894 GEOLOGY OF OHIO. 
It is generally light gray in color and semi crystalline in structure, and 
is heavily charged with fossils, some of which may prove to be charac- 
teristic of the stratum. It is frequently a Crinoidal Limestone for lim- 
ited areas. In composition it is pure enough to furnish an excellent 
furnace flux, for which purpose it is largely used along the whole line of 
its outcrop in this district. Though comparatively irregular and uncer- 
tain in the Hecking Valley, there has yet enough of it been raised in 
connection with the Baird ore to furnish many hundred tons of flux to 
the furnaces located here. Occasional exposures of it are darker colored 
than the main body of the rock, but it seldom acquires the dark shade of 
the underlying Blue or Zoar Limestone. There are phases, however, of 
the two that are liable to te confounded. 
Like the limestones already described, the Gray Limestone is fre- 
quently replaced by flint. The accumulations of flint along this horizon 
and that of the Putnam Hill Limestone, indeed, are the heaviest that 
occur in the Coal Measures of the State. The best known of all is Flint 
Ridge of Licking county, which belongs to the last named horizon. For 
a large area here, the limestone is gencrally wanting, its place being 
taken by six to eight feet of fossiliferous flint. The flint is of an endur- 
ing nature, and so it happens that along the margins of the ridge great 
blocks of it, which have been undermined by the waste of ages, are left 
to cover the slopes quite widely. 
There are flint ridges in Vinton county and also in Jackson, replacing 
the Gray Limestone, that are not at all inferior in thickness to the ridge of 
Licking county, just named. Their areas, however, are much less. 
Many of these deposits were extensively worked, by the earlier races 
that occupied the country, for arrow-heads and spear-points, and in cur 
own day mill-stones have been wrought from the stratum. Quite an 
important business ‘was carried on in this manufacture fifty years ago. 
This stratum constitutes, in part, the ‘‘calcareo-silicious rock” of the First 
Geological Survey (First Annual Report, page 31). 
The interval between the Blue Limestone and the Gray—counting 
from the Lower Zoar, or Lower Block ore—ranges in the Hocking Valley 
from one hundred and ten feet to one hundred and twenty feet. It in- 
creases gradually to the southward, becoming one hundred and twenty- 
five feet to one hundred and fifty feet in Vinton and northern Jackson, 
and as much as one hundred and sixty or one hundred and seventy feet in 
the southernmost portions of the district. | 
The Hanging Rock Limestone bears the famous limestone ore of the 
district, which is worked for the supply of fifty furnaces. It also 
covers. one of the most available coal seams of Jackson and Vinton coun- 
ties, and thus ts horizon is by far the best known of any in our western 
