Slate

Slate, a cleaved, compact, argillaceous rock, which has been to some extent metamorphosed, and is obtained generally from the older geological formations. The rock splits indefinitely in a direction which is generally uniform over a wide area, inclined at a high angle to the horizon, and altogether independent of the nearly obliterated original bedding of the rock. Under the microscope the component particles of the rock are seen not only to be rearranged with their long axes all in one direction, but also to be to some extent compressed, thus giving the "grain" to the rock. Slate differs in colour, being sometimes black, ferruginous, silvery, or green, but more often of a purplish-grey. It often contains scales of mica, minute crystals of garnet, or larger spots of chlorite, andalusite, kyanite, staurolite, or other minerals. The black slates may contain a considerable proportion of organic matter. Those containing garnets or other hard varieties are used as oilstones; but the chief use of the material is for roofing, for which the Bangor and other North Wales slates are the best in Britain. Several thousand tons are quarried annually, and over fifty million slates by tale are exported, chiefly to Europe. Immense numbers of small slates are used for writing purposes in schools.