Fig. 15.—Flow of lava invading a forest. A tree in the distance is not completely burned, showing that the molten rock had lost much of its original heat.Fig. 15.—Flow of lava invading a forest. A tree in the distance is not completely burned, showing that the molten rock had lost much of its original heat.
The effect of deep burial is to increase the heat of strata. This result is accomplished in two different ways. The direct effect arising from the imposition of weight, that derived from the mass of stratified material, is, as we know, to bring about a down-sinking of the earth's crust. In the measure of this falling, heat is engendered precisely as it is by the falling of a trip-hammer on the anvil, with which action, as is well known, we may heat an iron bar to a high temperature. It is true that this down-sinking of the surface under weight is in part due to the compression of the rocks, and in part to the slipping away of the soft underpinning of more or less fluid rock. Yet further it is in some measure brought about by the wrinkling of the crust. But all these actions result in the conversion of energy of position into heat, and so far serve to raise the temperature of the rocks which are concerned in the movements. By far the largest source of heat, however, is that which comes forth from the earth's interior, and which was stored there in the olden day when the matter forming the earth gathered into the mass of our sphere. This, which we may term the original heat, is constantly flowing forth into space, but makes its way slowly, because of the non-conductive, or, as we may phrase it, the "blanketing" effect of the outer rock. The effect of the strata is the same as that exercised by the non-conductive coatings which are put on steam boilers. A more familiar comparison may be had from the blankets used for bedclothing. If on top of the first blanket we put a second, we keep warmer because the temperature of the lower one is elevated by the heat from our body which is held in. In the crust of the earth each layer of rock resists the outflow of heat, and each addition lifts the temperature of all the layers below.
When water-bearing strata have been buried to the depth of ten miles, the temperature of the mass may be expected to rise to somewhere between seven hundred and a thousand degrees Fahrenheit. If the depth attained should be fifty miles, it is likely that the temperature will be five times as great. At such a heat the water which the rocks contain tends in a very vigorous way to expand and pass into the state of vapour. This it can not readily do, because of its close imprisonment; we may say, however, that the tendency toward explosion is almost as great asthat of ignited gunpowder. Such powder, if held in small spaces in a mass of cast steel, could be fired without rending the metal. The gases would be retained in a highly compressed, possibly in a fluid form. If now it happens that any of the strain in the rocks such as lead to the production of faults produce fissures leading from the surface into this zone of heated water, the tendency of the rocks containing the fluid, impelled by its expansion, will be to move with great energy toward the point of relief or lessened pressure which the crevice affords. Where rocks are in any way softened, pressure alone will force them into a cavity, as is shown by the fact that beds of tolerably hard clay stones in deep coal mines may be forced into the spaces by the pressure of the rocks which overlie them—in fact, the expense of cutting out these in-creeping rocks is in some British mines a serious item in the cost of the product.
The expansion of the water contained in the deep-lying heated rocks probably is by far the most efficient agent in urging them toward the plane of escape which the fissure affords. When the motion begins it pervades all parts of the rock at once, so that an actual flow is induced. So far as the movement is due to the superincumbent weight, the tendency is at once to increase the temperature of the moving mass. The result is that it may be urged into the fissure perhaps even hotter than when it started from the original bed place. In proportion as the rocky matter wins its way toward the surface, the pressure upon it diminishes, and the contained vapours are freer to expand. Taking on the vaporous form, the bubbles gather to each other, and when they appear at the throat of the volcano they may, if the explosions be infrequent, assume the character above noted in the little eruption of Vesuvius. Where, however, the lava ascends rapidly through the channel, it often attains the open air with so much vapour in it, and this intimately mingled with the mass, that the explosion rends the materials into an impalpably fine powder, which may float in the air for months beforeit falls to the earth. With a less violent movement the vapour bubbles expand in the lava, but do not rend it apart, thus forming the porous, spongy rock known as pumice. With a yet slower ascent a large part of the steam may go away, so that we may have a flow of lava welling forth from the vent, still giving forth steam, but with a vapour whose tension is so lowered that the matter is not blown apart, though it may boil violently for a time after it escapes into the air.
Although the foregoing relatively simple explanation of volcanic action can not be said as yet to be generally accepted by geologists, the reasons are sufficient which lead us to believe that it accounts for the main features which we observe in this class of explosions—in other words, it is a good working hypothesis. We shall now proceed in the manner which should be followed in all natural inquiry to see if the facts shown in the distribution of volcanoes in space and time confirm or deny the view.
