Chapter XXIV: Part I (3)

The reader will remember that Geologists divide the stratified rocks into three distinct classes, Mechanical, Chemical, and Organic. This distinction, they say, is founded on the actual operations of Nature. From a close examination of the natural agents now at work in the world, it appears that some strata are being formed chiefly by the action of mechanical force; others chiefly by the influence of chemical laws; and others again chiefly by the intervention of organic life. Thus we have three distinct classes of rock at present coming into existence, each exhibiting its own peculiar characteristics, and each, moreover, having its counterpart among the strata that compose the Crust of the Earth. We shall now proceed to set forth some of the evidence that may be advanced in favor of these important conclusions, beginning with those rocks that are called Mechanical.

And first it is important to have, at least, a general idea of the appearance which Mechanical Rocks present to the eye. We shall take three familiar examples, Conglomerate, Sandstone, and Clay. Conglomerate, or Pudding-stone as it is sometimes called, is composed of pebbles, gravel, and sand, more or less compacted together, and generally forming a hard and solid mass. The various materials of which it is composed, though united in the one rock, nevertheless remain their own external forms, and may be distinctly recognized even by the unpractised eye. Sandstone, as the name implies, is made up of grains of sand closely compressed and cemented together. The quality and appearance of this rock vary very much according to the size and character of its constituent particles. Often the grains of sand are as large as peas, or even larger; sometimes they are so minute that they cannot be distinguished without the aid of a lens. For the most part they consist of quartz, with grains of limestone intermixed; and they are usually rounded, as if by the action of running water. Clay is a rather vague and general term, now commonly employed to denote any finely-divided mineral matter which contains from ten to thirty per cent. of Alumina, and is thereby rendered plastic, and capable, when softened with water, of being moulded like paste with the hand. It occurs in many different forms among the strata of the Earth, according to the different minerals that enter into its composition and the different influences to which it has been subjected. Marl and Loam may be taken as well-known illustrations: the former is a clay in which there is a large proportion of calcareous matter; the latter is a mixture of clay and sand. Sometimes by pressure clay is condensed into a kind of slaty rock called Shale, which has the property of being easily split up into an immense number of thin plates or laminæ.

It should be remembered that there is not always a perfect uniformity in the structure of these rocks. In Conglomerate, for example, the pebbles may be as large as cannon balls, or they may be only the size of walnuts. So, too, we have every variety of fineness and coarseness in the quality of Sandstone. Again, both Conglomerate and Sandstone are often largely adulterated with clay, and on the other hand, clay will sometimes contain more than its usual proportion of sand or lime. Lastly, these materials are in one place compacted into hard and solid rock, in another they are found in a loose and incoherent condition.

But amidst all these varieties of form and texture, the rocks we have been describing generally preserve their peculiar characteristics, and with a little experience can be easily recognized. They are found to constitute a very large part, perhaps we might say the larger part, of the stratified rocks in every country that has hitherto been explored by Geologists. Wherever we go we are met by the same familiar appearances;--beds of Conglomerate, Sandstone, Clay, Marl, Shale, recurring again and again through a series of many hundred strata, sometimes in one order, and sometimes in another; sometimes without any formation of a different kind intervening, and sometimes alternating with limestone or other rocks of which we shall speak hereafter.

Such is the general character and appearance of those strata which are known among Geologists as Aqueous Rocks of Mechanical origin. Now, it must at once strike the reader, that these rocks are made up of just those very materials--the same both in kind and in form--that we have already shown to be daily prepared and fashioned by a vast and complex machinery in the great workshop of Nature. He will remember how enormous blocks are detached from the mountain side, or from the cliffs on the seashore, and broken up into fragments; how the fragments in time become pebbles, sand, and mud; and how these are caught up by rivers, tides, and currents, and carried far away to sea. Here we have certainly all the materials that are necessary for the building up of Conglomerate, Sandstone, and Shale. We have seen how they are prepared by the hand of Nature, how they are moulded into shape, how they are transported from place to place. Let us now pursue the sequel of their history, and follow them on to the end.

It is plain they cannot remain forever suspended in water; sooner or later they must fall to the bottom. Yet they will not all fall together. For though all are carried downward by the one force of gravity, those materials that are smaller and lighter will be more impeded by the resistance of the water. The pebbles and coarse gravel will be the first to reach the bottom, then the sand, and last of all the fine, impalpable mud. Thus, as the current sweeps along in its course, the sediment which it bears away from the land will be in a manner sorted, and three distinct layers of different materials will be deposited in the bed of the ocean;--first, nearest to the shore, a layer of pebbles and coarse gravel, then a layer of sand, and last of all a layer of fine mud or clay. This is the first step in the construction of stratified rock. To complete the work nothing more is necessary than the consolidation of these loose and incoherent materials. If this could be accomplished, then we should have a solid stratum of Conglomerate, a solid stratum of Sandstone, and a solid stratum of Shale formed in the bed of the ocean.

