Study of uraniferous silica precipitates in the Shirley Basin, Wyoming, identified areas where ancient uraniferous ground water once ponded. Chalcedony collected from and directly beneath thick accumulations of rhyolite ash contain as much as 250 ppm uranium in a pre-ash topographic low and lesser concentrations (10 to 160 ppm) elsewhere. Differences in the U concentration of chalcedony collected from approximately the same stratigraphic horizon reflect the enrichment of uranium in ground water as it percolated downward and basinward through the overlying rhyolite ash. Uranium is homogeneously distributed as a uranyl species within the chalcedony and reflects coprecipitation of dissolved uranium and colloidal silica in a uraniferous silica-gel. Laboratory measurements of the partitioning of uranium between various solutions and silica-gel precipitates indicate that, for ranges of pH and dissolved carbonate typical of ground water, dried silica-gel contains about 400 to 1,000 times the uranium concentration of the solution from which it forms. Uranium is postulated to be incorporated as an adsorbed uranyl-silica-hydroxyl complex. A 20-m.y.-minimum apparent age for uraniferous chalcedony was obtained by U-Pb isotope dating. Reported minimum ages for nearby sedimentary uranium deposits generally lie between this age and the age of rhyolite which hosts the silica (32.4 + or - 2.6 m.y.). Leaching of uranium from ash during the period 20 to 32 m.y. is therefore compatible with a volcanic source-rock hypothesis.