Part 7
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.
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.
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.”
[26] “Elements of Geology,” p. 732.
[27] Ibid, p. 733.
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.
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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]
[28] Lyell’s “Elements of Geology,” p. 617.
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]
[29] 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.[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.
[30] 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.