1922] on The Age of the Earth 501 



The complete tale is not yet told, but I think the balance of 

 probability is in favour of an age between 150 and 200 millions of 

 years for the earliest advent of geological conditions upon the globe. 



Astronomical investigation on the subject of the age of the 

 earth deals, generally, with that greater age which must be ascribed 

 to the earth as a planet. For this age vast periods have been 

 claimed. But it is possible to reconcile superior ages for the earth 

 as a planet with comparatively brief geological time. And, to my 

 mind, in doing so we proceed upon what is no more than a 

 necessary deduction based on our knowledge of the radio-activity 

 of terrestrial materials. I would go further — still, as I believe, 

 logically — and ascribe to radio-active energy an influence on 

 planetary and stellar evolution much greater than has hitherto been 

 admitted. 



The only planet we can investigate at all closely is, of course, our 

 earth. And what do we find ? In its surface materials there are 

 sufficient of the radio-active elements, as Lord Rayleigh first showed, 

 to account for the observed average temperature gradient if the 

 surface conditions extend a little way, about 19 kilometres, inwards. 

 It is, for many reasons, in the highest degree improbable that such 

 a definitely defined radio-active layer exists. Nor is it probable that 

 the earth's interior is free from radio-active substances. We find 

 both uranium and thorium in meteorites containing a large per- 

 centage of iron and nickel, and, although they have not ;is yet been 

 found in meteoric iron, we know from the mean density of the earth 

 that its interior cannot be composed of pure iron. It is probable 

 that a considerable proportion (some 40 per cent.) of siliceous 

 materials are intermingled ; and when such exists in meteorites 

 invariably we find the radio-active elements. By what conceivable 

 activity was all the uranium and thorium separated out and brought 

 to the surface ? 



The view that radio-active elements exist in the earth's interior 

 is sometimes met by a formal denial that the earth can be getting 

 hotter within. Upon what evidence is this denial based ? If the 

 central core of the earth for a radial distance of 2000 kilometres, 

 say, had risen in temperature by 1000° 0. over geological time — and 

 upon a low assumption of the interior radio-activity it might reach 

 this temperature in 150 million years — would we be aware of the 

 fact ? Would the day be appreciably lengthened ? Would there be 

 any effect at all if the outer parts were cooling due to loss of primal 

 heat ? We have further to consider that only over the short period 

 of historical time would any observations be available. The denial 

 is quite baseless so far as my estimates go. 



Well, then, if our earth is heating up within, is there not an 

 impending termination to our geological age ? Kelvin showed how 

 complete is the thermal isolation of the earth's interior, and it is 

 certain that interior heat is not now escaping. The rise of tempera- 



