Geologic Time and Isotopic Constitution of Radiogenic Lead

The isotopic constitutions of samples of lead from Colorado carnotite, Wilberforce uraninite, and Katanga and Great Bear Lake pitchblendes were found from densitometer measurements of the hyperfine structure of $\lambda 5372$. The probable geologic ages of these minerals calculated from the Ac D: Ra G ratios agree very well with the ages from Pb: U ratios with the exception of the Great Bear Lake pitchblende in which case leaching or some other method of alteration may have changed the Pb: U ratio.