892 GEOLOGY OF OHIO. 



In composition, it is often quite impure, containing a notable quantity 

 of silica, alumina, and iron, which generally forbids its profitable use as 

 a furnace flux. 



It is highly fossiliferou?, 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 solid portions of the stems have usually been replaced by calc spar, and 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 flint holds 

 the fossils of the limestone and maintains the same relations to overly- 

 ing and underlying rocks. In color it is generally black or dark-blue, 

 but there are often light-colored portions distributed through the maia. 

 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 abjve the Maxville limestone, its limits as measured vary- 

 ing bstween ninety and one hundred and thirty-five feet. Iq 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 countj', the iater- 

 val expands to twenty -two feet. Each of these courses when present, is cap- 

 ped with iron ore, and underlain with a coal seam. The ores continue after 

 the limestones fail, the lower becoming the one known as the Dresden, 

 Junction City, and Union Furnace RIock, in our section, and the upper be- 

 ing the Main Block or Hocking Furnace ore. This duplicat'on of the 

 limestone and its associated seams has ltd to some confusion where it 

 hag not been recognized. The upper layer is referred to in all measure- 

 irients that follow, where the Zoar Limestone is involved, unless exception 

 is expressly naade. 



The Hanging Rock Limestone — The Gray or Hanging Rock Limestone is 

 thei 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 



