Principles of Geology Part 46

Professor Bunsen of Marburg, after a.n.a.lyzing the gases which escape from the volcanic fumeroles and solfataras of Iceland, and after calculating the quant.i.ty of hydrogen evolved between two eruptions, affirms, in contradiction of opinions previously entertained, that the hydrogen bears a perfect relation in quant.i.ty to the magnitude of the streams of lava, a.s.suming the fusion of these last to have been the result of the heat evolved during the oxidation of alkaline and earthy metals, and this to have been brought about by the decomposition of water. Yet after having thus succeeded in removing the princ.i.p.al objection once so triumphantly urged against Davy's hypothesis, Bunsen concludes by declaring that the hydrogen evolved in volcanic regions cannot have been generated by the decomposition of water coming in contact with alkaline and earthy metallic bases. For, says the Professor, this process presupposes the prevalence of a temperature in which carbonic acid cannot exist in contact with hydrogen without suffering a partial reduction to carbonic oxide; "and not a trace of carbonic oxide is ever found in volcanic exhalations."[773] At the same time it will be seen, by consulting the able memoirs of the Marburg chemist, that he supposes many energetic kinds of chemical action to be continually going on in the interior of the earth, capable of causing the disengagement of hydrogen; and there can be no doubt that this gas may be a source of innumerable new changes, capable of producing the local development of internal heat.

_Cause of volcanic eruptions._--The most probable causes of a volcanic outburst at the surface have been in a great degree antic.i.p.ated in the preceding speculations on the liquefaction of rocks and the generation of gases. When a minute hole is bored in a tube filled with gas condensed into a liquid, the whole becomes instantly aeriform, or, as some writers have expressed it, "flashes into vapor," and often bursts the tube. Such an experiment may represent the mode in which gaseous matter may rush through a rent in the rocks, and continue to escape for days or weeks through a small orifice, with an explosive power sufficient to reduce every substance which opposes its pa.s.sage into small fragments or even dust. Lava may be propelled upwards at the same time, and ejected in the form of scoriae. In some places, where the fluid lava lies at the bottom of a deep fissure, communicating on the one hand with the surface, and on the other with a cavern in which a considerable body of vapor has been formed, there may be an efflux of lava, followed by the escape of gas. Eruptions often commence and close with the discharge of vapor; and, when this is the case, the next outburst may be expected to take place by the same vent, for the concluding evolution of elastic fluids will keep open the duct, and leave it un.o.bstructed.

The breaking out of lava from the side or base of a lofty cone, rather than from the summit, may be attributed to the hydrostatic pressure to which the flanks of the mountain are exposed, when the column of lava has risen to a great height. Or if, before it has reached the top, there should happen to be any stoppage in the main duct, the upward pressure of the ascending column of gas and lava may burst a lateral opening.

In the case however of Mount Loa, in the Sandwich Islands, there appears to be a singular want of connection or sympathy between the eruptions of the central and the great lateral vent. The great volcanic cone alluded to rises to the height of 13,760 feet above the level of the sea, having a crater at its summit, from which powerful streams of lava have flowed in recent times, and having another still larger crater, called Kilauea, on its southeastern slope, about 4000 feet above the sea. This lateral cavity resembles a huge quarry cut in the mountain's side, being about 1000 feet deep when in its ordinary state. It is seven miles and a half in circuit, and scattered over its bottom, at different levels, are lakes and pools of lava, always in a state of ebullition. The liquid in one of these will sometimes sink 100 or 150 feet, while it is overflowing in another at a higher elevation, there being, it should seem, no communication between them. In like manner, lava overflows in the summit crater of Mount Loa, nearly 14,000 feet high, while the great lateral cauldron just alluded to (of Kilauea) continues as tranquil as usual, affording no relief to any part of the gases or melted matter which are forcing their way upwards in the centre of the mountain.