With regard to this operation, however, we cannot hope for the advantage we have hitherto enjoyed, of actual observation. The process of consolidation, if it take place at all, is going on in the depths of the Sea. But though it is thus removed beyond the reach of our senses, it is not beyond the reach of our intelligence. We may borrow the torch of Science, and search even into the hidden recesses of Nature’s secret laboratory.

In the first place, a partial consolidation of clay and sand, and even of gravel, may take place under the influence of pressure alone. Many of us are familiar with this truth, but few, perhaps, are aware how extensively it is illustrated in the practical arts of life. Here are some curious and interesting examples. The minute fragments of coal which are produced by the friction of larger blocks against one another, and which may be obtained abundantly in the neighborhood of every coal mine, are now manufactured into a solid patent fuel by the simple process of forcible compression. Again, the dust and rubble of black lead, formerly cast aside as useless, are now carefully collected, and by no other force than pressure are converted into a solid mass, fit to be employed in the manufacture of lead-pencils. “The graphite or black lead of commerce,” says Sir Charles Lyell, “having become very scarce, Mr. Brockedon contrived a method by which the dust of the purer portions of the mineral found in Borrowdale might be recomposed into a mass as dense and as compact as native graphite. The powder of graphite is first carefully prepared and freed from air, and placed under a powerful press on a strong steel die, with air-tight fittings. It is then struck several blows, each of a power of a thousand tons; after which operation the powder is so perfectly solidified that it can be cut for pencils, and exhibits, when broken, the same texture as native graphite.”[35] An instance yet more to our purpose occurs in the experiments made to try the force of gunpowder. Leathern bags filled with sand are put into the mortar that is to receive the cannon-ball at a distance of fifty feet from the mouth of the gun; and the sand is often compressed by the percussion of the ball into a solid mass of Sandstone.[36] Now the deposits of which we are speaking cannot fail to be subjected to a very powerful and a very constant compressing force. For, since the process of deposition is always going on, the matter which is deposited to-day will to-morrow be covered with a new layer, and in the course of ages it may lie beneath an immense pile of mineral matter, hundreds or even thousands of feet in thickness.

But in fact there is another and more important agent at work. When the harder and more compact blocks of Conglomerate and Sandstone are subjected to a close analysis in the laboratory of the chemist, it is found that they are strongly cemented together, sometimes by a solution of lime filling up the interstices between the grains or pebbles, sometimes by a solution of silica, sometimes by a solution of iron. Now this discovery affords a useful clue when we come to study the present operations of Nature. It is to the agency of a mineral cement we must look for the perfect consolidation of Mechanical Rocks. Let us see if such a cement can be found.

It is well known that the water of rivers, lakes, and springs, is more or less charged with carbonic acid gas; and therefore, when it comes in contact with limestone, it dissolves a portion of the lime and holds it in solution. Hence it follows that in every part of the world there exists an abundant store of calcareous cement. Again, our readers must have observed the brownish, rusty color sometimes produced by streams on the surface of rocks and herbage. This is the result of the iron with which the streams are impregnated: and we are informed by scientific inquirers that water containing a solution of iron prevails very generally in almost all countries. The solution of silica in water is not so common; because pure silica cannot be dissolved by water except at a very high temperature. Nevertheless, it has been clearly demonstrated by observation, that silica, where it occurs in certain combinations with other mineral substances, may be dissolved readily enough: for instance, in the decomposition of felspar, and of all rocks in which felspar is an ingredient, silica is carried off in a state of solution.[37] And since these rocks are very numerous, and distributed over every part of the earth, we may fairly conclude that a solution of silica exists very abundantly in nature.

Now when we bear in mind that we have on the one hand in the Crust of the Earth, solid strata of Conglomerate and Sandstone, exhibiting the evident operation of these mineral cements; and on the other hand, near the surface, the loose materials of Conglomerate and Sandstone as if ready to be cemented, and close at hand the cementing mineral itself in a convenient form, it is not unreasonable to assume that the process should actually take place;--that water highly charged with iron, or lime, or silica, should filter through the loose gravel and sand, depositing its mineral cement as it passes along, and converting the newly-formed strata into compact and solid rock.

