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blende, mica, titaniferous iron, and occasionally augite. The name is derived from rpaXvc, rough, and expresses the harshness of the substance to the touch. Trachyte is of a whitish or grey colour, and appears to have been an abundant product of volcanoes during the tertiary period, which have continued to the present to erupt trachytic lava. It occurs largely in Hungary and Auvergne, and in vast quantities in South America, some of the loftiest heights of the Cordilleras being composed of it. The products of extinct volcanoes are entirely identical with those that are now active, and so strikingly resemble the older basalts and the other Plutonic rocks in general, that the common origin of the whole, modified by different circumstances, may be inferred.

There are no examples of igneous formation in process now within the bounds of our own island; but the granitic and trappean groups are abundantly disseminated, and bear witness to the fiery activity of former periods. Playfair estimated the granite in Scotland to occupy an area of 940 geographical square miles, equal to 1250 English miles, or about a twenty-fifth of the country; but a more recent estimate gives to the granite of North Britain an area of 1760 miles. In England it is supposed to be under 300 miles, or 2000 in all; while the trap in both parts of the kingdom is not less than 3000 square miles, making the whole extent of the igneous formations in Great Britain amount to 5000 miles; about one seventeenth of the whole surface, the larger proportion of which belongs to Scotland. We must look for the origin of the Plutonic rocks to those interior regions of the globe from whence the igneous products of the present era are erupted; but while the formation of the different groups has transpired from similar causes, it has taken place under different circumstances. The granites, excepting the most ancient, have been erupted, cooled, and solidified, under the pressure of superincumbent strata; the traps have undergone the same process, under the depth and pressure of the ocean; and the volcanic rocks have refrigerated and become solid masses at or near the surface, in contact with the atmosphere. These diverse conditions will explain many diversities of structure in these productions of the interior fires; and, as involving a different rate of cooling, they may account for the remarkable distinction between granitic veins and porphyritic and basaltic dykes, which has given rise to so much speculation. Granite and its varieties are found intruding into the minute fissures of the stratified masses, producing small diverging branches; while the trappean rocks generally pass through them without these ramifications. It seems likely that the circumstances under which the trap erupted admitted of a more rapid cooling and reduction to solidity than what obtained in the case of the granite. The latter retaining its fluidity longer, penetrated into the smail rents of the strata, which the former, for the reason assigned, had not the capacity to do.

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CHAPTER III.

GNEISS, MICA SCHIST, AND CLAY SLATE SYSTEMS.

T is an opinion sustained by the force of geological evidence, that a time has been, in the history of the earth, when it was in an incandescent condition-a molten mass of matter; but whether this was its first estate

the commencement of that cycle of change it has undergone-or merely another change from anterior solidity, is a point upon which geology can pronounce no verdict. The proof of the first hypothesis appears in the universal diffusion and igneous character of the granitic rocks, which, as we have had occasion to remark, while constituting some of the loftiest eminences of the globe, we are warranted in regarding as a vast crystalline floor, upon which all the stratified formations repose. The reader will bear in

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mind the fact, that at various eras subsequent to the first deposition of strata, the granite pavement has undergone fusion, and been protruded in a fluid state among the sedimentary formations. The production of a solid crust would be the result of this incandescent igneous mass losing its heat by radiation into surrounding space, and this crust would exhibit a surface diversified with inequalities, marked with projections and depressions, which may be called mountains, valleys, rents, and ravines, according as the various parts of the surface might radiate heat in an unequal manner. A strictly analogous case is the cooling down of a mass of metal after fusion in a furnace, upon which blisters and scoria are formed, presenting a similar proportion to the quantity of metal, as our highest mountains at present do to the mass of the planet. Upon the ordinary atmospheric and aqueous agencies, the decomposing energy of the air,— the fall and percolation of rains, and the flow of streams coming into play on a solid rind thus diversified - a consequent disintegration must have taken place, and the carrying away of the abraded material held in suspension or solution by the rivers, with its ultimate deposition at the bottom of those basins into which they discharged their waters, would be the result. In this manner we have strong grounds to believe that the first stratified rocks were produced-the gneiss and mica-schist systems as well as all subsequent deposits; for it is in harmony with operations that are clearly traceable as in progress in the existing constitution of the earth.

