STRATIGRAPHY OF FOSSILIFEROUS MEMBERS 117 



The McArthur member consists of impure, bluish-gray, argillaceous 

 limestone, gray to black calcareous shales, and shaly, calcareous sand- 

 stones. Where typically developed in Jackson and Vinton counties, 

 shales occupy the entire interval between the Tionesta coal and the 

 Black Flint, as the Homewood sandstone is often lacking or interbedded 

 in thin layers with the shale. The lower portion of these shales is cal- 

 careous and remarkably fossiliferous, and constitutes the McArthur 

 horizon. At the southern limit near Monroe Furnace in southern 

 Jackson County, 6 feet is present, while northward in Jackson, Vinton, 

 Hocking, and Perry counties, the member measures 2 to 14 feet, with 

 an average thickness of 4 feet. In the vicinity of McArthur the deposit 

 consists of 6 feet of massive limestone which is extremely rich in fossils 

 and forms by far the most fossiliferous horizon of the Pottsville forma- 

 tion with the possible exception of the black shales and impure shaly 

 limestones associated with the Lower Mercer member. The fauna is 

 of marine origin and is composed largely of brachiopods and pelecypods, 

 although gastropods are much more abundant individually than in 

 any of the older members. Collecting is excellent as the fossils are 

 well preserved often showing even the most delicate markings, and can 

 be easily obtained from the soft limestones and shales. It is common 

 for the substance of the shell to be decomposed to a soft, chalky, white 

 material which rapidly disintegrates on exposure. 



Conditions of Deposition 



At the close of the swamp conditions under which the Tionesta 

 coal was deposited, shallow waters prevailed over southern and east- 

 central Ohio which were favorable for the deposition of limestone such 

 as was deposited during Lower and Upper Mercer times. The sea 

 supported an abundant life as is testified by the richness of marine 

 fossils found in the stratum. However, sediments of various sorts, 

 such as sand and mud, were constantly carried to the sea, so that as a 

 result impure argillaceous limestones, calcareous shales, and shaly, cal- 

 careous sandstones were formed. Similar conditions existed in the 

 southern part of the outcrop until the bottom of the sea was built up 

 to a height sufficient for the swamp conditions necessary for the deposi- 

 tion of the Brookville coal to again prevail. 



Description of Geologic Sections and Collecting Localities 



Lawrence and Scioto Counties. In southern Ohio the horizon of 

 the McArthur member is occupied by hard, tough shales which enter 

 Ohio as a wedge from the south. Sandstone at some localities replaces 

 the shale or is interbedded with it. The shale measures 5 to 10 feet in 

 thickness and is reported to be sparingly fossiliferous. The section 



