Schist

or ScuistTus, in mineralogy, an argillaceous earth, including most of the varieties of slate, for which see the article ARDESIA. In the arrangement of Werner, who denominates the argillaceous slate thous chiefer, it consists of the three following species. 1. Common argillaceous schist. Colour smoke-grey, reddish, or blackish grey; sometimes also flesh red, reddish-brown, yellowish brown, or olive-yellow. The colour is, for the most part, uniform, but it sometimes presents spots, bands, and dendridical delineations. occurs in ma , di eminated, cr in roundish fragments, Lustre-silky or resinous, and varies from glimmering to slightly shining. Fracture slaty, generally straight, but sometimes curved or undulating. Some varieties are so compact as to present a dull, earthy fracture. Fragments tubular or rhomboidal, rarely large splintery. Varies from very soft to moderately hard, but even the hardest. may readily be scratched by a knife. It is easily frangible, but not brittle; and gives a greyish-white streak. Itis largely used for covering houses; and the straight foliated bluish grey varieties are employed as writing slates. ‘I'he softer and’ more compact varieties are made into slate pencils. 2. Hone-slate. Novaculite of Kirwan. Colour greenish grey, or smoke-grey pa ing into olive and mountain-green. Occurs in ma , and has a glimmering lustre. It is cut into hones for sharpening the finer kinds of steel instruments. It occurs at Lauenstein in Bareith, at Seifendorf in Saxony, and the Levant, whence it is called Turkish-hone; also in the valley of Llanberris, in North Wales. 3. Black-chalk. Colour greyish or bluish black. Occurs in ma ; the longitudinal fracture slaty and glimmering, the cro fracture earthy and dull. Its fragments are tubular or splintery. It stains the fingers, and gives a somewhat glo y grey streak. “It is employed for drawing and writing on paper and other materials. The best kinds come from Italy: it is also met with in Spain, France, Bareith, and the isle of Isla, in the He brides. Before the blow-pipe, without addition, it acquires a thin varnish, but does not melt, Its component parts, according to an analysis, by weight are» divs Silex’ 1125 Fa". ATonvva Carbon’ 2-75 Oxydof iron Sele! “wl: Water