GEOLOGY at Cromford the limestone forms short lenticular beds in the shale. In other places the limestone is intercalated in thin beds amongst the shales, and the proportion of limestone to shale varies greatly. Near Ashbourne the limestones are more evenly bedded, more numerous and closer to- gether, so that it is often difficult to say whether they are at the top of the Mountain Limestone or at the base of the Limestone Shales. In the neighbourhood of Kniveton and Tissington they are often contorted. One of the best exposures which gives evidence of the numerous folds into which these beds have been thrown was seen during the con- struction of the new railway from Buxton to Ashbourne near the village of Tissington. Although only about 60 feet of these shales were seen in a distance of 300 yards, they were bent into about six anticlines and the same number of synclines. The thin beds of limestones are local and soon thin out. It has been estimated that these beds may reach a thickness of 1,000 feet in the northern part of the county, but at Matlock they cannot be more than 400 feet thick. The sandstones in Edale which are mapped as Yoredale by the Geological Survey may possibly belong to the Shale Grit. In places further south they are absent, and the Shale Grit rests on the Yoredale Shales. IGNEOUS ROCKS The quiet deposition of the Mountain Limestone of Derbyshire was in various places disturbed by small volcanoes which poured out their lavas in a molten condition or discharged their fragments of volcanic detritus over the sea floor. The volcanic activity did not cease with the deposition of the Mountain Limestone, but continued whilst the lower portion of the Yoredale Shales was being formed. There was a later phase of volcanic activity when the molten material was no longer able to force its way to the surface, but intruded itself between the beds of limestone or successive lava flows. The igneous rocks of the county for more than a century have been known by the name of Toadstone. Some suppose it to have been derived from the German todtstein (deadstone), from the absence of ore in it ; others consider the rock was so named because of its resemblance to the back of a toad. The Toadstones vary not only in their character and appearance but also in their behaviour with regard to the limestone beds in which they occur. The rock is in places hard and dark in colour, in others soft and decomposed to a kind of clay. In some cases it is evidently interbedded with the limestones and in others cuts across them. Lithologically the Toadstones may be divided into two classes, viz. massive and fragmental. The massive kinds include lavas which are contemporaneous with the limestones in which they occur, and have a vesicular and often a slaggy appearance. The vesicles are frequently filled with amygdules of calcite and other minerals. The harder, more 13