But this conclusion does not rest upon antecedent probability alone. We have proof unquestionable that a process such as we have described is actually going on. In the dredging of the river Thames large masses of solid Conglomerate are found from time to time, firmly compacted together by a ferruginous cement. And there is internal evidence that the process of solidification has been effected by natural causes within historic times; for it happens not unfrequently that Roman coins and fragments of pottery are found embedded in the solid block of stone. Similar discoveries were made in deepening the bed of the river Dove in Derbyshire, about the year 1832. Thousands of silver coins were found about ten feet under the surface, firmly cemented into a hard Conglomerate. Several of these coins bear dates of the thirteenth and fourteenth centuries; and therefore the pebbles which form the rock must have been deposited and converted into a solid mass since that time. But we must not suppose that so long an interval is necessary for the consolidation of rocks. In the early part of the present century a vessel called the Thetis was wrecked off cape Frio on the coast of Brazil. A few months afterward, when an attempt was successfully made to recover the dollars and other treasures which had gone to the bottom with the wreck, they were found completely enveloped in solid masses of quartzose Sandstone. The materials of the newly-formed stone were in this case manifestly derived from the granite rocks of the Brazilian coast.[38]

In many parts of the Mediterranean, and along its shores, this process is known to be going on with equal rapidity. “The new-formed strata of Asia Minor,” writes Sir Charles Lyell, “consists of stone, not of loose, incoherent materials. Almost all the streamlets and rivers, like many of those in Tuscany and the south of Italy, hold abundance of carbonate of lime in solution, and precipitate Travertine, or sometimes bind together the sand and gravel into solid Sandstones and Conglomerates; every delta and sandbar thus acquires solidity, which often prevents streams from forcing their way through them, so that their mouths are constantly changing their position.”[39] In the Museum at Montpelier is exhibited a cannon embedded in a crystalline calcareous rock which was taken up from the bed of the Mediterranean near the mouth of the Rhone.[40]

To these examples of the solidification of rock within recent times we are tempted to add one more, taken from a Memoir published by the late Dr. Paris in the Transactions of the Royal Geological Society of Cornwall. “A sandstone occurs in various parts of the northern coast of Cornwall, which affords a most instructive example of a recent formation, since we here actually detect Nature at work in converting loose sand into solid rock. A very considerable portion of the northern coast of Cornwall is covered with calcareous sand, consisting of minute particles of comminuted shells, which in some places has accumulated in quantities so great, as to have formed hills of from forty to fifty feet in elevation. In digging into these sand-hills, or upon the occasional removal of some part of them by the winds, the remains of houses may be seen; and in places where the churchyards have been overwhelmed, a great number of human bones may be found. The sand is supposed to have been originally brought from the sea by hurricanes, probably at a remote period. It first appears in a state of slight but increasing aggregation on several parts of the shore in the Bay of St. Ives; but on approaching the Gwythian River it becomes more extensive and indurated.... It is around the promontory of New Kaye that the most extensive formation of Sandstone takes place. Here it may be seen in different stages of induration, from a state in which it is too friable to be detached from the rock on which it reposes, to a hardness so considerable that it requires a very violent blow from a sledge to break it. Buildings are constructed of it; the church of Cranstock is entirely built with it; and it is also employed for various articles of domestic and agricultural uses.”

No reasonable doubt can therefore remain that the loose beds of gravel, sand, and clay, which, as we have already seen, are deposited from day to day, and from year to year, and from century to century, beneath the waters of the ocean, may be converted in the course of time by natural agents into solid rocks of Conglomerate, of Sandstone, and of Shale. But this is not enough. It yet remains for us to explain how these solid rocks come to be arranged in a series of distinct layers or strata. The reader will remember that the supply of materials in any given area of the ocean is not fixed and continuous, but, on the contrary, variable and intermittent. During the periodical rains within the tropics, and during the melting of the snows in high latitudes or in mountain regions, the rivers become enormously swollen, and carry down a far greater quantity of sediment than at other seasons. The waste of cliffs, too, by the action of the waves, is much greater in winter than in summer. Thus, while at one season a particular river or current may be comparatively free from sediment, at another it will carry along in its turbid course an almost incredible freight of mineral matter. We have a notable example in the case of the Ganges. The bulk of earthy matter which this river discharges into the sea during the four months of rain, averages about 50,000,000 of cubic feet per day; whereas the daily discharge during the three months of hot weather is considerably less than one hundredth part of that amount.[41]

Besides this variety in the quantity of materials carried, there is also a great variety in the velocity both of rivers and of currents; and therefore they will not always carry the same materials to the same distance; for the less rapid the stream, the sooner will the sediment fall to the bottom. We may add that currents, as is well known, often change their direction from various causes, and thus at different times they will carry the waste of the land to different parts of the ocean.

From these considerations two conclusions may be fairly deduced: First, that the process of deposition may often go on very rapidly for a time over a given area, and then altogether cease, and after an interval begin again. In this way time may be allowed for one deposit to acquire more or less consistency before the next is superimposed; and thus a succession of distinct beds will be produced. Secondly, we may infer that the same precise materials will not always be deposited over the same area; at one time it will be sand, at another gravel, at another clay, at another some combination of these or other mineral substances. And thus it may happen that the strata deposited in successive periods of time shall not only be distinct one from the other, but composed of different materials;--that there shall be, in fact, as we so often see that there are, beds of Conglomerate, Sandstone, Clay, Marl, and other rocks, succeeding one another in every variety of order.

