516 GEOLOGY OF OHIO. 



CHAPTER V. 



FOSSILS OF THE CLINTON GROUP IN OHIO AND INDIANA. 

 By Aug. F. Foerste, Ph. D. 



Introduction. 



The Lower Silurian age is represented in Ohio almost exclusively 

 by the Cincinnati -Group, the equivalent of the Hudson River Group of 

 New York. Its upper part consists of a rapid alternation of thin lime- 

 stone layers with blue clays, usually terminating above in a blue clay bed 

 of variable magnitude, sometimes reaching 30 feet in thickness, at other 

 times almost disappearing. So far this blue clay bed has yielded only 

 such types of marine Lower Silurian fossils as are well known in the 

 limestones immediately below. These blue clay fossils however (for ex- 

 ample, Orthis occidtntalis and Orthis biforata), are slightly altered in type 

 from their earlier representatives, as though the shells had made an effort 

 to accomodate themselves to new conditions of life. If these consisted in 

 j. shallowing sea, the rise of a land surface, and the formation of an in- 

 termediate coastal region, the observations so far made in Ohio have not 

 made known any geological facts sufficiently attesting to the existence of 

 these conditions. The upper surface of the blue clay layer formed an 

 undulating surface upon which the rocks of Upper Silurian age were laid 

 down. In consequence, the lower Upper Silurian rocks also have undu- 

 lating surfaces. Little has been done, however, so far to determine what 

 share of this undulation in the various beds is due to post-Silurian 

 folding locally affecting all the rocks of Silurian age, and to what extent 

 folding may have taken place in the interval separating Lower Silurian 

 from Upper Silurian rocks. The rapid thickening or disappearance of 

 the blue clay bed, in places, is perhaps the only fair argument, at present, 

 for assuming any marked folding of Lower Silurian rocks in Ohio, before 

 the Clinton period of deposition. 



The lowest Upper Silurian deposits known in southwestern Ohio 

 are sandy beds of unknown age, presumably Medina. At Fa : r Haven 

 they are only two feet thick, and a lamellibranch shell (Pterinea?) ap- 

 parently belonging to this layer was lost before it could be determined. 



