KADIOACTIVITY AND THE EAETH's AGE 135 



wegian pegmatites, reaches the conclusion that the minerals of the thorite- 

 orangite group, tacluding urano-tKorite, crystallize in his first phase of 

 vela formation, that of "magmatic consolidation with the cooperation of 

 pneumatoljtic processes." In the second phase, or the "principal phase 

 of pneumatolytic minerals," galena crystallizes out. He has observed 

 galena in so many pegmatites that he considers it superfluous to enu- 

 merate them. ]^ow, every one who has had much to do with mraerals 

 knows how impure most of them are; how prone to include much more 

 than traces of whatever foreign substances may be present. Brogger's 

 observations show that lead compounds must be soluble in the menstruum 

 from which uraniferous minerals crystallize and indicate the probability 

 that lead may be occluded in them as an impurity, the amount of which 

 may vary from crystal to crystal. 



In view of these facts, the proportion of helium in uranium minerals 

 would perhaps afford a better basis than the lead content for estimates of 

 their age. If Sir William Eamsay's determination of the life of radium 

 is correct, Mr Eutherford's helium estimate of the age of fergusonite, 

 referred to above, would reduce to 66 million years, which seems not geo- 

 logically impossible. But I find no convincing evidence that the law of 

 decay is so simple as is assumed. Under the conditions in which uranium 

 compounds are stable, A must necessarily reduce to zero. It is in the 

 highest degree improbable that A. is a discontinuous function, and it is to 

 the same degree probable that the law of decay fails like Boyle's law, or 

 that A varies with circumstances such as may have environed a mineral 

 in a pegmatite, even though heat alone or pressure alone may be without 

 effect upon radioactivity. 



On the whole, then, the surface temperature gradients, taken in con- 

 nection with the age of the earth as determined stratigraphically, or 

 from the sodium content of the ocean, or from my theory of a cooling 

 earth, do not indicate that the excess of temperature within the earth is 

 due in any large measure to radioactivity. Something like a tenth of the 

 gradient, however, may perhaps be due to this cause — a question which a 

 more discriminating study of gradients will answer, at least in part. It 

 does not seem to me that geologists can possibly accept the ages of min- 

 erals as determined from the uranium-helium or the uranium-lead ratios, 

 which do not seem consistent and are far longer than stratigraphers could 

 admit. 



Distribution of Radioactivity in Depth 



That radioactivity is almost imiversally distributed over the earth's 

 terrestrial surface has been known for some years ; indeed, as Mr Ruther- 

 ford puts it, each blade of grass must be coated with an invisible deposit 