_CHAPTER VI._

STRATIFIED ROCKS OF MECHANICAL ORIGIN--FURTHER ILLUSTRATIONS.

_Impossible to witness the formation of stratified rocks in the
depths of the ocean--On a small scale examples are exhibited
by rivers and lakes--Alluvial plains--Their extraordinary
fertility--Great basin of the Nile--Experiments of the Royal
Society--The Mississippi and the Orinoco--Some rivers fill
up their own channels--Case of the river Po--Artificial
embankments--Large tract of alluvial soil deposited by the
Rhone in the Lake of Geneva--Deltas--The delta of the Ganges
and Brahmapootra--Delta of the Nile._

The argument set forth in the last chapter is simple, ingenious, and persuasive. Nay, we must fairly confess that to us it seems conclusive. We do not mean to say that it amounts to a rigorous demonstration. But it affords at least a strong presumption that the process of deposition, the process of consolidation, and the process of stratification, are going on to a vast extent beneath the waters of the ocean; and that, in these latter ages of the world’s history, Aqueous Rocks are slowly growing up under the influence of natural causes, which resemble in every important feature those that are now attracting so much attention within the Crust of the Earth. We are therefore prepared to accept this conclusion, if it be not found at variance with any well-established fact, or with any known and certain truth. But in matters of physical science the evidence of our senses is, after all, the most satisfactory argument. And our readers, no doubt, would like to witness, if possible, with their eyes, the building up of Stratified Rocks. Now, though it is not given to us to see this process in all its colossal magnitude as it goes on within the depths of the mighty ocean, it is yet possible to behold it exhibited, as it were, in miniature, in certain cases where the sediment of rivers is deposited within reach of observation.

Every one is familiar with the fact that many rivers overflow their banks at certain seasons, and spread themselves out over a wide area, sometimes reaching to the foot of the hills that bound the valleys through which they flow. This is the origin of those Alluvial Plains so remarkable for their surpassing richness and fertility. In each successive year a thin film of sediment is deposited on the surface of the land; and thus in the course of ages a soil is formed capable of producing, season after season, the most luxuriant crops without manifesting any symptoms of exhaustion. The soil of the Alluvial Plain near St. Louis, on the Mississippi, is thus spoken of by a modern traveller: “As to the quality of the land, any given number of crops might be grown off it. Corn has been raised on it for a hundred years together--as far back as the settlement is known. To inquire about the system of farming in the West is not productive of information which would be of service on the continent of Europe. There is no system: the farmer scratches the ground and throws in the seed, and his bountiful harvests come up year after year without further thought or trouble. Thousands of centuries have made the soil for him, and it defies him to make too heavy demands upon it. It gives him all he asks, and is never known to disappoint or fail.”[42]

The great basin of the Nile offers an admirable example of an Alluvial Plain on a scale of considerable magnitude. Even in the days of Herodotus, Egypt was regarded as the “gift of the Nile:” and the correctness of this opinion has been placed beyond all reasonable doubt by the investigations of modern science. The river bears along in its current, especially during the flood season, a large quantity of fine earthy sediment obtained by the process of Denudation from the mountains of central Africa. Once a year, between the months of July and November, it overflows its banks, and this sediment is deposited on the adjoining plains. Thus a new layer of rich soil is spread out every year over the existing surface; and the whole country is, in a manner, growing upward at the average rate, according to a rough estimate, of about six inches in the century. Near Cairo, where excavations have been made, the successive layers of annual deposit are distinctly visible to the eye. And it is worthy of remark that, although each one of these is no thicker than a sheet of paste-board, the stratum of alluvial soil which overlies the sands of the desert, and which to all appearance has come into existence by the very same process, is often forty, fifty, and even sixty feet in depth.

A series of interesting observations and experiments have been recently made under the auspices of the Royal Society, which afford some useful information on this subject. The colossal statue of Rameses, near Memphis, was found to be partly embedded in a stratum of mud which had gradually accumulated around it. Upon sinking a shaft, it was discovered that from the present surface of the plain to the base of the pedestal is a distance of nearly ten feet. Now, Rameses flourished, according to Lepsius, about one thousand three hundred and sixty years before the Christian Era; and therefore, since that time, or within a space of 3200 years, it is pretty clear that a thickness of ten feet has been added at this spot to the Alluvial Plain of the Nile. It is hard to resist the conclusion that the next stratum of ten feet as we proceed downward, which, in every respect, resembles the first, must have been produced in the same way by natural causes; and so on till we reach the barren sand of the desert, which is here just forty-two feet below the present level of the plain.[43]

It should seem, therefore, that Egypt is nothing more than a great Alluvial Plain, slowly built up in the long lapse of ages, by the annual inundations of the Nile. Vast tracts of the same kind are to be found in other parts of the world. The Mississippi, which drains about one-seventh of the whole North American continent, has formed an Alluvial Plain more than a thousand miles in length, and from thirty to eighty in breadth. And in South America, the Orinoco once a year spreads out its swollen and turbid waters over an area not unfrequently seventy miles broad; leaving behind, when it subsides, a substantial layer of muddy sediment to enrich the soil.[44] It would be easy to accumulate examples. But we shall be content with having referred the reader to the Great Basin of the Nile, which affords special opportunities for the study of alluvial phenomena; being illustrated at once by the historical monuments of remote antiquity and the scientific researches of recent times.

