ERUPTIVE ROCKS.

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Louis Figuier

Nothing is more difficult than to write a chronological history of the revolutions and changes to which the earth has been subjected during the ages which preceded the historic times. The phenomena which have concurred to fashion its enormous mass, and to give to it its present form and structure, are so numerous, so varied, and sometimes so nearly simultaneous in their action, that the records defy the powers of observation to separate them. The deposition of the sedimentary rocks has been subject to interruption during all ages of the world. Violent igneous eruptions have penetrated the sedimentary beds, elevating them in some places, depressing them in others, and in all cases disturbing their order of superposition, and ejecting masses of crystalline rocks from the incandescent centre to the surface. Amidst these perturbations, sometimes stretching over a vast extent of country, anything like a rigorous chronological record becomes impossible, for the phenomena are so continuous and complex that it is no longer possible to distinguish the fundamental from the accidental and secondary causes.

In order to render the subject somewhat clearer, the great facts relative to the progressive formation of the terrestrial globe are divided into epochs, during which the sedimentary rocks were formed in due order in the seas of the ancient world, the mud and sand in which were deposited day by day. Again, even where the line of demarcation is clearest between one formation and another, it must not be supposed there is any sharply defined line of separation between them. On the contrary, one system gradually merges into that which succeeds it. The rocks and fossils of the one gradually disappear, to be succeeded by those of the overlying series in the regular order of succession. The newly-made strata became the cemetery of the myriads of beings which lived and died in the bosom of the ocean. The rocks thus deposited were called Neptunian by the older geologists.

But while the seas of each epoch were thus building up, grain by grain, and bed by bed, the new formation out of the ruins of the older, other influences were at work, sometimes, to all appearance, impeding sometimes advancing, the great work. The Plutonic rocks—the igneous or eruptive rocks of modern geology, as we have seen above, were the great disturbing agents, and these disturbances occur in every age of the earth’s history. We shall have occasion to speak of these eruptive formations while describing the phenomena of the several epochs. But it is thought that the narrative will be made clearer and more instructive by grouping this class of phenomena into one chapter, which we place at the commencement, inasmuch as the constant reference to the eruptive rocks will thus be rendered more intelligible. To these are now added the section “Metamorphic Rocks,” from the fifth edition of the French work.

The rocks which issued from the centre of the earth in a state of fusion are found associated or interstratified with masses of every epoch, more especially with those of the more ancient strata. The formations which these rocks have originated possess great interest; first, because they enter into the composition of the terrestrial crust; secondly, because they have impressed on its surface, in the course of their eruption, some of the characteristics of its configuration and structure; finally, because, by their means, the metals which are the objects of human industry have been brought nearer to the surface. According to the order of their appearance, or as nearly so as can be ascertained, we shall class the eruptive rocks in two groups:—

I. The Volcanic Rocks, of comparatively recent origin, which have given rise to a succession of trachytes, basalts, and modern lavas. These, being of looser texture, are presumed to have cooled more rapidly than the Plutonic rocks, and at or near the surface.

II. The Plutonic Rocks, of older date, which are exemplified in the various kinds of granites, the syenites, the protogines, porphyries, &c. These differ from the volcanic rocks in their more compact crystalline structure, in the absence of tufa, as well as of pores and cavities; from which it is inferred that they were formed at considerable depths in the earth, and that they have cooled and crystallised slowly under great pressure.

Plutonic Eruptions.

The great eruptions of ancient granite are supposed to have occurred during the primary epoch, and chiefly in the carboniferous period. They present themselves sometimes in considerable masses, for the earth’s crust being still thin and permeable, it was prepared as it were for absorbing the granite masses. In consequence of its weak cohesion, the primitive crust of the globe would be rent and penetrated in all directions, as represented in the following section of Cape Wrath, in Sutherlandshire, in which the veins of granite ramify in a very irregular manner across the gneiss and hornblende-schist, there associated with it. (Fig. 3.)

Fig. 3.—Veins of granite traversing the gneiss of Cape Wrath.

Granite, when it is sound, furnishes a fine building-stone, but we must not suppose that it deserves that character of extreme hardness with which the poets have gratuitously gifted it. Its granular texture renders it unfit for road-stone, where it gets crushed too quickly to dust. With his hammer the geologist easily shapes his specimens; and in the Russian War, at the bombardment of Bomarsund, the shot from our ships demonstrated that ramparts of granite could be as easily demolished as those constructed of limestone.

The component minerals of granite are felspar, quartz, and mica, in varying proportions; felspar being generally the predominant ingredient, and quartz more plentiful than mica—the whole being united into a confusedly granular or crystalline mass. Occasionally it passes insensibly from fine to coarse-grained granite, and the finer grained is even sometimes found embedded in the more coarsely granular variety: sometimes it assumes a porphyritic texture. Porphyritic granite is a variety of granite, the components of which—quartz, felspar, and mica—are set in a non-crystallised paste, uniting the mass in a manner which will be familiar to many of our readers who may have seen the granite of the Land’s End, in Cornwall. Alongside these orthoclase crystals, quartz is implanted, usually in grains of irregular shape, more rarely crystallised, and seldom in the form of perfect crystals. To these ingredients are added thin scales or small hexagonal plates and crystals of white, brown, black, or greenish-coloured mica. Finally, the name of quartziferous porphyry is reserved for those varieties which present crystals of quartz; the other varieties are simply called porphyritic granite. True porphyry presents a paste essentially composed of compact felspar, in which the crystals of orthoclase—that is, felspar with a potash base—are abundantly disseminated, and sometimes with great regularity.

Granite is supposed to have been “formed at considerable depths in the earth, where it has cooled and crystallised slowly under great pressure, where the contained gases could not expand.”[11] “The influence,” says Lyell, “of subterranean heat may extend downwards from the crater of every active volcano to a great depth below, perhaps several miles or leagues, and the effects which are produced deep in the bowels of the earth may, or rather must, be distinct; so that volcanic and plutonic rocks, each different in texture, and sometimes even in composition, may originate simultaneously, the one at the surface, the other far beneath it.” Other views, however, of its origin are not unknown to science: Professor Ramsay and some other geologists consider granite to be metamorphic. “For my own part,” says the Professor, “I believe that in one sense it is an igneous rock; that is to say, that it has been completely fused. But in another sense it is a metamorphic rock, partly because it is impossible in many cases to draw any definite line between gneiss and granite, for they pass into each other by insensible gradations; and granite frequently occupies the space that ought to be filled with gneiss, were it not that the gneiss has been entirely fused. I believe therefore that granite and its allies are simply the effect of the extreme of metamorphism, brought about by great heat with presence of water. In other words, when the metamorphism has been so great that all traces of the semi-crystalline laminated structure have disappeared, a more perfect crystallisation has taken place.”[12] It is obvious that the very result on which the Professor founds his theory, namely, the difficulty “in many cases,” of drawing a line between the granite and the gneiss, would be produced by the sudden injection of the fluid minerals into gneiss, composed of the same materials. Moreover, it is only in some cases that the difficulty exists; in many others the line of separation is definable enough.[13]

The granitic rock called Syenite, in which a part of the mica is replaced by hornblende or amphibole, has to all appearance been erupted to the surface subsequently to the granite, and very often alongside of it. Thus the two extremities of the Vosges, towards Belfort and Strasburg, are eminently syenitic, while the intermediate part, towards Colmar, is as markedly granitic. In the Lyonnais, the southern region is granitic; the northern region, from Arbresle, is in great part syenitic. Syenite also makes its appearance in the Limousin.

Syenite, into which rose-coloured felspar often enters, forms a beautiful rock, because the green or nearly black hornblende heightens, by contrast, the effect of its colour. This rock is a valuable adjunct for architectural ornament; it is that out of which the ancient Egyptians shaped many of their monumental columns, sphinxes, and sarcophagi; the most perfect type of it is found in Egypt, not far from the city of Syene, from which it derives its name. The obelisk of Luxor now in Paris, several of the Egyptian obelisks in Rome, and the celebrated sphinxes, of which copies may be seen in front of the Egyptian Court at the Crystal Palace, the pedestal of the statue of Peter the Great at St. Petersburg, and the facing of the sub-basement of the column in the Place VendÔme in Paris, are of this stone, of which there are quarries in the neighbourhood of Plancher-les-Mines in the Vosges.

