FOSSILS OP THE CLINTON GROUP. 619 



gether and then consolidated into n. massive rock. On freshly fractured 

 surfaces this rock looked like the Dayton limestone elsewhere, suggest- 

 ing the origin of the Dayton limestone in general as an encrinital mass, 

 the original structure having generally disappeared in consequence of the 

 crystallization of the lime material. Beneath this lay two layers of thin 

 Dayton limestone, each varying from two to three inches in thickness. If 

 the Clinton does not occur immediately beneath, the quarrymen found some 

 other reason for stopping quarrying at this horizon. 



It is customary to begin the section of the Niagara series of south- 

 western Ohio with this Dayton limestone as the base. Above the Day- 

 ton limestone there is commonly a thickness of thirty feet or more of a 

 dolomitic rock with frequent partings known as the Niagara shales. It 

 is usually almost entirely destitute of fossils. The Dayton limestone is 

 in reality only a basal variation of these dolomitic limestones, be- 

 ing typically developed between Dayton and Centerville, merging into 

 the shales laterally and, locally, also vertically. Above the Niagara dolo- 

 mitic shales are more massive dolomitic limestones, containing Penta- 

 merus oblongus quite abundantly and for the first time, and other fossils 

 having a facies intermediate between the Niagara and Guelph. These 

 form the Springfield group, and may be seen along the Eaton pike about 

 three miles west of Dayton. The Cedarville dolomite, representing the 

 true Guelph, occupies the next higher horizon. 



In Indiana the succession of the Middle Silurian is not so well 

 known. At Hanover the blue clay at the top of the Lower Silurian seems 

 to be followed by 8 to 10 inches of Medina, and the Clinton, after twenty 

 feet of barren limestone, develops first into a cherty rock 2 to 3 feet thick, 

 and then into the tossiliferous Clinton 12 to 20 inches thick, the last present- 

 ing all the characteristics, lithological, and paleontological, of the upper 

 Ohio Clinton, where it is tinged reddish or brown by iron. This fossilif- 

 erous Clinton has not been traced north as far as the Waldron beds, by 

 means of its characteristic fossils, but all the detailed work so far done 

 indicates that the Clinton of Ohio and Indiana belongs just below the 

 base of the series of shales which in its upper horizons contains the fa- 

 mous Waldron Niagara fossils. Indeed, the Clinton is represented by 

 limestones at the base of sections not far from Waldron localities. The 

 Waldron shales are the only beds in Ohio or Indiana which closely imi- 

 tate the typical Niagara fauna of New York. The Clinton of Ohio and 

 Indiana carries a fauna showing many resemblances to Niagara fossils 

 elsewhere, but presenting also many distinctions, and the distinctions 

 have been considered sufficient to warrant the separation of the Clinton 

 fauna in Ohio, from the more marked Niagara fauna as exhibited at Wal- 

 dron, Indiana, and less perfectly known in Ohio. In general, it may be 

 said that the Ohio Clinton presents forms often closely allied to Niagara 

 forms of other states, but of an earlier type, and sufficiently distinct to 

 make their discrimination from Niagara forms desirable, so as to secure 



