Type section and use of name: The Perth Limestone Member was named by Hutchison (1960, p. 19-21) for exposures in the abandoned surface mine in the SW¼NE¼ sec. 3 T. 13 N., R. 7 W., near Perth, Clay County, Ind. Here and in adjacent counties this limestone had earlier been known as the Minshall Limestone. The name Minshall had also been mistakenly applied to other limestones and coals. The Perth lies 0.2 foot to 15 feet (0.06 to 4.6 m) above the Minshall Coal Member.
Description: At the type locality the lime-stone is gray, hard, argillaceous, and fossiliferous. It weathers brown and contains a 2-foot-thick (0.6-m) medial band of blue chert that is hard, fossiliferous, and vuggy. The limestone is not continuously present and varies considerably in character and ranges from 0.5 foot to 19 feet (0.2 to 5.8 m) in thickness. Where absent, its position is occupied by gray shale that is soft, calcareous in places, and fossiliferous or by gray to brown sandstone that is fine to medium grained and is calcareous in places.
Correlation: The Perth or a limestone of similar stratigraphic position has been recognized from central Warren County southward to the Ohio River. In southern Indiana the Perth seems to correspond closely to the limestone above the Buffaloville Coal Member, and drilling records indicate that a limestone of similar or identical position is present in the subsurface of most of the counties of southwestern Indiana.
Understanding of Perth correlation is complicated by the substitution of this name for the ambiguously used term Minshall, and in the guise of the older name, the Perth has been correlated as low as the Lead Creek Limestone Member and as high as the Curlew Limestone Member of western Kentucky. The Perth (old Minshall in part) lies below the lowest position of the fusulinid Wedekindellina, which is found in what at times has been called the Minshall Limestone (Thompson and Shaver, 1964, p. 21; Shaver and Smith, 1974, p. 12, 17). The Perth of this report, however, and the limestone above the Buffaloville Coal Member mark the lowest known Indiana position of the ostracod fauna characterized by Amphissites centronotus and A. girtyi, and it is a limestone close above the highest known Profusulnella and ostracods of the Amphissites rothi fauna. Collectively, the Perth microfauna is generally similar to that in the Seville Limestone Member of western Illinois, the Lower Mercer Limestone of Ohio, and the Curlew Limestone Member of western Kentucky. But a Kentucky limestone that is also a possible correlative is the limestone, possibly lower than the Curlew, containing Fusulinella and associated with what has been called the Lewisport Coal that was mined near Lewisport, Daviess County, Ky. On the basis of this microfauna the Perth has been assigned to the Desmoinesian Series (Shaver and Smith, 1974; Shaver, 1984).