Outlines of the Earth's History Part 13

The furious rains which beset the mountain in times of great eruptions excavate deep channels on its sides. The lava outbreaks which attend almost every eruption, and which descend from the base of the cinder cone at the height of from five to eight thousand feet above the sea, naturally find their way into these channels, where they course in the manner of rivers until the lower and less valleyed section of the cone is reached.

Such a lava flow naturally begins to freeze on the surface, the lava at first becoming viscid, much in the manner of cream on the surface of milk. Urged along by the more fluid lava underneath, this viscid coating takes a ropy or corrugated form. As the freezing goes deeper, a firm stone roof may be formed across the gorge, which, when the current of lava ceases to flow from the crater, permits the lower part of the stream to drain away, leaving a long cavern or scries of caves extending far up the cone. The nature of this action is exactly comparable to that which we may observe when on a frosty morning after rain we may find the empty channels which were occupied by rills of water roofed over with ice; the ice roofs are temporary, while those of lava may endure for ages. Some of these lava-stream caves have been disclosed, in the manner of ordinary caverns, by the falling of their roofs; but the greater part are naturally hidden beneath the ever-increasing materials of the cone.

The lava-stream caves of aetna are not only interesting because of their peculiarities of form, which we shall not undertake to describe, but also for the reason that they help us to account for a very peculiar feature in the history of the great cone. On the slopes of the volcano, below the upper cindery portion, there are several hundred lesser cones, varying from a few score to seven hundred feet in height. Each of these has its appropriate crater, and has evidently been the seat of one or more eruptions. As the greater part of these cones are ancient, many of them being almost effaced by the rain or buried beneath the ejections which have surrounded their bases since the time they were formed, we are led to believe that many thousands of them have been formed during the history of the volcano. The history of these subsidiary cones appears to be connected with the lava caves noted above. These caverns, owing to the irregularities of their form, contain water. They are, in fact, natural cisterns, where the abundant rainfall of the mountain finds here and there storage.

When, during the throes of an eruption, dikes such as we know often to penetrate the mountain, are riven outward from the crater through the ma.s.s of the cone, and filled with lava, the heated rock must often come in contact with these ma.s.ses of buried water. The result of this would inevitably be the local generation of steam at a high temperature, which would force its way out in a brief but vigorous eruption, such as has been observed to take place when these peripheral volcanoes are formed. Sometimes it has happened that after the explosion the lava has found its way in a stream from the fissure thus opened. That this explanation is sufficient is in a measure shown by observations on certain effects of lava flows from Vesuvius. The writer was informed by a very judicious observer, a resident of Naples, who had interested himself in the phenomena of that volcano, that the lava streams when they penetrated a cistern, such as they often encounter in pa.s.sing over villages or farmsteads, vaporized the water, and gave rise, through the action of the steam, to small temporary cones, which, though generally washed away by the further flow of the liquid rock, are essentially like those which we find on aetna. Such subsidiary, or, as they are sometimes called, parasitic cones, are known about other volcanoes, but nowhere are they so characteristic as on the flanks of that wonderful volcano.

A very conspicuous feature in the aetnean cone consists of a great valley known as the Val del Bove, or Bull Hollow, which extends from the base of the modern and ever-changeable cinder cone down the flanks of the older structure to near its base. This valley has steep sides, in places a thousand or more feet high, and has evidently been formed by the down-settling of portions of the cone which were left without support by the withdrawal from beneath them of materials cast forth in a time of explosion. In an eruption this remarkable valley was the seat of a vast water flood, the fluid being cast forth from the crater at the beginning of the explosion. In the mouths of this and other volcanoes, after a long period of repose, great quant.i.ties of water, gathering from rains or condensed from the steam which slowly escapes from these openings, often pours like a flood down the sides of the mountains. In the great eruption of Galongoon, in Java, such a ma.s.s of water, cast forth by a terrific explosion, mingled with ashes, so that the ma.s.s formed a thick mud, was shot forth with such energy that it ravaged an area nearly eighty miles in diameter, destroying the forests and their wild inhabitants, as well as the people who dwelt within the range of the amazing disaster. So powerfully was this water driven from the crater that the districts immediately at the base of the cone were in a manner overshot by the vast stream, and escaped with relatively little injury.

