CHAPTER XVIII.

ON EARTHQUAKES AND VOLCANOES.

Phenomena that precede the Shock of an Earthquake.-Extent to which the Waters in Lakes and Springs are agitated during Earthquakes.-Extent to which Earthquakes are felt on Land.-More severe in Mountains than in level Countries. Connection between Earthquakes and Volcanoes.—Electrical Earthquakes. First Appearance of a Volcano.-Common Phenomena attending Volcanic Eruptions.-Remarkable Eruption of Sumbawa in 1815.-Long Periods of Repose in some Volcanoes.-Volcano of Popocatept in Mexico.-Submarine Volcanoes; their Appearance preceded by violent Agitation of the Sea. -Submarine Volcanoes in the Azores-in the Grecian Archipelago.-Recent Submarine Volcano near Sicily.-Craters of Eruption.-Craters of Elevation.Theory of Von Buch confirmed by analogous Geological Facts.-Eruptions of Mud and Water from Volcanoes.-Groups of Volcanic Islands.-Fall and Extinction of a Volcano.-Vast Extent of some ancient Volcanoes.-Extinct Volcanoes of Central France.-Puy de Pariou, the best preserved of ancient Volcanoes.-Extinct Volcanoes in Germany and Asia.-Pseudo-Volcanoes.-Volcanic Rocks and Products.-Observations on Volcanic Fire.

ACCUSTOMED to view the hills in our own country in a state of profound repose, presenting in each succeeding year, the same unvaried outline, we can scarcely conceive the possibility of a whole district being covered, in the space of a single night, with new mountains and another soil; yet, within the limits of authentic history, such changes have been produced, by the united agency of earthquakes and volcanoes. For a particular description of recent volcanic eruptions, and the changes they have produced on the surrounding countries, I must refer the reader to the works of Spallanzani, Dolomieu, Sir William Hamilton, and M. Humboldt, and to the recent account of the Island of Java, by Lieutenant-Governor Raffles.

In the present chapter I propose, 1st, to describe those phenomena that indicate the connection between earthquakes and volcanoes, and between the volcanoes in distant countries; 2dly, to take a view of the most remarkable recent volcanic eruptions, and of the remains of ancient volcanoes, that prove the extensive action of internal heat on the crust of the globe; and, 3dly, to give a concise account of volcanic rocks and products.

Earthquakes and volcanoes may be considered as different effects, produced by the agency of subterranean fire. They frequently accompany each other; and, in all instances, that have been observed, the first eruption of a volcano is preceded by an earthquake of greater or less extent. Volcanoes do not make their appearance in every country where the shock of an earthquake is felt: but earthquakes are more frequent in volcanic districts than in any other. Earthquakes are, almost always, preceded by an uncommon agitation of the waters of the ocean, and of lakes. Springs send forth torrents

of mud, accompanied with a disagreeable stench. The air is generally calm, but the cattle discover inuch alarm, and seem to be, instinctively, aware of approaching calamity. A deep rumbling noise, like that of carriages over a rough pavement-a rushing sound like wind-or a tremendous explosion like the discharge of artillery,immediately precede the shock, which, suddenly, heaves the ground upwards, or tosses it, from side to side, with violent and successive vibrations. The shock seldom lasts longer than a minute; but it is frequently succeeded by others of greater or less violence, which, for a considerable time, continue to agitate the surface of the earth. During these shocks, large chasms and openings are made in the ground, through which smoke and flames are seen to issue: these sometimes break out where no chasms can be perceived. More frequently, stones, or torrents of water, are ejected from these openings. In violent earthquakes, the chasms are so extensive, that large cities have, in a moment, sunk down and for ever disappeared, leaving a lake of water in the place. Such was the fate of Euphemia in Calabria, in 1638, as described by Kircher, who was approaching the place, when the agitation of the ocean obliged him to land at Lopizicum. "Here," says he, "scenes of ruin every where appeared around me but my attention was quickly turned from more remote to contiguous danger, by a deep rumbling sound, which every moment grew louder. The place where we stood shook most dreadfully. After some time, the violent paroxysm ceasing, I stood up, and turning my eyes to look for Euphemia, saw only a frightful black cloud. We waited till it had passed away, when nothing but a dismal and putrid lake was to be seen where the city once stood."

