276 ME. H. H. AKNOLD-BEMKOSE ON [Aug. I907, 



Bonsall Sill (PI. XXI). — This mass of intrusive rock lies imme- 

 diately north-west of the village of Bonsall, covering the surface of 

 the ground for a distance of two-thirds of a mile from east to west 

 and over half a mile from north to south. The area covered by the 

 igneous rock is greater than that of any other sill in the county. 

 On the north it is bounded by the lower lava-flow of the Matlock 

 district, and probably dips under it. On the west it cuts across the 

 limestones between Bonsall Lane and Moorlands Lane, which dip 

 from 50° to 20° in a northerly direction. On the east it is bounded 

 by the quartz-rock and quartzose limestones of Pounder Lane. 1 



Numerous springs rise from the surface of this rock north of the 

 village, and the ground is in many places more or less swampy. 

 To the presence of the sill, which is impervious to water, the inhab- 

 itants of Bonsall owe their water-supply. Big blocks of the dolerite 

 are seen in the fields, and the loose stone walls are mainly composed 

 of it. There is no visible junction between the igneous rock and 

 the limestones above it ; but a cursory examination of the ground 

 shows that the igneous mass occurs at different horizons in the 

 limestone. Near its southernmost extension, in the upper part of 

 the village near the Methodist Chapel, the rock is much decomposed 

 and fine-grained, but in other places it is a very hard and coarse- 

 grained rock spangled with glittering golden-coloured patches of the 

 pseudomorphs of olivine described in a previous paper. 2 Seventeen 

 thin slices, from various parts of the mass, have been examined 

 under the microscope. The rock varies from a very coarse-grained 

 dolerite without olivine, through an ophitic and subophitic olivine- 

 dolerite, to an olivine-dolerite with granular augite. 



The coarse-grained dolerite is exposed at a distance of about a 

 sixth of a mile from the south-eastern boundary of the sill. It is 

 the only rock of this structure that I have found in the county. 



Thin slice 643 3 consists of augite, plagioclase- felspar, and 

 magnetite or ilmenite, with patches of (probably) interstitial 

 matter. In some portions of the thin slice the augite encloses the 

 large felspar- crystals and laths, forming the ophitic structure on 

 a large scale ; in others the augite is enclosed in the felspar, with 

 an approach to the pegmatitic structure. In others again, the 

 felspar and augite have interfered with each other's growths : some 

 felspars apparently crystallized before the augite, and others after. 

 The felspars attain a length of 2*25 millimetres, and both the felspars 

 and the augites are nearly as big as the idiomorphic olivines in the 

 ophitic parts of the igneous mass. 



The ophitic type is rich in olivine-crystals, which attain the size 

 of 3*3 x 2-25 millimetres. The -olivine occurs both as idiomorphic 

 crystals, and in groups or nests of crystals among the felspars and in 

 the augite-plates. In some specimens it is unaltered, except along 



i Quart. Journ. Geol. Soc. vol. liv (1898) pp. 169-82 & pis. xi-xii. 



2 Ibid. vol. 1 (1894) p. 616. 



3 This and other numerals applied to the slices correspond to the number 

 of the slides in the Author's collection. 



