Читать книгу The Subterranean World - G. Hartwig - Страница 13

Table of Contents

Extent of Misery inflicted by great Earthquakes—Earthquake Regions—Earthquakes in England—Great Number of Earthquakes—Vertical and undulatory Shocks—Warnings of Earthquakes—Sounds attending Earthquakes—Remarkable Displacements of Objects—Extent and Force of Seismic Wave Motion—Effects of Earthquakes on the Sea—Enormous Waves on Coasts—Oscillations of the Ocean—Fissures, Landslips, and shattering Falls of Rock caused by Earthquakes—Causes of Earthquakes—Probable Depth of Focus—Opinions of Sir Charles Lyell and Mr. Poulett Scrope—Impressions produced on Man and Animals by Earthquakes.

Of all the destructive agencies of nature there is none to equal the earthquake. The hurricane is comparatively weak in its fury; the volcanic eruption generally confines its rage to the neighbourhood of the labouring mountain, but a great earthquake may cover a whole land with ruins.

The terrible subterranean revolution which convulsed all Asia Minor and Syria in the reign of Tiberius, destroyed twelve celebrated cities in a single night. The sun, which on setting had gilded their temples and palaces with his parting rays, beheld them prostrate on the following morning.

In A.D. 115 Antioch was the centre of a great commotion. The city was full of soldiers under Trajan; heavy thunder, excessive winds, and subterranean noises were heard; the earth shook, the houses fell; the cries of people buried in the ruins passed unheeded. The Emperor leaped from a window, while mountains were broken and thrown down, and rivers disappeared, and were replaced by others in a new situation. Four centuries later (May 20, 526) the same doomed city was totally subverted by an earthquake, when it is reported that 250,000 persons perished.

Similar catastrophes, in which thousands and thousands of victims were suddenly destroyed, have frequently occurred in Peru and Chili, in the West Indies and Central America, in the Moluccas and Java, in the countries bordering on the Mediterranean and the Red Sea; but a bare mention of the loss of life conveys but a faint idea of the extent of misery inflicted by one of those great earthquakes which mark with an ominous shade many large tracts of the earth’s surface.

We must picture to ourselves the slow lingering death which is the fate of many—some buried alive, others burnt in the fire which almost invariably bursts out in a city where hundreds of dwellings have suddenly been laid prostrate—the numbers who escaped with loss of limbs or serious bodily injuries, and the surviving multitude, suddenly reduced to penury and want.

In the Calabrian earthquake of 1783, it is supposed that about a fourth part of the inhabitants of Polistena and of some other towns were buried alive, and might have been saved had there been no want of hands; but in so general a calamity, where each was occupied with his own misfortunes or those of his family, help could seldom be procured. ‘It frequently happened,’ says Sir Charles Lyell, ‘that persons in search of those most dear to them could hear their moans, could recognise their voices, were certain of the exact spot where they lay buried beneath their feet, yet could afford them no succour. The piled mass resisted all their strength, and rendered their efforts of no avail. At Terranuova four Augustin monks, who had taken refuge in a vaulted sacristy, the arch of which continued to support a vast pile of ruins, made their cries heard for the space of four days. One only of the brethren of the whole convent was saved, and of what avail was his strength to remove the enormous weight of rubbish which had overwhelmed his companions? He heard their voices die away gradually, and when afterwards their four corpses were disinterred, they were found clasped in each other’s arms.’arms.’

