The deposits of the Devonian period over a large area in the interior of North America to the south of the great lakes are known to be wholly of marine type and to have continued those of the Silurian period. They were formed in a great gulf open to the south. Along the western border of this gulf shore-deposits and, during Upper Devonian times, deposits of Old Red Sandstone type were accumulated, while in the middle of the gulf the resulting rocks were limestones and shales. In Ohio, following on a narrow band of what is regarded as Oriskany Sandstone (Lower Devonian), the Corniferous limestone and some local representatives of the Hamilton formation represent the Middle Devonian. Above this comes a great mass of black shale, which here represents the whole Upper Devonian and may continue up into black shales of the Lower Carboniferous. A black shale at the base of the Upper Devonian rocks has an extensive range in the central region of North America, being represented by the Huron shale in Canada and the Genessee shale in New York. Drifted land plants from the coast of the gulf, or from islands in it, have been found in the black shale and also in the underlying Corniferous limestone, and some other fossils are commonly spoken of as Algæ but have afforded little or no botanical information.