"How," asks Mr. Dana, "if there were any subterranean channel connecting the two great vents, could this want of sympathy exist? How, according to the laws of hydrostatic pressure, can a column of fluid stand 10,000 feet higher in one leg of the siphon than in the other?" The eruptions, he observes, are not paroxysmal; on the contrary, the lava rises slowly and gradually to the summit of the lofty cone, and then escapes there without any commotion manifesting itself in Kilauea, a gulf always open on the flanks of the same mountain. One conclusion, he says, is certain, namely, _that volcanoes are no safety valves_ as they have been called; for here two independent and apparently isolated centres of volcanic activity, only sixteen miles distant from each other, are sustained in one and the same cone.[774]

Without pretending to solve this enigma, I cannot refrain from remarking, that the supposed independence of several orifices of eruption in one crater like Kilauea, when adduced in confirmation of the doctrine of two distinct sources of volcanic action underneath one mountain, proves too much. No one can doubt, that the pools of lava in Kilauea have been derived from some common reservoir, and have resulted from a combination of causes commonly called volcanic, which are at work in the interior at some unknown distance below. These causes have given rise in Mount Loa to eruptions from many points, but princ.i.p.ally from one centre, so that a vast dome of ejected matter has been piled up. The subsidiary crater has evidently never given much relief to the imprisoned, heated, and liquefied matter, for Kilauea does not form a lateral protuberance interfering with the general shape or uniform outline of Mount Loa.

_Geysers of Iceland._--As aqueous vapor const.i.tutes the most abundant of the aeriform products of volcanoes in eruption, it may be well to consider attentively a case in which steam is exclusively the moving power--that of the Geysers of Iceland. These intermittent hot springs occur in a district situated in the southwestern division of Iceland, where nearly one hundred of them are said to break out within a circle of two miles. That the water is of atmospheric origin, derived from rain and melted snow, is proved, says Professor Bunsen, by the nitrogen which rises from them either pure or mixed with other gases. The springs rise through a thick current of lava, which may perhaps have flowed from Mount Hecla, the summit of that volcano being seen from the spot at the distance of more than thirty miles. In this district the rushing of water is sometimes heard in chasms beneath the surface; for here, as on Etna, rivers flow in subterranean channels through the porous and cavernous lavas. It has more than once happened, after earthquakes, that some of the boiling fountains have increased or diminished in violence and volume, or entirely ceased, or that new ones have made their appearance--changes which may be explained by the opening of new rents and the closing of pre-existing fissures.

Few of the Geysers play longer than five or six minutes at a time, although sometimes half an hour. The intervals between their eruptions are for the most part very irregular. The Great Geyser rises out of a s.p.a.cious basin at the summit of a circular mound composed of siliceous incrustations deposited from the spray of its waters. The diameter of this basin, in one direction, is fifty-six feet, and forty-six in another. (See fig. 94.) In the centre is a pipe seventy-eight feet in perpendicular depth, and from eight to ten feet in diameter, but gradually widening, as it rises into the basin. The inside of the basin is whitish, consisting of a siliceous crust, and perfectly smooth, as are likewise two small channels on the sides of the mound, down which the water escapes when the bowl is filled to the margin. The circular basin is sometimes empty, as represented in the following sketch; but is usually filled with beautifully transparent water in a state of ebullition. During the rise of the boiling water in the pipe, especially when the ebullition is most violent, and when the water is thrown up in jets, subterranean noises are heard, like the distant firing of cannon, and the earth is slightly shaken. The sound then increases and the motion becomes more violent, till at length a column of water is thrown up, with loud explosions, to the height of one or two hundred feet.

After playing for a time like an artificial fountain, and giving off great clouds of vapor, the pipe or tube is emptied; and a column of steam, rushing up with amazing force and a thundering noise, terminates the eruption.

[Ill.u.s.tration: Fig. 94.