When it comes forth from the base of the cinder cone, or from one of the small peripheral craters, the lava stream usually appears to be white hot, and to flow with almost the ease of water. It does not really have that measure of fluidity; its condition is rather that of thin paste; but the great weight of the material--near two and a half times that of water--causes the movement down the slope to be speedy.

The central portion of the lava stream long retains its high temperature; but the surface, cooling, is first converted into a tough sheet, which, though it may bend, can hardly be said to flow. Further hardening converts these outlying portions of the current into hard, gla.s.sy stone, which is broken into fragments in a way resembling the ice on the surface of a river. It thus comes about that the advancing front of the lava stream becomes covered, and its motion hindered by the frozen rock, until the rate of ongoing may not exceed a few feet an hour, and the appearance is that of a heap of stone slowly rolling down a slope. Now and then a crevice is formed, through which a thin stream of liquid lava pours forth, but the material, having already parted with much of its heat, rapidly cools, and in turn becomes covered with the coating of frozen fragments. In this state of the stream the lava flow stands on all sides high above the slope which it is traversing; it is, in fact, walled in by its own solidified parts, though it is urged forward by the contribution which continues to flow in the under arches. In this state of the movement trifling accidents, or even human interference, may direct the current this way or that.

Some of the most interesting chapters in the history of aetna relate to the efforts of the people to turn these slow-moving streams so that their torrents might flow into wilderness places rather than over the fields and towns. In the great flow of 1669, which menaced the city of Catania, a large place on the seash.o.r.e to the southeast of the cone, a public-spirited citizen, Senor Papallardo, protecting himself and his servants with clothing made of hides, and with large shields, set forth armed with great hooks with the purpose of diverting the course of the lava ma.s.s. He succeeded in pulling away the stones on the flank of the stream, so that a flow of the molten rock was turned in another direction. The expedient would probably have been successful if he had been allowed to continue his labours; but the inhabitants of a neighbouring village, which was threatened by the off-shooting current which Papallardo had created, took up arms and drove him and his retainers away. The flow continued until it reached Catania. The people made haste to build the city walls on the side of danger higher than it was before, but the tide mounted over its summit.

Although the lavas which come forth from the volcano evidently have a high temperature, their capacity for melting other rocks is relatively small. They scour these rocks, because of their weight, even more energetically than do powerful torrents of water, but they are relatively ineffective in melting stone. On aetna and elsewhere we may often observe lavas which have flowed through forests. When the tide of molten rock has pa.s.sed by, the trees may be found charred but not entirely burned away; even stems a few inches in diameter retain strength enough to uphold considerable fringes and clots of the lava which has clung to them. These facts bear out the conclusion that the fluidity of the heated stone depends in considerable measure on the water which is contained, either in its fluid or vaporous state, between the particles of the material.

If we consider the Italian volcanoes as a whole, we find that they lie in a long, discontinuous line extending from the northern part of the valley of the Po, within sight of the Alps, to aetna, and in subterranean cones perhaps to the northern coast of Africa. At the northern end of the line we have a beautiful group of extinct volcanoes, known as the Eugean Mountains. Thence southward to southern Tuscany craters are wanting, but there is evidence of fissures in the earth which give forth thermal waters. From southern Tuscany southward through Rome to Naples there are many extinct craters, none of which have been active in the historic period. From Naples southward the cones of this system, about a dozen in number, are on islands or close to the margin of the sea. It is a noteworthy fact that the greater part of these sh.o.r.e or insular vents have been active since the dawn of history; several of them frequently and furiously so, while none of those occupying an inland position have been the seat of explosions.

This is a striking instance going to show the relation of these processes to conditions which are brought about on the sea bottom.

aetna is, as we have noticed, a much more powerful volcano than Vesuvius. Its outbreaks are more vigorous, its emanations vastly greater in volume, and the ma.s.s of its constructions many times as great as those acc.u.mulated in any other European cone. There are, however, a number of volcanoes in the world which in certain features surpa.s.s aetna as much as that crater does Vesuvius. Of these we shall consider but two--Skaptar Jokul, of Iceland, remarkable for the volume of its lava flow, and Krakatoa, an island volcano between Java and Sumatra, which was the seat of the greatest explosion of which we have any record.

