GEOLOGICAL SCALE AND STRUCTURE. 19 



In chemical composition the Corniferous limestone is easily distin- 

 guished from all that underlie it. It is much less magnesian than the 

 other members of the Cliff limestone of Ohio,, already described. It is 

 never a true dolomite in composition, as the Waterlime and Niagara 

 limestones almost always are. The carbonate of magnesia ranges in it 

 from two to thirty-five per cent., reaching the latter figure in but few 

 cases. The composition of the typical, heavy-bedded lower Corniferous 

 may be taken as seventy per cent, carbonate of lime and twenty-five per 

 cent, carbonate of magnesia. The higher beds of the Columbus stone, 

 regularly yield ninety-one to ninety-five per cent, of carbonate of lime. 

 The upper division, or the Delaware stone, is much less pure in central 

 Ohio than the lower, a notable percentage of iron and alumina, as well 

 as silica, generally being contained in it. It is therefore, seldom or 

 never burned into lime. In northern Ohio, on the contrary, it is often 

 found a fairfy pure limestone. 



Both divisions, but particularly the lower one, carry occasional 

 courses of chert, that detract from the value of the beds in which they 

 occur. The chert is found in nodules which are easily detached 

 from the limestone for the most part. In some conditions in which the 

 chert occurs, fossils are found in it in a remarkably good state of preser- 

 vation. The percentage of chert and flint in any section would be con- 

 siderable, and this fact must be borne in mind in the analysis of drillings 

 from wells that penetrate the formation. The beds of the lower division 

 are prevailingly light-colored, ranging from whitish to gray, drab and 

 brown. The upper beds are oftener blue than otherwise. 



The beds of the lower division are, as a rule, much thicker than those 

 of the upper. The lowermost courses are sometimes quite massive. In 

 the state quarries the thickness of these courses is not less than five feet- 

 In the upper division the thickness of the several courses seldom reaches 

 one foot. 



Throughout the entire formation Devonian fossils abound in great 

 variety and in great numbers of individuals. They are often found in an 

 excellent state of preservation. The oldest vertabrate remains of the Ohio 

 rooks are found in the Corniferous limestone, a fact which gives special 

 interest to it. The uppermost beds of the lower or Columbus division is, 

 in many places, a genuine " bone bed "; the teeth and plates and spines of 

 ancient fishes, largely of the nearly extinct family of ganoids, constitut- 

 ing a considerable portion of the substance of the rock. Corals of vari- 

 ous types are also especially abundant and interesting in this limestone- 

 In fact, the formation is the most prolific in life of any in the Ohio scale. 

 At a few points in central Ohio, the upper division is found in a shaly 

 state and carrying characteristic fossils of the Marcellus slate. This fact 

 was first noticed in its true significance by Professor Whitfield. 



