STRATIGRAPHICAL ORDER. ; 19 
The Ferriferous Limestone is by no means as serviceable a guide 
for the entire coal field of Ohio as it has been shown to be in Western 
Pennsylvania. Great differences of opinion have existed in regard to 
what constituted its extension from Pennsylvania into Ohio, but although 
these questions have been definitely settled by the work of the recent 
Pennsylvania Survey, and although we now know which of the lime- 
stones of Mahoning and Columbiana counties are the proper and bodily 
continuations of the Ferriferous limestone of Lawrence and Beaver 
counties, Pennsylvania, there are still unsettled questions as to its west- 
ward extension through a number of counties. In Perry county, how- 
ever, the limestone comes in again with its flint and ore, in unmistak- 
able identity, and from this point southward to the Ohio river, it as- 
sumes the same central place in the Lower Coal Measures that it holds 
in Pennsylvania. 
In thickness it does not reach the highest measures attained in 
Pennsylvania, ranging here between 2 and 8 feet, but in chemical com- 
position, in fossil contents and in physical properties, it is the counter- 
part of the stratum in Pennsylvania. There is associated with it the 
same flint and ore, and it holds quite similar relations to the coal seams 
above and below it in the scale. 
This limestone received the same name in Southern Ohio that it 
bears in Pennsylvania, viz., the Ferriferous Limestone, but it was by a 
happy accident, for at the time that its name was given, its identity with 
the seam to the eastward was not suspected by the geologists who were 
at work upon the coal measures. (Report of Progress, 1870, p. 61, etc.) 
It is commonly known in the region referred to as the Gray Limestone. 
It was also named the Hanging Rock Limestone in Vol. III, Geology 
of Ohio, p. 892, et seq. 
The section of the coal measures of which the Ferriferous Lime- 
stone is the center, is thus seen to have had a symmetrical development 
on the opposite and widely separated sides of the basin in which and 
around which these various elements were formed, but a different state 
of things is found to have existed on the northern border of the basin. 
From the eastern side of Mahoning county to the center of Perry 
county, the Ferriferous limestone is either feebly developed and ob- 
scure, or is altogether wanting. So far as this one element is concerned, 
it has lost the character of a guide, and considered by itself it cannot 
be followed with ease or certainty across the interval. But in this very 
