Potassium-argon dates from a superpositional succession of igneous and sedimentary rocks of the Vieja, Garren, and Buck Hill Groups are critically evaluated. Five new faunas of vertebrate fossils collected from the interbedded tuffs are named, and tentative faunal lists are given. Misidentification of fossil vertebrates in an earlier collection are corrected. Abandonment of a previously published K-Ar date of 33.1 m.y. is urged because of lack of stratigraphic and geographic data. An Eocene-Oligocene boundary based upon vertebrates seems to fall within the Duchesnean Age.
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Wilson et al. (1968) studied this question.
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