DOLOMITE.
Dolomite is, chemically, composed of the carbonates of lime and magnesia in about equal proportions. It is found as rhombohedral crystals, the faces of which are often curved; also in granular and massive conditions. Its prevailing colour is light yellow both in crystals and rock masses, but, as with most other minerals, impurities occasionally make it assume other tints, principally red and green. Carbonate of iron is frequently present, sometimes to such an extent as to entirely alter the character of the substance. As separate crystals dolomite has very little interest for us, though rarely it may take the place of calcite or aragonite in the fossils of brick-earths and clays. But in its massive condition, as magnesian limestone, it is of increasing importance to the brickmaker. For many years it has been utilised in the manufacture of basic bricks, though at the present moment the market in these materials is attentively looking at the possibilities of the next mineral to be described.