834 Messrs. A. Holmes and R. W. Lawson on 



desiderata, lead, thorium, and uranium, one or more have not 

 been determined, or, at least, have not been placed on record. 

 It must not be supposed because a constituent has not been 

 recorded, that it is necessarily absent. This positive state- 

 ment should only be made where the record o£ the analysis 

 is equally positive. On this account large numbers of 

 analyses have been rejected by us, although in many cases 

 it has come to our notice by private information of a con- 

 fidential nature, that had the analyses been complete they 

 would have been strongly in favour of our thesis. For a 

 totally different reason, other series of analyses are not 

 given here; e.g. the uraninites of Brancheville, Conn., U.S.A., 

 and of Glastonbury, Conn., U.S.A. Although the analyses 

 of many of the minerals from these localities are unusually 

 accurate and complete, yet in them the ratio of thorium to 

 uranium is so nearly constant, that the test afforded by the 

 Pb/(U + 0'4Th) ratio becomes valueless. In other cases, 

 as for example the richly radioactive minerals of Llano Co., 

 Texas, the necessary condition of freshness fails, and the 

 variability of the Pb/U ratios is almost as great as that of 

 the Pb/Th ratios. 



In the following groups of analyses the variation of the ratio 

 of thorium to uranium is sufficiently wide to indicate that the 

 Pb/U ratio is independent of the proportion of thorium present. 



(b) Radioactive Minerals from the Moss District, Norway 

 (Table II.). 



The exact ages of the igneous complexes from which 

 this and the next following series of minerals (c) are taken 

 have not yet been satisfactorily determined by Scandinavian 

 geologists. Both groups of igneous rocks are, however, 

 younger than the metamorphosed sediments with which they 

 are associated, and both are certainly pre-Jatulian *. 



In Table II., the arrangement of which is uniform with 

 that of Table I., the contrast to which attention is parti- 

 cularly directed is between mineral 1 and any of the others. 



* The Fenno-Scandian Pre-Cambrian sequence is as follows: — 



1. Jotnian. 



Unconformity. 



2. Jatulian. 

 ... „ 



3. Upper Kalevian. 



V 



4. Lower Kalevian. 



?> 



5. Bottnian. 



» 



6. Ladogian. 



Irruptive contact. 



7. Kat-archsean. 