The extent to which earthquakes produce sensible effects on the waters of springs and lakes in distant parts of the world, is truly remarkable. During the earthquake of Lisbon, in 1775, almost all the springs and lakes in Britain, and every part of Europe, were violently agitated, many of them throwing up mud and sand, and emitting a fœtid odour. On the morning of the earthquake, the hot springs at Toplitz, in Bohemia, suddenly ceased for a minute to flow, and then burst forth with prodigious violence, throwing up turbid wa ter, the temperature of which was higher than before; it is said to have continued so, ever since. The hot wells at Bristol were coloured red, and rendered unfit for use, for some months afterwards. Even the distant waters of Lake Ontario*, in North America, were violently agitated at the time. These phenomena offer proofs of subterranean communications under a large portion of the globe; they also indicate, that a great quantity of gas or elastic vapour was, suddenly, generated and endeavouring to escape. From the fœtid odour

* It has been observed during many eathquakes in the Eastern States, that the subterranean noise and motion appeared to commence from the Lakes, and proceed towards the Atlantic Ocean, in a direction from the north-west.

perceived in some situations, it may be inferred, that this gas is bydrogen or sulphuretted hydrogen. In other instances, it may be stean, which condensing again would produce a vacuum, and occasion the external air to press downwards; this has been observed in mines, immediately after the shock of an earthquake.

The space over which the vibration of the dry ground is felt is very great, but generally wider in one direction than another; and where a succession of earthquakes has taken place in the same district, it is observed that the noise and shock approach from the same quarter. It has been before mentioned, that the earthquakes are most frequent in volcanic districts; but the shocks are not the most violent in the immediate vicinity of volcanoes. On the contrary, they are stronger in the more distant part of a volcanic country. The ground is agitated with greater force, as the surface has a smaller number of apertures communicating with the interior. "At Naples and Messina, and at the foot of Cotopaxi and Tungurahua, earthquakes are dreaded only when vapours and flames do not issue from the craters."Humboldt.

The connexion of earthquakes with volcanoes was noticed by ancient writers, and the latter were properly regarded as the openings through which the inclosed vapour and ignited matter, that occasion earthquakes, found a passage. Strabo, in his Geography, states, that "the town of Regium, situated on the Italian side of the Straits of Messina, was so called, according to Eschylus, from the circumstance that the island of Sicily was rent off from the continent by earthquakes. Proofs of this arise out of the phenomena attending Etna, and other parts of Sicily and the Lipara Islands, and even the opposite continent. Now, indeed, when craters are opened, through which fire and ignited matter and water are poured out, it is said that the land near the Straits is seldom shaken by earthquakes: but formerly, when all the passages to the surface were obstructed, the fire and vapour confined in the earth occasioned frequent earthquakes, and the land, being rent, admitted the ocean. At the same time, Prochyta and an adjacent island were also torn off from the continent, while other islands rose from the ocean, as frequently happens at this day."-(Strabo flourished in the reign of Augustus.)

It is highly probable that every extensive earthquake is followed by a volcanic eruption, more or less remote, unless (as not unfrequently happens) the elastic vapour immediately escapes from fissures made at the time, in the countries that are the most violently convulsed. An earthquake was strongly felt in Geneva when I was there, February 19, 1822, and did considerable damage, in several towns and villages in Savoy and France. A few weeks afterwards, I travelled from Geneva to Lyons, and from thence to the ancient volcanoes near Clermont. In the course of my route, I made frequent inquiries respecting the effects of the earthquake: it appeared to have been most strongly felt along the valley of the Rhone, and the

shock was not less severe in the volcanic district of Auvergne; its direction was from the south-east and on that day and the following there were several eruptions from Vesuvius.