Syenite disintegrates more readily than granite, and it contains indurated nodular concretions, which often remain in the form of large spherical balls, in the midst of the dÉbris resulting from disintegration of the mass. It remains to be added that syenitic masses are often very variable as regards their composition; the hornblende is sometimes wanting, in which case we can only recognise an ancient granite. In other instances the hornblende predominates to such a degree, that a large or small-grained diorite, or greenstone, results. The geologist should be prepared to observe these transitions, which are apt to lead him into error if passed over without being noticed.

Protogine is a talcose granite, composed of felspar, quartz, and talc or chlorite, or decomposed mica, which take the place of the usual mica. Excessively variable in its texture, protogine passes from the most perfect granitic aspect to that of a porphyry, in such a manner as to present continual subjects of uncertainty, rendering it very difficult to determine its geological age. Nevertheless, it is supposed to have come to the surface before and during the coal-period; in short, at Creusot, protogine covers the coal-fields so completely, that it is necessary to sink the pits through the protogine, in order to penetrate to the coal, and the rock has so manifestly acted on the coal-measure strata, as to have contorted and metamorphosed them. Something analogous to this manifests itself near Mont Blanc, where the colossal mass which predominates in that chain, and the peaks which belong to it, consist of protogine. But as no such action can be perceived in the overlying rocks of the Triassic period, it may be assumed that at the time of the deposition of the New Red Sandstone the protoginous eruptions had ceased.

It is necessary to add, however, that if the protogine rises in such bold pinnacles round Mont Blanc, the circumstance only applies to the more elevated parts of the mountain, and is influenced by the excessive rigour of the seasons, which demolishes and continually wears away all the parts of the rock which have been decomposed by atmospheric agency. Where protogine occurs in milder climates—around Creusot, and at Pierre-sur-Autre, in the Forez chain, for instance—the mountains show none of the scarped and bristling peaks exhibited in the chain of Mont Blanc. Only single isolated masses occasionally form rocking-stones, so called because, resting with a convex base upon a pedestal also convex, but in a contrary way, it is easy to move these naturally balanced blocks, although from their vast size it would require very considerable force to displace them. This tendency to fashion themselves into rounded or ellipsoidal forms belongs, also, to other granitic rocks, and even to some of the variegated sandstones. The rocking-stones have often given rise to legends and popular myths.The great eruptions of granite, protogine, and porphyry took place, according to M. Fournet, during the carboniferous period, for porphyritic pebbles are found in the conglomerates of the Coal-measure period. “The granite of Dartmoor, in Devonshire,” says Lyell,[14] “was formerly supposed to be one of the most ancient of the plutonic rocks, but it is now ascertained to be posterior in date to the culm-measures of that county, which from their position, and as containing true coal-plants, are regarded by Professor Sedgwick and Sir R. Murchison as members of the true Carboniferous series. This granite, like the syenitic granite of Christiana, has broken through the stratified formations without much changing their strike. Hence, on the north-west side of Dartmoor, the successive members of the Culm-measures abut against the granite, and become metamorphic as they approach. The granite of Cornwall is probably of the same date, and therefore as modern as the Carboniferous strata, if not newer.”

The ancient granites show themselves in France in the Vosges, in Auvergne, at Espinouse in Languedoc, at Plan-de-la-Tour in Provence, in the chain of the CÉvennes, at Mont Pilat near Lyons, and in the southern part of the Lyonnaise chain. They rarely impart boldness or grandeur to the landscape, as might be expected from their crystallised texture and hardness; for having been exposed to the effects of atmospheric changes from the earliest times of the earth’s consolidation, the rocks have become greatly worn away and rounded in the outlines of their masses. It is only when recent dislocations have broken them up that they assume a picturesque character.

The Christiania granite alluded to above was at one time thought to have belonged to the Silurian period. But, in 1813, Von Buch announced that the strata in question consisted of limestones containing orthoceratites and trilobites; the shales and limestone being only penetrated by granite-veins, and altered for a considerable distance from the point of contact.[15] The same granite is found to penetrate the ancient gneiss of the country on which the fossiliferous beds rest—unconformably, as the geologists say; that is, they rest on the edges of the gneiss, from which other stratified deposits had been washed away, leaving the gneiss denuded before the sedimentary beds were deposited. “Between the origin, therefore, of the gneiss and the granite,”[16] says Lyell, “there intervened, first, the period when the strata of gneiss were denuded; secondly, the period of the deposition of the Silurian deposits. Yet the granite produced after this long interval is often so intimately blended with the ancient gneiss at the point of the junction, that it is impossible to draw any other than an arbitrary line of separation between them; and where this is not the case, tortuous veins of granite pass freely through gneiss, ending sometimes in threads, as if the older rock had offered no resistance to their passage.” From this example Sir Charles concludes that it is impossible to conjecture whether certain granites, which send veins into gneiss and other metamorphic rocks, have been so injected while the gneiss was scarcely solidified, or at some time during the Secondary or Tertiary period. As it is, no single mass of granite can be pointed out more ancient than the oldest known fossiliferous deposits; no Lower Cambrian stratum is known to rest immediately on granite; no pebbles of granite are found in the conglomerates of the Lower Cambrian. On the contrary, granite is usually found, as in the case of Dartmoor, in immediate contact with primary formations, with every sign of elevation subsequent to their deposition. Porphyritic pebbles are found in the Coal-measures; porpyhries continue during the Triassic period; since, in some parts of Germany, veins of porphyry are found traversing the New Red Sandstone, or grÈs bigarrÉ of French geologists. Syenites have especially reacted upon the Silurian deposits and other old sedimentary rocks, up to those of the Lower Carboniferous period.

The term porphyry is usually applied to a rock with a paste or base of compact felspar, in which felspathic crystals of various sizes assume their natural form. The variety of their mineralogical characters, the admirable polish which can be given to them, and which renders them eminently useful for ornamentation, give to the porphyries 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 antico, 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 it had 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 in Scotland, 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.[17] 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 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,[18] “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 Camelford 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 following order, which is that of their relative antiquity, namely:—1. Trachytic; 2. Basaltic; 3. Volcanic or Lava formations.

Fig. 4

Fig. 4.—A peak of the Cantal chain.

Trachytic Formations.

Trachyte (derived from t?a???, rough), 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 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 form a white, grey, black, sometimes yellowish matrix, in which, as a 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 of Central France; the groups of Cantal and Mont Dore, and the chain of the Velay (Puy-de-DÔme).[19]

Plate I

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 phonolite, 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 scoriÆ, 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-DÔme, SarcouÏ, Clierzou, and Le Grand Suchet. The Puy-de-DÔme, 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 domite 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 Chimborazo being one of those trachytic cones.

Plate II

II.—Mountain and basaltic crater of La Coupe d’Ayzac, in the Vivarais.

Fig. 5

Fig. 5.—Theoretical view of a basaltic plateau.

Basaltic Formations.

Basaltic eruptions seem to have occurred during the Secondary and Tertiary periods. Basalt, according to Dr. Daubeny,[20] 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, with a chemical composition differing but slightly 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 alkaline matter and lime in their composition acting as a flux to the silica. There are examples of basalt existing in well-defined flows, which still adhere to craters visible at the present day, and with regard to the igneous origin of which there can be no doubt. One of the most striking examples of a basaltic cone is furnished by the mountain or crater of La Coupe d’Ayzac, in the Vivarais, in the south of France. Plate II., on the opposite page, gives an accurate representation of this curious basaltic flow. The remnants of the stream of liquefied basalt which once flowed down the flank of the hill may still be seen adhering in vast masses to the granite rocks on both sides of a narrow valley where the river Volant has cut across the lava and left a pavement or causeway, forming an assemblage of upright prismatic columns, fitted together with geometrical symmetry; the whole resting on a base of gneiss. Basaltic eruptions sometimes form a plateau, as represented in Fig. 5, where the process of formation is shown theoretically and in a manner which renders further explanation unnecessary. Many of these basaltic table-lands form plateaux of very considerable extent and thickness; others form fragments of the same, more or less dislocated; others, again, present themselves in isolated knolls, far removed from similar formations. In short, basaltic rocks present themselves in veins or dykes, more or less, in most countries, of which Central France and the banks of the Rhine offer many striking examples. These veins present very evident proofs that the matter has been introduced from below, and in a manner which could only result from injection from the interior to the exterior of the earth. Such are the proofs presented by the basaltic veins of Villeneuve-de-Berg, which terminate in slender filaments, sometimes bifurcated, which gradually lose themselves in the rock which they traverse. In several parts of the north of Ireland, chalk-formations with flints are traversed by basaltic dykes, the chalk being converted into granular marble near the basalt, the change sometimes extending eight or ten feet from the wall of the dyke, and being greatest near the surface of contact. In the Island of Rathlin, the walls of basalt traverse the chalk in three veins or dykes; the central one a foot thick, that on the right twenty feet, and on the left thirty-three feet thick, and all, according to Buckland and Conybeare, within the breadth of ninety feet.

