Gneiss is the predominating rock of Norway, and of all the north of Europe. It abounds in the Southern Alps, and the Pyrenees; in Greece; in the United States of America; and in the south of that continent, it reigns paramount over the loftiest chains of the Andes of Quito. Humboldt observed it also in the mountains of Parama and Venezuela.

This figure represents flexures in the gneiss beds of Bernera, an island in the Lewis; from Macculloch's Western Islands.

2. MICA SCHIST is a rock of a slaty texture, composed of mica and quartz. The mica almost always predominates, at least it is the most apparent. It is commonly gray, sometimes bordering upon yellow, more frequently on green, in which case it approaches to talc. Here it is not in spangles, as in granite, or in separate scales as in gneiss. The plates, or imperfect crystals, are almost always so blended or run together, that the whole forms continuous leaf-like pellicles. The quartz has its ordinary gray colour and vitreous aspect; it is in small plates of a nearly lenticular form, and interposed flatly between the leaflets of mica. These lens like pieces are often pretty thick, even half an inch in their centre, or become sometimes nearly

globular. The leaflets of mica are always wrapped round them; and when we break schists of this kind, as they always cleave in the middle, these balls or nodules of quartz, stand out on the fragment, and present, under the film of mica, a resemblance to tumours on the skin of an animal. At other times the grains of quartz are so small, and so enveloped in the micaceous matter, that they cannot be discerned; in which case, the rock seems to consist entirely of mica.

The mica passes frequently into talc, and the mica-schist, into talc-schist. One of the most striking examples of this transition is in Auvergne, near Saint-Sernin, on the road from Mauriac to Aurillac. The mica-schist there graduates, without changing even its colour, into a talcous schist, so unctuous to the touch, that if the rock slope ever so little, it is difficult to stand upright upon it. The foot slides, as if it were rubbed over with soap. On the southern slope of the great Alps, from Monte-Rosa to Mont Blanc, the predominant rock is talcous schist, but it becomes at intervals mica-schist, and exhibits all its geognostic characters, in other respects.

Mica-schist is often studded with garnets. Saussure descending the Simplon to Duomo D'Ossola, walked on a path that was almost paved over with them, standing out from its surface like heads of nails. Humboldt observed them, in the micaschists of America, but they were not so numerous as in the gneiss. The leaflets of the mica enclose the crystals of garnet, just as they do the nodules of quartz.

CLAY-SLATE FORMATION.

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As mica-slate differs from gneiss, chiefly in wanting felspar, so it readily passes into that rock by the admission of that mineral.

Mica-schist is very distinctly stratified; not, however, in so marked a manner as gneiss, and its strata are often contorted, and folded back in a great variety of forms, as with gneiss. See fig. p. 103.

Of all the primitive formations, mica-schist is the one which contains most foreign rocks; such as beds of talc, quartz, garnets, limestone, &c. Not only does it alternate with gneiss and clay-slate, but it even passes immediately into these rocks in the same district, and sometimes in the same stratum. This introduction of crystals of felspar, sometimes gives it the aspect of granite.

Mica-clate is peculiarly rich in metals, which exist more frequently in beds than in veins. Gold, silver, copper, tin, cobalt, and iron, are found in it.

Mica-slate is perhaps the most extensive of all the primitive strata, at least, in the Alps, in France, Germany, the Highlands, and Islands of Scotland, &c. In the Pyrenees, the formation of mica-schist presents a long ragged belt, resting immediately on granite, and much less extensive than it. In the north of Europe, mica-slate is almost as abundant as gneiss. It is found in different points of the mountains of the United States; and has been observed near Cumana, by Humboldt.

3. CLAY-SLATE.-This rock is well known from its extensive use in roofing houses. It is simple in its aspect; schistose; in the cross fracture dull, earthy, and fine grained. It is opaque, easily reduced to a gray powder, whatever be its colour in mass, and

melts before the blow-pipe into a blackish scoria. Its most ordinary colours are gray, more or less deep, and bluish-black, or sometimes greenish-gray. Iron is its usual colouring principle; but in the black varieties, there is carbonaceous matter. The greater number of the clay-slates split very readily into thin leaves, sometimes plane, sometimes more or less waved. Their surface is occasionally smooth, now and then deeply striated; and dull, or shining with a pearly or silky lustre.

The upper micaceous schists, those which differ most in position and characters from gneiss, appear composed, almost entirely, of mica. When its scales become more compact, and separate only in thicker leaflets, it passes into clay-slate. M. Cordier observed in the Pyrenees, an insensible passage of the one of these rocks, into the other, a fact noticed by Saussure, Von Buch, and many other mineralogists. In these cases the rock must undergo some modification of its constituents.

This is the most distinctly stratified of rocks; and its folia, or tables, are parallel to the fissures of the stratification, but they are sometimes so contorted or folded, and the folds continue in the same direction to such an extent, that a portion of the tables comes to be inclined or even perpendicular to the stratification in a part of the bed. There are, however, cases where the rock presents folia oblique to the plane of its beds. A remarkable one occurs in Saxony, four miles north of Freyberg. The mountain consists of very distinct layers, mingled with much limestone, alternating with other beds impregnated with plumbago, which renders the stratifica

PRIMITIVE LIMESTONE.

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tion very manifest. The first beds split asunder very readily, forming with the surface of superposition an angle of about 60°. Count Bournon has remarked that several schists split up very commonly under angles of 60° and 120°; a result which he thought due to the presence of mica.

The strata of the clay-slate formations are in general highly inclined; a position into which they must have been evidently raised by eruptive force. They contain a great number of foreign beds; as schistose talc, chlorite-schist, lydian stone, quartz, and limestone.

Clay-slate is rich in metals, particularly its modification called gray-wacke. Its distribution on the surface of the globe is very extensive. In Scotland it skirts the Highlands from Loch Lomond by Callender, Comrie, and Dunkeld. In the whole of this extensive district, it rests on, and graduates into mica-slate. On the continent it has been traced through a great extent of country; Saxony, Bohemia, Silesia, Franconia, Bavaria, the Alps of Switzerland, Austria, Hungary, and many other parts of Europe. It is found abundantly in the United States, and in immense quantity in South America. Nearly the whole territory between Potosi and Lima is said to consist of clay-slate.

4. PRIMITIVE LIMESTONE is always granular, of a semi-crystalline aspect, translucent on the edges, and in colour white or gray. The limestones formed among granite and gneiss rocks, are usually coarser grained, than those associated with clay slate. They often include masses of mica and quartz. Primitive limestone is not in general a stratified rock,