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Most coals in the United Kingdom were formed during the Carboniferous; the majority are of Westphalian age. The coalfields display a great diversity of size, geologic structure, and rank. Examples include: (a) certain coalfields of the Midland Valley of Scotland, where rank is generally low but capricious because of widespread igneous activity in the form of sills and dikes, (b) the contrasting low-rank Northumberland coalfield and higher rank Durham coalfield to the south, the former lying in a deep sedimentary trough, the latter situated on the eastern margin of a stable block that has a complicated geothermal history, and (c) the South Wales coalfield with its easterly low-rank coals extending to a western area of high-level anthracitization, the cause of which has nev r been explained satisfactorily. The small Kent coalfield, the Yorkshire coalfield with its northeasterly extension to the new working Selby coalfield, and the new Oxfordshire coalfield, which is at present undeveloped, will also be discussed. Rank variation will be illustrated by vitrinite reflectances measured in borehole sections and by reflectance maps that are not generally available for British coalfields. Some of these maps are based on actual measured reflectances undertaken for marketing purposes by the National Coal Board. Others have been prepared from coalfield seam maps, originally constructed from coal chemical parameters, using what are now acceptable chemical parameter-reflectance correlations.
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