Fig. 6

Fig. 6.—Basalt in prismatic columns.

Fig. 7

Fig. 7.—Basaltic Causeway, on the banks of the river Volant, in the ArdÈche.

One of the most striking characteristics of basalt is the prismatic and columnar structure which it often assumes; the lava being homogeneous and of very fine grain, the laws which determine the direction of the fissures or divisional planes consolidated from a molten to a solid state, become here very manifest—these are always at right angles to the surfaces of the rock through which the heat of the fused mass escaped. The basaltic rocks have been at all times remarkable for this picturesque arrangement of their parts. They usually present columns of regular prisms, having generally six, often five, and sometimes four, seven, or even three sides, whose disposition is always perpendicular to the cooling surfaces. These are often divided transversely, as in Fig. 6, at nearly equal distances, like the joints of a wall, composed of regularly arranged, equal-sided pieces adhering together, and frequently extending over a more or less considerable space. The name of Giant’s Causeway has been given, from time immemorial, to these curious columnar structures of basalt. In France, in the Vivarais and in the Velay, there are many such basaltic causeways. That of which Fig. 7 is a sketch lies on the banks of the river Volant, where it flows into the ArdÈche. Ireland has always been celebrated for its Giant’s Causeway, which extends over the whole of the northern part of Antrim, covering all the pre-existing strata of Chalk, Greensand, and Permian formations; the prismatic columns extend for miles along the cliffs, projecting into the sea at the point specially designated the Giant’s Causeway.

These columnar formations vary considerably in length and diameter. McCulloch mentions some in Skye, which “are about four hundred feet high; others in Morven not exceeding an inch (vol. ii. p. 137). In diameter those of Ailsa Craig measure nine feet, and those of Morven an inch or less.” Fingal’s Cave, in the Isle of Staffa, is renowned among basaltic rocks, although it was scarcely known on the mainland a century ago, when Sir Joseph Banks heard of it accidentally, and was the first to visit and describe it. Fingal’s Cave has been hollowed out, by the sea, through a gallery of immense prismatic columns of trap, which are continually beaten by the waves. The columns are usually upright, but sometimes they are curved and slightly inclined. Fig. 8 is a view of the basaltic grotto of Staffa.

Fig. 8

Fig. 8.—Basaltic cavern of Staffa—exterior.

Grottoes are sometimes formed by basaltic eruptions on land, followed by their separation into regular columns. The Grotto of Cheeses, at Bertrich-Baden, between TrÈves and Coblentz, is a remarkable example of this kind, being so called because its columns are formed of round, and usually flattened, stones placed one above the other in such a manner as to resemble a pile of cheeses.

Plate III

III.—Extinct volcanoes forming the Puy-de-DÔme Chain.

If we consider that in basalt-flows the lower part is compact, and often divided into prismatic columns, while the upper part is porous, cellular, scoriaceous, and irregularly divided—that the points of separation on which they rest are small beds presenting fragments of the porous stony concretions known under the name of Lapilli—that the lower portions of these masses present a multitude of points which penetrate the rocks on which they repose, thereby denoting that some fluid matter had moulded itself into its crevices—that the neighbouring rocks are often calcined to a considerable thickness, and the included vegetable remains carbonised—no doubt can exist as to the igneous origin of basaltic rocks. When it reached the surface through certain openings, the fluid basalt spread itself, flowing, as it were, over the horizontal surface of the ground; for if it had flowed upon inclined surfaces it could not have preserved the uniform surface and constant thickness which it generally exhibits.

Volcanic or Lava Formations.

The lava formations comprehend both extinct and active volcanoes. “The term,” says Lyell, “has a somewhat vague signification, having been applied to all melted matter observed to flow in streams from volcanic vents. When this matter consolidates in the open air, the upper part is usually scoriaceous, and the mass becomes more and more stony as we descend, or in proportion as it has consolidated more slowly and under greater pressure.”[21]

The formation of extinct volcanoes is represented in France by the volcanoes situated in the ancient provinces of Auvergne, Velay, and the Vivarais, but principally by nearly seventy volcanic cones of various sizes and of the height of from 500 to 1,000 feet, composed of loose scoriÆ, lava, and pozzuolana, arranged upon a granitic table-land, about twelve miles wide, which overlooks the town of Clermont-Ferrand, and which seem to have been produced along a longitudinal fracture in the earth’s crust, running in a direction from north to south. It is a range of volcanic hills, the “chain of Puys” nearly twenty miles in length, by two in breadth. By its cellular and porous structure, which is also granular and crystalline, the felspathic or pyroxenic lava which flowed from these volcanoes is readily distinguishable from the analogous lavas which belong to the basaltic or trachytic formations. Their surface is irregular, and bristles with asperities, formed by heaped-up angular blocks.

The volcanoes of the chain of Puys, represented on opposite page (Pl. III.) are so perfectly preserved, their lava is so frequently superposed on sheets of basalt, and presents a composition and texture so distinct, that there is no difficulty in establishing the fact that they are posterior to the basaltic formation, and of very recent age. Nevertheless, they do not appear to belong to the historic ages, for no tradition attests their eruption. Lyell places these eruptions in the Lower Miocene period, and their greatest activity in the Upper Miocene and Pliocene eras. “Extinct quadrupeds of those eras,” he says, “belonging to the genera mastodon, rhinoceros, and others, were buried in ashes and beds of alluvial sand and gravel, which owe their preservation to overspreading sheets of lava.”[22]

Fig. 9

Fig. 9.—Section of a volcano in action.

All volcanic phenomena can be explained by the theory we have already indicated, of fractures in the solid crust of the globe resulting from its cooling. The various phenomena which existing volcanoes present to us are, as Humboldt has said, “the result of every action exercised by the interior of a planet on its external crust.”[23] We designate as volcanoes all conduits which establish a permanent communication between the interior of the earth and its surface—a conduit which gives passage at intervals to eruptions of lava, and in Fig. 9 we have represented, in an ideal section, the geological mode of action of volcanic eruptions. The volcanoes on the surface of the globe, known to be in an occasional state of activity, number about three hundred, and these may be divided into two classes: the isolated or central, and the linear or those volcanoes which belong to a series.[24]

The first are active volcanoes, around which there may be established many secondary active mouths of eruption, always in connection with some principal crater. The second are disposed like the chimneys of furnaces, along fissures extending over considerable distances. Twenty, thirty, and even a greater number of volcanic cones may rise above one such rent in the earth’s crust, the direction of which will be indicated by their linear course. The Peak of Teneriffe is an instance of a central volcano; the long rampart-like chain of the Andes, presents, from the south of Chili to the north-west coast of America, one of the grandest instances of a continental volcanic chain; the remarkable range of volcanoes in the province of Quito belong to the latter class. Darwin relates that on the 19th of March, 1835, the attention of a sentry was called to something like a large star which gradually increased in size till about three o’clock, when it presented a very magnificent spectacle. “By the aid of a glass, dark objects, in constant succession, were seen in the midst of a great glare of red light, to be thrown up and to fall down. The light was sufficient to cast on the water a long bright reflection—it was the volcano of Osorno in action.” Mr. Darwin was afterwards assured that Aconcagua, in Chili, 480 miles to the north, was in action on the same night, and that the great eruption of Coseguina (2,700 miles north of Aconcagua), accompanied by an earthquake felt over 1,000 miles, also occurred within six hours of this same time; and yet Coseguina had been dormant for six-and-twenty years, and Aconcagua most rarely shows any signs of action.[25] It is also stated by Professor Dove that in the year 1835 the ashes discharged from the mountain of Coseguina were carried 700 miles, and that the roaring noise of the eruption was heard at San Salvador, a distance of 1,000 miles.

