Page:VCH Cornwall 1.djvu/75

 GEOLOGY This relation of the metamorphic aureole to cooling conditions finds substantial corroboration in the smaller granite mass of Godolphin, the more rapid consolidation of which is seen in its strongly developed chilled edge, while its limits of metamorphism are confined to a few hundred yards. These peculiar zones of metamorphism are sometimes observed where visible granite is absent ; Mr. E. Dixon has recorded two such metamorphic areas, one north of Gwinear, the other around Leeds- town, from which we may confidently infer that bodies of granite lie below the surface. Moreover the great elvan region which abuts against the granite at Gwennap contains large areas of spotted slate, but the meta- morphism is more feeble than that found bordering the granite mass. The granite of the Brown Willy district frequently contains large porphyritic crystals of felspar, and is more particularly schorlaceous towards the south. The granite district of Hensborough or St. Austell is far more variable and more schorlaceous. This granite was described by Dr. Boase and in more recent years by Mr. J. H. Collins. The latter observer alludes to the large size of the porphyritic felspars, and calls attention to veins almost entirely composed of that mineral, while others consist almost exclusively of mica. He also records the occurrence of both lithia and fluorine in the micas of this granite, and the relation which the latter element bears to the decomposition of felspar into kaolin, with which the very existence of our china clay industries is linked. As the industries and minerals of Cornwall will be treated in a separate section it will suffice here to state that china clay and china stone are the products of the decomposition of the granite in situ, and that this chemical alteration appears to be mainly brought about by the action of fluoric acid. The relative abundance of fluorine therefore in the original rock mass is probably the determining factor in the production of china clay, and its restriction as regards commercial purity to limited areas. In the Carn Menelez granite for instance the decomposition of its mass rarely yields china stone, and in conjunction with this fact we find an almost entire absence of fluor spar except in association with the mineral veins. According to Mr. Collins bands of greisen are very frequent in the Hensbarrow granite where decomposed. This mixture of quartz and white mica is common in the Cornish granites, and has in almost all cases been brought about by modification of the granite by subsequent chemical action resulting in the removal of the felspar. The schorlaceous portions of the granite are in many cases the products of a similar alteration but with a different result. The beautiful rock found in detached blocks in the neighbourhood of Luxullian is a variety of tour- maline granite, in which the conversion of a part of its felspar into clear quartz crowded with tourmaline needles has been described in its various stages by Professor Bonney, F.R.S., while brown grains of tourma- line are supposed to represent the mica of the granite. To attempt the 27