The whole of Iceland may be regarded as a volcanic ma.s.s composed mainly of lavas and ashes which have been thrown up by a group of volcanoes lying near the northern end of the long igneous axis which extends through the centre of the Atlantic. The island has been the seat of numerous eruptions; in fact, since its settlement by the Northmen in 1070 its st.u.r.dy inhabitants have been almost as much distressed by the calamities which have come from the internal heat as they have been by the enduring external cold. They have, indeed, been between frost and fire. The greatest recorded eruption of Iceland occurred in 1783, when the volcano of Skaptar, near the southern border of the island, poured forth, first, a vast discharge of dust and ashes, and afterward in the languid state of eruption inundated a series of valleys with the greatest lava flow of which we have any written record. The dust poured forth into the upper air, being finely divided and in enormous quant.i.ty, floated in the air for months, giving a dusky hue to the skies of Europe, which led the common people and many of the learned to fear that the wrath of G.o.d was upon them, and that the day of judgment was at hand. Even the poet Cowper, a man of high culture and education, shared in this unreasonable view.

The lava flow in this eruption filled one of the considerable valleys of the island, drying up the river, and inundating the plains on either side. Estimates which have been made as to the volume of this flow appear to indicate that it may have amounted to more than the bulk of the Mont Blanc.

This great eruption, by the direct effect of the calamity, and by the famine due to the ravaging of the fields and the frightening of the fish from the sh.o.r.es which it induced, destroyed nearly one fifth of the Icelandic people. It is, in fact, to be remembered as one of the three or four most calamitous eruptions of which we have any account, and, from the point of view of lava flow, the greatest in history.

Just a hundred years after the great Skaptar eruption, which darkened the skies of Europe, the island of Krakatoa, an isle formed by a small volcano in the straits of Java, was the seat of a vapour explosion which from its intensity is not only unparalleled, but almost unapproached in all accounts of such disturbances. Krakatoa had long been recognised as a volcanic isle; it is doubtful, however, if it had ever been seen in eruption during the three centuries or more since European ships began to sail by it until the month of May of the year above mentioned. Then an outbreak of what may be called ordinary violence took place, which after a few days so far ceased that observers landed and took account of the changes which the convulsion had brought about. For about three months there were no further signs of activity, but on the 29th of August a succession of vast explosions took place, which blew away a great part of the island, forming in its place a submarine crater two or three miles in diameter, creating world-wide disturbances of sea and air. The sounds of the outbreak were heard at a distance of sixteen hundred miles away. The waves of the air attendant on the explosion ran round the earth at least once, as was distinctly indicated by the self-recording barometers; it is possible, indeed, that, crossing each other in their east and west courses, these atmospheric tides twice girdled the sphere. In effect, the air over the crater was heaved up to the height of some tens of thousands of feet, and thence rolled off in great circular waves, such as may be observed in a pan of milk when a sharp blow pushes the bottom upward.

The violent stroke delivered to the waters of the sea created a vast wave, which in the region where it originated rolled upon the sh.o.r.es with a surf wall fifty or more feet high. In a few minutes about thirty thousand people were overwhelmed. The wave rolled on beyond its destructive limits much in the manner of the tide; its influence was felt in a sharp rise and fall of the waters as far as the Pacific coast of North America, and was indicated by the tide gauges in the Atlantic as far north as the coast of Europe.

Owing to the violence of the eruption, Krakatoa poured forth no lava, but the dust and ashes which ascended into the air--or, in other words, the finely divided lava which escaped into the atmosphere--probably amounted in bulk to more than twenty cubic miles.

