Micas are complex silicates of aluminum, with potassium, iron, lithium, magnesium and hydrogen. They are one of the principle components of many granites, gneisses, and schists. This mineral is always crystalline, being in the monoclinic system, but occurring in six-sided prisms. The cleavage is so dominant a character that the crystal form is usually overlooked, as it is seldom requisite in determining this mineral. The size of the sheets of mica depend on the size of the crystals, the larger sheets expressing great slowness in cooling from the original magmas. Sometimes the crystals may be two or even three feet in diameter. The hardness is not great, ranging between 2 and 3. The specific gravity lies between 2.7 and 3.2. The color varies according to the composition, from silvery-white, through gray, pink, and green to black. The luster is vitreous to pearly, sometimes gleaming in the darker-colored varieties. The commoner types of mica are as follows: Muscovite, H₂KAl₃(SiO₄)₃ or potash mica. Lepidolite, LiK(Al₂OH·F)Al(SiO₃)₃ or lithia mica. Biotite, (HK)₂(MgFe)₂Al₂(SiO₄)₃ or iron mica. Phlogopite, H₂KMg₃Al(SiO₈)₃ or magnesia mica. Muscovite is colorless, silvery-white, gray or sometimes pale-green or brown. It gets its name from Moscow where it was early used for window panes, and it is still used for stove and furnace doors, as well as in electric work, for a lubricant, etc. The best crystals occur in granites, in the coarse varieties of which large crystals may be obtained. It is found also as small scales in gneisses and schists, and when weathered from its original rocks it may be present in sandstones and shales. Muscovite is always in its origin an elementary component of deep-seated igneous rocks, like granite; but is never a component of extruded lavas. _Sericite_ is muscovite which has been secondarily produced by the alteration of other minerals into muscovite, as when feldspar, cyanite, topaz, etc., have been modified by the presence of heat and hot vapors, when near lavas that have come in contact with other rocks. Muscovite is very resistant to alteration by weathering, but when it does change, the greater part of it becomes kaolin. It is found at Acworth and Grafton, N. H., in plates, sometimes a yard across at Paris, Me., Chesterfield and Goshen, Mass., Portland and Middletown, Conn., at Warwick, Edenville, etc., N. Y., and all down the Appalachian Mts., also in the Rocky Mts., the Cascade Range, etc. Lepidolite is pink or lilac in color and occurs in scaly masses, mostly in granites. It does not come in large crystals. Lepidolite is found at Paris and Hebron, Me., Middletown, Conn., Pala, Calif., etc.