243 



igneous origin, never take the form of dykes cutting through the schists 

 but are associated with and sometimes pass into schists of similar chemical 

 composition. The original relations of these masses of " greenstone " to the 

 surrounding rocks have therefore been to a very great extent destroyed, 

 and both the " greenstones " themselves and the surrounding rocks so 

 metamorphosed that it is often difficult or impossible to determine their 

 original characters. The most easily recognizable igneous rock in this 

 district is a porphyritic epidiorite. The porphyritic crystals are now 

 represented by white pseudomorphs, often half an inch in length, the forms 

 of the originals having been perfectly preserved. These crystals appear as 

 flattish tables with conspicuous development of the clino-pinacoid (010) ; 

 the basal plane (001) and the prism faces (110 and 110) may also be recog- 

 nized. In short, the crystals, or rather the pseudomorphs, have the forms 

 characteristic of porphyritic dolerites. Under the microscope they are 

 turbid and rarely show any traces of individual action under polarized 

 light. They lie in a dark green ground-mass which is resolved under the 

 microscope into an aggregate of water-clear plagioclase, uralitic and 

 actinolitic hornblende and opaque iron-ore. The felspar of the ground -mass 

 usually occurs in more or less lath-shaped forms but sometimes shows the 

 mosaic structure. Quartz very probably occurs in association with this 

 plagioclase, but has not been recognized with certainty. The ground-mass 

 of this rock is exactly similar to the main mass of many of the epidiorites 

 described by Mr. PHILLIPS from Western Cornwall. In addition to the 

 porphyritic epidiorites above described we find also non- porphyritic rocks 

 of a similar character. The lenticles and bosses of definitely recognizable 

 igneous rock lie, for the most part, perfectly isolated in green actinolitic 

 and hornblendic schists. Sometimes the boundary lines are tolerably sharp, 

 at other times the external surfaces of the igneous rock are schistose and it 

 is impossible to say where the massive rock ends and the schist begins. In 

 studying the development of schistosity the porphyritic epidiorites are of 

 great interest because the porphyritic crystals remain as more or less 

 rounded " eyes " in the actinolitic schist. We will now describe one of 

 these "augen-schists." Examined macroscopically the rock is a green satiny 

 schist containing smaU white spots which sometimes show the outlines of 

 felspars but are as a rule more or less lenticular in form. In thin section 

 the ground-mass of the rock is seen to consist of very long actinolitic 

 needles, water-clear felspar, iron ores and grains or crystals of rutile/ 1 ) The 

 pleochroism of the actinolite is well marked (a very pale green, /3 green, 

 7 bluish green) but not so strong as that of the same mineral in the 

 metamorphosed dolerites round the granite masses of Cornwall. The needles 

 are arranged with their longest axes approximately parallel to each other 

 in the plane of schistosity. Where the streams of actinolite needles en- 



(1) In a paper, on the " Metamorphosis of Basic Igneous Rocks," by the author (Proc. Geol. 

 Ass., Vol. X. p. 76), the rutile was erroneously described as sphene. It is highly probable that 

 the rutile has been developed at the expense of the titaniferous iron-ores of the original rocks, 

 whether directly or through the intervention sphene (leucoxene) it is difficult to say. 



