Salina is the formation from which the salt is obtained at Syracuse. In northern Ohio it overlies the Niagara, and contains the gypsum of Sandusky. This deposition marks the era of a retiring sea, which left a series of shallow basins that became great evaporating pans.
Helderburg group is the surface rock of a large area in Ohio, and forms a summit of the Upper Silurian, and completes a circle of sedimentary formation corresponding, in a way, with that of the Lower Silurian.
The Trenton groups are nearly pure carbonate of lime, while those of the Niagara series—Clinton, Niagara, and Waterlime—are highly magnesian.
The Devonian age contains many strange forms of ancient life. In the Mississippi Valley, the Devonian strata are mostly calcareous, and much thinner than in New York and Pennsylvania, showing plainly that here, as in eastern Canada, open sea prevailed during this age, and that the Cincinnati Arch formed a land surface probably throughout all the Devonian ages. The Devonian system comprise:
Oriskany sandstones.
Corniferous limestone.—An open sea deposit. The average thickness in Ohio is 100 feet, and forms two belts of outcrop on opposite sides of the Cincinnati arch. The rock contains 20 per cent. of magnesia. Fragments of land plants and limbs of trees are found in this group.
Hamilton group.—A soft, blue limestone in Ohio.
Huron shale—exhibits a prevailing black color, and contains 10 per cent. of combustible matter. The line of its outcrop is marked by oil and gas springs. It is exposed in Kentucky and Tennessee, on both sides of Cincinnati anticlinal. It contains a large amount of carbon, derived from sea-weeds.
Erie shales is the name given to the Huron shale in northern Ohio and Lake Erie.
The carboniferous system is the highest group of rocks found in Ohio, and holds nearly all the beds of coal. As this period is not relative to Hamilton County, we shall only briefly refer to it.