SLATE, in geology, a fissile, fine-grained argillaceous rock which cleaves or splits readily into thin slabs having great tensile strength and durability. Some other rocks that occur in thin beds are improperly called slate, because they can be used for roofing and similar purposes. Stonefield slate, a thinly bedded limestone occurring near Oxford, is one of the best known. True slates do not, as a rule, split along the bedding, but along planes of cleavage, which may intersect the bedding at any angle, usually, in the case of good roofing slates, at high angles. The original material was a fine clay, sometimes with sand or volcanic dust, and the bedding of the sediment as originally laid down may be indicated by alternating bands, differing in colour or in litho logical character, sometimes to be seen on the cleavage faces of the slates. Cleavage is a superinduced structure, the result of pressure acting on the rock at some time when it was deeply buried beneath the earth's surface. On this account slates are found chiefly among the rocks of the older geological systems, although some occur in regions where comparatively recent rocks have been folded and compressed as a result of mountain build ing movements in the earth's crust.
In thin sections for microscopical examination, slates show much colourless mica in small, irregular scales, which in the best average about two thousand to the inch in breadth, and six thousand to the inch in thickness. Green chlorite in flakes is also usually abun dant, the principal other ingredient being quartz, in minute lens shaped grains. In colour, slates may be black, blue, purple, red, green or grey; dark slates usually owe their colour to carbonaceous material or to finely divided sulphide of iron, reddish and purple varieties to the presence of oxide of iron in the form of haematite, and green varieties to the presence of much chlorite. Slates are widely used for roofing purposes, for not only are they easily prepared and fixed, but they are weatherproof and durable.
North Wales provides most of the slate used in the British Isles, but slate of economic importance also occurs in North Devon, the Lake District, Scotland (Ballachulish) and Ireland (Kilkenny). There are also important quarries in France (the Ardennes), Bohemia, Germany (near Coblenz) and in the United States; it is sometimes obtained from open quarries, and some times from underground workings or mines.
The material is sometimes removed by means of "channelling machines," which make cuts in the face of the rock allowing a block to be wedged off; or, when blasting is resorted to, advantage is taken of the joints and other planes of weakness in the rock. The masses, dislodged by whatever means had been adopted, are divided into blocks small enough to be sent to the sheds where they are split and dressed.
In "making" slates, the splitter takes blocks about 3 inches thick, and a chisel, placed in a certain position against the edge of the block is lightly tapped with a mallet; a crack appears. in the direction of cleavage, and slight leverage with the chisel serves to split the block into two pieces with smooth and even surfaces. This is repeated until the original block is converted into 16 or 18 separate "slates," the thickness of which depends upon many circumstances, such as quality of rock, size required, and purpose for which it is to be used, the average thickness of a roofing tile of the best kinds of slate being about 6 in. The slates are after wards trimmed to size, either by hand, in which case they are cut between a fixed sharp edge and a movable knife acting on the principle of a printer's guillotine, or by means of machine-driven rotating knives.
A detailed bibliography of works relating to the origin, distribution and utilization of slate will be found in The Slates of Wales (National Museum of Wales, 1927). (F. J. N.)