A thorough mineralogical, chemical, and textural analysis of the Cincinnati Series shales was made by X-ray diffraction, X-ray spectrochemical, and gravitational methods to investigate the problem of the Ordovician existence of the Cincinnati arch. A summary of parameters is given for specimens collected laterally from three horizons in Ohio, Indiana, and Kentucky outcrops. The writer feels that the arch as a definite paleogeographic element should not be assigned to an age earlier than Silurian.

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