GKEENE 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 ctment. 



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. At a depth 

 of eight or ten feet below the surface of the stratum, a layer of shale, 

 several inches thick, occurs, which, from it's 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 Niagara 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- 

 eounted centuries. It often constitutes the substance of the rock for 

 eight or ten feet in thickness. No more perfect internal casts of this 

 shell seem possible than the quarries of W. Sroufe, Esq., 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 

 reticularis (shorter form), and Meristella Maria. None of these, however, 

 are confined to this division. The Niagara trilobite, Calymene Blumen- 

 hachii, var. Niagarensis, is also of frequent occurrence. 



(«.) 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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