A HISTORY OF DERBYSHIRE may mark the site of a volcanic vent occurs in the western part of the inlier. At Snelston Common south-west of Ashbourne is a small dome- shaped inlier of Mountain Limestone surrounded by Keuper Marl. Lead and copper ore were formerly worked here. Some portions of the limestone are dolomitized. At Diminsdale, Calke and Ticknall are three small inliers consisting of the upper beds of Mountain Limestone, which are succeeded con- formably by the Limestone Shales. They are brought to the surface through the combined agency of a series of gentle rolls and of a great fault which brings them against the Ashby Coal Measures. CAVERNS The upper beds of Mountain Limestone are in many places honey- combed with caverns. Some of the caverns of Derbyshire are old lead mines, but others are natural and connected with the underground drain- age of the district. The existence of unexplored caverns may be inferred by the numerous swallow holes found in the limestone. These swallows vary in depth and diameter and have generally the shape of a wide cone with a blunted point at the bottom. The sloping sides may be covered with grass with or without any signs of rock. The swallows are formed by the action of water containing carbonic acid finding its way into joints and enlarging them by dissolving away the rock and ultimately emerging at a lower level and at some distance from the place of entry. As the volume of water increases the subterranean course is enlarged by the solvent action of the water, and caverns are formed. The caverns are often enlarged by the falling in of the roof and the removal by water of the resulting debris. Poole's Hole or Cavern near Buxton illustrates the intimate relation between caverns and underground drainage. The river Wye, a tributary of the Derwent, rises on Axe Edge, and after flowing over the Yoredale Shales disappears down a swallow in the limestone called Plunge Hole. It reappears in Poole's Cavern, and after flowing for some distance in the part shown to visitors suddenly disappears again and finally emerges from a small hole called Wye Head a few fields away from the mouth of the cavern. The Speedwell Cavern near Castleton is an instance of those caverns which are partly artificial and partly natural. The entrance is at the foot of the Winnats. A level was driven into the limestone in order to reach some of the rakes which run through the hill. After passing several serins or calcite veins the level reached the New Rake at a hori- zontal distance of 750 yards from the entrance. The level entered a large narrow cavern which extends to a great height and was hollowed out in the New Rake by underground waters. A solid platform has now been built on the sloping floor of the cavern which is inclined at an angle of about 45 to the horizon. The level now contains water 8
