Abstract Silica occurs in the Edwards Limestone as microcrystalline quartz, chalcedonic quartz, megaquartz, detrital quartz grains, and opal. Microcrystalline quartz with minor chalcedonic quartz makes up chert nodules occurring in zones parallel to bedding planes. Many, but notmost, nodules contain abundant sponge spicules, but examination of 200 insoluble residues failed to reveal spicules in the associated carbonate rocks. It is postulated that spong-es and other organisms concentrated silica from sea water and that after death this silica was dissolved in places and precipitated in others as a function of pH. Persistence of beddingthrough nodules, presence of dolomite rhombs and rhomb ghosts, and perfection of preservation of fossils and carbonate fragments within the nodules are taken as proof of replacement. Cracks in some nodules are filled by the enclosing lime mud. This, together with the relationship with dolomite, shows replacement as a gel. Chert is too hard to break while enclosed in a plastic medium. Megaquartz occurs as irregular masses pervading and replacing shell material. Weathered surfaces often have a thin opal crust, and, in places, the outcrops are replaced by saccharoidal quartz to a depth of several inches. The surficial silicification is a result of weathering, and the source of silica is probably chert within the formation and above the water table.