354 FELSPAR FAMILY. 2. Four-sided prism, in which the lateral edges are rounded *. ,3. Four prisms arranged in the form of a cross "f. These crystals always appear as if they had been at one time hollow, and these hollows filled up with clay-slate, the position of which varies in regard to the crystals, and gives rise to the following varieties: a. In the centre of the crystal there is a small prism of clay-slate, the lateral planes of which are pa rallel with those of the crystal, and from the. angles of this prism black lines run to each angle of the crystal +, fig. 87. b. In this variety there is, in addition to the cen tral prism and black lines of the former, small er black-coloured prisms of clay-slate, one on ^each angle of the crystal, and their lateral planes are parallel with those of the crystal ||> fig 88. c. In this variety the terminal planes of the crystal are marked with black lines, which run from each of the lateral planes, parallel with the adja cent planes, to the black diagonal lines fig. 81L d. A black prism, in which the lateral planes are co vered with a thick or thin crust of the hollosv- spar <![. The black or clay-slate mass is often thickest in the middle, and becomes thinner towards the extremities oi the crystal: in other instances it is thinnest in the middle, and becomes gradually thicker towards the extremities of * Made cylindroide, Hauy. -|- Made quaternre, Ilaiiv. * Made tetragrammc, llauy. || Made ptntarhombique, Hauy. s Made polygramme, Ilam-. • Made drccnscrite, Hauy.