Form of Minerals Feuchtwanger: Treatise on Gems Page 39



	FORM OF MINERALS.	39

	lateral angles again replaced by another more acute pyramid of the second kind, so that its sign is In fig. 33 a prism of the second kind is first bounded by



	the fundamental pyramid, and then has its edges of combination replaced by a ditetragonal pyramid, and its sign is here In fig. 34 the polar edges of the pyramid are replaced by another pyramid, its sign being P.In fig. 35 a hemihedric form very characteristic of copper pyrites is represented, Ρ and P' being the two sphenoids, a the basal pinacoid, and b, c, two ditetragonl pyramids. III. The Hexagonal System.—The essential character of this system is, that it has four axes,—three equal lateral axes intersecting each other in one plane at 60°, and one principal axis at right angles to them. The extremities of the principal axis are named poles, and sections through it and one lateral axis, normal chief sections. The plane through the lateral axes is the basis, and from its hexagonal form gives the name to the system. As in the last system, its forms are either closed or open : and are divided into holohedral,