Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/402

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Dr. Mac Culloch on the Geology of

Rona without pointing out to future observers the singular contortions exhibited as well by the gneiss as by the hornblende rock where they come into contact. I may also venture to suggest that the rock of Blue Bay, described as a mixture of hornstone, chalcedony and quartz, appears to be a modification of gneiss, exceedingly intersected by veins of quartz and felspar, and probably of granite. It exhibits no genuine hornstone; that part of it which has at first sight this appearance, being an intimate mixture of quartz and felspar. The chalcedony too is only that modification of quartz which occurs in graphic granite, and which is well characterized by its peculiar waxy look. Exposure to the weather produces on its surface a glossy enamel, of an opacity and lustre still more nearly resembling that of chalcedony, but which does not penetrate into the stone. A similar enamel is to be observed investing the sandstone of Jura and of Schihallien, and I have also specimens of the granite of Rockall, which exhibit the same appearance. It seems to arise from a partial solution of the silex on the surface, and has been observed in certain foreign sandstones, being quoted as an instance of the solubility of silex in water. This quartz occasionally assumes a high red hue, which is evidently to be traced to the penetration and intimate mixture of red felspar. In some places it is of a green colour, being penetrated by threads and laminæ of a substance bearing a resemblance to steatite rather than to hornblende. It thus forms a greenish stone, probably different from quartz coloured by actinolite which is called prase, from the same stone penetrated by epidote which has obtained no name, or from quartz tinged by green earth, which occurs in Rum and which, as well as chalcedony when similarly coloured, is known by the name of heliotrope. Yet, as it possesses the external characters and aspect of prase, it may be safely referred to a species of which colour