In the sea the series of volcanoes show themselves in groups of islands disposed in longitudinal series.

Among these may be ranged the volcanic series of Sunda, which, according to the accounts of the matter ejected and the violence of the eruptions, seem to be among the most remarkable on the globe; the series of the Moluccas and of the Philippines; those of Japan; of the Marianne Islands; of Chili; of the double series of volcanic summits near Quito, those of the Antilles, Guatemala, and Mexico.

Among the central, or isolated volcanoes, we may class those of the Lipari Islands, which have Stromboli, in permanent activity, for their centre; Etna, Vesuvius, the volcanoes of the Azores, of the Canaries, of the Cape de Verde, of the Galapagos Islands, the Sandwich Islands, the Marquesas, the Society Islands, the Friendly Islands, Bourbon, and, finally, Ararat.

Fig. 10

Fig. 10.—Existing crater of Vesuvius.

The mouths of volcanic chimneys are, almost always, situated near the summit of a more or less isolated conical mountain; they usually consist of an opening in the form of a funnel, which is called the crater, and which descends into the interior of the volcanic chimney. But in the course of ages the crater becomes extended and enlarged, until, in some of the older volcanoes, it has attained almost incredible dimensions. In 1822 the crater of Vesuvius was 2,000 feet deep, and of a very considerable circumference. The crater of Kilauea, in the Sandwich Islands group, is an immense chasm 1,000 feet deep, with an outer circle no less than from two to three miles in diameter, in which lava is usually seen, Mr. Dana tells us, to boil up at the bottom of a lake, the level of which varies continually according to the active or quiescent state of the volcano. The cone which supports these craters, and which is designated the cone of ejection, is composed for the most part of lava or scoriÆ, the products of eruption. Many volcanoes consist only of a cone of scoriÆ. Such is that of Barren Isle, in the Bay of Bengal. Others, on the contrary, present a very small cone, notwithstanding the considerable height of the volcanic chain. As an example we may mention the new crater of Vesuvius, which was produced in 1829 within the former crater (Fig. 10).

Fig. 11

Fig. 11.—Fissures near Locarno.

The frequency and intensity of the eruptions bear no relation to the dimensions of the volcanic mountain. The eruption of a volcano is usually announced by a subterranean noise, accompanied by shocks, quivering of the ground, and sometimes by actual earthquakes. The noise, which usually proceeds from a great depth, makes itself heard, sometimes over a great extent of country, and resembles a well-sustained fire of artillery, accompanied by the rattle of musketry. Sometimes it is like the heavy rolling of subterranean thunder. Fissures are frequently produced during the eruptions, extending over a considerable radius, as represented in the woodcut on page 57 of the fissures of Locarno (Fig. 11), where they present a singular appearance; the clefts radiating from a centre in all directions, not unlike the starred fracture in a cracked pane of glass. The eruption begins with a strong shock, which shakes the whole interior of the mountain; masses of heated vapour and fluids begin to ascend, revealing themselves in some cases by the melting of the snow upon the flanks of the cone of ejection; while simultaneously with the final shock, which overcomes the last resistance opposed by the solid crust of the ground, a considerable body of gas, and more especially of steam, escapes from the mouth of the crater.

The steam, it is important to remark, is essentially the cause of the terrible mechanical effects which accompany volcanic eruptions. Granitic, porphyritic, trachytic, and sometimes even basaltic matters, have reached the surface without producing any of those violent explosions or ejections of rocks and stones which accompany modern volcanic eruptions; the older granites, porphyries, trachytes, and basalts were discharged without violence, because steam did not accompany those melted rocks—a sufficient proof of the comparative calm which attended the ancient as compared with modern eruptions. Well established by scientific observations, this is a fact which enables us to explain the cause of the tremendous mechanical effects attending modern volcanic eruptions, contrasted with the more tranquil eruptions of earlier times.

During the first moments of a volcanic eruption, the accumulated masses of stones and ashes, which fill the crater, are shot up into the sky by the suddenly and powerfully developed elasticity of the steam. This steam, which has been disengaged by the heat of the fluid lava, assumes the form of great rounded bubbles, which are evolved into the air to a great height above the crater, where they expand as they rise, in clouds of dazzling whiteness, assuming that appearance which Pliny the Younger compared to a stone pine rising over Vesuvius. The masses of clouds finally condense and follow the direction of the wind.

These volcanic clouds are grey or black, according to the quantity of ashes, that is, of pulverulent matter or dust, mixed with watery vapour, which they convey. In some eruptions it has been observed that these clouds, on descending to the surface of the soil, spread around an odour of hydrochloric or sulphuric acid, and traces of both these acids are found in the rain which proceeds from the condensation of these clouds.The fleecy clouds of vapour which issue from the volcanoes are streaked with lightning, followed by continuous peals of thunder; in condensing, they discharge disastrous showers, which sweep the sides of the mountain. Many eruptions, known as mud volcanoes, and watery volcanoes, are nothing more than these heavy rains, carrying down with them showers of ashes, stones, and scoriÆ, more or less mixed with water.

Passing on to the phenomena of which the crater is the scene at the time of an eruption, it is stated that at first there is an incessant rise and fall of the lava which fills the interior of the crater. This double movement is often interrupted by violent explosions of gas. The crater of Kilauea, in the Island of Hawaii, contains a lake of molten matter 1,600 feet broad, which is subject to such a double movement of elevation and depression. Each of the vaporous bubbles as it issues from the crater presses the molten lava upwards, till it rises and bursts with great force at the surface. A portion of the lava, half-cooled and reduced to scoriÆ, is thus projected upwards, and the several fragments are hurled violently in all directions, like those of a shell at the moment when it bursts.

The greater number of the fragments being thrown vertically into the air, fall back into the crater again. Many accumulating on the edge of the opening add more and more to the height of the cone of eruption. The lighter and smaller fragments, as well as the fine ashes, are drawn upwards by the spiral vapours, and sometimes transported by the winds over almost incredible distances.

In 1794 the ashes from Vesuvius were carried as far as the extremity of Calabria. In 1812 the volcanic ashes of Saint Vincent, in the Antilles, were carried eastward as far as Barbadoes, spreading such obscurity over the island, that, in open day, passengers could not see their way. Finally, some of the masses of molten lava are shot singly into the air during an eruption with a rapid rotatory motion, which causes them to assume the rounded shape in which they are known by the name of volcanic bombs.

We have already remarked that the lava, which in a fluid state fills the crater and the internal vent or chimney of the volcano, is forced upwards by gaseous fluids, and by the steam which has been generated from the water, entangled with the lava. In some cases the mechanical force of this vapour is so great as to drive the lava over the edge of the crater, when it forms a fiery torrent, spreading over the sides of the mountain. This only happens in the case of volcanoes of inconsiderable height; in lofty volcanoes it is not unusual for the lava thus to force an outlet for itself near the base of the mountain, through which the fiery stream discharges itself over the surrounding country. In such circumstances the lava cools somewhat rapidly; it becomes hard and presents a scoriaceous crust on the surface, while the vapour escapes in jets of steam through the interstices. But under this superficial crust the lava retains its fluid state, cooling slowly in the interior of the mass, while the thickening stream moves sluggishly along, impeded in its progress by the fragments of rock which this burning river drives before it.

The rate at which a current of lava moves along depends upon its mass, upon its degree of fluidity, and upon the inclination of the ground. It has been stated that certain streams of lava have traversed more than 3,000 yards in an hour; but the rate at which they travel is usually much less, a man on foot being often able to outstrip them. These streams, also, vary greatly in dimensions. The most considerable stream of lava from Etna had, in some parts, a thickness of nearly 120 feet, with a breadth of a geographical mile and a half. The largest lava-stream which has been recorded issued from the SkaptÁr Jokul, in Iceland, in 1783. It formed two currents, whose extremities were twenty leagues apart, and which from time to time presented a breadth of from seven to fifteen miles and a thickness of 650 feet.

