A discontinuous layer of lithified carbonate sand underlies a small portion of the Lac, a large lagoon on the southeastern coast of Bonaire. The layer lies thirty-five centimeters below the sediment surface, varies from five to twenty centimeters in thickness and is restricted to an area beneath a broad intertidal and subtidal flat. Beachrock crops out in the high intertidal zone. The lithified layer consists of grainstone cemented by acicular aragonite. Numerous lines of evidence indicate the cementation occurred in the marine environment. The lithified layer is at present continuously saturated with normal sea water. Evidence for earlier dissolution or erosion is lacking. The submarine cemented rocks lack the gray algal coating which is characteristic of beachrock and subaerially exposed coral rubble. Carbon 14 dating of the rock indicates cementation less than 900 years B.P. Study of the constituent particles of the lithified layer and the sediment above and below indicates continuous marine sedimentation. Four distinct types of micrite are present in the beachrock and submarine cemented layer: 1) structureless aragonite which fills or partially fills intraparticle pore space, 2) aragonite with a pellet-like fabric which fills or partially fills interparticle pore space, 3) aragonite coatings on single skeletal fragments with a gradational boundary between the fragment and coating and 4) high- and low-magnesium calcite which coats single and multiple skeletal fragments with, a sharp contact between the grain and micrite. This latter type is the most common form of micrite and might easily be misinterpreted as a cement. However, it can be shown that it results from micritization of encrusting coralline algae. The coralline algal encrustation was high-magnesium calcite. Electron microprobe and X-ray diffraction analysis of these coatings demonstrated that as micritization proceeds and the microstructure of the algal coating is destroyed the mineralogy changes from high- to low-magnesium calcite.