a genus of stones belonging to the argillaceous class, found chiefly in Cornwall in England. Its texture is either lamellar or coarsely granular; the specific gravity from 2630 to 2666. It contains 60 parts of siliceous earth, 25 of argillaceous, 9 of magnesia, and 6 of iron. The greenish kind contains more iron, and gives a green tincture to the nitrous acid.