THE CLINTON GROUP OF OHIO.— PART IV. 
BY A. F. FOERSTE. 
CHEMICAL GEOLOGY. 
The Silurian rocks of Ohio open with the Cincinnati Group. 
This group is composed of a series of limestones interbedded with 
shales or clay. Neither the limestones nor the shales are magnesian 
in character, the carbonate of magnesia rarely exceeding six per cent. 
The amount of silica, however, varies greatly. The lower beds of 
limestone contain from ten to twenty-five per cent, of silicious matter; 
there is a corresponding diminution of the amount of the carbonate 
of lime. The upper part of the series, however, contains very little 
silicious matter, usually less than two per cent.; the carbonate of lime, 
however, becomes a much more important constituent, averaging 
ninety per cent. The interbedded shales differ from the limestones 
chiefly in the amount of silica contained, there being a corresponding 
diminution of the carbonate of lime. Thus the amount of silicious 
matter in the shales varies between fifty-five and eighty per cent.; the 
amount of the carbonate of lime, between four and twenty per cent. 
The shales also contain more alumina than the limestones, but -the 
striking difference is evidently the amount of silicious matter contained. 
In one of the lower series of shales belonging to the Cincinnati Group, 
but found in Covington, Kentucky, the amount of silicious matter is 
somewhat less, forty-three per cent., and the amount of the carbonate 
of lime is much greater, forty-seven per cent. Otherwise the chemi- 
