MEASUREMENTS BASED ON RADIOACTIVITY 857 



preceding groups. Geologically the Ser-archean and post-Kalevian gran- 

 ites can not be distinguished in time, and as far as correlation has been 

 attempted they have been grouped together. 



VII. A valuable analysis of a Canadian Middle Precambrian mineral 

 is that of the uraninite from Yilleneuve, in Ontario. The writer is in- 

 debted to the Canadian Geological Survey for the information (due to 

 the work of Mr. M. E. Wilson) that the pegmatite in which the uraninite 

 occurs is associated with a granite which (a) is intrusive into the Gren- 

 ville series and into the pyroxene granites which penetrate the Grenville 

 series, and (h) is intruded by diabase dikes of Keweenawan age. These 

 details make it clear that the pegmatite belongs to one of the periods of 

 granite intrusions contained within the Middle Precambrian of the above 

 classification. This conclusion is completely in harmony with the age de- 

 duced from the single analysis available. 



Conclusions on accumulation of lead}^'^ — 1. The method of determining 

 geological time by the lead ratios of radioactive minerals gives results 

 consistent among themselves and in harmony with geological evidence, 

 wherever this is clear. Eejecting minerals in which alteration, or the 

 presence of primary lead, has vitiated the results in advance, the evidence 

 is conclusive that the ratio Pb/U is nearly constant for minerals of the 

 same age, and that the value of the ratio increases as the geological age 

 of the respective minerals increases. 



2. The results are in keeping with those deduced from other radioactive 

 methods. 



3. The age of the Carboniferous and Devonian intrusions is of the order 

 of 300 to 400 million years; the age of the granite intrusions of the Middle 

 Precambrian is of the order 1,000 to 1,200 million years. 



4. Where geological evidence is obscure, as, for example, in^he Ap- 

 palachians of the Carolinas, the metliod is capable (when suitable min- 

 erals are present) of being applied to the determination of the age of 

 granite and similar intrusive rocks. 



5. The method may ])e used comparatively for the 'correlation of igneous 

 intrusions in various pai'ts of the world, and in particular for the correla- 

 tion of the Precambrian rocks. 



6. Note added November 8, 1915. — In an address to the Geological 

 Society of America (Bull. 26, p. 171, 1915), G. F. Becker has attempted 

 to correlate recent developments in radiogeology and isostasy. He as- 

 sumes that Hayford^s level of isostatic compensation, at a depth of 121 

 km., is the depth of easiest fusion — that is, the depth at which the cooling 

 curve most nearly approaches the curve of fusion. On this assumption 



103 p,y Arthur Holmes. 



