233 



dusty magnetite and grains or well-formed crystals of epidote frequently 

 occur. The titaniferous iron-ore occurs in ragged masses often made 

 up of parallel rows of plates intersecting at angles of 60 and 120. It 

 is often transformed into leucoxene and sometimes into granular aggre- 

 gates of vividly polarizing sphene. Calcite and quartz often occur 

 as secondary products. Apatite is almost always recognizable. 



The rocks just described bear the closest resemblance to the ophitic 

 diabases of Ncrth and South Wales. Olivine, at any rate as a rule, is 

 conspicuous by its absence. Diabases with porphyritic crystals of felspar 

 appear to be rare. They occur, however, in some localities, as for example 

 at Addicornbe near Torquay. 



The diabases of the above type occasionally become schistose. A 

 good example of this occurs at Hope's Nose near Torquay. The rock is 

 of a dark greenish colour and possesses a marked schistosity. Under the 

 microscope it is seen to consist of pseudomorphs after ophitic augite, a 

 water-clear felspar (? albite) which is associated with much calcite in 

 the form of fine powder, and titaniferous iron partially replaced by 

 leucoxene. In some places the leucoxene consists of granules of vividly 

 polarizing sphene. In portions of the slide the ophitic structure is re- 

 placed by a micro-flaser structure which marks the planes of schistosity ; 

 that is the planes along which movement has taken place. This rock 

 has been profoundly affected by surface agencies as Avell as by dynamic 

 metamorphism. At the Town's Mills quarry near Liskeard there occurs 

 a green slaty rock of an exceptionally interesting character. It shows 

 a marked fissility and splits readily into thin slabs. A thin section cut 

 at right angles to the schistosity shows a well defined micro-flaser structure. 

 The constituents are plagioclase, chlorite and iron-ores mainly pyrites. 

 The felspar is very fresh but gives everywhere evidence of having 

 suffered from great mechanical disturbance. The twin lamellae are bent 

 and the individuals are often broken. The chlorite is arranged in many 

 planes which wind in and out amongst the broken felspathic matter 

 and thus define the flaser structure to which the schistosity of the rock 

 is due. The rock was in all probability a massive dolerite. It is now 

 a felspathic chlorite-schist. 



The dolerites (diabases) containing basaltic hornblende and rich 

 brown mica (not the pale brown mica so frequently developed near the 

 contact with granite) appear to be much less common than the normal 

 ophitic diabases. They have been recognized at Wearde, Ernsettle, Grove 

 and Trcluggan in the Plymouth district ; also near St. Minver in North 

 Cornwall. A rock from Treluggan consists of a matrix of cloudy felspar, 

 containing somewhat ill-formed crystals of basaltic hornblende, colourless 

 augite, plates of deep brown biotite, grains of titaniferous iron-ore and 

 large hexagonal prisms of apatite with central cores. A fair amount 

 of chlorite has been formed in consequence of the alteration of the 

 ferro-magnesian constituents. A rock from Wearde belonging to the 

 same group resembles a syenite in external appearance in consequence 

 of the pink colour of the felspar. In microscopic structure and com- 



