(_a_) HORNBLENDE.—Magnesium mica has extinction about parallel to the cleavage, while hornblende may have extinction angles of from 0° to 20°. Both have strong absorption, but biotite shows very slight pleochroism in basal sections, which also give approximately uniaxial interference figures in convergent light. (_b_) TOURMALINE.—Magnesium mica shreds show absorption parallel to elongation, while in tourmaline the absorption is at right angles to elongation. There is also an absence of cleavage in tourmaline. (_c_) CHLORITE.—By strong double refraction, the very high order colors, however, being often not noticed. Chlorite also shows aggregate structure and is almost always greenish in color. (_d_) TALC.—White mica by large axial angle of scales in convergent light and by micro-chemical tests. The distinction may be very difficult. REMARKS: Muscovite is a rare primary mineral in eruptive rocks, except in two-mica granite, etc. As a secondary mineral it occurs in dense scaly aggregates or as pseudomorphs after feldspar, nephelite, etc. It is frequent in crystalline schists and is probably also the mica in amphibolite and eclogite. Phlogopite is found chiefly in contact metamorphic limestone; and may be distinguished from muscovite by nearly uniaxial character and less sharp cleavage. Biotite is much more widely distributed, occurring especially in eruptive rocks, crystalline schists and contact rocks. Chemical corrosion occurs in original biotite of porphyritic rocks, producing a “resorption border” of augite and magnetite. Mechanical deformations, producing bending and slipping along “gliding” planes (oblique to cleavage), are common to all varieties of mica and may produce change to chlorite. The muscovites, together with the feldspars, are the most characteristic minerals of dynamo metamorphic origin. Biotites and phlogopites are attacked by sulphuric acid at high temperatures. Muscovite is but slightly attacked by acids. H., 2 to 3. Sp. gr., 2.7 to 3.2. The specific gravity separation between the micas is difficult on account of the scaly nature of the minerals. Other micas occur, some being alteration products of those already described. Among these may be mentioned: _Lithia Mica._—Both light and dark colored, occurring in granitic rocks and often only distinguished chemically from muscovite and biotite. _Damourite_ (_Sericite_) (hydrous K mica).—A secondary product usually in colorless, fine scaly aggregates in phillites, sericite-schists, etc. CHLORITE GROUP. Embracing the members of the Chlorite Group, commonly occurring in rocks. ANISOTROPIC. BIAXIAL. MONOCLINIC. The minerals of this group usually appear uniaxial, and crystallize in part with hexagonal symmetry. COMPOSITION: May be considered as isomorphous mixtures of H_{4}(MgFe)_{3}Si_{2}O_{9} and H_{4}(MgFe)_{2}(AlFe)_{2}SiO_{9} (Rosenbusch).