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. 1 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, 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 invali- 

 dating 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 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 Archssan 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 (b) 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 



* Brogger, Die Mineralien der Syemtpegmatitgaage, pt. 1, pp. 180, 164, and pt. 2, p. 10. 