View of the Crater of the Great Geyser in Iceland.[775]]

If stones are thrown into the crater, they are instantly ejected; and such is the explosive force, that very hard rocks are sometimes shivered by it into small pieces. Henderson found that by throwing a great quant.i.ty of large stones into the pipe of Strockr, one of the Geysers, he could bring on an eruption in a few minutes.[776] The fragments of stone, as well as the boiling water, were thrown in that case to a much greater height than usual. After the water had been ejected, a column of steam continued to rush up with a deafening roar for nearly an hour; but the Geyser, as if exhausted by this effort, did not send out a fresh eruption when its usual interval of rest had elapsed. The account given by Sir George Mackenzie of a Geyser which he saw in eruption in 1810 (see fig. 95), agrees perfectly with the above description by Henderson.

The steam and water rose for half an hour to the height of 70 feet, and the white column remained perpendicular notwithstanding a brisk gale of wind which was blowing against it. Stones thrown into the pipe were projected to a greater height than the water. To leeward of the vapor a heavy shower of rain was seen to fall.[777]

[Ill.u.s.tration: Fig. 95.

Eruption of the New Geyser in 1810. (Mackenzie.)]

Among the different theories proposed to account for these phenomena, I shall first mention one suggested by Sir. J. Herschel. An imitation of these jets, he says, may be produced on a small scale, by heating red hot the stem of a tobacco pipe, filling the bowl with water, and so inclining the pipe as to let the water run through the stem. Its escape, instead of taking place in a continued stream, is then performed by a succession of violent explosions, at first of steam alone, then of water mixed with steam; and, as the pipe cools, almost wholly of water. At every such paroxysmal escape of the water, a portion is driven back, accompanied with steam, into the bowl. The intervals between the explosions depend on the heat, length, and inclination of the pipe; their continuance, on its thickness and conducting power.[778] The application of this experiment to the Geysers merely requires that a subterranean stream, flowing through the pores and crevices of lava, should suddenly reach a fissure in which the rock is red hot or nearly so. Steam would immediately be formed, which, rushing up the fissure, might force up water along with it to the surface, while, at the same time, part of the steam might drive back the water of the supply for a certain distance towards its source. And when, after the s.p.a.ce of some minutes, the steam was all condensed, the water would return, and a repet.i.tion of the phenomena take place.

[Ill.u.s.tration: Fig. 96.

Supposed reservoir and pipe of a Geyser in Iceland.[779]]

There is, however, another mode of explaining the action of the Geyser, perhaps more probable than that above described. Suppose water percolating from the surface of the earth to penetrate into the subterranean cavity A D (fig. 96) by the fissures F F, while, at the same time, steam at an extremely high temperature, such as is commonly given out from the rents of lava currents during congelation, emanates from the fissures C. A portion of the steam is at first condensed into water, while the temperature of the water is raised by the latent heat thus evolved, till, at last, the lower part of the cavity is filled with boiling water and the upper with steam under high pressure. The expansive force of the steam becomes, at length, so great, that the water is forced up the fissure or pipe E B, and runs over the rim of the basin. When the pressure is thus diminished, the steam in the upper part of the cavity A expands, until all the water D is driven into the pipe; and when this happens, the steam, being the lighter of the two fluids, rushes up through the water with great velocity. If the pipe be choked up artificially, even for a few minutes, a great increase of heat must take place; for it is prevented from escaping in a latent form in steam; so that the water is made to boil more violently, and this brings on an eruption.

Professor Bunsen, before cited, adopts this theory to account for the play of the "Little Geyser," but says it will not explain the phenomena of the Great one. He considers this, like the others, to be a thermal spring, having a narrow funnel-shaped tube in the upper part of its course, where the walls of the channel have become coated over with siliceous incrustations. At the mouth of this tube the water has a temperature, corresponding to the pressure of the atmosphere, of about 212 Fahr., but at a certain depth below it is much hotter. This the professor succeeded in proving by experiment; a thermometer suspended by a string in the pipe rising to 266 Fahr., or no less than 48 degrees above the boiling point. After the column of water has been expelled, what remains in the basin and pipe is found to be much cooled.

