Named as middle of three formations of Thunderbird Group (rank raised). Name probably derived from Smugglers Pass located between North and South Franklin Mountain, El Paso Co, TX. Type locality is about 3000 ft northeast of Smugglers Pass in Fusselman Canyon, lat 3l deg 53' 06" and long 106 deg 29' 34". Outcrops in area of Tom Mays Memorial Park south to South Franklin Mountain in southern part of Franklin Mountains, Southwestern Basin-and-Range region. Consists of melanocratic porphyritic trachyte in lower part with phenocrysts predominantly of albitized microcline perthites with very minor plagioclase in dark-gray to olive-black groundmass, ranges up to 150 ft thick; porphyritic grayish-red trachytes with phenocrysts of albitized microcline perthite, ranges up to 200 ft; tuffaceous sandstone and conglomerate with detrital quartz, pumice and lithics and minor perthite, albitized perthite and magnetite, ranges up to 90 ft thick; and silicified ignimbrites with microcline perthite phenocrysts and rare quartz phenocrysts in a dark-gray groundmass, varies in thickness from several ft to about 20 ft. Overlies Coronado Hills Conglomerate (new) of Thunderbird; underlies Tom Mays Peak Formation (new). [Contact relationships not specified]. Geologic map. Geochemistry. Stratigraphic column. Late Precambrian age (950 m.y. old).

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