There is another process by which Alluvial Plains are formed. It often happens that a river fills up the channel in which it has been moving for years, and is forced to shift its course and seek a new passage to the sea. In progress of time this channel is filled up like the former and deserted, and then a third, and then a fourth. At each change a new stratum is formed, almost always distinguished for its extraordinary fertility. This phenomenon is chiefly to be looked for when an extensive and almost level plain lies between some lofty range of mountains and the sea. In such a case, the river which bears away the waste of the mountains, will move onward in its course with a sluggish current, and will, of necessity, deposit the greater part of its burden on the way. There is scarcely a country in the world that does not abound in formations of this kind; and we could point to many notable instances in which herds of cattle are now grazing on the very spot where, within quite recent times, the turbid waters of some great stream flowed sullenly along.

The river Po, which receives through a thousand mountain torrents an enormous quantity of mineral sediment from the Alps, affords an instructive example. Since the beginning of the fifteenth century it has many times changed its course, often committing great devastations, and always leaving behind unmistakable traces of its movements. Several towns that once stood on the left bank of the river are now on the right. In some instances parish churches and religious houses were pulled down when the devouring stream was seen slowly to approach, and then rebuilt with the same materials at a greater distance. An old channel may be easily recognized at the present day near Cremona, which bears the name of Po Morto, and another called Po Vecchio, in the territory of Parma.

It may be interesting to our readers to learn that these movements have been checked in modern times. By a system of artificial embankment the waters of the river are now confined within definite and narrow limits: thus the velocity of the current is increased and a very considerable portion of the sediment is carried on to the sea. Nevertheless, much is still deposited in the bed of the river, which is, in consequence, raised higher and higher each successive year. Hence it has become necessary, in order to prevent inundations, to add every season to the height of the embankments, so that the river now presents the appearance of an enormous aqueduct, of which some idea may be formed from the fact that, in the neighborhood of Ferrara, the surface of the stream is higher than the roofs of the houses. This system of embankment is carried on very extensively in Northern Italy to check the overflowing of rivers, and to prevent them from changing their courses. It is as old as the time of Dante, who tells us that the inhabitants of Padua erected barriers along the Brenta when the snows began to melt and the season of the floods was approaching,

“Per difender lor ville e lor castelli,
Anzi che Chiarentana il caldo senta.”

_Inferno_, Canto xv.

As a river sometimes fills up its own channel, so too may it fill up a lake through which it flows, and convert it likewise into a great Alluvial Plain. Thus it is said several extensive lakes have been transformed into dry land in modern times near Parma, Piacenza, and Cremona. Elsewhere the process may be seen in actual operation. The Rhone when it enters the lake of Geneva is a turbid discolored stream; the natural consequence of the immense quantity of earthy sediment with which it is charged. But as it slowly moves along, the sediment falls to the bottom, and when, at length, “by Leman’s waters washed,” it emerges at the town of Geneva, and shoots beneath the magnificent bridge that joins the opposite shores, it has already assumed that beautiful azure blue which travellers love to gaze on, and poets love to sing. The sediment left behind goes to form a great alluvial tract which is slowly but steadily advancing into the lake. An ancient town called Port Vallais, which, eight centuries ago, stood at the water’s edge, is now a mile and a half inland. And if the world were to last long enough, and the natural agents at present in operation were to remain unchanged, the time would come, we can scarcely doubt, when the whole lake of Geneva would have been converted into an Alluvial Plain of vast extent and inexhaustible fertility.

This last example leads us on to the phenomenon of Deltas, which afford, perhaps, the best opportunity of observing the actual formation of stratified rocks. Some large rivers, as we have already seen, enter the sea with such extreme velocity as to bear away their sediment to a distance of several hundred miles from the land. But in other cases the onward rush of the stream is much sooner arrested, and the sediment, if it be not caught up by ocean currents, is deposited near the mouth of the river, and forms a triangular tract of alluvial land. This kind of deposit is called a Delta, from the resemblance it bears to the letter (Δ) of that name in the Greek Alphabet. The apex of the triangle points up the stream, the base is toward the sea. Hence, when a Delta is formed the river naturally divides into two branches, one flowing to the right, the other to the left. In progress of time new channels are almost always made, and the great stream empties itself into the sea by many mouths.

