Terrestrial Surface Materials. 697 



the igneous rocks, but it supports the value of: the constant 

 assumed above. 



In the following table of results the quantities of radium 

 are, as usual, expressed in billionths of a gram per gram of 

 rock, and must, therefore, be multiplied by 10 ~ 12 . 



Igneous Rocks. 



Acid. 



63 Granites 2-71 Mean 3-4)1 for 



23 Intrusive and volcanic 3*9 J S6 rocks. 



Intermediate. 



12 Syenites 2'-i] 



8 Diorites 16 I Mean 2*57 for 



18 Trachytes 3*0 [ 48 rocks. 



10 "Porphyries" 2'8 J 



Basic. 



5 Gabbros and Norites 1*3 ~"| 



8 Diabases and Dolerites 1*0 j 



14 Basalts and Melaphyres chiefly") ~. I Mean 1*28 for 



Deccan and Antarctic J | 31 rocks. 



11 Basalts chiefly Hebridean 0'5 j 



18 Basalts generally representative 1'4J 



Additional. 



7 Yesuvian lavas 12*6 



14 Gneisses, various 2*1 



In selecting rocks for the foregoing determinations I have 

 endeavoured to include material as representative as possible. 

 In the composites of the intrusive and volcanic series, differen- 

 tiation products are excluded ; i. e. the chemical extremes of 

 the groups. But it would seem from the results as if dif- 

 ferentiation had operated to segregate radium in these 

 rocks. The interesting question is raised as to how far such 

 differentiation is indicative of more general effects of the 

 kind, or may serve to indicate general differentiation. I 

 hope to return to this question at a later date, when compo- 

 sites of the complementary differentiation products are being 

 examined. 



The three composites of basalts serve to illustrate the 

 divergence which exists in the radium content of these rocks. 

 Thus the fourteen rocks, of which one only is from the great 

 Hebridean outflow and ten are from the Antarctic and Deccan 



