GEOLOGIC TIME. 64 1 



geologic time of 200,000,000 of years. 1 Dr. James Croll esti- 

 mated 72,000,000 years for the time duration since the first 

 deposition of sedimentary rocks, while Sir Alfred R. Wallace 

 thought that 28,000,000 years would suffice. 2 'Of the value 

 of this estimate he says : " It is not of course supposed that the 

 calculation here given makes any approach to accuracy, but it 

 is believed that it does indicate the order of magnitude of the 

 time required." 3 Dr. Alexander Winchell reduced geologic 

 time still more in his estimate of 3,000,000 years for the whole 

 incrusted age of the world. 4 Later writers, however, do not 

 accept this, as we find Sir Archibald Geikie concluding on the 

 basis of denudation and deposition that the sedimentary rocks 

 would have required 73,000,000 of years for their deposition, if 

 denudation was at the rate of one foot in 730 years ; or of 680,- 

 000,000 of years if at the slower rate of one foot in 6,800 years. 5 

 Mr. T. Mellard Reade adopted one foot in 3,000 years as the rate 

 of average denudation throughout geologic time, and obtained a 

 result of 95,000,000 of years as the time that had elapsed 

 since the beginning of Cambrian time. 6 M. A. de Lapparent 

 is one of the few European continental geologists that has 

 written on geologic time. On the basis of mechanical denuda- 

 tion and sedimentation he thinks that from 67,000,000 to 

 90,000,000 of years would suffice, at the present rate of sedi- 

 mentation for everything that has been produced since the 

 consolidation of the crust. 7 The two most recent writers who 

 have taken their initial datum point or "geochrone" from 

 the consideration of late Cenozoic or Pleistocene phenomena 



1 Nature, Vol. 18, 1878, pp. 267-268. 



2 Stella Evolution and its Relations to Geological Time, 1889, pp. 48-49. 



3 Island Life, 2d. Ed., 1892, pp. 222-223. 



4 World Life, or Comparative Geology. Chicago, 1883, p. 378. 



5 Presidential Address; report of 62d meeting British Assoc. Adv. Sci., 1892, p. 21. 



6 Measurement of Geological Time. Geol. Mag., Vol. 10, 1893, pp. 99-100. 



7 De la mesure du temps paries phenomenes de sedimentation. Bull. Soc. Geol. 

 France, 3d ser., Vol. 18, 1890, pp. 351-355. La Destinee de la terre ferme et duree 

 des temps geologiques. Revue des questions scientifiques, July, 189 1. Pamphlet. 

 Bruxelles. Pp. 1-38. 



