z 4 2 Siliceous Genus. The forms of lts cryftals are various, the moft ufualisthat of hexangular prifms, furmountcd with hexagonal pyramids on one or both ends the angles of the prifm correfponding with thofe of the pyramid; fometimes the pyramids are joined bafe to bafe, and frequently it exhibits but one pyramid, or a prifm without a pyramid. The prifms are generally feamed perpendicularly to their axis, the pyramids fcarce ever. For other forms I muff refer to Mr. de Lifle. Fragments rounded by rolling in water are alfo frequent. Its fradfure is conchoidal, but frequently fo flatly as to refemble the foliated. Its fragments, 4 or 3. Its hardnefs, n. Its fp. grav. a,653, when colourlefs; that of the red 2,67. 2d Family. Quartz. Its colours are various, white, grey, yellowifh, reddilh, or greemlh white, or yellowifh, bluifh or pearl grey, or yellow, or fwarthy brown or pale or blood red. * Its external luftre cafual, the internal, 3, 2 or I. Its tranfparency, 2,3, rarely 1. ' * Commonly amorphous, frequently cryßallized as mountain cryftal. The prifms have fometimes’ been found crooked. N. A&. Petrop. 1785, p. 265. Cubic cryftals of quartz have alfo been difcovered.