Vol. 63.'] THE XOADSTONES OP DEKBYSHIEE. 251 



dolerite with minute felspar-microliths and laths in an isotropic 

 groundmass. A fuller description appears in my previous paper. 1 



Monk's-Dale Vent. — Monk's-Dale Vent is near the centre of 

 the northern tract. In Monk's Dale, where the road from Tideswell 

 to Buxton crosses the dale, is a small exposure of agglomerate 

 almost elliptical in area. The valley loses its gorge-like character, 

 and for a short distance opens out into an oval-shaped flat-bottomed 

 valley. The bottom and the lower parts of the south-western slope 

 are occupied by the agglomerate which cuts across the strike of the 

 limestone-beds. The prevailing dip of the limestones on the 

 northern, north-western, and north-eastern boundaries of the 

 igneous rock is from 15° to 20° north-eastward, and on the south 

 the dip is about 10° southward. The agglomerate consists of 

 lapilli in a limestone- or calcite-cement. 2 This vent is probably the 

 oldest in the northern area. On both sides of the dale the lower 

 lava is seen. Barometer-measurement proved the bottom of this 

 lava to be 190 feet above the highest part of the agglomerate. 

 Allowing for the dip of the beds between them, there must be 

 about 300 feet of limestone between the agglomerate and the lava- 

 flow above it. As the bottom of the lower lava is about 620 feet 

 below the top of the limestone, Monk's-Dale Vent is some 970 feet 

 down in the Mountain-Limestone Series. 



Calton-Hill Vent. — This vent is the largest in the county, 

 and probably furnished the upper lava-flow of the northern tract. 

 About a mile west of Taddington, two-thirds of a mile north 

 of Chelmorton, and a mile and a half south-west of Miller's-Dale 

 Station, Calton Hill forms a well-marked feature in the land- 

 scape, and can be seen at a distance standing out clearly in front 

 of the escarpments of Chelmorton Low and the other hills to the 

 south of it. Its external form is like that of a volcanic neck. 

 The sides of the hill rise to a height of about 200 feet above the 

 ground on its northern, western, and eastern flanks ; while, for a 

 short extent on the south, it is connected with the higher ground or 

 escarpment between Chelmorton Low and Sough Top. The summit 

 is 1319 feet above sea-level. The base of the hill measures about 

 two-thirds of a mile from east to west, and half a mile from north 

 to south. The igneous rock of which the hill is composed cuts 

 across the strike of the limestones, which in its neighbourhood 

 steadily dip in a direction nearly due south at an angle of 10°. On 

 the Geological-Survey map it is marked as a pear-like extension of 

 the lava-stream in a northerly direction. The ground covered by 

 the igneous rock is, however, much greater than that indicated by 

 the Geological Survey. The hill consists of olivine-basalt and 

 dolerite, with a small patch of agglomerate a short distance north- 

 east of its summit. The slopes are strewn with large blocks of 



1 ' On the Microscopical Structure of the Carboniferous Dolerites & Tuffs 

 of Derbyshire' Quart. Journ. Geol. Soc. vol. 1 (1894) pp. 625-20. 



2 Ibid. pp. 029-30. 



