SLATE, (stegania), a stone of a compact texture, and laminated structure, splitting into fine plates.
Dr Hill distinguishes four species of stegania. 1. The whitish steganium, being a soft, friable, flaty stone, of a tolerably fine and close texture, considerably heavy, perfectly dull and destitute of brightness, variegated with a pale brown or brownish yellow. This species is common in many counties of England, lying near the surface of the ground. It is generally very full of perpendicular as well as horizontal cavities, many of which are filled up with a spar a little purer and more crystalline than the rest; and is commonly used for covering houses. 2. The red steganium is a very fine and elegant slate, of a smooth surface, firm and compact texture, considerably heavy, and of a very beautiful pale purple, glittering all over with small glossy spangles: it is composed of a multitude of very thin plates or flakes, laid closely and evenly over one another, and cohering pretty firmly: this is very common in the northern parts of England, and is much valued as a strong and beautiful covering for houses. 3. The common blue steganium, is very well known as an useful and valuable stone, of a fine smooth texture and glossy surface, moderately heavy, and of a pale greyish blue; composed of a multitude of even plates, laid close upon one another, and easily splitting at the commissures of them: this is also very common in the north parts of England, and is used in most places for the covering of houses. There are other species of this slate, viz. the brownish blue friable steganium, usually called coal-slate; the greyish black friable steganium, commonly called shiver; and the greyish blue sparkling steganium. 4. The friable, aluminous, black steganium, being the Irish slate of the shops: this is composed of a multitude of thin flakes, laid very evenly and regularly over one another, and splits very readily at the commissures of them. It is common in many parts of Ireland, and is found in some places in Eng-
land, always lying near the surface in very thick strata. In medicine it is used in haemorrhages of all kinds with success, and is taken often as a good medicine in fevers.