TRANSACTIONS OF SECTION C. 433 



wliidi has been prepared from one of them has an atomic weight of 20606 

 (Houigschmid, 1914), a fact which ensures its radioactive origin. 



Hitherto all time periods have been calculated on the assumption that the 

 radioactive ' constants ' of uranium and its daughter elements have not varied 

 with time. Keeping this in mind, we may, I think, tentatively accept some 

 period such as 1,500,000,000 yeai-e, as representing the time that has elapsed 

 since the crystallisation of the oldest plutonic rocks of the earth's crust. The 

 actual age of the earth must, of course, be greater even than this — on the assump- 

 tion specified — for wherever geological evidence is clear, the oldest plutonic rocks 

 are found to be intrusive into a pre-existing sedimentary or volcanic series. 



Yet geological attempts to determine the age of the earth, founded on sedi- 

 mentation and salt accumulation, point to considerably lower figures, say 

 100,000,000 to 400,000,000 years. Certainly the data on which the geological 

 methods depend are of doubtful value ; and the assumption of uniformity which 

 underlies their interpretation is clearly far from being justified. Nevertheless, 

 although the geological results are thus discredited at their source, they make it 

 advisable to examine carefully the assumption of uniformity lying behind the 

 radioactive figures. 



The third method of determining geological time, that based on the now 

 hypothetical cooling of the earth's ' crust,' would probably dissolve the contro- 

 versy, if it did not, unfortunately, precipitate a fresh one. The new difficulty is 

 that the traditional view of a surface originally molten, or practically so, is not 

 altogether regarded with favour in the light of the planetesLmal hypothesis. 

 However, ignoring .this for the moment, it is safe to say, with the existing 

 <listribution of uranium and thorium in the surface rocks, that our planet cannot 

 have cooled down to its present condition (from a stage when the temperature 

 near the surface was approximately 1,000° C.) in less than 1,600,000,000 or 

 1,700,000,000 years. These figures again imply that the rates of decay of the 

 radioactive parent elements have not varied during the earth's history. 



As Professor Joly has suggested, the discrepancy between the numerical 

 results of the geological, and lead and helium methods would disappear, in favour 

 of the former, if it could be shown that the rate of decay of uranium had 

 decreased with the lapse of time. Eeconciliation would then be effected from 

 the fact that the present accumulation of lead in uranium minerals would have 

 required a shorter period of time than could have been possible on the assump- 

 tion of uniformity. However, an ec[ually certain deduction is that during any given 

 time in the past more heat of radioactive origin would have been generated 

 in the rocks than the amount calculated on the basis of uniformity. As a result, 

 the secular cooling of the earth would have been slowed down to such an extent 

 that it would be either reversed, or so prolonged that the age would become 

 ridiculously high. The only alternative to this reductio ad ahsurdum is to 

 conclude that the earth never has been molten at or near the surface. 



Although belief in an all-time solid earth does not of itself invalidate the 

 highest figures based on lead-ratios and uniformity of decay, yet it is an assump- 

 tion that must be granted before acceptance of a lower figure for the age of the 

 earth becomes consistent. On the other hand, if geologists favour an earth 

 initially molten near the surface, then the low figures afforded by the accumula- 

 tion of salt and sediment must of necessity be rejected in favour of the higher 

 results afforded by the accumulation of lead generated at a constant rate. 



It should be observed, in passing, that the lead-ratio, quite apart from the 

 age which it represents, is a factor of extreme value to the geologist. With its 

 help the great granite intrusions of the Archaean platforms in various parts of 

 the world may be correlated, and in other cases when suitable minerals are avail- 

 able for analysis, the period to which an igneous intrusion should be assigned 

 may be determined approximately, even though field evidence is totally lacking. 



Professor Soddy has suggested that the final product of the thorium 

 series is probably an isotope of lead. This view is unsupported by mineralogical 

 evidence. However, while the lead-ratios of minerals rich in thorium may be 

 reliable, and may give ju.st the same result as that of a contemporaneous 

 uranmite, yet in the majority of examples they are remarkably variable. This 

 fact, and the evidence of atomic weights, indicate that thorium tends to be 

 accompanied by primary lead of the ordinary type, in much the same way, 

 1915. J, J, 



