The Rattlesnake Hills of central Wyoming contain approximately 40 volcanic vents and intrusive bodies of alkalic and calc-alkalic composition in a 125-sq-mi area. Two samples, a phonolite and a rhyodacite (quartz latite), dated by the K-Ar method, yielded dates of 43.6 ± 1.0 and 44.0 ± 2.6 m.y., respectively. These dates, combined with stratigraphic evidence of Van Houten (1955) and Carey (1959), indicate a minimum time span of 5 m.y. for the Rattlesnake Hills volcanism.

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