The most noteworthy feature in the distribution of volcanoes is that, at the present time at least, all active vents are limited to the sea floors or to the shore lands within the narrow range of three hundred miles from the coast. Wherever we find a coast line destitute of volcanoes, as is the case with the eastern coast of North and South America, it appears that the shore has recently been carried into the land for a considerable distance—in other words, old coast lines are normally volcanic; that is, here and there have vents of this nature. Thus the North Atlantic, the coasts of which appear to have gone inland for a great distance in geologically recent times, is non-volcanic; while the Pacific coast, which for a long time has remained in its present position, has a singularly continuous line of craters near the shore extending from Alaska to Tierra del Fuego. So uninterrupted is this line of volcanoes that if they were all in eruption it would very likely be possible to journey down the coast without ever being out of sight of the columns of vapour which they would sendforth. On the floor of the sea volcanic peaks appear to be very widely distributed; only a few of them—those which attain the surface of the water—are really known, but soundings show long lines of elevations which doubtless represent cones distributed along fault lines, none of the peaks of sufficient height to break the surface of the sea. It is likely, indeed, that for one marine volcano which appears as an island there are scores which do not attain the surface. Volcanic islands exist and generally abound in the ocean and greater seas; every now and then we observe a new one forming as a small island, which is apt to be washed away by the sea shortly after the eruption ceases, the disappearance being speedy, for the reason that the volcanic ashes of which these cones are composed drift away like snow before the movement of the waves.
If the waters of the ocean and seas were drained away so that we could inspect the portion of the earth's surface which they cover as readily as we do the dry lands, the most conspicuous feature would be the innumerable volcanic eminences which lie hidden in these watery realms. Wherever the observer passed from the centres of the present lands he would note within the limits of those fields only mountains, much modified by river action; hills which the rivers had left in scarfing away the strata; and dales which had been carved out by the flowing waters. Near the shore lines of the vanished seas he would begin to find mountains, hills, and vales occasionally commingled with volcanic peaks, those structures built from the materials ejected from the vents. Passing the coast line to the seaward, the hills and dales would quickly disappear, and before long the mountains would vanish from his way, and he would gradually enter on a region of vast rolling plains beset by volcanic peaks, generally accumulated in long ranges, somewhat after the manner of mountains, but differing from those elevations not only in origin but in aspect, the volcanic set of peaks being altogether made up of conical, cup-topped elevations.
A little consideration will show us that the fact of volcanoes being in the limit to the sea floors and to a narrow fringe of shore next certain ocean borders is reconcilable with the view as to their formation which we have adopted. We have already noted the fact that the continents are old, which implies that the parts of the earth which they occupy have long been the seats of tolerably continuous erosion. Now and then they have swung down partly beneath the sea, and during their submersion they received a share of sediments. But, on the whole, all parts of the lands except strips next the coast may be reckoned as having been subjected to an excess of wearing action far exceeding the depositional work. Therefore, as we readily see, underneath such land areas there has been no blanketing process going on which has served to increase the heat in the deep underlying rocks. On the contrary, it would be easy to show, and the reader may see it himself, that the progressive cooling of the earth has probably brought about a lowering of the temperature in all the section from the surface to very great depths, so that not only is the rock water unaffected by increase of heat, but may be actually losing temperature. In other words, the conditions which we assume bring about volcanic action do not exist beneath the old land.
Beneath the seas, except in their very greatest depths, and perhaps even there, the process of forming strata is continually going on. Next the shores, sometimes for a hundred or two miles away to seaward, the principal contribution may be the sediment worn from the lands by the waves and the rivers. Farther away it is to a large extent made up of the remains of animals and plants, which when dying give their skeletons to form the strata. Much of the materials laid down—perhaps in all more than half—consist of volcanic dust, ashes, and pumice, which drifts very long times before it finds its way to the bottom. We have as yet no data of a precise kind for determining the average rate of accumulation of sediments upon the seafloor, but from what is known of the wearing of the lands, and the amount of volcanic waste which finds its way to the seas, it is probably not less than about a foot in ten thousand years; it is most likely, indeed, much to exceed this amount. From data afforded by the eruptions in Java and in other fields where the quantity of volcanic dust contributed to the seas can be estimated, the writer is disposed to believe that the average rate of sedimentation on the sea floors is twice as great as the estimate above given.
Accumulating at the average rate of one foot in ten thousand years, it would require a million years to produce a hundred feet of sediments; a hundred million to form ten thousand feet, and five hundred million to create the thickness of about ten miles of bed. At the rate of two feet in ten thousand years, the thickness accumulated would be about twenty miles. When we come to consider the duration of the earth's geologic history, we shall find reasons for believing that the formation of sediment may have continued for as much as five hundred million years.
