Page:VCH Derbyshire 1.djvu/71

 GEOLOGY removed the upper carboniferous rocks from the central portions of Derbyshire and Yorkshire, and thus separated the coalfields on the two sides of the Pennine Chain one from the other. The Yorkshire, Notting- hamshire and Derbyshire coalfield was considered by Professor Ramsay to lie in the form of a basin, the northern, southern and eastern edges of which lie underneath the New Red, Permian and other overlying secondary strata. He estimated the area of available Coal Measures under these strata to be approximately equal to the area of the coal- field exposed at the surface. The depth at which the coal is worked varies considerably. Some shafts are sunk through the Magnesian Limestone which lies unconform- ably on the Coal Measures. Under Bolsover the depth to the ' top hard ' coal is 900 to 950 feet, at Pleasley colliery near Mansfield it was reached at 1,545 feet. In Derbyshire the Coal Measures may be divided into the middle coal measures, which are about 2,300 feet in thickness, and the lower or gannister series, which is about 1,000 feet in thickness. The middle coal measures consist of sandstones, shales and clays with ironstones and coal seams. The gannister series consists of flagstones and shales with thin coal seams, under which are floors or beds of gannister. The seams of coal vary from 2 to 7 feet in thickness. According to the horizontal section of the Geological Survey the succession of strata from Kirby Woodhouse, through Alfreton Common and Wingfield Manor to Crich are as follows in descending order : Permian Rocks Marls and Sandstone. Magnesian Limestone (lower) Marls and Sandstone. ft. 40 60 3 Middle Coal Measures Strata to Top Hard Coal, about Waterloo Ell Lower Hard Furnace Black Shale or Clod. . . Kilburn Shales with Ironstone 700 1,600 Lower Coal or Gannister Series Flagstones of Wingfield Manor Shales and Flaggy Sandstones, with two coals underlaid by Gannister floors .... Near the southern extremity of the coalfield at Cinderhill colliery the depth to the top hard coal was 655 feet, to the deep hard coal 1,183 feet, and to the Kilburn coal 1,995 ^ eet - The principal coals in Derby- shire are the ' top hard ' and ' lower hard ' seams, which produce the glossy coal with long fracture known as splint ; and the ' upper soft ' and ' lower soft ' coals. Different seams have different qualities, and are suited for household, steam or gas purposes. The coal output of Derby- 23