Lead and the End Product of 'Thorium. 833 



optical properties obscure. If it should be a paulo-post 

 mineral formed by the action of igneous exudations imme- 

 diately following the earlier manifestations of igneous 

 activity, it would be expected that in some cases a little lead 

 might be associated with it, as with other pneumatolytic 

 minerals. In the case of thorites from other localities, and 

 also of thorianite, the same variability of ratio is to be 

 noticed. Too much stress must therefore not be placed on 

 evidence based upon analyses of thorites, the variable 

 composition and obscure history of which at once arouse 

 suspicion as to their true origin. 



Taking the results as a whole, it is clearly evident that the 

 value of the lead-uranium ratio is independent of the thorium 

 content. Comparing the results for pyrochlore (2) and 

 thorite (2) it will be seen that in the latter the ratio of 

 thorium to uranium is 180 times that of the former. The 

 lead-uranium ratio, however, is almost identical in the two 

 cases. 



There is absolutely no indication of constancy in the lead- 

 thorium ratios ; so we may definitely conclude that the 

 hypothesis adopted for the calculation of those ratios is not 

 fulfilled (viz. Th E relatively stable, Ea G relatively unstable). 



As pointed out previously, if both the thorium and 

 uranium lead isotopes are stable, the ratio Pb^/(U Wi + , 4Th w ) 

 should be constant for all the Devonian minerals examined. 

 While there are not such wide divergencies in this column 

 as in the last considered, it is clear that this ratio is far from 

 being constant. The maximum and minimum values are 

 respectively 0*052 and 0*0012, the former of which is more 

 than forty times the latter. 



We may therefore conclude that — 



(1) Radium G- is a stable isotope of lead ; 



(2) Radium G and Thorium E cannot both be stable 



isotopes of lead ; 

 and therefore that 



(3) Thorium E is an unstable isotope of lead. 



If (3) is correct, then Th E must gradually disintegrate, 

 losing radiations which have so far eluded detection, and 

 thus becoming transformed into a simpler element. 



§ 6. The Association of Lead with Cranium and Thorium in 

 other series of Rock Minerals. 

 (a) Introduction. — It is unfortunate that there are very few 

 series of published analyses of original thorium- and uranium- 

 bearing minerals suitable for our present purpose. When 

 the mineral is suitable it frequently happens that of the three 



