Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

Volcanoes.

Of old it was believed that volcanoes represented the outpouring of fluid rock which came forth from the central realm of the earth, a region which was supposed still to retain the liquid state through which the whole mass of our earth has doubtless passed. Recent studies, however, have brought about a change in the views of geologists which is represented by the fact that we shall treat volcanic phenomena in connection with the history of rock water.

In endeavouring to understand the phenomena of volcanoes it is very desirable that the student should understand what goes on in a normal eruption. The writer may, therefore, be warranted in describing some observations which he had an opportunity to make at an eruption of Vesuvius in 1883, when it was possible to behold far more than can ordinarily be discerned in such outbreaks—in fact, the opportunity of a like nature has probably not been enjoyed by any other person interested in volcanic action. In the winter of 1882-'83 Vesuvius was subjected to a succession of slight outbreaks. At the time of the observations about to be noted the crater had been reduced to a cup about three hundred feet in diameter and about a hundred feet deep. The vertical shaft at the bottom, through which the outbursts were taking place, was about a hundred feet across. Taking advantage of a heavy gale from the northwest, it was practicable, notwithstanding the explosions, to climb to the edge of the crater wall. Looking down into the throat of the volcano, although the pit was full of whirling vapours and the heat was so great that the protection of a mask was necessary, it was possible to see something of what was going on at the moment of an explosion.

The pipe of the volcano was full of white-hot lava. Even in a day of sunshine, which was only partly obscured by the vapours which hung about the opening, the heat of the lava made it very brilliant. This mass of fluid rock was in continuous motion, swaying violently up and down the tube. From four to six times a minute, at the moment of its upswaying, it would burst as by the explosion of a gigantic bubble. The upper portion of the mass was blown upward in fragments, the discharge being like that of shot from a fowling piece; the fragments, varying in size from small, shotlike bits to masses larger than a man's head, were shot up sometimes to the height of fifteen hundred feet above the point of ejection. The wind, blowing at the rate of about forty miles an hour, drove the falling bits of rock to the leeward, so that there was no considerable danger to be apprehended from them. Some seconds after the explosion they could be heard rattling down on the farther slope of the cone. Observations on the interval between the discharge and the fall of the fragments made it easy to compute the height to which they were thrown.

At the moment when the lava in the pipe opened for the passage of the vapour which created the explosion the movement, though performed in a fraction of a second, was clearly visible. At first the vapour was colourless; a few score feet up it began to assume a faint, bluish hue; yet higher, when it was more expanded, the tint changed to that of steam, which soon became of the ordinary aspect, and gathered in swift-revolving clouds. The watery nature of the vapour was perfectly evident by its odour. Though commingled with sulphurous-acid gas, it still had the characteristic smell of steam. For a half hour it was possible to watch the successive explosions, and even to make rough sketches of the scene. Occasionally the explosions would come in quick succession, so that the lava was blown out of the tube; again, the pool would merely sway up and down in a manner which could be explained only by supposing that great bubbles of vapour were working their way upward toward the point where they could burst. Each of these bubbles probably filled a large part of the diameter of the pipe. In general, the phenomena recalled the escape of the jet from a geyser, or, to take a familiar instance, that of steam from the pipe of a high-pressure engine. When the heat is great, steam may often be seen at the mouth of the pipe with the same transparent appearance which was observed in the throat of the crater. In the cold air of the mountain the vapour was rapidly condensed, giving a rainbow hue in the clouds when they were viewed at the right angle. The observations were interrupted by the fact that the wind so far died away that large balls of the ejected lava began to fall on the windward side of the cone. These fragments, though cooled and blackened on their outside by their considerable journey up and down through the air, were still so soft that they splashed when they struck the surface of cinders.

Watching the cone from a distance, one could note that from time to time the explosions, increasing in frequency, finally attained a point where the action appeared to be continuous. The transition was comparable to that which we may observe in a locomotive which, when it first gets under way, gives forth occasional jets of steam, but, slowly gaining speed, finally pours forth what to eye and ear alike seem to be a continuous outrush. All the evidence that we have concerning volcanic outbreaks corroborates that just cited, and is to the effect that the essence of the action consists in the outbreak of water vapour at a high temperature, and therefore endowed with very great expansive force. Along with this steam there are many other gases, which always appear to be but a very small part of the whole escape of a vaporous nature—in fact, the volcanic steam, so far as its chemical composition has been ascertained, has the composition which we should expect to find in rock water which had been forced out from the rock by the tensions that high temperature creates.

Because of its conspicuous nature, the lava which flows from most volcanoes, or is blown out from them in the form of finely divided ash, is commonly regarded as the primary feature in a volcanic outbreak. Such is not really the case. Volcanic explosions may occur with very little output of fluid rock, and that which comes forth may consist altogether of the finely divided bits of rock to which we give the name of ash. In fact, in all very powerful explosions we may expect to find no lava flow, but great quantities of this finely divided rock, which when it started from the depths of the earth was in a fluid state, but was blown to pieces by the contained vapour as it approached the surface.

If the student is so fortunate as to behold a flood of lava coming forth from the flanks of a volcano, he will observe that even at the very points of issue, where the material is white-hot and appears to be as fluid as water, the whole surface gives forth steam. On a still day, viewed from a distance, the path of a lava flow is marked by a dense cloud of this vapour which comes forth from it. Even after the lava has cooled so that it is safe to walk upon it, every crevice continues to pour forth steam. Years after the flowing has ceased, and when the rock surface has become cool enough for the growth of certain plants upon it, these crevices still yield steam. It is evident, in a word, that a considerable part of a lava mass, even after it escapes from the volcanic pipes, is water which is intimately commingled with the rock, probably lying between the very finest grains of the heated substance. Yet this lava which has come forth from the volcano has only a portion of the water which it originally contained; a large, perhaps the greater part, has gone forth in the explosive way through the crater. It is reasonably believed that the fluidity of lava is in considerable measure due to the water which it contains, and which serves to give the mass the consistence of paste, the partial fluidity of flour and rock grains being alike brought about in the same manner.

So much of the phenomena of volcanoes as has been above noted is intended to show the large part which interstitial water plays in volcanic action. We shall now turn our attention again to the state of the deeply buried rock water, to see how far we may be able by it to account for these strange explosive actions. When sediments are laid down on the sea floor the materials consist of small, irregularly shaped fragments, which lie tumbled together in the manner of a mass of bricks which have been shot out of a cart. Water is buried in the plentiful interspaces between these bits of stone; as before remarked, the amount of this construction water varies. In general, it is at first not far from one tenth part of the materials. Besides the fluid contained in the distinct spaces, there is a share which is held as combined water in the intimate structure of the crystals, if such there be in the mass. When this water is built into the stone it has the ordinary temperature of the sea bottom. As the depositing actions continue to work, other beds are formed on the top of that which we are considering, and in time the layer may be buried to the depth of many thousand feet. There are reasons to believe that on the floors of the oceans this burial of beds containing water may have brought great quantities of fluid to the depth of twenty miles or more below the outer surface of the rocks.

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 as that 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 before it 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 send forth. 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 sea floor, 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 Cretaceous and 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 and deep-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 500 b.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 the interior 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. These facts 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 was very 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 cultivated land, 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.)

From the nature of the recent eruptions of Vesuvius it appears 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 had long 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 fact that 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 lava stands 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 the débris accumulated about them have been bored by certain shellfish, known as Lithodomi, 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 be planted 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 thousand cubic 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.

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 where this 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 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 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, 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 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 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, 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 lava mass. 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 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 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 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 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 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 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—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 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 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 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 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 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 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 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 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 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 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 coke 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 mass, 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 mass 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 Massachusetts, having an area of about twenty square miles, the writer, with the assistance of his colleague, Prof. R.S. Tarr, found about four hundred distinct dikes exhibited on the shore 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 passage 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 quantity 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 assemble these scattered bits of metal into the aggregation which constitutes 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 superincumbent 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.

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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 Ætna, 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 quantity. In its history Ætna 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 quantity 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 shore, the volcanic waste, because of its fine division or because of the blebs of air which its masses 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.

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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 débris, 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 quantity 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 mass, 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 mass, 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.

There is another element in the movement of the soils which, though less appreciable, is still of great importance. The agents of decay which produce and remove the detritus, the chemical changes of the bed rock, and the mechanical action which roots apply to them, along with the solutional processes, are constantly lowering the surface of the mass. In this way we can often prove that a soil continuously existing has worked downward through many thousand feet of strata. In this process of downgoing the country on which the layer rests may have greatly changed its form, but the deposit, under favourable conditions, may continue to retain some trace of the materials which it derived from beds which have long since disappeared, their position having been far up in the spaces now occupied by the air. Where the slopes are steep and streams abound, we rarely find detritus which belonged in rock more than a hundred feet above the present surface of the soil. Where, however, as on those isolated table-lands or buttes which abound in certain portions of the Mississippi Valley, as well as in many other countries, we find a patch of soil lying on a nearly level surface, which for geologic ages has not felt the effect of streams, we may discover, commingled in the débris, the harder wreckage derived from the decay of a thousand feet or more of vanished strata.

When we consider the effect of organic life on the processes which go on in the soil, we first note the large fact that the development of all land vegetation depends upon the existence of this detritus—in a word, on the slow movement of the decaying rocky matter from the point where it is disrupted to its field of rest in the depths of the sea. The plants take their food from the portion of this rocky waste which is brought into solution by the waters which penetrate the mass. On the plants the animals feed, and so this vast assemblage of organisms is maintained. Not only does the land life maintain itself on the soil, and give much to the sea, but it serves in various ways to protect this detrital coating from too rapid destruction, and to improve its quality. To see the nature of this work we should visit a region where primeval forests still lie upon the slopes of a hilly region. In the body of such a wood we find next the surface a coating of decayed vegetable matter, made up of the falling leaves, bark, branches, and trunks which are constantly descending to the earth. Ordinarily, this layer is a foot or more in thickness; at the top it is almost altogether composed of vegetable matter; at the bottom it verges into the true soil. An important effect of this decayed vegetation is to restrain the movement of the surface water. Even in the heaviest rains, provided the mass be not frozen, the water is taken into it and delivered in the manner of springs to the larger streams. We can better note the measure of this effect by observing the difference in the ground covered by this primeval forest and that which we find near by which has been converted into tilled fields. With the same degree of rapidity in the flow, the distinct stream channels on the tilled ground are likely to be from twenty to a hundred times in length what they are on the forest bed. The result is that while the brook which drains the forested area maintains a tolerably constant flow of clean water, the other from the tilled ground courses only in times of heavy rain, and then is heavily charged with mud. In the virgin conditions of the soil the downwear is very slow; in its artificial state this wearing goes on so rapidly that the sloping fields are likely to be worn to below the soil level in a few score years.

