part 1] AND ASSOCIATED ROCKS OF MOZAMBIQUE. 85 



then it produced lead at the same rate at the same time in Canada, 

 Scandinavia, India, and in other parts of the world. That is to 

 say. the amount of lead generated by uranium during the time 

 that has elapsed since any given period is — so far as the Earth is 

 concerned — independent of the location of the uranium. Thus, 

 if the ratio of lead to uranium in a uranium-bearing mineral of 

 igneous origin occurring in Mozambique is equal to the Pb U ratio 

 for igneous minerals occurring in any other part of the world, then 

 these minerals are of the same age, and consequently the rocks in 

 which they occur are also of the same age. The requisite con- 

 ditions that the minerals used as criteria must be fresh, primary. 

 free from original lead, and free from considerable amounts of 

 thorium, have been fully dealt with in previous papers, and do not 

 here require further discussion. 



The chief mineral investigated from Mozambique is zircon, 

 which was obtained in the field during 1911 by crushing consider- 

 able quantities of the parent rocks and separating the heavier 

 minerals by panning. 



- (1) Correlation of the Gneissose Granites. 



Zircon was obtained from the gneissose granites of the Eibawe 

 Mountains (Sawa Valley) and Etipoli Hills. The two crops 

 together provided sufficient material for duplicate analyses of lead 

 and uranium. The mean results, which are set forth in Table XI 

 (p. 86), give a lead-ratio of 0*21. A later analysis for thorium 

 was made in Vienna by my friend. Mr. E. W. Lawson, and the 

 amount found i 0*0013 per cent.) is so small that the possibility 

 of the lead-ratio being appreciably affected by thorium-lead is 

 completely dispelled. 



The only rocks with which an approximate correlation, may be 

 made are the Laurentian gneissose granites of Canada. In 1909, 

 zircon and sphene from the Smart Mine, Sebastopol (Ontario), were 

 investigated by Prof. Strutt, 1 and the amount of helium that he 

 obtained from these minerals indicated that they were older than 

 any of the minerals that he examined from other localities. His 

 results therefore serve to confirm the corresponding high value, 

 0*24, found for the lead-ratio of zircon from the Smart Mine. 

 Very little information is published regarding the geology of the 

 immediate neighbourhood of Sebastopol, but I am indebted to the 

 Geological Survev of Canada for the following information : — 



The minerals mentioned [zircon and sphene] are the ?ame as those associ- 

 ated with orthoclase-pegmatite masses that occur in the phlogopite-apatite 

 bearing pyroxenites of the Templeton and Buckingham districts. Quebec. 

 These pegmatites are regarded hy Mr. M. E. Wilson (of the Canadian 

 Geological Survey) as related in origin to the charnockite-like pyroxene- 

 granite of the Buckingham Series. 2 and would therefore be of early Pre- 

 Cambrian age and older than the Yilleneuve pegmatite. 



1 R. J. Strutt. Proc. Roy. Soc. vol. lxxxiii (1909) p. 300 k vol. lxxxiv 

 (1910) p. 195. 



2 M. E. Wilson, South-Eastern Portion of Buckingham Map Area. Quebec : 

 Summary Rep. Geol. Surv. Canada for 1913 (1914) p. 202. 



