Royal Geological Society of Cornwall

1921 Geological Magazine  
The granite of the island is not of normal character, but shows idiomorphic crystals of quartz and felspar in a fine-grained crystalline ground-mass. The predominant type of alteration is greisening, the granite being traversed by a series of narrow parallel veins of greisen, with a central portion of pegmatite or of quartz, which carries such minerals as cassiterite, blende, wolfram, chalcopyrite, molybdenite, apatite, lithia mica, fluor, tourmaline, beryl, etc. The slate in contact with the
more » ... contact with the granite shows evidence of intense thermal metamorphism, and in some cases there is evidence of the granite having absorbed the slate along the contact. Impregnation of the slate by minute granitic veins is also common. The lodes are seen in the form of greisen veins carrying mineral and in the form of more typical fissure lodes, and they occur in great numbers in both granite and clay-slate, but are all narrow. A clean ferruginous sand occurs in the island, which in many respects resembles St. Erth Sand. Tourmalinization is the exception, and the author concludes that the veins were filled by magmatic solutions which were highly siliceous and contained fluorine to a considerable extent, but little boron.
doi:10.1017/s0016756800090221 fatcat:htsvxmwqvbeaxck5yk6vnmy6si