Mercer Limestone in Newark-Zanesville Region 305 



It will be observed that the greatest variation is in the interval 

 between the Berea grit and the top of the limestone. This place 

 in the geological scale is occupied by the Bedford and Ohio shales. 

 The difference of the thickness of these shales at Flint Ridge and 

 Roseville is 460 feet, and as the two wells are approximately 17 

 miles apart, this would mean an average increase of 27 feet per 

 mile in the thickness of the shales, from northwest to southeast. 

 This agrees closely with Dr. Bownocker's statement in regard to 

 the thickening of these formations toward the east, in his bul- 

 letin on oil and gas {Geological Survey of Ohio, Fourth Series, Bul- 

 letin No. 1, 1903, p. 119). 



SUMMARY 



The exposures of the Mercer limestone at Bald Knob and Flint 

 Ridge show the same lithological character and carry the same 

 fauna. The thickness at Bald Knob could not be measured, but 

 at Flint Ridge the greatest thickness is about 14 feet, and the lime- 

 stone is underlain by cannel coal of workable thickness. The 

 greater part of the limestone is shaly and fossihferous, but there is 

 a layer of impure, very fossihferous limestone at the top and a 

 somewhat thicker part of heavy, only moderately fossihferous 

 limestone at the bottom. The fossils are mostly brachiopods and 

 small pelecypods, the number of species of pelecypods being two 

 or three times as great as the number of species of brachiopods. 



In the Fultonham region the greatest thickness of the Mercer 

 limestone is a little less than 3 feet, and it consists of a single mas- 

 sive layer that does not become shaly on weathering. The under- 

 lying coal is not more than 6 inches thick. Above the limestone 

 is a gray calcareous shale in which are found many of the pelecy- 

 pods and other fossils of the Flint Ridge region. The limestone 

 itself carries principally a brachiopod and gasteropod fauna, only 

 one pelecypod being found. The interval from the base of the 

 Sharon conglomerate to the Mercer limestone is much less than 

 at Bald Knob, and the whole section seems to be shortened. 



At Somerset the limestone is a little less than 5 feet thick, and 

 its general appearance when in place is similar to that of the Mer- 



