Pg. 13. Fairbury Trachyte. Ash-flow tuff, possibly erupted from speculative caldera source in southern Wylie Mountains. Relation to Davis Mountains stratigraphy is unknown. Trachyte sample from Chispa Mountain quadrangle, Jeff Davis and Culberson Counties, southwestern Texas, yielded a K-Ar age of 37.8 +/-0.8 Ma (alkali feldspar). Data from Hugh Hay-Roe, 1957 (Univ. Texas-Austin Bur. Econ. Geol., Geol. Quad. Map No. 21), and C.D. Henry and J.G. Price, 1984 (Jour. Geophysical Res., v. 89, p. 8765-8786); 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).]

Source: Publication.