The Niagaran rocks of southwestern Ohio and southeastern Indiana exhibit several lithologic and paleontologic facies on their respective flanks of the Cincinnati Arch. Detailed stratigraphie zoning of these rocks, subsequent laboratory investigation of thin sections, insoluble residues, and rock powders, and chemical analyses of the samples collected show (1) the lower part of the Dayton limestone of Ohio is equivalent to the entire Dayton limestone of Indiana; (2) the Osgood shale of Ohio is equivalent to the upper Osgood shale of Indiana; (3) a portion of the Laurel limestone of Indiana is equivalent to the entire Laurel limestone of Ohio; (4) the Euphemia, Springfield, and Cedarville dolomites are true dolomites, whereas the Dayton and Laurel limestones are dolomitic limestones; and (5) the water in which the Niagaran sediments were deposited appears to have varied in depth between 150 and 500 feet.