The foregoing inquiries concerning the origin of volcanoes show that at the present time they are clearly connected with some process which goes on beneath the sea. An extension of the inquiry indicates that this relation has existed in earlier geological times; for, although the living volcanoes are limited to places within three hundred miles of the sea, we find lava flows, ashes, and other volcanic accumulations far in the interior of the continents, though the energy which brought them forth to the earth's surface has ceased to operate in those parts of the land. In these cases of continental volcanoes it generally, if not always, appears that the cessation of the activity attended the removal of the shore line of the ocean or the disappearance of great inland seas. Thus the volcanoes of the Yellowstone district may have owed their activity to the immense deposits of sediment which were formed in the vast fresh-water lakes which during the later Cretaceousand early Tertiary times stretched along the eastern face of the Rocky Mountains, forming a Mediterranean Sea in North America comparable to that which borders southern Europe. It thus appears that the arrangement of volcanoes with reference to sea basins has held for a considerable period in the past. Still further, when we look backward through the successive formations of the earth's crust we find here and there evidences in old lava flows, in volcanic ashes, and sometimes in the ruins of ancient cones which have been buried in the strata, that igneous activity such as is now displayed in our volcanoes has been, since the earliest days of which we have any record, a characteristic feature of the earth. There is no reason to suppose that this action has in the past been any greater or any less than in modern days. All these facts point to the conclusion that volcanic action is due to the escape of rock water which has been heated to high temperatures, and which drives along with it as it journeys toward a crevice the rock in which it has been confined.
We will now notice some other explanations of volcanic action which have obtained a certain credence. First, we may note the view that these ejections from craters are forced out from a supposed liquid interior of the earth. One of the difficulties of this view is that we do not know that the earth's central parts are fluid—in fact, many considerations indicate that such is not the case. Next, we observe that we not infrequently find two craters, each containing fluid lava, with the fluid standing at differences of height of several thousand feet, although the cones are situated very near each other. If these lavas came from a common internal reservoir, the principles which control the action of fluids would cause the lavas to be at the same elevation. Moreover, this view does not provide any explanation of the fact that volcanoes are in some way connected with actions which go on on the floors of great water basins. There is every reason to believe that the fractures in the rocks under the land are as numerous anddeep-going as those beneath the sea. If it were a mere question of access to a fluid interior, volcanoes should be equally distributed on land and sea floors. Last of all, this explanation in no wise accounts for the intermixture of water with the fluid rock. We can not well believe that water could have formed a part of the deeper earth in the old days of original igneous fusion. In that time the water must have been all above the earth in the vaporous state.
Another supposition somewhat akin to that mentioned is that the water of the seas finds its way down through crevices beneath the floors of the ocean, and, there coming in contact with an internal molten mass, is converted into steam, which, along with the fluid rock, escapes from the volcanic vent. In addition to the objections urged to the preceding view, we may say concerning this that the lava, if it came forth under these circumstances, would emerge by the short way, that by which the water went down, and not by the longer road, by which it may be discharged ten thousand feet or more above the level of the sea.
The foregoing general account of volcanic action should properly be followed by some account of what takes place in characteristic eruptions. This history of these matters is so ample that it would require the space of a great encyclopædia to contain them. We shall therefore be able to make only certain selections which may serve to illustrate the more important facts.
By far the best-known volcanic cone is that of Vesuvius, which has been subjected to tolerably complete record for about twenty-four hundred years. About 500b.c.the Greeks, who were ever on the search for places where they might advantageously plant colonies, settled on the island of Ischia, which forms the western of what is now termed the Bay of Naples. This island was well placed for tillage as well as for commerce, but the enterprising colonists were again and again disturbed by violent outbreaks of one or more volcanoes which lie in theinterior of this island; at one time it appears that the people were driven away by these explosions.
In these pre-Christian days Vesuvius, then known as Monte Somma, was not known to be a volcano, it never having shown any trace of eruption. It appeared as a regularly shaped mountain, somewhat over two thousand feet high, with a central depression about three miles in diameter at the top, and perhaps two miles over at the bottom, which was plainlike in form, with some lakes of bitter water in the centre. The most we know of this central cavity is connected with the insurrection of the slaves led by Spartacus, the army of the revolters having camped for a time on the plain encircled by the crater walls. The outer slopes of the mountain afforded then a remarkably fertile soil; some traces, indeed, of the fertility have withstood the modern eruptions which have desolated its flanks. This wonderful Bay of Naples became the seat of the fairest Roman culture, as well as of a very extended commerce. Toward the close of the first century of our era the region was perhaps richer, more beautifully cultivated, and the seat of a more elaborate luxury than any part of the shore line of Europe at the present day. At the foot of the mountain, on the eastern border of the bay, the city of Pompeii, with a population of about fifty thousand souls, was a considerable port, with an extensive commerce, particularly with Egypt. The charming town was also a place of great resort for rich Egyptians who cared to dwell in Europe. On the flanks of the mountain there was at least one large town, Herculaneum, which appears to have been an association of rich men's residences. On the eastern side of the bay, at a point now known as Baiæ, the Roman Government had a naval station, which in the year 79 was under the command of the celebrated Pliny, a most voluminous though unscientific writer on matters of natural history. With him in that year there was his nephew, commonly known as the younger Pliny, then a student of eighteen years, but afterward himself an author. Thesefacts are stated in some detail, for they are all involved in the great tragedy which we are now to describe.