The coa.r.s.er part of this material, including much pumice, fell upon the seas in the vicinity, where, owing to its lightness, it was free to drift in the marine currents far and wide throughout the oceanic realm. The finer particles, thrown high into the air, perhaps to the height of nearly a hundred thousand feet--certainly to the elevation of more than half this amount--drifted far and wide in the atmosphere, so that for years the air of all regions was clouded by it, the sunrise and sunset having a peculiar red glow, which the dust particles produce by the light which they reflect. In this period, at all times when the day was clear, the sun appeared to be surrounded by a dusky halo. In time the greater part of this dust was drawn down by gravity, some portion of it probably falling on every square foot of the earth. Since the disappearance of the characteristic phenomena which it produced in the atmosphere, European observers have noted the existence of faint clouds lying in the upper part of the air at the height of a hundred miles or more above the surface. These clouds, which were at first distinctly visible in the earliest stage of dawn and in the latest period of the sunset glow, seemed to be in rapid motion to the eastward, and to be mounting higher above the earth. It has been not unreasonably supposed that these shining clouds represent portions of the finest dust from Krakatoa, which has been thrown so far above the earth's attraction that it is separating itself from the sphere. If this view be correct, it seems likely that we may look to great volcanic explosions as a source whence the dustlike particles which people the celestial s.p.a.ces may have come. They may, in a word, be due to volcanic explosions occurring on this and other celestial spheres.

The question suggested above as to the possibility of volcanic ejections throwing matter from the earth beyond the control of its gravitative energy is one of great scientific interest. Computations (not altogether trustworthy) show that a body leaving the earth's surface under the conditions of a cannon ball fired vertically upward would have to possess a velocity at the start of at least seven miles a second in order to go free into s.p.a.ce. It would at first sight seem that we should be able to reckon whether volcanoes can propel earth matter upward with this speed. In fact, however, sufficient data are not obtainable; we only know in a general way that the column of vapour rises to the height of thirty or forty thousand feet, and this in eruptions of no great magnitude. In an accident such as that at Krakatoa, even if an observer were near enough to see clearly what was going on, the chance of his surviving the disturbance would be small.

Moreover, the ascending vapours, owing to their expansion of the steam in the column, begin to fly out sideways on its periphery, so that the upper part of the central section in the discharge is not visible from the earth.

It is in the central section of the uprushing ma.s.s, if anywhere, that the dust might attain the height necessary to put it beyond the earth's attraction, bringing it fairly into the realm of the solar system, or to the position where its own motion and the attraction of the other spheres would give it an independent orbital movement about the sun, or perhaps about the earth. We can only say that observations on the height of volcanic ejections are extremely desirable; they can probably only be made from a balloon. An ascension thus made beyond the cloud disk which the eruption produces might bring the observer where he could discern enough to determine the matter. Although the movements of the rocky particles could not be observed, the colour which they would give to the heavens might tell the story which we wish to know. There is evidence that large ma.s.ses of stone hurled up by volcanic eruption have fallen seven miles from the base of the cone. a.s.suming that the ma.s.ses went straight upward at the beginning of their ascent, and that they were afterward borne outwardly by the expansion of the column, computations which have a general but no absolute value appear to indicate that the ma.s.ses attained a height of from thirty to fifty miles, and had an initial velocity which, if doubled, might have carried them into s.p.a.ce.

Last of all, we shall note the conditions which attend the eruptions of submarine volcanoes. Such explosions have been observed in but a few instances, and only in those cases where there is reason to believe that the crater at the time of its explosion had attained to within a few hundred feet of the sea level. In these cases the ejections, never as yet observed in the state of lava, but in the condition of dust and pumice, have occasionally formed a low island, which has shortly been washed away by the waves. Knowing as we do that volcanoes abound on the sea floor, the question why we do not oftener see their explosions disturbing the surface of the waters is very interesting, but not as yet clearly explicable. It is possible, however, that a volcanic discharge taking place at the depth of several thousand feet below the surface of the water would not be able to blow the fluid aside so as to open a pipe to the surface, but would expend its energy in a hidden manner near the ocean floor. The vapours would have to expand gradually, as they do in pa.s.sing up through the rock pipe of a volcano, and in their slow upward pa.s.sage might be absorbed by the water. The solid materials thrown forth would in this case necessarily fall close about the vent, and create a very steep cone, such, indeed, as we find indicated by the soundings off certain volcanic islands which appear only recently to have overtopped the level of the waters.

As will be seen, though inadequately from the diagrams of Vesuvius, volcanic cones have a regularity and symmetry of form far exceeding that afforded by the outlines of any other of the earth's features.

