Opal is a common constituent in rocks of Paleocene-Eocene, and Miocene age in the Atlantic and eastern Gulf coastal plains. In Georgia and South Carolina the opal occurs as biogenic material, dominantly sponge spicules recrystallized to opal-C; as primary sediment in opaline cherts; as rim cement, or massive cement, in opal-cemented sandstones, siltstones, spiculites, and silicified shell hash; and replaces shell carbonate either pseudomorphically, or epimorphically (new term, defined in text). The rocks described were deposited in shallow marine and estuarine environments, at water depths within the zone of effective wave action, and at normal, or near normal, marine salinites. The associated clay mineral is normally montmorillonite, but there is no evidence that volcanic ash was a major component of the original sediment. Opal is the dominant form of precipitated silica in all of the rocks examined, but chalcedony and microcrystalline quartz occur as rim cement and replace shell carbonates pseudomorphically or epimorphically. Chalcedony also occurs as a late stage pore filling. As a general rule, chalcedony and microcrystalline quartz postdate opal cement or carbonate replacement, but the cement sequence opal-chalcedony-opal does occur. Alternating hands of length-fast and length-slow chalcedony in pore fillings suggest frequent changes in pore water chemistry during late stages of diagenesis. It is undoubtedly the pore water chemistry that determines which form of silica will be deposited, but I have found no evidence relating to the nature of the differences in pore waters. The rocks described have never been deeply buried and the d(101) spacing is immature, ranging from 4.09 to 4.11 Angstroms. The opal is, in some cases, faintly birefringent, reflecting various modes of orientation of cristobalite crystals (radial, linear, or random). Even in the oldest rocks (Paleocene-Eocene) opal shows no evidence of progressive recrystallization to chalcedony or microcrystalline quartz.