62 Arthur Holmes — Radio-activity 



up to 160 tons to the square inch, 1 which are quite sufficient for 

 geological purposes, since they represent average conditions which 

 cannot be generally reached at depths of less than 50 miles, or 

 80 kilometres, below the surface of the earth. To that depth, at 

 least, the assumption of complete radio-active independence is 

 thoroughly justified by direct experimental evidence. Since for 

 thermal reasons the radio-active elements must be largely concentrated 

 in an outer shell, it is of very little geological importance to decide 

 the question of possible inhibition at depths greater than 50 miles. 



2. Distribution - of the Radio-active Elements. 



As a result of a large number of experimental determinations of 

 radium 3 in rocks and meteorites, first initiated by Strutt 3 and since 

 continued by other workers, 4 it is now possible to state with confidence 

 the salient features of the distribution of radium in rocks and 

 meteorites of different types. 



(1) Igneous rocks as a whole average more radium than sedi- 

 mentary rocks. 



(2) Metamorphic rocks agree in their averages with those of the 

 igneous or sedimentary rocks from which they have been formed. , 



(3) The average for alkaline igneous rocks is higher than that 

 for calc-alkaline rocks, and this important fact must be remembered 

 in connexion with local exceptions to statement 5. 



(4) The average for volcanic rocks is higher than that for plutonic 

 rocks. 5 



(5) Acid igneous rocks average a higher content of radium than 

 those of intermediate composition, and the latter, in turn, are richer 

 than basic and ultra-basic rocks. 



(6) The ultra-basic stony material of meteorites is poorer in radium 

 than are terrestrial ultra-basic rocks. 



(7) The metallic material of meteorites contains no radium, and in 

 keeping with this significant fact terrestrial native iron is also free 

 from radium. 



Estimations of thorium have not been so numerous as those of 

 radium, and most of the work has been done in the laboratories of 



1 Eve & Adams, Nature, p. 209, July, 1907. 



2 One part of radium in equilibrium with its parent uranium implies the 

 presence of 3,000,000 parts of the latter element. 



3 Proc. Eoy. Soc, A, vol. lxxvii, p. 472, 1906. 



4 See Joly, Phil. Mag., vol. xxiv, p. 694, 1906 ; Holmes, loc. cit., p. 15, 1914, 

 for references to literature. 



5 The unusually high radium content of recent Vesuvian lavas found by Joly 

 is probably due to — 



(1) Gravitational differentiation. 



(2) Concentration by volatile fluxes. 



(3) Association of radium, unaccompanied by its parent uranium, with 



potash minerals. It is well known that barium is commonly associated 

 with magmas rich in potash, and it is therefore suggested that free 

 radium may be similarly concentrated. In old lavas any such free 

 radium would almost entirely have disintegrated, leaving to be 

 measured only the radium which is directly related to the uranium 

 present in the rock. 