Previously to these experiments of Bunsen and Descloizeaux, made in Iceland in 1846, it would scarcely have been supposed possible that the lower part of a free and open column of water could be raised so much in temperature without causing a circulation of ascending and descending currents, followed by an almost immediate equalization of heat. Such circulation is no doubt impeded greatly by the sides of the well not being vertical, and by numerous contractions of its diameter, but the phenomenon may be chiefly due to another cause. According to recent experiments on the cohesion of liquids by Mr. Donny of Ghent, it appears that when water is freed from all admixture of air, its temperature can be raised, even under ordinary atmospheric pressure, to 275 Fahr., so much does the cohesion of its molecules increase[780] when they are not separated by particles of air. As water long boiled becomes more and more deprived of air, it is probably very free from such intermixture at the bottom of the Geysers.

Among other results of the experiments of Bunsen and his companion, they convinced themselves that the column of fluid filling the tube is constantly receiving accessions of hot water from below, while it becomes cooler above by evaporation on the broad surface of the basin.

They also came to a conclusion of no small interest, as bearing on the probable mechanism of ordinary volcanic eruptions, namely that the tube itself is the main seat or focus of mechanical force. This was proved by letting down stones suspended by strings to various depths. Those which were sunk to considerable distances from the surface were not cast up again, whereas those nearer the mouth of the tube were ejected to great heights. Other experiments also were made tending to demonstrate the singular fact, that there is often scarce any motion below, when a violent rush of steam and water is taking place above. It seems that when a lofty column of water possesses a temperature increasing with the depth, any slight ebullition or disturbance of equilibrium in the upper portion may first force up water into the basin, and then cause it to flow over the edge. A lower portion, thus suddenly relieved of part of its pressure, expands and is converted into vapor more rapidly than the first, owing to its greater heat. This allows the next subjacent stratum, which is much hotter, to rise and flash into a gaseous form; and this process goes on till the ebullition has descended from the middle to near the bottom of the funnel.[781]

In speculating, therefore, on the mechanism of an ordinary volcanic eruption, we may suppose that large subterranean cavities exist at the depth of some miles below the surface of the earth, in which melted lava acc.u.mulates; and when water containing the usual mixture of air penetrates into these, the steam thus generated may press upon the lava and force it up the duct of a volcano, in the same manner as a column of water is driven up the pipe of a Geyser. In other cases we may suppose a continuous column of liquid lava mixed with _red-hot water_ (for water may exist in that state, as Professor Bunsen reminds us, under pressure), and this column may have a temperature regularly increasing downwards. A disturbance of equilibrium may first bring on an eruption near the surface, by the expansion and conversion into gas of entangled water and other const.i.tuents of what we call lava, so as to occasion a diminution of pressure. More steam would then be liberated, carrying up with it jets of melted rock, which being hurled up into the air may fall in showers of ashes on the surrounding country, and at length, by the arrival of lava and water more and more heated at the orifice of the duct or the crater of the volcano, expansive power may be acquired sufficient to expel a ma.s.sive current of lava. After the eruption has ceased, a period of tranquillity succeeds, during which fresh accessions of heat are communicated from below, and additional ma.s.ses of rock fused by degrees, while at the same time atmospheric or sea water is descending from the surface. At length the conditions required for a new outburst are obtained, and another cycle of similar changes is renewed.

