It is usually formed by low-grade regional metamorphism of mudrock. Slate is a fine-grained, metamorphic rock that shows no obvious compositional layering, but can easily be split into thin slabs and plates. The phrases "clean slate" and " blank slate" come from this usage. It may mean a single roofing tile made of slate, or a writing slate, which was traditionally a small, smooth piece of the rock, often framed in wood, used with chalk as a notepad or notice board, and especially for recording charges in pubs and inns. The word "slate" is also used for certain types of object made from slate rock. Slate is not to be confused with shale, from which it may be formed, or schist. However, slate occurs in a variety of colors even from a single locality for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green, or cyan. Slate is frequently grey in color, especially when seen en masse covering roofs. When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates display a property called fissility, forming smooth, flat sheets of stone, which have long been used for roofing, floor tiles, and other purposes. The foliation in slate, called " slaty cleavage", is caused by strong compression in which fine-grained clay forms flakes to regrow in planes perpendicular to the compression. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. It is the finest-grained foliated metamorphic rock. Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism.
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