494 Professor John Joly [Feb. 24, 



surface materials during geological time. The validity of the correc- 

 tion is, however, open to doubt. Others considered that the organic 

 changes recorded in the rocks required a longer period. Sollas gave, 

 as I think, a clear answer to this objection in his " Age of the 

 Earth." Both Lyell and Geikie, and Poulton, had in past years 

 upheld the doctrine of Uniformity. But the advent of the radio- 

 active method, as founded on the uranium family of elements, 

 seemed to point to a vastly greater age ; leading, in fact, to the 

 extraordinary conclusion that the present rate of solvent denudation 

 is not less than four times, and may be eight (or even more) times, in 

 excess of the average rate obtaining during the past. 



The earliest suggestion of the possibility of using the stored-up 

 products of radio-active change came from Rutherford. He, and 

 later Strutt (now Lord Rayleigh), applied the accumulation of helium 

 to the evaluation of geological time. Strutt laid out a geological 

 chronology, the first of its kind, but considered he was dealing with 

 minor limits. Boltwood used the residual product of uranium — lead 

 — and for Archaean (?) materials reached as much as 164<) million 

 years. As I have already said, the denudative method cannot be 

 regarded as extending to those remote times. But such results as 

 480 x 10 6 years for Silurian or Ordovician deposits, and 1200 x 10 & 

 years for Post-Jatulian, are quite out of harmony with the denudative 

 method. To-day the matter stands thus : — A number of results are 

 available based upon the use of carefully selected material, and when 

 the material is thus selected the ratio of lead to uranium — the " lead 

 ratio " as it is termed — increases as we go downwards and diminishes 

 as we go upwards in the strata, preserving a fair degree of agreement 

 even for widely separated localities. 



Those who would rest content with this result, however, can do 

 so only by ignoring the very interesting and suggestive fact that 

 when we base the results on the lead ratio of selected thorium 

 minerals, we arrive at ages which are in substantial agreement with 

 the results reached by the denudative method. On the face of it 

 this agreement gives strong support to the conclusions reached by 

 methods absolutely different in nature. 



For long it was known that thorium minerals — such as thorite — 

 gave persistently lower ages than uranium minerals. It became the 

 custom with some to treat these ages as untrustworthy. But we 

 know now that this attitude is not justified, but rather that the onus 

 of explaining away the impressive agreement between the indications 

 of thorium lead and denudative statistics rests with those who would 

 reject the age supported by both. 



Soddy's determination of the atomic weight of the thorium lead 

 isotope in 1917 afforded material for an age determination on a 

 very large scale, and from the nature of the research, one of special 

 value. The material was a thorite from Ceylon ; from rocks imme- 

 diately overlying the Charnockite series. The latter is extremely 



