SYMPOSIUM OX RADIOACTIVITY. 73 



high viscosity for the small mobile masses naturally arose from 

 the dose balance between the liquid and solid states. Such a con- 

 dition seems equally to be implied by the remarkable mixtures of 

 dense and light matter often seen in the igneous rocks.^* 



The matter forced early to the surface is held to have been 

 buried by later accretions to the growing planet, to have later 

 been subject to a second liquefaction and extrusion, a second 

 burial and so on. Progressive selection and reselection are postu- 

 lated until the growth essentially ceased. Since then a more 

 complete selection and concentration of the eutectic material at 

 the surface has been in progress as far as further generation of 

 internal heat has furnished the actuating agency. 



Now if this picture in its working details and in its rather 

 sharp antithesis to the older view is clearly in mind, the part 

 which the radioactive substances may be supposed to play in 

 co-operation with this mechanism without changing the general 

 conception is little less than self-evident. The radioactive particles 

 are sources of self-generated heat. Under the planetesimal view 

 the radioactive substances were promiscuously scattered through 

 the mixed mass as it was gathered in heterogeneously from the 

 nebula by the crossing of the planetesimal orbits. Xo original 

 segregation of this class of matter more than of any other hea\'y 

 material is assignable. The relative amount of the radioactive 

 matter, at least of the classes now known to be radioactive, must 

 have been extremely small and its influence on the specific gravity 

 of the matter with which it was mixed must have been negligible. 

 The self-heating effects of these disseminated particles were neces- 

 sarily expended first upon themselves and next upon adjacent 

 matter and, other things being equal, this home-made heat should 

 have given these parts precedence in passing into the mobile state. 

 Normally the mixed units that enclosed a radioactive particle 

 should have been as susceptible of partially passing into the liquid 

 state as similar units that were free from radioactive matter. The 

 special source of heat should have turned the balance in favor 

 of the unit immediately surrounding the radioactive particle. 

 Thus the radioactive matter normally became involved in the 

 mobile matter and passed with it to or toward the surface. 



With ever}- stage in the growth of the earth and with every 

 reburial of the radioactive material, a second similar preferential 

 action should have followed. On the essential completion of the 



"Ctamberlin and Salisbury's Geology, Vol. II, pp. 121-2. 



