Detailed clay mineralogical analyses were conducted on shales and mudstones of the Kope and Fairview Formations in the Upper Ordovician standard of the Cincinnati area. X-ray-diffraction, electron-microscopy, cation-exchange, and DTA studies reveal a suite composed mainly of illite, chlorite, and lesser amounts of illite-montmorillonite and vermiculite.

Clay-mineral ratios show consistent changes approximately 3-4 ft above the presently accepted and mappable boundary between the Kope and Fairview Formations. Increases in combined chlorite-vermiculite amount to about 15 percent and are accompanied by a general increase in the proportion of limestone in the section.

In addition, cation-exchange and X-ray data reveal up to 3 percent vermiculite in the upper part of the Fairview, compared with 0-1 percent in the lower part. These clays probably represent degraded flakes of chlorite due to weathering in the source area.

Paleocurrent measurements reported by Hofmann (1966) show a dominantly eastern source direction for the Kope carbonates and a dominantly northern source for the Fairview carbonates. Differences in clay-mineral constituents in the associated shales and mudstones appear to correspond to these data and may indicate differences in source-area lithologic characteristics in those directions. Further, it is not unlikely that some volcanic influence may account for variations in mixed-layer illite-montmorillonite.

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