Pg. 12. Aquilla Creek Formation. Sample from trachyte lava flow, in Blue Mountain quadrangle, Jeff Davis County, southwestern Texas, yielded a K-Ar age of 37.6 +/-0.8 Ma (alkali feldspar). Data from J.E. Anderson, 1968 (Univ. Texas-Austin Bur. Econ. Geol., Geol. Quad. Map No. 36), and D.F. Parker and F.W. McDowell, 1979 (GSA Bull., v. 90, p. 1100-1110); age recalculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362). [Age considered Eocene. 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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