SLATE, Olt CLAY-SLATE (Fr. celat, a shiver or splinter), is a highly metamorphosed argillaceous rock. fine-grained and fissile, and of a dull blue, gray, green, or black color. It splits into thin dentine or plates, that are altogether independent of the lavers pf deposit; though sometimes ceinciding with them, they more frequently cross them at, different angles. See CLE tVAGE. Some rocks that split into the thin plates of the stratification are popularly but errotte.msly 'mined slate, as the thin bedded sand:tones properly called flagstones or tilesto ies, the fissile shales of Cambrian and Silurian age, and the metaphoric, gneiss, and mica schist. whose planes of division cor respond to their stratification. True slate is If very compact rock, little liable to be acted upon by atmospheric agencies. It is chiefly obtained from paleozoic strata, but it is fonnd.also among more fecent rocks. It is used fur various purposes, being split into thin slabs of small size for the -roofing of houses, and into larger slabs for fitting up dairies, etc., and even for making billiard-tahles, and split and polished by means of pumice for writing-slates. There are extensive quarries of roofing-slate in Wales and in
the western highlands of Scotland, and in the Ardennes in France, sonic of which have been wrought for a long time, and give employment to an great, number of workmen. A hard compact slate is best for roofing; that which is porous imbibes water, the freezing of which splits it in winter, while it affords also a soil for mosses, which soon injure the roof.
In roofing with slates it is necessary to put on the slates in two thicknesses, so that the sloping joints in:1y be covered by the overlap of the course above. Besides this, the third course must also cover the first by on inch or two, to prevent rain from penetrat ing. Slates are laid upon boarding, and bedded in lime. and nailed with malleable-iron nails, japanned, so as to prevent them from rusting. When large strong slates are used, they may be nailed to strong laths in place of hoarding. Welsh slates are the cheapest and most generally used but Eosdale or Ballachulish slates, from the vr. of Scotland, are stronger and better when the roofs are liable to be injured.