Lead and the End Product of Thorium. 831 



Devonian age *. A recent discovery of Osteolepis proves 

 that the sediments upon which the igneous rocks are intruded 

 belong to the Middle rather than the Lower Devonian, and 

 the age assigned to the nepheline syenites from which our 

 minerals were taken is not far removed from this, being 

 almost certainly Middle Devonian. 



Table I. gives the results of the analyses of Devonian 

 minerals made by the authors. The first two columns give 

 the numbers and names of the minerals respectively, and the 

 next three columns their respective contents of uranium, 

 thorium, and lead. In the sixth column is the ratio between 

 the amounts of thorium and uranium in the mineral, which 

 serves to emphasise several points that will be mentioned 

 in the next section. The two following columns give the 

 lead-uranium and lead-thorium ratios respectively, referred 

 to the time-average quantities of uranium and thorium in 

 each mineral. The former of these should be constant if 

 uranium lead were stable and thorium lead unstable, whilst 

 the latter should be constant in the inverse case. Finally, 

 if both uranium lead and thorium lead were stable, the 

 corrected expression given in the last column should be 

 constant. 



§ 5. Discussion of Results. 



The minerals used are evidently well adapted to the 

 examination of the question of the stability of thorium lead, 

 as the ratio of thorium to uranium varies between very wide 

 limits, reaching a maximum value of 120. In spite of this, 

 however, the lead-uranium ratios are, with the exception 

 of thorites (3) and (1), remarkably constant, having a 

 representative value 0'04«i'. It is true that several of the 

 ratios give a higher value than this, but in every such 

 case the uranium content of the mineral is less than 

 O'l per cent., and the lead content is correspondingly 

 low. In such cases the effect of original lead in increasing 

 the ratios must be of considerable importance, and the 

 explanation of the divergency is almost certainly to be 

 found in that possibility. The reason for the excessive 

 values of the ratio in the cases of thorites (3) and (1) is not 

 easy to find. There is again the possibility of original lead. 

 In accordance with this suggestion it should be pointed out 

 that thorite is not always as fresh as could be desired. 

 Thorite frequently appears to be pseudomorphous after some 

 unknown original form, its structure being fibrous and its 



* Briigger, Zeit. fiir Krut xvi. (1890); Holmes, Proc. Koy. Soc. A. 

 Ixxxv. p. 251(1911). 