The Delta formed in the Bay of Bengal by the two great rivers of India, the Ganges and the Brahmapootra, offers an illustration of this phenomenon on a scale of unusual magnitude. Indeed, strictly speaking, it is not one Delta only, but rather two Deltas lying side by side; the one deriving its origin from the Ganges, the other from the Brahmapootra. This double Delta extends its base for two hundred and fifty miles along the Bay of Bengal, and stretches inward into the continent of India to an almost equal distance. Here, then, is a vast tract of country manifestly composed of earthy sediment, obtained by the process of Denudation from the Himalayan mountains, and afterward transported to its present site by the agency of moving water. But the deposition of earthy matter does not suddenly come to an end when we reach the present line of the coast. The sea is visibly discolored by the sediment far beyond the actual base of the Delta; and a sloping bank of mud is found to stretch beneath the waters of the Bay to a distance of a hundred miles.

Even within the short period of a man’s life the domain of dry land is often visibly enlarged. Sandbanks are first formed in some of those numerous winding channels through which the two rivers find their way to the sea. The sandbanks, receiving fresh accessions during each succeeding flood, in a short time become islands; and the islands have been known, in a few years, to attain a superficial extent of many square miles. Then begins to appear a wild and luxuriant vegetation--reeds, long grass, shrubs, and trees; and those impenetrable thickets are formed, to which the buffalo, the rhinoceros, and the tiger soon resort for shelter. A very extensive tract of this kind, adjoining the sea-coast, and known as the Sunderbunds, is said to be as large as the principality of Wales.

The Delta of the Nile, though not quite one-half as large as the Delta of the Ganges, presents nevertheless some features of peculiar interest. In many places where a vertical section is exposed to view, the phenomenon of stratification may be distinctly recognized. The upper part of the deposit belonging to each year is composed of earth of a lighter color than the lower part; and the whole forms a distinct layer of hardened clay, which may be easily separated from those above and below. This formation, therefore, corresponds exactly with those strata of shale which we so often meet with in the Crust of the Earth. Again, many of the old channels through which the Nile made its way to the sea in ancient times, have been since filled up and converted into solid land. The two extreme arms of the river, which formerly enclosed the Delta, were two hundred miles apart where they entered the Mediterranean. But these channels are now Alluvial Plains, and the base of the Delta is but ninety miles in length. Hence, though the quantity of land which has been formed by the sediment of the Nile is much greater now than it formerly was, the size of the Delta properly so called has not been increased but diminished.

If we turn to the great continent of America, we are met by results not less striking and important. The Delta of the Mississippi is two hundred miles in length, and one hundred and forty in breadth. This vast stratum of mud is between five and six hundred feet thick, and covers an area twelve thousand square miles in extent. Each year it receives from the great _Father of Rivers_ a new accession of sediment which is computed at 3,700,000,000 of cubic feet. And besides this annual deposit of inorganic matter, we must not omit from our estimate the countless trees of various species and of gigantic size, which are torn up by the floods, carried along by the impetuous stream, and buried at last with the bones of animals, and works of human art, and other spoils of the land, in the mud of the Delta at the river’s mouth.[45]

_CHAPTER VII._

STRATIFIED ROCKS OF CHEMICAL ORIGIN.

_Chemical agency employed in the formation of mechanical
rock--But some rocks produced almost exclusively by the
action of chemical laws--Difference between a mixture
and a solution--A saturated solution--Stalactites and
Stalagmites--Fantastic columns in limestone caverns--The grotto
of Antiparos in the Grecian Archipelago--Wyer’s cave in the
Blue Mountains of America--Travertine rock in Italy--Growth of
limestone in the Solfatara Lake near Tivoli--Incrustations of
the Anio--Formation of travertine at the baths of San Filippo
and San Vignone._

The Aqueous Rocks of which we have spoken in the last two chapters are called by Geologists Mechanical; inasmuch as they owe their existence chiefly to the agency of Mechanical force. It should be observed, however, that a very considerable share in the production of these rocks must be ascribed, not unfrequently, to Chemical influence. Chemical action helps to prepare the materials of which they are composed; and Chemical action likewise furnishes the calcareous, siliceous, and other mineral cements by which they are, in a great measure, consolidated. There is, however, a second class of Aqueous Rocks which are produced almost exclusively by the operation of Chemical laws, and which we have accordingly denominated Stratified Rocks of Chemical Origin. It is of these that we purpose to speak in the present chapter. They constitute a much smaller proportion of the Earth’s Crust than either the Mechanical or the Organic Rocks. But the history of their formation is curious and instructive. We shall confine ourselves to one or two simple and familiar illustrations.