Affecting narratives are preserved of mothers saved after the fifth, sixth, and even seventh day of their interment, when their infants or children had perished with hunger. In his work on the great Neapolitan earthquake of 1857, Mr. Mallet, from innumerable narratives of personal peril and sad adventure, selects the distressing case of a noble family of Monte Murro, as affording a vivid picture of the terrors of an earthquake night. Don Andrea del Fino, the owner of one of the few houses in the city which escaped total destruction, was with his wife in bed, his daughter sleeping in an adjacent chamber on the principal floor. At the first shock his wife, who was awake, leaped from bed, and immediately after, a mass of the vaulting above came down, and buried her sleeping husband. At the same moment, the vault above their daughter’s room fell in upon her. From the light and hollow construction of the vaults neither was at once killed. The signora escaped by leaping from the front window, she scarcely knew how. For more than two hours she wandered, unnoticed, amongst the mass of terrified survivors in the streets, before she could obtain aid from her own tenants and dependants, to extricate her husband. They got him out after more than eighteen hours’ entombment—alive, indeed, but maimed and lame for life. His daughter was dead. As he lay longing despairingly for release from the rubbish, which a second shock, an hour after the first, had so shaken and closed in around him that he could scarcely breathe, he heard, but a few feet off, her agonising cries and groans grow fainter and fainter, until at last they died away. His wife, to whose devotion his own life was owing, had escaped unhurt.

Unfortunately man too often vies with the brute forces of nature to increase the horrors of a great earthquake. As the arm of the law is paralysed by the general panic, thieves and ruffians are not slow to avail themselves of their opportunity. Thus in the Calabrian catastrophe of 1783, nothing could be more atrocious than the conduct of the peasants, who abandoned the farms and flocked in great numbers into the towns—not to rescue their countrymen from a lingering death, but to plunder. They dashed through the streets amid tottering walls and clouds of dust, trampling beneath their feet the bodies of the wounded and half buried, and often stripping them, while yet living, of their clothes.

From the vast ruin and misery they entail, it is evident that where earthquakes are frequent, there can never be perfect security of property even under the best government; and as the fruits collected by the labour of many years may be lost in an instant, the progress of civilisation and national wealth must necessarily be retarded.

‘Earthquakes alone,’ says Mr. Darwin, ‘are sufficient to destroy the prosperity of any country. If beneath England the now inert subterranean forces should exert those powers which most assuredly in former geological ages they have exerted, how completely would the entire condition of the land be changed! What would become of the lofty houses, thickly packed cities, great manufactories, the beautiful public and private edifices? If the new period of disturbance were first to commence by some great earthquake in the dead of the night, how terrific would be the carnage! England would at once be bankrupt; all papers, records, and accounts would from that moment be lost. Government, being unable to collect the taxes, and failing to maintain its authority, the hand of violence and rapine would remain uncontrolled. In every large town famine would go forth, pestilence and death following in its train.’

Fortunately the experience of many ages shows that the regions subject to these terrible catastrophes are confined to a comparatively small part of the surface of the globe. Thus Southern Italy and Sicily; the tract embracing the Canaries, the Azores, Portugal, and Morocco; Asia Minor, Syria, and the Caucasus; the Arabian shore of the Red Sea; the East Indian Archipelago; the West Indies, Nicaragua, Quito, Peru, and Chili, are particularly liable to destructive shocks.

But beyond these limits slighter earthquakes are of far more common occurrence than is generally supposed, and probably they leave no part of the world entirely undisturbed. From the year 1821 to 1830 no less than 115 earthquakes have been felt to the north of the Alps, and since the year 1089, 225 are cited in the annals of England. Some of these earthquakes seem to have but just stopped at the point when a slight increase of their force would have covered the land with ruins. In 1574, on the 26th of February, between five and six in the evening, an earthquake was felt at York, Worcester, Hereford, Gloucester, and Bristol. Norton Chapel was filled with worshippers; they were nearly all overthrown, and fled in terror, thinking that the dead were unearthed or that the chapel was falling. Six years later, on the 6th of April, at 6 P.M., all England was thrown into consternation. The great bell at Westminster began to toll; the students at the Temple started up from table and rushed into the street, knives in hand; a part of the Temple Church fell, and stones dropped from St. Paul’s. Two stones fell in Christ’s Church, and crushed two persons. In rushing out of the church many were lamed, and there was a shower of chimneys in the streets. At Sandwich, the occurrence was marked by the violence of the sea, which made ships run foul of each other; and at Dover a part of the fortifications fell with the rock which supported it.

