First used. Shown on stratigraphic chart of Missouri. Overlies Iron Mountain porphyries; unconformably underlies Pilot Knob conglomerates. Described as a diabase. No thickness given. Placed within Francoisian Series of Late Algomic (Archeozoic) age.

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

Skrainka diabase. A basic post-batholithic intrusive rock commonly found in irregular tabular sill-like masses and in small bosses, mainly in east-central part of St. Francois Mountains, southeastern Missouri. Intrudes granite rocks of Musco and Bevos groups and into the felsite rocks of Middlebrook and Van East groups. Consists of dark, bluish-gray diabase; has medium- to fine-grained diabasic texture with gray feldspar laths. Most known exposures are within granites; unit is believed to be a differentiate of granitic batholith. Exposures are few due to extreme chemical weathering; generally exposures consist of a series of isolated, rudely spheroidally, weathered, dark-gray boulders in roadcuts or along stream banks; boulders have characteristic knob and pit surface; rock is fresh below knobby surface and intensely to completely weathered above surface. Unconformably underlies Lamotte formation. Age is Precambrian.
Typically developed at Skrainka quarry, in sec. 3, T. 33 N., R. 6 E., Madison Co., MO.

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