Not only does the natural coating of vegetation, such as our forests impose upon the country, protect the soil from washing away, but the roots of the larger plants are continually at work in various ways to increase the fertility and depth of the stratum. In the form of slender fibrils these underground branches enter the joints and bed planes of the rock, and there growing they disrupt the materials, giving them a larger surface on which decay may operate. These bits, at first of considerable size, are in turn broken up by the same action. Where the underlying rocks afford nutritious materials, the branches of our tap-rooted trees sometimes find their way ten feet or more below the base of the true soil. Not only do they thus break up the stones, but the nutrition which they obtain in the depths is brought up and deposited in the parts above the ground, as well as in the roots which lie in the true soil, so that when the tree dies it becomes available for other plants. Thus in the forest condition of a country the amount of rock material contributed to the deposit in general so far exceeds that which is taken away to the rivers by the underground water as to insure the deepening of the soil bed to the point where only the strongest roots—those belonging to our tap-rooted trees—can penetrate through it to the bed rocks.

Almost all forests are from time to time visited by winds which uproot the trees. When they are thus rent from the earth, the underground branches often form a disk containing a thick tangle of stones and earth, and having a diameter of ten or fifteen feet. The writer has frequently observed a hundred cubic feet of soil matter, some of it taken from the depth of a yard or more, thus uplifted into the air. In the path of a hurricane or tornado we may sometimes find thousands of acres which have been subjected to this rude overturning—a natural ploughing. As the roots rot away, the débris which they held falls outside of the pit, thus forming a little hillock along the side of the cavity. After a time the thrusting action of other roots and the slow motion of the soil down the slope restore the surface from its hillocky character to its original smoothness; but in many cases the naturalist who has learned to discern with his feet may note these irregularities long after it has been recovered with the forest.

Great as is the effect of plants on the soil, that influence is almost equalled by the action of the animals which have the habit of entering the earth, finding there a temporary abiding place. The number of these ground forms is surprisingly great. It includes, indeed, a host of creatures which are efficient agents in enriching the earth. The species of earthworms, some of which occupy forested districts as well as the fields, have the habit of passing the soil material through their bodies, extracting from the mass such nutriment as it may contain. In this manner the particles of mineral matter become pulverized, and in a measure affected by chemical changes in the bodies of the creatures, and are thus better fitted to afford plant food. Sometimes the amount of the earth which the creatures take in in moving through their burrows and void upon the surface is sufficient to form annually a layer on the surface of the ground having a depth of one twentieth of an inch or more. It thus may well happen that the soil to the depth of two or three feet is completely overturned in the course of a few hundred years. As the particles which the creatures devour are rather small, the tendency is to accumulate the finer portions of the soil near the surface of the earth, where by solution they may contribute to the needs of the lowly plants. It is probably due to the action of these creatures that small relics of ancient men, such as stone tools, are commonly found buried at a considerable depth beneath the earth, and rarely appear upon the surface except where it has been subjected to deep ploughing or to the action of running streams.

Along with the earthworms, the ants labour to overturn the soil; frequently they are the more effective of the two agents. The common species, though they make no permanent hillocks, have been observed by the writer to lay upon the surface each year as much as a quarter of an inch of sand and other fine materials which they have brought up from a considerable depth. In many regions, particularly in those occupied by glacial drift, and pebbly alluvium along the rivers, the effect of this action, like that of earthworms, is to bring to the surface the finer materials, leaving the coarser pebbles in the depths. In this way they have changed the superficial character of the soil over great areas; we may say, indeed, over a large part of the earth, and this in a way which fits it better to serve the needs of the wild plants as well as the uses of the farmer.

Many thousand species of insects, particularly the larger beetles, have the habit of passing their larval state in the under earth. Here they generally excavate burrows, and thus in a way delve the soil. As many of them die before reaching maturity, their store of organic matter is contributed to the mass, and serves to nourish the plants. If the student will carefully examine a section of the earth either in its natural or in its tilled state, he will be surprised to find how numerous the grubs are. They may often be found to the number of a score or more of each cubic foot of material. Many of the species which develop underground come from eggs which have carefully been encased in organic matter before their deposition in the earth. Thus some of the carrion beetles are in the habit of laying their eggs in the bodies of dead birds or field mice, which they then bury to the depth of some inches in the earth. In this way nearly all the small birds and mammals of our woods disappear from view in a few hours after they are dead. Other species make balls from the dung of cattle in which they lay their eggs, afterward rolling the little spheres, it may be for hundreds of feet, to the chambers in the soil which they have previously prepared. In this way a great deal of animal matter is introduced into the earth, and contributes to its fertility.

Many of our small mammals have the habit of making their dwelling places in the soil. Some of them, such as the moles, normally abide in the subterranean realm for all their lives. Others use the excavations as places of retreat. In any case, these excavations serve to move the particles of the soil about, and the materials which the animals drag into the earth, as well as the excrement of the creatures, act to enrich it. This habit of taking food underground is not limited to the mammals; it is common with the ants, and even the earthworms, as noted by Charles Darwin in his wonderful essay on these creatures, are accustomed to drag into their burrows bits of grass and the slender leaves of pines. It is not known what purpose they attain by these actions, but it is sufficiently common somewhat to affect the conditions of the soil.

The result of these complicated works done by animals and plants on the soil is that the material to a considerable depth are constantly being supplied with organic matter, which, along with the mineral material, constitutes that part of the earth which can support vegetation. Experiment will readily show that neither crushed rock nor pure vegetable mould will of itself serve to maintain any but the lowliest vegetation. It requires that the two materials be mixed in order that the earth may yield food for ordinary plants, particularly for those which are of use to man, as crops. On this account all the processes above noted whereby the waste of plant and animal life is carried below the surface are of the utmost importance in the creation and preservation of the soil. It has been found, indeed, in almost all cases, necessary for the farmer to maintain the fertility of his fields to plough-in quantities of such organic waste. By so doing he imitates the work which is effected in virgin soil by natural action. As the process is costly in time and material, it is often neglected or imperfectly done, with the result that the fields rapidly diminish in fertility.

The way in which the buried organic matter acts upon the soil is not yet thoroughly understood. In part it accomplishes the results by the materials which on its decay it contributes to the soil in a state in which they may readily be dissolved and taken up by the roots into their sap; in part, however, it is believed that they better the conditions by affording dwelling places for a host of lowly species, such as the forms which are known as bacteria. The organisms probably aid in the decomposition of the mineral matter, and in the conversion of nitrogen, which abounds in the air or the soil, into nitrates of potash and soda—substances which have a very great value as fertilizers. Some effect is produced by the decay of the foreign matter brought into the soil, which as it passes away leaves channels through which the soil water can more readily pass.

By far the most general and important effect arising from the decay of organic matter in the earth is to be found in the carbon dioxide which is formed as the oxygen of the air combines with the carbon which all organic material contains. As before noted, water thus charged has its capacity for taking other substances into solution vastly increased, and on this solvent action depends in large part the decay of the bed rocks and the solution of materials which are to be appropriated by the plants.

Having now sketched the general conditions which lead to the formation of soils, we must take account of certain important variations in their conditions due to differences in the ways in which they are formed and preserved. These matters are not only of interest to the geologist, but are of the utmost importance to the life of mankind, as well as all the lower creatures which dwell upon the lands. First, we should note that soils are divisible into three great groups, which, though not sharply parted from each other, are sufficiently peculiar for the purposes of classification. Where the earth material has been derived from the rocks which nearly or immediately underlie it, we have a group of soils which may be entitled those of immediate derivation—that is, derived from rocks near by, or from beds which once overlaid the level and have since been decayed away. Next, we have alluvial soils, those composed of materials which have been transported by streams, commonly from a great distance, and laid down on their flood plains. Third, the soils the mineral matters of which have been brought into their position by the action of glaciers; these in a way resemble those formed by rivers, but the materials are generally imperfectly sorted, coarse and fine being mingled together. Last of all, we have the soils due to the accumulation of blown dust or blown sand, which, unlike the others, occupy but a small part of the land surface. It would be possible, indeed, to make yet another division, including those areas which when emerging from the sea were covered with fine, uncemented detritus ready at once to serve the purposes of a soil. Only here and there, and but seldom, do we find soils of this nature.

It is characteristic of soils belonging to the group to which we have given the title of immediate derivation that they have accumulated slowly, that they move very gradually down the slopes on which they lie, and that in all cases they represent, with a part of their mass at least, levels of rock which have disappeared from the region which they occupied. The additions made to their mass are from below, and that mass is constantly shrinking, generally at a pretty rapid rate, by the mineral matter which is dissolved and goes away with the spring water. They also are characteristically thin on steep slopes, thickening toward the base of the incline, where the diminished grade permits the soil to move slowly, and therefore to accumulate.

In alluvial soils we find accumulations which are characterized by growth on their upper surfaces, and by the distant transportation of the materials of which they are composed. In these deposits the outleaching removes vast amounts of the materials, but so long as the floods from time to time visit their surfaces the growth of the deposits is continued. This growth rarely takes place from the waste of the bed rocks on which the alluvium lies. It is characteristic of alluvial soils that they are generally made up of débris derived from fields where the materials have undergone the change which we have noted in the last paragraph; therefore these latter deposits have throughout the character which renders the mineral materials easily dissolved. Moreover, the mass as it is constructed is commonly mingled with a great deal of organic waste, which serves to promote its fertility. On these accounts alluvial grounds, though they vary considerably in fertility, commonly afford the most fruitful fields of any region. They have, moreover, the signal advantage that they often may be refreshed by allowing the flood waters to visit them, an action which but for the interference of man commonly takes place once each year. Thus in the valley of the Nile there are fields which have been giving rich grain harvests probably for more than four thousand years, without any other effective fertilizing than that derived from the mud of the great river.

