OUTLINE OF ROCK FORMATIONS. 



53 



disturbance that have sufficient prominence to affect seriously the distribu- 

 tion of the rock formations being- the Cincinnati arch with its western and 

 eastern branches. Along its broad, flat-topped crest the oldest formations 

 of Ohio and Indiana are exposed. The strata dip westward from the arch 

 and its western arm toward the coal basin of Indiana and Illinois, while 

 between the arms thej dip northward toward the coal basin of Michigan. 

 From the borders of Lakes Ontario and Erie the strata dip southward and 

 southeastward to the coal basin of the Allegheny Plateau, while from the 

 Cincinnati arch they dip eastward toward the same basin. In passing 

 toward the coal basins either from the Cincinnati arch and its branches or 

 from the north border of the region, a succession of rock formations is 

 encountered which have an imbricate arrangement, and the outcrops 

 present an ascending series in the geological scale. The series embraces 

 that part of the , geological time scale which falls between the Trenton 

 epoch of the Lower Silurian or Ordovician and the Permian epoch of the 

 Carboniferous. It includes the groups of rocks set forth in the following 

 series, which are given in their order of development from older to newer. 



Silurian. 



Upper . 



Devonian , 



Tahle of rock formations. 



(Trenton limestone. 

 Utica shale. 

 Hudson River group. 

 Medina group. 

 Clinton group. 

 Niagara group. 

 Salina and VVaterlime. 

 Lower Helderberg limestone. 

 Oriskany sandstone. 

 Corniferous limestone. 

 Marcellus shale and Hamilton formation. 

 , Genesee, Portage, and Chemung, or Ohio shale. 



Classification unsettled J ^averly or Bedford shale (CatskiU ? ) . 



[ Berea grit and shale (Catskill?). 

 Cuyahoga shale (equivalent of part of Pocono sandstone). 

 Logan conglomerate (equivalent of part of Pocono sandstone). 

 Pottsville, or Conglomerate Coal Measures. 

 Allegheny, or Lower Productive Coal Measures. 

 Conemaugh, or Lower Barren Coal Measures. 

 Monongahela, or Upper Productive Coal Measures. 

 Dunkard beds (Permian?). 



Trenton limestone and Utica shale. — The two lower members of the above series, 

 although clearly represented in a large part of this region beneath later ro.ck 



Carboniferous . 



