BEARINGS OF RADIOACTIVITY ON GEOLOGY 693 



will perhaps suffice to point out the line of co-operation of the new 

 thermal agency with the new genetic hypothesis. The two seem 

 to co-operate happily. Jointly they seem to furnish a promising 

 basis for a revised thermal geology in harmony with accumulating 

 geologic data in various lines and with the growing evidence of the 

 elastic rigidity of the earth-body as a whole. At least the con- 

 centration of the radioactive substances at the surface seems to 

 be aptly explained, and the mechanism that conserves the solidity 

 of the earth falls into consonance with the new experimental evi- 

 dence of an elastico-rigid body-tide which seems scarcely less than 

 decisive. 



There is perhaps one further point, among the many remaining, 

 that should be briefly touched here lest there seem to be an out- 

 standing incongruity in the present distribution of vulcanism. If 

 there is a progressive supply of heat in the earth's crust springing 

 from radioactivity and if it is this that actuates vulcanism, why 

 are not volcanoes more uniformly distributed over the face of the 

 globe ? A general sub-uniform distribution is a natural deduction 

 from the postulates. The distribution of pits on the moon, assum- 

 ing that they are volcanic craters, fairly fits the picture that nor- 

 mally arises from the action of such an agency. Especially is this 

 true if vulcanism is effected in so selective and so individual a way 

 as we have indicated. Why has not such a distribution persisted 

 on the earth? It will perhaps be conceded that the prevalence 

 of vulcanism in Archaean times fairly satisfies the terms of the case. 

 But at present volcanoes are rare in the primitive shields that form 

 the nuclei of the continents while volcanoes are concentrated about 

 the borders of the continents and in the deep basins and are par- 

 ticularly abundant where the great segments of the crust join one 

 another. The primitive shields are indeed intimately scarred and 

 shotted with igneous intrusions of the early ages, but they are 

 almost immune now. 



There seem to be two lines of plausible explanation. These 

 old embossments have suffered denudation from an early date 

 and the matter removed has been carried to the borders of the 

 adjacent basins. According to the hypothesis of concentration 

 at the surface, this lost matter carried a relatively high proportion 



