Earth as an Abode fitted for Life. 73 



tion, had bean rotating twice as fast as at present, or even 

 20 per cent, faster than at present, traces of it's present figure 

 must have been left in a great preponderance of land, and 

 probably no sea at all, in the equatorial regions. Taking 

 into account all uncertainties, whether in respect to Adams' 

 estimate of the rate of frictional retardation of the earth's 

 rotatory speed, or to the conditions as to the rigidity of the 

 earth once consolidated, we may safely conclude that the 

 earth was certainly not solid 5,000 million years ago, and 

 was probably not solid 1,000 million years ago*. 



§ 13. A second argument for limitation of the earth's age, 

 which was really my own first argument, is founded on the 

 consideration of underground heat. To explain a first rough 

 and ready estimate of it I shall read one short statement. 

 It is from a very short paper that I communicated to the 

 Eoyal Society of Edinburgh on the 18th December, 1865, 

 entitled, " The Doctrine of Uniformity in Geology brief! y 

 refuted." 



" The ' Doctrine of Uniformity ' in Geology, as held by many of the 

 most eminent of British Geologists, assumes that the earth's surface and 

 upper crust have been nearly as they are at present in temperature, and 

 other physical qualities, during millions of millions of years. But the heat 

 which we knoio, by observation, to be now conducted out of the earth yearly 

 is so great, that if this action had been going on with any approach to 

 uniformity for 20,000 million years, the amount of heat lost out of the 

 earth would have been about as much as would heat, by 100° C, a 

 quantity of ordinary surface rock of 100 times the earth's bulk. This 

 would be more than enough to melt a mass of surface rock equal in bulk 

 to the whole earth. No hypothesis as to chemical action, internal fluidity, 

 effects of pressure at great depth, or possible character of substances in 

 the interior of the earth, possessing the smallest vestige of probability, 

 can justify the supposition that the earth's upper crust has remained 

 nearly as it is, while from the whole, or from any part, of the earth, so 

 great a quantity of heat has been lost." 



§ 14. The sixteen words which I have emphasized in read- 

 ing this statement to you (italics in the reprint) indicate the 

 matter-of-fact foundation for the conclusion asserted. This 

 conclusion suffices to sweep away the whole system of geolo- 

 gical and biological speculation demanding an " inconceiv- 

 ably " great vista of past time, or even a few thousand million 

 years, for the history of life on the earth, and approximate 

 uniformity of plutonic action throughout that time ; which, 

 as we have seen, was very generally prevalent thirty years 



* " The fact that the continents are arranged along meridians, rather 

 than in an equatorial belt, affords some degree of proof that the consoli- 

 dation of the earth took place at a time when the diurnal rotation differed 

 but little from its present value. It is probable that the date of consoli- 

 dation is considerably more recent than a thousand million years ago." — 

 Thomson and Tait, 'Treatise on Natural Philosophy,' 2nd ed., 1883, § 830. 



