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 14
