896 GEOLOGY OF OHIO. 



ability, and made use of it to some extent, but greater stress has been 

 laid upon it by Prof. Andrews and the other members of the present 

 cbrps who have worked within its limits. In the district now under 

 consideration it is especially serviceable, as it can be followed by a contin- 

 uous line of outcrops from Nelsonville to the river hills above Ironton. 

 The westernmost exposures of it pass through Starr township, Hocking 

 county ; through Brown, Madison, Vinton, and Wilkesville townships, 

 Vinton county; through Huntington and Kaccoon townships, Gallia 

 county ; through Madison township, Jackson county, and through Wash- 

 ington, Dicatur, Elizabeth, and Upper townships, Lawrence county. It 

 overhangs the Ohio River on the Monitor Furnace lands, above Ironton. 

 It is found at an elevation of one hundred to one hundred and ten feet 

 above the Shawnee Limestone in the Hocking Valley, and the interval 

 increases slowly to the southward and eastward, being about one hundred 

 and twenty feet in Lawrence county, and one hundred and fifty feet in 

 Gallia county. It is seldomless than twofeetin thickness, and isfrequent- 

 ly found eight to ten feet thick through extensive tracts. It is highly fos- 

 siliferous in most of its outcrop, and, like the fossiliferous limestones 

 below it in the scale, is very frequently replaced by flint. It constitutes 

 flint ridges, indeed, scarcely inferior in thickness and extent to the 

 deposits of this sort along the outcrop of the Putnam Hill Limestone. 

 This phase is shown very distinctly in Lawrence county, on Mt. Vernon 

 Furnace lands. Dr. Hildreth, of the first survey, supposed the Cambridge 

 flint, the Putnam Hill flint, and the Hanging Pvock flint, to be one 

 stratum, to which he gave the name of the " calcareo-silicious stratum." 

 It is possible that he counted in also one or two exposures of the Zoar 

 flint. He clearly recognized ,however, the difierences in quality between 

 the flints from these separate horizons. The buhr stone, which was 

 manufactured into millstones quite extensively, thirty years ago, was 

 derived from the Putnam Hill and Hanging Rock horizons, exclusively. 

 The formation generally gains in volume when flint takes the place 

 of lime. This statement can be made, in fact, for all of the similar 

 deposits that have here been described. 



The limestone agrees in composition with the one last named, holding 

 in its best phases from eighty to ninety per cent, of carbonate of lime, 

 with very little magnesia. Itcontains, locally, a notable quantity of iron. 

 It is frequently a crinoidal limestone, and is generally fossiliferous. 

 It is crystalline in structure for the most part. The prevailing shade of 

 color is a grayish brown, which is quite characteristic. It is the same in 

 Athens, Vinton, and Lawrence counties. In the Hocking Valley, how- 

 ever, the limestone is often black for considerable areas, and gets a local 

 name from this fact. It is also sometimes reddish in tint, and sometimes 