On the 6th October 1863, a movement, though gentle when compared to the preceding instances, was felt from the English Channel to the Mersey, and from Hereford to Leamington and Oxford. The Malvern range was about the centre of the area, as it has often been before. Even in alluvial Holland, six or eight slight earthquakes have been felt during the last century. The industrious researches of Kluge show that, during the eight years from 1850 to 1857, no less than 4,620 earthquakes—a great proportion of which (509) fell to the share of Southern Europe—have been noticed in both hemispheres; and when we remember that a very considerable part of the globe is still either totally unknown or removed by the barbarous condition of its inhabitants from all intercourse with the scientific world, and that, consequently, the above list must necessarily be incomplete, it is very probable that not a day passes without some agitation of the surface of the earth in some place or other.

A violent earthquake almost always consists of several shocks following each other in rapid succession. Sometimes they are preceded by slighter vibrations; at other times they suddenly convulse the land without any previous notice. In most instances, each shock lasts but a few seconds; but this is enough to ruin the work of ages. Three violent commotions within five minutes destroyed the town of Caraccas on March 26, 1812; and the earthquake which, in 1692, desolated Jamaica lasted but three minutes. On January 11, 1839, two shocks within thirty seconds covered Martinique, and the whole range of the Lesser Antilles, with ruins. But a violent earthquake, though itself but of short duration, is generally followed by a series of secondary shocks, which are repeated at gradually widening intervals and with decreasing energy, so that if these subsequent tremors be taken into account, it may often be said to last for weeks, or even months. Thus, to cite but one instance, the earthquake of October 21, 1766, destroyed the whole town of Cumana in a few minutes, but during the following fourteen months the earth was in a constant vibratory motion, and scarce an hour passed without a shock being felt.

In countries where earthquakes are comparatively rare (for instance in the south of Europe) the belief is very general that oppressive heat, stillness of the air, and a misty horizon are always forerunners of the phenomenon. But this popular opinion is not confirmed by the experience of trustworthy observers, who have lived for years in countries such as Cumana, Quito, Peru, and Chili, where the ground trembles frequently and violently. Humboldt experienced earthquakes during every state of the weather, serene and dry, rainy and stormy.

Brute animals, being more sensitive than men of the slightest movement of the earth, are said to evince extraordinary alarm, and it has been often observed that even the dull hog shows symptoms of uneasiness previous to the shock. During the great Neapolitan earthquake of 1857 an unusual halo-like light was seen in the sky before, and not long before, the shock. Mr. Mallet was at first inclined to look upon this notion as a superstitious tale; but, finding it widely diffused in a country where communication is bad and news travels slowly, no longer doubted that it was founded on fact.^[10] Conjectures would be useless as to its nature, but future observation directed to the point may determine whether some sort of auroral light may emanate from the vast depths of rock formation under the enormous tensions and compressions that must precede the final crash and rupture; or whether volcanic action, going on in the unseen depths below, may give rise to powerful disturbances of electric equilibrium, and hence to the development of light; just as from volcanic mountains in eruption lightnings continually flash from the huge volumes of steam and floating ashes above the crater. Humboldt is also of opinion that, though in general the revolutions which take place below the surface of the earth are not announced beforehand by any meteorological process, or a peculiar appearance of the sky, it is not improbable that during violent shocks some change may occur in the condition of the atmosphere. Thus, during the earthquake in the Piedmontese valleys of Pelis and Clusson, great alterations were observed in the electrical tension of the atmosphere without any appearance of a thunder-storm.

