A number of selected uranium ore specimens from several localities have been analyzed for uranium and lead in both pitchblende and galena phases by isotope dilution techniques and the lead isotopic abundance determined. It is shown that the hypothesis of hydrothermal deposition of uranium accompanied by old radiogenic lead from the basement at one time about 60 m.y. ago does not satisfy the isotopic data. A new hypothesis is presented which requires local sources with high U/Th and U/Pb ratios, variable radon leakage, suitable ground water movement, and deposition at the site of H 2 S production at low temperature. This hypothesis can explain the age discordances and the lead isotope abundances in galena. It is possible from the isotopic data to have all deposition occurring within the last five million years but it does not preclude other periods of deposition such as in Laramide time. The isotopic ages are apparent ages only and bear no direct relation to the time of deposition. The isotopic ratios, however, provide information which may be used to restrict theories of origin.

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