m 



Prof. J. Joly on the Radioactivity of 



ureas, possess four times as much radium as the Hebridean 

 basalt. The result for the latter confirms Strutt's low figure 

 for this rock *. Finally, a carefully made up composite, in 

 which basalts from the most widely separated areas are 

 equally represented, gives a value approximating to a mean 

 between the high and low results. The general mean for 

 basic rocks includes the more representative series of basalts 

 only. Of the Additional rocks, the gneisses not being in all 

 cases of certain igneous derivation, are not available for 

 general deductions. 



For comparison with the determinations by fusion I have 

 classified below the results obtained in some of the leading 

 investigations in which the method bv solution has been 

 employed. The numbers in brackets given after the results 

 indicate the number of rocks dealt with. 



Granites, 



Acid Intru- 

 sives and 

 Volcanics. 



Syenites. : Diorites. 



Trachytes 



and 

 Andesites. 



Gabbros jDolerites 



and and 



Norites. 'Diabases. 



Basalts. 



Strutt 1 2-74(10) 



Florance 2 j ' ' 75 ( 2 ) 

 Scblundt 

 & Moore 

 Buchner-t ...3-70(5) 

 Fletcher 5 ...085(4) 



1-03(1) 

 1-90 (1) 



1-95 (7) 

 0-80 (3) 



2-67 (3) 



99(1) 



0-80 (3) 



1-68 (4) 



1-85(12) 



0-85 (20) 



0-63 (1) 0-59 (2) 0-44 (8) 

 033 (2) j 0-59 (3) 0-81 (1) 



0-70 (1) \ 0-87 (2) 



Mean 



•51 (21) 



1-58 (12) 



2-67 (3) ! 0-85 (4) ! 1-21 (36) 0-50 (4) | 0-67 (7) 



0-50 (1) 

 0-71 (5) 



0-56 (15) 



1 Strutt, Proc. E. S. lxxvii. A. p. 472, and Proc. P. S. lxxxiv. A. p. 377. 



2 Farr & Florance, Phil. Mag. Nov. 1909. 



3 Schlundt & Moore, Bull. U.S. Geol. Survey, p. 395 (1909). 



4 Buchner, Konink. A/cad. van Wetensch. te Amsterdam, Proc. Oct. 1910, 

 and April 1912. 



s Fletcher, Phil. Mag. July 1910; Jan. 1911; June 1911. In the analysis 

 given above the series of observations on the Leinster granite are treated as 

 representing but one rock. Otherwise a considerable lowering of the mean 

 lor granite must arise. I have not dealt similarly with the series on the 

 Andine lavas, as these appear to include distinct outflows and petrologically 

 distinct varieties of trachytes and andesites. 



When the means for acid, intermediate, and basic rocks 

 are calculated from the above table, giving equal weight to 

 each separate rock determination, we have: — 



Acid, 



Intermediate, 



Basic, 



mean for 33 rocks 2*17 



43 



26 



1-28 

 0-58 



* Strutt. Prop. JR. S. lxxxiv. A. 



p. o( t 



