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 Hocking 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 be 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 beet known of all is Flint 

 Ridge of Licking county, which belongs to the last named horizon. For 

 a large area here, the limestone is generally 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 Ihe earlier races 

 that occupied the country, for arrow-heads and spear-points, and in our 

 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-siliciousrock" 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 dSfty feet in Vinton and northern Jackson, 

 and as much as one hundred alid 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 



