SLATE. The slaty cleavage or structure in stone appears to be the result of a general cause acting subsequently to the deposition and disturbance of the strata, capable of per vading and re-arranging the particles so as to systematise their mutual attractions, but not to fuse them together, destroy their original distinctness, or obliterate the evidence of their original condition. This force was so general, that along many miles of country, as, for example, in the whole Snowdonian chain, one particular direction (north-north east), in North Devon and Pembrokeshire another (nearly east and west), is found to prevail more or less distinctly in all the rocks. This dependence of the slaty structure on the nature of the rock is sometimes very positively pronounced, as in some classes of rock the cleavage does change and even reverse its inclination where contortions prevail. Pres sure, in some peculiar application seems to be the grand agent in the production of slaty cleavage.
For economical purposes there appears little chance of obtaining in the British Islands good Slate (properly so called) from any but the ancient argillaceous strata superposed on mica schist and gneiss, and covered by old red-sandstone or mountain limestone. From these strata in Scotland, Cumberland, West moreland, Yorkshire, Charnwood Forest, North Wales abundantly, South Wales, Devonshire, Cornwall, the north and south of Ireland, slates of various value are dug. The thin j flagstone of the coal formation in many parts of England and Wales, the laminated sandy limestone of Stonesfield, Collywiston, &c., which are often called slates, and are exten sively used in roofing, are all obtained by natural partings parallel to the stratification. True slate is split by wedges from the appa rently solid rock along planes often no more discoverable than those of a real crystal. In colour it is purple, blue, green, yellowish, or almost white, or striped across the planes. In some slates (west of Scotland, Ingleton, &e.) crystals of cubical iron pyrites are scattered. Much of the Cumberland slate (13orrowdale) appears full of fragments.
Some slate quarries are remarkably situated, and are wrought under circumstances of diffi culty. At Honister Crag, between Borrowdale and Buttermere, there is a quarry near the top of a mountain whose sides are so steep, that the quarrymen take up a week's provision with tlfem, to avoid the necessity of ascending and descending every day. The slates are lowered by sledges in a mode which would be perilous to any not accustomed to it. In all slate quarries the slate is extracted in large blocks ; and the lamellar structure of the slate enables the blocks to be easily separated into thin layers by the use of a chisel and mallet. All the slates in ordinary use, both for roofing and for writing on, are brought to the thin slate so familiar to us by these simple means.
A considerable increase has taken place during the last half of 1850 in the export of slate from the various quarries in North Wales ; a circumstance attributed to the repeal of the brick duty causing an impetus to the erection of houses. From the port of Caer narvon alone, the exports for the half year were 01,400 tons, being an increase of 11,000 tons over the prior half year, and for the month of February, 1851, the shipment has been to the extent of 0,200 tons. In other ports a large increase is apparent.
Slate is now used for a variety of purposes as a substitute for stone in some cases, and for lead in others. Pavements, cisterns, walls, partitions, &c., are constructed. A superior production called enamelled slate is used for ornamental tabletops, slabs for consoles and Pier-tables, wash-stand tops, columns, pilasters, door furniture, mural tables, monuments, &c.
Slate pencils are simply narrow slips of a very soft kind of slate. An ingenious kind of slate pencil has been lately devised, in which the material, brought to a very small thickness, is slid into a groove in a wooden stem, through which it is propelled by a spiral apparatus, somewhat on the principle of the ever-pointed lead pencils. In 1810 Mr. Cohen patented a new kind of slate pencil, made from a mixture of alumina, French chalk, soap-stone, and water.