Abstract
Jarosite-bearing Tertiary sediments crop out along the west side of the San Joaquin Valley in the foothills of Diablo Range, about 10 miles south of Los Banos, California. The mineral has been observed microscopically in Paleocene siltstone of the Martinez formation, and in Eocene glauconitic and laminated sandstone and in diatomaceous shale in the lower portion of the Kreyenhagen formation. In all but the glauconitic sandstone, the grains of jarosite are so minute as to prevent positive identification by ordinary microscopic means; however, larger idiomorphic crystals intimately associated with glauconite yield grains sufficiently large to be identified with the petrographic microscope.
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