CHAPTER VII. EARTHQUAKES.
After speaking of the wonders which are found on the surface of the earth, and in its recesses, it now seems natural to say a few words on those violent convulsions of nature, which appear occasionally to shake it to its centre, and to effect the most extraordinary changes in its outward appearance. There appears but little doubt that earthquakes are intimately connected with volcanoes, though it is very difficult to understand how the subterraneous vapours which occasion them are generated, and in what way they gain sufficient force, to produce the extraordinary effects which we see result from them, at such a great depth from the surface of the ground. In the great earthquake which took place in Asia Minor in the seventeenth year of the Christian era, and which destroyed thirteen great cities in one night, and shook a mass of earth 300 miles in diameter, it has been calculated that the moving power, supposing it to have been internal fire or vapour, must have been launched 200 miles below the surface of the earth.
Another great earthquake destroyed the city of Antioch. Another threw down the famous Colossus of Rhodes. In more modern times, we find descriptions of the earthquake at Puteoli, which occasioned the sea to retire 200 yards from its former bed; the earthquake in Calabria in 1638, which was evidently connected with an eruption of Mount Etna, and by which the city of Euphemia was swallowed up; and the great earthquake of 1755, which completely destroyed the city of Lisbon.
The Lisbon earthquake extended over a space nearly equal to four millions of square miles; but the city of Lisbon appears to have been in the centre of its fury. On the 1st of November, about forty minutes past nine in the morning, the shock was felt at Lisbon; and though it only lasted six seconds, it overthrew every church and large building in the city.
It is a remarkable circumstance attending this earthquake, that it was felt almost as violently at sea as it was on land, and that the master of a ship, fifty leagues from Lisbon, felt the shock of the first earthquake so decidedly, that he fancied he had mistaken his reckoning and had struck on a rock. This great earthquake was felt in many other places, even in England, particularly at Eyam Bridge, near the Peak in Derbyshire, where the overseer of the lead-mines, sitting in his writing-room, felt the chair in which he was sitting raised up and set down again, so decidedly that he distinctly felt his feet were raised from the ground. Of course he was excessively frightened, and ran out to see if anything had happened, when he met the miners running in different directions from the mine all excessively terrified, and each having something to relate which had happened to himself. One had seen the rocks move, another had heard them grind one upon another, and all had been terrified with a fearful rumbling like groans, which appeared to come from the bowels of the earth. At White Rock, in Glamorganshire, about three-quarters past six in the evening, and about two hours after the ebb of the tide, an immense quantity of water rushed up with a prodigious noise, floated two large vessels, the least of which was of 200 tons burden, and swept them from their moorings, driving them down the river with such extraordinary impetuosity that it almost upset them. "The whole rise and fall of this extraordinary body of water did not last above ten minutes, nor was it felt in any other part of the river, so that it seemed to have gushed out of the earth at that place." Among the other remarkable phenomena which attended this earthquake, was the drying up of numerous springs in various parts of the world, and the breaking forth of others.
THE CALABRIAN EARTHQUAKES.
These earthquakes began on the 5th of February, 1783; and they continued till the latter end of the following May, doing a great deal of mischief in Sicily and the two Calabrias. During the first month after their commencement, the earth appeared in a constant tremour, and its motions were various, sometimes moving horizontally, and sometimes by pulsations or beatings from the bottom upwards. "This variety increased the apprehensions of the unfortunate inhabitants, who momentarily expected that the earth would open beneath their feet, and swallow them up. The rains had been continual and violent, often accompanied by lightning and furious gusts of wind. There were many openings and cracks in the earth; and several hills had been lowered, while others were made quite level. In the plains, the chasms were so deep, that many roads were rendered impassable. Huge mountains were severed, and portions of them driven into the valleys, which were thus filled up. The course of several rivers was changed; and many springs of water appeared in places which had before been perfectly dry."
