Page:VCH Derbyshire 1.djvu/63

 GEOLOGY much disturbed, and their upturned edges may be seen striking at the agglomerate. To the east of the vents is a small valley with its eastern slopes running up to a well marked escarpment of limestone higher in the series. The escarpment reaches a height of 1,000 feet above the sea, and may be traced for about a mile. West of the vents a similar but not so well marked an escarpment may be traced. On both sides of the valley in which the vents are situate a bed of laminated tuff under- lies the limestone of the escarpments. Its thickness is about 90 feet, and it is made up of alternations of finely and coarsely laminated bands of volcanic lapilli in a cement of calcite. This tuff was probably thrown out from the vents. That the volcanic action then became very feeble and intermittent is proved by the irregular distribution of minute lapilli in the limestone immediately above the tuff up to a height of 1 8 feet. Several small dykes of dolerite traverse the southern vent, one of them being several feet in width. Near the northern boundary of the Moun- tain Limestone at Castleton is a small elliptical vent which forms a slight feature in the landscape. It pierces the limestones near the northern slope of Cow Low. The limestones may be seen dipping about 20 degrees north on the north, south and west of the agglomerate and within a short distance of it, so that the igneous rock undoubtedly cuts across the beds of limestone. The agglomerate forms a low ridge covered with grass, and is about 80 feet in length. It consists of a mass of minute lapilli with included blocks of a doleritic type containing minute felspars in an isotropic ground mass. The lapilli contain crystals and are seldom vesicular. The vent near the village of Hopton presents no feature in the landscape and is not separated by any difference of contour from the surrounding limestones. It penetrates the upper beds of the limestone. A short distance to the east and also immediately north of the agglom- erate the limestones are seen dipping 10 to 20 south-east. Good sections of the igneous rock are seen along the Wirksworth and Carsington road, and also on the side of the road which has been cut in the east bank of the ravine leading down from Via Gellia on the north. The rock consists of a coarse tumultuous agglomerate composed of masses of small lapilli and angular blocks of basalt and dolerite. It is pierced by several veins or small dykes of a black fine grained rock enclosing red coloured fragments which contain felspars and are probably small pieces of agglomerate caught up by the dykes. Near the village of Bonsall a coarse agglomerate is exposed in the banks of Ember Lane and in the field above. There is a large admixture of calcareous material with the volcanic detritus. It consists of an intimate mixture of limestone fragments and of volcanic lapilli. This mass of agglomerate probably marks the site of a vent, though there are no exposures very near to it showing the dip of the limestone. The adjacent limestones consist of a quartz rock or silicified limestone from which all traces of bedding have been obliterated. Near the village of Kniveton are several exposures of Toadstone 15