Yd. 6$.~] THE TOADSTONES OF DERBYSHIRE. 273 



where he states that it was 17 fathoms thick, the shaft was sunk 

 through a vesicular lava (blocks of which are to be found on the old 

 hillocks), probably the lower lava of the district. 



The Black-Hillock shaft was in the coarse dolerite, and probably 

 in the pipe up which the intrusive rock came. On the "Hillock are 

 blocks of ophitic olivine-dolerite, and of a dolerite with granular 

 augite. 



The intrusive rock is generally a typical ophitic olivine-dolerite ; 

 but in places the augite loses its ophitic character, and forms larger 

 or smaller irregularly-shaped grains between the big felspar-laths. 

 In an old limekiln north of the sill are seen blocks of a laminated 

 calcareous tuff, consisting of small rounded pebbles of limestone and 

 lapilii, some of which are minutely vesicular, others containing 

 felspar-laths cemented by carbonate of lime. They are probably 

 from a tuff connected with the lower lava. 



The Potluck sill is about 800 feet higher in the Mountain Lime- 

 stone Series than the Peak-Forest sill. This estimate is based on 

 the dip of the limestone-beds and the fall of the ground between the 

 two exposures. 



Waterswallows Sill (PI. XIX). — A little more than 1 mile 

 north-east of^ Buxton and south of Waterswallows, is a boss of 

 dolerite about half a mile in diameter. It forms a slight eminence, 

 the highest parts of which are 40 feet above the surrounding country. 

 A few small exposures are seen, and numerous blocks of the hard 

 igneous rock are distributed over the ground. Apparently it cuts 

 through the lower lava of the northern area and the limestone-beds 

 on the north-east. This is the only large intrusive mass in which 

 I have not found the ophitic type of dolerite, although the coarse- 

 grained structure met with in other sills is present. 



The rock varies from a coarse-grained olivine-dolerite in which 

 felspar predominates, through an intermediate type, to a fine-grained 

 dolerite or basalt with small portions of interstitial matter. The 

 three types may be thus tabulated : — 



Eig felspars. Smaller felspars. Very small felspars. 



Augite in big grains and Smaller augite-grains. Small augites. 



prisms. 

 Olivine in small pheno- Big olivines. Big olivines. 



crysts. 



Tideswell-Dale Sill (PI. XIX) was described in vol. lv (1899) 

 of the Society's Journal, pp. 239-49 & pis. xix-xx. 



The intrusive mass attains a thickness of about 60 feet, and may 

 be divided into five bands or zones. The central portion, which is 

 at least 6 feet thick, consists of a coarsely-crystalline ophitic olivine- 

 dolerite ; above and below it are bands of the type with granular 

 augites and big felspars (compare the Waterswallows Sill, above). 

 The base and top consist of a fine-grained olivine-dolerite, with 

 granular augite and felspars. If the measurements of the two lowest 



Q.J. G.S. No. 251. u 