In the course of these illustrations we shall have a good deal to say about Carbonate of Lime in a state of solution; and it may perhaps be useful to explain, first of all, what is meant by a solution, in the technical language of Chemistry. If a spoonful of salt is put into a tumbler of water, the particles of salt, after a little time, cease to cohere together, and become so diffused through the water as to be no longer visible to the eye, although their presence in every part may be easily discerned by the taste. The salt is then said to be _dissolved_, and the water in which it is dissolved is called a _solution_ of salt. It is important to distinguish the case of a solution from the case of a mere mechanical mixture. If, instead of the salt, we were to put into the tumbler of water a spoonful of very fine sand, then we should have a _mixture_ but not a _solution_. By stirring briskly the contents of the tumbler we might, indeed, effect a very close union between the particles of water and the particles of sand: but this union would be altogether different in kind from the union that was observed in the former case between the particles of water and the particles of salt. First, the sand would remain visible to the eye, making the water turbid and discolored; whereas the salt entirely disappeared, leaving the water limpid and transparent as before. Again, if the water be allowed to rest, the sand will in time fall to the bottom, whereas the salt will not.

But there is a limit to the capacity of water for holding salt in solution. If spoonful after spoonful be added, it will be found, when a certain point has been reached, that the water can at length dissolve no more. It is then called a _saturated solution_ of salt. If, in this case, a portion of the water were to pass away by evaporation, it is clear, we should have the same quantity of salt as before, in a smaller quantity of water. The consequence would be that _all_ the salt could not then be held in solution, and some of it would fall to the bottom; or, in chemical language, a precipitate of salt would be formed on the bottom of the tumbler. Now, according to the theory of Geologists, many rocks, hundreds of feet thick, and solid enough to form the walls of our palaces, our churches, and our castles, have been produced in the Crust of the Earth by just such a process as this. In support of their theory we are about to show that the process is actually going on in our own time, and is open to the examination of all who may desire to study it for themselves.

We shall begin with the formation of Stalactites and Stalagmites. The mode in which these singular masses of rock are brought into existence is very clearly explained, and the picturesque appearance they so often present to the eye is very graphically described, by Dr. Mantell, in his Wonders of Geology, from which the following passages are taken:--“One of the most common appearances in limestone caverns is the formation of what are called Stalactites, from a Greek word signifying distillation or dropping. Whenever water filters through a limestone rock it dissolves a portion of it; and on reaching any opening, such as a cavern, oozes from the sides or roof, and forms a drop, the moisture of which is soon evaporated by the air, and a small circular plate or ring of calcareous matter remains; another drop succeeds in the same place, and adds, from the same cause, a fresh coat of incrustation. In time, these successive additions produce a long, irregular, conical projection from the roof, which is generally hollow, and is continually being increased by the fresh accession of water, loaded with calcareous or chalky matter: this is deposited on the outside of the Stalactite already formed, and, trickling down, adds to its length by subsiding to the point, and evaporating as before; precisely in the same manner as, during frosty weather, icicles are formed on the edges of the eaves of a roof. When the supply of water holding lime in solution is too rapid to allow of its evaporation at the bottom of the Stalactite, it drops on the floor of the cave, and drying up gradually, forms in like manner a Stalactite rising upward from the ground, instead of hanging from the roof; this is called for the sake of distinction Stalagmite.

“It frequently happens, where these processes are uninterrupted, that a Stalactite hanging from the roof, and a Stalagmite formed immediately under it from the super-abundant water, increase until they unite, and thus constitute a natural pillar, apparently supporting the roof of the grotto. It is to the grotesque forms assumed by Stalactites and these natural columns, that caverns owe the interesting appearances described in such glowing terms by those who witness them for the first time. One of the most beautiful stalactitic caverns in England is at Clapham, near Ingleborough. In the Cheddar Cliffs, Somersetshire, there has been discovered a similar cave richly incrusted with sparry concretions. There are others in Derbyshire.

“The grotto of Antiparos in the Grecian Archipelago, not far from Paros, has long been celebrated. The sides and roof of its principal cavity are covered with immense incrustations of calcareous spar, which form either Stalactites depending from above or irregular pillars rising from the floor. Several perfect columns reaching to the ceiling have been formed and others are still in progress, by the union of the Stalactite from above with the Stalagmite below. These, being composed of matter slowly deposited, have assumed the most fantastic shapes; while the pure, white, and glittering spar beautifully catches and reflects the light of the torches of the visitors to this subterranean palace, in a manner which causes all astonishment to cease at the romantic tales told of the place--of its caves of diamonds and of its ruby walls; the simple truth, when deprived of all exaggeration, being sufficient to excite admiration and awe.

