Abstract: Shales of marine, brackish-water, and nonmarine origin, collected from cyclothems of Pennsylvanian age, were analyzed chemically and mineralogically. Clay mineral identifications were accomplished using x-ray diffraction and differential thermal analysis techniques. Summations of the structure amplitudes of the individual clay minerals formed a basis for semi-quantitative ratios between illite and kaolinite-chlorite. Illite was found to be enriched in the marine shales. The origin of the authigenic clay minerals in the shales is discussed from two aspects. The first and probably the more correct uses the thesis that the ultimate clay mineral is built up from a skeletal aluminum-silicate structure; the second discusses the origin of the clay minerals from a purely colloidal point of view. Trace element determinations, pH values, and pipette analyses are also included as a part of this study.