STRATIGRAPHY OF FOSSILIFEROUS MEMBERS 95 



When the limestone is found it measures only a few inches in thickness. 

 In the central part of the outcrop, where the flinty phase is developed, 

 the stratum is about 1 foot 6 inches thick, while in northeastern 

 Ohio the heavy limestones reach a maximum thickness of 3 feet 6 inches 

 with an average of over 2 feet. Locally in a few places in Ohio the 

 member attains 15 feet in thickness. 



Conditions of Deposition 



The Upper Mercer limestone is of marine origin and was deposited 

 in relatively shallow waters. The fossils, although not abundant, are 

 all marine. The ore of southern Ohio, and the flints of the central 

 part of the outcrop are likewise of marine origin as is shown by the 

 fauna which is practically identical with that of the limestone. The 

 origin of the flint is discussed by Stout in his report on the Geology of 

 Muskingum County in which three views are given as tenable for the 

 origin of the flints associated with the limestones of the Pennsylvanian 

 system : l 



(a) "That the flint was formed by direct precipitation of the sili- 

 ceous matter by silica-secreting organisms. 



(b) That the flint was the resultant of chemical action of soluble 

 silica and other components in the sea water upon the calcium car- 

 bonate of the newly formed limestone. In this case the change took 

 place while the limestone was forming or while it was yet under the 

 direct influence of the salt brines. 



(c) That circulating ground waters, charged with silica and 

 organic components which acted upon the limestone deposited under 

 normal conditions and buried by later sediments, slowly removed the 

 calcium carbonate and deposited silica in its place. This action began 

 as soon as the beds were covered by other material and is still effective. 

 Under this condition the flint is entirely of secondary origin." 



After a discussion of these three views, the following conclusions 

 are reached: 2 "It appears that these flint beds were original deposits 

 laid down in shallow water under conditions similar to those where 

 limestones were deposited, but where siliceous material from organic 

 life was present in considerable quantities, and was either deposited 

 directly or was substituted for calcium carbonate of newly formed lime- 

 stone. The action of ground waters in the silicification of limestones 

 appears, therefore, relatively small; and this view is further strength- 

 ened by evidence afforded by the higher beds of this (Muskingum) 

 county." 



'Stout, W., Geol. Surv. Ohio, Fourth Ser., Bull. 21, p. 109, 1918. 

 2 Idem., p. 113. 



