April 15, 1922J 



NATURE 



481 



There was, indeed, some scanty sedimentation in 

 Archaean times. We cannot form any estimate of its 

 effects either upon our numerator or upon our de- 

 nominator save that we seem entitled to conclude that 

 they were small. " The Archaean was essentially a 

 period of world-wide vulcanism, and in the relative 

 proportions of rocks of igneous and sedimentary origin 

 represents a departure from the uniformity of con- 

 tlitions of later geological time." I quote from the 

 monograph of Van Hise and Leith. 



Before passing on to the results based upon radio- 

 activity I must refer to one point in particular which 

 has been urged against accepting present-day rates of 

 denudation as a basis of time measurement. It is said 

 we live in a period of abnormal continental elevation 

 which, it is asserted, involves excessive solvent denuda- 

 tion. A little attention to the nature and conditions 

 of solvent denudation should have sufficed to forestall 

 the argument. But a ready method of dealing with it 

 is available. The continent of North America has a 

 mean elevation of 700 metres : it is being denuded at 

 : iie rate of 79 tonnes per square mile pei annum : for 

 ^outh America the corresponding figures are 650 metres 

 and 50 tonnes. Now Europe has a much lower mean 

 elevation — 300 metres. Its rate of denudation is, 

 however,' 100 tons per square mile per annum. The 

 rate of solvent denudation is, in fact, by measurement 

 found to be less for the more elevated land, as, theoretic- 

 ally, it should be. The argument then, if it has any 

 basis, would indicate that the age as found from 

 M)lvent denudation is excessive. 



Prior to the advent of those methods for investigating 

 ihe earth's age, which are based on radio-active 

 changes in the elements, no serious objections to the 

 results reached by the geological methods were raised, 

 o far as I know. There were some, indeed, who 

 regarded the age as excessive. Thus Becker arrived at 

 a lesser figure by taking into account the progressive im- 

 poverishment of the surface materials during geological 

 time. The validity of the correction is, however, open 

 to doubt. Others considered that the organic changes 

 recorded in the rocks required a longer period. Sollas 

 _cive, as I think, a clear answer to this objection in 

 lis " Age of the Earth." Both Lyell and Geikie, and 

 Poulton, had in past years upheld the doctrine of 

 I'niformity. But the advent of the radio-active 

 method, as founded on the uranium family of ele- 

 ments, 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 

 : !ie stored-up products of radio-active change came 

 irom Rutherford. He, and later Strutt (now Lord 

 Rayleigh), applied the accumulation of helium to the 

 evaluation of geological time. Strutt laid out a 

 ecological chronology, the first of its kind, but con- 

 idered he was dealing with minor limits. Boltwood 

 ised the residual product of uranium — lead — and for 

 Archx'an (?) materials reached as much as 1640 

 million years. As I have already said, the denudative 

 method cannot be regarded as extending to those 

 remote times; But such results as 430 x io« years 



NO. 2737, VOL. 109] 



for Silurian or Ordovician deposits, and 1200x10' 

 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 191 7, 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 

 immediately overlying the Charnockite series. The 

 latter is extremely 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 communicating 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 spectro- 

 scopic 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 

 Institute 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 



