- Usage in publication:
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- Alum Rock rhyolite
- Modifications:
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- Original reference
- Mapped
- Dominant lithology:
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- Rhyolite
- AAPG geologic province:
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- Northern Coast Range province
Summary:
Pg. 47-48, pls. 1, 3. Alum Rock rhyolite. Pale-colored aphanitic rock; strongly jointed; more or less silicified. Fresh surfaces range in color from nearly white to brownish red or purple. Intruded into Cretaceous Oakland conglomerate and Berryessa formation (new). [Age is] known only to be post-Cretaceous, but by analogy with Leona rhyolite of the Oakland, it is more likely post-Orinda and possibly Quaternary.
Type area: San Jose-Mount Hamilton area in Coast Ranges about 50 mi southeast of San Francisco, west-central CA. Mapped area is on west slope of Diablo Range. Rhyolite is intrusive body about 1000 feet wide and 400 feet long which crops out in lower Alum Rock Canyon, [Calaveras Reservoir 7.5-min quadrangle, Santa Clara Co., western CA].
Source: US geologic names lexicon (USGS Bull. 1200, p. 70).
For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.
Asterisk (*) indicates published by U.S. Geological Survey authors.
"No current usage" (†) implies that a name has been abandoned or has fallen into disuse. Former usage and, if known, replacement name given in parentheses ( ).
Slash (/) indicates name conflicts with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). May be explained within brackets ([ ]).