SPARRY GYPSUM OR SELENITE. 243 Yellowish-white, it passes into wax, honey, and ochre yellow, and clove and yellowish brown ; and it is some times brownish-black. It occurs massive, coarsely disseminated; and crys tallised in the following figures: 1 • Six-sided prism *, generally broad, and oblique an gular, with two opposite broad, and four smaller lateral planes; or with tw-o opposite very small, and four broader planes ; or with alternate broader and narrower lateral planes: the terminal planes or faces are conical or spherical convex, or obtusely bevelled, and the bevelling planes set on obliquely, but parallelly on the broader lateral planes ; or acuminated with four planes, which are set on the smaller lateral planes J. 2. Lens. 3. Twin-crystals. These are either formed by two lenses, which are attached by their faces, or by two six-sided prisms pushed into each other in the direction of their breadth, in such a manner that the united summits at one extremity form a re-entering angle, but at the other a salient angle, or four-planed acumination. When two such twin- crystals are pushed into each other in the direction of their length, a ^ Quadruple crystal is formed. Q 2 Thc * The primitive figure, according to Ilauy, is a four-sided prism, whoso **** are oblique parallelograms, with angles of 113° 7^48"and 66® 1- t f Chaux sulphatec trapezienne, Ilauy. UImni* sulphatec equivalcnte, Ilairw