For many years there had been no eruption about the Bay of Naples. The volcanoes on Ischia had been still for a century or more, and the various circular openings on the mainland had been so far quiet that they were not recognised as volcanoes. Even the inquisitive Pliny, with his great learning, was so little of a geologist that he did not know the signs which indicate the seat of volcanic action, though they are among the most conspicuous features which can meet the eye. The Greeks would doubtless have recognised the meaning of these physical signs. In the year 63 the shores of the Bay of Naples were subjected to a distinctive earthquake. Others less severe followed in subsequent years. In an early morning in the year 79, a servant aroused the elder Pliny at Baiæ with the news that there was a wonderful cloud rising from Monte Somma. The younger Pliny states that in form it was like a pine tree, the common species in Italy having a long trunk with a crown of foliage on its summit, shaped like an umbrella. This crown of the column grew until it spread over the whole landscape, darkening the field of view. Shortly after, a despatch boat brought a message to the admiral, who at once set forth for the seat of the disturbance. He invited his nephew to accompany him, but the prudent young man relates in his letters to Tacitus, from whom we know the little concerning the eruption which has come down to us, that he preferred to do some reading which he had to attend to. His uncle, however, went straight forward, intending to land at some point on the shore at the foot of the cone. He found the sea, however, so high that a landing was impossible; moreover, the fall of rock fragments menaced the ship. He therefore cruised along the shore for some distance, landing at a station probably near the present village of Castellamare. At this point the fall of ashes and pumice wasvery great, but the sturdy old Roman had his dinner and slept after it. There is testimony that he snored loudly, and was aroused only when his servants began to fear that the fall of ashes and stones would block the way out of his bedchamber. When he came forth with his attendants, their heads protected by planks resting on pillows, he set out toward Pompeii, which was probably the place where he sought to land. After going some distance, the brave man fell dead, probably from heart disease; it is said that he was at the time exceedingly asthmatic. No sooner were his servants satisfied that the life had passed from his body than they fled. The remains were recovered after the eruption had ceased. The younger Pliny further relates that after his uncle left, the cloud from the mountain became so dense that in midday the darkness was that of midnight, and the earthquake shocks were so violent that wagons brought to the courtyard of the dwelling to bear the members of the household away were rolled this way and that by the quakings of the earth.
Save for the above-mentioned few and unimportant details concerning the eruption, we have no other contemporaneous account. We have, indeed, no more extended story until Dion Cassius, writing long after the event, tells us that Herculaneum and Pompeii were overwhelmed; but he mixes his story with fantastic legends concerning the appearance of gods and demons, as is his fashion in his so-called history. Of all the Roman writers, he is perhaps the most untrustworthy. Fortunately, however, we have in the deposits of ashes which were thrown out at the time of this great eruption some basis for interpreting the events which took place. It is evident that for many hours the Vesuvian crater, which had been dormant for at least five hundred years, blew out with exceeding fury. It poured forth no lava streams; the energy of the uprushing vapours was too great for that. The molten rock in their path was blown into fine bits, and all the hard material cast forth as free dust. In the course of the eruption, which probably did not endure more than two days, possibly not more than twenty-four hours, ash enough was poured forth to form a thick layer which spread far over the neighbouring area of land and sea floor. It covered the cities of Herculaneum and Pompeii to a depth of more than twenty feet, and over a circle having a diameter of twenty miles the average thickness may have been something like this amount. So deep was it that, although almost all the people of these towns survived, it did not seem to them worth while to undertake to excavate their dwelling places. At Pompeii the covering did not overtop the higher of the low houses. An amount of labour which may be estimated at not over one thirtieth of the value, or at least the cost which had been incurred in building the city, would have restored it to a perfectly inhabitable state. The fact that it was utterly abandoned probably indicates a certain superstitious view in connection with the eruption.
