A petrographic study was made of the rocks near the boundary of the lower Pennsylvanian Quadrant and the Permian Phosphoria formations. The mineralogy is described in detail and some of the detrital minerals, quartz, chert, tourmaline, and zircon, are subdivided into a variety of species. The chemical minerals described in detail are chert, calcite, dolomite, collophane, kaolinite, and illite. The Quadrant in this area is composed largely of fine-grained siltstones (dolomitic, illitic, and kaolinitic) and carbonate rocks. There is little significant difference in the detrital minerals of the two formations. However, the Phosphoria contains a much larger percentage of chemical rocks than the Quadrant. The detritus of the two formations was primarily derived by the reworking of older sediments. The minerals of the two formations, in this basin-edge area, comprise a quartzite-limestone series and are typical of a period of near peneplaination in the source area and belong in the shallow water marine facies of sedimentation. It is indicative of epeirogenic tectonic control both in the source area and in the locus of deposition. The rocks in the Phosphoria are modified by the deposition of an abundance of authigenic minerals and a decrease in detrital minerals. This modification was likely caused by an abundance of volcanic material in the source area.