14 GEOLOGY OF OHIO. 
almost continuously from the Pennsylvania line to the Ohio, and that 
it is one of the most reliable and useful guides in the exploration of the 
country traversed by it. 
Andrews pronounced it ‘‘everywhere a guide to the stratigraphical 
position of the rocks below it” throughout Hocking, Vinton and Jack- 
son counties. (Report of Progress, 1870, page 93.) 
It is generally known in Ohio in the regions where it occurs as the 
Blue Limestone, but the name fixed upon it by Newberry, viz., the 
Zoar Limestone, has also come into common use. 
It is so nearly uniform in its leading characteristics that an ade- 
quate description of it in any one locality will answer without change 
for its whole extent. 
In color it is dark-blue, occasionally almost black. In thickness, its 
usual range is from one to three feet, but it sometimes reaches a thick- 
ness of ten feet. It is frequently doubled, a second stratum coming in 
a few feet above the main bed. It does not le in massive nor in even 
beds. It is often shaly in structure. It contains a notable quantity of 
iron, alumina and silica as a rule, but it is sometimes pure enough to be 
used for lime or even for furnace flux. It is highly fossiliferous, con- 
taining a considerable variety of the usual coal measure forms. It is 
especially characterized by the large stems of crinoids, which make one 
of its noticeable features. When replaced by flint, as it often is locally, 
the flint holds the fossils of the limestone. 
The limestone is overlain throughout: its whole extent by an excel- 
lent iron ore. The ore sometimes rests immediately on the limestone, 
and sometimes it is separated by a few feet of clay or shale. It is 
extensively worked in several] districts of Ohio, and this fact makes the 
horizon much better known than it would otherwise be. 
In addition to the ore borne by the limestone, it is also to be noted 
that a widely distributed coal seam belongs to the Lower Mercer 
horizon. The coal is sometimes directly covered by the limestone, but 
it often lies ten or fifteen feet below the latter. It is of mineable thick- 
ness in many localities, but it is worked mainly in country banks, so 
far as Ohio is concerned. | 
The Lower Mercer Limestone, thus definitely characterized as a 
stratum, and thus re-enforced by the well-known and widely-worked ore 
that it bears, and also by the coal seam that it covers, is, beyond 
question, the best marked formation in the Lower Coal Measures of Ohio, 
