PRESIDENTIAL ADDRESS SECTION V>. 2^ 



and radio-active substances in the earth as a whole is a hancHcap 

 in this task. It will therefore be of interest first of all to 

 review briefly what is known of this branch of the subject. 



Radio-active matter in the form of compounds is now known 

 to be widely distributed over the face of the globe, a distribution 

 which has, no doubt, been considerably influenced by the surface 

 changes which the Earth has undergone. It is found in minute 

 quantities in practically all the rocks of the Earth's surface, in 

 nearly all the waters and in the atmosphere, while hand in hand 

 with a general tendencv to diffusion there has also been, as we 

 shall see, certain tendencies to concentration. 



So far as the crust of the Earth is concerned, the igneous rocks 

 may be taken as the source of the radio-active sul)stances. Thev 

 have a higher radium content than the sedimentary rocks, since 

 the latter have been derived from the former, and in the pro- 

 cesses of denudation and deposition a certain amount of the 

 radio-active contents of the igneous rocks is taken up by the 

 atmosphere and the waters. With regard to the igneous rocks 

 Professor Strutt has found a higher radium content in the acid 

 than in the basic rocks, and from the luunber of analyses so far 

 completed, estimating the radium content in billionths of a 

 gramme per gramme of rock, we get the further interesting 

 generalisation that a combination of silica with a high proportion 

 of alkali, such as is found in the phonolites and rocks of a similar 

 class, favours the relative al)undance of radium in the igneous 

 rocks. The extent to which the alkalis are present is. apparently, 

 according to the later results of Holmes, the predominating 

 factor in determinating tlie ([uantity of radium present. 



Joly's results indicate the probability also that volcanic and, 

 to a less extent, hyperbyssal rocks on the average have a higher 

 radium content than their plutonic equivalents. This may be 

 accounted for by the fact. demcMistrated 1)\' numerous analyses, 

 that volcanic rocks contain, as a rule, more soda and more silica 

 than the corresponding plutonic rocks. 



Radium, therefore, shows a marked preference for alkaline 

 and acid rocks, and also for volcanic rocks as compared with 

 plutonic. Apparently, therefore, the processes of differentia- 

 tion responsible for such rocks have also been favourable for the 

 concentration of uranium, and conse(|uentK- c;f radium. The 

 same may also 1)e said of thorium, as indicated bv the associa- 

 tion of uranium and thorium-bearing minerals witli pegmatites 

 genetically related to granites and syenites uf an alkaline 

 character. Besides pegmatites, radio-active matter also tends 

 to be concentrated in certain rock constituents, such as zircon, 

 pyromorphite, apatite, etc. 



These results regarding the concentration of radio-active 

 compounds are certainly significant, but the data are. perhaps, as 

 yet hardly sufficient to enable us to draw any final conclusion 

 from them. The close association with acid pegmatites, how- 

 ever, suggests that the same process of differentiation may have 



