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Slate is an argillaceous rock of various colors-blue, green, purple, grey and black-and a peculiar structure by which it readily splits into thin plates or laminae. of sedimentary origin, being primarily deposited on ocean floors as fine mud formed by the waste and denudation of pre-existing rocks, and afterwards compressed, hardened and altered into compact rock. Slate beds occur mainly in the Cambrian, Silurian and Devonian formations-frequently alternating with bands of grit and lime stone, or interstratified with felspathic lava or ashes, and being tilted up from their original horizontal or nearly horisontal position, stretch across wide districts in a series of undulations, which rise to the surface in crests, or dip into troughs underground and form angles of every inclination with the horizon (Encyclopedia Britannica).

It is a rock of peculiar composition which is easily separated or split into thin sheets or laminae. The term is applicable to many kinds of rock which is possessed of the fissile character, but it is it. more properly ascribed to that of argillaceous or clay state which is the most serviceable for roofing and other purposes. This is eminently characterized by the ease with which it is split into

smooth plates, with a uniform degree of hardness and smoothness of various degree of lustre and tints, blackish gray, bluish black, bluish and reddish brown, purplish and greenish. The best slates are distinguished by an appearance of compactness and solidity in the blocks, with nothing to suggest their fissilecharacter, and this is so perfect that when fresh from the quarry these may be split with greater ease than pine timber, and into sheets of any desired thickness. When one is balanced on the finger and struck with the hammer, it should give a clear ringing sound; and after being dried in an oven or immersed in water, it should absorb but little. This is an excellent test of the comparative values of different slates. The powder of slates is of a light color,. and when a pointed piece is rubbed upon a smooth slate surface a mark is left which is easily washed away, and it is this quality which makes the slate so serviceable for writing. and drawing, and creates such a demand for them for school use.

Slate rocks are of common occurrence in districts of the metamorphic and Silurian formations; but they are not of good quality for working, and very rarely are found hard and sound until they have been followed to some depth beyond the reach of

atmosphere influences. Quarries of great extent are worked in North Wales; in Cumberland, Westmoreland, Cornwall and other counties in England; in Argyleshire, Scotland; and others have been opened in Waterford, and other counties in Ireland. The product of the Welsh quarries were largely exported to the United States, but this business has been of late years decreased by the opening of slate quarries in Vermont, New York, Pennsylvania and Maryland. The Welsh quarries were opened toward the end of the last century by Lord Penrhyn, who went to great expense in establishing the business by constructing roads to the water, and providing the necessary facilities for loading vessels. Lord Penrhyn was succeeded by his relative, Col. Pennant, who greatly extended the business, giving employment to 2,500 or 3,000 and producing an enormous annual revenue. The beds extend into the sides of the hills in a succession of terraces, one above the other, to the number of 12, each of them 60 feet high; and railroad tracks are laid upon each for loading the material.

In different localities the beds occur variously inclined from a horizontal to a vertical position, producing various plates or laminae, suitable for use as gravestones, flagging, billiard tables, mantelpieces, partitions, floors, roofing and school slates. In one instance in Wales, near Derwenlas, an attempt has been made to mine the slate without uncovering the earth; and in Cornwall,

at the Delabole quarries, works have been sunk below the sea, to a depth of over 300 feet. In the United States slate quarries were opened in 1839 on the Piscataquis river in Maine, but the difficulty of transportation prevented their coming into competition with Welsh and Vermont slates. Though the quarries in Vermont are shallow, the slates are fully equal in quality to the Welsh slates, and are obtained with facility by open cuts along and into the sides of the hills. Slabs are readily produced of 20 feet long, and as thin as 11⁄2 inches. In New York, slate quarries are worked at Granville and Hoosic, &c., Washington Co. The slate formation crosses New Jersey, and is worked in Sussex Co., and on the Delaware river. On the other side, it stretches across Northampton and Lehigh Cos. The earliest quarry opened was on the Blue Mountain in 1826, about a mile from the Delaware Gap. The product proved excellent for school slates, and a manufactory was established, the first of the kind in the country. It originated the village of Slateford. It worked roofing slate also, which was shipped down the river to Philadelphia. As the business increased, new quarries were opened, and a dozen factories for making school slates came into operation, until the production in roofing and other slates became very large. In 1854, over 300,000 feet in lumber was consumed in slate frames alone. In Lehigh Co., the slate. works multiplied, producing a large

amount of different kinds. The demand was large among the German farming population, who preferred slate to shingles, and a great quantity was also shipped to the neighboring mining towns, as also by water and rail to towns and cities on the seaboard. In Maryland, slate quarries have been worked to a considerable extent in Harford Co., chiefly shipped to Baltimore or other neighboring localities. Slate is found also in Southern States.

Slates are quarried by blasting out large slabs, or by splitting them off with gads or large wedges. The slabs from a foot to a foot and a half thick, or may be 8 or 10 feet long, and I and 2 wide, are set on edge, and grooves are cut across the top and down the sides to determine the lines of fracture, which is done by blows from a wooden beetle directed upon the top near the furrow. The splitting is done by driving wide thin chisels between the laminae, and the sizes of the slates are reduced when

ever desirable by cutting cross grooves, and then breaking the pieces with the chisel. When reduced to the required thinness, the slates are dressed over the edge of a block of wood by the blows of a short chopping knife called sack, sax and

zax. On the back of this tool is a sharp tapering point with which the workman pecks holes through the slates for nails to hold them down to the roof. Much of this work is now done by machinery; especially, slabs made for table tops, billiard tables, etc. Various articles are prepared of slate in imitation of marbles, granites, for ornamental purposes, through processes of varnishing, coloring, polishing, enameling, etc.

Roofing slates in Great Britain are classified, and are denominated according to sizes-Queens, Imperials, Duchesses, Countesses, Ladies, Doubles, etc.

Before machinery was applied to the manufacture of school slates, the slates were shaved like shingles, the operator seated on a wooden horse, and using a drawing knife. The smoothing was finished by rubbing the slate with a rag filled with its own dust. The slate was then washed, and was ready for the frame. At first wild cherry was used for frames, but a scarcity led to the use of birch. The grooves and mortices are cut by circular saws, and holes. are drilled in the corners for securing the frame. The work has been greatly improved and facilitated by mechanical inventions.