SLATE, a very remarkable form of clay rock, frequently fossiliferous and not confined to one geological period. Consisting essentially of clay, the par tides of slate are so mechanically ar ranged that the rock splits with perfect facility into almost indefinitely thin layers in one direction only, and in all others either breaks with a jagged edge, or in well-defined joints at some distance from each other. Mineralogically slate is nothing more than a pure clay; nor does there seem any reason to suppose that any approach is made in it toward crys talline structure. As, however, no other rock shows this tendency to split indefi nitely, the case is one of great interest. Practically slate is very valuable, owing to its peculiar facility of splitting and the perfectly smooth natural face which it presents. Its hardness and compact ness preserve it from all weathering by mere exposure, though, when ground down, it easily passes back into fine clay. Slate is always, and properly, regarded as a metamorphic rock. For a long time slate was used almost exclusively for roofing. For this purpose the slates are cut into sizes varying from a few square inches to two square feet, though some are much larger. This is done with very
simple tools and extreme rapidity. The rest is in slabs or thick slates, often very large. The use of slate and slabs has increased considerably of late years. Slabs are now used in house fittings; as in strong rooms, powder magazines, lar ders, partitions, baths, stables, floors, etc. It is very largely used also for enamel ing; the surface of enameled slate being made to represent marble of all kinds with wonderful accuracy, and resisting almost all wear. Quarries of great mag nitude are worked in Cornwall, Wales, Scotland, and Ireland. The product of the Welsh quarries was formerly largely exported to the United States, but this business received a serious check on the opening of valuable quarries in Vermont.
New York, Pennsylvania, Maryland, and other states.
Adhesive slate is a kind of greenish gray slate, quickly absorbent of water, and remarkable for its adhesiveness. Bi tuminous slate, a soft species of sectile slate clay impregnated with bitumen; hornblende slate, a slaty rock consisting of hornblende and feldspar with some chlorite, principally used for pavements, etc.