A HISTORY OF DERBYSHIRE in what is mapped by the Geological Survey as an inlier of Mountain Limestone. They consist of an agglomerate of more or less rounded blocks of a vesicular dolerite and pieces of limestone mingled with a finer deposit of lapilli. The largest of these patches covers an area of about 800 feet by 400 feet, and is cut by two streams and forms a ridge of high ground between them. The beds surrounding it are very much contorted. On the north and south the beds dip east and west at high angles, forming an anticline through which the agglomerate has made its way, and on the west the beds are vertical with a north-north-west strike. Half a mile north is another small patch of agglomerate, and south-east of the large mass is another small patch which apparently cuts across the strike of limestones a short distance from it and may mark the site of a vent. TUFFS In addition to the fragmental igneous rocks or tuffs found in the vents, there are bands of tuff interbedded with the limestones and there- fore contemporaneous with them. As far as has been at present ascer- tained, five of these bedded tuffs were unaccompanied by lava flows. The thickest is probably that at Ashover, in the inlier of Mountain Limestone. The river Amber has cut its way through the limestone down into the tuff. A shaft was sunk into it to a depth of 210 feet without reaching the bottom. This tuff is laminated, contains fragments of chert, of limestone often rounded, and blocks of amygdaloidal dolerite, and is traversed by veins of calcite. The matrix of the rock is composed of lapilli cemented with volcanic dust and calcite. At the village of Litton near Tideswell is a well banded tuff intercalated with the limestone. It probably attains a thickness of 200 feet. It consists of alternations of fine and coarse laminae of a green and yellow colour, with pebbles of coralline limestone and blocks of dolerite or basalt up to 18 inches in length. The matrix is formed of very vesicular lapilli cemented by calcite. Near the Peep o' Day, the highest house in Litton, the road has been cut through several feet of the rock, and the alternating bands of coarse and fine volcanic detritus, which are clearly visible, point to the varying length and intensity of the volcanic explosions. The limestones immediately above the tuff are very fossiliferous, and contain volcanic detritus in small quantities up to a height of about 1 8 feet. Another laminated tuff is seen in a valley a short distance north- west of Tideswell, but its thickness is not known, as the base is not visible. The tuff at Shothouse Spring has been described above. The latest series of tuffs are found in the neighbourhood of Tissing- ton in the Limestone Shales. Though they cover a large area of ground, they were not discovered until the new railway line from Ashbourne to Buxton was in course of construction. The rocks in this part of the district are very much contorted, and the numerous folds have caused a repetition of the beds to be seen in the cuttings, and the tuff is brought 16