The group of glaciated soils differs in many ways from either of those mentioned. In it we find the mineral matter to have been broken up, transported, and accumulated without the influence of those conditions which ordinarily serve to mix rock débris with organic matter during the process by which it is broken into bits. When vegetation came to preoccupy the fields made desolate by glacial action, it found in most places more than sufficient material to form soils, but the greater part of the matter was in the condition of pebbles of very hard rock and sand grains, fragments of silex. Fortunately, the broken-up state of this material, by exposing a great surface of the rocky matter to decay, has enabled the plants to convert a portion of the mass into earth fit for the uses of their roots. But as the time which has elapsed since the disappearance of the glaciers is much less than that occupied in the formation of ordinary soil, this decay has in most cases not yet gone very far, so that in a cubic foot of glaciated waste the amount of material available for plants is often only a fraction of that held in the soils of immediate derivation.

In the greater portion of the fields occupied by glacial waste the processes which lead to the introduction of organic matter into the earth have not gone far enough to set in effective work the great laboratory which has to operate in order to give fertile soil. The pebbles hinder the penetration of the roots as well as the movement of insects and other animals. There has not been time enough for the overturning of trees to bring about a certain admixture of vegetable matter with the soil—in a word, the process of soil-making, though the first condition, that of broken-up rock, has been accomplished, is as yet very incomplete. It needs, indeed, care in the introduction of organic matter for its completion.

It is characteristic of glacial soils that they are indefinitely deep. This often is a disadvantageous feature, for the reason that the soil water may pass so far down into the earth that the roots are often deprived of the moisture which they need, and which in ordinary soils is retained near the surface by the hard underlayer. On the other hand, where the glacial waste is made up of pebbles formed from rocks of varied chemical composition, which contain a considerable share of lime, potash, soda, and other substances which are required by plants, the very large surface which they expose to decay provides the soil with a continuous enrichment. In a cubic foot of pebbly glacial earth we often find that the mass offers several hundred times as much surface to the action of decay as is afforded by the underlying solid bed rock from which a soil of immediate derivation has to win its mineral supply. Where the pebbly glacial waste is provided with a mixture of vegetable matter, the process of decay commonly goes forward with considerable rapidity. If the supply of such matter is large, such as may be produced by ploughing in barnyard manure or green crops, the nutritive value of the earth may be brought to a very high point.

It is a familiar experience in regions where glacial soils exist that the earth beneath the swamps when drained is found to be extraordinarily well suited for farming purposes. On inspecting the pebbles from such places, we observe that they are remarkably decayed. Where the masses contain large quantities of feldspar, as is the case in the greater part of our granitic and other crystalline rocks, this material in its decomposition is converted into kaolin or feldspar clay, and gives the stones a peculiar white appearance, which marks the decomposition, and indicates the process by which a great variety of valuable soil ingredients are brought into a state where they may be available for plants.

In certain parts of the glacial areas, particularly in the region near the margin of the ice sheet, where the glacier remained in one position for a considerable time, we find extensive deposits of silicious sand, formed of the materials which settled from the under-ice stream, near where they escaped from the glacial cavern. These kames and sand plains, because of the silicious nature of their materials and the very porous nature of the soil which they afford, are commonly sterile, or at most render a profit to the tiller by dint of exceeding care. Thus in Massachusetts, although the first settlers seized upon these grounds, and planted their villages upon them because the forests there were scanty and the ground free from encumbering boulders, were soon driven to betake themselves to those areas where the drift was less silicious, and where the pebbles afforded a share of clay. Very extensive fields of this sandy nature in southeastern New England have never been brought under tillage. Thus on the island of Martha's Vineyard there is a connected area containing about thirty thousand acres which lies in a very favourable position for tillage, but has been found substantially worthless for such use. The farmers have found it more advantageous to clear away the boulders from the coarser drift in order to win soil which would give them fair returns.

Those areas which are occupied by soil materials which have been brought into their position by the action of the wind may, as regards their character, be divided into two very distinct groups—the dunes and loess deposits. In the former group, where, as we have noted (see page [123]), the coarse sea sands or those from the shores of lakes are driven forward as a marching hillock, the grains of the material are almost always silicious. The fragments in the motion are not taken up into the air, but are blown along the surface. Such dune accumulations afford an earth which is even more sterile than that of the glacial sand plains, where there is generally a certain admixture of pebbles from rocks which by their decomposition may afford some elements of fertility. Fortunately for the interests of man, these wind-borne sands occupy but a small area; in North America, in the aggregate, there probably are not more than one thousand square miles of such deposits.

Where the rock material drifted by the winds is so fine that it may rise into the air in the form of dust, the accumulations made of it generally afford a fertile soil, and this for the reason that they are composed of various kinds of rock, and not, as in the case of dunes, of nearly pure silica. In some very rare cases, where the seashore is bordered by coral reefs, as it is in parts of southern Florida, and the strand is made up of limestone bits derived from the hard parts which the polyps secrete, small dunes are made of limy material. Owing, however, in part to the relatively heavy nature of this substance, as well as to the rapid manner in which its grains become cemented together, such limestone dunes never attain great size nor travel any distance from their point of origin.

As before noted, dust accumulations form the soil in extended areas which lie to the leeward of great deserts. Thus a considerable part of western China and much of the United States to the west of the Mississippi is covered by these wind-blown earths. Wherever the rainfall is considerable these loess deposits have proved to have a high agricultural value.

Where a region has an earth which has recently passed from beneath the sea or a great lake, the surface is commonly covered by incoherent detritus which has escaped consolidation into hard rock by the fact that it has not been buried and thus brought into the laboratory of the earth's crust. When such a region becomes dry land, the materials are immediately ready to enter into the state of soil. They commonly contain a good deal of waste derived from the organic life which dwelt upon the sea bottom and was embedded in the strata as they were formed. Where these accumulations are made in a lake, the land vegetation at once possesses the field, even a single year being sufficient for it to effect its establishment. Where the lands emerge from the sea, it requires a few years for the salt water to drain away so that the earth can be fit for the uses of plants. In a general way these sea-bottom soils resemble those formed in the alluvial plains. They are, however, commonly more sandy, and their substances less penetrated by that decay which goes on very freely in the atmosphere because of the abundant supply of oxygen, and but slowly on the sea floor. Moreover, the marine deposits are generally made up in large part of silicious sand, a material which is produced in large quantities by the disruption of the rocks along the sea coast. The largest single field of these ocean-bottom soils of North America is found in the lowland region of the southern United States, a wide belt of country extending along the coast from the Rio Grande to New York. Although the streams have channelled shallow valleys in the beds of this region, the larger part of its surface still has the peculiar features of form and composition which were impressed upon it when it lay below the surface of the sea.

Local variations in the character of the soil covering are exceedingly numerous, and these differences of condition profoundly affect the estate of man. We shall therefore consider some of the more important of these conditions, with special reference to their origin.

The most important and distinctly marked variation in the fertility of soils is that which is produced by differences in the rainfall. No parts of the earth are entirely lacking in rain, but over considerable areas the precipitation does not exceed half a foot a year. In such realms the soil is sterile, and the natural coating of vegetation limited to those plants which can subsist on dew or which can take on an occasional growth at such times as moisture may come upon them. With a slight increase in precipitation, the soil rapidly increases in productivity, so that we may say that where as much as about ten inches of water enters the earth during the summer half of the year, it becomes in a considerable measure fit for agriculture. Observations indicate that the conditions of fertility are not satisfied where the rainfall is just sufficient to fill the pores of the soil; there must be enough water entering the earth to bring about a certain amount of outflow in the form of springs. The reason of this need becomes apparent when we study the evident features of those soils which, though from season to season charged with water, do not yield springs, but send the moisture away through the atmosphere. Wherever these conditions occur we observe that the soil in dry seasons becomes coated with a deposit of mineral matter, which, because of its taste, has received the name of alkali. The origin of this coating is as follows: The pores of the soil, charged from year to year with sufficient water to fill them, become stored with a fluid which contains a very large amount of dissolved mineral matter—too much, indeed, to permit the roots of plants, save a few species which have become accustomed to the conditions, to do their appointed work. In fact, this water is much like that of the sea, which the roots of only a few of our higher plants can tolerate. When the dry season comes on, the heat of the sun evaporates the water at the surface, leaving behind a coating composed of the substances which the water contains. The soil below acts in the manner of a lamp-wick to draw up fluid as rapidly as the heat burns it away. When the soil water is as far as possible exhausted, the alkali coating may represent a considerable part of the soluble matter of the soil, and in the next rainy season it may return in whole or in part to the under-earth, again to be drawn in the manner before described to the upper level. It is therefore only when a considerable share of the ground water goes forth to the streams in each year that the alkaline materials are in quantity kept down to the point where the roots of our crop-giving plants can make due use of the soil. Where, in an arid region, the ground can be watered from the enduring streams or from artificial reservoirs, the main advantage arising from the process is commonly found in the control which it gives the farmer in the amount of the soil water. He can add to the rainfall sufficient to take away the excess of mineral matter. When such soils are first brought under tillage it is necessary to use a large amount of water from the canals, in order to wash away the old store of alkali. After that a comparatively small contribution will often keep the soil in excellent condition for agriculture. It has been found, however, in the irrigated lands beside the Nile that where too much saving is practised in the irrigation, the alkaline coating will appear where it has been unknown before, and with it an unfitness of the earth to bear crops.

