PAGE Tourmaline crystals, growing amid feldspar crystals in a cavity in granite, from Paris, Me. 279 Plate 1.—Basal forms of the isometric system 311 Plate 2.—Basal forms of the tetragonal system. Basal forms of the orthorhombic system 312 Plate 3.—Basal forms of the monoclinic system. A cross section of the prism with its edges beveled so that a six-sided prism is formed (pseudo-hexagonal). Basal form of the triclinic system. 313 Plate 4.—Basal forms of the hexagonal system 314 Plate 5.—Gold in quartz from California (_in color_) 280 Plate 6.—Native silver in calcite. Argentite, the black masses throughout the white quartz (_in color_) 281 Plate 7.—Pyrargyrite as it appears after moderate exposure to the light; streak at left. Crystal form of pyrargyrite. Prousite as it appears after moderate exposure to the light; streak at left (_in color_) 282 Plate 8.—Native copper from Michigan. Chalcopyrite in tetrahedrons and an occasional octahedron; streak to the left (_in color_) 283 Plate 9.—Chalcocite crystals with the bluish tarnish. Tetrahedrite crystals; streak to left (_in color_) 284 Plate 10.—Tetrahedrons showing characteristic manner in which tetrahedrite occurs. A cube with the edges beveled and the corners cut in a form characteristic of cuprite 315 Plate 11.—Cuprite, the red crystals showing characteristic color, others showing the green tarnish of malachite. Malachite (green) and azurite (blue), the two minerals shown together as they very commonly occur (_in color_) 285 Plate 12.—Limonite. The crystal form in which goethite is found (_in color_) 286 Plate 13.—Hematite. Clinton iron ore, oolitic. Siderite crystals (_in color_) 287 Plate 14.—Crystal forms of hematite. A typical crystal of magnetite. The rhombohedron typical of siderite 317 Plate 15.—Pyrite crystals. Marcasite in concretionary form with radiate structure (_in color_) 288 Plate 16.—The pyritohedron. The pyritohedron with certain of its edges beveled by the cube faces, to show the relationship of these two forms 318 Plate 17.—Galena in crystals. Pyromorphite crystals (Green) (_in