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Show 370 ALTERATION OF STRATA [Ch. XXVI. traces of organic remains are effaced in that part of the lime. stone which is most ct·ysta1line. As the carbonic acid has not been expelled, in-this instance, from that part of the rock which must be supposed to ~ave been melted, the change must have taken place under considerable pressure; for we know, by the experiments of Sir James Hall, that it would require the weight of about 1700 feet of sea-wa ter ' un rl1ich would be equivalent to the pressm·e of a column of liquid lava 600 feet high, to prevent this acid from being given off. Another of the dikes of the north-east of Ireland has con-verted a mass of red sandstone into hornstone *. lly another, the slate-clay of the coal-measures has been indurated, and has assumed the character of flinty slate t ; and in another place the slate-clay of the lias has been changed into flinty slate, which still retains numerous impressions of ammonites+· One of the greenstone dikes of the same. country passes throug~ a bed of coal, which it reduces to a cmder for the space ofmne feet on each side~· The secondary sandstones in Sky are converted into solid quartz in several places where they come in contact ·with veins or masses o f ti·ap ·, and a bed of quartz, says Dr. Macculloch, has been found near a mass of trap, among the coal-strata of Fife, which was in all probability a stratum of ordinary sandstone subsequently indurated by the act~on of he~t II· . cr .Alterations of strata in contact wtth grantte .. -Havmo selected these from innumerable examples of mutatwns caused by volcanic dikes, we may next consider the changes produced by the contiguity of plutonic rocks: To some. of tl~ese we have already adverted, when speakmg of gramte ~ems, and endeavouring to establish the igneous origin of gramte. We t . d that the main body of the Cornish granite sends men wne .r arse forth veins through the killas of that country ll' a co . argillaceous schist, which is converted into hornblende-schist * Rev. W. Conybcare, Geol. Trans., 1st scrif.!s, vol. iii. P· 20l.r ' 253 • r. t lhirl. Jl· 213, and Playfair, lllust. of llutt. Thea),§ , t lhul., p. 20;J. + • .,. Sec diagram, No. 87. § lbiu., p. 206. II Spt. of Geol.,_vol. l.J>· 206. ll Ch.XXVI.], IN CONTACT WITH GRANITE. 371 near the contact with the veins. These appearances arc well seen at the junction of the granite and killas in St. Michael's Mount, .a small island nearly 300 feet high, situated in the bay, at the distance of about three miles from Penzance. In the department of the Hautes Alpes, in France, near Vizille, M. Elic de Beaumont traced a black argillaceous limestone, charged with belemnites to within a few yards of a mass of granite. Here the limestone begins to put on a No. 90, Junction of granite wit!~ JurussitJ o1· oolite st1·ata in tlle Alps, nea1• Cham pol eon. granular texture, but is extremely :fine-grained. When nearer the junction it becomes grey and has a saccharoid structure. In another locality, near Champoleon, a granite composed of quartz, black mica, and I'ose-coloured felspar, is observed partly to overlie the secondary rocks, producing an alteration which extends for about thirty feet downwards, diminishing in the inferior beds which lie farthest from the granite. (See woodcut No. 90.) In the altered mass the argillaceous beds are hardened, the limestone is saccharoid, the grits quartzose, and in the midst of them is a thin layer of an imperfect granite. It is also an important circumstance, that near the point of contact both the granite and the secondary rocks become metalliferous, and contain nests and small veins of blende, galena, iron, and copper pyrites. The stratified rocks become harder and more 2 B 2 |