Pg. 41, 51 (table 1), 82, pl. 1. Bonpas limestone member of Mattoon formation (new) of McLeansboro group. Proposed to replace name Calhoun limestone, in order to restrict name Calhoun to underlying coal. Stratigraphically below Omega limestone member. Medium gray, weathers with reddish cast; dense to finely crystalline, thick-bedded; tends to be shaly in lower part. Thickness 3 feet. Recognized in southeastern Illinois. Age is Pennsylvanian. Report presents new rock-stratigraphic classification of Pennsylvanian strata in Illinois. Cyclical classification retained but is entirely independent of rock-stratigraphic classification.
Type locality: in NE/4 NE/4 NE/4 sec. 6, T. 2 N., R. 14 W., Olney quadrangle, Richland Co., southeastern IL. Named from Bonpas Creek, about 1.5 mi east of type outcrop.

Source: Publication; US geologic names lexicon (USGS Bull. 1200, p. 428).