Slate

(slāt,) or CLAY-SLATE. [From A. S. slith, flat.] (Geol. and Min.) A highly metamorphosed argillaceous rock, fine-grained and fi ile, and of a dull blue, gray, green, or black color. It splits into thin lam inæ or plates, that are altogether independent of the layers of deposit; though sometimes coinciding with them, they more frequently cro them at different angles. True slate is avery compact rock, little liable to be acted upon by atmospheric agencies. It is chiefly obtained from Palæozoic strata, but it is found also among more recent rocks. It is used for various purposes, being split into thin slabs of small size for the roofing of houses, and into larger slabs for fitting up dairies, ., and even for making billiard-tables, and split and polished by means of pumice for writing-slates. It is very largely used also for enamelling; the surface of enamelled slate being made to represent marble of all kinds with wonderful accuracy, and resisting almost all wear. Thus,for mantelpieces, billiard - tables, ornamental slabs, and furniture, it has no equal, its cheapne being such as to drive other material out of the market. There are valuable quarries of S. in Vermont, New York, Pennsylvania, Maryland, . 5 9 12 13 6 Fig. 596. BASE OF THE SKULL. 1,Hard palate, formed by the palate proce bones; 3, Vomer, dividing the openings of the posterior nostrils; 4, Zygomatic fo a; 5, Basilar proce of the occipital bone; 6. Foramen magnum, through which the spinal cord pa es; 7, Foramen ovale; 8, Glenoid fo a, in which the head of the lowerjaw-bone lies; 9, External auditory foramen: 10, Carotid foramen of the left side; 11, Styloid proce ; 12. Mastoid proce ; 13, One of the condyles of the occipital bone. [s. 831]