Vol. S^.'] THE TOADSTONES OF DERBYSHIRE. 261 



the beds. Although Pilkingfcon gives 138 feet between the first 

 and second toadstones, his section, apparently drawn to scale, shows 

 only 115 feet. 1 



The foregoing evidence is sufficient to show that the two lavas of 

 the Matlock area are separated by from 85 to 100 feet of limestone, 

 instead of the 150 feet estimated by the writers of more than a 

 century ago. 



The section of the Mill-Close Shaft made by the officers of the 

 Geological Survey 2 is perhaps the most reliable estimate of the 

 thickness of the limestones above the upper lava, namely, 150 feet. 

 The section in the railway-cutting at Matlock Bath showed 97 

 feet of thickly-bedded white limestone. 3 A generalized section 

 would therefore give the following thicknesses : — 



Feet. 

 Thinly-bedded limestones, with chert. ... 53 



Thickly-bedded white limestones 97 



Upper lava 70 



Thickly-bedded limestones, 80 to 100 100 



Total 320 



Lower lava — 



[See vertical section No. II, p. 249.] 



The upper lava is therefore about 150 feet down in the series, 

 and the lower lava about 250 feet. 



Vents. 



With one exception, the vents in the Matlock area occur in the 

 Mountain Limestone, and consist of agglomerate. The Hopton 

 Yent, which is the southernmost of all, is partly in the upper beds of 

 the Mountain Limestone, and partly in the Toredale Shales, and 

 consists of a basalt-breccia. 



Cracknowl Yent, Bakewell. — Like the Speedwell Yent, this 

 is in the upper beds of the Mountain Limestone. It is about a 

 mile north-west of Bakewell and half a mile east of Ashford Hall, 

 and occupies a horseshoe-shaped slope on the eastern side of the 

 valley between Eowdale House and Lumford Mills. The shape of 

 the slope has been determined by the denudation of the softer rock, 

 which is surrounded by the harder limestone. The area is elliptical 

 in shape, the longer axis extending in the direction from north- 

 north-west to south- south-east. 



The agglomerate, which consists of small lapilli and included 

 blocks of a vesicular rock, cuts across the strike of the limestone- 

 beds to the north-north-west and south-south-east of the vent. At 

 these places the transition from limestone to the soft igneous rock 



1 ' View of the Present State of Derbyshire ' vol. i, pp. 51-55 & fig. i, p. 51. 



2 Geol. Surv. Mem. ' North Derbyshire ' 2nd ed. (1887) p. 146. 



3 Ibid. p. 23. 