Earthquakes are generally attended with sounds, sometimes like the howling of a storm, or the rumbling of subterranean thunder, at others like the clashing of iron chains, or as if a number of heavily laden waggons were rolling rapidly over the pavement, or as if enormous masses of glass were suddenly shivered to pieces. As solid bodies are excellent conductors of sound (burnt clay, for instance, propagating it ten or twelve times more rapidly than the air), the subterranean noise may be heard at a vast distance from the primary seat of the earthquake. In Caraccas, in the grass-plains of Calabozo, and on the banks of the Rio Apure, which falls into the Orinoco, a dreadful thunder-like sound was everywhere heard on April 30, 1812, without any simultaneous trembling of the earth, at the time when, at the distance of 158 geographical miles, the volcano of St. Vincent, in the Lesser Antilles, was pouring out of its crater a mighty lava-stream. This was, according to distance, as if an eruption of Vesuvius were heard in the north of France. In the year 1744, during the great eruption of the volcano Cotopaxi, a subterranean noise like the firing of cannon was heard at Honda on the Magdalena river. But the crater of Cotopaxi is 17,000 feet higher than Honda, and both points, situated at a distance of 109 geographical miles, are moreover separated by the colossal mountain masses of Quito, Pasto, and Popayan, and by numberless ravines and deep valleys. The sound was certainly transmitted, not through the air, but through the earth, and must have proceeded from a very considerable depth.

But noise is not the necessary attendant of an earthquake, for many instances are known in which the most violent shocks have been completely noiseless. No subterranean sounds were heard during the terrific earthquake which destroyed Riobamba on February 4, 1797, and the same circumstance is mentioned in the narratives of many of the Chilian earthquakes.

The phenomenon of sound, when unaccompanied by any perceptible vibration, makes a peculiarly deep impression on the mind, even of those who have long inhabited a country subject to frequent earthquakes. They tremble at the idea of the catastrophe which may follow. A remarkable instance of a long protracted noise without any trembling of the earth occurred in 1784, at the wealthy mining town of Guanaxuato in Mexico, where the rolling of subterranean thunder, with now and then a louder crash, was heard for more than a month, without the slightest shock, either on the surface of the earth, or in the neighbouring silver mines, which are 1,500 feet deep. The noise was confined to a small space, so that a few miles from the town it was no longer audible. Never before had this phenomenon been known to occur in the Mexican highlands, nor has it been repeated since.

Earthquake shocks are either vertical or undulatory. A vertical shock, which is felt immediately above the seat or focus of the subterranean disturbance, causes a movement up and down. Like an exploding mine, it frequently jerks movable bodies high up into the air. Thus, during the great earthquake of Riobamba, the bodies of many of the inhabitants were thrown upon the hill of La Culla, which rises to the height of several hundred feet at the other side of the Lican torrent; and during the earthquake of Chili in 1837, a large mast, planted thirty feet deep in the ground at Fort San Carlos, and propped with iron bars, was thrown upwards, so that a round hole remained behind.

Although to the inhabitants of a shaken district the undulatory wave or vibration of an earthquake appears to radiate horizontally outwards from the spot on the surface where it is first felt, the force does not really operate in a horizontal direction, like a wave caused by a pebble on the surface of a pond; for at every point, except that immediately above the focus of the shock, it comes up obliquely from below, causing the ground to move forwards and then backwards in a, more or less horizontal direction. As a ship, yielding to the oscillatory movements of the waves, alternately inclines to one side or the other, so, during the more violent undulations of the soil, the objects on its surface are momentarily moved from their vertical position, and often considerably inclined towards the horizon. Thus during the great earthquake which convulsed the valley of the MississippiMississippi in 1811–12, Mr. Bringier, an engineer of New Orleans, who was on horseback near New Madrid, where some of the severest shocks were experienced, saw the trees bend as the wave-motion of the earthquake passed under them, and immediately afterwards recover their position. The transit of the wave through the woods was marked by the crash of countless branches, first heard on one side and then on the other. It must have been awful to see the giants of the forest thus move to and fro like a corn-field agitated by the wind!

Very remarkable displacements of objects are not seldom caused by earthquakes, such as the rotation of the blocks of columns or the turning of statues on their pedestals.

