The term porphyry is usually applied to a rock with a paste or base of compact felspar, in which felspathic crystals of various sizes ;sume their natural form. The variety of their mineralogical cha-

Lcters, the admirable polish which can be given to them, and which tenders them eminently useful for ornamentation, give to the porphy-ies an artistic and industrial importance, which would be greatly enhanced if the difficulty of working such a hard material did not •render the price so high.

The porphyries possess various degrees of hardness and compactness. When a fine dark-red colour — which contrasts well with the white of the felspar — is combined with hardness, a magnificent stone is the result, susceptible of taking a polish, and fit for any kind of ornamental work; for the decoration of buildings, for the construction of vases, columns, &c. The red Egyptian porphyry, called Rosso ajitico, was particularly sought after by the ancients, who made sepulchres, baths, and obelisks of it. The grandest known mass of this kind of porphyry is the Obelisk of Sextus V. at Rome. In the Museum of the Louvre, in Paris, some magnificent basins and statues, made of the same stone, may also be seen.

In spite of its compact texture porphyry disintegrates, like other rocks, when exposed to air and water. One of the sphinxes transported from Egypt to Paris, being accidentally placed under a gutter of the Louvre, soon began to exhibit signs of exfoliation, notwithstanding ithad remained sound for ages under the climate of Egypt. In this country, and even in France, where the climate is much drier, the porphyries frequently decompose so as to become scarcely recognisable. They crop out in various parts of France, but are only abundant in the north-eastern part of the central plateau, and in some parts of the south. They form mountains of a conical form, presenting, nearly always, considerable depressions on their flanks. In the Vosges they attain a height of from three to four thousand feet. The Serpentine rocks are a sort of compact talc^ which owe their soapy texture and greasy feel to silicate of magnesia. Their softness permits of their being turned in a lathe and fashioned into vessels of various forms. Even stoves are constructed of this substance, which bears heat well. The serpentine quarried on the banks of Lake Como, which bears the name of pierre ollaire, or pot-stone, is excellently adapted for this purpose. Serpentine shows itself in the Vosges, in the Limousin, in the Lyonnais, and in the Var; it occupies an immense tract in the Alps, as well as in the Apennines. Mona marble is an example of serpentine; and the Lizard Point, Cornwall, is a mass of it. A portion of the stratified rocks of Tuscany, and also those of the Island of Elba, have been upheaved and overturned by eruptions of it.

As for the British Islands, plutonic rocks are extensively developed inScotland, where the Cambrian and Silurian rocks, often of gneissic character — associated here and there with great bosses of granite and syenite — form by far the greater part of the region known as the Highlands. In the Isle of Arran a circular mass of coarse-grained granite protrudes through the schists of the northern part of the island; while, in the southern part, a finer-grained granite and veins of porphyry and coarse-grained granite have broken through the stratified rocks.* In Devonshire and Cornwall there are four great bosses of granite; in the southern parts of Cornwall the mineral axis is defined by a line drawn through the centre of the several bosses from south-west to north-east; but in the north of Cornwall, and extending into Devonshire, it strikes nearly east and west. The great granite

♦ " Geology of the Island of Arran," by Andrew C. Ramsay. ** Geology of Arran and Clydesdale," by James Bryce.

mass in Cornwall lies on the moors north of St. Austell, and indicates the existence of more than one disturbing force. "There was an elevating force," says Professor Sedgwick,* "protruding from the St. Austell granite; and, if I interpret the phenomena correctly, there was a contemporaneous elevating force acting from the south; and between these two forces, the beds, now spread over the surface from the St. Austell granite to the Dodman and Narehead, were broken, contorted, and placed in their present disturbed position. Some great disturbing forces," he observes, " have modified the symmetry of this part of Cornwall, affecting," he believes, " the whole transverse section of the country from the headlands near Fowey to those south of Padstow." This great granite-axis was upheaved in a line commencing at the west end of Cornwall, rising through the slate-rocks of the older Devonian group, continuing in association with them as far as the boss north of St. Austell, producing much confusion in the stratified masses; the granite-mass between St. Clear and Camel-ford rose between the deposition of the Petherwin and that of the Plymouth group; lastly, the Dartmoor granite rose, partially moving the adjacent slates in such a manner that its north end abuts against and tilts up the base of the Culm-trough, mineralising the great Culm-limestone, while on the south it does the same to the base of the Plymouth slates. These facts prove that the granite of Dartmoor, which was formerly thought to be the most ancient of the Plutonic rocks, is of a date subsequent to the Culm-measures of Devonshire, which are now regarded as forming part of the true carboniferous series.

Volcanic Rocks.

Considered as a whole, the volcanic rocks may be grouped into three distinct formations, which we shall notice in the follo^ving order, which is that of their relative antiquity, namely: — i. Trachytic; 2. Basaltic; 3. Volcanic or Lava for matiotis

Trachytic Formations.

