[Illustration: Crystal of Spinel] Crystal forms in which dolomite occurs [Illustration: _A_ the cleavage form, rhombohedron with the faces curved] [Illustration: _B_ the rhombohedron with the corners cut, as it often occurs] [Illustration: _C_ the form found in gypsum or anhydrite] Plate 34 [Illustration: Orthoclase, a cleavage piece, _a_ and _b_ the perfect cleavage planes, and _c_ the imperfect cleavage plane] Crystal forms of orthoclase [Illustration: _A_ the simple crystal] [Illustration: _B_ the twinned form] [Illustration: _C_ the twinned form in which the crystals are intergrowing] [Illustration: Diagram of a multiple twin of a plagioclase feldspar] Plate 37 Diagrams of amphibole crystals [Illustration: _A_ a typical crystal] [Illustration: _B_ cross section showing the intersection of cleavage planes] [Illustration: _C_ and _D_ cross sections to show variations in outline] [Illustration: Tremolite in silky fibrous crystals. Asbestos] [Illustration: Hornblende crystals in quartzite] Plate 42 [Illustration: Epidote crystals] [Illustration: Typical forms of epidote crystals; _p_ prism faces, _m_, _n_, _x_, and _y_ beveled edges of the prism, _o_ octahedral faces] Typical forms of tourmaline [Illustration: _A_ side view; _B_ and _C_ ends to show terminations; _p_ prism faces, _m_ beveling of prism edges, _r_ a low rhombohedron on the end, _s_ the opposite rhombohedron, _b_ basal face, and the other faces represent bevelings] Plate 48 [Illustration: A group of barite crystals] [Illustration: Outline of the typical tabular barite crystal] [Illustration: The six-sided double pyramid, composed of three interpenetrating crystals, typical of witherite and strontianite] Plate 44 [Illustration: The typical form of analcite] [Illustration: A typical natrolite crystal] [Illustration: The typical crystal form of stilbite] [Illustration: A sheaf-like bundle of fibrous crystals, typical of stilbite] Plate 45 [Illustration: A group of calcite crystals] Typical forms of calcite [Illustration: _A_ the rhombohedron formed by cleavage] [Illustration: _B_ a rhombohedral crystal truncated by the basal plane] [Illustration: _C_ the scalenohedron] [Illustration: _D_ the scalenohedron truncated by the rhombohedron] [Illustration: _E_ the scalenohedron on a prism] Plate 46 Typical forms of aragonite [Illustration: _A_ the simple crystal] [Illustration: _B_ a needle-like form, twinned] [Illustration: _C_ cross section to show how the form may appear six-sided] [Illustration: Typical form of the anhydrite crystal] Plate 47