a stone of a compact texture and laminated structure, splitting into fine plates

Dr Hill distinguishes four species of slate steganium:

1. The whitish steganium, being a soft, friable, flaky 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 very common in many counties in 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 thiver; 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 England, always lying near the surface in very thick strata. In medicine, it is used in hemorrhages of all kinds with success, and is taken often as a good medicine in fevers.

There is a sort of slate-stones called, by Dr Hill, ammoschiftum. Of this kind there are only two species: 1. That composed only of sparry and crystalline particles; or the grey, friable, dull ammoschiftum; being a coarse, harsh, and rough stone, of a very loose texture, considerably heavy; and composed of a large, coarse, obtusely angular grit, surrounded, and in part held together, by a loose earthy spar. This stone is very common in most countries, and is frequently used to cover houses, instead of tiles: it bears the weather but badly, and is apt to crumble after frosts. 2. That composed of talcy, sparry, and crystalline particles. This comprehends five species, viz. the brownish white glittering ammoschiftum; the greenish grey shining ammoschiftum; the yellowish grey glittering ammoschiftum; the hard purple and white laminated ammoschiftum; and the bluish glittering slate stone. These sorts of slate-stone are very common in the northern countries, and are used in covering houses, paving, building, &c.