Named as a unit of the "Upper volcanic series" for Razorback Mountain in sec. 36, T 18 S, R 11 W, Grant Co., NM in the Basin-and-Range province. No type locality designated. Mapped (geologic map) in northern part of quad. from Peloncillo Mountain north and west to quad. border, and in north central part of quad in Mimbres River and Mimbres Hot Springs areas. Two members mapped; both are black or dark gray and fine-grained in contrast to the rocks in the "Lower volcanic series". The lower member has black, fine-grained iddingsite andesite flows and flow breccias containing quartz and oligoclase inclusions. The upper member has black aphanitic rhyolite flows with spherulitic, perlitic, and brecciated zones, and minor tuffs. Some green, fine grained tuffs, and some white pumiceous tuffs locally. Total thickness for both members is 300 ft west of Mimbres Peak, 700 ft at Caballo Blanco , and 800 ft in Donahue Canyon. Most of the andesite is gray to black with small red hematite stains; the andesite flows tend to split into slightly irregular layers 2-3 inches thick, parallel to the surface over which it flowed; is locally vesicular. The upper member is fine grained, flow layered, in thinner layers (one half to 1 inch thick) than the lower member, and is glassy, characterized by vitreous luster. No source found. Lies on the irregular surface of the Caballo Blanco rhyolite (named), on Peloncillo sediments, or on Mimbres Peak rhyolite (named), depending on location. Overlain by Bear Spring basalt. Tertiary age assigned.

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