Pg. 137 (fig. 1), 138 (fig. 2), 140-141. Bobtail quartz latite member of Gem Hill formation of Tropico Group; all new. Felsitic to porphyritic rock predominantly cream white in color, but locally tan, pink, brown, or pale green. Locally contains chilled marginal zones of gray perlite. Latite makes up several large volcanic plugs, a number of small plugs, pods, and dikes intrusive through pre-Tertiary granitic rocks into the Gem Hill; also occurs as several short lentils of flow breccia in the formation. Older than Fiss fanglomerate (new) of Tropico Group which it does not intrude. "Age of most if not all of Bobtail quartz latite member must be the same as that of Gem Hill formation which it intrudes and in which it occurs as extrusive masses, or probably middle Miocene." p. 141
Type locality: Soledad Mountain, Rosamond quadrangle, in secs. 6 and 7, T. 10 N., R. 12 W., San Bernardino Base and Meridian, Kern Co., south-central CA. Named from Bobtail Mines on west slope of Soledad Mountain. Many scattered buttes within 10 mi of Soledad Mountain are remnants of volcanic plugs made up of Bobtail quartz latite.

Source: US geologic names lexicon (USGS Bull. 1200, p. 409-410); GNU records (USGS DDS-6; Menlo GNULEX).

Four facies of Bobtail Quartz Latite Member of Gem Hill Formation recognized and mapped as follows: porphyry, felsite and porphyritic felsite, and perlite (all of intrusive origin), and felsite breccia (of probable extrusive origin). Map legend shows Gem Hill Formation as Oligocene(?) to middle Miocene(?) age. [not clear if this age applies to member]

Source: GNU records (USGS DDS-6; Menlo GNULEX).