REVIEW OF GEOLOGICAL STRUCTURE. 5 



development of the Medina Group in Pennsylvania and West Virginia, 

 at points not more than two hundred miles directly east from the eastern 

 base of the Cincinnati arch. ' 



THE CLINTON GROUP. 



A large number of fossils, new to science, have been collected from the 

 Clinton Group since the publication of our first volume of Geology, and 

 most of these have been described in the volumes on Palseontology, but 

 nothing has been discovered that requires any modification of the full 

 and accurate description of the character and extent of the Clinton lime- 

 stone given by Professor Orton. 



It will be remembered that the stratum of beautiful building Stone 

 known as the Dayton stone, is made in our reports to form the base of 

 the Niagara Group. It has been suggested that it should rather be re- 

 garded as the cap-stone of the Clinton, but no proof has been adduced 

 which would justify the change ; indeed, any line separating the Clinton 

 and the Niagara formations in Ohio, must be very faintly drawn, as they 

 form subordinate parts of one whole. Even in New York the ties that 

 bind the Clinton and Niagara are much more numerous than would be 

 inferred from their distinctness in the tables of geological classification. 

 A large part of the fossils of the Clinton run up into the Niagara, and, 

 while there are certain lithological distinctions there, as well as a few 

 fossils that serve to separate them, they are plainly portions of one great 

 formation. In Ohio, where shores that afforded silicious sediments were 

 remote, the physical conditions were more uniform throughout the Upper 

 Silurian age, and the Clinton and Niagara rocks are more alike, both in 

 " lithological character and fossils, than in New York. 



To any one who will even briefly review the facts presented by the 

 Clinton and Niagara Groups as they are shown in New York and Ohio, 

 the history of their deposition, and consequently their relations, will be 

 easily understood. 



In Ohio the disturbances which resulted in the elevation of the Cin- 

 cinnati arch, not only produced a great fold in the sediments which had 

 been deposited in the Lower Silurian Sea, but caused a withdrawal of 

 this sea, leaving its bed nearly bare throughout the interior of the conti- 

 nent, and forming a broad plain traversed by a low mountain chain. 

 After remaining in this condition for an indefinite period, the eastern 

 half of the continent was again invaded by the advancing sea. This 

 flowed up between the Cincinnati arch and the old land of the Allegheny 

 belt, producing the Medina as its first sediment, then the Clinton and 

 then the Niagara, as the depth and spread of the water became greater. 



