20 GEOLOGY OF OHIO. 
interval where the Ferriferous limestone has grown weak and uncer- 
tain, another limestone of the same general character is found, which 
completely bridges the chasm, and by means of which we are able to 
maintain the unity of the series unbroken. ‘This is the Gray limestone 
of Newberry and the original Putnam Hill limestone of Andrews. 
Newberry made constant use of it as a guide through Stark, Tusca- 
rawas, Holmes, and Coshocton counties. He conjoins it with the Lower 
Mercer limestone as to steadiness and extent, claiming for it as for the 
latter, that it can be followed almost uninterruptedly from the Pennsyl- 
vania line to the Ohio river. (Vol. II, p. 1380). This claim cannot be 
substantiated in the light of what is now known, so far as the southward 
extension of it is concerned, but the limestone can be followed by fre- 
quently recurring and unmistakable exposures from the western side 
of Mahoning county as far as New Lexington, Perry county. It can 
be traced, indeed, further than: this in both directions, but it is no 
longer a guide. It must itself be followed by the aid of other and bet- 
ter marked strata, the Ferriferous limestone being the most available 
of all. 
The Putnam Hill limestone underlies the Ferriferous limestone by 
15 to 50 feet. The usual interval may be counted 30 feet. A coal 
seam occurs directly below it, which is often of workable thickness and 
sometimes of great economic value. It is Coal No. 4, of Newberry, in 
the counties already named. A considerable bed of plastic clay is found 
below the coal, which is worked in some instances. 
Like the Mercer limestones below it, the Putnam Hill limestone 
generally bears a block ore. The ore is of good quality and is some- 
times mined in a small way. 
In color, the Putnam Hill limestone is intermediate between the 
Lower Mercer and the Ferriferous limestones. If it were not for the 
contrast with the former, it could as properly be called blue as gray. 
If it had been contrasted with the Ferriferous limestone instead, it 
would certainly have received the former designation. It is indistin- 
guishable from the better phases of the Upper Mercer limestone, and 
has often been confounded with it. Both of them figure in Ohio geology 
as gray limestones. When it is remembered that both of them overlie 
coals, that both bear block ores, that both are charged with the same 
species of fossils, it can be seen that the stratigraphical order may be 
necessary to determine to which horizon any given outcrop belongs. 
