ROCK.-BUTTER. 307 It occurs massive, and tuberose. Internally it is strongly glimmering, and resinous. The fracture is straight foliated. The fragments are blunt-edged. It is translucent on the edges. It feels rather greasy. It is easily frangible. ' Constituent Parts, It is Alum, mixed with Alumina and Oxide of Iron. • _ ■ i Geognostic Situation. It oozes out of rocks that contain' alum, or its consti tuents, as alum-slate, bituminous-shale impregnated with iron-pyrites, or alum-earth. Geographic Situation. It occurs at the Ilurlet Alum-work, near Paisley; °ozing out of rocks of alum-slate in the island of Born holm, in the Baltic; at Muskau in Upper Lusatia; . ® a alfeld in Thuringia; and, according to Pallas, in alu minous rocks on the banks of the river Jenisei, in Sibe ria. Genus II.—Salts ok Magnesia. This Genus contains but one species, viz. Epsom Salt. Natural