Early Tertiary sediments in the Gulf Coastal province contain numerous glauconitic zones. Studies of the characteristic types of glauconitic sediments in the field and laboratory disclose that they were generally deposited as fossiliferous sands, silts, and clays. Secondary textures and minerals indicate that some glauconitic clastic sediments were altered to marlstones and limestones after compaction. The larger size of glauconite grains relative to associated detritals, the specific gravity, the rarity of broken grains, the organic shapes of pellets, and the altered mud matrices indicate that most glauconite forms early, after little or no transportation. In aggregates of glauconite the sharp contact between pellets and matrix suggests 2 stages of glauconitization. Multiple stages of glauconitization are also indicated by cryptocrystalline glauconite pellets having ordered glauconitic rims. Micas, feldspars, and quartz replaced by glauconite indicate possibly later glauconization. Forms resulting from colloidal deposition may have developed at any time during and after sedimentation of the glauconitic beds. All evidence accumulated suggests that glauconitization may occur at different times during petrogenesis. X-ray analyses performed in connection with this study support Burst's conclusions that glauconite may be Fe-rich clays of the illite-, montmorillonite-, kaolinite-, or chlorite-types. On the other hand, no correlation of glauconitic clay-types with stratigraphic units or depositional environments is apparent. Total evidence developed from this study suggests that the problem of glauconite genesis will be solved by studying the physical-chemical properties of modern sediments.