A peculiar effect, and which only simulates volcanic activity, is observable in localities where mud volcanoes exist. Volcanoes of this class are for the most part conical hills of low elevation, with a hollow or depression at the centre, from which they discharge the mud which is forced upwards by gas and steam. The temperature of the ejected matter is only slightly elevated. The mud, generally of a greyish colour, with the odour of petroleum, is subject to the same alternating movements which have been already ascribed to the fluid lava of volcanoes, properly so called. The gases which force out this liquid mud, mixed with salts, gypsum, naphtha, sulphur, sometimes even of ammonia, are usually carburetted hydrogen and carbonic acid. Everything leads to the conclusion that these compounds proceed, at least in great part, from the reaction produced between the various elements of the subsoil under the influence of infiltrating water between bituminous marls, complex carbonates, and probably carbonic acid, derived from acidulated springs. M. Fournet saw in Languedoc, near Roujan, traces of some of these formations; and not far from that neighbourhood is the bituminous spring of Gabian.

IV.—Mud volcano at Turbaco, South America.

Mud volcanoes, or salses, exist in rather numerous localities. Several are found in the neighbourhood of Modena. There are some in Sicily, between Aragona and Girgenti. Pallas observed them in the Crimea—in the peninsula of Kertch, and in the Isle of TamÀn. Von Humboldt has described and figured a group of them in the province of Cartagena, in South America. Finally, they have been observed in the Island of Trinidad and in Hindostan. In 1797 an eruption of mud ejected from Tunguragua, in Quito, filled a valley 1,000 feet wide to a depth of 600 feet. On the opposite page is represented the mud volcano of Turbaco, in the province of Cartagena (Plate IV.), which is described and figured by Von Humboldt in his “Voyage to the Equatorial Regions of America.”

In certain countries we find small hillocks of argillaceous formation, resulting from ancient discharges of mud volcanoes, from which all disengagement of gas, water, and mud has long ceased. Sometimes, however, the phenomenon returns and resumes its interrupted course with great violence. Slight shocks of earthquakes are then felt; blocks of dried earth are projected from the ancient crater, and new waves of mud flow over its edge, and spread over the neighbouring ground.

To return to ordinary volcanoes, that is to say, those which eject lava. At the end of a lava-flow, when the violence of the volcanic action begins to subside, the discharge from the crater is confined to the disengagement of vaporous gases, mixed with steam, which make their escape in more or less abundance through a multitude of fissures in the ground.

The great number of volcanoes which have thus become extinct form what are called solfataras. The sulphuretted hydrogen, which is given out through the fissures in the ground, is decomposed by contact with the air, water being formed by the action of the oxygen of the atmosphere, and sulphur deposited in considerable quantities on the walls of the crater, and in the cracks of the ground. Such is the geological source of the sulphur which is collected at Pozzuoli, near Naples, and in many other similar regions—a substance which plays a most important part in the industrial occupations of the world. It is, in fact, from sulphur extracted from the ground about the mouths of extinct volcanoes, that is to say from the products of solfataras, that sulphuric acid is frequently made—sulphuric acid being the fundamental agent, one of the most powerful elements, of the manufacturing productions of both worlds.

The last phase of volcanic activity is the disengagement of carbonic acid gas without any increase of temperature. In places where these continued emanations of carbonic acid gas manifest themselves, the existence of ancient volcanoes may be recognised, of which these discharges are the closing phenomenon. This is seen in a most remarkable manner in Auvergne, where there are a multitude of acidulated springs, that is to say, springs charged with carbonic acid. During the time when he was opening the mines of Pontgibaud, M. Fournet had to contend with emanations which sometimes exhibited themselves with explosive power. Jets of water were thrown to great heights in the galleries, roaring with the noise of steam when escaping from the boiler of a locomotive engine. The water which filled an abandoned mine-shaft was, on two separate occasions, upheaved with great violence—half emptying the pit—while vast volumes of the gas overspread the whole valley, suffocating a horse and a flock of geese. The miners were compelled to fly in all haste at the moment when the gas burst forth, holding themselves as upright as possible, to avoid plunging their heads into the carbonic acid gas, which, from its low specific gravity, was now filling the lower parts of the galleries. It represented on a small scale the effect of the Grotto del Cane, which excites such surprise among the ignorant near Naples; passing, also, for one of the marvels of Nature all over the world. M. Fournet states that all the minute fissures of the metalliferous gneiss near Clermont are quite saturated with free carbonic acid gas, which rises plentifully from the soil there, as well as in many parts of the surrounding country. The components of the gneiss, with the exception of the quartz, are softened by it; and fresh combinations of the acid with lime, iron, and manganese are continually taking place. In short, long after volcanoes have become extinct, hot springs, charged with mineral ingredients, continue to flow in the same area.

The same facts as those of the Grotto del Cane manifest themselves with even greater intensity in Java, in the so-called Valley of Poison, which is an object of terror to the natives. In this celebrated valley the ground is said to be covered with skeletons and carcases of tigers, goats, stags, birds, and even of human beings; for asphyxia or suffocation, it seems, strikes all living things which venture into this desolate place. In the same island a stream of sulphurous water, as white as milk, issues from the crater of Mount Idienne, on the east coast; and on one occasion, as cited by Nozet in the Journal de GÉologie, a great body of hot water, charged with sulphuric acid, was discharged from the same volcano, inundating and destroying all the vegetation of a large tract of country by its noxious fumes and poisonous properties.

Plate V

V.—Great Geyser of Iceland.

It is known that the alkaline waters of PlombiÈres, in the Vosges, have a temperature of 160° Fahr. For 2,000 years, according to DaubrÉe, through beds of concrete, of lime, brick, and sandstone, these hot waters have percolated until they have originated calcareous spar, aragonite, and fluor spar, together with siliceous minerals, such as opal, which are found filling the interstices of the bricks and mortar. From these and other similar statements, “we are led,” says Sir Charles Lyell,[26] “to infer that when in the bowels of the earth there are large volumes of molten matter, containing heated water and various acids, under enormous pressure, these subterraneous fluid masses will gradually part with their heat by the escape of steam and various gases through fissures producing hot springs, or by the passage of the same through the pores of the overlying and injected rocks.” “Although,” he adds,[27] “we can only study the phenomena as exhibited at the surface, it is clear that the gaseous fluids must have made their way through the whole thickness of the porous or fissured rocks, which intervene between the subterraneous reservoirs of gas and the external air. The extent, therefore, of the earth’s crust which the vapours have permeated, and are now permeating, may be thousands of fathoms in thickness, and their heating and modifying influence may be spread throughout the whole of this solid mass.”

The fountains of boiling water, known under the name of Geysers, are another emanation connected with ancient craters. They are either continuous or intermittent. In Iceland we find great numbers of these gushing springs—in fact, the island is one entire mass of eruptive rock. Nearly all the volcanoes are situated upon a broad band of trachyte, which traverses the island from south-west to north-east. It is traversed by immense fissures, and covered with masses of lava, such as no other country presents. The volcanic action, in short, goes on with such energy that certain paroxysms of Mount Hecla have lasted for six years without interruption. But the Great Geyser, represented on the opposite page (Plate V.), is, perhaps, even more an object of curiosity. This water-volcano projects a column of boiling water, eight yards in diameter, charged with silica, to the height, it has been said, of about 150 feet, depositing vast quantities of silica as it cools after reaching the earth.

The volcanoes in actual activity are, as we have said, very numerous, being more than 200 in number, scattered over the whole surface of the globe, but mostly occurring in tropical regions. The island of Java alone contains about fifty, which have been mapped and described by Dr. Junghahn. Those best known are Vesuvius, near Naples; Etna, in Sicily; and Stromboli, in the Lipari Islands. A rapid sketch of a few of these may interest the reader.

Vesuvius is of all volcanoes that which has been most closely studied; it is, so to speak, the classical volcano. Few persons are ignorant of the fact that it opened—after a period of quiescence extending beyond the memory of living man—in the year 79 of our era. This eruption cost the elder Pliny his life, who fell a sacrifice to his desire to witness one of the most imposing of natural phenomena. After many mutations the present crater of Vesuvius consists of a cone, surrounded on the side opposite the sea by a semicircular crest, composed of pumiceous matter, foreign to Vesuvius properly speaking, for we believe that Mount Vesuvius was originally the mountain to which the name of Somma is now given. The cone which now bears the name of Vesuvius was probably formed during the celebrated eruption of 79, which buried under its showers of pumiceous ashes the cities of Pompeii and Herculaneum. This cone terminates in a crater, the shape of which has undergone many changes, and which has, since its origin, thrown out eruptions of a varied character, together with streams of lava. In our days the eruptions of Vesuvius have only been separated by intervals of a few years.