_Causes of earthquakes--wave-like motion._--I shall now proceed to examine the manner in which the heat of the interior may give rise to earthquakes. One of the most common phenomena attending subterranean movements, is the undulatory motion of the ground. And this, says Mich.e.l.l, will seem less extraordinary, if we call to mind the extreme elasticity of the earth and the compressibility of even the most solid materials. Large districts, he suggests, may rest on fluid lava; and, when this is disturbed, its motions may be propagated through the inc.u.mbent rocks. He also adds the following ingenious speculation:--"As a small quant.i.ty of vapor almost instantly generated at some considerable depth below the surface of the earth will produce a vibratory motion, so a very large quant.i.ty (whether it be generated almost instantly, or in any small portion of time) will produce a wave-like motion. The manner in which this wave-like motion will be propagated may, in some measure, be represented by the following experiment:--Suppose a large cloth, or carpet (spread upon a floor), to be raised at one edge, and then suddenly brought down again to the floor; the air under it, being by this means propelled, will pa.s.s along till it escapes at the opposite side, raising the cloth in a wave all the way as it goes. In like manner, a large quant.i.ty of vapor may be conceived to raise the earth in a wave, as it pa.s.ses along between the strata, which it may easily separate in a horizontal direction, there being little or no cohesion between one stratum and another. The part of the earth that is first raised being bent from its natural form, will endeavor to restore itself by its elasticity; and the parts next to it being to have their weight supported by the vapor, which will insinuate itself under them, will be raised in their turn, till it either finds some vent, or is again condensed by the cold into water, and by that means prevented from proceeding any farther."[782] In a memoir published in 1843, on the structure of the Appalachian chain, by the Professors Rogers,[783] the following hypothesis is proposed as "simpler and more in accordance with dynamical considerations, and the recorded observations on earthquakes."--"In place," say they, "of supposing it possible for a body of vapor or gaseous matter to pa.s.s horizontally between the strata, or even between the crust and the fluid lava upon which it floats, and with which it must be closely entangled, we are inclined to attribute the movement to an _actual pulsation_, engendered in the _molten matter itself_, by a linear disruption under enormous tension, giving vent explosively to elastic vapors, escaping either to the surface, or into cavernous s.p.a.ces beneath. According to this supposition, the movement of the subterranean vapors would be _towards_, and not from, the disrupted belt, and the oscillation of the crust would originate in the tremendous and sudden disturbance of the previous pressure on the surface of the lava ma.s.s below, brought about by the instantaneous and violent rending of the overlying strata."

This theory requires us to admit that the crust of the earth is so flexible, that it can a.s.sume the form, and follow the motion of an undulation in the fluid below. Even if we grant this, says Mr. Mallet, another more serious objection presents itself, viz. the great velocity attributed to the transit of the wave in the subterranean sea of lava.

We are called upon to admit that the speed of the wave below equals that of the true earthquake shock at the surface, which is so immense, that it is not inferior to the velocity of sound in the same solids. But the undulation in the fluid below must follow the laws of a tidal wave, or of the great sea-wave already spoken of. "Its velocity, like that of the tidal wave of our seas, will be a function of its length and of the depth of the fluid, diminished in this case by certain considerations as to the density and degree of viscidity of the liquid; and although it would be at present impossible, for want of data, to calculate the exact velocity with which this subterraneous lava-wave could move, it may be certainly affirmed that its velocity would be immeasurably short of the observed or theoretic velocity of the great earth-wave, or true shock in earthquakes."[784]

_Liquid gases._--The rending and upheaving of continental ma.s.ses are operations which are not difficult to explain, when we are once convinced that heat, of sufficient power, not only to melt but to reduce to a gaseous form a great variety of substances, is acc.u.mulated in certain parts of the interior. We see that elastic fluids are capable of projecting solid ma.s.ses to immense heights in the air; and the volcano of Cotopaxi has been known to throw out, to the distance of eight or nine miles, a ma.s.s of rock about one hundred cubic yards in volume. When we observe these aeriform fluids rushing out from particular vents for months, or even years, continuously, what power may we not expect them to exert in other places, where they happen to be confined under an enormous weight of rock?

The experiments of Faraday and others have shown, within the last twelve years, that many of the gases, including all those which are most copiously disengaged from volcanic vents, as the carbonic, sulphurous, and muriatic acids, may be condensed into liquids by pressure. At temperatures of from 30 to 50 F., the pressure required for this purpose varies from fifteen to fifty atmospheres; and this amount of pressure we may regard as very insignificant in the operations of nature. A column of Vesuvian lava that would reach from the lip of the crater to the level of the sea, must be equal to about three hundred atmospheres; so that, at depths which may be termed moderate in the interior of the crust of the earth, the gases may be condensed into liquids, even at very high temperatures. The method employed to reduce some of these gases to a liquid state is, to confine the materials, from the mutual action of which they are evolved, in tubes hermetically sealed, so that the acc.u.mulated pressure of the vapor, as it rises and expands, may force some part of it to a.s.sume the liquid state. A similar process may, and indeed must, frequently take place in subterranean caverns and fissures, or even in the pores and cells of many rocks; by which means, a much greater store of expansive power may be _packed_ into a small s.p.a.ce than could happen if these vapors had not the property of becoming liquid. For, although the gas occupies much less room in a liquid state, yet it exerts exactly the same pressure upon the sides of the containing cavity as if it remained in the form of vapor.

