26 PRESIDENTIAL ADDRESS SECTION I',. 



been in operation in the case of the radio-active elements as has 

 also been responsible for the selective concentration of the 

 various minerals so characteristic of the final activities of an 

 igneous magma, and frequently characteristic of pegmatites, such 

 as tourmaline, topaz, zircon, beryl, cassiterite, etc. It is con- 

 sidered by Holmes in this connection that the action of magmatic 

 gases and vapours has largely controlled the process of differen- 

 tiation, which has resulted in the extraction of the radio-active 

 elements and their concentration in the subsidiary portions of 

 the magma, such as the pegmatite veins. 



Chamberlin considers that since in the radio-active elements 

 we have a thermal agency of very high efficiency, it is probable 

 that, in virtue of the heat-producing activities of these elements, 

 arising from continuous atomic disintegration, they have had 

 a decided influence upon the production of igneous intrusion and 

 extrusion. In his opinion, they thus assist by their action 

 other agencies such as compression, not only in the liquefaction 

 of rock matter, but also in facilitating a passage for it towards 

 the surface. In short, the peculiar activities of the radio-active 

 elements have been one of the principal agents in effecting their 

 concentration towards the surface of the earth. 



Leaving out of consideration, however, the actual methods 

 of distribution and concentration, let us return to the relation 

 between radio-activity and the earth's internal temperature. We 

 have seen that radium is widely distributed over the more super- 

 ficial portions of the earth. If, then, radio-active matter was 

 distributed in a similar proportion throughout the entire mass 

 of the earth, from the surface to the centre, it has been estimated 

 that the observed temperature gradient as the eartli is penetrated, 

 would be very much higher than it is ; in fact, the earth would 

 never ha\e become fit for habitation, and would actual! v be 

 growing hotter ! But as this is not the case, we can only con- 

 clude either that radio-active elements are absent from the more 

 central portion of the earth, or possibly present to a cjuite in- 

 appreciable extent, or that, if there is in the earth an equable 

 distribution of radio-active matter in de]jth, then there must be 

 some agency, such as pressure, which is able to restrain its 

 activity in depth or altogether prevent its atomic disintegration. 



The results of various observations, deductions, and experi- 

 ments, however, tend very clearly to show that radio-active 

 substances can, apparently, undergo disintegration persistently 

 and uniformly under all known terrestrial conditions, and so 

 their action is probably not controlled or influenced in any way 

 by pressure or temperature. These considerations certainly 

 point to the conclusion that the radio-active elements are practi- 

 cally confined in their occurrence to the crustal portion of the 

 globe. The evidence of concentration that I have already 

 referred to tends to support this view, althouo-h the very fact 

 that concentration and diffusion has. and no doubt still is, taking 



