The Isotopic Constitution of Radiogenic Leads and the Measurement of Geological Time. II

The isotopic constitution of 21 samples of radiogenic lead has been determined with a mass spectrometer and the results have been correlated with mineral analysis data available for the specimens from which the leads were extracted. The common lead contamination in the samples was found, on the whole, to agree closely with that estimated from earlier atomic weight determinations. The AcD/RaG ratios (${\mathrm{Pb}}^{207}$/${\mathrm{Pb}}^{206}$ ratios corrected for common lead contamination) appear to be in better agreement with a value 4.6 percent for the present day ratio of the activity of the actinium series to that of the uranium series than with the directly determined and commonly accepted value 4.0 percent. The results indicate that AcU was not nearly as abundant in the early days of the earth as was at one time believed.