308 GEOLOGIC TIME. 



know. Different men may put different value on the three factors, bulk 

 ot sediment, rate of denutlation, and area of denudation; but I tliink a 

 fair and impartial examination of the reasoning- involved in this paper 

 will sliowthat the luinciple of the calculation is sound. 



"It must not be forgotten that to arrive at the earth's age Archnean 

 time has to be added to my estimate of 9.5,000,000 years, which very 

 materially increases the margin of geologic time on which we are 

 allowed to draw."* 



In an earlier paper Reade assembles much valuable data on chemical 

 denudation,! and later reviews the results obtained by the geologist and 

 the mathematician. J 



M. A. de Lapparent is one of the/ew European continental geologists 

 who have written on geologic time. On the basis of mechanical deiuida- 

 tion and sedimentation he thinks that from (57,000,000 to 90,000,000 

 years, at the present rate of sedimentation, would account for every- 

 thing that has been produced since the consolidation of the crust.§ 



Df(H((. — In some observations on the length of geologic time. Prof. 

 James I). Dana says that geology has no means of substituting posi- 

 tive lengths of time in place of the time ratios he has deduced from 

 the relative thicknesses of the rock series pertaining to the several geo- 

 logic ages, but tliat it affords facts sufficient to j)rove the general prop- 

 osition that geologic " //>He is long.'''' lie cites examples, such as the 

 retreat of Niagara Falls and the recent growth of coral reefs. Accord- 

 ing to his time ratio, if 48,000,000 years is assigned since the commence- 

 ment of the Silurian, the I*aleozoic, Mesozoic, and Cenozoic time would 

 represent, respectively, 30,000,000, 9,000,000, and 3,000,000 years. || 



Me Gee. — In an article on comparative chronology by Mr. W J McGee, 

 the conclusion is reached that the antiquity of the glacial dei^osits 

 margined by the great terminal moraine is about 7,000 years, and of 

 the Columbian formation and of the ice invasion to which it is ascribed, 

 200,000 years, and of the Lafayette formation of late Tertiary age, 

 10,000,000 years. On this basis the mean estimate of the age of the 

 earth is 15,000,000,000 years, and 7,000,000,000 years have elapsed 

 since the beginning of Paleozoic time.^ In a subsequent " Note on the 

 Age of the Earth" Mr. McGee modifies his former statement, and gives 

 as a mean estimate of the age of the earth 0,000,000,000 years, and of 

 the duration of time since the beginning of the Paleozoic, 2,400,000,000 

 years, which is based on a minimum estimate for the age of the earth of 



* "Measnremeat of Geological Time." (icoJoy. Mag.. 1893, vol. x, p. 100. 



\ Proc. Liverpool Ueol. Soc, 1877, vol. iii, pi. iii, pp. 211-235. 



X Geol. Mag., 1878, vol. V, pp. 145-164. 



§De la mesure du temps par les pbenomeues de s^dimeutatiou. BvU. Soc. Geol. 

 France, 1890, 3d ser., vol. xviii, pp. 351-355. Ladestin^e de laterre ferme, et durce 

 des temps gi'ologiqiies. Revue des qixestious scientiliqiies, July, 1891. Pamphlet, 

 Rriixelles, pp. 1-38. 



\\ Manual of Geology, 2d ed., pp. 690,591. 



^Am. Anthropologist, 1892, vol. v, p. 340. 



