OUTLINE OF ROCK FORMATIONS. 55 



limestone.-' Beneath this is the Rochester shale/ which includes the Dayton 

 building stone. 



The Niagara group covers in its outcroi^s or as a surface rock not less 

 than 6,000 square miles in western Ohio. The main area of outcrop is 

 immediately north and east of the area of the Hudson River group. But 

 there are extensive outcrops farther north in a belt leading from Hardin 

 County northward to the western end of Lake Erie. In Indiana the group 

 appears as the surface rock in an area of fulh^ 5,000 square miles along 

 the north and west borders of the area of the Hudson River group. There 

 is a small area of outcrop in the northwestern part of the State, but that lies 

 outside the i-egion under discussion. A considerable area in northern 

 Kentucky, contiguous to the areas of Indiana and Ohio, also lies outside 

 this region. 



Salina and Waterlime, or the Onondaga series. TllCSO tWO SOmOwliat UuHke fomia- 



tions have been refeiTed by Hall, Dana, and others to a single geological 

 epoch, the Onondaga. The outcrop of the Salina formation in New York 

 covers a narrow belt leading eastward from Grand Island in Niagara River 

 to the Hudson River Valley. Its thickness in the eastern part of the State 

 is only 100 to 200 feet, but in Onondaga County, in central New York, it 

 reaches a thickness of 800 feet and continues thick from that locality west- 

 ward. Like the Hudson River group, it consists largely of soft rocks, 

 shales, and shaly limestone, with marls and beds or veins of gypsum. As a 

 result of its soft texture the area of outcrop of this formation has become 

 the site of a shallow basin. The Waterlime formation in western New 

 York rests upon the Salina without a perceptible break or line of demarca- 

 tion. It consists of an impux-e silico-argillaceous limestone, which is more 

 enduring than the remainder of the salt group and can be distinctly traced 

 through the district. 



Orton considers it doubtful whether the Salina beds occur in the Ohio 

 rock series. The Waterlime outcrops extensively in northwestern Ohio and 

 also in the north-central part of the State, its outcrops being about as 

 extensive as those of the Niagara group. It is in the main a compact 

 magnesian limestone of drab or brown color. 



' The Lockport limestone and the Rochester shale were formerly called the Niagara limestone 

 and the Niagara shale, but by the usage of the United States Geological Survey the term Niagara is 

 now applied only to the group or higher classiflc unit. 



