376 Prof. J. Joly on the 



Holmes enters into the question of the geological positions 

 of the results cited by Boltwood, and concludes that they may 

 be tabulated as follows. His own mean result is included in 

 the table. 



Geological Period. 



Pb/U. 



Millions of 

 Years. 





0-041 

 0045 

 0050 

 0-053 



0-125 

 0-155 

 0-160 

 0-175 

 0-20 



340 

 370 

 410 

 430 



1025 

 1270 

 1310 

 1435 

 1640 



Devonian "" 



Pre-Carboni f'erous 





Pre-Cambrian : — 



Sweden \ 



United States \ 

 Ceylon 





The Swedish minerals are from pegmatites of an age 

 younger than Jatulian. The results obtained from them 

 show, among 17 specimens examined, two well-marked 

 groups, having the ratios tabulated. There is nothing in the 

 rocks to indicate any difference in age. Of the United 

 States minerals those having the lesser ratio are from 

 granites intruded into the Llano group (Texas) of meta- 

 morphosed sediments. Their age is, therefore, younger 

 than the sediments, which are early Algonkian. Those with 

 the higher ratio are from Burnet Co. (Texas) and Douglas Co. 

 (Colorado). The geological evidence is similar to that of 

 Llano Co. 



The evidence for the pre-Cambrian age of the Ceylon 

 thorianite is the resemblance of the rocks to the fundamental 

 complex of India. The tabulated values are the means of 

 several results cited by Boltwood, some of which are in closer 

 mutual agreement than others. 



These results greatly transcend S truths in the antiquity 

 they assign to Palaeozoic and pre-Cambrian time. This fact 

 can be explained by the escape of helium. The possibility 

 of occluded lead entering seriously into such determinations 

 will, doubtless, form the subject of future research. Mean- 

 while it seems improbable that the higher average ratios of 

 the oldest minerals can find explanation in this manner. I 

 have already dwelt sufficiently, in view of our very deficient 

 knowledge, on these points. 



The discordance between the radioactive indications of time 

 and those derived from the stratigraphical column appears 

 clearly when we plot one against the other (Plate III.). The 



