THE TERRESTRIAL DISTRIBUTION OF RADIUM 19 



Summing up, we may conclude that in general radium shows 

 a marked preference for alkaline and acid rocks, and to a less 

 extent for volcanic as compared with plutonic rocks. That is 

 to say, the processes of differentiation which are responsible for 

 the production of rocks rich in alkalies (particularly soda) and 

 silica, and incidentally for the difference in composition between 

 volcanic and corresponding plutonic rocks, are also responsible 

 for the relative concentration of uranium, and therefore of 

 radium. Although the evidence in the case of thorium is less 

 abundant than that for radium, it tends to show that similar 

 statements are equally true of that element. This is indicated 

 very forcibly by the well-known association of uranium-and- 

 thorium-bearing minerals with pegmatites, which, in turn, are 

 genetically related to granites and syenites of an alkaline 

 character. Occurrences in Norway, Finland, Greenland, Con- 

 necticut, and Ceylon sufficiently illustrate the statement. 



The problem now arises whether there is any common 

 factor in the conditions governing the formation ot pegmatites 

 and alkaline rocks, which is also capable of extracting uranium 

 and concentrating it. The writer believes that such a common 

 factor may be found in the so-called " mineralising agents." 

 Among the minerals which are unable to crystallise except 

 under the influence of magmatic gases and vapours {e.g. water, 

 chlorine, fluorine, boron, sulphur oxides, etc.), are quartz, albite, 

 orthoclase, sodalite, haiiyne, amphiboles, micas, tourmaline, 

 topaz, zircon, sphene, beryl, and cassiterite. It is significant 

 that these are among the most characteristic minerals of pegma- 

 tites and alkaline rocks, and of acid rocks in general. Further, 

 we may observe, that among the "mineralising agents" are 

 just those gases which would combine with uranium and 

 thorium to form volatile and mobile compounds. It is therefore 

 suggested that, in a normal magma, selective differentiation 

 proceeds in such a way that the radioactive parent elements 

 are concentrated in those subsidiary portions of the magma 

 which ultimately give rise to pegmatites or to alkaline rocks, 

 the process of differentiation being largely controlled by 

 magmatic gases and vapours. For the same reason, the gaseous 

 emanations of active volcanoes may be the agents originally 

 responsible for the relatively higher percentages of soda and 

 silica carried by the lavas from which they escape. 



How far similar principles may help to explain the origin of 



