May 24, 191;] 



NATURE 



245 



point, but 1 understand Prof. Stefan Meyer may be 

 making some examination of the radiations of the 

 material, and the resuhs he obtains will there fcM"e- be 

 of very great value in deciding this point. 



Frederick Soddy. 

 Aberdeen, May 14. 



Prof. Soddy having given me the privilege of read- 

 ing his letter in advance, I should like to take the 

 opportunity of directing attention to the geological age 

 of the thorium minerals of Ceylon, and to a few further 

 statistics bearing on the suggestion that only 35 per 

 cent, of thorium produces a stable isotope of lead. 

 I am indebted to my friend, Mr. E. J. Wayland, late 

 assistant mineral surveyor of Ceylon, for the follow- 

 ing provisional classification (in order of age) of the 

 older rocks of the island : — 



(6) Newer pegmatites and quartz reefs. 



(5) Welipatanwila series of sediments. 



(4) Pyroxenites. 



(3) Hornblende, zircon, and other f>egmatites of the 

 Balangoda series (source of thorite and thorianite). 

 Galle group, and crystalline limestones (?) 



(2) Charnockite series (pyroxene granulites). 



(i) Older rocks of the basal complex, including 

 gneisses, with monazite and zircon, pegmatitic secre- 

 tions, and dioritic intrusions. 



This classification clears up much of the mystery 

 in which the thorium minerals of Ceylon have hitherto 

 been shrouded. It was thought at first that they be- 

 longed to two distinct periods (Nos. (i) and (,'?) in the 

 above list), and as the figures in the table given below 

 clearly indicate, the belief was curiously supported by 

 the Pb L' ratios. However, it is now fairly estab- 

 lished that thorite and thorianite do not occur in the 

 older rocks of the basal complex, and therefore the 

 higher lead-ratios are misleading for age measure- 

 ment. The only lead-ratios of any value for this pur- 

 pose are those of uraninite and zircon, the former 

 being of the same age as the thorium minerals, while 

 the latter belongs to the pre-Charnockite zirconiferous 

 rocks, and is therefc«-e ven,' much older. 



In the following table I have recalculated the lead- 

 ratios on the assumption that 35 per cent, of thorium 

 gives lead as a stable end product ; cm", in other words, 

 that, weight for weight, thorium produces in any given 

 time only one-seventh as much lead as uranium. 



(For references see Proc. Geol. Assoc., vol. xxvi., 

 p. 301, 1915.) 



The recalculated ratios approach , that of uraninite 

 much more closely than do the simple Pb'U ratios, 

 and thus they support Prof. Soddv's suggestion better 

 than might have been expected from analyses of 

 thorium minerals. 



NO. 2482, VOL. 99] 



I have elsewhere pointed out the unsuitability of 

 thorium minerals for age determination or correlation, 

 . and this is particularly so in the case of mmerals from 

 the Palaeozoic igneous rocks of Langesundfjord, Nor- 

 way. Mr. Lawson and myself based our former con- 

 clusion that lead could not be the end product of 

 thorium largely on analyses of these minerals. How- 

 ever, I have now recalculated the ratios on the assump- 

 tion that thorium has one-seventh th€ lead-producing 

 power of uranium, and it is satisfactor\' to find that, 

 when thorium is less than five times as abundant as 

 uranium, the ratios agree as closely on this calculation 

 as do the simple lead-ratios. When thorium is more 

 than five times as abundant as uranium neither set 

 of ratios gives any approach to agreement, although 

 the minerals from anv one locality agree among them- 

 selves. (For the analyses referred to, see Phil. Mag., 

 vol. xxviii., p. 832, 1914; and Proc. Geol. .Assoc., 

 vol. xxvi., p. 302, 1915). Thus, having found from 

 experience that the pernicious and irregular behaviour 

 of thorium minerals is apt to be very misleading, I 

 must admit that their evidence is worthless in the 

 absence of atomic weight determinations. 



An atomic weight determination bv Richards and 

 Lembert on lead from Ceylon thorianite mav be used 

 (in the same way -as Prof. Soddv has used his own 

 and Hiinigschmid's measurements) to test the question 

 of end product. The thorianite referred to contained 

 60 per cent. Th and 20 per cent. U, and therefore if 

 the whole of the thorium disintegrated into lead, the 

 atomic weight should be 207-32 ; whereas if only 35 per 

 cent, of the thorium formed lead, then the atomic 

 weight should be 206-73. Remembering that the pre- 

 sence of original lead is implied by the high lead-ratios 

 of the above table, and that such exogenous lead 

 would raise slightly the latter figure, one finds with 

 pleasure that the atomic weight actually found was 

 206-82. Prof. Soddv's suggestion thus affords a happv 

 compromise as to the end products of thorium; there 

 is already accumulative evidence in its favour, and 

 as yet there is none against it. In particular it is satis- 

 factory to observe that if the suggestion should receive 

 decisive demonstration, then the estimates of geological 

 time already based on lead-ratios are not appreciablv 

 affected. .Arthur Holmes. 



Imperial College of Science and Technology, S.W.7, 

 May 16. 



THE SUSPEXDED PUBLICATION OF THE 

 "KEW BULLETIN." 



WE learn with astonishment that it appears to 

 have been decided to suspend the publica- 

 tion of the Kew Bulletin. We say "appears," 

 because it seems almost incredible to anyone with 

 a sense of proportion of the issues involved that 

 such an unfortunate step can really be seriously 

 contemplated. It is, however, announced that 

 the Controller of H.M. Stationer}- Office (the 

 publisher) has been instructed to form a priority 

 list of printed books and to defer the publication 

 of everything- which is not essential, that it has 

 been ruled that the Ke^jo Bulletin is not essential, 

 and that its publication has therefore been sus- 

 pended. It would be of interest to know what 

 steps were taken to enable a considered judgment 

 to be arrived at as regards this useful publication, 

 and to what extent those responsible for its dis- 

 continuance are com|jetent to form a just opinion 

 on the merits of what is mainly a teciinical 

 journal. 



The objects sensed by the Keiv Bulletin since 



