OUTLINE OF ROCK FORMATIONS. 57 



In Ohio a Devonian limestone occurs, which is thought by Orton 

 probably to cover more than the single epoch known as the Corniferous 

 in New York. It forms the surface rock in a narrow strip, 5 to 15 miles in 

 width, extending from Sandusk}' southward past Columbus into north- 

 western Pickaway County, lying immediately east of the outcrop of the 

 Waterlime. It appears in a strip of similar width in northwestern Ohio, on 

 the northwest border of the Waterlime. It also appears on some of the 

 islands in the western end of Lake Erie, and on the elevated tract in Logan 

 and parts of neighboring counties in west-central Ohio. Its thickness 

 seldom exceeds 75 feet. 



In Indiana the Corniferous or Upper Helderberg outcrops at intervals 

 in a narrow belt extending northward from the Ohio River at the Louisville 

 Rapids to Logansport. It is shown by well drillings to immediately 

 underlie the drift in several counties north of the Wabash River in a belt 

 extending from the Ohio-Indiana line westward about to Rensselaer. It 

 is found to present considerable variation in color, composition, and texture 

 in different localities. It is thought by Phinney to range from 30 to 65 

 feet in thickness, but as the data are largely from drillings this estimate of 

 range may be only an approximation. 



Marcellus shale and Hamilton formation. The MarCelluS slialc of NcW York 



occupies a narrow depression along the southern border of the Cornif- 

 erous limestone, from the vicinity of Cayuga Lake westward to the city of 

 Buffalo. It is exposed only in a few valleys, the drift being so heavy as to 

 conceal it elsewhere. The greatest observed thickness in western New 

 York does not exceed 50 feet A portion of it is very bituminous and of 

 black color. This formation is considered by Hall to be a part of the 

 Hamilton group, there being no well-marked line of separation from that 

 group.^ 



The Hamilton group of New York consists of a series of shales with 

 occasional thin beds of limestone, and attains a thickness in central New 

 York of nearly 1,000 feet. Each member of the series thins gradually to 

 the west, until at the border of Lake Erie tlie thickness of the entire group 

 is scarcely 500 feet. The shales are of blue-gray or green color, and thus 

 contrast strikingly with the dark Marcellus shale. The outcrop is confined 

 to a narrow strip, scarcely 10 miles in average width, which extends from the 



'New York Geol. Survey, Fourth Geol. District, 1843, p. 177. 



