284. GEOLOGY OF OHIO. 
The black slate also appears in the bed of the river about a mile below 
Cole’s, on the land of David Dix and Henry Main, and in the bed of Horse- 
shoe Run, in the south-east part of Troy township. 
In No. 1, above, are large concretions of hard, black limestone, which 
are horizontally marked with parallel ridges or rings. When broken, 
some of them show small gashes or wafer-like cavities. These concre- 
tions are common near the base of Huron shale, and are often quite 
round, though they more usually show the form illustrated by the figure 
below. 
CONCRETION IN THE BLACK SLATE, DELAWARE County, THREE AND A HALF FEET IN DIAMETER. 
Besides those black concretions, there are some irregular calcareous 
masses that are blue and very hard. Thesecalcareous masses in the lower 
portion of the Huron seem to indicate the waning of that condition of 
the ocean that deposited the Hamilton. 
At Delaware, a quarter of a mile below the railroad bridge over the 
Olentangy, the Huron shale appears in the left bank of the river, under- 
lain by the shale which has been regarded the equivalent of the Hamil- 
ton. There are no fossils in this underlying shale at Delaware proving 
its Hamilton age, and it will be referred to in the following pages, to 
avoid a possible misuse of terms, as the Olentangy shale. The slate is 
of its usual thin beds, with some calcareous layers, which are black, and 
about half an inch thick, hardly distinguishable from the slate itself. 
Here are also the round, calcareous concretions, technically called septaria, 
common to the lower portion of the black slate. The line of contact 
of the slate with the shale underlying is quite conspicuous at some dis- 
tance from the bluff, the shale weathering out faster, allowing the tough 
beds of slate to project. 
