Pg. 15, 44+. Derby formation. Dolomite, fine-grained, crystalline, slightly calcareous, light-gray to yellowish-gray or reddish-brown; some alternating soft porous beds. Thickness 38 to 40 feet. Conformably underlies Doerun formation and conformably overlies Davis formation. [Age is Late Cambrian.]
Type locality: on Harris Branch near Derby Mine (Federal Mine No. 2), about 1 mi south of railroad stations at Elvins, [Flat River 7.5-min quadrangle], St. Francois Co., eastern MO. Named from Derby Mine.

Source: Publication; US geologic names lexicon, USGS Bull. 896, p. 600.

Used as [/]Derby-Doe Run Member of Franconia Formation in IL. Name is hyphenated because the two units were named separately, in MO, but have similar lithology and conformable relationship. In north-central IL, Derby-Doe Run is silty, argillaceous dolomite. Farther south it becomes quite pure, except for a thin zone at the top that is slightly sandy, argillaceous, and glauconitic. Member is about 20 ft thick in the north and thickness to probably more than 500 ft to the southeast. Age is Late Cambrian (Franconian).
[The use of a hyphen between undifferentiated [members] is not considered proper notation (CSN, 1933).]

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

In subsurface of MO and IL, mapped as Derby and Doe Run Formations, undivided. Age is Late Cambrian (Croixan).

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

Cambrian (Croixan) Derby Formation occurs on Ozark dome in MO. Correlates with Franconia Formation of Knox Group in IL. Difficult to differentiate Derby from overlying Doe Run Formation, Potosi Dolomite, or Eminence Formation in subsurface.

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