ROCK-SALT is common salt (chloride of sodium) occurring as a mineral and in a solid .form. It is always mixed with various impurities. It is found massive or crystallized, its crystals generally cubes, its masses very often either granular or fibrous. It is white, gray, or, owing to the presence of impurities, more rarely red, violet, blue, or striped. For its chemical and other qualities, see SALT. It is a very extensively-diffused mineral, and in some places forms great rock and even mountain masses. A hill of rock-salt near Montserrat, in Spain, is 500 ft. high. The island of Ormuz, in the Persian gulf, is formed of rock-salt. The Indus, in the upper part of its course, forces its way through hills of rock-salt, rising in cliffs 100 ft. above the river. In many parts of the world rock-salt is found in beds under the soil or other rocks. Those of Cheshire in England are particularly celebrated, as at present yielding almost all the salt used in Britain, great .part of which is pumped from them in the form of brine. Part is also obtained by
mining, as at Northwich. The mines of Wieliczka in Poland are of great extent. The workings are at depths varying from 200 to 740 ft., and the salt at the deepest working is the purest. Some of the chambers in the mines are said to be 300 ft. high. Blasting by gunpowder is often necessary in the mining operations. The mines give employment to 1200 or 1400 workmen; and they have been wrought for centuries. Vast quantities of rock-salt occur in many parts of Asia, Africa, and America. In Caramania and Arabia, rock-salt is sometimes used for building houses, the dryness of the climate rendering its solubility unimportant.—The salt which crystallizes on the margins and bottoms of salt lakes may be regarded as a variety of rock-salt. Concerning the salt of the ocean, the salt found in many desert regions as an effervescence on the ground or on rocks, the salt with which sandstone and other rocks are impregnated, etc., see SALT.