The Curtis Formation, a lithostratigraphic unit of the San Rafael Group of east-central Utah, was deposited mainly by tide- and wave-currents in a nearshore shelf depositional system during Late Jurassic time. In the lowermost 6 to 8 meters (20 to 26 feet) of the Curtis lies a mudstone facies, for which an analysis for composition and origin of clay minerals has not been done. A week was spent systemically collecting samples at four locations. Three are along the western San Rafael Swell (Sid and Charley, Horn Silver Gulch, and South Sand Bench at I-70), and one is on the east side of the Swell (San Rafael Swell East). Samples were collected at half meter intervals from just above the J-3 regional unconformity on the underlying Entrada Sandstone up to where the facies become silty and sandy. Because the samples are cemented with calcium carbonate, they were disaggregated with acetic acid to yield an insoluble residue of less than 4 μm (0.004 mm) silt- and clay-sized particles. Silt-sized grains separated from clay-sized grains in a settling tube over a two-hour period. The clay-liquid suspension was drawn with a syphon tube, smeared on glass slides, and allowed to dry. Each of the clay smears was then further prepared for four different analyses on an X-ray diffractometer: 1) untreated, 2) glycolated with ethylene glycol, 3) heat treated to 4000 C, and 4) heat treated to 550o C. Substances identified are the clay minerals smectite, illite, and chlorite, plus the non-clay minerals quartz and the zeolite phillipsite. The 3 clay minerals are detrital and were formed by chemical weathering of minerals in a parent rock. Sparse andesitic rock fragments in conglomeratic units within the mudstone facies together with the presence of the zeolite mineral phillipsite suggest that smectite probably formed from the chemical weathering of volcanic materials. Illite and chlorite clays could not have originated at the expense of smectite during burial diagenesis because the relative abundance of smectite increases while illite decreases upward stratigraphically, and chlorite abundance is uniform throughout the mudstone facies. Chlorite clays probably formed by the of Illite to smectite as weathering conditions shifted from cooler and drier to warmer and humid over time weathering of minerals in either volcanic or metamorphic rocks. The decrease in the illite to smectite ratio from the base to top of the mudstone facies can be explained by the chemical alteration.