Nearly all literature relating to flint clays of Pennsylvanian age in the eastern United States concerns deposits occurring in the lower part of the Pennsylvanian System. This report describes the geologic occurrence and the physical and mineralogical features of flint clay deposits of middle and late Pennsylvanian age in the eastern United States, many of which have not previously been described as flint clay. The brecciated texture, conchoidal fracture, and variegated color--including shades of yellow, brown, gray, green, and red--of these clays make them distinctive. They are composed primarily of well crystallized kaolinite. The flint clays are often closely associated with coals, and at several localities they occur in channels or lenses where the normal continuity of the coal bed has been interrupted by erosion or nondeposition. The flint clay is believed to have formed by alteration of ordinary fine-grained Pennsylvanian sediments in acid swamps in a manner similar to that recently proposed by Patterson and Hosterman (1960) to explain the formation of early Pennsylvanian flint clay in the Olive Hill district, Kentucky.