BEARINGS OF RADIOACTIVITY ON GEOLOGY 679 



solution of the lime from the original matter of the abysmal deposits, 

 leaving them residual concentrates, and in part to the collection 

 in the depths, in relatively high proportions, of phosphate-bearing 

 relics (teeth, bones, etc.) with which radioactive substances are 

 associated. It is a suggestive fact that the phosphatic nodules 

 of the great deeps are highly radioactive compared with ordinary 

 sedimentary material. A part of this is clearly due to the con- 

 centration of the radioactive substances after the phosphates were 

 deposited, for fresh phosphatic material is notably less radioactive 

 than fossilized phosphates. 1 



It appears then that the radioactive substances on the surface 

 of the earth are subject to special agencies that lead in part to 

 greater concentration and in part to wider distribution, and that 

 these act co-ordinately with the general dispersing agencies that 

 give radioactivity to the 'derivative rocks, to the waters, and to 

 the air. 



If it were permissible to reason from what is known of surface 

 phenomena, particularly from the broad fact that radioactivity 

 increases as we go from air to water, from water to sediment, and 

 from sediment to igneous rock, it might be inferred very plausibly 

 that radioactivity would be found to reach its maximum concen- 

 tration in the heart of the earth, and certainly that the deeper 

 parts would be as rich as the superficial ones. This presumption 

 might very justly be felt to be strengthened by the fact that the 

 atoms of uranium, radium, and thorium are among the heaviest 

 known and that if the earth were ever gaseous or liquid, these 

 heavy atoms might naturally be expected to be concentrated 

 toward its center unless the viscosity of the fluid mass were too 

 great to permit this, in which case the distribution should be either 

 equable or indifferent to depth. 



But Strutt 2 early called attention to the fact that if such an 

 increasing abundance exists toward the center of the earth, or 

 if there were an equable distribution in depth, the heat gradient 

 as the earth is penetrated would be higher than observation 

 shows it to be. By computations on the data then available he 



1 Strutt, Proc. Roy. Soc, LXXX A, 582. 



2 Proc. Roy. Soc, LXXVII A (1906), 472; LXXVIII A, 150. 