Although the crust of mineral matters formed in the manner above described is characteristic of arid countries, and in general peculiar to them, a similar deposit may under peculiar conditions be formed in regions of great rainfall. Thus on the eastern coast of New England, where the tidal marshes have here and there been diked from the sea and brought under tillage, the dissolved mineral matters of the soil, which are excessive in quantity, are drawn to the surface, forming a coating essentially like that which is so common in arid regions. The writer has observed this crust on such diked lands, having a thickness of an eighth of an inch. In fact, this alkali coating represents merely the extreme operation of a process which is going on in all soils, and which contributes much to their fertility. When rain falls and passes downward into the earth, it conveys the soluble matter to a depth below the surface, often to beyond the point where our ordinary crop plants, such as the small grains, can have access to it, and this for the reason that their roots do not penetrate deeply. When dry weather comes and evaporation takes place from the surface, the fluid is drawn up to the upper soil layer, and there, in process of evaporation, deposits the dissolved materials which it contains. Thus the mineral matter which is fit for plant food is constantly set in motion, and in its movement passes the rootlets of the plants. It is probably on this account—at least in part—that very wet weather is almost as unfavourable to the farmer as exceedingly dry, the normal alternation in the conditions being, as is well known, best suited to his needs.

So long as the earth is subjected to conditions in which the rainfall may bring about a variable amount of water in the superficial detrital layer, we find normal fruitful soils, though in their more arid conditions they may be fit for but few species of plants. When, by increasing aridity, we pass to conditions where there is no tolerably permanent store of water in the débris, the material ceases to have the qualities of a soil, and becomes mere rock waste. At the other extreme of the scale we pass to conditions where the water is steadfastly maintained in the interstices of the detritus, and there again the characteristic of the soil and its fitness for the uses of land vegetation likewise disappear. In a word, true soil conditions demand the presence of moisture, but that in insufficient quantities, to keep the pores of the earth continually filled; where they are thus filled, we have the condition of swamps. Between these extremes the level at which the water stands in the soil in average seasons is continually varying. In rainy weather it may rise quite to the surface; in a dry season it may sink far down. As this water rises and falls, it not only moves, as before noted, the soluble mineral materials, but it draws the air into and expels it from the earth with each movement. This atmospheric circulation of the soil, as has been proved by experiment, is of great importance in maintaining its fertility; the successive charges of air supply the needs of the microscopic underground creatures which play a large part in enriching the soil, and the direct effect of the oxygen in promoting decay is likewise considerable. A part of the work which is accomplished by overturning the earth in tillage consists in this introduction of the air into the pores of the soil, where it serves to advance the actions which bring mineral matters into solution.

Mountain gorge, Himalayas, India. Note the difference in the slope of the eroded rocks and the effect of erosion upon them; also the talus slopes at the base of the cliffs which the torrent is cutting away. On the left of the foreground there is a little bench showing a recent higher line of the water.

In the original conditions of any country which is the seat of considerable rainfall, and where the river system is not so far developed as to provide channels for the ready exit of the waters, we commonly find very extensive swamps; these conditions of bad drainage almost invariably exist where a region has recently been elevated above the level of the sea, and still retains the form of an irregular rolling plain common to sea floors, and also in regions where the work done by glaciers has confused the drainage which the antecedent streams may have developed. In an old, well-elaborated river system swamps are commonly absent, or, if they occur, are due to local accidents of an unimportant nature.

For our purpose swamps may be divided into three groups—climbing bogs, lake bogs, and marine marshes. The first two of these groups depend on the movements of the rain water over the land; the third on the action of the tides. Beginning our account with the first and most exceptional of these groups, we note the following features in their interesting history:

Wherever in a humid region, on a gentle slope—say with an inclination not exceeding ten feet to the mile—the soil is possessed by any species of plants whose stems grow closely together, so that from their decayed parts a spongelike mass is produced, we have the conditions which favour the development of climbing bogs. Beginning usually in the shores of a pool, these plants, necessarily of a water-loving species, retain so much moisture in the spongy mass which they form that they gradually extend up the slope. Thus extending the margin of their field, and at the same time thickening the deposit which they form, these plants may build a climbing bog over the surface until steeps are attained where the inclination is so great that the necessary amount of water can not be held in the spongy mass, or where, even if so held, the whole coating will in time slip down in the manner of an avalanche.

The greater part of the climbing bogs of the world are limited to the moist and cool regions of high latitudes, where species of moss belonging to the genus Sphagnum plentifully flourish. These plants can only grow where they are continuously supplied with a bath of water about their roots. They develop in lake bogs as far south as Mexico, but in the climbing form they are hardly traceable south of New England, and are nowhere extensively developed within the limits of the United States. In more northern parts of this continent, and in northwestern Europe, particularly in the moist climate of Ireland, climbing bogs occupy great areas, and hold up their lakes of interstitially contained water over the slopes of hills, where the surface rises at the rate of thirty feet or more to the mile. So long as the deposit of decayed vegetable matter which has accumulated in this manner is thin, therefore everywhere penetrated by the fibrous roots of the moss, it may continue to cling to its sloping bed; but when it attains a considerable thickness, and the roots in the lower part decay, the pulpy mass, water-laden in some time of heavy rain, break away in a vast torrent of thick, black mud, which may inundate the lower lands, causing widespread destruction.

In more southern countries, other water-loving plants lead to the formation of climbing bogs. Of these, the commonest and most effective are the species of reeds, of which our Indian cane is a familiar example. Brakes of this vegetation, plentifully mingled with other species of aquatic growth, form those remarkable climbing bogs known as the Dismal and other swamps, which numerously occur along the coast line of the United States from southern Maryland to eastern Texas. Climbing bogs are particularly interesting, not only from the fact that they are eminently peculiar effects of plant growth, but because they give us a vivid picture of those ancient morasses in which grew the plants that formed the beds of vegetable matter now appearing in the state of coal. Each such bed of buried swamp material was, with rare exceptions, where the accumulation took place in lakes, gathered in climbing bogs such as we have described.

Lake bogs occur in all parts of the world, but in their best development are limited to relatively high latitudes, and this for the reason that the plants which form vegetable matter grow most luxuriantly in cool climates and in regions where the level of the basin is subject to less variation than occurs in the alternating wet and dry seasons which exist in nearly all tropical regions. The fittest conditions are found in glaciated regions, where, as before noted, small lakes are usually very abundant. On the shores of one of these pools, of size not so great that the waves may attain a considerable height, or in the sheltered bay of a larger lake, various aquatic plants, especially the species of pond lilies, take root upon the bottom, and spread their expanded leaves on the surface of the water. These flexible-leaved and elastic-stemmed plants can endure waves which attain no more than a foot or two of height, and by the friction which they afford make the swash on the shore very slight. In the quiet water, rushes take root, and still further protect the strand, so that the very delicate vegetation of the mosses, such as the Sphagnum, can fix itself on the shore.

As soon as the Sphagnum mat has begun its growth, the strength given by its interlaced fibres enables it to extend off from the shore and float upon the water. In this way it may rapidly enlarge, if not broken up by the waves, so that its front advances into the lake at the rate of several inches each year. While growing outwardly it thickens, so that the bottom of the mass gradually works down toward the floor of the basin. At the same time the lower part of the sheet, decaying, contributes a shower of soft peat mud to the floor of the lake. In this way, growing at its edge, deepening, and contributing to an upgrowth from the bottom, a few centuries may serve entirely to fill a deep basin with peaty accumulation. In general, however, the surface of the bog closes over the lake before the accumulation has completely filled the shoreward portions of the area. In these conditions we have what is familiarly known as a quaking bog, which can be swayed up and down by a person who quickly stoops and rises while standing on the surface. In this state the tough and thick sheet of growing plants is sufficient to uphold a considerable weight, but so elastic that the underlying water can be thrown into waves. Long before the bog has completely filled the lake with the peaty accumulations the growth of trees is apt to take place on its surface, which often reduces the area to the appearance of a very level wet wood.

Fig. 17.—Diagram showing beginning of peat bog: A, lake; B, lilies and rushes; C, lake bog; D, climbing bog.

Climbing and lake bogs in the United States occupy a total area of more than fifty thousand square miles. In all North America the total area is probably more than twice as great. Similar deposits are exceedingly common in the Eurasian continent and in southern Patagonia. It is probable that the total amount of these fields in different parts of the world exceeds half a million square miles. These two groups of fresh-water swamps have an interest, for the reason that when reduced to cultivation by drainage and by subsequent removal of the excess of peaty matter, by burning or by natural decay, afford very rich soil. The fairest fields of northern Europe, particularly in Great Britain and Ireland, have been thus won to tillage. In the first centuries of our era a large part of England—perhaps as much as one tenth of the ground now tilled in that country—was occupied by these lands, which retained water in such measure as to make them unfit for tillage, the greater portion of this area being in the condition of thin climbing bog. For many centuries much of the energy of the people was devoted to the reclamation of these valuable lands. This task of winning the swamp lands to agriculture has been more completely accomplished in England than elsewhere, but it has gone far on the continent of Europe, particularly in Germany. In the United States, owing to the fact that lands have been cheap, little of this work of swamp-draining has as yet been accomplished. It is likely that the next great field of improvement to be cultivated by the enterprising people will be found in these excessively humid lands, from which the food-giving resources for the support of many million people can be won.

Fig. 18.—Diagram showing development of swamp: A, remains of lake; B, surface growth; c, peat.

The group of marine marshes differs in many important regards from those which are formed in fresh water. Where the tide visits any coast line, and in sheltered positions along that shore, a number of plants, mostly belonging to the group of grasses, species which have become accustomed to having their roots bathed by salt water, begin the formation of a spongy mat, which resembles that composed of Sphagnum, only it is much more solid. This mat of the marine marshes soon attains a thickness of a foot or more, the upper or growing surface lying in a position where it is covered for two or three hours at each visit of the tide. Growing rapidly outward from the shore, and having a strength which enables it to resist in a tolerably effective manner waves not more than two or three feet high, this accumulation makes head against the sea. To a certain extent the waves undermine the front of the sheet and break up masses of it, which they distribute over the shallow bottom below the level at which these plants can grow. In this deeper water, also, other marine animals and plants are continually developing, and their remains are added to the accumulations which are ever shallowing the water, thus permitting a further extension of the level, higher-lying marsh. This process continues until the growth has gone as far as the scouring action of the tidal currents will permit. In the end the bay, originally of wide-open water, is only such at high tide. For the greater part of the time it appears as broad savannas, whose brilliant green gives them the aspect of rare fertility.

