Pg. 24. Tule Mountain Trachyandesite [Member] of Chisos Formation. Trachyte lava flow sample from Smoky Creek quadrangle, Brewster County, southwestern Texas, yielded a K-Ar age of 29.3 +/-1.5 Ma. Age is too young, based on comparison with extensively dated overlying units, including South Rim Formation (33 Ma) and Mitchell Mesa Rhyolite (32.3 Ma). Probable age is about 34 Ma. Data from R.A. Maxwell and others, 1967 (Univ. Texas-Austin Pub. 6711, 320 p.); age 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).]

Source: Publication.