74 ILLINOIS ACADEMY OF SCIENCE. 



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. 



Reciprocally, radioactivity greatly eases the burden laid on 

 compression in the outer part of the earth where it is least compe- 

 tent and where resort was had to igneous intrusions from below 

 to give the crust its observed temperatures. With the addition of 

 the new thermal agency the intrusions are presumed to play much 

 the same part as before, but more actively, as they must now be 

 supposed to meet the liquefying effects both of compression and 

 of radioactivity. If there was ground before to question the 

 efficiency of compressional heat, aided by such other sources as 

 were formerly assignable, to give rise to the high degree of igneous 

 activity that marked the Archean ages and to sustain the lesser 

 igneous action of later periods down to the present, this doubt is 

 amply resolved by the combined efficiency of compression and 

 radioactivity. In any case it is certain that a large amount of 

 energy has been brought to the surface and radiated into space. 



Radioactivity also comes to the aid of other agencies of extru- 

 sion in the peculiar service it renders in opening a path for the 

 outward movement of the liquid matter. In the liquefying 

 process, as we have seen, the radioactive particles should have 

 been gathered by their self-heating action into the liquid vesicles 

 and have been forced outward with them. The self-heating prop- 

 erty thus became an endowment of the liquid and gave to it 

 thermal efficiency in dissolving and fluxing its way. This efficiency 

 was continually renewed by the progressive disintegration of the 



