700 Prof. J. Joly on the Radioactivity of 



remains that the great body of acid rocks are the richest in 

 radium, and are over twice as rich as the typically basic 

 group. I have shown that th'> thorium content of the acid 

 rocks is also in excess of that of the basic rocks *. No ex- 

 planation of tliis is forthcoming. It may arise in the primary 

 differentiation of a parent magma into granitoid and gabbroid 



•divisions. It has been maintained that these divisions re- 

 present in themselves fundamental magmas, and that they 



-are present in about equal proportions in the lithospheret- 



The connexion between radioactivity and silica content 

 may turn out to be closer than a rough proportionality. If 

 we take the three chemical rock-divisions as characterized by 

 74, 60, and 48 per cent, of silica and assign to the most acid 

 division a radium content of 3, we find the radium in the 

 other groups, if proportional to the silica, would be 2*4 

 and I' 9. The basic division departs most from the propor- 

 tionality, but considering the extreme variations both in 

 silica content and radium content which obtain in this 

 division, the agreement seems sufficiently close to suggest 

 some connexion between the segregation of uranium and of 

 silica in the history of the magma. A similar proportionality 

 is indicated in the thorium content J: but here the number of 

 observations is insufficient. I hope to be shortly in a position 

 to enter more fully into this question. 



Coming now to the availability for geological discussion 

 of such results as the foregoing, we are met by the difficulty 

 that we really do not know the quantitative distribution of 

 the several rock families in the lithosphere. When, there- 

 fore, we seek to derive a general mean radium content our 

 conclusion may be attended with a certain amount of error. 

 On the result of a very considerable number of analyses of 

 igneous rocks, Clarke § concludes that the lithosphere to a 

 depth of ten miles probably approximates in composition to 

 . that of a diorite or andesite ; or is, in short, " intermediate 9> 

 in chemical character. Jf this is correct we should assign 

 to it such a radium content as we find in intermediate rocks, 

 i. e. 2*57. If we combine the gabbros and granites we get 

 a mean of 2*0, and if basic and acidic generally are combined 

 the average is 2'2. It would appear as if the radium content 

 of average igneous rocks at the surface is not below 2'0, and 

 may probably be about 2 5. If the entire number of 105 



* Covg. Interuat. de Had. et (V Elect. 1011, p. 370 . 

 t Loewinson-Lessing , J Geol. Mag. 1911, p. ^4S. 

 J Joly, Joe. eit. 

 § ' Data of rjeucHemistrvs' 2nd ed. 1911, p. 26, 