The fact that the people had time to flee from Herculaneum and Pompeii, bearing with them their more valuable effects, is proved by the excavations at these places which have been made in modern times. The larger part of Pompeii and a considerable portion of Herculaneum have been thus explored; only rarely have human remains been found. Here and there, particularly in the cellars, the labourers engaged in the work of disinterring the cities note that their picks enter a cavity; examining the space, they find they have discovered the remains of a human skeleton. It has recently been learned that by pouring soft plaster of Paris into these openings a mould may be obtained which gives in a surprisingly perfect manner the original form of the body. The explanation of this mould is as follows: Along with the fall of cinders in an eruption there is always a great descent of rain, arising from the condensation of the steam which pours forth from the volcano. This water, mingling with the ashes, forms a pasty mud, which often flows in vast streams,and is sometimes known as mud lava. This material has the qualities of cement—that is, it shortly "sets" in a manner comparable to plaster of Paris or ordinary mortar. During the eruption of 79 this mud penetrated all the low places in Pompeii, covering the bodies of the people, who were suffocated by the fumes of the volcanic emanations. We know that these people were not drowned by the inundation; their attitudes show that they were dead before the flowing matter penetrated to where they lay.
It happened that Pompeii lay beyond the influence of the subsequent great eruptions of Vesuvius, so that it afterward received only slight ash showers. Herculaneum, on the other hand, has century by century been more and more deeply buried until at the present time it is covered by many sheets of lava. This is particularly to be regretted, for the reason that, while Pompeii was a seaport town of no great wealth or culture, Herculaneum was the residence place of the gentry, people who possessed libraries, the records of which can be in many cases deciphered, and from which we might hope to obtain some of the lost treasures of antiquity. The papyrus rolls on which the books of that day were written, though charred by heat and time, are still interpretable.
After the great explosion of 79, Vesuvius sank again into repose. It was not until 1056 that vigorous eruptions again began. From time to time slight explosions occurred, none of which yielded lava flows; it was not until the date last mentioned that this accompaniment of the eruption began to appear. In 1636, after a repose of nearly a century and a half, there came a very great outbreak, which desolated a wide extent of country on the northwestern side of the cone. At this stage in the history of the crater the volcanic flow began to attain the sea. Washing over the edge of the old original crater of Monte Somma, and thus lowering its elevation, these streams devastated, during the eruption just mentioned and in various other outbreaks, a wide field of cultivatedland, overwhelming many villages. The last considerable eruption which yielded large quantities of lava was that of 1872, which sent its tide for a distance of about six miles.
Since 1636 the eruptions of Vesuvius have steadily increased in frequency, and, on the whole, diminished in violence. In the early years of its history the great outbreaks were usually separated by intervals of a century or more, and were of such energy that the lava was mostly blown to dust, forming clouds so vast that on two occasions at least they caused a midnight darkness at Constantinople, nearly twelve hundred miles away. This is as if a volcano at Chicago should completely hide the sun in the city of Boston. In the present state of Vesuvius, the cone may be said to be in slight, almost continuous eruption. The old central valley which existed before the eruption of 79, and continued to be distinct for long after that time, has been filled up by a smaller cone, bearing a relatively tiny crater of vent, the original wall being visible only on the eastern and northern parts of its circuit, and here only with much diminished height. On the western face the slope from the base of the mountain to the summit of the new cone is almost continuous, though the trained eye can trace the outline of Monte Somma—its position in a kind of bench, which is traceable on that side of the long slope leading from the summit of the new cone to the sea. The fact that the lavas of Vesuvius have broken out on the southwestern side, while the old wall of the cone has remained unbroken on the eastern versant, has a curious explanation. The prevailing wind of Naples is from the southwest, being the strong counter trades which belong in that latitude. In the old days when the Monte Somma cone was constructed these winds caused the larger part of the ashes to fall on the leeward side of the cone, thus forming a thicker and higher wall around that part of the crater.
Fig. 16.—Diagrammatic sections through Mount Vesuvius, showing changes in the form of the cone. (From Phillips.)Fig. 16.—Diagrammatic sections through Mount Vesuvius, showing changes in the form of the cone. (From Phillips.)
From the nature of the recent eruptions of Vesuvius itappears likely that the mountain is about to enter on a second period of inaction. The pipes leading through the new cone are small, and the mass of this elevation constitutes a great plug, closing the old crater mouth. To give vent to a large discharge of steam, the whole of this great mass, having a depth of nearly two thousand feet, would have to be blown away. It seems most likely that when the occasion for such a discharge comes, the vapours of the eruption will seek a vent through some other of the many volcanic openings which lie to the westward of this great cone. The history of these lesser volcanoes points to the conclusion that when the path by way of Vesuvius is obstructed they may give relief to the steam which is forcing its course to the surface. Two or three times since the eruption of Pliny, during periods when Vesuvius hadlong been quiet, outbreaks have taken place on Ischia or in the Phlægræn Fields, a region dotted with small craters which lies to the west of Naples. The last of these occurred in 1552, and led to the formation of the beautiful little cone known as Monte Nuovo. This eruption took place near the town of Puzzuoli, a place which was then the seat of a university, the people of which have left us records of the accident.
