lines of the fissures that traverse it in every possible direction. Large lumps, though broken into small morsels, the size of a nut, do not show the least appearance of a fresh fracture. It is astonishing, to trace the degree in which this mountain is cracked and cleft; and still more so, to examine the surface of these fissures. They are every where covered with small rhombohedrons, which sometimes present their faces, at others, their edges and their angles. These rhomboids are crystallized dolomite (magnesian limestone). The masses of augite rock which are the causes of so extraordinary a change, are not far off. They are seen at the foot of the hill of Santa Agatha; they cross the gorge of the Fersina, and re-appear on the great road, in the very village of Cognola. They constitute black coloured balls with concentric coats, and a solid nucleus of augite greenstone or basalt.

M. Von Buch has given a drawing of Langkofel, a mountain of Dolomite in the valley of Groden in the Tyrol, of very singular aspect. One can hardly imagine how such an obelisk could protrude itself through the strata, and rise up alone to a vertical height of 4000 feet. It is entirely inaccessible, and even the boldest chamois can hardly find on it a blade of grass to browse. Here we behold a vast pyramid of snow-white semi-crystallized magnesian limestone, standing alone above the dark green base of augite porphyry. Towards the right, the nearly horizontal limestone strata are observable which contributed their calcareous matter towards the formation of the dolomite. From an extensive induction M. Von Buch concludes that the augite

ERUPTIVE ORIGIN OF THE DOLOMITE.

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porphyry can transform into dolomites any calcareous bed, which it may chance to traverse under suitable conditions, whether that bed be the limestone crag, or of the oolitic series of Jura.

Below the place called Sotto i Sassi in the valley of Fassa there are some striking eruptive phenomena. Here the raising of a particular rock is not the question; but of the total mass of the mountains, and consequently an extensive territory. In fact, the strata of red sandstone and shell limestone occur in such abrupt positions, and at such different heights, that we try in vain to refer all these separate portions to a general level, or even to a common inclination, from their elevation of more than 7500 feet above Saint-Pellegrin, to that of 900 feet above Cattern and Tramin. But wherever these sandstones, and calcareous beds, present such lofty escarpments, they are crowned with Dolomite, in which case we are at no loss to discover somewhere at hand, the augite rock which has raised them. It thence appears natural to conclude that all these beds have been elevated by the augitic porphyry, as well as the dolomite, especially since we can hardly conceive how the porphyry could pierce them without pushing them up. A proof of this penetration is given in the geological profile of the Duron; where that prodigious variety of level and façade are seen which accompanies masses violently erupted. Reflecting on the effects of these upheav ings, we shall be less surprised to meet with petrifactions of anomia in the sandstones and calcareous beds 8000 feet high near Sasso in Val Fredda. These petrifactions, prior to the eruptive catastrophe

These

were placed far below the level of the seas. elevations belong therefore to periods subsequent to the emergence of the primeval land; and are introduced by me here merely to prove the cause and manner of that emergencę.

On reflecting on these striking facts, we must not forget that the calcareous ejections of Vesuvius are also constantly composed of granular dolomite. Their aspect alone shows their nature; and the chemical analyses of Smithson Tennant, and Leopold Gmelin, fully confirm this indication from external character. Entirely analogous phenomena are presented in the environs of Rome, on the borders of the lake of Albano; the peperino (an indurated volcanic tuffa) is replete with blocks of dolomite of a dazzling whiteness. The limestones of Vesuvius are cleft and split, like the dolomites of Fassa. These dolomites are not confined to that Alpine valley; but they are to be found every where, and under every form, between Pusterthal and the frontiers of Italy. They surround, like rocky islands, with peaks of prodigious elevation, the upper part of the valley of Gröden and Wolkenstein; and thence stretch far and near, over many a valley of augitic greenstone, or pyroxenic porphyry.-Annales de Chim. et de Phys. xxiii. 71.

Having premised these general observations, I shall give a particular description of each of the three great primitive envelopes of the earth.

1. GNEISS. As this schistose body serves no particular purpose in the arts of life, it attracted little attention till in the progress of modern geology, the importance of its scientific relations appeared.

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The name, gneiss, was the technical term by which the miners of Freyberg distinguished the roof of their veins, when it was altered, and of a steatitic or greenish appearance, whatever might be its mineral nature, whether granite, porphyry, mica-slate, or our actual gneiss. The little treatise of Werner, on the classification of rocks, which appeared in 1787, finally fixed the acceptation now attached to the word gneiss. He describes gneiss, as a rock composed of felspar, quartz, and mica, immediately adhering to each other, possessing at once a granitic and a schistose texture. The felspar and the quartz occur in grains aggregated together, which produce the granitic structure. These aggregates are assembled in small plates, betwixt which intervene scales of mica. In this way, the slaty texture is produced. Gneiss differs from granite not only in texture, but also from containing commonly a greater proportion of mica; to the abundance of which mineral, it owes its foliated aspect. Felspar is the predominating principle in gneiss, especially in the deepest seated strata; though it is in less quantity than in granite, relatively to the two other ingredients. It occurs in grains of middle size, or even very small, and of a white or grayish-white colour. The quartz is for the most part in smaller grains. It has a vitreous aspect, and an ash-gray colour. The mica. of gneiss is in small spangles, often distinct; but sometimes intimately attached to each other, so as to form continuous leaves of moderate size. Its most usual colour is gray, which passes often through every shade to black.. To its differences the chief variations in the appearance of gneiss are due;

because this rock splitting always in the line of these leaflets, has its fragments coated with them. Gneiss frequently contains crystals of hornblende.

Garnet is the mineral most commonly included in gneiss, occurring thus in the north of Europe, in Norway, and in Greenland, where the crystals are often as large as a nut, and in prodigious number. Humboldt remarked them in great quantities, both red and green, in the gneiss of America, particularly at the Caracas.

Gneiss is very distinctly stratified, and whenever it reposes on a granitic mass, it wraps itself round it, and follows its sinuosities.

It is the rock which contains, at least in Europe, most metallic substances. There is hardly one which has not been found in it, and in such abundance as to become an object of mining. The metallic ores are sometimes in veins, but more commonly in beds. Gold is found in it in Dauphiny, at the foot of Monte-Rosa, and in the territory of Salzburg. The threads and veins of the mountain of Chalances, near Allemont, that afford silver, cobalt, antimony, &c., are in gneiss. This rock, in the Vosges, includes many metals. In the gneiss of Auvergne, threads of lead, silver, and antimony occur. At Freyberg, as also in Bohemia, rich and celebrated mines of silver, lead, &c., are worked. The famous copper mines of Fahlun in Sweden occur in the same rock. It contains iron ore in profusion; as in the iron mines of Scandinavia, at Dannemora, Utoë, and Arendal.

In America, according to Humboldt, gneiss is much less metalliferous.