A certain amount of magnesium carbonate is present in the skeletons of some marine organisms. This has been shown both by Forchammer and Walther[911]. A foraminifer, _Nubecularia novorossica_, has been found with 26 per cent. of magnesium carbonate, and a serpula with 7·64 per cent.; alcyonarian corals contain up to 9·32 per cent., while calcareous algæ, such as Lithothamnium and Halimeda, contain about 12 per cent.[12]. The magnesium salt is not, however, here combined with calcium carbonate to form the mineral dolomite; none the less it is clear that such organisms introduce magnesium in appreciable quantities into the constitution of marine limestones. Marine limestones are very commonly "dolomitised." Dolomite, the joint carbonate, CaMg(CO_{3})_{2}, contains 54·35 per cent. of calcium carbonate and 45·65 per cent. of magnesium carbonate, or carbon dioxide 47·8, lime 30·4, and magnesia 21·8. Its specific gravity is 2·85. The occurrence of dolomite in intimate association with calcite has been proved by E. W. Skeats[13] in the case of modern coral-reefs, and the secondary deposition of the mineral has been made clear. The skeletons of the corals themselves may now consist of dolomite, while calcite has crystallised in their interstices, or remains as part of the original infilling of mud. The presence of dolomite in reefs has, of course, long been known, having been observed by J. D. Dana in 1849, and it has been realised that, by prolonged alteration, masses of _Dolomite Rock_ become built up[14]. Commonly, the process produces a _Dolomitic Limestone_, in which calcium carbonate is still in excess of the 54 per cent. which is present in the mineral dolomite. The alteration of the original limestone is, however, sufficiently profound. The ready crystallisation of dolomite as rhombohedra destroys the organic structure, and traces of corals or molluscan shells disappear from great thicknesses of rock. It is uncertain whether the process of dolomitisation proceeds most rapidly in the evaporating waters of the lagoons, or, as Pfaff believes, at considerable depths, where the pressure may reach 100 atmospheres. Magnesium carbonate, as we shall note later, may be removed from dolomite in solution under pressure at a greater rate than calcium carbonate. If this occurs in sea-water, it would seem to militate against the production of dolomite in the lower levels of a reef.