Sir William Hamilton, who has given a very circumstantial account of these earthquakes, is decidedly of opinion that they were caused by some great operation of nature of a volcanic kind. He found that in every case the shocks had been preceded by a rumbling noise from the westward, and that they had begun with a horizontal motion, ending with a whirling movement so violent that the tops of the largest trees almost touched the ground from side to side. The oxen and horses seemed to prepare themselves for each shock, standing with their legs as wide asunder as possible, in order to prevent themselves from being thrown down. In one situation, a piece of ground a mile in length, and half a mile in breadth, with two cottages, and several large olive and mulberry trees, was lifted up and floated about a mile down the valley, where it remained with the houses and most of the trees erect. In another place, two huge portions of ground, on which stood several hundred houses, were detached from the town to which they belonged, and carried half a mile across a ravine. Of course most of the houses which made this extraordinary leap were thrown down; but many of the inhabitants who were in them were dug out alive, and several of them completely unhurt. Many similar accidents were found to have happened in various places, and the same effects were produced on the springs as had been noticed in other earthquakes.
EARTHQUAKE OF JAMAICA.
The earthquake of Jamaica, in 1692, is one of the most dreadful that history has to record. It was attended with a hollow rumbling noise like that of thunder, and in less than a minute all the houses on one side of the principal street in the town of Port Royal sank into a fearful gulf forty fathoms deep, and water came roaring up where the houses had been. On the other side of the street the ground rose up and down like the waves of the sea, raising the houses and throwing them into heaps as it subsided. In another part of the town the street cracked along all its length, and the houses appeared suddenly twice as far apart as they were before. In many places the earth opened and closed again, so that several hundred of these openings were to be seen at the same time; and, as the wretched inhabitants ran out of their tottering dwellings, the earth opened under their feet, and in some cases swallowed them up entirely; while in others, the earth suddenly closing, caught them by the middle, and thus crushed them to death. In some cases these fearful openings spouted up cataracts of water, which were attended by a most noisome stench. It is not possible for any place to exhibit a scene of greater desolation than the whole island presented at this period. The thundering bellowing of the distant mountains, the dusky gloom of the sky, and the crash of the falling buildings gave unspeakable horror to the scene. Such of the inhabitants as were saved sought shelter on board the ships in the harbour, and remained there for more than two months, the shocks continuing with more or less violence every day. When, at length, the inhabitants were enabled to return, they found the whole face of the country changed. Very few of the houses which had not been swallowed up were left standing, and what had been cultivated plantations were converted into large pools of water. The greater part of the rivers had been choked up by the falling in of detached masses of the mountains, and spreading over the valleys, they had changed what was once fertile soil into morasses, which could only be drained by cutting new channels for the rivers; while the mountains themselves had changed their shapes so completely, that it was conjectured that they had formed the chief seat of the earthquake.
CHAPTER VIII. ISLANDS.
An account has already been given of the sudden appearance of several islands that were evidently of volcanic origin; but new islands are occasionally formed under other circumstances. Of this nature are the coral reefs of the Indian and Pacific Oceans, and the floating island of one of the lakes of Cumberland.
CORAL REEFS.
Throughout the whole range of the Indian and Pacific Oceans, are to be seen in different places what are called lagoon islands in different stages of growth, from coral banks, just gleaming through the deep water, like the rocks of Alacran, on which the Tweed was wrecked on the 12th of February, 1847, to perfect islands covered with a variety of trees and shrubs, which give food to paroquets, pigeons, and other birds.
The nature of the polypes which form coral has been already described when speaking of fossils, (p. 158,) and those which form the coral islands are of exactly the same nature. They belong to the lowest order of animated beings; they are radiated and star-like in their forms, and are generally incapable of locomotion; yet they possess the power of excreting from the lower part of their bodies a large quantity of calcareous matter, which is necessary to each animal, in order to form the cells or polypidom, in one of whose hollows it lives, its companions living in other similar hollows of the same polypidom, and being all affected alike by everything that affects the mass, though each polyp has a separate existence, and may be termed a distinct individual. Thus these little animals build their stone houses, adding compartment after compartment; and, as Professor Ansted observes, "erecting in succession one story after another," they continue at their work "month after month, year after year, and century after century;" one generation following another, till in time these little insignificant and feeble animals have actually added numerous islands to the solid matter of our globe. "The prodigious extent of the combined and unintermitting labours of these little world-architects," says Professor Ansted, "must be witnessed, in order to be adequately conceived or realized. They have built