iiantVof a' Oh'ce continuous deposit laid down at the end of Mississippian 

 time. * Uplift, however, caused the withdrawal of the sea, and for 

 a long period the region was subjected to erosion. The duration of 

 this period of erosion was great enough to remove most of the Max- 

 ville limestone, and in some places streams had cut for considerable 

 distances into the Logan formation. Remnants of the Maxville 

 limestone appear at the present time only as isolated patches or 

 islands surrounded by younger rocks of Pottsville age. At the end 

 of these land conditions, the region was again submerged and the 

 basal members of the Pottsville formation, the Harrison ore and the 

 Sharon conglomerate, were deposited in the low troughs and basins. 



The Pottsville formation varies in thickness from 100 to 350 

 feet, but averages about 255 feet. The deposit thickens toward the 

 south, and in southern Ohio attains its maximum development; there 

 is also a general thickening from the western to the eastern part of 

 the outcrop. The extremely irregular line of contact between the 

 Pottsville formation and the Mississippian system below is responsible 

 for the great variation in the thickness of the formation in different 

 regions. Where the Maxville limestone has been largely or entirely 

 eroded, the lowest members of the Pottsville formation, the Harrison 

 ore and the Sharon conglomerate, are present, while successively 

 higher members form the base of the formation where increasing 

 thicknesses of Maxville limestone are found. In Hopewell Township, 

 Muskingum County, on Poverty Run, the Quakertown coal forms 

 the base of the Pottsville, while elsewhere any of the horizons from 

 the Harrison ore to the Quakertown coal may be in contact with 

 rocks of Mississippian age. 



The Pottsville formation in Ohio is composed for the most part 

 of shales and sandstones, interbedded with clays, coals, iron ores, 

 and marine limestones. The accompanying generalized section of 

 the formation shows the succession of the various members and their 

 relations to each other. The members vary greatly in character 

 and thickness from place to place. Some, such as the Black Flint 

 at the top of the formation, are local in their occurrence and are found 

 only in the southern part of the outcrop; others, such as the Mc- 

 Arthur limestone and Sciotoville clay, are more extensively developed 

 in southern Ohio but to the northward become thin and finally dis- 

 appear. There are certain strata, however, of great importance for 

 stratigraphic study, as they are very persistent and can be traced from 

 the Ohio river northward along the Pottsville outcrop to the Ohio- 

 Pennsylvania state line. At the base occurs the Sharon conglomerate 

 which, although patchy in appearance, can be found everywhere in 

 the deepest troughs eroded in pre-Pottsville times. Among the 

 coals, the Quakertown or No. 2 coal, the Lower Mercer or No. 3 

 coal, the Middle Mercer or No. 3a coal, and the Tionesta or No. 3b 

 coal are the well-defined stratigraphic units. By far the best devel- 



iMorse, W. C., Geol. Sur. Ohio, Fourth Ser., Bull. 13, p. 99, 1910. 



6 



