62 GEOLOGICAL BIOLOGY. 



Sir Archibald Geikie, the Director of the Geological Sur- 

 vey of Great Britain, has expressed the opinion that the for- 

 mation of all stratified rocks of the earth's crust required be- 

 tween 73,000,000 and 680,000,000 of years.* 



SirWm. Thomson (Lord Kelvin), on the basis of radiation 

 of heat from the surface, and the present underground tem- 

 perature of the earth, estimated that the time since the con- 

 solidation of the crust is between 20,000,000 and 400,000,000, 

 and that all geological history showing continuity of life must 

 be limited within some such period of past time as 100,000,000 

 years, f 



A more recent estimate made by Clarence King gave ap- 

 proximately 24,000,000 for the same period; this estimate 

 has recently been approved by Lord Kelvin, after the debate 

 arising from Prof. Perry's criticism of the validity of Kelvin's 

 primary assumptions.;}: Geo. H. Darwin estimated, from the 

 rate of retardation of the earth's rotation by tidal friction, 

 that not over 57,000,000 years have elapsed since the moon 

 separated off from the mass of the earth ; and Prof. Tait, 

 from these and other physical grounds, estimates not over 

 10,000,000 years for all the geological work on the surface 

 of the earth. Houghton's estimate from erosion gives 

 28,000,000 for the deposition of the rock strata; Wallace 

 accepts approximately the latter estimate. 



Dana's estimate, as we have seen, is 48,000,000 years. 

 Upham's § estimate, based upon glacial phenomena, finds 

 Glacial and Postglacial time to be 30,000 to 40,000 years, 

 Quarternary 100,000, and thence, by estimating the relative 

 length of the faunal life periods. Tertiary 50 or 100 times 

 longer than the ice age, or 2,000,000 to 4,000,000 years; 

 this brings the mean approximately to the same figures given 

 by Dana. 



* Address before British Association, in i8g2. See Naitire, August 4, 1892, 

 vol. XLVi. pp. 317-323- 



f Address before the Geol. Soc. of Glasgow, February 27, 1868. See 

 " Popular Lectures and Addresses of Sir Wm. Thomson (Baron Kelvin)," vol. 

 II. p. 64. 



X Nature, vol. 1,1. pp. 224, 341, and 582. See also Lord Kelvin's reply, 

 pp. 227 and 438. 



%, Am. Journal of Science, vol. XLV. pp. 209-220. 



