BEARINGS OF RADIOACTIVITY ON GEOLOGY 68 1 



the surface might naturally have taken place, or must we find a 

 new geological process to fit the new thermal difficulty ? 



The rigor of the dilemma is softened somewhat by noting that 

 the deductions of Strutt, Joly, and their colleagues are based 

 simply on comparisons between the heat-generating power of 

 radioactive substances in the crust and the conductive power of 

 the crust. The functions of igneous extrusion as a mode of trans- 

 fer of internal heat do not seem to be taken into account. This 

 is not unnatural since the heat carried out by extrusive matter 

 and by waters heated by igneous intrusions has not usually been 

 regarded as an important factor in reducing the high temperature 

 inherited by the earth under the older view. But the movement 

 of igneous matter and of waters and gases heated by it has 

 been made to play an essential part in the working concepts that 

 have been based on the planetesimal hypothesis. There will be 

 occasion to return to this critical difference of view. 



When the apparent excess of thermal riches arising from the 

 new source was first realized an escape from the dilemma raised 

 by it was sought in the natural supposition that the disintegration 

 of uranium and thorium was restrained by pressure in the depths 

 of the earth, and that, though present there, their activity was 

 greatly subdued or possibly inhibited altogether. This plausible 

 explanation was diligently tested; but the general tenor of experi- 

 ments on the effects of pressure, notably those of Eve and Adams 1 

 in which the pressures were carried to intensities sufficient to cover 

 earth-pressures to the depths supposed to limit radioactivity and 

 beyond, showed no appreciable restraint on the disintegrating 

 process. It seems necessary, therefore, in the present state of 

 evidence, to accept the inference that the radioactive substances 

 are really concentrated toward the surface, and that the radio- 

 active content in the depths of the earth is of a much lower order. 



It does not fall to me to adjust the new requirements to the 

 older view of the earth's internal temperatures based on a molten 

 earth, for other considerations led me to the abandonment of this 

 view before the advent of the new issue. I must leave it to those 

 who hold to the molten hypothesis to battle with its new perils. 

 I Nature ) July, 1907, p. 269. 