Owing to the conditions of their growth, the deposits formed in marine marshes contain no distinct peat, the nearest approach to that substance being the tangle of wirelike roots which covers the upper foot or so of the accumulation. The greater part of the mass is composed of fine silt, brought in by the streams of land water which discharge into the basin, and by the remains of animals which dwelt upon the bottom or between the stalks of the plants that occupy the surface of the marshes. These interspaces afford admirable shelter to a host of small marine forms. The result is, that the tidal marshes, as well as the lower-lying mud flats, which have been occupied by the mat of vegetation, afford admirable earth for tillage. Unfortunately, however, there are two disadvantages connected with the redemption of such lands. In the first place, it is necessary to exclude the sea from the area, which can only be accomplished by considerable engineering work; in the second place, the exclusion of the tide inevitably results in the silting up of the passage by which the water found its way to the sea. As these openings are often used for harbours, the effect arising from their destruction is often rather serious. Nevertheless, in some parts of the world very extensive and most fertile tracts of land have thus been won from the sea; a large part of Holland and shore-land districts in northern Europe are made up of fields which were originally covered by the tide. Near the mouth of the Rhine, indeed, the people have found these sea-bottom soils so profitable that they have gone beyond the zone of the marshes, and have drained considerable seas which of old were permanently covered, even at the lowest level of the waters.

On the coast of North America marine marshes have an extensive development, and vary much in character. In the Bay of Fundy, where the tides have an altitude of fifty feet or more, the energy of their currents is such that the marsh mat rarely forms. Its place, however, is taken by vast and ever-changing mud flats, the materials of which are swept to and fro by the moving waters. The people of this region have learned an art of a peculiar nature, by which they win broad fields of excellent land from the sea. Selecting an area of the flats, the surface of which has been brought to within a few feet of high tide, they inclose it with a stout barrier or dike, which has openings for the free admission of the tidal waters. Entering this basin, the tide, moving with considerable velocity, bears in quantities of sediment. In the basin, the motion being arrested, this sediment falls to the bottom, and serves to raise its level. In a few months the sheet of sediment is brought near the plane of the tidal movement, then the gates are closed at times when the tide has attained half of its height, so that the ground within the dike is not visited by the sea water, and can be cultivated.

Along the coast of New England the ordinary marine marshes attain an extensive development in the form of broad-grassed savannas. With this aspect, though with a considerable change in the plants which they bear, the fringe of savannas continues southward along the coast to northern Florida. In the region about the mouth of the Savannah River, so named from the vast extent of the tidal marshes, these fields attain their greatest development. In central and southern Florida, however, where the seacoast is admirably suited for their development, these coastal marshes of the grassy type disappear, their place being taken by the peculiar morasses formed by the growth of the mangrove tree.

In the mangrove marshes the tree which gives the areas their name covers all the field which is visited by the tide. This tree grows with its crown supported on stiltlike roots, at a level above high tide. From its horizontal branches there grow off roots, which reach downward into the water, and thence to the bottom. The seeds of the mangrove are admirably devised so as to enable the plant to obtain a foothold on the mud flats, even where they are covered at low tide with a depth of two or three feet of water. They are several inches in length, and arranged with booklets at their lower ends; floating near the bottom, they thus catch upon it, and in a few weeks' growth push the shoot to the level of the water, thus affording a foundation for a new plantation. In this manner, extending the old forests out into the shallow water of the bays, and forming new colonies wherever the water is not too deep, these plants rapidly occupy all the region which elsewhere would appear in the form of savannas.

The tidal marshes of North America, which may be in time converted to the uses of man, probably occupy an area exceeding twenty thousand square miles. If the work of reclaiming such lands from the sea ever attains the advance in this country that it has done in Holland, the area added to the dry land by engineering devices may amount to as much as fifty thousand square miles—a territory rather greater than the surface of Kentucky, and with a food-yielding power at least five times as great as is afforded by that fertile State. In fact, these conquests from the sea are hereafter to be among the great works which will attract the energies of mankind. In the arid region of the Cordilleras, as well as in many other countries, the soil, though destitute of those qualities which make it fit for the uses of man, because of the absence of water in sufficient amount, is, as regards its structure and depth, as well as its mineral contents, admirably suited to the needs of agriculture. The development of soils in desert regions is in almost all cases to be accounted for by the former existence in the realms they occupy of a much greater rainfall than now exists. Thus in the Rocky Mountain country, when the deep soils of the ample valleys were formed, the lakes, as we have before noted, were no longer dead seas, as is at present so generally the case, but poured forth great streams to the sea. Here, as elsewhere, we find evidence that certain portions of the earth which recently had an abundant rainfall have now become starved for the lack of that supply. All the soils of arid regions where the trial has been made have proved very fertile when subjected to irrigation, which can often be accomplished by storing the waters of the brief rainy season or by diverting those of rivers which enter the deserts from well-watered mountain fields. In fact, the soil of these arid realms yields peculiarly ample returns to the husbandman, because of certain conditions due to the exceeding dryness of the air. This leads to an absence of cloudy weather, so that from the time the seed is planted the growth is stimulated by uninterrupted and intense sunshine. The same dryness of the air leads, as we have seen, to a rapid evaporation from the surface, by which, in a manner before noted, the dissolved mineral matter is brought near the top of the soil, where it can best serve the greater part of our crop plants. On these accounts an acre of irrigated soil can be made to yield a far greater return than can be obtained from land of like chemical composition in humid regions.

Fig. 20.—Diagram showing mode of growth of mangroves.

In many parts of the world, particularly in the northern and western portions of the Mississippi Valley, there are widespread areas, which, though moderately well watered, were in their virgin state almost without forests. In the prairie region the early settlers found the country unwooded, except along the margins of the streams. On the borders of the true prairies, however, they found considerable areas of a prevailingly forested land, with here and there a tract of prairie. There were several of these open fields south of the Ohio, though the country there is in general forested; one of these prairie areas, in the Green River district of Kentucky, was several thousand square miles in extent. At first it was supposed that the absence of trees in the open country of the Mississippi Valley was due to some peculiarity of the soil, but experience shows that plantations luxuriantly develop, and that the timber will spread rapidly in the natural way. In fact, if the seeds of the trees which have been planted since the settlement of the country were allowed to develop as they seek to do, it would only be a few centuries before the region would be forest-clad as far west as the rainfall would permit the plants to develop. Probably the woods would attain to near the hundredth meridian.

In the opinion of the writer, the treeless character of the Western plains is mainly to be accounted for by the habit which our Indians had of burning the herbage of a lowly sort each year, so that the large game might obtain better pasturage. It is a well-known fact to all those who have had to deal with cattle on fields which are in the natural state that fire betters the pasturage. Beginning this method of burning in the arid regions to the west of the original forests, the natural action of the fire has been gradually to destroy these woods. Although the older and larger trees, on account of their thick bark and the height of their foliage above the ground, escaped destruction, all the smaller and younger members of the species were constantly swept away. Thus when the old trees died they left no succession, and the country assumed its prairie character. That the prairies were formed in this manner seems to be proved by the testimony which we have concerning the open area before mentioned as having existed in western Kentucky. It is said that around the timberless fields there was a wide fringe of old fire-scarred trees, with no undergrowth beneath their branches, and that as they died no kind of large vegetation took their place. When the Indians who set these fires were driven away, as was the case in the last decade of the last century, the country at once began to resume its timbered condition. From the margin and from every interior point where the trees survived, their seeds spread so that before the open land was all subjugated to the plough it was necessary in many places to clear away a thick growth of the young forest-building trees.

The soils which develop on the lavas and ashes about an active volcano afford interesting subjects for study, for the reason that they show how far the development of the layer which supports vegetation may depend upon the character of the rocks from which it is derived. Where the materials ejected from a volcano lie in a rainy district, the process of decay which converts the rock into soil is commonly very rapid, a few years of exposure to the weather being sufficient to bring about the formation of a fertile soil. This is due to the fact that most lavas, as well as the so-called volcanic ashes, which are of the same material as the lavas, only blown to pieces, are composed of varied minerals, the most of which are readily attacked by the agents of decay. Now and then, however, we find the materials ejected from a particular volcano, or even the lavas and ashes of a single eruption, in such a chemical state that soils form upon them with exceeding slowness.

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The foregoing incomplete considerations make it plain that the soil-covering of the earth is the result of very delicate adjustments, which determine the rate at which the broken-down rocks find their path from their original bed places to the sea. The admirable way in which this movement is controlled is indicated by the fact that almost everywhere we find a soil-covering deep enough for the use of a varied vegetation, but rarely averaging more than a dozen feet in depth. Only here and there are the rocks bare or the earth swathed in a profound mass of detritus. This indicates how steadfast and measured is the march of the rock waste from the hills to the sea. Unhappily, man, when by his needs he is forced to till the soil, is compelled to break up this ancient and perfect order. He has to strip the living mantle from the earth, replacing it with growth of those species which serve his needs. Those plants which are most serviceable—which are, indeed, indispensable in the higher civilization, the grains—require for their cultivation that the earth be stripped bare and deeply stirred during the rainy season, and thus subjected to the most destructive effect of the rainfall. The result is, that in almost all grain fields the rate of soil destruction vastly surpasses that at which the accumulation is being made. We may say, indeed, that, except in alluvial plains, where the soil grows by flood-made additions to its upper surface, no field tilled in grain can without exceeding care remain usable for a century. Even though the agriculturist returns to the earth all the chemical substances which he takes away in his crops, the loss of the soil by the washing away of its substance to the stream will inevitably reduce the region to sterility.

It is not fanciful to say that the greatest misfortune which in a large way man has had to meet in his agriculture arises from this peculiar stress which grain crops put upon the soil. If these grains grew upon perennial plants, in the manner of our larger fruits, the problem of man's relation to the soil would be much simpler than it is at present. He might then manage to till the earth without bringing upon it the inevitable destruction which he now inflicts. As it is, he should recognise that his needs imperil this ancient and precious element in the earth's structure, and he should endeavour in every possible way to minimize the damage which he brings about. This result he may accomplish in certain simple ways.

