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 



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