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The gneiss and mica-schist with part of the slate systems form the primary strata of the improved Wernerian classification; the inferior order of Mr. Conybeare; the inferior stratified, non-fossiliferous group of Sir H. De la Beche; the metamorphic rocks of Sir C. Lyell; and the Agalysian rocks of Alexander Brongniart.

Gneiss System.The essential ingredients of gneiss are the same as granite-quartz, felspar and mica, or hornblende but sometimes one or more of these minerals are absent, and other substances supply their place, which contributes to vary the general

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compound. The elements of gneiss, however, show decided symptoms of being less entire than in granite, having lost their asperity, had the sharp angles rounded or broken. off, as if water-worn, and display, more or less, a laminated and stratified structure. These circumstances mark the aqueous origin of the rock, which, though obscure in some cases, so as to render it difficult to distinguish it from granite, are in general very well developed. Three principal varieties have been noticed: --

Mica Schist.

Gneiss.

Granite.

The granitic, closely resembling granite, the crystalline grain being large and distinct. The schistose, having a slaty appearance, and easily splitting into layers.

The laminar, having the various minerals disposed in distinct alternating lamina, which give a striped appearance to the rock.

The last variety is represented above in contrast with granite. The lamina are frequently undulating and contorted in a very high degree, indicating "a troubled condition of the water from which the ingredients fell; or a source of agitation in the still yielding sediment, which seems scarcely ever to have occurred among the secondary and later strata. The only plausible explanation of this remarkable i circumstance which has occurred to us," says Mr. Phillips, "is the agitation of the sea or i the soft sediment on its bed by heat; exactly as in the bottoms of steam-boilers, the calcareous sediment is formed in irregular undulating lamina, which appear on a cross | section very similar to the flexures in the lamina of gneiss." Except in the variety which closely resembles granite, gneiss is very distinctly stratified, as in the beds about Loch Sunart in Argyleshire, the strata exhibiting considerable contortions and convolu- | tions. The colour is usually of a greyish or reddish white, but has a darker tinge when hornblende instead of mica enters into its composition. Almost all the metals are met with in gneiss; the Saxon, Bohemian, and Saltzburg mines are worked in this rock; but in Great Britain the metallic ores are not rich in such situations. At Strontian in the Scotch highlands, a lead vein, occurring at the junction of the gneiss and the granite, was formerly celebrated.

Gneiss has a very wide geographical range, for hardly any considerable range of mountains occurs without it. In England and Wales it is scarcely known at all, but in Scotland it occupies an estimated area of 9600 square miles. The external aspect of the country in which it predominates has been thus described: "Occupying large tracts of the central highlands, the characters which gneiss impresses on the scenery are very distinctly seen. It is the least picturesque and most monotonous of the primary rocks, the hills being flat and shapeless, and their sides embossed with small round protuberances, between which the water stagnates and moss accumulates. Most gneiss districts seem but a repetition of these features on a great scale, the hills being seldom serrated in outline, or broken into rocky cliffs; whilst the valleys or straths are wide and flat, full of small lakes or pools, and disfigured by brown heaths and dark morasses. It has altogether the aspect of a land newly raised from the ocean, in which the rivers have not had time to hollow out channels for themselves, or to complete its drainage. In a word, all who are in quest of the picturesque should avoid the pure gneiss districts, as the few spots worthy of notice will be found separated by long dreary uninteresting tracks." One of these excepted spots, we may suppose to be Cape Wrath, the north

western point of Scotland. This is a lofty headland of gneiss, interstratified with hornblende, and intersected with granite veins. It projects far out into the rough Atlantic. The waves driven by the northern storms have cut deep fissures and caverns in the promontory; the pinnacles around it, and the rocks at its base starting up from the bottom of the ocean, striking the imagination as so many guards against the further invasion of the Cape.

