GREENE COUNTY. | 673 
called to the fact that Greene county possesses an ample supply of 
hydraulic limestone fully equal in quality to the cement which serves a 
district of France most satisfactorily. The great obstacle to the intro- 
duction of a new cement lies in the fact that masons, after becoming used 
to one particular product, are very loth to adopt the changes in practice 
which a new article renders necessary. The product here furnished is a 
hydraulic lime, and not a hydraulic cement. 
The silicious concretions and nodules often replacing fossils, and the 
silicious layers which are so abundant in the quarries of Clarke county, 
are almost entirely wanting here. 
Shaly partings are occasionally found between the courses. Ata depth 
of eight or ten feet below the surface of the stratum, a layer of shale, 
several inches thick, occurs, which, from its impervious nature, becomes 
an important water-bearer. 
There is not the same paucity of fossils in this stratum which marks 
the Dayton stone or the Niavara shale, but compared with the limestones 
of the Clinton and Cincinnati groups, and also with the overlying division, 
it may yet be said to be poor in this respect. The most striking forms 
by far that it contains are the casts of the monstrous brachiopod shell, 
Pentamerus oblongus, which sometimes completely cover the surface of the. 
layers. This interesting and characteristic fossil begins its great devel: 
opment in the rocks of the Mississippi valley at this particular horizon. 
At the east it characterizes the Clinton group, but it has never yet been 
found in the Clinton limestone of Ohio. A single overgrown specimen 
was obtained from the bottom of the Niagara series by the late Col. Greer, 
of Dayton, and a few specimens have been found in the West Union 
cliff of Adams county, but throughout the periods represented: by this 
and the succeeding formation it had a wonderful expansion, literally 
paving the ancient sea-floor for hundreds of square miles. through un- 
counted centuries. It often constitutes the substance cf the rock for 
eight or ten feet in thickness. No more perfect imternal casts of this 
shell seem possible than the quarries of W. Sroufe, Iisq., of Yellow Springs, 
have furnished. 
A few other brachiopod shells are occasionally met with in this division. 
Among them may be named Pentamerus ventricosus, Orthis biforata, Atrypa 
retucularis (shorter form), and Meristella Maria. None of these, however, 
are confined to this division. The Niagara trilobite, Calymene Blumen- 
bachu, var. Nvagarensis, is also of frequent occurrence. 
(e.) Overlying the Springfield stone, there is found in southern Ohio the 
representative of a formation the place of which was a subject, of much 
discussion in the earlier days of American geology, The discussion has 
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