1922] on The Age of the Earth 495 



ancient — Lewisian or Lower Archaean. Upon reading in " Nature " 

 Prof. Soddy's account of his determination of the atomic weight of 

 the lead derived from these rocks, I estimated that the quantity of 

 lead extracted from the thorite gave an age of 130 millions of years 

 for the time since this mineral had been generated ; and on com- 

 municating with Prof. Soddy I found that he had reached a somewhat 

 similar conclusion. 



At this time, however, there was the possibility that thorium lead 

 was not altogether stable. Suspicion fell more especially on thallium 

 as the final product. Two experimental results, however, laid this 

 doubt to rest ; experiments upon a thorianite made in my laboratory 

 by J. R. Cotter failed to detect even spectroscopic traces of this 

 element, and there was insufficient thallium found in the thorite 

 dealt with by Prof. Soddy. In a subsequent letter to " Nature " 

 Prof. Soddy states that a research carried out at the Radium Insti- 

 tute of Vienna supported the view that the lead isotopes derived from 

 thorium were both stable. I shall refer presently to yet additional 

 evidence that the transformations of the thorium family cease with 

 lead. 



Writing to " Nature " in support of the hypothesis then under 

 discussion — i.e. that thorium lead was unstable — A. Holmes cited a 

 result on a selected specimen of uraninite, showing that the rocks in 

 which Soddy's thorite occurred were, according to the uranium-lead 

 ratio, 512 millions of years old. Previous uranium-lead ratios had 

 assigned a much greater age to them. Here, then, the results join 

 issue : the uranium result is just four times as great as the thorium. 

 We notice, too, that on the uranium-scale of time this thorite must 

 be older than Silurian or Ordovician, which have been determined by 

 uranium lead as 430 millions of years ago. Probably its age dates 

 back to Cambrian or even to pre-Cambrian time. From what we 

 have already inferred we cannot regard 130 millions of years for 

 early Palaeozoic time as irreconcilable with the maxima which denuda- 

 tive methods afford. More recently, lead derived from a Norwegian 

 thorite of Langesundfiord — also of lower Palaeozoic age — seems to 

 reveal an age of 150 millions of years. In this case, also, there is 

 the added security of a determination of the atomic weight of the 

 lead. 



We cannot discredit these results on the score of radio-active 

 instability of the lead. Why, then, set them aside in favour of 

 results reached on uranium lead, which are in hopeless contradiction 

 to the indications of the record of the surface activities of the 

 globe ? It is, indeed, not too much to say that the whole position 

 is now reversed, and that to-day suspicion attaches to the uranium- 

 lead ratio. And, as we shall see, there is much unknown about the 

 earlier radio-active sequence in the uranium series, while the dis- 

 covery of isotopes opens the way to possibilities unthought of in the 

 earlier days of radio-active science. 



Vol. XXIII. (No. 116) 2 n 



