Mica-schist and Gneiss.-The stratified, laminated, or foliated metamorphic * rocks consist of two chief groups. The first, and usually uppermost, is Mica-schist, a schistose rock, abounding in a mineral called mica (from its glittering appearance), and in quartz, a substance of which rock-crystals and the semi-transparent pebbles common in most beds of shingle or beach are examples. These two minerals are disposed in alternate layers, forming laminae of greater or less thickness, which are extremely contorted and undulated. Some of these masses, especially in the upper portion, bear a considerable resemblance to the metamorphic argillaceous schists; the lower are of a more quartzose character, probably from having been subjected to a greater degree of igneous action.

Gneisst consists of laminated and contorted bed-like masses of quartz, felspar, and mica, irregularly alternating; and may, in truth, be regarded as laminated granite, for the same substances enter into their composition as prevail in the amorphous masses of that rock. Gneiss is often found associated and alternating with mica-schist, quartz-rock, clay-slate, and hard granular limestone. The whole series of stratified metamorphic beds may therefore be considered as partaking of one common mineralogical character, and, with the exception of the calcareous rocks, may have originated from the disintegration and subsequent consolidation of more ancient crystalline masses.

The stratified appearance of gneiss and mica-schist is attributed by some geologists to an arrangement of crystals of different specific gravities in horizontal planes; their subsequent softening by heat admitting of the flexuosities

York; Brooke and Miller's edition of Phillips' Mineralogy; Nicol's Mineralogy; and Mitchell and Tennant's Mineralogy (Orr's Circle), will be found among the best works on the subject in the English language.

* The metamorphic rocks are termed "hypozoic" (or below life) rocks by Phillips: see his "Manual," chap. 5.

+ A German mining term.

of these rocks. Others * believe that melted granite, upon cooling under particular circumstances, would assume a stratified or ribboned appearance, analogous to that of gneiss; or might even resemble in structure some aqueous sediments.

From the researches of Mr. H. C. Sorby it is evident that, whilst some mica-schists are truly sedimentary laminated beds, formed of the debris of older granitic rocks, others are altered rocks, which owe their foliated character, as in the case of cleavable slate-rocks (p. 835), to violent pressure (and perhaps chemical changes) at periods subsequent to their original formation.†

There are various substances associated with this group, as hornblende-schist, a black or grey rock, chlorite-schist, a green slaty rock, and the beautiful mottled magnesian rock called Serpentine, with steatite; the latter are often connected with trap.

Granite (so named from its granular structure) is the foundation upon which all the strata of which we have spoken are superimposed, and the framework of the earth's crust; rising to the loftiest heights, and stretching intc mountain-chains, which mark the grand natural divisions of the physical geography of the globe.

Although presenting great variety in the proportion and colour of its ingredients, granite is essentially composed of three substances, which may be easily recognised in the blocks of which many of our pavements, bridges, roads,

* Especially Naumann and Scrope; "Considerations on Volcanos; Neues Jahrb. 1847, p. 297; Quart. Journ. Geol. Soc. vol. iv. part 2, Miscell. p. 1; ibid. vol. xii. p. 345; and above, p. 856.

Report Brit. Assoc. 1856, rep. sect. p. 78. In a paper by Mr. D. Forbes, "On the Relations of the Silurian and Metamorphic Rocks of the South of Norway," in the Edinburgh New. Phil. Journ. new series, January, 1856, we have an ingenious attempt to show that the foliated rocks of much of that district can be hypothetically related to certain more or less horizontal lines, at right angles to the existing vertical planes of the schists, and representing the lines of bedding previous to the metamorphic changes which have induced their present vertical foliation.

and other works are constructed. These are mica, known by its silvery or glittering aspect,-quartz, by its glassy appearance, and felspar,* which forms the opaque white, pink, or yellowish masses, oftentimes seen in sections as long angular crystals, which from their size and colour may be readily detected, even by the unscientific observer. In some species of granite, tale and hornblende occur, and the mica is wanting; in the former case the rock is called protogine; in the latter syenite: those masses which are composed of crystals of felspar in a base of compact felspar constitute porphyry.

M. Bischof states that it may be demonstrated mathematically that all the sedimentary strata, and all the substances enclosed in drusy cavities, are derived from the plutonic crystalline rocks; these have furnished the materials, and water has conveyed the quartz, calcareous spars, heavy spars, metals, and other substances which fill such cavities.

One of the most important advances towards a knowledge of the exact nature and origin of granite has been made by Mr. H. C. Sorby, in his memoir, "On some Peculiarities in the Microscopical Structure of Crystals, applicable to the determination of the Aqueous or Igneous Origin of Minerals and Kocks," read before the Geological Society in December, 1857. In this paper many curious facts were described, relating to the minute cavities contained in nearly all crystals, and sometimes occupied by fluid, sometimes by stony matter; and it was shown that many crystals must have formed when both melted rock and water were present; this water being in such an expanded state as would indicate a red heat, since some of the "fluid-cavities" in the crystals serve as self-registering thermometers. It turns out that granite is a rock that was formed in this way. It is an

* Kaolin or porcelain-earth is formed by the decomposition of alkaline felspar. "See Mémoire sur les Kaolins ou argiles à porcelaine," by M. Alex. Brongniart; Archives du Muséum d'Hist. Nat. 1839 and 1841.

aqueo-igneous rock; being really due to combined agency of both water and fire: and hence, observes Mr. Sorby, have arisen the discussions as to which it is-Neptunian or Plutonic. Indeed Mr. Sorby finds in some minerals ejected from volcanos, and probably formed deep down at their base, conditions analogous to those which he observes in granite.*

22. MICA-SCHIST AND GNEISS.-These rocks are widely spread over and around the masses of unstratified plutonic rocks. They occur in Caernarvon and Cumberland, but are of inconsiderable extent in England. In Scotland they extend over great part of the Highlands, and largely prevail in the Hebrides; they form mountain-ranges in the north of Ireland, and cover large areas in Londonderry and Donegal.

The gneiss of the Northern Highlands is of two distinct ages. There is the old gneiss of Cape Wrath and the shores of Rosshire, on which rests the Cambrian conglomerates of the Ross Mountains (see p. 806); and the younger gneissose rocks of the eastern part of Sutherland, which overlie the fossiliferous limestones of Durness, and are altered strata of Silurian age. An instance of secondary gneiss resting on lias occurs in Switzerland.

The most striking features of these rocks are the flexures and contortions in which they are so generally folded; prov ing the soft and ductile state in which the component materials must have existed; for they present every variety of sinuosity and curvature imaginable.

The Isle of Lewis (one of the Hebrides), so admirably illustrated by Dr. Macculloch, is remarkable for the contortions observable in its precipitous cliffs of gneiss, and the innumerable granite-veins with which they are traversed. The face of the rocky cliffs appears like veined marbled paper; and the imagination can scarcely conceive an intri* See Appendix A.

cacy or interlamination of this nature, of which a resemblance could not be found in the cliffs of Lewis.* From the decomposition and falling away of the sur

rounding parts of the rocks near Oreby, an interesting, perhaps solitary, example occurs of a bent and detached mass of gneiss, about thirty or forty feet high (Lign. 203), and which forms a highly interesting and picturesque object.

23. CONTORTIONS OF CRYSTALLINE ROCKS.-The curvatures and flexures of rocks largely composed of quartz is a subject of great interest in another point of view, because it bears upon the question as to the solution and deposition of silex; a process which appears to have been going on in the crust of the earth from the formation of the most ancient granitic rocks to the deposits now in progress. I have before remarked, that the appearance of some of the * Western Isles, p. 193.