Mica-schist System. The mica-schist is a compound essentially of quartz and mica, and thus differs from granite and gneiss by the absence of felspar as a constituent. Sometimes the mica predominates, or the quartz, and occasionally felspar occurs with other substances, as hornblende, chlorite, and talc, producing hornblende, chlorite, and talcose schists. Common garnet, also, and staulorite, are often so abundant as to become constituents, and originate the varieties of garnetiferous and staurolitiferous mica-schist. In some varieties, it exhibits a fine and regular laminar structure, which gives to the rock a fibrous aspect, as on the preceding page. In talcose and chlorite schist, the distinctive elements take the place of mica, quartz forming the other ingredient. They are usually found associated together, but the chlorite is more abundant than the talc, and easily distinguishable by the green colour, flexible structure, and soapy feel of its characteristic mineral. Hornblende-schist has a dark-green colour approaching to black, but it is of rare occurrence in an independent form, Ben Lair in Ross-shire being the principal mass in Great Britain. Mica-schist, on the contrary, is often silvery or pearly white with intermingling shades of grey. The same able Scotch geologist, whose description of the gneiss districts of his native country has been given, thus speaks of the associate system: "Mica slate has but little economical value, though in exposed situations forming a useful substitute for roofing slate. Either alone or mixed with other beds, it forms extensive districts, whose scenery assumes various features, according to the power of the rock to resist decomposition. Where soft and easily destroyed, the mountain districts to which it is mostly confined are low, tame, undulating, and seldom broken by precipices or projecting cliffs; where it is hard and quartzose, they have a more picturesque character, the hills rising to 4000 feet or upwards, and running out in long serrated edges, with peaked or dome-shaped summits, abrupt precipices, and deep craggy ravines, overhung with birch, or partially covered with vegetation. As illustrations of these, it is enough to mention Killicrankie, Loch Katterin, and the Trosachs, the fantastic ridges of Glencroe, or the bold, rugged, and sterile mountains on the west coast." The annexed sketch exhibits the two great formations in their order of succession

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above the granite. Classifying both groups together, the careful geologist just quoted proposes the following arrangement, introducing the subordinate strata, which form component parts of them in the order of superposition.

UPPER SERIES. Chlorite slates, apparently uniting the clay-slate and mica-slate deposits. No organic remains. It ranges on the whole south-east border of the Grampians, from near Aberdeen to Argyleshire.

MIDDLE SERIES. Mica-schists, primary limestone, quartz-rock, in various combinations, the former by far the most predominant, the others only locally important; the limestone occurs in different parts of the series. That of Loch Earn, Inverary, and Balahulish, is in the upper part, approaching to chlorite-slate; that of Glen Tilt, and of the vale of Loch Tay, is in the lower part. No organic remains. This occupies a great part of the eastern and southern Highlands, the north-west of Ireland.

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LOWER SERIES. Gneiss, with primary limestone, quartz-rock, hornblende slate, &c. Gneiss is the predominant rock, and varies much in all respects; the others are of local Mica-schist alternates with gneiss. No organic remains. The gneiss series occupies nearly all the Hebrides, and very large tracts of the northern and north-western Highlands.

We have no important examples of mica-schist in South Britain, but in Scotland it occupies a calculated area of 4150 square miles. In connection with gneiss, most of the considerable mountain ranges on the face of the globe present rocks belonging to these systems, uplifted, as Professor Phillips remarks, upon an axis of unstratified granitic masses, so as to be inclined at high angles to the horizon. "The great European basin is defined by irregular elevations of this kind from the frozen sea to the Atlantic; by the Uralian and Caucasian chains, the ranges of Asia Minor, Greece, South Italy, and the Atlas; the irregular western border of Spain, Ireland, the north of Scotland, and Scandinavia, is of similar structure. Within this area, the Sierras of Spain, the Pyrenees, the Alps, and many minor mountains, show the extremely wide expansion of these oldest known systems of stratified rocks."

The primary limestone, mentioned as a subordinate member of both systems, forms irregular beds, indistinctly stratified, or large lenticular masses thinning out, and again increasing. It is composed of calcareous earth in its pure state, is generally white and crystalline, and bears such a striking resemblance to loaf-sugar as to be called saccharine, or chaux carbonatée saccharoid, by the French. The white variety is the statuary

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