At Lima, which, owing to its repeated destructions by earthquakes, is properly a city of ruins, Professor Dana saw two obelisks with the upper stone on each displaced and turned round on its axis about fifteen degrees in a direction from north to east. These rotations by earthquakes have been attributed by some authors to an actual rotatory movement in the earthquake vibration; but it has lately been shown by Mr. Mallet that this hypothesis is untenable and unnecessary, as a simple vibration back and forth is all that is required to produce a rotatory motion in the stone of a column, provided that stone be attached below more strongly on one side of the centre than on the opposite.

The wave-motion of an earthquake sometimes spreads over enormous spaces. The shocks of the earthquake of New Granada which took place in the night from the 16th to the 17th of June 1826, were noticed over a surface of 750,000 square miles. The earthquake of Valdivia (February 20, 1835) was felt southwards on the distant island of Chiloe to the north as far as Copiapo, in Mendoza to the east of the Andes, and on the Island of Juan Fernandez, 300 miles from the coast. Supposing these effects to have taken place at corresponding distances in Europe, all the land would have trembled from the North Sea to the Mediterranean, and from Ireland to the centre of France.

It is evident that the extent and force of the wave-motion of an earthquake must in a great measure depend upon the nature of the rocks through which it is transmitted. It will vibrate more easily through solid homogeneous masses, while in alluvial deposits, or in a soil composed of sand and loose conglomerate, its undulations will be propagated irregularly and its effects be far more destructive. This is particularly the case where the alluvial deposits repose on a substratum of hard rock. Thus the devastations of the Calabrian earthquake of 1783 were most apparent in the plain of Oppido, in those parts where the newer tertiary strata rest upon granite. The earthquake wave generally follows the direction of mountain-chains, and but rarely crosses them. The great Chilian earthquakes, which often propagate their vibrations to distances of many hundred miles along the western foot of the Andes, remain unfelt on their eastern border; while the earthquakes along the shores of Venezuela, Caraccas, and New Granada rarely transmit their vibrations beyond the high mountain-chains which run parallel with the coast. This is probably due to the numerous dislocations, rents, and caverns which are produced by the elevation of the mountain-chains, and necessarily serve as barriers to the propagation of the earthquake wave.

Severe earthquakes are not seldom accompanied by a violent agitation of the sea. First, at the instant of the shock, the water swells high up on the beach with a gentle motion, and then as quietly recedes; secondly, some time afterwards, the whole body of the sea retires from the coast, and then returns in waves of overwhelming force. The first movement seems to be an immediate consequence of the earthquake affecting differently a fluid and a solid, so that their respective levels are slightly deranged; but the second is a far more important phenomenon. ‘Some authors,’ says Mr. Darwin, ‘have attempted to explain it by supposing that the sea retains its level, while the land oscillates upwards; but surely the water close to the land, even on a rather steep coast, would partake of the motion of the bottom; moreover, similar movements of the sea have occurred at islands far distant from the chief line of disturbance. I suspect (but the subject is a very obscure one) that a wave, however produced, first draws the water from the shore on which it is advancing to break. I have observed that this happens with the little waves from the paddles of a steamboat. From the great wave not immediately following the earthquake, but sometimes after the interval of even half-an-hour, and from distant islands being affected similarly with the coasts near the focus of the disturbance, it appears that the wave first rises in the offing, and, as this is of general occurrence, the cause must be general. I suspect we must look to the line where the less disturbed waters of the deep ocean join the water nearer the coast which has partaken of the movements of the land, as the place where the great wave is first generated; it would also appear that the wave is larger or smaller according to the extent of shoal water which has been agitated together with the bottom on which it rested.’

The following examples sufficiently prove that no storm, however violent, is capable of raising such prodigious waves as an earthquake.

