Stratigraphic and petrographic study of 32 sections of the Mercer and Connoquenessing formations [Pennsylvanian] in western Pennsylvania has shown that certain consistent relationships exist between the presence or absence of high-alumina Mercer clay and the stratigraphy and petrography of underlying outcropping sandstones. In sections where flint or diaspore clays are present, the clay is underlain by 0-150 ft. of fine-to-medium-grained, very light-gray lower Connoquenessing sandstone. In sections where plastic clay or silty claystone occur below the Mercer coal, the clay is underlain by 20-150 ft. of fine-grained to conglomeratic, medium-light-gray upper Connoquenessing sandstone. The distribution of the high-alumina clay is believed to be related to ancient topographic highs where flint clay was deposited directly on the lower Connoquenessing sandstone. The upper Connoquenessing sandstone was deposited in adjacent topographic lows, while the flint clay still present on the topographic highs was leached to form diaspore. Areas where the upper Connoquenessing sandstone is thin or absent would be the most favorable regions for further prospecting for high-alumina Mercer clay in western Pennsylvania.