CONVERGENCE OF EVIDENCE 



877 



but little is present. The geological age can not be later than the close 

 of the Permian. Uraninites from the Carolinas gave Eichards and Lem- 

 bert an atomic weight of 206.4. This suggests that some of the lead of 

 those uraninites may be primary, as this is higher than the lowest atomic 

 weights which have been found for uranium lead. The 300,000,000 years 

 for the Connecticut uraninites should therefore be a maximum In the 

 table as given on a following page for the column of maximum ages it 

 is seen that for any probable correction in age due to original lead the 

 minerals would still fall within the Carboniferous, including Pennsyl- 

 vanian and Permian. In the column of minimum ages the Glastonbury 

 uraninites fall into the close of the Devonian. Although this does not 

 seem so probable, it is not positively excluded by the evidence. The 

 column of minimum ages is based for the Paleozoic on the Middle 

 Devonian age of the uranium minerals from Brevig, ?^orway. The two 

 determinations, although in the proper sequence of age, do not agree 

 closely with each other in the amount of time which elapsed between them. 

 Another possibility is that the Glastonbury uraninites are in reality late 

 Carboniferous, with an age of about 200,000,000 years, but with one- 

 third of the lead original, giving an age about 50 per cent too great. 

 More probably the truth lies in some adjustment between several of these 

 factors. On the whole, the column of minimum ages should for the 

 present be regarded as probably nearer to the truth. Nevertheless, urani- 

 nites, from the Carolinas, with an estimated correction for primary lead 

 made by means of the atomic weights, give an age between 310,000,000 

 and 345,000,000 years. Their geologic age is, however, unknown, further 

 than that they are now regarded as Paleozoic and probably Upper Paleo- 

 zoic. Their most probable position is Carboniferous, either Pennsylva- 

 nian or Permian. ^ 



The next older determination is of Devonian minerals from Brevig, 

 Christiania district, Norway. It is seen from the tables previously given, 

 page 853, that the per cent of lead is very small, ranging in twelve deter- 

 minations from .001 to .037 per cent. The lead-uranium ratio, ranging 

 from .040 to .062 in different minerals, nearly constant to the second 

 decimal place, considering the small amounts of lead, may be taken as 

 evidence of the accuracy of the analytical work by Holmes, and also of 

 the absence of more than a trace of original lead. Holmes and Lawson 

 give an elaborate discussion of this question in a recent paper,^^^ showing 

 that thorium-lead is not present. Holmes gives the mean age as 370,- 

 000,000 to 380,000,000 years, which should be corrected to 345,000,000 



"^Lead and the end product of thorium. Philosophical Mag., 6 ser., vol. 28, 1914, pp. 

 823-840. 