The outbreak which formed Monte Nuovo was slight but very characteristic. It occurred in and beside a circular pool known as the Lucrine Lake, itself an ancient crater. At the beginning of the disturbance the ground opened in ragged cavities, from which mud and ashes and great fragments of hard rock were hurled high in the air, some of the stones ascending to a height of several thousand feet. With slight intermissions this outbreak continued for some days, resulting in the formation of a hill about five hundred feet high, with a crater in its top, the bottom of which lay near the level of the sea. Although this volcanic elevation, being made altogether of loose fragments, is rapidly wearing down, while the crater is filling up, it remains a beautiful object in the landscape, and is also noteworthy for the fact that it is the only structure of this nature which we know from its beginning. In the Phlægræn Field there are a number of other craters of small size, with very low cones about them. These appear to have been the product of brief, slight eruptions. That known as the Solfatara, though not in eruption during the historic period, is interesting for the fact that from the crevices of the rocks about it there comes forth a continued efflux of carbonic-acid gas. This substance probably arises from the effect of heat contained in old lavas which are in contact with limestone in the deep under-earth. We know such limestones are covered by the lavas of Vesuvius, for the reason that numerous blocks of the rock are thrown out during eruptions, and are often found embedded in the lava streams. It is an interesting factthat these craters of the Phlægræn Field, lying between the seats of vigorous eruption on Ischia and at Vesuvius, have never been in vigorous eruption. Their slight outbreaks seem to indicate that they have no permanent connection with the sources whence those stronger vents obtain their supply of heated steam.
The facts disclosed by the study of the Vesuvian system of volcanoes afford the geologist a basis for many interesting conclusions.
In the first place, he notes that the greater part of the cones, all those of small size, are made up of finely divided rock, which may have been more or less cemented by the processes of change which go on within it. It is thus clear that the lava flows are unessential—indeed, we may say accidental—contributions to the mass. In the case of Vesuvius they certainly do not amount to as much as one tenth of the elevation due to the volcanic action. The share of the lava in Vesuvius is probably greater than the average, for during the last six centuries this vent has been remarkably lavigerous.[8]Observation on the volcanoes of other districts show that the Vesuvian group is in this regard not peculiar. Of nearly two hundred cones which the writer has examined, not more than one tenth disclose distinct lavas.
An inspection of the old inner wall of Monte Somma in that portion where it is best preserved, on the north side of the Atria del Cavallo, or Horse Gulch—so called for the reason that those who ascended Vesuvius were accustomed to leave their saddle animals there—we perceive that the body of the old cone is to a considerable extent interlaced with dikes or fissures which have been filled with molten lava that has cooled in its place. It is evident that during the throes of an eruption, when the lavastands high in the crater, these rents are frequently formed, to be filled by the fluid rock. In fact, lava discharges, though they may afterward course for long distances in the open air, generally break their way underground through the cindery cone, and first are disclosed at the distance of a mile or more from the inner walls of the crater. Their path is probably formed by riftings in the compacted ashes, such as we trace on the steep sides of the Atria del Cavallo, as before noted. For the further history of these fissures, we shall have to refer to facts which are better exhibited in the cone of Ætna.
The amount of rock matter which has been thrown forth from the volcanoes about the Bay of Naples is very great. Only a portion of it remains in the region around these cones; by far the greater part has been washed or blown away. After each considerable eruption a wide field is coated with ashes, so that the tilled grounds appear as if entirely sterilized; but in a short time the matter in good part disappears, a portion of it decays and is leached away, and the most of the remainder washes into the sea. Only the showers, which accumulate a deep layer, are apt to be retained on the surface of the country. A great deal of this powdered rock drifts away in the wind, sometimes in great quantities, as in those cases where it darkened the sky more than a thousand miles from the cone. Moreover, the water of the steam which brought about the discharges and the other gases which accompanied the vapour have left no traces of their presence, except in the deep channels which the rain of the condensing steam have formed on the hillsides. Nevertheless, after all these subtractions are made, the quantity of volcanic matter remaining on the surface about the Bay of Naples would, if evenly distributed, form a layer several hundred feet in thickness—perhaps, indeed, a thousand feet in depth—over the territory in which the vents occur. All this matter has been taken in relatively recent times from the depths of the earth. The surprising fact is that no considerable and,indeed, no permanent subsidence of the surface has attended this excavation. We can not believe that this withdrawal of material from the under-earth has resulted in the formation of open underground spaces. We know full well that any such, if it were of considerable size, would quickly be crushed in by the weight of the overlying rocks. We have, indeed, to suppose that these steam-impelled lavas, which are driven toward the vent whence they are to go forth in the state of dust or fluid, come underground from distances away, probably from beneath the floors of the sea to the westward.
