Reeflike structures similar to the knoll-reefs of north England are described from lower Carboniferous (Glencar and Dartry) limestone localities in the Benbulbin mountain region, northwest Ireland. In most instances they are elliptical mounds, elongated parallel to inferred current direction, and are composed of sloping concentric layers of very fine-grained massive or bedded limestone. The original sediment was a clotted calcite mud; the organic component, chiefly bryozoan remains, makes up only a small part, and remains of true reef-building organisms are absent. Patches of coarsely crystalline calcite found in both the massive and bedded reef facies are interpreted as fillings of a network of cavities developed in early stages of diagenesis, probably as a result of sediment creep or slumping on the reef slopes. Dolomite is present as a clastic component, as primary growths in cavities, and as metasomatic replacements of calcite. The reefs appear to be mainly inorganic in origin,

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