282 PITCHSTONE FAMILY. It occurs massive, vesicular; and the vesicles are some times round, and sometimes so much elongated, that the mass appears fibrous. Its lustre is shining and pearly. Its fracture, on account of the thinness of the distinct concretions, is hardly observable, but appears to be small and imperfect conchoidal. The fragments are angular and blunt-edged. It occurs in large and coarse angulo-granular distinct concretions, that include'small and round granular con cretions, which are again composed of very thin concen tric lamellar concretions. The surface of the concretions, particularly in the large and coarse granular, is smooth, shining, and pearly, and has a striking resemblance to that of pearl. In the centre of these concretions, we frequently meet with roundish balls of obsidian. It is translucent on the edges, sometimes even trans lucent. It is uncommonly easily frangible. It is soft, passing into very soft; and Is rather heavy ; approaching to light. Specific gravity, Mexican, 2.254, Vauquelin. Hunga rian, 2.340, Klaproth. Hungarian, 2.343, Hoff, Chemical Characters. Before the blowpipe it intumesces very much, and i* converted into a white spumaceous glass. - Constituent