STRATIGRAPHY OP FOSSILIFEROUS MEMBERS 47 



of the members of the Pottsville formation, as it enters southern Ohio 

 from Kentucky and extends northward across the state, being present 

 in every county where the Pottsville occurs, and finally extends across 

 the Ohio-Pennsylvania bolder line from Mahoning County. It is 

 found in the following counties beginning at the Ohio River, Lawrence, 

 Scioto, Jackson, Vinton, Hocking, Perry, Muskingum, Licking, Coshoc- 

 ton, Tuscarawas, Holmes, Wayne, Stark, Summit, Portage, Mahoning, 

 and Columbiana. On account of its wonderful persistence it forms an 

 extremely valuable horizon for stratigraphic and correlative purposes. 



The Lower Mercer limestone occurs above a thin but rather per- 

 sistent bed of coal, known as the Middle Mercer coal, and is either 

 directly overlain by an iron ore, designated as the Lower Mercer or 

 Little Red Block, or is separated from it by an interval of only a few 

 feet. The ore is fossiliferous but to a much less extent than the lime- 

 stone; the faunas of the two horizons, however, are practically the 

 same, indicating a marine origin for the ore and conditions of deposi- 

 tion similar to those under which the limestone was deposited. 



Outcrop in Southern Ohio. In southern Ohio the Lower Mercer 

 member is a hard, tough, dark blue or almost black limestone, with 

 thick or thin layers which are often shaly in character. In Scioto and 

 Lawrence counties the limestone measures 1 foot in thickness; north- 

 ward in Jackson County two benches are present, each 1 foot thick, 

 separated by about 2 feet of black shale. In Vinton, Hocking, and 

 Perry counties similar conditions exist, and the two benches together 

 with the intervening shales attain a thickness of 5 feet. Dr. Edward 

 Orton, in his description of the Hanging Rock District in Lawrence 

 and Scioto counties, describes the member thus: 1 "The color of the 



limestone is dark-blue. In thickness it occasionally rises to 10 feet, 



but it, as often, shrinks to 10 inches. The usual measure for it in the 

 district is from 1 to 3 feet. It is generally shaly in structure, at 

 least for a part of the stratum. It does not lie in massive or even 

 beds, and does not endure weather well." 



The limestone is everywhere fossiliferous, but at some localities 

 fossils are present in extreme abundance. The soft black shales which 

 occur between the benches or on top of the limestone are especially 

 fossiliferous and afford excellent collecting. The fauna is everywhere 

 characterized by a richness of large crinoid stems. 



Outcrop in East-Central Ohio. In east-central Ohio the Lower 

 Mercer limestone also foims the most persistent and well marked 

 stratigraphic unit of the Pottsville formation. It is particularly well 

 developed in western Muskingum County as far east as Zanesville and 

 in the adjoining parts of Licking County. Thence the deposit ex- 

 tends northward into Coshocton, Tuscarawas, Holmes, and Wayne 

 counties. The average thickness in Muskingum County is 3 feet 7 



'Orion, Edward, Geol. Surv. Ohio, Vol. Ill, p. 891, 1878. 