Where, as is generally the case, the shape of the cone is determined by the distribution of the falling cinders or divided lava which const.i.tutes the ma.s.s of most cones, the slope is in general that known as a catenary curve--i.e., the line formed by a chain hanging between two points at some distance from the vertical. It is interesting to note that this graceful outline is a reflection or consequence of the curve described by the uprushing vapour. The expansion in the ascending column causes it to enlarge at a somewhat steadfast rate, while the speed of the ascent is ever diminishing. Precisely the same action can be seen in the like rush of steam and other gases and vapours from the cannon's mouth; only in the case of the gun, even of the greatest size, we can not trace the movement for more than a few hundred feet. In this column of ejection the outward movement from the centre carries the bits of lava outwardly from the centre of the shaft, so that when they lose their ascending velocity they are drawn downward upon the flanks of the cone, the amount falling upon each part of that surface being in a general way proportional to the thickness of the vaporous ma.s.s from which they descend. The result is, that the thickest part of the ash heap is formed on the upper part of the crater, from which point the deposit fades away in depth in every direction. In a certain measure the concentration toward the centre of the cone is brought about by the draught of air which moves in toward the ascending column.

Although, in general, ejections of volcanic matter take place through cones, that being the inevitable form produced by the escaping steam, very extensive outpourings of lava, ejections which in ma.s.s probably far exceed those thrown forth through ordinary craters, are occasionally poured out through fissures in the earth's crust. Thus in Oregon, Idaho, and Washington, in eastern Europe, in southern India, and at some other points, vast flows, which apparently took place from fissures, have inundated great realms with lava ejections. The conditions which appear to bring about these fissure eruptions of lava are not yet well understood. A provisional and very probable account of the action can be had in the hypothesis which will now be set forth.

Where any region has been for a long time the seat of volcanic action, it is probable that a large amount of rock in a more or less fluid condition exists beneath its surface. Although the outrushing steam ejects much of this molten material, there are reasons to suppose that a yet greater part lies dormant in the underground s.p.a.ces. Thus in the case of aetna we have seen that, though some thousands of miles of rock matter have come forth, the base of the cone has been uplifted, probably by the moving to that region of more or less fluid rock. If now a region thus underlaid by what we may call incipient lavas is subjected to the peculiar compressive actions which lead to mountain-building, we should naturally expect that such soft material would be poured forth, possibly in vast quant.i.ties through fault fissures, which are so readily formed in all kinds of rock when subject to irregular and powerful strains, such as are necessarily brought about when rocks are moved in mountain-making. The great eruptions which formed the volcanic table-lands on the west coast of North America appear to have owed the extrusion of their materials to mountain-building actions. This seems to have been the case also in some of those smaller areas where fissure flows occur in Europe. It is likely that this action will explain the greater part of these ma.s.sive eruptions.

It need not be supposed that the rock beneath these countries, which when forced out became lava, was necessarily in the state of perfect fluidity before it was forced through the fissures. Situated at great depth in the earth, it was under a pressure so great that its particles may have been so brought together that the material was essentially solid, though free to move under the great strains which affected it, and acquiring temperature along with the fluidity which heat induces as it was forced along by the mountain-building pressure.

As an ill.u.s.tration of how materials may become highly heated when forced to move particle on particle, it may be well to cite the case in which the iron stringpiece on top of a wooden dam near Holyoke, Ma.s.s., was affected when the barrier went away in a flood. The iron stringer, being very well put together, was, it is said, drawn out by the strain until it became sensibly reddened by the motion of its particles, and finally fell hissing into the waters below. A like heating is observable when metal is drawn out in making wire. Thus a ma.s.s of imperfectly fluid rock might in a forced journey of a few miles acquire a decided increase of temperature.

Although the most striking volcanic action--all such phenomena, indeed, as commonly receives the name--is exhibited finally on the earth's surface, a great deal of work which belongs in the same group of geological actions is altogether confined to the deep-lying rock, and leads to the formation of dikes which penetrate the strata, but do not rise to the open air. We have already noted the fact that dikes abound in the deeper parts of volcanic cones, though the fissures into which they find their way are seldom riven up to the surface. In the same way beneath the ground in non-volcanic countries we may discover at a great depth in the older, much-changed rock a vast number of these crevices, varying from a few inches to a hundred feet or more in width, which have been filled with lavas, the rock once molten having afterward cooled. In most cases these dikes are disclosed to us through the down-wearing of the earth that has removed the beds into which the dikes did not penetrate, thus disclosing the realm in which the disturbances took place.

