A HISTORY OF DERBYSHIRE 



compact and coarsely crystalline dolerites and finer grained basalts are 

 intrusive, transgressing the limestone beds, and form either sills, vents 

 or dykes. 



The fragmental igneous rocks comprise tuffs, which are inter- 

 bedded with the limestone, and vents, which cut across the beds of 

 limestone. 



The massive Toadstones belong to the basic division of igneous 

 rocks. They consist of olivine, augite, plagioclase felspar, magnetite and 

 secondary products, and vary from an ophitic dolerite or diabase to a 

 basalt. The fragmental rocks sometimes consist of fragments of the 

 massive rocks, but in the majority of cases are made up of lapilli, which 

 either are or have been of a glassy nature, due to rapid cooling. 



An examination of the maps of the Geological Survey will show 

 that the Toadstones may be divided into a northern and southern group. 

 Each of these groups consists of lava flows and bedded tuff contem- 

 poraneous with the limestone and the vents or remains of pipes up 

 which the showers of tuff and flows of lava came to the surface. Both 

 groups contain the sills or intrusive rocks which transgress the beds of 

 limestone. 



VOLCANIC VENTS 



Some of the Toadstones consist of a coarse agglomerate or a fine 

 tuff with or without blocks of dolerite and limestone. They are not 

 intercalated with the limestone but are found to cut across the beds. 

 They may form hills with a dome-shaped outline, occupy a hollow, or 

 even mark no feature on the surface of the ground. Their relation to the 

 surrounding limestone beds and their lithological structure are sufficient 

 evidence to prove that they are the remains of the pipes or vents which 

 have been filled with volcanic debris and received their present contour 

 through the action of denudation. 



The most interesting group of vents is found at Grange Mill near 

 Winster and about five miles west of Matlock Bath. They form two 

 dome-shaped hills with grassy and well marked contours, which rise from 

 the valley to heights of 100 feet and 200 feet respectively. The larger 

 one covers an area of 2,400 feet by 1,300 feet, and the smaller one an 

 area of 1,300 feet by 900 feet. Good views of these hills are seen on 

 the roads from the village of Aldwark and from Longcliffe Wharf to 

 Grange Mill. The road from Grange Mill to Winster passes close by 

 them, and several good exposures of the rock are seen. The smooth and 

 steep grassy slopes consist of a grey rock with green lapilli and a few 

 limestone pebbles. On the hillsides are found a few blocks of saccha- 

 roidal limestone. The rocks in the neighbourhood of the vents form part 

 of a small dome, the longer axis of which ranges north-north-west ; the 

 two vents lie on this anticline. The limestone is seen within a few feet 

 of the agglomerate. On the south-west of the northern vent it dips north 

 or north-west at an angle of 10 to 15, so that the agglomerate cuts 

 across the strike of the limestone. South of the larger vent the beds are 



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