44 



ADVENTURES IN RADIOISOTOPE RESEARCH 



Table I. — Lead Coxtent of Igneoxts Rocks 



Rock Types 



gm Lead per 

 gm Rock 



Basalt, Giant's Causeway 4 X 10"" ^ 



Gabbros and related types (composite of 67 samples) 5 X 10"^ 



Essexites and related types (composite of 40 samples) 10 X 10~^ 



Shonkinites (average of 2 samples) 10 X 10~^ 



Soda-granites and soda-syenites (composite of 2(5 



samples) U X 10"" ^ 



Potash-granites and potash-syenites (composite of 24 



samples) 14 X 10~ ® 



Amphibolite, inclusion in Kimberlite, Wesselton 



Mine, S. Africa 15 X 10^ « 



Kimberlite ('basaltic ' type). Dyke from 1350-foot 



level, Dutoitspan Mine 16 X 10~ " 



Lherzolite, Baltimore, Maryland 19 X 10~" 



Granitic rocks (composite of 58 samples of widely 



different localities) ' 30 X 10 « 



Dunite, Jackson Co., Nortli Carolina 42 X 10"'' 



second decimal place. That the ore-lead must have been formed in 

 the ancestral sun, or during the events that attended the birth of 

 the solar system, was already pointed out some years ago b}^ Prof. 

 Holmes^ . 



We conclude from the above determinations that the greater part of 

 rock-lead is also of the same origin. Although acid rocks, which have a 

 relatively high uranium and thorium content, are found to contain more 

 lead than basic rocks, this difference is not to be interpreted as an 

 argument in favour of the radioactive origin of the whole of the lead 

 in rocks, but an expression of the fact that lead, like uranium and thorium, 

 shows a marked affinity to siliceous magmas. 



It is of interest to compare the lead content of basic and ultrabasic 

 rocks with that of meteorites as determined by Noddack'^. Tlie lead 

 content of stony meteorites is near that of basalt and average gabbro, 

 and is markedly lower than that of terrestrial ultrabasic rocks. The lead 

 content of iron meteorites, as confirmed in this laboratory, is about ten 

 times greater than that of stony meteorites, while that of troilite (the 

 high lead content of which was predicted by Prof. V. M. Goldschmidt) 

 is more than a hundred times greater (700xlO~^). These results show 

 1 hat when the earth was formed the silicate shell received only a modest 



^Holmes, Nature 117, 482 (1926). 

 2NODDACK, Die Naturwiss. 18, 761 (1930). 



