AGE OF THE EARTH—JOLY. 285 
thorite, crystallized in the first phase of vein formation, “that of 
magmatic consolidation with the cooperation of pneumatolytic proc- 
esses,” and that in the second and principal phase of pneumatolytic 
activity galena crystallized out.’ If the 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 cer- 
tain proportionality 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 segre- 
gated lead is, relatively to the uranium, more for, say, Archean 
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 invali- 
dating the results regarding Archean time. To take a concrete 
example. The assumption that of the total lead found in Devonian 
minerals a quantity equal to 2 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 corresponding 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 Archean minerals will 
not be very important, as will presently be seen. Unless, then, we 
have some reason to infer that the conditions attending the forma- 
tion 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 (6) may exist. The volatile escape of helium 
has been demonstrated by Strutt. Under past conditions of heat- 
ing and percolation, etc., its escape is very probable. On the other 
hand, the accretion of radium is not impossible, for radium is known 
to migrate from its parent elements, and in considerable amounts. 
Lead is certainly at least equally liable to migration under suitable 
—— 
i Broeger, Dis Mineralien der Syenitpesmatitgange, pt. 1, pp. 160, 164, and pt. 2, p. 10. 
