14) Darwin (Chas) :-" Origin of Species,'' 5th Ed., 1871, p. 291. 



15) Wallace:—" Island Life," zd Ed., 1892, pp. 222-223. 



1^6) Geikie:- "Presidential Address," 62d Meeting Brit. Assoc. Adv. Sci., 1892 ; (Nature, 

 Aug. 4, 1892, and in Sinithsoiiian Report for 1892). 



(7) '^e.z.Ae:: — Geological Magazine, Vol. 10, 1893, PP- 99-100. 



(8) Dana : — " Manual of Geology," 3d Ed., 1880, pp. 590-591. 



(9) Le Conte: — " Elements of Geology." 1888, pp. 275-276. 



Cio) Upham:— yl ;«(■?-._/(?«>-. Sci., Vol. XLV, March, 1893, pp. 209-220. 



(11) Walcott: — Ainer. Geologist, Vol. XII, December, 1893, pp. 333-368. 



(12) McGee: — Science, June 9, 1893, P- 3°9- 



18, Pkoc. Roch. Acad. Sci., Vol. 2, July, i£ 



