The present study has three objectives: (1) to show that cements form within centimeters of depth below surfaces of reefs and eliminate pore space almost entirely within less than 60 cm below the surfaces of reefs, (2) to describe and illustrate the fabrics and the kinds of minerals of marine cements in Red Sea reefs, and (3) to infer a possible mechanism of precipitation of carbonate cement which is based on observations of strong etching, corrosion, and partial replacement by calcium-carbonate cement of quartz particles in Red Sea reefs. Although seawater is generally undersaturated with respect to silica corrosion and embayment as a result of dissolution, such as observed in the Red Sea reefs, are not normally prevalent in quartz particles lodged on the sea bottom. Thus a mechanism other than undersaturation of silica must explain the partial dissolution of quartz; as carbonate cement replaced quartz particles, carbonate precipitation and quartz solution must proceed simultaneously. These reactions are under the control of pH: a pH level exceeding 9 and even 10 appears to be a necessary condition for the dissolution of quartz and the precipitation of carbonate. Photosynthesis and respiration of the biomass in the reef cause a shift in the carbonate buffer system of seawater with the uptake of CO 2 . Although pH values in excess of 9 can be measured in the waters of reefs, microlevels of pH of 10 and even 10.5 may likely be maintained in thin gellike films or monomolecular layers ("skin effect") that cling to the surfaces of the framework builders of reefs. Such high pH levels may trigger the precipitation of carbonate cement in reefs. The crucial evidence in this study is petrographic (the replacement of quartz particles by carbonate cement); the pH control is inferred.