Although the shores of the Bay of Naples have remained in general with unchanged elevation for about two thousand years, they have here and there been subjected to slight oscillations which are most likely connected with the movement of volcanic matter toward the vents where it is to find escape. The most interesting evidence of this nature is afforded by the studies which have been made on the ruins of the Temple of Serapis at Puzzuoli. This edifice was constructed in pre-Christian times for the worship of the Egyptian god Serapis, whose intervention was sought by sick people. The fact that this divinity of the Nile found a residence in this region shows how intimate was the relation between Rome and Egypt in this ancient day. The Serapeium was built on the edge of the sea, just above its level. When in modern days it began to be studied, its floor was about on its original level, but the few standing columns of the edifice afford indubitable evidence that this part of the shore has been lowered to the amount of twenty feet or more and then re-elevated. The subsidence is proved by the fact that the upper part of the columns which were not protected by thedébrisaccumulated about them have been bored by certain shellfish, known asLithodomi, which have the habit of excavating shelters in soft stone, such as these marble columns afford. At present the floor on which the ruin stands appears to be gradually sinking, though the rate of movement is very slow.
Another evidence that the ejections may travel for a great distance underground on their way to the vent is afforded by the fact that Vesuvius and Ætna, though near three hundred miles apart, appear to exchange activities—that is, their periods of outbreak are not simultaneous. Although these elements of the chronology of the two cones may be accidental, taken with similar facts derived from other fields, they appear to indicate that vents, though far separated from each other, may, so to speak, be fed from a common subterranean source. It is a singular fact in this connection that the volcano of Stromboli, though situated between these two cones, is in a state of almost incessant activity. This probably indicates that the last-named vent derives its vapours from another level in the earth than the greater cones. In this regard volcanoes probably behave like springs, of which, indeed, they may be regarded as a group. The reader is doubtless aware that hot and cold springs often escape very near together, the difference in the temperature being due to the depth from which their waters come forth.
As the accidents of volcanic explosion are of a nature to be very damaging to man, as well as to the lower orders of Nature, it is fit that we should note in general the effect of the Neapolitan eruptions on the history of civilization in that region. As stated above, the first Greek settlements in this vicinity—those on the island of Ischia—were much disturbed by volcanic outbreaks, yet the island became the seat of a permanent and prosperous colony. The great eruption of 79 probably cost many hundred lives, and led to the abandonment of two considerable cities, which, however, could at small cost have been recovered to use. Since that day various eruptions have temporarily desolated portions of the territory, but only in very small fields have the ravages been irremediable. Where the ground was covered with dust, it has in most places been again tillable, and so rapid is the decay of the lavas that in a century after their flow has ceased vines can in most cases beplanted on their surfaces. The city of Naples, which lies amid the vents, though not immediately in contact with any of them, has steadfastly grown and prospered from the pre-Christian times. It is doubtful if any lives have ever been lost in the city in consequence of an eruption, and no great inconvenience has been experienced from them. Now and then, after a great ash shower, the volcanic dust has to be removed, but the labour is less serious than that imposed on many northern cities by a snowstorm. Through all these convulsions the tillage of the district has been maintained. It has ever been the seat of as rich and profitable a husbandry as is afforded by any part of Italy. In fact, the ash showers, as they import fine divided rock very rich in substances necessary for the growth of plants, have in a measure served to maintain the fertility of the soil, and by this action have in some degree compensated for the injury which they occasionally inflict. Comparing the ravages of the eruptions with those inflicted by war, unnecessary disease, or even bad politics, and we see that these natural accidents have been most merciful to man. Many a tyrant has caused more suffering and death than has been inflicted by these rude operations of Nature.
From the point of view of the naturalist, Ætna is vastly more interesting than Vesuvius. The bulk of the cone is more than twenty times as great as that of the Neapolitan volcano, and the magnitude of its explosions, as well as the range of phenomena which they exhibit, incomparably greater. It happens, however, that while human history of the recorded kind has been intimately bound up with the tiny Vesuvian cone, partly because the relatively slight nature of its disturbances permitted men to dwell beside it, the larger Ætna has expelled culture from the field near its vent, and has done the greater part of its work in the vast solitude which it has created.[9]
Ætna has been in frequent eruption for a very much longer time than Vesuvius. In the odes of Pindar, in the sixth century before Christ, we find records of eruptions. It is said also that the philosopher Empedocles sought fame and death by casting himself into the fiery crater. There has thus in the case of this mountain been no such long period of repose as occurred in Vesuvius. Though our records of the outbreaks are exceedingly imperfect, they serve to show that the vent has maintained its activity much more continuously than is ordinarily the case with volcanoes. Ætna is characteristically a lava-yielding cone; though the amount of dust put forth is large, the ratio of the fluid rock which flows away from the crater is very much greater than at Vesuvius. Nearly half the cone, indeed, may be composed of this material. Our space does not permit anything like a consecutive story of the Ætnean eruptions since the dawn of history, or even a full account of its majestic cone; we can only note certain features of a particularly instructive nature which have been remarked by the many able men who have studied this structure and the effects of its outbreak.