The frequency of earthquakes, at particular periods, is well deserving of notice. In the fourth and fifth centuries, some of the most civilized parts of the world were almost desolated by these awful visitations. Thrace, Asia Minor, and Syria, according to cotemporary historians, suffered most severely: the earth was agitated continually for long periods, and flames were seen to burst from the earth, over a vast extent of surface. On the 26th of January, A. D. 447, subterranean thunders were heard from the Black to the Red Sea, and the earth was convulsed, without intermission, for the space of six months; in many places, the air seemed to be on fire; towns and large tracts of ground were swallowed up in Phrygia. On the 20th of May, A. D. 520, the city of Antioch was overturned by a dreadful earthquake, and two hundred and fifty thousand of its inhabitants are said to have been crushed in ruins. A raging fire covered the ground on which the city was built, and the district around; spreading over an extent of forty-two miles in diameter, and a surface of fourteen hundred square miles.

About the middle of the last century, after the earthquake at Lisbon, Europe, Africa, and America, were, for some time, repeatedly agitated, by subterranean explosions; as may be seen by referring to the journals of that time. Etna, which had been in a state of profound repose for eighty years, broke out with great activity; and, according to Humboldt, some of the most tremendous earthquakes and volcanic eruptions ever recorded in history were witnessed in Mexico. In the night of the 19th of September, 1759, a vast volcano broke out in a lofty cultivated plain; a tract of ground more than twelve miles in extent, rose up like a bladder to the height of five hundred and twenty-four feet, and six new mountains were formed, higher than the Malvern Hills, in Worcestershire. More recently (in 1812) the tremendous earthquakes in the Caraccas were followed by an eruption in the Island of St. Vincent's, from a volcano that had not been burning since the year 1718; and violent oscillations of the ground were felt both in the islands and on the coasts of America. It may be inferred from these circumstances, that the cause of earthquakes and volcanic eruptions is seated deep below the surface of the earth; in confirmation of which, it will only be necessary to state, that on the same day on which Lisbon was nearly destroyed, all Europe, and a great part of northern Africa, felt the shock more or less severely: its effects were also sensible across the Atlantic, both in the United States and the West Indies. Incredible as it may seem, one fourth of the northern hemisphere was agitated by the same earthquake. The bed of the Atlantic was raised above the surface of the ocean, and flame and vapour were discharged: this was observed by vessels at sea. If we take a terrestrial globe, and

cover those parts of it that were thus affected by the earthquake with black crape, we shall obtain a more distinct idea of the extent of surface shaken, than a mere verbal description can convey. This appears to have been one of the most severe shocks that the old continent had experienced for several centuries. The cause which could effect a simultaneous concussion over such a vast extent, must probably have been seated nearly midway between the centre of the globe and its surface.

It has been remarked, that in general, earthquakes are more severely felt in mountainous than in low countries: this might be expected from the structure of the earth.* In alpine districts, the primary mountains are not pressed with the incumbent mass of secondary rocks; and, consequently, in such situations, the resistance to a force acting from beneath will be much less, as all the weight of secondary rock is removed. In very violent earthquakes, the secondary strata are broken or agitated; but proofs are not wanting, of lesser vibrations being stopped by their pressure. Humboldt says, he has seen workmen hasten from the mines of Marienburgh, in Saxony, alarmed by agitations of the earth that were not felt at the surface. During the earthquake at Lisbon, the miners in Derbyshire felt the rocks move, and heard noises which were scarcely perceived by those above. That an expansive force, acting from beneath, is the proximate cause of earthquakes, can scarcely be denied ; and the prodigious power of steam, when suddenly generated, seems equal to their production, if the quantity be sufficiently great. It is said, that a single drop of water falling into a furnace of melted copper, will blow up the whole building. This may be an exaggerated statement: but the prodigious force of steam at high temperatures is well known, and there can be no difficulty in admitting, that if a current of subterranean water were to find access to a mass of lava ma ny miles in extent, and most intensely heated, it would produce an earthquake more or less violent, in proportion to the quantity of steam generated, and its distance from the surface. When the hydrogen gas exploded in a mine near Workington, in Cumberland, a shock like that of an earthquake was felt by ships in the river, at two miles' distance.

The horrid crash, like the rattling of carriages, which precedes earthquakes, may be occasioned by the rending of the rocks, or parting of the strata through which the confined vapour is forcing a passage.

All the phenomena that accompany earthquakes indicate the intense operation of elastic vapour, expanding and endeavouring to escape where the least resistance is presented, and producing vibra

* See a paper on Earthquakes, by the Rev. Mr. Mitchell, Philosophical Trans actions, 1759.