Age of the Earth. 373 



undifferentiated magma has been fairly radioactive, may not 

 the pegmatitic substances, representing a large part of the 

 rejected elements of the magma, be rich in the products of 

 radioactive decay ? It would seem that we are reasonably 

 entitled to expect this. There might even be a certain pro- 

 portionality between the amounts of radioactive bodies and 

 segregated products of decay. 



The results of the experiments themselves alone can indicate 

 how far sources of error of this kind have operated. The 

 final ratio — whether of helium or lead — to the parent radio- 

 active substance is, we may suppose, compounded of two 

 ratios, a segregation ratio which obtained from the first, and 

 a generative ratio which kept on increasing throughout 

 geological time. Consider the case of lead. We have no 

 prima facie right to conclude that the originally segregated lead 

 is, relatively to the uranium, more for, say, Archaean minerals 

 than for Devonian. If then the gross lead ratio for the 

 former is very much greater than for the latter, the effect of 

 the occluded lead must only exercise an insignificant influence 

 in invalidating the results regarding Archsean time. To take 

 a concrete example. The assumption that of the total lead 

 found in Devonian minerals a quantity equal to two per cent. 

 of the uranium present in each case is not of radioactive 

 origin but was originally introduced, amounts to saying that 

 one half the ratio (about) is due to original segregation and 

 one half to radioactive genesis. The time value of the cor- 

 responding deduction from Devonian time (as derived from 

 the gross ratio) is about 160 million years. A quantitatively 

 equal correction applied to the ratio observed in Archa?an 

 minerals will not be very important, as will presently be seen. 

 Unless, then, we have some reason to infer that the conditions 

 attending the formation of the minerals having the higher 

 ratios were such as to lead to the inclusion of greater relative 

 amounts of lead, the objection under this head is not of 

 serious weight, at least in the case of the higher ages which 

 have been arrived at. 



Acting either to increase or diminish the observed deduced 

 age, errors under the head (b) may exist. The volatile escape 

 of helium has been demonstrated by Strutt. Under past 

 conditions of heating and percolation, &c, its escape is very 

 probable. Acting the other way, radium is known to migrate 

 from its parent elements, and in considerable amounts. Lead 

 is certainly at least equally liable to migration under suitable 

 conditions. These sources -of error would also tend to go on 

 augmenting with the lapse of time. Unless, however, it can 



Phil Mag. S. 6. Vol. 22. No. 129. Sept. 1911. 2 C 



