678 T. C. CHAMBERLIN 



their temperatures of mutual solution or of fusion. But it must 

 be observed that even if such a casual distribution prevailed in 

 the rock-matter when first it took the liquid form, this distribution 

 might not persist indefinitely, for selective segregation has appar- 

 ently taken place during the later processes. It is quite clear 

 that the radioactivity is concentrated in some constituents rather 

 than others, as for example in zircon, pyromorphite, apatite, and 

 some other minerals, and in pegmatite and some other 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 segregated rock- 

 material may rationally be expected to differ from one another 

 and from the parent magma in radioactive content. The deter- 

 minations thus far made, though not adequate to demonstrate 

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

 fore 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 material 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 



