in mineralogy, a name given to several different kinds of stones, but more especially to some of the argillaceous kind; as,

1. The bluish purple schistus, schistus tegularis, or common roof-flate. This is so soft that it may be slightly scraped with the nail, and is of a very brittle lamellated texture, of the specific gravity of 2.876. It is fusible per se in a strong heat, and runs into a black scoria. By a chemical analysis it is found to consist of 26 parts of argillaceous earth, 46 of filaceous earth, 8 of magnesia, 4 of calcareous earth, and 14 of iron. The dark-blue flate, or schistus scriptorius, contains more magnesia and less iron than the common purple schistus, and effervesces more briskly with acids. Its specific gravity is 2.701.

2. The pyritaceous schistus is of a grey colour, brown, blue, or black; and capable of more or less decomposition by exposure to the air, according to the quantity of pyritous matter it contains and the state of the iron in it. When this last is in a semi-phlogisticated state it is easily decomposed; but very slowly, or not at all, if the calx is much dephlogisticated. The aluminous schistus belongs to this species.

3. The bituminous schistus is generally black, and of a lamellated texture, of various degrees of hardness, not giving fire with steel, but emitting a strong smell when heated, and sometimes without being heated. M. Magellan mentions a specimen which burns like coal, with a strong smell of mineral bitumen, but of a yellowish brown, or rather dark ash-colour, found in Yorkshire.—This kind of schistus does not show any white mark when scratched like the other schistus.