Where, as is occasionally the case in deep mines, or on some bare rocky cliff of great height, we can trace a dike in its upward course through a long distance, we find that we can never distinctly discover the lower point of its extension. No one has ever seen in a clear way the point of origin of such an injection. We can, however, often follow it upward to the place where there was no longer a rift into which it could enter. In its upward path the molten matter appears generally to have followed some previously existing fracture, a joint plane or a fault, which generally runs through the rocks on those planes. We can observe evidence that the material was in the state of igneous fluidity by the fact that it has baked the country rocks on either side of the fissure, the amount of baking being in proportion to the width of the dike, and thus to the amount of heat which it could give forth. A dike six inches in diameter will sometimes barely sear its walls, while one a hundred feet in width will often alter the strata for a great distance on either side. In some instances, as in the coal beds near Richmond, Va., dikes occasionally cut through beds of bituminous coal. In these cases we find that the coal has been converted into c.o.ke for many feet either side of a considerable injection. The fact that the dike material was molten is still further shown by the occurrence in it of fragments which it has taken up from the walls, and which may have been partly melted, and in most cases have clearly been much heated.

Where dikes extend up through stratified beds which are separated from each other by distinct layers, along which the rock is not firmly bound together, it now and then happens, as noted by Mr. G.K. Gilbert, of the United States Geological Survey, that the lava has forced its way horizontally between these layers, gradually uplifting the overlying ma.s.s, which it did not break through, into a dome-shaped elevation. These side flows from dikes are termed laccolites, a word which signifies the pool-like nature of the stony ma.s.s which they form between the strata.

In many regions, where the earth has worn down so as to reveal the zone of dikes which was formed at a great depth, the surface of the country is fairly laced with these intrusions. Thus on Cape Ann, a rocky isle on the east coast of Ma.s.sachusetts, having an area of about twenty square miles, the writer, with the a.s.sistance of his colleague, Prof. R.S. Tarr, found about four hundred distinct dikes exhibited on the sh.o.r.e line where the rocks had been swept bare by the waves. If the census of these intrusions could have been extended over the whole island, it would probably have appeared that the total number exceeded five thousand. In other regions square miles can be found where the dikes intercepted by the surface occupy an aggregate area greater than that of the rocks into which they have been intruded.

Now and then, but rarely, the student of dikes finds one where the bordering walls, in place of having the clean-cut appearance which they usually exhibit, has its sides greatly worn away and much melted, as if by the long-continued pa.s.sage of the igneous fluid through the crevice. Such dikes are usually very wide, and are probably the paths through which lavas found their way to the surface of the earth, pouring forth in a volcanic eruption. In some cases we can trace their relation to ancient volcanic cones which have worn down in all their part which were made up of incoherent materials, so that there remains only the central pipe, which has been preserved from decay by the coherent character of the lava which filled it.

The hypothesis that dikes are driven upward into strata by the pressure of the beds which overlie materials hot and soft enough to be put in motion when a fissure enters them, and that their movement upward through the crevice is accounted for by this pressure, makes certain features of these intrusions comprehensible. Seeing that very long, slender dikes are found penetrating the rock, which could not have had a high temperature, it becomes difficult to understand how the lava could have maintained its fluidity; but on the supposition that it was impelled forward by a strong pressure, and that the energy thus transmitted through it was converted into heat, we discover a means whereby it could have been retained in the liquid condition, even when forced for long distances through very narrow channels.

Moreover, this explanation accounts for the fact which has long remained unexplained that dikes, except those formed about volcanic craters, rarely, if ever, rise to the surface.