First, as regards the fertility of the soil, as distinguished from the thickness of the coating, it may be said that modern discoveries enable us to see the ways whereby we may for an indefinite period avoid the debasement of our great heritage, the food-giving earth. We now know in various parts of the world extensive and practically inexhaustible deposits, whence may be obtained the phosphates, potash, soda, etc., which we take from the soil in our crops. We also have learned ways in which the materials contained in our sewage may be kept from the sea and restored to the fields. In fact, the recent developments of agriculture have made it not only easy, but in most cases profitable, to avoid this waste of materials which has reduced so many regions to poverty. We may fairly look forward to the time, not long distant, when the old progressive degradation in the fertility of the soil coating will no longer occur. It is otherwise with the mass of the soil, that body of commingled decayed rock and vegetable matter which must possess a certain thickness in order to serve its needs. As yet no considerable arrest has been made in the processes which lead to the destruction of this earthy mass. In all countries where tillage is general the rivers are flowing charged with all they can bear away of soil material. Thus in the valley of the Po, a region where, if the soil were forest-clad, the down-wearing of the surface would probably be at no greater rate than one foot in five thousand years, the river bears away the soil detritus so rapidly that at the present time the downgoing is at the rate of one foot in eight hundred years, and each decade sees the soil disappear from hillsides which were once fertile, but are now reduced to bare rocks. All about the Mediterranean the traveller notes extensive regions which were once covered with luxuriant forests, and were afterward the seats of prosperous agriculture, where the soil has utterly disappeared, leaving only the bare rocks, which could not recover its natural covering in thousands of years of the enforced fallow.

Within the limits of the United States the degradation of the soil, owing to the peculiar conditions of the country, is in many districts going forward with startling rapidity. It has been the habit of our people—a habit favoured by the wide extent of fertile and easily acquired frontier ground—recklessly to till their farms until the fields were exhausted, and then to abandon them for new ground. By shallow ploughing on steep hillsides, by neglect in the beginning of those gulches which form in such places, it is easy in the hill country of the eastern United States to have the soil washed away within twenty years after the protecting forests have been destroyed. The writer has estimated that in the States south of the Ohio and James Rivers more than eight thousand square miles of originally fertile ground have by neglect been brought into a condition where it will no longer bear crops of any kind, and over fifteen hundred miles of the area have been so worn down to the subsoil or the bed rock that it may never be profitable to win it again to agricultural uses.

Hitherto, in our American agriculture, our people have been to a great extent pioneers; they have been compelled to win what they could in the cheapest possible way and with the rudest implements, and without much regard to the future of those who were in subsequent generations to occupy the fields which they were conquering from the wilderness and the savages. The danger is now that this reckless tillage, in a way justified of old, may be continued and become habitual with our people. It is, indeed, already a fixed habit in many parts of the country, particularly in the South, where a small farmer expects to wear out two or three plantations in the course of his natural life. Many of them manage to ruin from one to two hundred acres of land in the course of half a century of uninterrupted labour. This system deserves the reprobation of all good citizens; it would be well, indeed, if it were possible to do so, to stamp it out by the law. The same principle which makes it illegal for a man to burn his own dwelling house may fairly be applied in restraining him from destroying the land which he tills.

There are a few simple principles which, if properly applied, may serve to correct this misuse of our American soil. The careful tiller should note that all soils whatever which lie on declivities having a slope of more than one foot in thirty inevitably and rapidly waste when subject to plough tillage. This instrument tends to smear and consolidate the layer of earth over which its heel runs, so that at a depth of a few inches below the surface a layer tolerably impervious to water is formed. The result is that the porous portion of the deposit becomes excessively charged with water in times of heavy rain, and moves down the hillside in a rapid manner. All such steep slopes should be left in their wooded state, or, if brought into use, should be retained as pasture lands.

Where, as is often the case with the farms in hilly countries, all the fields are steeply inclined, it is an excellent precaution to leave the upper part of the slope with a forest covering. In this condition not only is the excessive flow of surface water diminished, but the moisture which creeps down the slope from the wooded area tends to keep the lower-lying fields in a better state for tillage, and promotes the decay of the underlying rocks, and thus adds to the body and richness of the earth.

On those soils which must be tilled, even where they tend to wash away, the aim should be to keep the detritus open to such a depth that it may take in as much as possible of the rainfall, yielding the water to the streams through the springs. This end can generally be accomplished by deep ploughing; it can, in almost all cases, be attained by under-drainage. The effect of allowing the water to penetrate is not only to diminish the superficial wearing, but to maintain the process of subsoil and bed-rock decay by which the detrital covering is naturally renewed. Where, as in many parts of the country, the washing away of the soil can not otherwise be arrested, the progress of the destruction can be delayed by forming with the skilful use of the plough ditches of slight declivity leading along the hillsides to the natural waterways. One of the most satisfactory marks of the improvement which is now taking place in the agriculture of the cotton-yielding States of this country is to be found in the rapid increase in the use of the ditch system here mentioned. This system, combined with ploughing in the manner where the earth is with each overturning thrown uphill, will greatly reduce the destructive effect of rainfall on steep-lying fields. But the only effective protection, however, is accomplished by carefully terracing the slopes, so that the tilled ground lies in level benches. This system is extensively followed in the thickly settled portions of Europe, but it may be a century before it will be much used in this country.

The duty of the soil-tiller by the earth with which he deals may be briefly summed up: He should look upon himself as an agent necessarily interfering with the operations which naturally form and preserve the soil. He should see that his work brings two risks; he may impoverish the accumulation of detrital material by taking out the plant food more rapidly than it is prepared for use. This injurious result may be at any time reparable by a proper use of manures. Not so, however, with the other form of destruction, which results in the actual removal of the soil materials. Where neglect has brought about this disaster, it can only be repaired by leaving the area to recover beneath the slowly formed forest coating. This process in almost all cases requires many thousands of years for its accomplishment. The man who has wrought such destruction has harmed the inheritance of life.

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CHAPTER IX.
the rocks and their order.

In the preceding chapters of this book the attention of the student has been directed mainly to the operations of those natural forces which act upon the surface of the earth. Incidentally the consequences arising from the applications of energy to the outer part of the planet have been attended to, but the main aim has been to set forth the work which solar energy, operating in the form of heat, accomplishes upon the lands. We have now to consider one of the great results of these actions, which is exhibited in the successive strata that make up the earth's crust.

The most noteworthy effect arising from the action of the solar forces on the earth and their co-operation with those which originate in our sphere is found in the destruction of beds or other deposits of rock, and the removal of the materials to the floors of water basins, where they are again aggregated in strata, and gradually brought once more into a stable condition within the earth. This work is accomplished by water in its various states, the action being directly affected by gravitation. In the form of steam, water which has been built into rocks and volcanically expelled by tensions, due to the heat which it has acquired at great depths below the surface, blows forth great quantities of lava, which is contributed to the formation of strata, either directly in the solid form or indirectly, after having been dissolved in the sea. Acting as waves, water impelled by solar energy transmitted to it by the winds beats against the shores, wearing away great quantities of rock, which is dragged off to the neighbouring sea bottoms, there to resume the bedded form. Moving ice in glaciers, water again applying solar energy given to it by its elevation above the sea, most effectively grinds away the elevated parts of the crust, the débris being delivered to the ocean. In the rain the same work is done, and even in the wind the power of the sun serves to abrade the high-lying rocks, making new strata of their fragments.

As gravity enters as an element in all the movements of divided rock, the tendency of the waste worn from the land is to gather on to the bottoms of basins which contain water. Rarely, and only in a small way, this process results in the accumulation of lake deposits; the greater part of the work is done upon the sea floor. When the beds are formed in lake basins, they may be accumulated in either of two very diverse conditions. They may be formed in what are called dead seas, in which case the detrital materials are commonly small in amount, for the reason that the inflowing streams are inconsiderable; in such basins there is normally a large share of saline materials, which are laid down by the evaporation of the water. In ordinary lakes the deposits which are formed are mostly due to the sediment that the rivers import. These materials are usually fine-grained, and the sand or pebbles which they contain are plentifully mingled with clay. Hence lake deposits are usually of an argillaceous nature. As organic life, such as secretes limestone, is rarely developed to any extent in lake basins, limy beds are very rarely formed beneath those areas of water. Where they occur, they are generally due to the fact that rivers charged with limy matter import such quantities of the substance that it is precipitated on the bottom.

As lake deposits are normally formed in basins above the level of the sea, and as the drainage channels of the basins are always cutting down, the effect is to leave such strata at a considerable height above the sea level, where the erosive agents may readily attack them. In consequence of this condition, lacustrine beds are rarely found of great antiquity; they generally disappear soon after they are formed. Where preserved, their endurance is generally to be attributed to the fact that the region they occupy has been lowered beneath the sea and covered by marine strata.

The great laboratory in which the sedimentary deposits are accumulated, the realm in which at least ninety-nine of the hundred parts of these materials are laid down, is the oceanic part of the earth. On the floors of the seas and oceans we have not only the region where the greater part of the sedimentation is effected, but that in which the work assumes the greatest variety. The sea bottoms, as regards the deposits formed upon them, are naturally divided into two regions—the one in which the débris from the land forms an important part of the sediment, and the other, where the remoteness of the shores deprives the sediment of land waste, or at least of enough of that material in any such share as can affect the character of the deposits.

What we may term the littoral or shore zone of the sea occupies a belt of prevailingly shallow water, varying in width from a few score to a few hundred miles. Where the bottom descends steeply from the coast, where there are no strong off-shore setting currents, and where the region is not near the mouth of a large river which bears a great tide of sediment to the sea, the land waste may not affect the bottom for more than a mile or two from the shore. Where these conditions are reversed, the débris from the air-covered region may be found three or four hundred miles from the coast line. It should also be noted that the incessant up-and-down goings of the land result in a constant change in the position of the coast line, and consequently in the extension of the land sediment, in the course of a few geological periods over a far wider field of sea bottom than that to which they would attain if the shores remained steadfast.

