Consolidation and cementation are effected by finer calcareous material or subsequent crystallization of the carbonate of lime. Limestones are very susceptible to chemical changes and may be replaced by iron oxide or carbonate, yielding ores of great com mercial value. They may be silicified and become cherts, often with the retention of their organic structures. On certain islands in the Pacific (Ocean I. etc.), they become phosphatized from the replacement of carbonate by the phosphates removed by perco lating waters from guano. Such rock-phosphate is utilized for artificial fertilizers.
Limestones are used for a large variety of economic purposes. They are "burnt" for lime, or when argillaceous, hydraulic lime; when intimately mixed with sandy clay and burnt, they yield portland cement; in their purer forms they are widely used in the chemical, glass, soap-making and silicate industries; they serve as fluxes in the preparation of basic steel; and oolitic, dolomitic and ornamental limestones are utilized for building-stones, and less pure varieties for road-stones.
Dolomite-rock, also known as dolomitic or magnesian limestone, consists principally of the mineral dolomite, which is a carbonate of magnesium and calcium, but often contains admixture of other substances such as calcite, quartz, carbonates and oxides of iron, argillaceous material and chert or chalcedony. Dolomites when pure and well crystallized may be snowy white (e.g., some ex amples from the eastern Alps) but are commonly yellow, creamy, brownish or grey from the presence of impurities. Dolomite dis
solves only very slowly in dilute hydrochloric acid in the cold, but readily when the acid is warmed.
Dolomites of compact structure have a higher specific gravity than limestones, but they often have a cavernous or drusy charac ter, the walls of the hollows being lined with small crystals of dolomite having a pearly lustre and rounded faces. They are also slightly harder, and for this reason and on account of their greater resistance to weathering, they last better as building-, paving- and road-stones. Dolomites are rarely fossiliferous, as the process of dolomitization (which may be either pene-contemporaneous with, or subsequent to, the formation of the limestone) tends to destroy any organic remains originally present. Many dolomites, particu larly those of Permian age in the north of England (Sunderland) show remarkable concretionary structures. The beds look as if made up of rounded balls of all sizes. These are composed of radiating calcite crystals.
Dolomites furnish excellent building-stones, such, for example, as those of the north-east of England (Mansfield stone, etc.).
Parts of the houses of parliament at Westminster are built of dolomite. Granular dolomite is also used, on account of its re fractoriness to heat, for lining the hearths of furnaces in which basic steel is prepared. (P. G. H. B.) LIMICOLAE, the shore-birds, plovers, etc. ; the name is now usually superseded by the term Charadriformes (q.v.).