Oak Mountain felsite (new) of Middlebrook group (new). Briefly summarized in reports by W.C. Hayes and J.A. Martin, p. 53, and W.C. Hayes, p. 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)]. Consists of series of nearly black flows containing small phenocrysts of potash feldspar and quartz. Age is Precambrian.
Occurs in St. Francois Mountain area. Oak Mountain is in St. Francois Co., southeastern MO.

Source: Modified from US geologic names lexicon (USGS Bull. 1350, p. 536).

Oak Mountain rhyolite of Middlebrook group. Megascopically, rocks are felsites or trachytes; microscopically, rocks are rhyolite, leucorhyolite, and sodaclase rhyolite. Flows are dark-brown and gray to almost black, weather to gray, tan, brown, and dull red, making identification of unit difficult. Contains quartz and light-gray feldspar phenocrysts; potash feldspar crystals are few. Flow structure and flow breccia are well developed in upper part of flows. Exposed in St. Francois Mountains, southeastern Missouri, notably on Oak Mountain, Brown Mountain, and Reservoir Hill, St. Francois County; outcrops are less abundant than most other felsite units. Age is Precambrian.
Named from exposures on north flank of Oak Mountain, in sec. 8, T. 34 N., R. 4 E., St. Francois Co., MO. (Type locality not designated.)
[GNU remark (Feb. 16, 1990, USGS DDS-6; Denver GNULEX): Stratigraphic relationships of units of Middlebrook not discussed in text; geologic map (plate 1) missing from library copy of report.]

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