Page:VCH Derbyshire 1.djvu/66

 A HISTORY OF DERBYSHIRE a curiously shaped hill near Miller's Dale station. In this neighbour- hood it attains a thickness of more than 100 feet. This lava flow was preceded by showers of tuff. A few years ago a section along a tram line leading to some limekilns east of the station showed the junction of the lava with the limestone beneath it. On the limestone are 2 feet of clay, followed by about 2 feet of a tufaceous limestone ; this was succeeded by a small lava flow about 9 feet thick, and then by a well bedded and coarse tuff of several feet in thickness, and above this is the main lava flow. A section in the quarry on the opposite side of the valley at the foot of Knot Low shows several feet of a decomposed tufaceous limestone below the lava. Traces of tuff have been found at several other places below this lava flow. The lower and earlier lava flow of this district, which we have already said appears at the bottom of Miller's Dale, is also seen in Monk's Dale, from whence it is carried up by the rise of the rocks through Hargatewall to Tunstead and Small Dale. In the Matlock district there were at least two flows of lava. The largest, which forms the summit of Masson Hill, may be traced for several miles, and was succeeded by a fall of tuff. This tuff is well banded, and is best seen near Tearsall farm. It extends for at least a mile. It is about 20 feet thick. There were probably a number of lava flows, and there is plenty of evidence that some of them thin out rapidly. Some of them were very thin, and do not extend any distance. In Cressbrook Dale a small flow 10 to 20 feet thick is seen on the eastern slope of the valley. Immediately south of the fault which bounds the Tideswell Dale inlier on the south are two small flows 15 to 20 feet feet, and separated by about 1 5 feet of limestone. At Pethills, near Kniveton, are two bands of igneous rock, which probably represent lava flows. They are bedded with Yoredale Shales, and consist of vesicular dolerite. The beds dip at a high angle, and the limestones and shales between the two igneous bands are nearly vertical with a north-north-west strike. The beds probably form an anticline, and the two bands of Toadstone on either side of the axis may therefore be parts of the same flow. The limestone is seen in contact with the igneous rock, and there are no evidences of metamorphism. THE SILLS Some of the Toadstones differ in several respects from those which we have just described. They weather less rapidly, and consist of a hard coarsely crystalline dolerite or a basalt, and are neither vesicular nor slaggy. They often project in large compact pieces above the surface of the ground. Whilst in some places they lie between two beds of lime- stone, in others there is evidence that they transgress or cut across the limestone beds. In some places they have altered the limestones in contact with them to a marble or a saccharoidal condition. They are undoubtedly intrusive sheets or sills. We will briefly describe several of 18