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Holocene reefs in Bermuda, called "boilers," are presently being cemented by fibrous and fine-grained "mud textured" magnesium calcite to produce a rock with low porosity and permeability. As much as 50% of the resulting rock consists of cement and internal sediment. Internal composition varies from pelleted ostracod- and foram-rich sediment to oolites and pisolites. Such sediments occur in vugs measuring from a few millimeters to caves up to more than 1 m across. These submarine caves are formed by overgrowth and sheetlike layers of coralline algae and the hydrozoan Millepora sp. (constructional formation), and by organic enlargement of cave walls by boring organisms (destructional formation). Such caves could be mistaken easily for subaerially formed caves, and the inte nal oolites and pisolites might be mistaken for freshwater cave pearls.
Syncementation fractures up to 1.5 m in width crosscut one of the "boiler" reefs. Fractures are believed to result from instability due to organic growth which commonly produces overhanging ledges along the margins of these reefs.
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