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Show 372 PASSAGE OF GNE1SS INTO GRANITE. [Ch. XXVI. crystalline, but the granite, ~n th~ co;trary, softer and less perfectly crystallized near the JUncuon. • • . It will appear from sections .described by M. Hug~, that some of the secondary beds of hmestone and slate, whiCh are in a similar manner overlaid by granite, have been altered into gneiss and mica-schist t· Some of th.ese altered sedimentary formations are supposed, by M. El,lC de Beaumont, to be of the age of the lias of England, and others to be even as modern as the jurassic or oolite formations. We can scarcely doubt, in these cases, that the heat communicated by the granitic mass reduced the contiguous strata to semi-fusion, and that on cooling slowly the rock assumed a ca·ystaUine texture. 'l~he experiments of Gregory Watt prove, distinctly, that a rock n~ed not be perfect}! melted in order that a re-arrangement of 1ts component particles should take place, and that a more crystalline texture should ensue. We may easily suppose, therefore, that all traces of shells ~nd other organic remains may be destroyed, an~ that new chemical combinations may arise, without the mass bemg so fused as that the lines of stratification should be wholly obliterated. In allusion to the passage from granite to gneiss before described, Dr. Macculloch remarks, that' in numerous parts of Scotland, where the leading masses of gneiss are schistose, evenly stratified, and scarcely ever traversed by granite veins, they become contorted and irr~g~lar as they approach ~he granite; assuming also the gr~mt1c cha~acter, and ~e~~mmg intersected by veins, numerous m proportiOn to the v1c1mty ~f the mass. The conclusion,' he adds, 'is obvious; the flmd 'te has invaded the aqueous stratum as far as its influence gram . . · l' could reach, and thus far has filled it w1th vems, d1sturbec 1ts reo-ularity and generated in it a new mineral character, often ab~olutely confounded with its own. And if the more remote beds, and those alternating with other rocks, are not thus "' Elie de B~aumont, Sur les Montagnes de l'Oisans, &c., Mem. de Ia Soc. d'Ilist. Nnt. de Paris, tome v. 1· Natur. Historischc A.lpenrcisc, Solcme, 1830. Ch.XXVI.J AL'rERED ROCKS, t 373 affected, it is not only that it has acted less on those, but that, if it had equally affected them, they never could have existed, or would have been a11 granitic and venous gneiss*. According to these views, gneiss and mica-schist may be nothing more than micaceous and argillaceous sandstones altered by heat, and certainly, in their mode of stratification and lamination, they correspond most exactly. Granular quartz may have been derived from siliceous sandstone, compact quartz from the same. Clay-slate may be altered shale, and shale appears to be clay which has been subjected to great pressure. Granular marble has probably originated in the form of ordinary limestone, having in many instances been replete with shelJs and corals now obliterated, while calcareous sands and marls have been changed into impure crystalline limestones. Associated with the rocks termed primary we meet with anthracite, just as we find . beds of coal in sedimentary formations, and we know that, in the vicinity of some trap dikes, coal is converted into anthracite. ' Hornblende schist,' says Dr. Macculloch, 'may at first have been mere clay, for clay or shale is found altered by trap into Lydian stone, a substance differing from hornblende-schist almost solely in compactness and uniformity of texturet.' 'In Shetland,' remarks the same author, 'argilJaceous schist (or clay-slate), when in contact with granite, is sometimes converted into hornblende-schist, the schist becoming first siliceous, fmd ultimately, at the contact, hornblende-schist t.' This theory, if confirmed by observation and experiment, may enable us to account for the high position in the series usually held by clay slate relatively to hornblende-schist, as also to gneiss and mica-schist, which so commonly alternate with hornblende-schist. For we must suppose the heat which alters the strata to proceed, in almost all cases, from below upwards, and to act with greatest intensity on the inferior strata. If, therefore, several sets of argillaceous strata or shales be superimposed upon each other in a vertical series of beds in the same * Syst. of Gcol., vol. ii. p. 145. t Ibid., vol. i, p. 210. t Ibid., p. 211 |