250 ME. H. H. ARNOLD-BEMKOSE ON - [Aug. I907, 



and proves, I think, conclusively that the lava seen in Chee Dale 

 and at the entrance to Chee-Tor Tunnel is the lower or second lava 

 of the district, and not a third bed of toadstone as suggested in the 

 Geological-Survey Memoir. 



Thickness of Limestones between the Upper and Lower Lava-Flows 

 in the Valleys hereunder named. 



Valley. Hills or Valleys. Thickness in feet. 



Chee Dale Knot Low. Chee Tor 142 to 160 



Miller's Dale Priestcliffe Lees. Miller's Dale 143 



Monk's Dale Knot Low. Monk's Dale 130 



Monk's Dale Crichley Wood. Monk's Dale 140 to 149 



Sandy Dale Priestcliffe Low. Sandy Dale, at Black- 

 well ; two measurements 130 to 143 



Chelmorton Flat ... Chelmorton Low. Chelmorton Flat ... 130 



I made the following measurements of the thickness of the upper 

 and lower lavas. The vertical distances were measured by the 

 barometer, corrected for changes in the weather : due allowance 

 being made for the dip of the limestones, and for the horizontal 

 distance between the outcrops. 



Thickness in feet. 



Upper lava . . . Knot Low, Miller's Dale 100 



Crichley Hill, Miller's Dale 97 



Lower lava . . . Monk's Dale, Miller's Dale 70 to 80 



Near Wheston, north-west of Tideswell ... 84 



The upper and lower lavas of Fin Cop near Ashford are about 30 feet thick, 

 and are separated by about 150 feet of limestone. 



Vents. 



The three vents in the northern area occur in the Mountain 

 Limestone. Two of them consist of fragmental rock or agglomerate, 

 and the third (which is by far the largest) mainly of massive 

 basalt. 



Speedwell Vent, Castleton. — This vent is the northernmost 

 exposure of igneous rock in Derbyshire, and stands on the extreme 

 northern verge of the limestone-area. 1 It lies south of the road 

 from Castleton to the Winnats, about a quarter of a mile south-east 

 of Speedwell Mine, and may be distinguished as a slight feature in 

 the landscape. It is elliptical in area, the longer axis extending 

 from west to east for a distance of about 80 feet, and evidently cuts 

 across the strike of the limestone-beds by which it is surrounded. 

 The limestones north, west, and south of the vent dip into the 

 valley at angles varying from 15° to 20°, and a short distance to 

 the westward, the limestones are seen to strike at the agglomerate. 

 The material filling the vent consists of minute lapilli containing* 

 felspars but seldom vesicular, and blocks of a fine-grained olivine- 



1 Sir A. Geikie, ' Ancient Volcanoes of Great Britain ' vol. ii (1897) p. 16. 



