Page:VCH Derbyshire 1.djvu/56

 A HISTORY OF DERBYSHIRE Heavy spar or cauk (barytes) is often obtained from old mining hillocks. A stalactitic form of barytes has been found near Youlgreave. Fluor- spar (calcium fluoride) is often found in small cubes lining cavities in the limestone. At Windy Knoll near Castleton large cubes of this mineral occurred in a vein a few years ago. ' Blue John,' the purple variety, arranged in layers of different shades of colour, is found at Castle- ton in the Blue John mine and worked into ornaments. Calc-spar is abundant in veins, and in some places is crushed for making footpaths. It is often found in large dog-tooth crystals, which show traces of the rhombohedral cleavage. It is rarely that the specimens are clear enough to show double refraction. Bitumen is found in small quantities filling cavities in the limestone and the interior of fossils. The hard variety is black and has a shining surface when fractured. The softer variety is known as elaterite and adheres to the fingers, but is sometimes soft like indiarubber. It slowly oozes out of the limestone in Windy Knoll quarry and is a rare mineral in England. It consists mainly of carbon, hydrogen and oxygen. Quartz is found in the siliceous or quartzose limestone in numerous small bipyrimidal crystals only just visible to the naked eye. Larger quartz crystals called ' Buxton diamonds ' have been found lining the interior of cavities in the Toadstone. YOREDALE ROCKS The upper beds of the Mountain Limestone are sometimes inter- stratified with shales. These shales become more numerous, until the rock may be described as shale with intercalated beds or nodules of limestone. There is thus a gradual passage from the calcareous to the argillaceous deposits. These shale beds, in which the amount of lime- stone varies considerably, are passage beds from the Mountain Limestone to the Millstone Grit series. They were called by Farey in 1 8 1 1 the Great or Limestone Shale. In 1860 they were in the neighbourhood of Ticknall called Upper Limestone Shales by the Geological Survey officers, but later in other parts of the county were designated by them as Yoredale Shales. Dr. Wheelton Hind considers that the upper parts of the Carboniferous Limestone of Derbyshire and not the Yoredale Shales of the Geological Survey are the palaeontological equivalent of the Yoredales of Wensley Dale, and suggests that the name Pendleside Group should include the shales and limestones which rest on the Mountain Limestone. 1 As the question of the name is under dispute it will save confusion to adhere to that by which these rocks are at present known on the maps and in the memoir of the Geological Survey. The black shales and thin limestones of the Yoredale series vary in character and thickness in different parts of the county. In Edale they consist of shales with nodules of limestone containing goniatites, whilst 1 Geol. Mag. 1897, p. 159 ; Quart. Journ. Geol. Soc. Ivii. 347. 12