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slate
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. It is the finest grained foliated metamorphic rock.〔Essentials of Geology, 3rd Ed, Stephen Marshak〕 Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.〔
A very strong foliation is called "slaty cleavage".〔 It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression.〔 When expertly "cut" by striking parallel to the foliation, with a specialized tool in the quarry, many slates will form smooth flat sheets of stone which have long been used for roofing, floor tiles, and other purposes.〔 Slate is frequently grey in color, especially when seen, en masse, covering roofs. 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 not to be confused with shale, from which it may be formed, or schist.
The word "slate" is also used for certain types of object made from slate rock. It may mean a single roofing tile made of slate, or a writing slate. This was traditionally a small smooth piece of the rock, often framed in wood, used with chalk as a notepad or noticeboard, and especially for recording charges in pubs and inns. The phrases "clean slate" and "blank slate" come from this usage.
==Historical mining terminology==
Before the mid-19th century, the terms ''slate'', ''shale'' and ''schist'' were not sharply distinguished.〔R. W. Raymond, Slate, (A Glossary of Mining and Metallurgical Terms ), American Institute of Mining Engineers, 1881; page 78.〕 In the context of underground coal mining in the United States, the term slate was commonly used to refer to shale well into the 20th century.〔Albert H. Fay, Slate, (A Glossary of the Mining and Mineral Industry ), United States Bureau of Mines, 1920; page 622.〕 For example, ''roof slate'' referred to shale above a coal seam, and ''draw slate'' referred to shale that fell from the mine roof as the coal was removed.〔J. Marvin Weller, ed.,(Supplement to the Glossary of Geology and Related Sciences ), American Geological Institute, 1960; page 18.〕
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