Part 9

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.

Lyell’s “Elements of Geology,” p. 620.

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. 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.

Ibid, p. 620.

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.