Abstract
The Leona rhyolite crops out discontinuously for 21 miles along the front of the hills from Berkeley to a point 4 miles southeast of Hayward, Calif. Long ago, A. C. Lawson and C. W. Clark agreed that it represents a single extensive lava flow of late Tertiary, probably Pliocene age. Because so many large rhyolite extrusive rocks are being interpreted nowadays as welded tuffs, re-examination of the Leona is timely.
In its southern half, recently studied, the rhyolite belt exhibits continuous groundmass fabric with unfragmented phenocrysts and practically lacks pyroclastic debris, suggesting origin as lava. However, downward extension of several broad hill cappings into narrow feeder-like masses, the presence of rhyolite dikes, and other features suggest extrusion as a series of domes and short flows rising more or less simultaneously along subsidiary shears in a major fault zone rather than as a single flow.
Commonly, the rock is a weathered mixture of iron oxide-stained quartz and argillized feldspar. Where fresh, it typically contains a few corroded phenocrysts of sodic plagioclase and quartz in spherulitic glass or in a felted microcrystalline groundmass of quartz and albite. A little chlorite and altered biotite are the only ferromagnesian minerals. Pyrite crystals, many of them rounded but not altered chemically, are abundant in fresh rhyolite but extremely rare in the underlying rocks; the pyrite appears to be primary.
In previous descriptions, orthoclase and oligoclase have been identified as major components, and albite has not been reported. Despite the modified mineralogy, the name soda rhyolite, originally assigned by Lawson, still seems appropriate.
The formation is little deformed and thus apparently is younger than the intense regional post-Pliocene orogeny; it is overlain by Pleistocene sedimentary rocks. This and other indirect evidence favor emplacement of the rhyolite rather early in the Pleistocene. Since then, it has played an important role in the geomorphic development of the east side of San Francisco Bay.
