BEARINGS OF RADIOACTIVITY ON GEOLOGY 691 



by the crossing of the planetesimal orbits. No original segrega- 

 tion of this class of matter more than of any other heavy 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 necessarily 

 expended first upon themselves and next upon adjacent matter, 

 and, other things being equal, this homemade heat should have 

 given these parts precedence in passing into the mobile state. 

 Normally the mixed units that inclosed 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 every 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 

 growth of the earth a more complete concentration of the self- 

 heating matter should have followed, for additional weighting by 

 accretion had essentially ceased and compression had become 

 essentially static while the self-heating competency of the radio- 

 active matter, though no doubt somewhat reduced by consump- 

 tion, was probably more efficient relatively in the production of 

 heat than it had been during the more active stages of growth. 



It seems clear, therefore, that at all times after the volcanic 

 process was well under way radioactivity should have been rela- 

 tively most active in the outer part of the earth and should have 

 become especially so in the latest stages of the earth. It is there- 

 fore not too much, perhaps, to claim that a specific basis in favor- 

 able conditions and a definite working mechanism for an effective 

 concentration of self-liquefying matter at the surface was postu- 

 lated in a singularly apt way before radioactivity was discovered, 

 and quite irrespective of the dilemma which its discovery has 

 involved. 