The materials contained in dikes differ exceedingly in their chemical and mineral character. These variations are due to the differences in Nature of the deposits whence they come, and also in a measure to exchanges which take place between their own substance and that of the rocks between which they are deposited. This process often has importance of an economic kind, for it not infrequently leads to the formation of metalliferous veins or other aggregations of ores, either in the dike itself or in the country rock. The way in which this is brought about may be easily understood by a familiar example. If flesh be placed in water which has the same temperature, no exchange of materials will take place; but if the water be heated, a circulation will be set up, which in time will bring a large part of the soluble matter into the surrounding water. This movement is primarily dependent on differences of temperature, and consequently differences in the quant.i.ty of soluble substances which the water seeks to take up. When a dike is injected into cooler rocks, such a slow circulation is induced. The water contained in the interstices of the stone becomes charged with mineral materials, if such exist in positions where it can obtain possession of them, and as cooling goes on, these dissolved materials are deposited in the manner of veins. These veins are generally laid down on the planes of contact between the two kinds of stone, but they may be formed in any other cavities which exist in the neighbourhood. The formation of such veins is often aided by the considerable shrinkage of the lava in the dike, which, when it cools, tends to lose about fifteen per cent of its volume, and is thus likely to leave a crevice next the boundary walls. Ores thus formed afford some of the commonest and often the richest mineral deposits. At Leadville, in Colorado, the great silver-bearing lodes probably were produced in this manner, wherein lavas, either those of dikes or those which flowed in the open air, have come in contact with limestones.

The mineral materials originally in the once molten rock or in the limy beds was, we believe, laid down on ancient sea floors in the remains of organic forms, which for their particular uses took the materials from the old sea water. The vein-making action has served to a.s.semble these scattered bits of metal into the aggregation which const.i.tutes a workable deposit. In time, as the rocks wear down, the materials of the veins are again taken into solution and returned to the sea, thence perhaps to tread again the cycle of change.

In certain dikes, and sometimes also, perhaps, in lavas known as basalts, which have flowed on the surface, the rock when cooling, from the shrinkage which then occurs, has broken in a very regular way, forming hexagonal columns which are more or less divided on their length by joints. When worn away by the agencies of decay, especially where the material forms steep cliffs, a highly artificial effect is produced, which is often compared, where cut at right angles to the columns, to pavements, or, where the division is parallel to the columns, to the pipes of an organ.

What we know of dikes inclines us to the opinion that as a whole they represent movements of softened rock where the motion-compelling agent is not mainly the expansion of the contained water which gives rise to volcanic ejection, but rather in large part due to the weight of superinc.u.mbent strata setting in motion materials which were somewhat softened, and which tended to creep, as do the clays in deep coal mines. It is evident, however; it is, moreover, quite natural, that dike work is somewhat mingled with that produced by the volcanic forces; but while the line between the two actions is not sharp, the discrimination is important, and occurs with a distinctness rather unusual on the boundary line between two adjacent fields of phenomena.

We have now to consider the general effects of the earth's interior heat so far as that body of temperature tends to drive materials from the depths of the earth to the surface. This group of influences is one of the most important which operates on our sphere; as we shall shortly see, without such action the earth would in time become an unfit theatre for the development of organic life. To perceive the effect of these movements, we must first note that in the great rock-constructing realm of the seas organic life is constantly extracting from the water substances, such as lime, potash, soda, and a host of other substances necessary for the maintenance of high-grade organisms, depositing these materials in the growing strata. Into these beds, which are buried as fast as they form, goes not only these earthy materials, but a great store of the sea water as well. The result would be in course of time a complete withdrawal into the depths of the earth of those substances which play a necessary part in organic development. The earth would become more or less completely waterless on its surface, and the rocks exposed to view would be composed mainly of silica, the material which to a great extent resists solution, and therefore avoids the dissolving which overtakes most other kinds of rocks. Here comes in the machinery of the hot springs, the dikes, and the volcanoes. These agents, operating under the influence of the internal heat of the earth, are constantly engaged in bearing the earthy matter, particularly its precious more solvent parts, back to the surface. The hot springs and volcanoes work swiftly and directly, and return the water, the carbon dioxide, and a host of other vaporizable and soluble and fusible substances to the realm of solar activity, to the living surface zone of the earth. The dikes operate less immediately, but in the end to the same effect.

They lift their materials miles above the level where they were originally laid, probably from a zone which is rarely if ever exposed to view, placing them near the surface, where the erosive agents can readily find access to them.

Of the three agents which serve to export earth materials from its depths, volcanoes are doubtless the most important. They send forth the greater part of the water which is expelled from the rocks.

