[Illustration: Labradorite, showing multiple twinning (the striation), and the iridescent play of colors] Plate 36 [Illustration: Crystal form of a pyroxene; _a_ and _b_ prism faces, _m_ the beveled edge between two prism faces] [Illustration: Cross section of a pyroxene crystal showing the lines of intersection of the two cleavage planes] [Illustration: Cross sections of pyroxenes, showing typical forms taken by crystals] [Illustration: Augite crystals, in crystalline limestone] Plate 38 [Illustration: The dodecahedron and the 24-sided figure characteristic of garnets] [Illustration: The garnet, grossularite] [Illustration: The garnet alamandite] Plate 39 [Illustration: Beryl of gem quality] [Illustration: Zircon in syenite] Plate 40 [Illustration: Cyanite crystals in schist] [Illustration: A crystal of mica, showing basal cleavage] Plate 41 [Illustration: Crystal form typical of topaz] [Illustration: A topaz crystal from Brazil] [Illustration: Crystal form typical of staurolite when simple] [Illustration: A typical twin of staurolite] Plate 43 [Illustration: Serpentine] [Illustration: Chlorite] Plate 49 [Illustration: Apatite crystals in crystalline calcite] [Illustration: The ends of apatite crystals showing common modes of termination] Plate 50 [Illustration: A group of fluorite crystals] [Illustration: A group of halite crystals] Plate 61 [Illustration: Amber] [Illustration: Two bottles of petroleum, the left hand one with a paraffin base, the right hand one with an asphalt base] Plate 65 [Illustration: Mica schist, with garnets] [Illustration: Chlorite schist] Plate 67 [Illustration: Serpentine, composed of serpentine, hematite, and some calcite] Plate 1 Basal forms of the isometric system [Illustration: Cube] [Illustration: Octahedron] [Illustration: Dodecahedron] Plate 2 Basal forms of the tetragonal system [Illustration: A square prism] [Illustration: Octahedron] Basal forms of the orthorhombic system [Illustration: A Rectangular prism] [Illustration: Octahedron] Plate 3 Basal forms of the monoclinic system [Illustration: The rectangular prism askew] [Illustration: The octahedron] [Illustration: A cross section of the prism with its edges beveled so that the _b_ faces are obliterated by the _m_ faces, and a six-sided prism is formed (pseudo-hexagonal)] [Illustration: Basal form of the triclinic system] Plate 4 Basal forms of the hexagonal system [Illustration: The six-sided prism] [Illustration: The double pyramid] [Illustration: The rhombohedron] Plate 10 [Illustration: Tetrahedrons showing characteristic manner in which tetrahedrite occurs]