Trachyte (derived from fpo-x^^^ ^oi^g^i), having a coarse, cellular appearance, and a rough and gritty feel, belongs to the class of volcanic rocks. The eruptions of trachyte seem to have commenced towards the middle of the Tertiary period, and to have

See Quarterly Journal of Geological Society ^ vol. viii., pp. 9 and 10.

continued up to its close. The trachytes present considerable analogy in their composition to the felspathic porphyries, but their mineralogical characters are different. Their texture is porous; they fomi a white, grey, black, sometimes yellowish matrix, in which, as a

i'ig. 4.— A peak of the Cantal chain.

rule, felspar predominates, together with disseminated crystals of felspar, some hornblende or augite, and dark-coloured mica. In its external appearance trachyte is very variable. It forms the three most elevated mountain ranges ot Central France; the groups of Cantal and Mont Dore, and the chain of the Velay (Puy-de- Dome).*

• For full information in reference to the rocks and geolofjy of this part of France, the reader is referred to the masterly work on "The Geology and Extinct Volcanoes of Central France," by G. Poulett Scrope, 2nd edition, 1858. '

I.—Peak of Sancy in the Mont Dore group, Auvergne.

The igneous group of Cantal may be described as a series of lofty summits, ranged around a large cavity, which was at one period probably a volcanic crater, the circular base of which occupies an area of nearly fifteen leagues in diameter. The strictly trachytic portion of the group rises in the centre, and is composed of high mountains, throwing off spurs, which gradually decrease in height, and terminate in plateaux more or less inclined. These central mountains attain a height varying between 4,500 and 5,500 feet above the level of the sea. A scaly or schistose variety of trachyte, called pho7iolite^ or clinkstone (from the ringing metallic sound it emits when struck with the hammer), with an unusual proportion of felspar, or, according to Gmelin, composed of felspar and zeolite, forms the steep trachytic escarpments at the centre, which enclose the principal valleys; their abrupt peaks giving a remarkably picturesque appearance to the landscape. In the engraving on p. 40 (Fig. 4) the slaty, laminated character of the clinkstone is well represented in one of the phonolitic peaks of the Cantal group. The group at Mont Dore consists of seven or eight rocky summits, occupying a circuit of about five leagues in diameter. The massive trachytic rock, of which this mountainous mass is chiefly formed, has an average thickness of 1,200 to 2,600 feet; comprehending over that range prodigious layers of scoriae, pumiceous conglomerates, and detritus, interstratified with beds of trachyte and basalt, bearing the signs of an igneous origin, tufa forming the base; and between them occur layers of lignite, or imperfectly mineralised woody fibre, the whole being superimposed on a primitive plateau of about 3,250 feet in height. Scored and furrowed out by deep valleys, the viscous mass was gradually upheaved, until in the needle-like Pic de Sancy (Plate I.), a pyramidal rock of porphyritic trachyte, which is the loftiest point of Mont Dore, it attains the height of 6,258 feet. The Pic de Sancy, represented on page 40 (Fig. 4), gives an excellent idea of the general appearance of the trachytic mountains of Mont Dore.

Upon the same plateau with Mont Dore, and about seven miles north of its last slopes, the trachytic formation is repeated in four

rounded domes — those of Puy-de-D6me, Sarcoui", Clierzou, and Le Grand Suchet. The Puy-de-D6me, one of the most remarkable volcanic domes in Auvergne, presents another fine and very striking example of an eruptive trachytic rock. The rock here assumes a peculiar mineral character, which has caused the name of doinite to be given to it. The chain of the Velay forms a zone, composed of independent plateaux and peaks, which forms upon the horizon a long and

Strangely intersected ridge. The bareness of the mountains, their forms — pointed or rounded, sometimes terminating in scarped plateaux — give to the whole landscape an appearance at once picturesque and characteristic. The peak of Le Mezen, which rises 5,820 feet above the sea, forms the culminating point of the chain. The phonolites of which it consists have been erupted from fissures which present themselves at a great number of points, ranging from north-north-west to south-south-east. On the banks of the Rhine and in Hungary the trachytic formation presents itself in features identical with those which indicate it in France. In America it is principally represented by some immense cones, superposed in the chain of the Andes; the colossal Chim-borazo being one of those trachytic cones.

Basaltic Formations.

Basaltic eruptions seem to have occurred during the Secondary and Tertiary periods. Basalt, according to Dr. Daubeny,* in its more strict sense, " is composed of an intimate mixture of augite with a zeolitic mineral, which appears to have been formed out of labradorite (felspar of Labrador), by the addition of water — the presence of water being in all zeolites the cause of that bubbling-up under the blow-pipe to which they owe their appellation." M. Delesse and other mineralogists are of opinion that the idea of augite being the prevailing mineral in basalt, must be abandoned; and that although its presence gives the rock its distinctive character, as compared with trachytic and most other trap rocks, still the principal element in their composition is felspar. Basalt, a lava consisting essentially of augite, labradorite (or nepheline) and magnetic iron-ore is the rock which predominates in this formation. It contains a smaller quantity of silica than the trachyte, and a larger proportion of lime and magnesia. Hence, independent of the iron in its composition, it is heavier in proportion, as it contains more or less silica. Some varieties of basalt contain very large quantities of olivine, a mineral of an olive-green colour, vith a chemical composition differing but slighdy from serpentine. Both basalts and trachyte contain more soda and less silica in their composition than granites; some of the basalts are highly fusible, the alkahne matter and lime in their composition acting as a flux to the silica. There are examples of basalt

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