THE TERRESTRIAL DISTRIBUTION OF RADIUM 17 



high values obtained by Joly. However, Joly's recent work 

 has replaced his earlier determinations by results more in 

 accordance with those of other analysts. Taking the averages 

 of the results obtained by the solution method, it will now be 

 seen, that with one exception, they indicate an unbroken pro- 

 portionality between the average amount of radium and the 

 average amount of silica in igneous rocks. The one exception is 

 afforded by the three determinations of Eve and Mcintosh, 

 which are as follows : 



(Tinguaite .... 

 Tinguaite .... 

 Nepheline Syenite 



Average . 



A similar exception is illustrated by three radium analyses of 

 alkaline rocks made by the present writer : 



{Phonolite 

 Phonolite 

 Solvsbergite 



2*83 x io -12 grams per gram. 

 4-10 x io- 12 „ „ „ 

 2-26 x io- 12 



and by the first of the following averages for alkaline rocks of 

 intermediate composition : 



13 Volcanic (and Hypabyssal) Rocks 

 1 1 Plutonic Rocks 



-12 



Average 



2"94 x 10^'- grams per gram. 

 1-87 x io- 12 „ „ „ 



2-45 x io~ 12 „ „ „ 



These results lead on to a further generalisation : that 

 alkaline rocks tend to be richer in radium than normal or 

 calc-alkaline rocks of similar acidity. Indeed, the extent to 

 which alkalies are present, and in particular, the extent to which 

 sodium is present, would appear to be the predominating factor 

 in determining the quantity of radium in a rock magma. The 

 association of radium with alkalies is more fundamental than its 

 association with silica, for it is the combination of silica with a 

 high proportion of alkali that favours the relative abundance of 

 radium. 



Joly's results, which bring out very clearly the sympathetic 

 relation between the radium and silica contents of a rock, 

 also indicate the probability that volcanic, and, to a less extent, 

 hypabyssal rocks, are on the average more richly charged with 

 radium than their plutonic equivalents. My own determinations, 



