The Eureka Valley Tuff consists of two major ash-flow sheets and a local over-lying sequence of ash-flow tuff erupted from vents within the Little Walker caldera 11 mi west-northwest of Bridgeport, in east-central California. The lower of the two major ash-flow sheets, here named the Tollhouse Flat Member, is the “biotite-augite-latite” of Ransome (1898). The overlying By-Day Member can readily be identified by the absence of phenocrystic biotite and by paleomagnetic and other petrographic criteria. The recognition of the distinctive By-Day Member above the Tollhouse Flat Member both in the Bridgeport area and west of the Sierra crest unequivocally demonstrates the generally accepted correlation of the latitic ash-flow tuffs of the two areas. K-Ar age determinations indicate that the three members of the Eureka Valley Tuff were erupted within a very short interval of time about 9.5 m.y. ago.

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