62 ILLINOIS ACADEMY OF SCIENCE. 



rocks. The pegmatitic material, in segregating from a granitic 

 magma, seems to have gathered into itself an unusual proportion 

 of the radioactive substance of the parent mass. In the details 

 of final distribution, therefore, the different parts of the segre- 

 gated rock-material may rationally be expected to differ from one 

 another and from the parent magma in radioactive content. The 

 determinations thus far made, though not adequate to demon- 

 strate this, seem to be in consonance with it. Much interest will 

 therefore gather about the forthcoming determinations as they 

 multiply and contribute their quota of evidence bearing on the 

 radioactive qualities of the various species of igneous rocks. 



Among the derivative and sedimentary processes it seems clear 

 that there are modes of concentration also which have given to 

 different sediments different contents of radioactive substances. 

 It appears from the determinations already made that the radio- 

 active substances are leached out of the parent igneous rocks 

 faster than the average minerals of those rocks, for weathered 

 igneous rocks are found to carry less radioactive matter than fresh 

 rocks. This is in accord with the aptitude for chemical change 

 already noted ; and yet soils which are almost the type of ultra- 

 weathered material still retain notable radioactivity, but a part 

 of this is probably a redeposit from the atmosphere. In general, 

 it appears that the clayey element carries more radioactive mate- 

 rial than the quartzose sands or the calcareous derivatives. 



In the deep sea deposits, radioactive matter is higher than in 

 the deposits of the shallow parts of the ocean. In the red clays 

 and radiolarian oozes of the abysmal depths the content is 

 markedly greater than in the land-girting muds and sands, or the 

 calcareous oozes of mid-depths. This is assigned in part to the 

 removal by solution of the Hme 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 radio- 

 active substances are associated. It is a suggestive fact that the 

 phosphatic nodules of the great deeps are highly radioactive com- 

 pared with ordinary sedimentary material. A part of this is 

 clearly due to the concentration of the radioactive substances 

 after the phosphates were deposited, for fresh phosphatic material 

 is notably less radioactive than fossihzed phosphates.^ 



It appears, then, that the radioactive substances on the surface 



•Strutt: Proc. Roy. Soc, 80A, p. 582. 



