The minerals of the mica family proper are especially distinguished by their one very perfect basal cleavage, which is developed to a degree not known in any other mineral group. This cleavage affords remarkably thin laminae, each plate of which is flexible and usually elastic, and exhibits a lustre that is generally splendid in degree and metalloidal to pearly in kind. Although a few fairly complete crystals have been found, the micas, as a group, cannot be said to crystallise in any one crystallographic system. The chemical composition of the various members of the family also differs very considerably. Speaking in general terms, they may be considered to be complex silicates of alumina and the alkalis, potash or soda, with or without lithia, or the alkaline earth, magnesia (and rarely baryta or lime), or a combination of silica with the whole three, viz., alumina, alkali, and alkaline earth. Iron may likewise be looked upon as a constant constituent, which, in some species, attains a position of the first importance. In the analyses of the six typical varieties of anhydrous mica given below it will be noticed how fairly constant the percentage of alkali is (potash), while quantitatively the other ingredients vary very considerably. The typical hydrous micas have the same general composition as the above, but contain more than 4 per cent of water. The specific gravity of the micas usually ranges from about 2·7 to about 3, and their hardness from 2 to 3, according to Moh's scale. The