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 oi 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 Cajie, 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 islatnd a stream of sulphurous water, as white as milk, issues from the crater of Mount Idienrie, on the east coast; and on one occasion, as cited by Nozet in the Journal de Geelogie, a great body of hot water, charged with aulphuric acid, was discharged from the same volcano, inundating and destroying all the \egetation of a large tract of country by its noxious fumes and poisonous properties. It is known that the alkaline waters of Plombieres, in the Vosges, have a temperature of i6o° Fahr. For 2,000 years, according

v.— Great Geyser of Iceland.

VOLCANIC OR LAVA FORMATIONS. 6/

to Daubr^e, 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,* "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,t "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 northeast. 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 mth 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

** Elements of Geology," p. 732. f^hid, p. 733. F 2

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 i^v^ 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, thro^vn 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 ina state of ignition, and forms the natural lighthouse of the Tyirhenian 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 ebuUirion 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 neariy 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 temiinates 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 inch-nation. 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 woooied crater which it partly

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