Need for more extensive application of thin-section petrography to study of sediments, especially rock phosphates, is emphasized. The petrographic microscope alone is inadequate for investigation of these rocks, and often it will be necessary for the petrographer to employ X-ray diffraction, qualitative microchemistry, and other special methods. The mineralogy of rock phosphates is discussed in light of present limitations of knowledge of these minerals, and data are furnished on thirty-eight minerals that can occur in these rocks. A list of recently discredited species is also furnished, and several textures and microstructures are illustrated. The geochemical and mineralogical data on rock phosphates are regrettably meager. Theories on the origin of some of the primary, sedimentary rock phosphates cannot progress beyond the realm of speculation until the conditions which bring about precipitation of varieties of apatite from sea water have been more thoroughly investigated by laboratory techniques. Data...