If a tube, whether of gla.s.s or other materials, filled with condensed gas, have its temperature slightly raised, it will often burst; for a slight increment of heat causes the elasticity of the gas to increase in a very high ratio. We have only to suppose certain rocks, permeated by these liquid gases (as porous strata are sometimes filled with water), to have their temperature raised some hundred degrees, and we obtain a power capable of lifting superinc.u.mbent ma.s.ses of almost any conceivable thickness; while, if the depth at which the gas is confined be great, there is no reason to suppose that any other appearances would be witnessed by the inhabitants of the surface than vibratory movements and rents, from which no vapor might escape. In making their way through fissures a very few miles only in length, or in forcing a pa.s.sage through soft yielding strata, the vapors may be cooled and absorbed by water. For water has a strong affinity to several of the gases, and will absorb large quant.i.ties, with a very slight increase of volume. In this manner, the heat or the volume of springs may be augmented, and their mineral properties made to vary.

_Connection between the state of the atmosphere and earthquakes._--The inhabitants of Stromboli, who are mostly fishermen, are said to make use of that volcano as a weather-gla.s.s, the eruptions being comparatively feeble when the sky is serene, but increasing in turbulence during tempestuous weather, so that in winter the island often seems to shake from its foundations. Mr. P. Scrope, after calling attention to these and other a.n.a.logous facts, first started the idea (as long ago as the year 1825) that the diminished pressure of the atmosphere, the concomitant of stormy weather, may modify the intensity of the volcanic action. He suggests that where liquid lava communicates with the surface, as in the crater of Stromboli, it may rise or fall in the vent on the same principle as mercury in a barometer; because the ebullition or expansive power of the steam contained in the lava would be checked by every increase, and augmented by every diminution of weight. In like manner, if a bed of liquid lava be confined at an immense depth below the surface, its expansive force may be counteracted partly by the weight of the inc.u.mbent rocks, and also in part by atmospheric pressure acting contemporaneously on a vast superficial area. In that case, if the upheaving force increase gradually in energy, it will at length be restrained by only the slightest degree of superiority in the antagonist or repressive power, and then the equilibrium may be suddenly destroyed by any cause, such as an ascending draught of air, which is capable of depressing the barometer. In this manner we may account for the remarkable coincidence so frequently observed between the state of the weather and subterranean commotions, although it must be admitted that earthquakes and volcanic eruptions react in their turn upon the atmosphere, so that disturbances of the latter are generally the consequences rather than the forerunners of volcanic disturbances.[785]

From an elaborate catalogue of the earthquakes experienced in Europe and Syria during the last fifteen centuries, M. Alexis Perrey has deduced the conclusion that the number which happen in the winter season preponderates over those which occur in any one of the other seasons of the year, there being, however, some exceptions to this rule, as in the Pyrenees. Curious and valuable as are these data, M. d'Archiac justly remarks, in commenting upon them, that they are not as yet sufficiently extensive or accordant in different regions, to ent.i.tle us to deduce any general conclusions from them respecting the laws of subterranean movements throughout the globe.[786]

_Permanent elevation and subsidence._--It is easy to conceive that the shattered rocks may a.s.sume an arched form during a convulsion, so that the country above may remain permanently upheaved. In other cases gas may drive before it ma.s.ses of liquid lava, which may thus be injected into newly opened fissures. The gas having then obtained more room, by the forcing up of the inc.u.mbent rocks, may remain at rest; while the lava congealing in the rents may afford a solid foundation for the newly raised district.

