RELATION OF RADIOACTIVITY TO VULCANISM 749 



There are other highly suggestive features to Strutt's paper, but 

 let us consider the geological bearing of those already presented. 

 First, it seems probable that we have a hitherto unconsidered supply 

 of heat capable of maintaining for an exceedingly long period the 

 known temperature gradient within the earth, and, in fact, necessi- 

 tating the introduction of restrictions as to interior distribution in 

 order to account for the lack of a higher than the observed gradient. 

 As Strutt suggests, an assumption that the earth is getting hotter is 

 "not likely to be regarded with favor. " But that we have a condition 

 of equilibrium seems quite possible, and, if not equilibrium, there must 

 at least be a condition of very much slower cooling than the calculated 

 loss by conduction to the surface would indicate. In this latter case 

 we may still assume that the earth's heat has been derived from two 

 groups of sources: one, the initial condition and relative positions of 

 its constituent atoms, molecules, and masses, acting as outlined by 

 one of the nebular or solid accretion hypotheses; the other, radio- 

 activity or, more broadly, sub-atomic changes. This latter has prob- 

 ably varied but little for a long period of time. ^ If the condition at 

 present is one of practical equilibrium, or not far removed therefrom, 

 it means that the effects of any original high temperature (such as 

 assumed by the nebular hypotheses) are now exceedingly slight, and as, 

 if such original temperature ever existed, it must have died out with 

 a gradually decreasing rate, its effects must have been slight for a long 

 period of time. It would seem, therefore, that the utmost caution 

 should be used in any attempt to explain characteristics of the geo- 

 logical record, especially in its later part, in terms of any cosmic 

 hypothesis assuming initial high temperatures. 



The folding of zones of the earth's crust into mountain ranges or 

 mountain systems and certain other diastrophic changes, in particular 

 those giving rise to earthquakes, are very commonly referred, in whole 

 or in part, to the contraction of the earth as it cools. But the indica- 



I It is supposed on very suggestive evidence that uranium is the parent of radium. 

 Geologically considered, radium has a comparatively short life, breaking down with a 

 velocity that decreases according to an exponential law and reaching half value in 1,300 

 years. The supply of radium is apparently maintained by the transformation of ura- 

 nium, which is an exceedingly slow process, varying in velocity similarly to radium and 

 reaching half value, according to Rutherford's calculation (!oc. cit., p. 458), in about 

 600,000,000 years or more. 



