PROF. AETHUB HOLMES points out that there is a serious discrepancy between the data for lead in rocks (a) as given by Prof. Hevesy and Dr. Hobble and (b) as cited by him in the letters on this subject which appeared in NATURE of Dec. 19, 1931. The latter figures (under Pb on p. 1040) refer to the preliminary results obtained by the gravimetric method, whereas those announced on p. 1039 are the final results obtained by the use of the radioactive indicator method. Both sets of figures will appear in a forth-coming detailed record of the investigation. It appears that Prof. Hevesy substituted the final results for the preliminary figures after Prof. Holmes had passed the proofs of his own letter. The effect of the corrections, however, is to emphasise the validity of the conclusions based on the original figures. It now appears that in the composite sample of granitic rocks the total lead is eight (not five) times the radioactively generated lead. Thus, if the atomic weight of the lead originally present in the earth be taken as 207.22, that of the lead now present in granitic rocks should be about 207.19. Actual determinations on ore-lead correspond very closely with these figures.