Experiments have recently been made in America, by Colonel Totten, to ascertain the ratio according to which some of the stones commonly used in architecture expand with given increments of heat.[787] It was found impossible, in a country where the annual variation of temperature was more than 90 F., to make a coping of stones, five feet in length, in which the joints should fit so tightly as not to admit water between the stone and the cement; the annual contraction and expansion of the stones causing, at the junctions, small crevices, the width of which varied with the nature of the rock. It was ascertained that fine-grained granite expanded with 1 F. at the rate of 000004825; while crystalline marble 000005668; and red sandstone 000009532, or about twice as much as granite.

Now, according to this law of expansion, a ma.s.s of sandstone a mile in thickness, which should have its temperature raised 200 F., would lift a superimposed layer of rock to the height of ten feet above its former level. But, suppose a part of the earth's crust, one hundred miles in thickness and equally expansive, to have its temperature raised 600 or 800, this might produce an elevation of between two and three thousand feet. The cooling of the same ma.s.s might afterwards cause the overlying rocks to sink down again and resume their original position. By such agency we might explain the gradual rise of Scandinavia or the subsidence of Greenland, if this last phenomenon should also be established as a fact on farther inquiry.

It is also possible that as the clay in Wedgwood's pyrometer contracts, by giving off its water, and then, by incipient vitrification; so, large ma.s.ses of argillaceous strata on the earth's interior may shrink, when subjected to heat and chemical changes, and allow the inc.u.mbent rocks to subside gradually.

Moreover, if we suppose that lava cooling slowly at great depths may be converted into various granitic rocks, we obtain another source of depression; for, according to the experiments of Deville and the calculations of Bischoff, the contraction of granite when pa.s.sing from a melted or plastic to a solid and crystalline state must be more than ten per cent.[788] The sudden subsidence of land may also be occasioned by subterranean caverns giving way, when gases are condensed, or when they escape through newly-formed crevices. The subtraction, moreover, of matter from certain parts of the interior, by the flowing of lava and of mineral springs, must, in the course of ages, cause vacuities below, so that the undermined surface may at length fall in.

_The balance of dry land, how preserved._--In the present state of our knowledge, we cannot pretend to estimate the average number of earthquakes which may happen in the course of a single year. As the area of the ocean is nearly three times that of the land, it is probable that about three submarine earthquakes may occur for one exclusively continental; and when we consider the great frequency of slight movements in certain districts, we can hardly suppose that a day, if, indeed, an hour, ever pa.s.ses without one or more shocks being experienced in some part of the globe. We have also seen that in Sweden, and other countries, changes in the relative level of sea and land may take place without commotion, and these perhaps produce the most important geographical and geological changes; for the position of land may be altered to a greater amount by an elevation or depression of one inch over a vast area, than by the sinking of a more limited tract, such as the forest of Aripao, to the depth of many fathoms at once.[789]

It must be evident, from the historical details above given, that the force of subterranean movement, whether intermittent or continuous, whether with or without disturbance, does not operate at random, but is developed in certain regions only; and although the alterations produced during the time required for the occurrence of a few volcanic eruptions may be inconsiderable, we can hardly doubt that, during the ages necessary for the formation of large volcanic cones, composed of thousands of lava currents, shoals might be converted into lofty mountains, and low lands into deep seas.

In a former chapter (p. 198), I have stated that aqueous and igneous agents may be regarded as antagonist forces; the aqueous laboring incessantly to reduce the inequalities of the earth's surface to a level, while the igneous are equally active in renewing the unevenness of the surface. By some geologists it has been thought that the levelling power of running water was opposed rather to the _elevating_ force of earthquakes than to their action generally. This opinion is, however, untenable; for the sinking down of the bed of the ocean is one of the means by which the gradual submersion of land is prevented. The depth of the sea cannot be increased at any one point without a universal fall of the waters, nor can any partial deposition of sediment occur without the displacement of a quant.i.ty of water of equal volume, which will raise the sea, though in an imperceptible degree, even to the antipodes. The preservation, therefore, of the dry land may sometimes be effected by the subsidence of part of the earth's crust (that part, namely, which is covered by the ocean), and in like manner an upheaving movement must often tend to destroy land; for if it render the bed of the sea more shallow, it will displace a certain quant.i.ty of water, and thus tend to submerge low tracts.

