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. 



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