24 GEOLOGY OF OHIO. 



Newberry first proved, is certainly the equivalent in the general scale of 

 the Genessee slate, the Portage group and the Chemung group, the last 

 named being itself a formation of great thickness and extent in New- 

 York and Pennsylvania. In other words, the shales of our column fill 

 the entire interval between the Hamilton proper and the Catskill group, 

 and in the judgment of some geologists a wider interval even than that 

 named above. As Newberry was the first to show, the oil sands of 

 Pennsylvania are banks of pebble rock that are buri-d in the eastward 

 extension of the Ohio shale, but which make no sign within our own 

 limits. 



But while, definite boundaries for the division proposed can not be 

 laid down or applied within the shale formation, the facts that the top and 

 bottom of the column, on their western out-crops, are prevailingly black, 

 and that the middle of the series is oftener interrupted with light colored 

 beds, are important ones in the history of the formation and deserve to be 

 held in mind. From what has been already stated, it is seen that the com- 

 position and thickness of the shale series depend upon where it is meas- 

 ured, whether on the border of the formation or in the interior of the old 

 sea-basin in which it was formed. On the western border of the shales in 

 southern Ohio, in Highland county, for example, the interval between .the 

 Upper Silurian limestone, on which the shales here rest by overlap, and 

 the Berea grit, is three hundred feet. In Ross county the same interval 

 is nearly four hundred feet. From both of the measurements fifty feet 

 must be deducted for the thickness of the Bedford shale, in order to give 

 the real thickness of the series now under consideration. In the sections 

 named the shales are mainly black, although blue beds are still recogiza- 

 ble in the series. Passing northwards to Crawford county, the series is 

 found about four hundred and fifty feet thick. In Lorain county, at Elyria, 

 it is about nine hundred and fifty feet, and at Cleveland, about one thou- 

 sand three hundred feet thick, while in Tuscarawas county, at Canal Do- 

 ver, the drill descended through one thousand eight hundred and sixty feet 

 of alternating beds of blue and black shale without reaching the bottom 

 of the series, and in the Ohio Valley, at Wellsville, through two thousand 

 six hundred feet of shales, without reaching bottom. In the last two sec- 

 tions the blue shales decidedly preponderate, though the separate black 

 beds can be counted by the score. 



The shales are for the most part poor in fossils, except in those of mi- 

 croscopic size. Banks representing a score or more of feet in vertical 

 column often fail to reward a careful search with a single specimen of ver- 

 tebrate', molluscan or articulate life, and so far as the unaided eye is con- 

 cerned, they are almost equally barren of vegetable remains. Occasion- 

 ally, however, fossiliferous bands are found, the contents of which serve 

 to determine the geological age and equivalence of the portion of the 

 series in which they occur. 



