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 
corps who have worked within its limits. In the district now under 
consideration it is especially serviceable, as it can be followed bya contin- 
uous iine of outcrops from Nelsonville to the river hills above Ironten. 
The westernmost exposures of it pass through Starr township, Hocking 
county; through Brown, Madison, Vinton, and Wilkesville townships, 
Vinton county; through Huntington and Raccoon townships, Gallia 
county ; through Madison township, Jackson county, and through Wash- 
ington, Decatur, 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 seldom less than twofeet in 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 Rock 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 ,jhowever, the differences 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 Reck 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. It contains, 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 
