﻿678 



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



Hofmann * has given the results o£ his analyses of two 

 samples of Broggerite, from which the following percentage 

 constituents have been calculated. It is clear that if the end 



Table VII. 



Sample. 



U. 



Pb. 



Th. 



Bi. 



Vm. 



Thm. 



Age of mineral in millions 

 of years, 



from Pb/Um 

 ratio l . 



from Bi/Th„, 

 ratio. 



I 



II 



67-40 

 67*08 



8-61 

 8-49 



4-10 

 4-63 



0-30 

 0-33 



72-35 

 71-96 



4-27 



4-82 



1000 

 990 



1490 

 1440 



1 The lead ratios agree with those of the minerals given in Table II. 

 Part I. p. 835, Moss district, Norway, Age pre-Jatulian. 



product of the thorium series is a stable isotope of bismuth f, 

 we may use the Bi/Th m ratio to determine the age of the 

 mineral. Moreover, the age as calculated in this manner 

 should agree with that as found from the Pb/IJ m J ratio. 

 The two expressions for the age of the mineral will be 

 respectively Bi t /Th w x 2'09 . 10 10 years, and Pb,/U™ x 8'58 . 10* 

 years. Th m and U m are the respective time-average values- 

 of the thorium and the uranium contents of the mineral, and 

 the ages as found in the case of the mineral under considera- 

 tion are given in the last two columns of Table VII. In 

 consideration of the relatively small percentages of thorium 

 and bismuth present, and the difficulty of their estimation,, 

 it might be thought that the agreement between the ages were 

 sufficiently satisfactory to favour the view that thorium- 

 bismuth ("thorium F ") might be a stable product. 



Opposed to this view, however, is the fact that in the 

 collection of analyses of thorite and allied minerals cited 

 by Hintz in his ' Mineralogie,' vol. i. p. 1675, there is not a 

 single determination of bismuth. Moreover, the Devonian 

 minerals already examined by us have been tested for bis- 

 muth with negative results. Hillebrand (Bull. 220, p. lll r 

 U.S.G-.S., 1903) gives two analyses of uraninite which 



* Hofrnann, Ber. d. d. Chem. Ges.xxxir. p. 914 (1901). 



t The part played by original bismuth in such minerals is almost 

 certainly quite negligible. For instance: Vogt (loc. cit.) suggests 

 0*00000 x grm. as the average amount of bismuth in 100 grams of rock 

 (cf. Holmes, loc. cit. 1911, p. 253). 



X If actinium lead is unstable, the results obtained for the age of a 

 mineral from the Pb/U ratio will be rather low, and if the end product 

 of actinium is bismuth, the age obtained from the Bi/Th ratio will be- 

 siightly high. 



