892 GEOLOGY OF OHIO. 
In composition, it is often quite impure, containing a notable quantity 
of silica, alumina, and iron, which generally forbids iss profitable use as 
a furnace flux. 
It is highly fossiliferous, being charged with common Coal Measure 
forms, The large stems of crinoids are specially noticeable in it. The 
solid portions of the stems have usually been replaced by calc spar, and 
their white, crystalline sections make a marked contrast to the dark and 
earthy rock in which they are imbedded. The center of the stem is 
often blue, like the mass of the rock. This peculiarity holds in every 
county of the State in which the limestone is shown, but in the counties 
eastward, it is shared with the two limestones next above it in the series. 
This limestone is very often replaced by flint. Sometimes layers of 
flint are interstratified with the limestone layers, and sometimes the 
limestone entirely disappears for miles in succession. The fliat holds 
the fossils of the limestone and maintains the same relations to overly- 
ing and underlying rocks. In color it is general'y black or dark-blue, 
but there are often light-colored portions distributed through the mazs. 
Where the interbedding of limestone and flint referred to above, occurs, 
there is sometimes a notable expansion of the series, its varied members 
filling fifteen or even twenty feet. The position of the Zoar Lime- 
stone in the series has been already plainly indicated. It lies about one 
hundred feet above the Maxville limestone, its limits as measured vary- 
ing between ninety and one hundred and thirty-five feet. In the Hock- 
ing Valley it is from one hundred and sixty-five feet to two hundred feet 
above the lowest coal, and these are also common measures through Vin- 
ton and Jackson counties. | 
It has been spoken of as a single seam; but it must be distinctly ad- 
ded that two courses, separated by an interval of fifteen feet, belong to 
the horizon to the northward and eastward. In Vinton county, the inter- 
val expands to twenty-two feet. Each of these courses when present, is cap- 
ped with ironore, and underlain withacoal seam. Theorescontinue after 
the limestones fail, the lower becoming the one known as the Dresden, 
Junction City,and Union Furnace Block, in our section, and the upper be- 
ing the Main Block or Hocking Furnace ore. This duplication of the 
limestone and its associated seams has led to some confusion where it 
has not been recognized. The upper layer is referred to in all measure- 
ments that follow, where the Zoar Limestone is involved, unless once pun 
is expressly made. 
The Hanging Rock Limestone—The Gray or Hanging Rock Limestone is 
the next one of the main series to be met in ascending the scale. It is 
the limestone named “ Ferriferous,” by Prof. Andrews, in the southern 
