634. GEOLOGY OF OHIO. 
gea in which the same general conditions were maintained, while very 
different strata were in process of formation at the east. . The Columbus 
and Delaware limestones probably cover the age in which the Corniferous 
limestone, and the Hamilton group, in part, of New York were forming; 
but there seems no warrant whatever for identifying the subdivisions of 
our scale with the subdivisions recognized flve hundred or a thousand 
miles away. A disturbance of previous conditions took place in this 
interior sea, which is marked by the change from the Columbus lime- 
stone to the Delaware limestone, but any correlation of this change, with 
epochal changes at the eastward, is, so far as the facts appealed to indi- 
cate, entirely arbitrary. 
Nor does it seem necessary to restrict the application of the name 
‘‘Hamilton” to the ten or twelve feet that underlie the Huron shale 
proper. The “ Hudson River age” of the upper limestones of Cincinnati 
is universally recognized, though the most characteristic of Trenton fos- 
sils are found in the same beds. On very. nearly the same grounds we 
can believe that the Hamilton group of New York covers a part of the 
Devonian limestones, as well as a part of the Devonian shales of Ohio. 
The Olentangy shale is shown in but very few sections in this county. 
The best of them is on Slate Run, in Perry township, but north of the 
county line, in the Olentangy Valley, numerous sections are exposed. 
It contains calcareous concretions in considerable numbers. They are 
less regular than the concretions of the Huron shale proper, and are len- 
ticular, rather than globular. Only obscure traces of fossils have been 
noted in its beds, and these were fragments of corals that have no signif- 
icance in marking horizons. 
The Huron shale proper begins with a boundary as definite as a black 
mark on paper. Its lowest layer is as characteristic of the formation as 
any layer in it, and from this point up, with wonderful sameness of com- 
position, layer after layer testifies to conditions of an ancient sea that 
remained unchanged until the floor had been covered with at least five 
hundred feet of fine, and, probably, slowly, accumulating sediments. 
The shale is brownish or blueish black in fresh exposures, but weath- 
ered surfaces have a distinctly blue color. The leaves of the shale are 
quite thin and fragile in all natural sections, but where the rock is 
freshly opened for ten or fifteen feet, the beds havea very solid and substan- 
tial look. They are unable, however, to resist the action of the atmosphere, 
and the solidest portions need but a winter to be turned into crumbling 
fragments that are excellently adapted to the making of sidewalks. The 
last stage in their decomposition is a very tenacious, light-colored clay, 
of which mention will be made again. 
