Pg. 24. Mule Ear Spring Tuff [Member] of Chisos Formation. Rhyolite samples from Castolon and Santana Mesa quadrangles, Brewster County, southwestern Texas, yielded K-Ar ages 35.0 +/-6.0 Ma (material unknown) and 33.7 +/-0.7 Ma (alkali feldspar), respectively. Rhyolite sample (=Matadero tuff of Chuchla, 1981, and Gregory, 1981) from Chihuahua, Mexico, yielded a K-Ar age of 34.1 +/-0.7 Ma (alkali feldspar). Data from R.J. Chuchla, 1981 (Univ. Texas-Austin, MS thesis, 199 p.), J.L. Gregory, 1981 (Univ. Texas-Austin, MS thesis, 78 p.), C.D. Henry and J.G. Price, 1984 (Jour. Geophysical Res., v. 89, p. 8765-8786), and R.A. Maxwell and others, 1967 (Univ. Texas-Austin Pub. 6711, 320 p.); ages recalculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362).
[Age considered Oligocene; Eocene-Oligocene boundary 36.6 (38-34) Ma (from Geologic Names Committee, USGS, 1983 ed. geol. time scale, with additions from N.J. Snelling, 1985, The Geol. Soc. Mem., no. 10).]

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