The Lipari Isles contain the volcano of Stromboli, which is continually in a state of ignition, and forms the natural lighthouse of the Tyrrhenian Sea; such it was when Homer mentioned it, such it was before old Homer’s time, and such it still appears in our days. Its eruptions are incessant. The crater whence they issue is not situated on the summit of the cone, but upon one of its sides, at nearly two-thirds of its height. It is in part filled with fluid lava, which is continually subjected to alternate elevation and depression—a movement provoked by the ebullition and ascension of bubbles of steam which rise to the surface, projecting upwards a tall column of ashes. During the night these clouds of vapour shine with a magnificent red reflection, which lights up the whole isle and the surrounding sea with a lurid glow.

Situated on the eastern coast of Sicily, Etna appears, at the first glance, to have a much more simple structure than Vesuvius. Its slopes are less steep, more uniform on all sides; its vast base nearly represents the form of a buckler. The lower portion of Etna, or the cultivated region of the mountain, has an inclination of about three degrees. The middle, or forest region, is steeper, and has an inclination of about eight degrees. The mountain terminates in a cone of an elliptical form of thirty-two degrees of inclination, which bears in the middle, above a nearly horizontal terrace, the cone of eruption with its circular crater. The crater is 10,874 feet high. It gives out no lava, but only vomits forth gas and vapour, the streams of lava issuing from sixteen smaller cones which have been formed on the slopes of the mountain. The observer may, by looking at the summit, convince himself that these cones are disposed in rays, and are based upon clefts or fissures which converge towards the crater as towards a centre.

But the most extraordinary display of volcanic phenomena occurs in the Pacific Ocean, in the Sandwich Islands, and in Java. Mauna Loa and Mauna Kea, in Hawaii, are huge flattened cones, 14,000 feet high. According to Mr. Dana, these lofty, featureless hills sometimes throw out successive streams of lava, not very far below their summits, often two miles in breadth and six-and-twenty in length; and that not from one vent, but in every direction, from the apex of the cone down slopes varying from four to eight degrees of inclination. The lateral crater of Kilauea, on the flank of Mauna Loa, is from 3,000 to 4,000 feet above the level of the sea—an immense chasm 1,000 feet deep, with an outer circuit two to three miles in diameter. At the bottom lava is seen to boil up in a molten lake, the level of which rises or falls according to the active or quiescent state of the volcano; but in place of overflowing, the column of melted rock, when the pressure becomes excessive, forces a passage through subterranean communications leading to the sea. One of these outbursts, which took place at an ancient wooded crater six miles east of Kilauea, was observed by Mr. Coan, a missionary, in June, 1840. Another indication of the subterranean progress of the lava took place a mile or two beyond this, in which the fiery flood spread itself over fifty acres of land, and then found its way underground for several miles further, to reappear at the bottom of a second ancient wooded crater which it partly filled up.[28]

The volcanic mountains of Java constitute the highest peaks of a mountain-range running through the island from east to west, on which Dr. Junghahn described and mapped forty-six conical eminences, ranging from 4,000 to 11,000 feet high. At the top of many of the loftiest of these Dr. Junghahn found the active cones and craters of small size, and surrounded by a plain of ashes and sand, which he calls the “old crater wall,” sometimes exceeding 1,000 feet in vertical height, and many of the semicircular walls enclosing large cavities or calderas, four geographical miles in diameter. From the highest parts of many of these hollows rivers flow, which, in the course of ages, have cut out deep valleys in the mountain’s side.[29]

To this rapid sketch of actually existing volcanic phenomena we may add a brief notice of submarine volcanoes. If these are known to us only in small numbers, the circumstance is explained by the fact that their appearance above the bosom of the sea is almost invariably followed by a more or less complete disappearance; at the same time such very striking and visible phenomena afford a sufficient proof of the continued persistence of volcanic action beneath the bed of the sea-basin. At various times islands have suddenly appeared, amid the ocean, at points where the navigator had not before noticed them. In this manner we have witnessed the island called Graham’s, Ferdinanda, or Julia, which suddenly appeared off the south-west coast Sicily in 1831, and was swept away by the waves two months afterwards.[30] At several periods also, and notably in 1811, new islands were formed in the Azores, which raised themselves above the waves by repeated efforts all round the islands, and at many other points.

The island which appeared in 1796 ten leagues from the northern point of Unalaska, one of the Aleutian group of islands, is specially remarkable. We first see a column of smoke issuing from the bosom of the ocean, afterwards a black point appears, from which bundles of fiery sparks seem to rise over the surface of the sea. During the many months that these phenomena continue, the island increases in breadth and in height. Finally smoke only is seen; at the end of four years, even this last trace of volcanic convulsion altogether ceases. The island continued, nevertheless, to enlarge and to increase in height, and in 1806 it formed a cone, surmounted by four other smaller ones.

In the space comprised between the isles of Santorin, Tharasia, and Aspronisi, in the Mediterranean, there arose, 160 years before our era, the island of Hyera, which was enlarged by the upheaval of islets on its margin during the years 19, 726, and 1427. Again, in 1773, Micra-Kameni, and in 1707, Nea-Kameni, made their appearance. These islands increased in size successively in 1709, in 1711, in 1712. According to ancient writers, Santorin, Tharasia, and Aspronisi, made their appearance many ages before the Christian era, at the termination of earthquakes of great violence.

Metamorphic Rocks.

The rocks composing the terrestrial crust have not always remained in their original state. They have frequently undergone changes which have altogether modified their properties, physical and chemical.

When they present these characteristics, we term them Metamorphic Rocks. The phenomena which belong to this subject are at once important and new, and have lately much attracted the attention of geologists. We shall best enlighten our readers on the metamorphism of rocks, if we treat of it under the heads of special and general metamorphism.

When a mass of eruptive rock penetrates the terrestrial crust it subjects the rocks through which it passes to a special metamorphism—to the effects of heat produced by contact. Such effects may almost always be observed near the margin of masses of eruptive rock, and they are attributable either to the communicated heat of the eruptive rock itself, or to the disengagement of gases, of steam, or of mineral and thermal waters, which have accompanied its eruption. The effects vary not only with the rock ejected, but even with the nature of the rock surrounding it.

In the case of volcanic lava ejected in a molten state, for instance, the modifications it effects on the surrounding rock are very characteristic. Its structure becomes prismatic, full of cracks, often cellular and scoriaceous. Wood and other combustibles touched by the lava are consumed or partially carbonised. Limestone assumes a granular and crystalline texture. Siliceous rocks are transformed, not only into quartz like glass, but they also combine with various bases, and yield vitreous and cellular silicates. It is nearly the same with argillaceous rocks, which adhere together, and frequently take the colour of red bricks.

The surrounding rock is frequently impregnated with specular iron-ore, and penetrated with hydrochloric or sulphuric acid, and by divers salts formed from these acids.

At a certain distance from the place of contact with the lava, the action of water aided by heat produces silica, carbonate of lime, aragonite, zeolite, and various other minerals.

From immediate contact with the lava, then, the metamorphic rocks denote the action of a very strong heat. They bear evident traces of calcination, of softening, and even of fusion. When they present themselves as hydrosilicates and carbonates, the silica and associated minerals are most frequently at some distance from the points of contact; and the formation of these minerals is probably due to the combination of water and heat, although this last ceases to be the principal agent.

The hydrated volcanic rocks, such as the basalts and trappean rocks in general, continue to produce effects of metamorphism, in which heat operates, although its influence is inconsiderable, water being much the more powerful agent. The metamorphosis which is observable in the structure and mineralogical composition of neighbouring rocks is as follows:—The structure of separation becomes fragmentary, columnar, or many-sided, and even prismatic. It becomes especially prismatic in combustibles, in sandstones, in argillaceous formations, in felspathic rocks, and even in limestones. Prisms are formed perpendicular to the surface of contact, their length sometimes exceeding six feet. Most commonly they still contain water or volatile matter. These characters may be observed at the junction of the basalts which has been ejected upon the argillaceous strata near Clermont in Auvergne, at Polignac, and in the neighbourhood of Le Puy-en-Velay.

If the vein of Basalt or Trap has traversed a bed of coal or of lignite, we find the combustible strongly metamorphosed at the point of contact. Sometimes it becomes cellular and is changed into coke. This is especially the case in the coal-basin of Brassac. But more frequently the coal has lost all, or part of, its bituminous and volatile matter—it has been metamorphosed into anthracite—as an example we may quote the lignite of Mont Meisner.

