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Show 354 GRANITE VEINS. [Ch.XXV. t1l emse1 v es m· t o gt·an1' te , in a manner analogous to that of the vo1 c am·e d1'k e s o f E~ t11a and Vesuvius, where they cut and shift each other, or pass through alternating beds of lava and tufF. No. 85. The annexed diagram will explain to the reader the manner in which these granite veins often branch off from the principal mass. Those on the right-hand side, and in the middle, are taken from Dr. Macculloch's representation of veins passing through the gneiss at Cape Wrath, in Scotland*. The veins on the left are described, by Captain Basil Hall, as traversing the argillaceous schist of the Table-Mountain at the Cape of Good Hopet. No. 86. o1·anite f!eins traf!crsing gneiss at Cape Wrath, in Scotla11d. We subjoin another sketch from Dr. Macculloch's interesting • Western Islands, plate 31. . t Account of the structure of the Table-Mountain, &c., Trans. Roy. Soc. Edm., vol. vii. Ch.XXV.] ORANITE VEINS. 355 representations of the granite veins in Scotland, in which the contrast of colour between the vein and some of the dark varieties of hornblende-schist associated with the O'neiss renders 0 the phenomena more conspicuous. The following sketch of a group of granite veins in Cornwall is given by Messieurs Von Oeynhausen and Von Dec hen*. No. 87. Granite veins passing tliro-ugh Hornblende slate, Carnsilver Cove, Cornwall. The main body of the granite here is of a porphyritic appearance with large crystals of felspar j but in the veins it is nne-grained and without these large crystals. The general height of the veins is from 16 to 20 feet, but some are much higher. The vein-granite of Cornwall very generally assumes a finer grain, and frequently undergoes a change in mineral composition, as is very commonly observed in other countries. Thus, according to Professor Sedgwick, the main body of the Cornish granite is an aggregate of mica, quartz, and felspar; but the veins are sometimes without mica, being a granular aggregate of quartz and felspar. In other varieties quartz prevails to the almost entire exclusion both of felspar and mica; in others, the mica and quartz both disappear, and the vein is simply composed of white granular felspart. Changes are sometimes caused in the intersected strata very * Phil. Mag. and Annals, No. 27, new Series, March, 1829. t On Geol. of Cornwall, Trans, of Cambridge Soc., vol, i. p. 124. 2A2 |