URANIUM AND GEOLOGY JOLY. 359 



clients, then these radio-active nuclei heating and expanding beyond 

 the capacity of surrounding materials would rise to the surface of a 

 world in which convective actions were still possible, and, very con- 

 ceivably, even after such conditions had ceased to be general; and in 

 this way the surface materials would become richer than the interior. 

 For instance, the extruded mass of the Deccan basalt would fill a 

 sphere 36 miles in radius. Imagine such a sphere located originally 

 somewhere deep beneath the surface of the earth surrounded by mate- 

 rials of like density. The ultimate excess of temperature, due to its 

 uranium, attained at the central parts would amount to about 1,000° 

 C, or such lesser temperature as convective effects within the mass 

 would permit. This might take some thirty million years to come 

 about, but before so great an excess of temperature was reached the 

 force of buoyancy developed in virtue of its thermal expansion must 

 inevitabh^ bring the entire mass to the surface. This reasoning would, 

 at any rate, apply to material situated at a considerable distance in- 

 ward, and may possibly be connected with vulcanicity and other 

 crustal disturbances observed at the surface.*^ The other view, that 

 the addition of uranium to the earth was mainly an event subsequent 

 to its formation in bulk, so that radio-active substances were added 

 from without and, possibly, from a solar or cosmic source, has not the 

 same a priori probability in its favor.^ 



I have in this part of my address briefly to place before you an 

 account of my experiments on the amounts of radium distributed in 

 surface materials. Here, indeed, direct knowledge is not attainable; 

 but this knowledge takes us but a very few miles inward toward the 

 center of the earth. 



The igneous roclxs. 



The basalt of the Deccan, to which I have referred, known to cover 

 some 200,000 square miles to a depth of from 4,000 to G,000 feet or 

 more, appears to be radio-active throughout. A fine series of tunnel 

 and surface specimens sent to me by the Director of the Indian Geo- 

 logical Survey has enabled me to examine the radio-activity at various 

 points. It is remarkable that the mean result does not depart much 

 from that afforded by a long series of experiments on north of Ire- 

 land basalt and on the basalt of Greenland. 



Again, the granites and syenites — and those of Mourne, Aberdeen, 

 Leinster, Plauen, Finsteraarhorn have been examined — while variable, 

 yet approximate to the same mean result. 



In the Simplon and St. Gothard tunnels igneous rocks have been 

 penetrated at considerable depth beneath the surface. The greatest 

 true depth is attained, I think, in the central St. Gothard massif. 



" See Appendix A to this article, 

 6 Nature, Vol. LXXV, p, 294. 

 88292— SAi 1908 24 