Again, in some exceptional cases, the combustible may even be changed into graphite near to its junction with Trap. This is observed at the coal-mine of New Cumnock in Ayrshire.

When near its junction with a trappean rock, a combustible has been metamorphosed into coke or anthracite, it is also frequently impregnated by hydrated oxide of iron, by clay, foliated carbonate of lime, iron pyrites, and by various mineral veins. It may happen that the combustible has been reduced to a pulverulent state, in which case it is unfit for use. Such is the case in a coal-mine at Newcastle, where the coal lies within thirty yards of a dyke of Trap.

When Basalt and Trap have been ejected through limestone rock, the latter becomes more or less altered. Near the points of contact, the metamorphism which they have undergone is revealed by the change of colour and aspect, which is exhibited all around the vein, often also by the development of a crystalline structure. Limestone becomes granular and saccharoid—it is changed into marble. The most remarkable instance of this metamorphism is the Carrara marble, a non-fossiliferous limestone of the Oolite series, which has been altered and the fossils destroyed; so that the marble of these celebrated quarries, once supposed to have been formed before the creation of organic beings, is now shown to be an altered limestone of the Oolitic period, and the underlying crystalline schists are sandstones and shales of secondary age modified by plutonic action.

The action of basalt upon limestone is observable at Villeneuve de Berg, in Auvergne; but still more in the neighbourhood of Belfast, where we may see the Chalk changed into saccharoid limestone near to its contact with the Trap. Sometimes the metamorphism extends many feet from the point of contact; nay, more than that, some zeolites and other minerals seem to be developed in the crystallised limestone.

When sandstone is found in contact with trappean rock, it presents unequivocal traces of metamorphism; it loses its reddish colour and becomes white, grey, green, or black; parallel veins may be detected which give it a jaspideous structure; it separates into prisms perpendicular to the walls of the injected veins, when it assumes a brilliant and vitreous lustre. Sometimes it is even also found penetrated by zeolites, a family of minerals which melt before the blowpipe with considerable ebullition. The mottled sandstones of Germany, which are traversed by veins of basalt, often exhibit metamorphism, particularly at Wildenstern, in WÜrtemberg.

Argillaceous rocks, like all others, are subject to metamorphism when they come in contact with eruptive trappean rocks. In these circumstances they change colour and assume a varied or prismatic structure; at the same time their hardness increases, and they become lithoidal or stony in structure. They may also become cellular—form zeolites in their cavities with foliated carbonate of lime, as well as minerals which commonly occur in amygdaloid. Sometimes even the fissures are coated by the metallic minerals, and the other minerals which accompany them in their metalliferous beds. Generally they lose a part of their water and of their carbonic acid. In other circumstances they combine with oxide of iron and the alkalies. This has been asserted, for example, at Essey, in the department of the Meurthe, where a very argillaceous sandstone is found, charged with jasper porcellanite, near to the junction of the rock with a vein of basalt.

Hitherto we have spoken only of the metamorphosis the result of volcanic action. A few words will suffice to acquaint the reader with the metamorphism exercised by the porphyries and granites. By contact with granite, we find coal changed into anthracite or graphite. It is important to note, however, that coal has seldom been metamorphosed into coke. As to the limestone, it is sometimes, as we have seen, transformed into marble; we even find in its interior divers minerals, notably silicates with a calcareous base, such as garnets, pyroxene, hornblende, &c. The sandstones and clay-slates have alike been altered.

The surrounding deposit and the eruptive rock are both frequently impregnated with quartz, carbonate of lime, sulphate of baryta, fluorides, and, in a word, with the whole tribe of metalliferous minerals, which present themselves, besides, with the characteristics which are common to them in the veins.

General Metamorphism.

Sedimentary rocks sometimes exhibit all the symptoms of metamorphism where there is no evidence of direct eruptive action, and that upon a scale much grander than in the case of special metamorphism. It is observable over whole regions, in which it has modified and altered simultaneously all the surrounding rocks. This state of things is called general, or normal, metamorphism. The fundamental gneiss, which covers such a vast extent of country, is the most striking instance known of general metamorphism. It was first described by Sir W. E. Logan, Director of the Canadian Geological Survey, who estimates its thickness at 30,000 feet. The Laurentian Gneiss is a term which is used by geologists to designate those metamorphic rocks which are known to be older than the Cambrian system. They are parts of the old pre-Cambrian continents which lie at the base of the great American continent, Scandinavia, the Hebrides, &c.; and which are largely developed on the west coast of Scotland. In order to give the reader some idea of this metamorphism, we shall endeavour to trace its effects in rocks of the same nature, indicating the characters successively presented by the rocks according to the intensity of the metamorphism to which they have been subjected.

Combustibles, which have a special composition, totally different from all other rocks, are obviously the first objects of examination. When we descend in the series of sedimentary deposits, the combustibles are observed completely to change their characters. From the peat which is the product of our own epoch, we pass to lignite, to coal, to anthracite, and even to graphite; and find that their density increases, varying up to at least double. Hydrogen, nitrogen, and, above all, oxygen, diminish rapidly. Volatile and bituminous matters decrease, while carbon undergoes a proportionate increase.

This metamorphism of the combustible minerals, which takes place in deposits of different ages, may also be observed even in the same bed. For instance, in the coal formations of America, which extend to the west of the Alleghany mountains, the Coal-measures contain a certain proportion of volatile matter, which goes on diminishing in proportion as we approach the granite rocks; this proportion rises to fifty per cent. upon the Ohio, but it falls to forty upon the Manon-Gahela, and even to sixteen in the Alleghanies. Finally, in the regions where the strata have been most disturbed, in Pennsylvania and Massachusetts, the coal has been metamorphosed into anthracite and even into graphite or plumbago.

Limestone is one of the rocks upon which we can most easily follow the effects of general metamorphism. When it has not been modified, it is usually found in sedimentary rocks in the state of compact limestone, of coarse limestone, or of earthy limestone such as chalk. But let us consider it in the mountains, especially in mountains which are at the same time granitic, such as the Pyrenees, the Vosges, and the Alps. We shall then see its characters completely modified. In the long and deep valleys of the Alps, for example, we can follow the alterations of the limestone for many leagues, the beds losing more and more their regularity in proportion as we approach the central chain, until they lose themselves in solitary pinnacles and projections enclosed in crystalline schists or granitic rocks. Towards the upper regions of the Alps the limestone divides itself into pseudo-regular fragments, it is more strongly cemented, more compact, more sonorous; its colour becomes paler, and it passes from black to grey by the gradual disappearance of organic and bituminous matter with which it has been impregnated, at the same time its crystalline structure increases in a manner scarcely perceptible. It may even be observed to be metamorphosed into an aggregate of microscopic crystals, and finally to pass into a white saccharoid limestone.

This metamorphism is produced without any decomposition of the limestone; it has rather been softened and half melted by the heat, that is, rendered plastic, so to speak, for we find in it fossils still recognisable, and among these, notably, some Ammonites and Belemnites, the presence of which enables us to state that it is the greyish-black Jurassic limestone, which has been transformed into white saccharoid or granular limestone. If the limestone subjected to this transformation were perfectly pure, it would simply take a crystalline structure; but it is generally mixed with sand and various argillaceous matters, which have been deposited along with it, matters which go to form new minerals. These new minerals, however, are not disseminated by chance; they develop themselves in the direction of the lamination, so to speak, of the limestone, and in its fissures, in such a manner that they present themselves in nodules, seams, and sometimes in veins.

Among the principal minerals of the saccharoid limestone we may mention graphite, quartz, some very varied silicates, such as andalusite, disthene, serpentine, talc, garnet, augite, hornblende, epidote, chlorite, the micas, the felspars; finally, spinel, corundum, phosphate of lime, oxide of iron and oligiste, iron pyrites, &c. Besides these, various minerals in veins figure among those which exist more commonly in the saccharoid limestone.

