Slate, a rock of no definite composition, distinguished by its structure, which is of parallel sheets or laminae, easily separated. The term is in common use also applied to various rocks which do not possess the fissile character in so eminent a degree, and which are sometimes distinguished from the true slates by the name of schists; such are the mica, tal-cose, hornblende, and chlorite schists or slates. Shale differs from slate in its more earthy texture and less tenacity, as well as want of the perfect slaty structure. But its composition is like that of the argillaceous or clay slate, which is the well known roofing and writing slate. This variety, which is the only slate of economical importance, is found among the met-amorphic rocks passing into mica slate, and with the strata of the Silurian period, and sometimes with those of still later origin. It is eminently characterized by splitting with ease into large smooth plates, which have a uniform degree of hardness, possess a dull or feeble lustre, and are blackish gray, bluish black, bluish or reddish brown, purplish, or greenish.

The rock is often traversed by thin seams of quartz, but the prepared slates should be entirely free from foreign minerals, and especially from iron pyrites, which are too often seen in yellow cubical crystals scattered over the surface of what would otherwise be excellent roofing slates. Such are unfit for writing or school slates; and for roofing slates they are objectionable on account of the pyrites weakening the slates, and also being liable to decompose after exposure for some time, and cause unsightly stains of oxide of iron. Carbonate of lime is also sometimes present, and is likewise injurious. The best slates are distinguished by an appearance of compactness and solidity in the blocks, with nothing to suggest their fissile character; and yet this should be so perfect, that when fresh from the quarry these blocks may be split with greater ease than pine timber, and into sheets of any desired thinness. The faces should be perfectly smooth and parallel, without any curvatures or irregularities. There should be no lines of cross fracture that should prevent their breaking in any one direction more than another.

When one is balanced on the finger and struck with a hammer, it should give a clear ringing sound; and after being dried in an oven and immersed in water, it should absorb but little, as may be ascertained by weighing it before and after immersion. This is an excellent test of the comparative values of different slates. The powder of slates is light gray, and when a pointed piece is rubbed upon a smooth slate surface a portion of the powder remains behind, leaving a plain mark that is easily wiped or washed off. It is this property which renders the slates serviceable for drawing and writing upon. Argillaceous slates, like the clays which they originally were, are essentially composed of silex and alumina, and the following is the result of the analysis of a common Scotch variety: silex, 50 parts in 100; alumina, 27; oxide and sulphate of iron, 11; potash, 4; magnesia, 1; water, 7; carbon, a trace. The slates are found often in beds of great extent, associated with other beds of similar character; and this singular feature is observed in the structure of the rocks, that the cleavage, or lines along which the slates naturally separate, has no relation to the lines of stratification.

However much the beds themselves may be contorted and follow irregular waving planes, each system of cleavage lines, in case there are more than one, as sometimes occurs, maintains its own direction and rarely coincides with the plane of dip. It is evident that the cleavage seams must have been produced subsequently to the time when the beds acquired their final position. This structure is what is known as slaty cleavage; and sometimes when the strata are themselves thinly bedded and the stratification is regular over extended areas, it is not easy to distinguish immediately the two sets of planes one from the other. - Slates are quarried either by blasting out large slabs, or, when practicable, splitting them off with gads and large wedges. The slabs from a foot to a foot and a half thick, and it may be 8 or 10 ft. long and 1 or 2 ft. wide, are set on edge, and grooves are cut across the top and down the sides to determine the lines of fracture for separating them into rectangular blocks, which is clone by blows from a wooden beetle directed upon the top near the furrow. The splitting is effected by driving wide, thin chisels between the laminae, and the sizes of the slates are reduced whenever desirable by cutting cross grooves and then breaking the pieces with the chisel.

When reduced to the required thinness, the slates are roughly dressed over the edge of a block of wood by the blows of a sort of chopping knife called a sack, sax, or zax. On the back of this tool is a sharp tapering steel point, with which the workman when preparing roofing slates pecks two holes through the slates near what is to be the head or upper edge, for the nails which are to hold it down to the roof. In Vermont machines have been applied to cutting grooves in the slate in the ledge to facilitate the quarrying, and the cutting and trimming are also done by machinery. It is important that all this work should be done while the blocks are fresh from the quarry, as in drying they are apt to lose their property of splitting freely, though freezing may restore this; but a succession of frosts and thaws has the effect of thorough seasoning. Slabs for internal decoration, as mantelpieces, and for articles of furniture, as table tops, billiard tables, sinks, etc, are cut by circular saws which are made to revolve slowly. The sheets when thus squared to suitable sizes are planed in machines similar to those used for planing metals; and pieces for mouldings are shaped by tools of the desired figure.

Various ornamental articles are prepared of slate in imitation of marbles, granites, and other stones, by the application of colors, which are baked in, varnished, and polished, the applications being several times repeated. (See Enamelling, vol. vi., p. 591.) - Slate Pencils are made from argillaceous slate rock, sometimes from talcose slate, and sometimes from various materials ground together and compressed. Near the town of Castleton, and near one extremity of the western Vermont slate belt, is found an argillaceous slate from which the finest pencils are made. The stone is sawed into blocks 7 in. long by 6 in. wide, and split into slabs a little more than a quarter of an inch thick. These are then planed and placed in a machine, in winch a series of grooved knives cut through one half the thickness of the slab, when it is placed in a second machine having a bed with grooves corresponding to the sides of the pencils cut, and a cutter like the one in the first machine completes the operation. The pencils are then counted and put up in boxes of 100 each, and packed in cases of 10,000. There are three sizes, 6, 5, and 4 in. in length.

The waste of this slate has been utilized by grinding it into flour and making it into artificial pencils.