In the year 1692 the town of Kingston in Jamaica was almost totally destroyed by a huge earthquake wave. A frigate which lay in port was carried forwards over the houses and stranded in the middle of the town. In his ‘Principles of Geology,’ Sir Charles Lyell relates that, during the Calabrian earthquake of 1783 the Prince of Scilla had persuaded a great part of his vassals to betake themselves to their fishing boats for safety, and he himself had gone on board. On the night of February 5, when some of the people were sleeping in the boats, and others on a level plain slightly elevated above the sea, the earth rocked and large masses of rock were thrown down with a dreadful crash upon the plain. Immediately afterwards the sea, rising more than twenty feet above the level of this low tract, rolled foaming over it and swept away the multitude. It then retreated, but soon rushed back again with greater violence, bringing back with it some of the bodies it had carried away. At the same time every boat was sunk or dashed against the beach, and some of them were swept far inland. The aged prince was killed, with 1,430 of his people.

After the earthquake which devastated the town of Lima on the 28th of October 1746, the sea rose on the evening of the same day eighty feet above its usual level in the neighbouring Bay of Callao, overwhelmed the town, and destroyed nearly all the inhabitants. Of the twenty-three ships which were lying in the harbour at the time, nineteen immediately sank, while the four others were thrown upon the land at the distance of nearly a league.

Shortly after the shock which desolated Chili on the 20th of February 1835, a great wave was seen from the distance of three or four miles, approaching in the middle of the Bay of Talcahuano with a smooth outline, but tearing up cottages and trees along the shore, as it swept onwards with irresistible force. At the head of the bay it broke in a fearful line of white breakers, which rushed up to a height of twenty-three vertical feet above the highest spring tides. Their force must have been prodigious, for at the Fort a cannon with its carriage, estimated at four tons in weight, was moved fifteen feet inwards. The whole coast was strewed over with timber and furniture, as if a thousand ships had been wrecked. As Mr. Darwin walked along the shore, he observed that numerous fragments of rock, which, from the marine productions adhering to them, must recently have been lying in deep water, had been cast up high on the beach. One of these was six feet long, three broad, and two thick.

During the dreadful earthquake which in 1868 raised the strip of land at the western foot of the Andes from Iburra in Ecuador, to Iquique in Peru, 1,200 miles in length, the receding sea uncovered the bay at Iquique to the depth of four fathoms, and then, returning in an immense wave, a mass of dark blue water, forty feet high, rushed over the already ruined city, and swept away every trace of what had been a town. One spectator, seeing the whole surface of the sea rise like a mountain, ran for his life to the Pampa. The waves overtook him. Fighting with the dark water, amidst wreck and ruin of every kind, carried back into the bay, and again thrown back to the Pampa, wounded and half-naked, he crept for safety into a hole of the sand, and waited sadly for the dawn. At Arica, the British Vice-Consul, alarmed at the first shock, rushed out of the house with his family, and made for the high ground, in just terror of the expected sea-wave. Through the ruined town, amidst dead and dying, half stifled with dust, they reached rising ground, and, looking back, saw a dreadful sequel—the sea rushed in and left not a vestigevestige remaining of the lower part of Arica. Six vessels were lost in the bay or tossed over rocks and houses; an American gunboat was whirled away from her moorings, and laid, without a broken spar or tarnished flag, high and dry on the sand-hills, a quarter of a mile from the sea.

As might be expected from the movable nature of water, the wave-motion of earthquakes is frequently propagated to surprising distances over the sea. The Chilian earthquake of 1835 produced oscillations of the ocean that made themselves felt on the Sandwich Islands at a distance of 5,000 nautical miles. On Mauai, the sea retreated 120 feet, and then suddenly returned with a tremendous wave that swept away the trees and houses on the beach. In Hawaii, a large congregation had assembled for divine service near Byron’s Bay. Suddenly the water began to sink, so that soon a great part of the harbour was laid dry. The spectators hurried to the shore to admire the astonishing spectacle, when a wave, rising twenty feet above the usual tide-mark, inundated the land, destroyed sixty-six huts, and drowned eleven of the islanders, though the best swimmers in the world. So far from its starting-point did the South American earthquake seek its victims. Fifteen hours and a half after the great earthquake of Arica (1868) the water-wave undulating over the vast Pacific was felt at Chatham Islands, a distance of 6,300 miles, and an hour later at New Zealand.

