37i REPORT— 1900. 



of heredity a I'econciliation of the demands of biologists and the restric- 

 tions placed by the physicist on geological time may be found. His 

 address to the Zoological Section last year will be fresh in the minds of 

 all, and I need not further press the point. 



Turning to physical methods we have Professor Darwin's age of the 

 moon, suggesting a minimum of fifty-seven millions of years, to which 

 Professor Sollas has referred. Lord Kelvin's method, based on the rate of 

 cooling of the globe and the observed fall of temperature in the terrestrial 

 crust, depends for the accuracy of its indication on data regarding the 

 physical properties of the deeper- lying materials of the earth which we do 

 not ay yet possess. This has been fully discussed lately by Professor 

 Perry. The distinguished author of the method has at no time denied 

 the restrictions placed by our present ignorance on the indications of the 

 method. The eflfect is not to deprive the method of value, but to restrict 

 its present functions to the delimitation of certain bounds to our 

 speculations, which bounds may on the minor estimate be taken as some 

 twenty millions of years, but which may, according to the density, specific 

 heat, and conductivity of the deeper-lying materials at elevated tempera- 

 tures, allow of a much moi'e extended estimate. 



It must be admitted that no one method of approaching the delicate 

 question of the age of the earth can claim to Imve reached that con- 

 sistentior status which we look for in scientific results. So much may 

 possibly have happened during the long past vista which it is hoped to 

 penetrate that more than a considerable degree of probability may never 

 be attained by our results. Admitting this, I have to appear perhaps in 

 the light of an advocate when I state my belief that the method by solvent 

 denudation is not discredited by the conclusions arrived at by other 

 methods, in so far as these assign major or minor limits to the age of the 

 earth, and that none other approaches the question so directly or on such 

 easily obtained data. 



APPENDIX. 



Tlie Lleoloiiical Age of the Earth. 

 [Read before the Congres Geologique International, I'JOO.] 



The method of determining the age of the earth summarised in this 

 paper is based on the assumption that the ocean has retained substantially 

 the whole of the sodium committed to it by the solvent denudation of 

 geological time, and that the supply of the element sodium by the rivers 

 has on the whole lieen uniform in rate.^ 



Hence we derive as a numerator a number expressing the mass of 

 sodium at present in the ocean, and a denominator expressing in the same 

 units the amount of this element annually discharged into the ocean by 

 the rivers of the world. 



The quotient is the geological age of the earth. 



Corrections are applied to both numerator and denominator for any 

 certain or very probable source of error. These corrections are approxi- 

 mate only, upper and lower limits of their values being suggested. ^ 



' No other dissolved substance in the ocean conforms to the first of these con- 

 ditions. 



- A more amplified account of what follows will be found in a paper by the 

 author, ' An Estimate of the Geological Ago of the Earth,' Tram. lioyal Bvllin 