up 400 miles of barrier reef on the shores of New Caledonia; and on the north-east coast of Australia their labours extend for 1000 miles in length; and their reefs, which may average, perhaps, a quarter of a mile in breadth, and 150 feet in depth, have been built amidst the waves of the ocean, and in defiance of its fiercest storms." Similar reefs are in progress in the Indian Ocean; and a group of these, called the Keeling or Cocos Islands, are described at length by Mr. Darwin, who visited them as naturalist on board the ship Beagle in 1836. Nothing can be more singular than the appearance of these islands: a ring is first formed consisting only of coral reefs, and enclosing a lagoon, or lake of salt water. The effect of the scenery thus produced is very striking, though it depends almost entirely upon colour. "The shallow, clear, and still water of the lagoon, resting in its greater part on white sand, is, when illumined by a vertical sun, of the most vivid green. This brilliant expanse, several miles in width, is on all sides divided, either by a line of snow-white breakers from the dark heaving waters of the ocean, or from the blue vault of heaven by the strips of land, crowned by the level tops of the cocoanut trees. As a white cloud here and there affords a pleasing contrast with the azure sky, so, in the lagoon, bands of living coral occasionally darken the emerald green water." On reaching one of these islands, or rather reefs, Mr. Darwin went on shore, and found the strip of dry land was only a few hundred yards in width. On the lagoon side there was a white calcareous beach; and on the outer coast a solid broad flat mass of coral rock, which served to break the violence of the open sea. Where the island consists only of a ring of coral reefs, it is called an atoll, or lagoon island; but gradually the coral reefs become covered with earth, as they serve as a barrier to arrest the sand, which is continually being carried by the great rivers into the sea, and held in solution by its waters. As soon as a little earth has been collected on the coral, the seeds of different trees, which have been carried onwards by the currents of the ocean, are deposited, and cocoa-palms and other plants spring up; while the leaves and fruit of these, as they fall and decay, form fresh soil, from which additional plants spring; and thus by degrees the lagoons become filled, and a complete island formed. At the border near the sea the lagoon islands present large fields of coral rising above the water, and interspersed with gigantic shells of the common Chama (Tridacna giganted); but when Mr. Darwin examined these coral fields, he found that the whole forest of delicately branching corals, though standing upright, were all dead and rotten, as it appears that corals are not able to survive even a short exposure in the air to the sun's rays; so that their upward limit of growth is determined by that of lowest water at spring tides. The coral when thus rotted crumbles into mud, and this mud helps to fill up the lagoons. In some of the islands stones of various kinds have occasionally been found mixed with the mud, but they have been apparently carried there by the currents of the ocean, probably entangled in the roots of some tree; as stones are so rare on the lagoon islands generally, that some of the natives of a group of lagoon islands in the midst of the Pacific, when carried by Kotzebue to Kamtschatka, collected stones there to take back with them to their own country. Besides the atolls, or rings of coral reefs, the corals sometimes extend in straight lines in front of the shores of a continent or of a large island, or they encircle smaller islands; "in both cases being separated from the land by a broad and rather deep channel of water analogous to the lagoon within an atoll." These are called barrier reefs, and though they are of very great extent, in some cases "the whole line of reef has been converted into land, but usually a snow-white line of great breakers, with only here and there a single low islet crowned with cocoa-nut trees, divides the dark heaving waters of the ocean from the light green expanse of the lagoon-channel. And the quiet waters of the channel generally bathe a fringe of low alluvial soil, loaded with the most beautiful productions of the Tropics, and lying at the foot of the wild, abrupt, central mountains. Encircling barrier reefs are of all sizes, from three miles to no less than forty-four miles in diameter, and that which fronts one side, and encircles both ends, of New Caledonia is 400 miles long. Each reef includes one or more rocky islands of various heights, and in one instance even as many as twelve separate islands." The reefs vary in their distance from the shore, in some cases approaching even within one mile of it, and in others being upwards of twenty miles, with all the varying distances between these two extremes. The third class consists of what are called the fringing reefs, which, as Mr. Darwin observes, "will require a very short notice. Where the land slopes abruptly under water, these reefs are only a few yards in width, forming a mere ribbon or fringe round the shores; but where the land slopes gently under the water, the reef extends farther, sometimes even as much as a mile from the land." Mr. Darwin adds, that, "from the corals growing more vigorously on the outside, and from the noxious effect of the sediment washed inwards, the outer edge of the reef is the highest part, and between it and the land there is generally a shallow sandy channel, a few feet in depth. Where banks of sediment have accumulated near to the surface, as in parts of the West Indies, they sometimes become fringed with corals, and hence in some degree resemble lagoon-islands, or atolls; in the same manner as fringing reefs, surrounding gently sloping islands, in some degree resemble barrier reefs."