Three varieties of limestone are distinguished: (1) microcrystalline or granular, more than 50 percent of the rock granular; (2) crystalline, composed mainly of sutured anhedra; (3) fragmental, consists chiefly of calcite fragments, quartz grains, and organic remains. Limestones especially susceptible to metasomatic replacement include the medium-bedded crystalline and fragmental varieties. Thin sections and photomicrographs show that diagenesis of various constituents follows a definite sequence: (1) for carbonates, aragonite or calcite to calcite to dolomite; (2) for silica, opal to chalcedony to quartz. Metasomatic materials are derived in part from associated sediments and in part from the host rocks.

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