in natural history, the name of a genus of spars, thus called, because regularly of a paralleloped form.

They are pellucid crystalline spars externally of a determinate and regular figure, always found loose, detached, and separate from all other bodies, and in form of an oblique paralleloped, with six parallelogram sides and eight solid angles, easily fissile either in an horizontal or perpendicular direction, being composed of numbers of thin plates, and those of very elegantly and regularly arranged bodies, each of the same form with the whole mass, except that they are thinner in proportion to their horizontal planes; and naturally fall into these and no other figures, on being broken with a slight blow.