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.

Abstract One of the most significant developments in recent years in South Texas has been the exploration for and exploitation of gas and oil in the deep Edwards Limestone of Comanchean age. Although the possibility of production from the deep Edwards was suggested by a blowout in 1941 northeast of the San Marcos arch in the shallow Edwards fields in the Darst Creek-Luling area, and by development of Edwards production along the Jourdanton-Pleasanton trend, it was not until 1953 that production was established in the San Miguel pool, McMullen County. The discovery of the huge Fashing Edwards gas and oil pool in October 1956 accelerated exploration for and development of deep Edwards production. At the end of 1964, 47 deep and intermediate-depth Edwards pools had been discovered in a belt more than 200 mi long, extending from central Lavaca County south- westward to western Webb County on the Rio Grande. Most of the production is gas or gas condensate ; several pools produce oil, but generally have high gas/oil ratios. Edwards Limestone production is found in both a reef and back-reef facies of coquina-like fossiliferous limestone and dolomotized limestone. Porosity and permeability are markedly varied. Part of the accumulation in the northeastern part of the productive area is controlled structurally by closures against up-to-the-coast faults along the Charlotte-Sample and deep Edwards fault trends. Southward and southwestward, along the Stuart City reef trend, accumulation is controlled primarily by reefs. Three fields appear to produce from traps resulting from local deep-seated salt domes. Gas is marketed readily where found near a pipeline, but several smaller accumulations still are unconnected. Some of the gas contains enough hydrogen sulfide to warrant sulfur extraction. Possibilities for additional deep Edwards and pre-Edwards Cretaceous and Jurassic discoveries appear to be good, but exploration and development will be slow because of depth and the expense of finding production and drilling and completing wells.