It is characteristic of the sediments deposited within the influence of the continental detritus that they vary very much in their action, and that this variation takes place not only horizontally along the shores in the same stratum, but vertically, in the succession of the beds. It also may be traced down the slope from the coast line to deep water. Thus where all the débris comes from the action of the waves, the deposits formed from the shore outwardly will consist of coarse materials, such as pebbles near the coast, of sand in the deeper and remoter section, and of finer silt in the part of the deposit which is farthest out. With each change in the level of the coast line the position of these belts will necessarily be altered. Where a great river enters the sea, the changes in the volume of sediment which it from time to time sends forth, together with the alternations in the position of its point of discharge, led to great local complexities in the strata. Moreover, the turbid water sent forth by the stream may, as in the case of the tide from the Amazon, be drifted for hundreds of miles along the coast line or into the open sea.

The most important variations which occur in the deposits of the littoral zone are brought about by the formations of rocks more or less composed of limestone. Everywhere the sea is, as compared with lake waters, remarkably rich in organic life. Next the shore, partly because the water is there shallow, but also because of its relative warmth and the extent to which it is in motion, organic life, both that of animals and plants, commonly develops in a very luxuriant way. Only where the bottom is composed of drifting sands, which do not afford a foothold for those species which need to rest upon the shore, do we fail to find that surface thickly tenanted with varied forms. These are arranged according to the depth of the bottom. The species of marine plants which are attached to fixed objects are limited to the depth within which the sunlight effectively penetrates the water; in general, it may be said that they do not extend below a depth of one hundred feet. The animal forms are distributed, according to their kinds, over the floor, but few species having the capacity to endure any great range in the pressure of the sea water. Only a few forms, indeed, extend from low tide to the depth of a thousand feet.

The greatest development of organic life, the realm in which the largest number of species occur, and where their growth is most rapid, lies within about a hundred feet of the low-tide level. Here sunlight, warmth, and motion in the water combine to favour organic development. It is in this region that coral reefs and other great accumulations of limestone, formed from the skeletons of polyps and mollusks, most abundantly occur. These deposits of a limy nature depend upon a very delicate adjustment of the conditions which favour the growth of certain creatures; very slight geographic changes, by inducing movements of sand or mud, are apt to interrupt their formation, bringing about a great and immediate alteration in the character of the deposits. Thus it is that where geologists find considerable fields of rock, where limestones are intercalated with sandstones and deposits of clay, they are justified in assuming that the strata were laid down near some ancient shore. In general, these coast deposits become more and more limy as we go toward the tropical realms, and this for the reason that the species which secrete large amounts of lime are in those regions most abundant and attain the most rapid growth. The stony polyps, the most vigorous of the limestone makers, grow in large quantities only in the tropical realm, or near to it, where ocean streams of great warmth may provide the creatures with the conditions of temperature and food which they need.

As we pass from the shore to the deeper sea, the share of land detritus rapidly diminishes until, as before remarked, at the distance of five hundred miles from the coast line, very little of that waste, except that from volcanoes, attains the bottom of the sea. By far the larger part of the contributions which go to the formation of these deep-sea strata come from organic remains, which are continually falling upon the sea floor. In part, this waste is derived from creatures which dwell upon the bottom; in considerable measure, however, it is from the dead bodies of those forms which live near the surface of the sea, and which when dying sink slowly through the intermediate realm to the bottom.

Owing to the absence of sunlight, the prevailingly cold water of the deeper seas, and the lack of vegetation in those realms, the growth of organic forms on the deep-sea floor is relatively slow. Thus it happens that each shell or other contribution to the sediment lies for some time on the bottom before it is buried. While in this condition it is apt to be devoured by some of the many species which dwell on the bottom and subsist from the remains of animals and plants which they find there. In all cases the fossilization of any form depends upon the accumulation of sediment before the processes of destruction have overtaken them, and among these processes we must give the first place to the creatures which subsist on shells, bones, or other substances of like nature which find their way to the ocean floor. In the absolute darkness, the still water, and the exceeding cold of the deeper seas, animals find difficult conditions for development. Moreover, in this deep realm there is no native vegetation, and, in general, but little material of this nature descends to the bottom from the surface of the sea. The result is, the animals have to subsist on the remains of other animals which at some step in the succession have obtained their provender from the plants which belong on the surface or in the shallow waters of the sea. This limitation of the food supply causes the depths of the sea to be a realm of continual hunger, a region where every particle of organic matter is apt to be seized upon by some needy creature.

In consequence of the fact that little organic matter on the deeper sea floors escapes being devoured, the most of the material of this nature which goes into strata enters that state in a finely divided condition. In the group of worms alone—forms which in a great diversity of species inhabit the sea floor—we find creatures which are specially adapted to digesting the débris which gathers on the sea bottom. Wandering over this surface, much in the manner of our ordinary earthworms, these creatures devour the mud, voiding the matter from their bodies in a yet more perfectly divided form. Hence it comes about that the limestone beds, so commonly formed beneath the open seas, are generally composed of materials which show but few and very imperfect fossils. Studying any series of limestone beds, we commonly find that each layer, in greater or less degree, is made up of rather massive materials, which evidently came to their place in the form of a limy mud. Very often this lime has crystallized, and thus has lost all trace of its original organic structure.

One of the conspicuous features which may be observed in any succession of limestone beds is the partings or divisions into layers which occur with varied frequency. Sometimes at vertical intervals of not more than one or two inches, again with spacings of a score of feet, we find divisional planes, which indicate a sudden change in the process of rock formation. The lime disappears, and in place of it we have a thin layer of very fine detritus, which takes on the form of a clay. Examining these partings with care, we observe that on the upper surface on the limestone the remains of the animal which dwelt on the ancient sea floor are remarkably well preserved, they having evidently escaped the effect of the process which reduced their ancestors, whose remains constitute the layer, to mud. Furthermore, we note that the shaly layer is not only lacking in lime, but commonly contains no trace of animals such as might have dwelt on the bottom. The fossils it bears are usually of species which swam in the overlying water and came to the bottom after death. Following up through the layer of shale, we note that the ordinary bottom life gradually reappears, and shortly becomes so plentiful that the deposit resumes the character which it had before the interruption began. Often, however, we note that the assemblage of species which dwelt on the given area of sea floor has undergone a considerable change. Forms in existence in the lower layer may be lacking in the upper, their place being taken by new varieties.

So far the origin of these divisional planes in marine deposits has received little attention from geologists; they have, indeed, assumed that each of these alterations indicates some sudden disturbance of the life of the sea floors. They have, however, generally assumed that the change was due to alterations in the depth of the sea or in the run of ocean currents. It seems to the writer, however, that while these divisions may in certain cases be due to the above-mentioned and, indeed, to a great variety of causes, they are in general best to be explained by the action of earthquakes. Water being an exceedingly elastic substance, an earthquake passes through it with much greater speed than it traverses the rocks which support the ocean floor. The result is that, when the fluid and solid oscillate in the repeated swingings which a shock causes, they do not move together, but rub over each other, the independent movements having the swing of from a few inches to a foot or two in shocks of considerable energy.

When the sea bottom and the overlying water, vibrating under the impulse of an earthquake shock, move past each other, the inevitable result is the formation of muddy water; the very fine silt of the bottom is shaken up into the fluid, which afterward descends as a sheet to its original position. It is a well-known fact that such muddying of water, in which species accustomed to other conditions dwell, inevitably leads to their death by covering their breathing organs and otherwise disturbing the delicately balanced conditions which enable them to exist. We find, in fact, that most of the tenants of the water, particularly the forms which dwell upon the bottom, are provided with an array of contrivances which enable them to clear away from their bodies such small quantities of silt as may inconvenience them. Thus, in the case of our common clam, the breathing organs are covered with vibratory cilia, which, acting like brooms, sweep off any foreign matter which may come upon their surfaces. Moreover, the creature has a long, double, spoutlike organ, which it can elevate some distance above the bottom, through which it draws and discharges the water from which it obtains food and air. Other forms, such as the crinoids, or sea lilies, elevate the breathing parts on top of tall stems of marvellous construction, which brings those vital organs at the level, it may be, of three or four feet above the zone of mud. In consequence of the peculiar method of growth, the crinoids often escape the damage done by the disturbance of the bottom, and thus form limestone beds of remarkable thickness; sometimes, indeed, we find these layers composed mainly of crinoidal remains, which exhibit only slight traces of partings such as we have described, being essentially united for the depth of ten or twenty feet. Where the layers have been mainly accumulated by shellfish, their average thickness is less than half a foot.

When we examine the partitions between the layers of limestone, we commonly find that, however thin, they generally extend for an indefinite distance in every direction. The writer has traced some of these for miles; never, indeed, has he been able to find where they disappeared. This fact makes it clear that the destruction which took place at the stage where these partings were formed was widespread; so far as it was due to earthquake shocks, we may fairly believe that in many cases it occurred over areas which were to be measured by tens of thousands of square miles. Indeed, from what we know of earthquake shocks, it seems likely that the devastation may at times have affected millions of square miles.

Another class of accidents connected with earthquakes may also suddenly disturb the mud on the sea bottom. When, as elsewhere noted, a shock originates beneath the sea, the effect is suddenly to elevate the water over the seat of the jarring and the regions thereabouts to the height of some feet. This elevation quickly takes the shape of a ringlike wave, which rolls off in every direction from its point of origin. Where the sea is deep, the effect of this wave on the bottom may be but slight; but as the undulation attains shallower water, and in proportion to the shoaling, the front of the surge is retarded in its advance by the friction of the bottom, while the rear part, being in deeper water, crowds upon the advancing line. The action is precisely that which has been described as occurring in wind-made waves as they approach the beach; but in this last-named group of undulations, because of the great width of the swell, the effect of the shallowing is evident in much deeper water. It is likely that at the depth of a thousand feet the passing of one of these vast surges born of earthquakes may so stir the mud of the sea floor as to bring about a widespread destruction of life, and thus give rise to many of the partitions between strata.