The enormous powers which come into action during a great earthquake show themselves not only in the destruction of edifices and the wide-spread ruin so produced, but in the changes which they effect in the configuration of the soil. Wherever masses of earth rest loosely upon a sloped surface of subjacent rock, or where steep mountain crests overlie wet and unctuous beds of shale, or where the rock itself is composed of coherent material, or where river-banks are formed of precipitous masses of clays, or where the corroding waters have undermined the ground, the violent commotion caused by an earthquake cannot fail to produce landslips, fissures, and falls of rock. In 1571, on the 17th of February, the ground opened all at once at the ‘Wonder,’ near Putley, not far from Marcle in Herefordshire; and a large part of the sloping surface of the hill—twenty-six acres, it is said—descended with the trees and sheep-folds, and continued in motion from Saturday to Monday, masses of ground being turned round through half a circle in their descent. This was a great landslip, said to have been occasioned by an earthquake.

Earth-fissures were formerly supposed to be occasioned by a stretching of the ground, occasioned by the wavy nature of the shocks; but Mr. Mallet has shown that no earthquake wave can possibly produce any such stretching, and considers them as cases of small and incipient landslips caused by the shaking downwards of a loose mass. His own observations left no doubt in his mind that the descriptions, given by the Neapolitan Academy in their Historical Account of the Earthquake of 1783, of the earth-fissures therein produced, and designated constantly by the pompous term ‘voragines,’ are gross exaggerations, and that the well-known Jamaica earth-fissures, that were said to have opened and closed with the wave, and to have bitten people in two, must be regarded as audacious fables.

‘The vulgar mind, filled from infancy with superstitious terrors as to “the things under the earth,” is seized at once by the notion of these fissures of profound and fabulous depth with fire and vapour of smoke issuing from within their murky abysses; but they should cease to belong to science.’

Enormous landslips are sometimes occasioned by earthquakes, but their extent depends less upon the power and energy of the shock than upon the conditions of unstable equilibrium presented by great masses of loose material, through the configuration of the country. In consequence of landslips or dislodgements of large masses of rock, alterations in the flow or distribution of the waters frequently take place. Thus, brooks or rivers are not seldom dammed, and temporary ponds or lakes created.

Permanent elevations of the land have been observed after some earthquakes. Thus, after the violent shocks of November 19, 1822, a great part of the coast of Chili was found to be raised several feet above its previous level, and after the great earthquake which occurred in New Zealand in the night of January 23, 1855, a large tract of land was found to be permanently upraised from one to nine feet. Before the shock there had been no room to pass between the sea and the base of a perpendicular cliff called Muka-Muka, except for a short time at low water, and the herdsmen were obliged to wait for low tide in order to drive their cattle past the cliff. But immediately after the upheaval, a gently sloping raised beach, more than 100 feet wide, was laid dry, affording ample space at all states of the tide for the passage of man and beast.

These permanent elevations have often been attributed to the immediate agency of earthquakes; but Mr. Mallet proves this assumption to be a fallacy, as the impulse of the earthquake wave even right above the focus is utterly incapable of raising the level of the land by a height much more than instrumentally appreciable, and there is not the least evidence that any part of even this elevation is permanent. That earthquakes occur along with, and as part of, a train of other circumstances which do produce permanent elevation occasionally, and that earthquakes are probably always the signals that the forces producing elevation are operative, is another matter, with which that erroneous or loosely expressed view should not be confounded.

