HARKER: NOTES ON NORTH OK p;NGLAND ROCKS. 200. 



(iii) Grcisen of Grainsgill, near Carrock Fell, Cumberland. — 

 This rock, probably of very local occurrence, is seen in hand- 

 specimens to consist of silvery white mica and quartz. Owing to the 

 tendency of the rock to break along the soft, well-cleaved mica-flakes, 

 these seem to make up a much greater part of the rock than is 

 actually the case. 



In a thin section these two minerals are seen to constitute the 

 main bulk of the rock, but little felspar being present, and only a few 

 grains of opaque magnetite. The mica is in thick flakes or in confused 

 sheaf-like bundles of smaller scales, and is recognised at once by its 

 marked cleavage traces and bright red and green polarisation-tints. 

 It is partly moulded by the quartz, and partly wedged in between 

 the grains of that mineral. The quartz, with dust-like inclusions, 

 forms irregularly-shaped grains, often enclosing or moulding round 

 one another. 



(iv) Granophyre of Buckbarrow, Cumberland. — This rock is 

 the ' syenite ' of some early writers and the ' chloritic granite ' of 

 Mr. Clifton Ward, but, although a red granite in general appearance, 

 its true structure, as revealed by the microscope, is that known as the 

 granophyric or micro-pegmatitic. The bulk of the slide is seen to 

 consist of this micro-pegmatite, a minute intergrowth of felspar and 

 quartz. Over considerable areas of the slide the little wedges of 

 quartz have a common optical orientation, as is shown by their 

 simultaneous extinction between crossed Nicols ; the same is 

 true of the felspar intergrown with the quartz. In this granophyric 

 ground-mass are embedded crystals of felspar, grains of quartz, and 

 scales of biotite. The felspars are partly orthoclase, but the longer 

 and narrower crystals by their striation and low extinction-angles are 

 proved to be an acid variety of plagioclase (albite or oligoclase). 

 A very beautiful feature of the rock is the evident relation of the 

 micropegmatite to the felspar crystals about which it has grown. 

 The felspathic constituent of the micropegmatite is in crystalline 

 continuity with the felspar crystal nearest to it, and extinguishes in 

 the same position with it between crossed Nicols. Some of the 

 felspar crystals (orthoclase) show twinning on the ' Carlsbad law,' 

 and in these cases the surrounding micropegmatite divides into 

 two parts, in crystalline and optical continuity, as regards its 

 felspathic portion, with the two halves of the twin crystal 

 respectively. 



The rock contains some biotite flakes, but these are all more or 

 less converted by secondary changes into a green mineral of the 

 chlorite family. Another secondary mineral is epidote, which 

 appears in nearly colourless, brilliantly polarising grains, patches, 



July 1889. O 