If we examine with the microscope the fine-grained silts which make up the shaly layers between limestones, we find the materials to be mostly of inorganic origin. It is hard to trace the origin of the mineral matter which it contains; some of the fragments are likely to prove of Volcanic origin; others, bits of dust from meteorites; yet others, dust blown from the land, which may, as we know, be conveyed for any distance across the seas. Mingled with this sediment of an inorganic origin we almost invariably find a share of organic waste, derived not from creatures which dwelt upon the bottom, but from those which inhabited the higher-lying waters. If, now, we take a portion of the limestone layer which lies above or below the shale parting, and carefully dissolve out with acids the limy matter which it contains, we obtain a residuum which in general character, except so far as the particles may have been affected by the acid, is exactly like the material which forms the claylike partition. We are thus readily led to the conclusion that on the floors of the deeper seas there is constantly descending, in the form of a very slow shower, a mass of mineral detritus. Where organic life belonging to the species which secrete hard shells or skeletons is absent, this accumulation, proceeding with exceeding slowness, gradually accumulates layers, which take on a shaly character. Where limestone-making animals abound, they so increase the rate of deposition that the proportion of the mineral material in the growing strata is very much reduced; it may, indeed, become as small as one per cent of the mass. In this case we may say that the deposit of limestone grew a hundred times as fast as the intervening beds of shale.

The foregoing considerations make it tolerably clear that the sea floor is in receipt of two diverse classes of sediment—those of a mineral and those of an organic origin. The mineral, or inorganic, materials predominate along the shores. They gradually diminish in quantity toward the open sea, where the supply is mainly dependent on the substances thrown forth from volcanoes, on pumice in its massive or its comminuted form—i.e., volcanic dust, states of lava in which the material, because of the vesicles which it contains, can float for ages before it comes to rest on the sea bottom. Variations in the volcanic waste contributed to the sea floor may somewhat affect the quantity of the inorganic sediments, but, as a whole, the downfalling of these fragments is probably at a singularly uniform rate. It is otherwise with the contributions of sediment arising from organic forms. This varies in a surprising measure. On the coral reefs, such as form in the mid oceans, the proportion of matter which has not come into the accumulation through the bodies of animals and plants may be as small as one tenth of one per cent, or less. In the deeper seas, it is doubtful whether the rate of animal growth is such as to permit the formation of any beds which have less than one half of their mass made up of materials which fell through the water.

In certain areas of the open seas the upper part of the water is dwelt in by a host of creatures, mostly foraminifera, which extract limestone from the water, and, on dying, send their shells to the bottom. Thus in the North Atlantic, even where the sea floor is of great depth beneath the surface, there is constantly accumulating a mass of limy matter, which is forming very massive limestone strata, somewhat resembling chalk deposits, such as abundantly occur in Great Britain, in the neighbouring parts of Europe, in Texas, and elsewhere. Accumulations such as this, where the supply is derived from the surface of the water, are not affected by the accidents which divide beds made on the bottom in the manner before described. They may, therefore, have the singularly continuous character which we note in the English chalk, where, for the thickness of hundreds of feet, we may have no evident partitions, except certain divisions, which have evidently originated long after the beds were formed.

We have already noted the fact that, while the floors of the deeper seas appear to lack mountainous elevations, those arising from the folding of strata, they are plentifully scattered over with volcanic cones. We may therefore suppose that, in general, the deposits formed on the sea floor are to a great extent affected by the materials which these vents cast forth. Lava streams and showers represent only a part of the contributions from volcanoes, which finally find their way to the bottom. In larger part, the materials thrown forth are probably first dissolved in the water and then taken up by the organic species; only after the death of these creatures does the waste go to the bottom. As hosts of these creatures have no solid skeleton to contribute to the sea floor, such mineral matter as they may obtain is after their death at once restored to the sea.

Not only does the contribution of organic sediment diminish in quantity with the depth which is attained, but the deeper parts of the ocean bed appear to be in a condition where no accumulations of this nature are made, and this for the reason that the water dissolves the organic matter more rapidly than it is laid down. Thus in place of limestone, which would otherwise form, we have only a claylike residuum, such as is obtained when we dissolve lime rocks in acids. This process of solution, by which the limy matter deposited on the bottom is taken back into the water, goes on everywhere, but at a rate which increases with the depth. This increase is due in part to the augmentation of pressure, and in part to the larger share of carbonic dioxide which the water at great depths holds. The result is, that explorations with the dredge seem to indicate that on certain parts of the deeper sea floors the rocks are undergoing a process of dissolution comparable to that which takes place in limestone caverns. So considerable is the solvent work that a large part of the inorganic waste appears to be taken up by the waters, so as to leave the bottom essentially without sedimentary accumulations. The sea, in a word, appears to be eating into rocks which it laid down before the depression attained its present great depth.

We should here note something of the conditions which determine the supply of food which the marine animals obtain. First of all, we may recur to the point that the ocean waters appear to contain something of all the earth materials which do not readily decompose when they are taken into the state of solution. These mineral substances, including the metals, are obtained in part from the lands, through the action of the rain water and the waves, but perhaps in larger share from the volcanic matter which, in the form of floating lava, pumice, or dust, is plentifully delivered to the sea. Except doubtfully, and at most in a very small way, this chemical store of the sea water can not be directly taken into the structures of animals; it can only be immediately appropriated by the marine plants. These forms can only develop in that superficial realm of the seas which is penetrated by the sunlight, or say within the depth of five hundred feet, mostly within one hundred feet of the surface, about one thirtieth of the average, and about one fiftieth of the maximum ocean depth. On this marine plant life, and in a small measure on the vegetable matter derived from the land, the marine animals primarily depend for their provender. Through the conditions which bring about the formation of Sargassum seas, those areas of the ocean where seaweeds grow afloat, as well as by the water-logging and weighting down of other vegetable matter, some part of the plant remains is carried to the sea floor, even to great depths; but the main dependence of the deep-sea forms of animals is upon other animal forms, which themselves may have obtained their store from yet others. In fact, in any deep-sea form we might find it necessary to trace back the food by thousands of steps before we found the creature which had access to the vegetable matter. It is easy to see how such conditions profoundly limit the development of organic being in the abysm of the ocean.

The sedentary animals, or those which are fixed to the sea bottom—a group which includes the larger part of the marine species—have to depend for their sustenance on the movement of the water which passes their station. If the seas were perfectly still, none of these creatures except the most minute could be fed; therefore the currents of the ocean go far by their speed to determine the rate at which life may flourish. At great depths, as we have seen, these movements are practically limited to that which is caused by the slow movement which the tide brings about. The amount of this motion is proportional to the depth of the sea; in the deeper parts, it carries the water to and fro twice each day for the distance of about two hundred and fifty feet. In the shallower water this motion increases in proportion to the shoaling, and in the regions near the shores the currents of the sea which, except the massive drift from the poles, do not usually touch the bottom, begin to have their influence. Where the water is less than a hundred feet in depth, each wave contributes to the movement, which attains its maximum near the shore, where every surge sweeps the water rapidly to and fro. It is in this surge belt, where the waves are broken, that marine animals are best provided with food, and it is here that their growth is most rapid. If the student will obtain a pint of water from the surf, he will find that it is clouded by fragments of organic matter, the quantity in a pound of the fluid often amounting to the fiftieth part of its weight. He will thus perceive that along the shore line, though the provision of victuals is most abundant, the store is made from the animals and plants which are ground up in the mill. In a word, while the coast is a place of rapid growth, it is also a region of rapid destruction; only in the case of the coral animals, which associate their bodies with a number of myriads in large and elaborately organized communities, do we find animals which can make such head against the action of the waves that they can build great deposits in their realm.

It should be noted that a part of the advantage which is afforded to organic life by the shore belt is due to the fact that the waters are there subjected to a constant process of aëration by the whipping into foam and spray which occurs where the waves overturn.

It will be interesting to the student to note the great number of mechanical contrivances which have been devised to give security to animals and plants which face these difficult conditions arising from successive violent blows of falling water. Among these may be briefly noted those of the limpets—mollusks which dwell in a conical shell, which faces the water with a domelike outside, and which at the moment of the stroke is drawn down upon the rock by the strong muscle which fastens the creature to its foundation. The barnacles, which with their wedge-shaped prows cut the water at the moment of the stroke, but open in the pauses between the waves, so that the creature may with its branching arms grasp at the food which floats about it; the nullipores, forms of seaweed which are framed of limestone and cling firmly to the rock—afford yet other instances of protective adaptations contrived to insure the safety of creatures which dwell in the field of abundant food supply.

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The facts above presented will show the reader that the marine sediments are formed under conditions which permit a great variety in the nature of the materials of which they are composed. As soon as the deposits are built into rocks and covered by later accumulations, their materials enter the laboratory of the under earth, where they are subjected to progressive changes. Even before they have attained a great depth, through the laying down of later deposits upon them, changes begin which serve to alter their structure. The fragments of a soluble kind begin to be dissolved, and are redeposited, so that the mass commonly becomes much more solid, passing from the state of detritus to that of more or less solid rock. When yet more deeply buried, and thereby brought into a realm of greater warmth, or perhaps when penetrated by dikes and thereby heated, these changes go yet further. More of the material is commonly rearranged by solution and redeposition, so that limestone may be converted into crystalline marble, granular sandstones into firm masses, known as quartzites, and clays into the harder form of slate. Where the changes go to the extreme point, rocks originally distinctly bedded probably may be so taken to pieces and made over that all traces of their stratification may be destroyed, all fossils obliterated, and the stone transformed into mica schist, or granite or other crystalline rock. It may be injected into the overlying strata in the form of dikes, or it may be blown forth into the air through volcanoes. Involved in mountain-folding, after being more or less changed in the manner described, the beds may become tangled together like the rumpled leaves of a book, or even with the complexity of snarled thread. All these changes of condition makes it difficult for the geologist to unravel the succession of strata so that he may know the true order of the rocks, and read from them the story of the successive geological periods. This task, though incomplete, has by the labours of many thousand men been so far advanced that we are now able to divide the record into chapters, the divisions of the geologic ages, and to give some account of the succession of events, organic and geographic, which have occurred since life began to write its records.