88 DR. A. HOLMES ON THE PRE-CAMBEIAN [vol. lxxiv,. 



specimen, I wish also to thank Dr. L. L. Fermor for the informa- 

 tion that the pegmatites in which the mineral occurs (together 

 with samarskite, columbite, and other radioactive minerals) are 

 regarded as the latest of the Vedic (that is, Archaean) rocks of 

 India. The}* therefore belong to the post-Dharwar intrusives 

 of Sir Thomas Holland's classification of the Pre-Cambrian rocks 

 of India. 1 The lead-ratio found from the analysis of the Singar 

 uraninite (014) is in harmony with this correlation, and indicates 

 that the rocks are of the same age as those of the Moss district of 

 Norway (described below) . 



In Scandinavia there are three areas of Middle Pre-Cambrian 

 granites and pegmatites in which radioactive minerals are found. 

 These are the districts of Ytterby in Sw T eden, and of Moss and 

 Arendal in Southern Norway. The Ytterby pegmatites belong to 

 the Ser-Archa?an granites of A. Gr. Hogbom, 2 and cannot geologically 

 be distinguished in age from the Moss or Arendal granites. The 

 Moss granites are post-Kalevian, and in their case the lead-ratio is 

 very accurately known, for the atomic weight of broggerite from 

 Moss has been found by Honigschmid to be 206*06 ; the lead-ratio, 

 0T3, calculated from two analyses of the same mineral by 

 Hofmann, is therefore thoroughly reliable. 3 The Arendal granites 

 probably belong to the same period of intrusion as those of the 

 Moss district, though it is not impossible that they may be 

 of post-Bothnian and pre-Kalevian age. The lead-ratio, 017,. 

 suggests the latter correlation ; but it is not conclusive, 4 for the 

 Y"tterby fergusonite also gives a ratio of 0*17, and yet is definitely 

 of Ser- Archaean age. 



The ratios may always be too high, if either ordinary lead or 

 thorium-lead happens to be present, and therefore in a series of lead- 

 ratios of minerals of the same geological age, those giving the 

 lowest 5 ratios are most likely to be strictly correct. The only 

 direct method of determining whether the lead present is 'uranium'- 

 lead in its entirety involves a measurement of its atomic Aveight ; 

 and as this is a task which few chemists are capable of undertaking,, 

 and for which, moreover, it is particularly difficult to obtain 

 material, it cannot be expected that a complete chain of evidence 

 should always be forthcoming. Such data as are now available 

 are expressed in Tables XI & XII ; and it is thought that the 



1 Congres Geol. Internat. C.R. Xll^me Sess. Canada, 1913 (1914) p. 376. 



2 ' Pre-Cambrian Geology of Sweden,' Bull. Geol. Inst. Upsala, vol. x 

 (1910-11) p. 2. 



3 A. Holmes & E. W. Lawson, Phil. Mag. ser. 6, vol. xxix (1915) p. 678. 



4 [More conclusive evidence is provided by an atomic-weight estimation 

 for lead extracted from cleveite, Arendal, carried out by Richards & Wads- 

 worth. Although the Pb/U ratio is nearly 0'19, the atomic weight is not 

 unduly high, being 206'08.— A. H., March 1919.] 



5 It should be noticed that in the case of helium-ratios the highest values 

 are considerably below the lowest values of the corresponding lead-ratios,, 

 this being due to the ease with which helium escapes. 



6 For a list of such determinations, see A. Holmes & R. W. Lawson, op. cit* 

 1915, p. 682 ; and for a general discussion, see pp. 679 et aeqq. & pp. 687-88- 



