Abstract
Average values have been obtained by gamma-ray spectrometry for the thorium and uranium contents of some orthoquartzites and graywackes. The Atoka and Jackfork quartz sandstones of western Arkansas average approximately 4 ppm thorium and 1 ppm uranium. The Umpqua and Tyee graywackes of western Oregon average approximately 7 ppm thorium and 2 ppm uranium. Weighted average concentrations for all sandstones are 5·5 ppm thorium and 1·7 ppm uranium. Much of the radioactivity, particularly in the graywackes, is carried by heavy minerals such as biotite and zircon. The average concentrations in sand-sized material are. therefore, lower and may be estimated as 3 ppm thorium and 1 ppm uranium. Sand-sized quartz and other minerals deposited in hydraulic equilibrium probably have average concentrations in the neighborhood of those of the Atoka and Jackfork orthoquartzitos, namely: 4 ppm thorium and 1 ppm uranium.
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