The most important feature exhibited by Ætna is the vast size of its cone. At its apex its height, though variable from the frequent destruction and rebuilding of the crater walls, may be reckoned as about eleven thousand feet. The base on which the volcanic material lies is probably less than a thousand feet above the sea, so that the maximum thickness of the heap of volcanic ejections is probably about two miles. The average depth of this coating is probably about five thousand feet, and, as the cone has an average diameter of about thirty miles, we may conclude that the cone now contains about a thousandcubic miles of volcanic materials. Great as is this mass, it is only a small part of the ejected material which has gone forth from the vent. All the matter which in its vaporous state went forth with the eruption, the other gases and vapours thus discharged, have disappeared. So, too, a large part of the ash and much of the lava has been swept away by the streams which drain the region, and which in times of eruption are greatly swollen by the accompanying torrential rains. The writer has estimated that if all the emanations from the volcano—solid, fluid, and gaseous—could be heaped on the cone, they would form a mass of between two and three thousand cubic miles in contents. Yet notwithstanding this enormous outputting of earthy matter, the earth on which the Ætnean cone has been constructed has not only failed to sink down, but has been in process of continuous, slow uprising, which has lifted the surface more than a thousand feet above the level which it had at the time when volcanic action began in this field. Here, even more clearly than in the case of Vesuvius, we see that the materials driven forth from the crater are derived not from just beneath its foundation, but from a distance, from realms which in the case of this insular volcano are beneath the sea floors. It is certain that here the migration of rock matter, impelled by the expansion of its contained water toward the vent, has so far exceeded that which has been discharged through the crater that an uprising of the surface such as we have observed has been brought about.
Mount Ætna, seen from near Catania. The imperfect cones on the sky line to the left are those of small secondary eruptions.Mount Ætna, seen from near Catania. The imperfect cones on the sky line to the left are those of small secondary eruptions.
There are certain peculiarities of Mount Ætna which are due in part to its great size and in part to the climatal conditions of the region in which it lies. The upper part of the mountain in winter is deeply snow-clad; the frozen water often, indeed, forms great drifts in the gorges near the summit. Here it has occasionally happened that a layer of ashes has deeply buried the mass, so that it has been preserved for years, becoming gradually more inclosed by the subsequent eruptions. At one point wherethis compact snow—which has, indeed, taken on the form of ice—has been revealed to view, it has been quarried and conveyed to the towns upon the seacoast. It is likely that there are many such masses of ice inclosed between the ash layers in the upper part of the mountain, where, owing to the height, the climate is very cold. This curious fact shows how perfect a non-conductor the ash beds of a volcano are to protect the frozen water from the heat of the rocks about the crater.
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 Ætna are not only interestingbecause 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 mass of the cone, and filled with lava, the heated rock must often come in contact with these masses 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 passing over villages or farmsteads, vaporized the water, and gave rise, throughthe 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 Ætna. 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 Ætnean 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 quantities 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 mass of water, cast forth by a terrific explosion, mingled with ashes, so that the mass 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 withalmost 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, glassy 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 Ætna 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 seashore to the southeast of the cone, a public-spirited citizen, Señor 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 lavamass. 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 Ætna and elsewhere we may often observe lavas which have flowed through forests. When the tide of molten rock has passed 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 Ætna, 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 throughRome 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 shore 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.
Ætna 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 mass of its constructions many times as great as those accumulated in any other European cone. There are, however, a number of volcanoes in the world which in certain features surpass Ætna 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 mass 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 sturdy 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 thelanguid 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 quantity, 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 God 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 shores 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 whichthe 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 shores 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 coarser 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—driftedfar 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 spaces 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 space. 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 vapourrises 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 mass, 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 masses of stone hurled up by volcanic eruption have fallen seven miles from the base of the cone. Assuming that the masses 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 masses attained a height of from thirty to fifty miles, and had an initial velocity which, if doubled, might have carried them into space.
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 craterat 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 passing up through the rock pipe of a volcano, and in their slow upward passage 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 constitutes the mass 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 steamand 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 mass 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 mass 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 spaces. Thus in the case of Ætna we have seen that, though some thousandsof 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 quantities 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 massive 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 illustration 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, Mass., 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 mass ofimperfectly 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.