The causes of earthquakes are still hidden in obscurity, and probably will ever remain so, as these violent convulsions originate at depths far below the reach of human observation. Mr. Mallet came to the conclusion that the depth of the original Calabrian shock in 1857 did not exceed seven or eight miles, and deduces from all the facts known as to the movements of earthquakes, that the subterranean points where the shocks originate perhaps never exceed thirty geographical miles, so that, even supposing the central nucleus of the earth to be fluid, they cannot possibly be due to the reaction of the internal ocean of molten stone upon the solid shell with which it is enveloped, but must have their seat within the latter. The existence of reservoirs of fused matter at various depths in the solid earth-rind is quite sufficient to account for all seismic and volcanic phenomena; for it is evident that whenever rain-water, or the waters of the sea percolating through rocks, gain access to these subterranean lakes of molten stone, steam must be generated, the pressure of which will in many cases rend and dislocate the incumbent masses.

‘During such movements,’ says Sir Charles Lyell, ‘fissures may be formed and injected with gaseous or fluid matter, which may sometimes fail to reach the surface, while at other times it may be expelled through volcanic vents, stufas, and hot springs. When the strain on the rocks has caused them to split, or the roofs of pre-existing fissures or caverns have been made to fall in, vibratory jars will be produced and propagated in all directions, like waves of sound through the crust of the earth, with varying velocity, according to the violence of the original shock, and the density or elasticity of the substance through which they pass. They will travel, for example, faster through granite than through limestone, and more rapidly through the latter than through wet clay, but the rate will be uniform through the same homogeneous medium.’

According to Mr. Poulett Scrope, the originating cause of the earthquake must be sought in the expansion of some deeply seated mass of mineral matter, owing to augmentation of temperature or diminution of pressure. By this expansive force, the solid rocks above are suddenly rent asunder, and whether below the sea or not, their violent disruption produces a jarring vibration, which is propagated on either side through their continuous masses in undulatory pulsations.

Some geologists are of opinion that earthquakes are frequently the result of the subsiding, sinking in, or cracking of subterranean cavern roofs, in consequence of the pressure of the superincumbent rocks. Small local earthquakes may be explained by this theory; but terrible convulsions which shake a whole continent evidently proceed from a far more formidable cause, and are more satisfactorily explained by the agency of subterranean heat and elastic vapours.

If, even during an ordinary storm, the black clouds, the howling of the wind, the flashes of lightning, and the loud claps of thunder strike men and brutes with fear, we may naturally expect to see terror carried to its highest pitch by so dreadful a phenomenon as an earthquake. All creatures living or burrowing under the earth—rats, mice, moles, snakes—hastily creep forth from their subterranean abodes, though many no doubt are gripped and suffocated by the suddenly moved soil before they can effect their escape; the crocodile, generally silent, like our little lizards, rushes out of the river and runs bellowing into the woods; the hogs show symptoms of uneasiness; the horses tremble; the oxen huddle together; and the fowls run about with discordant cries. On man, the phenomenon makes a peculiarly deep impression.

‘A bad earthquake,’ says Mr. Darwin, ‘at once destroys our oldest associations. The earth, the very emblem of solidity, has moved beneath our feet like a thin crust over a fluid. One second of time has created in the mind a strange idea of insecurity, which hours of reflection would not have produced.’ We can no longer trust the soil on which we stand, and feel ourselves completely at the mercy of some unknown destructive power, which at any moment, without forewarning, can destroy our property or our lives. But as first impressions are always the deepest, so habit renders man callous even to the terrors of an ordinary earthquake. In countries where slight shocks are of frequent occurrence, almost every vestige of fear vanishes from the minds of the natives, or of the strangers whom a long residence has familiarised with the phenomenon.

On the rainless coast of Peru, thunderstorms and hail are unknown. The thunder of the storm is there replaced by the thunder which accompanies the earthquake. But the frequent repetition of this subterranean tumult, and the general belief that dangerous shocks occur only twice or thrice in the course of a century, produce in Lima so great an indifference towards slighter oscillations of the soil that they hardly attract more attention than a hail-storm in Northern Europe.