Brown Mountain rhyolite porphyry (new) of Bevos group (new). Briefly summarized in reports by W.C. Hayes, p. 8 (strat. column), 81, 82, 83 (table 1), this volume. New names are credited to Tolman and Robertson (in prep.) [1969, Missouri Div. Geol. Survey Water Res. Rpt. Inv., no. 44 (Contrib. to Precambrian Geol., no. 1)]. Age is Precambrian.
Occurs in St. Francois Mountain area. Brown Mountain is in St. Francois Co., southeastern MO.

Source: Modified from US geologic names lexicon (USGS Bull. 1350, p. 95).

Pg. 50. Brown Mountain rhyolite porphyry of Bevos group. An intrusive sill-like body of salmon red, aphanitic to very fine-grained porphyritic rhyolite containing salmon red feldspar phenocrysts and clear quartz, small amount of altered biotite, and occasional miarolitic cavities, often lined with quartz. Thickness up to about 300 feet. Exposed in St. Francois Mountains, southeastern Missouri. Outcrop area is about 4 sq mi. Formed during later phase (of 2) of batholithic intrusion. Appears to have differentiated in place. Represents a rapidly chilled marginal facies of Breadtray granite of Bevos group; Breadtray was last of batholithic rocks of St. Francois Mountains to differentiate. Age is Precambrian.
Named from exposures on Brown Mountain, in sec. 1, T. 33 N., R. 4 E., Iron Co., southeastern MO. Notable exposures in sec. 12, where it contains dark felsitic xenolithic inclusions close to contact. (Type locality not designated.)
See C.E. Robertson, 1995, Precambrian rocks, IN T.L. Thompson, Stratigraphic succession in Missouri (revised-1995), Missouri Division of Geology and Land Survey, 2nd series, v. 40 Revised, p. 5-14.

Source: Strat. succession in Missouri (Thompson, 1995, p. 14); GNU records (USGS DDS-6; Denver GNULEX).

Brown Mountain Rhyolite Porphyry, oldest of four Precambrian Y hypabyssal rock units assigned to the /St. Francois Mountains Intrusive Suite (revised). [Previously assigned to Bevos Group by Tolman and Robertson (1969); status of Bevos not discussed.] Present in St. Francois Mountains, southeast Missouri (Midcontinent region). Older than Carver Creek Granite Porphyry (revised) of St. Francois Mountains. Nomenclature listed in table 1.
[Conflicts with nomenclature guidelines (ACSN, 1970; NACSN, 1983, 2005, 2021): name St. Francois Mountains applied concurrently to two different units in same area.]

Source: Modified from GNU records (USGS DDS-6; Denver GNULEX).

Brown Mountain rhyolite porphyry not used in this report. Mineralogy is similar to Breadtray facies of Butler Hill granite and is considered part of Butler Hill. Study area in St. Francois Mountains, southeastern Missouri (Midcontinent region).
[Authors' intentions are unclear as to which, if any, of the geologic units discussed should have formal status.]

Source: Modified from GNU records (USGS DDS-6; Denver GNULEX).