When metamorphic limestone is sufficiently pure, it is employed as statuary marble. Such is the geological origin of Carrara marble, which is quarried in the Apuan Alps on a great scale; such, also, was the marble of Paros and Antiparos, still so celebrated for its purity. On examination, however, with the lens the Carrara marble exhibits blackish veins and spangles of graphite; the finest blocks, also, frequently contain nodules of ironstone, which are lined with perfectly limpid crystals of quartz. These accidental defects are very annoying to the sculptor, for they are very minute, and nothing on the exterior of the block betrays their existence. In the marble of Paros, even when it is strongly translucent, specks of mica are often found. In the ancient quarries the nodules are so numerous as to have hindered their being worked, up even to the present time.

When the mica which occurs in granular limestone takes a green colour and forms veins, it constitutes the Cipoline marble, which is found in Corsica, and in the Val Godemar in the Alps. Some white marbles are quarried in France, chiefly at Loubie, at Sost, at Saint-BÉat in the Pyrenees, and at Chippal in the Vosges. In our country, and especially in Ireland, there are numerous quarries of marble, veined and coloured of every hue, but none of a purity suitable for the finest statuary purposes. All these marbles are only metamorphosed limestones.

The white marbles employed almost all over the world are those of Carrara. They result from the metamorphism of limestone of the Lias. They have not been penetrated by the eruptive rocks, but they have been subjected upon a great scale to a general metamorphism, to which their crystalline structure may be attributed.

It is easily understood that the calcareous strata have not undergone such an energetic metamorphism without the beds of sandstone and clay, associated with them, having also undergone some modification of the same kind. The siliceous beds accompanying the saccharoid limestone have, in short, a character of their own. They are formed of small grains of transparent quartz more or less cemented one to the other in a manner strongly resembling those of the saccharoid limestone. Between these grains are usually developed some lamellÆ of mica of brilliant and silky lustre, of which the colour is white, red, or green; in a word, it has produced a quartzite. Some veins of quartz frequently traverse this quartzite in all directions. Independent of the mica, it may contain, besides, the different minerals already mentioned as occurring in the limestone, and particularly silicates—such as disthene, andalusite, staurotide, garnet, and hornblende.

The argillaceous beds present a series of metamorphisms analogous to the preceding. We can follow them readily through all their gradations when we direct our attention towards such granitic masses as those which constitute the Alps, Pyrenees, the Bretagne Mountains, or our own Grampians. The schists may perhaps be considered the first step towards the metamorphism of certain argillaceous rocks; in fact, the schists are not susceptible of mixing with water like clay; they become stony, and acquire a much greater density, but their chief characteristic is a foliated structure.

Experiment proves that when we subject a substance to a great pressure a foliated structure is produced in a direction perpendicular to that in which the pressure is exercised. Everything leads us, therefore, to believe that pressure is the principal cause of the schistous texture, and of the foliation of clay-slates, the most characteristic variety of which is the roofing-slate which is quarried so extensively in North Wales, in Cumberland, and various parts of Scotland in the British Islands; in the Ardennes; and in the neighbourhood of Angers, in France.

In some localities the slate becomes siliceous and is charged with crystals of felspar. Nevertheless, it still presents itself in parallel beds, and contains the same fossil remains still in a recognisable state. For example, in the neighbourhood of Thann, in the Vosges, certain vegetable imprints are perfectly preserved in the metamorphic schist, and in their midst are developed some crystals of felspar.

Mica-schist, which is formed of layers of quartz and mica, is found habitually associated with rocks which have taken a crystalline structure, proceeding evidently from an energetic metamorphism of beds originally argillaceous. Chiastolite, disthene, staurotide, hornblende, and other minerals are found in it. Mica-schists occur extensively in Brittany, in the Vosges, in the Pyrenees. In all cases, as we approach the masses of granite, in these regions, the crystalline structure becomes more and more marked.

In describing the various facts relating to the metamorphism of rocks, we have said little of the causes which have produced it. The causes are, indeed, in the region of hypothesis, and somewhat mysterious.

In what concerns special metamorphism, the cause is supposed to admit of easy explanation—it is heat. When a rock is ejected from the interior of the earth in a state of igneous fusion, we comprehend readily enough that the strata, which it traverses, should sustain alterations due to the influence of heat, and varying with its intensity. This is clear enough in the case of lava. On the other hand, as water always exists in the interior of the earth’s crust, and as this water must be at a very high temperature in the neighbourhood of volcanic fires, it contributes, no doubt, largely to the metamorphism. If the rocks have not been ejected in a state of fusion, it is evidently water, with the different mineral substances it holds in solution, which is the chief actor in the special metamorphism which is produced.

In general metamorphism, water appears still to be the principal agent. As it is infiltered through the various beds it will modify their composition, either by dissolving certain substances, or by introducing into the metalliferous deposits certain new substances, such as may be seen forming, even under our eyes, in mineral springs. This has tended to render the sedimentary deposits plastic, and has permitted the development of that crystalline structure, which is one of the principal characteristics of metamorphic rocks. This action has been seconded by other causes, notably by heat and pressure, which would have an immense increase of power and energy when metamorphism takes place at a great depth beneath the surface. Dr. Holl, in an able paper descriptive of the geology of the Malvern Hills, read before the Geological Society in February, 1865, adopts this hypothesis as explanatory of the vast phenomena which are there displayed. After describing the position of this interesting and strangely-mingled range of rocks, he adds: “These metamorphic rocks are for the most part highly inclined, and often in a position nearly vertical. Their disturbance and metamorphism, their being traversed by granitic veins, and still later their invasion by trap-dykes and their subsequent elevation above the sea-level, were all events which must have occupied no inconsiderable period, even of geological time. I presume,” he adds, “that it will not be maintained in the present day that the metamorphism of rocks over areas of any but very moderate extent is due to the intrusion of veins and erupted masses. The insufficiency of such agency becomes the more obvious when we consider the slight effects produced by even tolerably extensive outbursts, such as the Dartmoor granite; while in the case of the Malverns there is an absence of any local cause whatever. The more probable explanation in the case of these larger areas is, that they were faulted down, or otherwise depressed, so as to be brought within the influence of the earth’s internal heat, and this is the more likely as they belong to an epoch when the crust is believed to have been thinner.” When it is considered that, according to the doctrine of modern geology, the Laurentian rocks, or their equivalents, lie at the base of all the sedimentary deposits; that this, like other systems of stratified rocks, was deposited in the form of sand, mud, and clay, to the thickness of 30,000 feet; and that over an area embracing Scandinavia, the Hebrides, great part of Scotland, and England as far south as the Malverns, besides a large proportion of the American continent, with certain forms of animal life, as recent investigations demonstrate—can the mind of man realise any other cause by which this vast extent of metamorphism could have been produced?

Electric and galvanic currents, circulating in the stratified crust, are not to be overlooked. The experiments of Mr. R. W. Fox and Mr. Robert Hunt suggest that, in passing long-continued galvanic currents through masses of moistened clay, there is a tendency to produce cleavage and a semi-crystalline arrangement of the particles of matter.[31]

[11] Lyell’s “Elements of Geology,” p. 694.[12] “Physical Geology and Geography of Great Britain,” by A. C. Ramsay, p. 38, 2nd ed.[13] At the same time it may be safely assumed (as Professor Ramsay believes to be the case) that granite in most cases is a metamorphic rock; yet are there many instances in which it may with greater truth be considered as a true plutonic rock.[14] “Elements of Geology,” p. 716, 6th edition.[15] “Elements of Geology,” p. 717.[16] Ibid, p. 718.[17] “Geology of the Island of Arran,” by Andrew C. Ramsay. “Geology of Arran and Clydesdale,” by James Bryce.[18] See Quarterly Journal of Geological Society, vol. viii., pp. 9 and 10.[19] For full information in reference to the rocks and geology 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.[20] “Volcanoes,” 2nd ed.[21] “Elements of Geology,” p. 596.[22] Ibid, p. 677.[23] “Cosmos,” vol. i., p. 25. Bohn.[24] “Cosmos,” vol. i., p. 237.[25] Darwin’s “Journal,” p. 291, 2nd edition.[26] “Elements of Geology,” p. 732.[27] Ibid, p. 733.[28] Lyell’s “Elements of Geology,” p. 617.[29] Lyell’s “Elements of Geology,” p. 620.[30] Ibid, p. 620.[31] Report of the Royal Cornwall Polytechnic Society for 1837. Robert Hunt, in “Memoirs of the Geological Survey of Great Britain,” vol. i., p. 433.

                                                                                                                                                                                                                                                                                                           

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