488 J. A. PHILLIPS ON THE SO-CALLED 



But little unaltered felspar was observed in the sections prepared 

 from this rock ; but patches of quartz, which are apparently imper- 

 fect pseudomorphs after that mineral, are sometimes present. These 

 and the various crystalline grains of quartz which occur in this 

 rock are traversed in all directions by hornblendic belonites. Mag- 

 netite is moderately plentiful ; but apatite is not frequently met 

 with. 



The fine-grained varieties, or " hornblende -slates " of Mr. Eogers, 

 have a finely schistose structure, but are exceedingly hard, and 

 break somewhat more easily along certain parallel planes than in 

 other directions. When examined under the microscope they are 

 found to be composed of a colourless material, through which very 

 minute crystalline flakes of hornblende are thickly disseminated 

 together with a little magnetite, quartz, and a nebulous green 

 mineral which high powers resolve into imperfect crystalline forms. 

 These rocks are often traversed by strings and veins of quartz. 



At Merrifield, two miles north-east of Liskeard, a quarry is 

 worked on a stone which diners from the coarser rock on St.-Cleer 

 Down, inasmuch as it contains crystals of plagioclase still exhibiting 

 the usual coloured striations when seen in polarized light. This rock 

 also occasionally encloses a few nearly colourless and imperfectly 

 formed crystals of epidote as a product of alteration. 



On the left bank of the stream, flowing through the valley half a 

 mile west of Trewolland, is a quarry on the road-side, which is 

 worked for a stone of the usual character, but considerably lighter 

 in colour. 



When thin sections of this rock are examined, they are found to 

 consist of crystals of plagioclase which have become externally de- 

 composed into a nearly amorphous substance, which cements them 

 together. If viewed between crossed prisms, these crystals exhibit 

 the usual coloured striations of triclinic felspar, while the cementing 

 material, apparently resulting from their decomposition, frequently 

 exercises no action upon polarized light. In this are dissemi- 

 nated patches and crystalline grains of quartz and caleite, a few 

 flakes of partially decomposed mica, occasional needles of apatite, 

 and a considerable amount of either magnetite or ilmenite. 



At Pope's Mill, two miles south-east of Liskeard, is a compara- 

 tively coarse-grained dark-coloured rock, consisting of plagioclase, 

 hornblende, and augite with brown mica, a little quartz, apatite, and 

 magnetite. The crystals of these minerals are usually imperfect ; 

 but some of the hornblende is evidently pseudomorphic after augite, 

 and, such being the case, must be regarded as uralite. 



Two miles south of Callington, on a farm called Scatchell, is a 

 dolerite of which both the felspar and augite have undergone con- 

 siderable alteration. This rock contains some of the augitic mineral 

 found in that from Hendra Chapel, Camelford, and numerous pseu- 

 domorphs after ilmenite. 



On Balstone Down, a mile south-east of the town of Callington, 

 stone of a light greyish-green colour is obtained from a quarry 