Various computations which the writer has made indicate that an ordinary volcano, such as aetna, in times of most intense explosion, may send forth in the form of steam one fourth of a cubic mile or more of water during each day of its discharge, and in a single great eruption may pour forth several times this quant.i.ty. In its history aetna has probably returned to the atmosphere some hundred cubic miles of water which but for the process would have remained permanently locked up in its rock prison.

The ejection of rock material, though probably on the average less in quant.i.ty than the water which escapes, is also of noteworthy importance. The volcanoes of Java and the adjacent isles have, during the last hundred and twenty years, delivered to the seas more earth material than has been carried into those basins by the great rivers.

If we could take account of all the volcanic ejections which have occurred in this time, we should doubtless find that the sum of the materials thus cast forth into the oceans was several times as great as that which was delivered from the lands by all the superficial agents which wear them away. Moreover, while the material from the land, except the small part which is in a state of complete solution, all falls close to the sh.o.r.e, the volcanic waste, because of its fine division or because of the blebs of air which its ma.s.ses contain, may float for many years before it finds its way to the bottom, it may be at the antipodes of the point at which it came from the earth. While thus journeying through the sea the rock matter from the volcanoes is apt to become dissolved in water; it is, indeed, doubtful if any considerable part of that which enters the ocean goes by gravitation to its floor. The greater portion probably enters the state of solution and makes its way thence through the bodies of plants and animals again into the ponderable state.

If an observer could view the earth from the surface of the moon, he would probably each day behold one of these storms which the volcanoes send forth. In the fortnight of darkness, even with the naked eye, it would probably be possible to discern at any time several eruptions, some of which would indicate that the earth's surface was ravaged by great catastrophes. The nearer view of these actions shows us that although locally and in small measure they are harmful to the life of the earth, they are in a large way beneficent.

CHAPTER VIII.

THE SOIL.

The frequent mention which it has been necessary to make of soil phenomena in the preceding chapters shows how intimately this feature in the structure of the earth is blended with all the elements of its physical history. It is now necessary for us to take up the phenomena of soils in a consecutive manner.

The study of any considerable river basin enables us to trace the more important steps which lead to the destructure and renovation of the earth's detrital coating. In such an interpretation we note that everywhere the rocks which were built on the sea bottom, and more or less made over in the great laboratory of the earth's interior, are at the surface, when exposed to the conditions of the atmosphere, in process of being taken to pieces and returned to the sea. This action goes on everywhere; every drop of rain helps it. It is aided by frost, or even by the changes of expansion and contraction which occur in the rocks from variations of heat. The result is that, except where the slopes are steep, the surface is quickly covered with a layer of fragments, all of which are in the process of decay, and ready to afford some food to plants. Even where the rock appears bare, it is generally covered with lichens, which, adhering to it, obtain a share of nutriment from the decayed material which they help to hold on the slope. When they have retained a thin sheet of the _debris_, mosses and small flowering plants help the work of retaining the detritus.

Soon the strong-rooted bushes and trees win a foothold, and by sending their rootlets, which are at first small but rapidly enlarge, into the crevices, they hasten the disruption of the stones.

If the construction of soil goes on upon a steep cliff, the quant.i.ty retained on the slope may be small, but at the base we find a talus, composed of the fragments not held by the vegetation, which gradually increases as the cliff wears down, until the original precipice may be quite obliterated beneath a soil slope. At first this process is rapid; it becomes gradually slower and slower as the talus mounts up the cliff and as the cliff loses its steepness, until finally a gentle slope takes the place of the steep.

From the highest points in any river valley to the sea level the broken-up rock, which we term soil, is in process of continuous motion. Everywhere the rain water, flowing over the surface or soaking through the porous ma.s.s, is conveying portions of the material which is taken into solution in a speedy manner to the sea. Everywhere the expansion of the soil in freezing, or the movements imposed on it by the growth of roots, by the overturning of trees, or by the innumerable borings and burrowings which animals make in the ma.s.s, is through the action of gravitation slowly working down the slope. Every little disturbance of the grains or fragments of the soil which lifts them up causes them when they fall to descend a little way farther toward the sea level. Working toward the streams, the materials of the soil are in time delivered to those flowing waters, and by them urged speedily, though in most cases interruptedly, toward the ocean.