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Show 368 ALTERATIONS PRODUCED [Ch. XXVI. theory by direct experiments on the fusion o~ rocks in the l~?o· ratory, and by observation of the changes m the compos1t1~n and texture of stratified masses, as they approach or cotne m contact with igneous veins and dikes. In studying t?e latter class of phenomena, we have the advantage of examming the condition of the rock at some distance from the dike where it has escaped the influence of heat, and its state where it has been near to, or in contact with, the fused mass. The changes thus exhibited may be regarded as the results of a series of experiments, made on a great scale by nat.ure u~der e~ery variety of condition, both as relates to the mmeral mgredtents of the rocks, the intensity of heat or pressure, the celerity or slowness of the cooling process, and other circumstances. St·rata altered by volcanic dikes-Plas Newydd.-We shall select a few examples of these alterations in illustration of our present argument. One of the most interesting is the modification of strata in the proximity of a volcanic dike near Plas Newydd, in Anglesea, described by Professor Henslow. The dike is 134 feet wide, and consists of basalt (dolerite of some authors), a compound of felspar and augite. Strata of shale and argillaceous limestone, through which it cuts perpen· dicularly, are altered to a distance of thirty, or even in some places to thirty-five feet, from the edge of the dike. The shale, as it approaches the basalt, becomes gradua~ly ~ore compact, and is most indurated where nearest the JUncti?n. Here it loses part of its schistose structure, but the separat10n into parallel layers is still discernible. In several places the shale is converted into hard porcellanous jasper. In the most hardened part of the mass the fossil shells, principally ProductlB, are nearly obliterated, yet even here their impressions may frequently be traced. 'I.'he argillaceous limesto~e undergoes analogous mutations, losing its earthy texture as. 1t appl'Oaches the dike, and becoming granular and crystallme. But the most extraordinary phenomenon is the appearance in t~e shale of numerous crystals of analcime and garnet, wh1ch are " I• I Ch. XXVI.] BY VOLCANIC DIKES. 369 d!sti~ctly confined to those portions of the rock affected by the d.tke . Garnet~ have been observed, under very analogous circumstances, m High 'reesdale, by Professor Sedgwick, whet·e. the! also oc.cur in shale and limestone, altered by a basaltic d1ke. This discovery is most interesting, because garnets often abound in mica-schist, and we see in the instances above cited, that they did not previously exist in the shale and limestone, and that they have evidently been produced b heat in rocks in which the marks of strati£cation have no; been effaced. Stirling Castle.-To select another example: we £nd in the rock of Stirling Castle, a calcareous sandstone fractured and forcibly displaced by a mass of green-stone, which has evidentl invaded the strata in a melted state. 'l'he sandstone has bee~ indurated, and has assumed a texture approaching to hornstone near the~unction. So also in ~rthur's Seat and Salisbury Craig, near Edmburgh, a sandstone IS seen to come in contact with greenstone, and to be converted into a jaspidcous rock t. Antrim.-In the north of Ireland, in several parts of the c~unty of Antrim, .chalk, with flints, is traversed by basaltic dtkes. The chalk IS converted into granular marble near the basalt, the change sometimes extending eight or ten feet from the 'vall of the dike, being greatest at that point, and thence gradually decreasing till it becomes evanescent. ' 'l,he extreme ~ffect,' says Dr. Berger, ' presents a dark brown crystalJinc hmestone, the crystals running in flakes as large as those of coarse primitive limestone; the next state is saccharine, then fine-grained and arenaceous ; a compact variety having a porcellanous aspect, and a bluish-grey colour succeeds ; this, towards the outer edge, becomes yellowish-white, nnd insensibly graduates into the unaltered chalk. '!'he flints in the altet·ed chalk usually assume a grey yellowish colour :t.' All • Trans. of Cambridge Phil. Soc., vol. i. p. 406. ~ lllust..~f Hutt. Theory, § 253 and 261. Dr.l\faccnlloclJ, Gcol. Tmns. 1st senes, vol. 11• p. 305. ' t Dr. Btlrger, Geol. Trans., 1st series, vol. iii. p. 172. Vo1., III. 2B |