Astronomers having proved (see above, p. 129) that there has been no change in the diameter of the earth during the last two thousand years, we may a.s.sume it as probable, that the dimensions of the planet remain uniform. If, then, we inquire in what manner the force of earthquakes must be regulated, in order to restore perpetually the inequalities of the surface which the levelling power of water tends to efface, it will be found, that the amount of depression must exceed that of elevation.

It would be otherwise if the action of volcanoes and mineral springs were suspended; for then the forcing outwards of the earth's envelope ought to be no more than equal to its sinking in.

To understand this proposition more clearly, it must be borne in mind, that the deposits of rivers and currents probably add as much to the height of lands which are rising, as they take from those which have risen. Suppose a large river to bring down sediment to a part of the ocean two thousand feet deep, and that the depth of this part is gradually reduced by the acc.u.mulation of sediment till only a shoal remains, covered by water at high tides; if now an upheaving force should uplift this shoal to the height of 2000 feet, the result would be a mountain 2000 feet high. But had the movement raised the same part of the bottom of the sea before the sediment of the river had filled it up; then, instead of changing a shoal into a mountain 2000 feet high, it would only have converted a deep sea into a shoal.

It appears, then, that the operations of the earthquake are often such as to cause the levelling power of water to counteract itself; and, although the idea may appear paradoxical, we may be sure, wherever we find hills and mountains composed of stratified deposits, that such inequalities of the surface would have had no existence if water, at some former period, had not been laboring to reduce the earth's surface to one level.

But, besides the transfer of matter by running water from the continents to the ocean, there is a constant transportation from below upwards, by mineral springs and volcanic vents. As mountain ma.s.ses are, in the course of ages, created by the pouring forth of successive streams of lava, so stratified rocks, of great extent, originate from the deposition of carbonate of lime, and other mineral ingredients, with which springs are impregnated. The surface of the land, and portions of the bottom of the sea, being thus raised, the external accessions due to these operations would cause the dimensions of the planet to enlarge continually, if the amount of depression of the earth's crust were no more than equal to the elevation. In order, therefore, that the mean diameter of the earth should remain uniform, and the unevenness of the surface be preserved, it is necessary that the amount of subsidence should be in excess. And such a predominance of depression is far from improbable, on mechanical principles, since every upheaving movement must be expected either to produce caverns in the ma.s.s below, or to cause some diminution of its density. Vacuities must, also, arise from the subtraction of the matter poured out from volcanoes and mineral springs, or from the contraction of argillaceous ma.s.ses by subterranean heat; and the foundations having been thus weakened, the earth's crust, shaken and rent by reiterated convulsions, must, in the course of time, fall in.

If we embrace these views, important geological consequences will follow; since, if there be, upon the whole, more subsidence than elevation, the average depth to which former surfaces have sunk beneath their original level must exceed the height which ancient marine strata have attained above the sea. If, for example, marine strata, about the age of our chalk and greensand, have been lifted up in Europe to an extreme height of more than eleven thousand feet, and a mean elevation of some hundreds, we may conclude that certain parts of the surface, which existed when those strata were deposited, have sunk to an extreme depth of _more than_ eleven thousand feet below their original level, and to a mean depth of _more than_ a few hundreds.

In regard to faults, also, we must infer, according to the hypothesis now proposed, that a greater number have arisen from the sinking down than from the elevation of rocks.

To conclude: it seems to be rendered probable, by the views above explained, that the constant repair of the land, and the subserviency of our planet to the support of terrestrial as well as aquatic species, are secured by the elevating and depressing power of causes acting in the interior of the earth; which, although so often the source of death and terror to the inhabitants of the globe--visiting in succession every zone, and filling the earth with monuments of ruin and disorder--are nevertheless the agents of a conservative principle above all others essential to the stability of the system.

BOOK III.

CHAPTER x.x.xIII.