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 



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