[Illustration: A cube with the edges beveled and the corners cut in a form characteristic of cuprite] Plate 30 [Illustration: Two intergrowing or twinned quartz crystals] [Illustration: Diagram of the typical quartz crystal, _p_ prism faces, _l_ left hand rhombohedron, _r_ right hand rhombohedron] [Illustration: A quartz crystal on which the left hand rhombohedron is represented by small faces while the right hand rhombohedron has large faces] Plate 14 [Illustration: Crystal forms of hematite, _A_ the rhombohedron with the edges beveled; _B_ the tabular form, resulting from the excessive development of the two _o_ faces opposite each other] [Illustration: A typical crystal of magnetite] [Illustration: The rhombohedron typical of siderite] Plate 16 [Illustration: The pyritohedron] [Illustration: The pyritohedron with certain of its edges beveled by the cube faces, to show the relationship of these two forms] Plate 18 Typical forms for cerrusite [Illustration: The pyramid, _n_ the prism face, _m_ the beveled prism, _p_ the octahedral face, and _o_ the edge of the octahedral faces beveled] [Illustration: The simple type of twinning] [Illustration: A multiple twin where three crystals grow through each other] [Illustration: Forms in which anglesite occurs: _l_ the pyramid face, _p_ the prism face, _o_ the vertical edge of the prism beveled, _m_ the horizontal edge of the prism beveled, _n_ a further beveling of the horizontal edge of the prism. _D_ the tabular, _E_ the prismatic form] Plate 20 [Illustration: A characteristic form in which sphalerite may occur; being the combination of, _d_ the dodecahedron, _o_ the octahedron, and _t_, a 24-sided figure] [Illustration: Characteristic form for zincite crystals, _n_ the hexagonal prism, and _p_ pyramidal faces on it] [Illustration: Typical form of crystal of willemite: _p_ the prism, _r_ rhombohedron faces on end, ½ _r_ a second lower rhombohedron] Plate 22 [Illustration: Moss agates, showing the dendritic growth of manganitic minerals, like manganite or pyrolusite] [Illustration: Moss agates] [Illustration: Crystal form of manganite] Plate 23 [Illustration: Crystals of green corundum in syenite, from Montana] [Illustration: Typical crystal forms of corundum: _A_ the elongated prism with the alternate corners cut by rhombohedral faces, _B_ the tabular prism, _C_ the double pyramid] Plate 28 [Illustration: Cassiterite, twinned crystals] [Illustration: The crystal form in which both cassiterite and rutile occur when in simple crystals, _p_ prism faces, _m_ beveling of the prism, _o_ octahedral face, _n_ beveling of the edge between octahedral faces] [Illustration: Multiple twinning characteristic of rutile] Plate 29