“Sometimes a linear fissure in the roof, by the direction it gives to the dropping of the lapidifying water, forms a perfectly transparent curtain or partition. A remarkable instance of this kind occurs in a cavern in North America called Wyer’s Cave. This cave is situated in a ridge of limestone hills running parallel to the Blue Mountains. A narrow and rugged fissure leads to a large cavern, where the most grotesque figures, formed by the percolation of water through beds of limestone, present themselves, while the eye, glancing onward, watches the dim and distant glimmers of the lights of the guides--some in the recess below, and others in the galleries above. Passing from these recesses, the passage conducts to a flight of steps that leads into a large cavern of irregular form and of great beauty. Its dimensions are about thirty feet by fifty. Here the incrustations hang just like a sheet of water that was frozen as it fell; there they rise into a beautiful stalactite pillar; and yonder compose an elevated seat, surrounded by sparry pinnacles. Beyond this room is another more irregular, but more beautiful; for besides having sparry ornaments in common with the others, the roof overhead is of the most admirable and singular formation. It is entirely covered with Stalactites, which are suspended from it like inverted pinnacles; and they are of the finest material, and most beautifully shaped and embossed. In another apartment an immense sheet of transparent Stalactite, which extends from the floor to the roof, emits, when struck, deep and mellow sounds like those of a muffled drum.

“Farther on is another vaulted chamber, which is one hundred feet long, thirty-six wide, and twenty-six high. Its walls are filled with grotesque concretions. The effect of the lights placed by the guides at various elevations, and leaving hidden more than they reveal, is extremely fine. At the extremity of another range of apartments, a magnificent hall, two hundred and fifty feet long, and thirty-three feet high, suddenly appears. Here is a splendid sheet of rock-work running up the centre of the room, and giving it the aspect of two separate and noble galleries. This partition rises twenty feet above the floor, and leaves the fine span of the arched roof untouched. There is here a beautiful concretion, which has the form and drapery of a gigantic statue; and the whole place is filled with stalagmitical masses of the most varied and grotesque character. The fine perspective of this room, four times the length of an ordinary church, and the amazing vaulted roof spreading overhead, without any support of pillar or column, produce a most striking effect. In another apartment, which has an altitude of fifty feet, there is at one end an elevated recess ornamented with a group of pendant Stalactites of unusual size and singular beauty. They are as large as the pipes of a full-sized organ, and ranged with great regularity: when struck they emit mellow sounds of various keys, not unlike the tones of musical glasses. The length of this extraordinary group of caverns is not less than one thousand six hundred feet.”

In the case of Stalactites and Stalagmites the actual formation of limestone by the influence of Chemical action is brought home forcibly to the mind, and, in a manner, made palpable to the senses. We shall now pass to other examples in which the process is scarcely less open to observation, and in which the limestone assumes a somewhat more massive and rock-like form. Every one who has been in Italy is familiar with the limestone rock called Travertine. It is seen in the ancient walls and the venerable temples of Pæstum, which have withstood unharmed the wasting hand of time for upward of twenty centuries. In Rome, too, this stone is associated in our minds as well with the enduring monuments of antiquity, as with the imposing splendor of Christian art. The Coliseum, the most stupendous of ruins, and St. Peter’s, the most sublime of temples, are built of Travertine. In fact it seems to have been, in every age, the chief building stone employed in the architecture of the Eternal City; and the quarries from which it was taken in ancient times may still be seen at Ponte Lucano, near Tivoli. Now it is an interesting fact, that close to this very spot, at the Solfatara lake on the one side, and at Tivoli itself on the other, the formation of Travertine is going on in our own time, by the precipitation of lime from a state of solution.

The Solfatara lake, situated about fourteen miles from Rome, on the road to Tivoli, is supplied with an unfailing stream of tepid water, impregnated with carbonic acid gas and saturated with carbonate of lime. The amount of carbonate of lime which the water is capable of holding in solution depends chiefly on three things: first, on the presence of carbonic acid; secondly, on the high temperature of the water; and thirdly, on its quantity. Now the carbonic acid is ever rising in bubbles to the surface and passing away; the temperature of the water is lowered by contact with the cooler atmosphere; and its quantity is diminished by evaporation. Thus the capacity which the water at first had for holding the carbonate of lime in solution is notably diminished, and a part of the lime is precipitated to the bottom in a solid form, or clings to the vegetable matter with which it comes in contact.

A very simple and interesting experiment, made in the early part of the present century by Sir Humphrey Davy, will illustrate the rapidity with which the formation of solid stone is even now taking place. In the month of May he fixed a stick in the bed of the lake, and left it standing until the following April, when he found that it was covered with an incrustation of limestone several inches thick.[46] In precisely the same way new layers of Travertine are annually deposited in the bed of the lake, and incrusted on its rocky margin; and so the lake itself is becoming smaller and smaller from year to year. We are told that in the middle of the seventeenth century it was a mile in circuit, and now it is a little more than a quarter of a mile.[47] Here, therefore, we have an immense mass of compact limestone rock, built up by natural agents within the last two centuries.

At Tivoli, about four miles beyond the Solfatara, and two miles from the quarries of Ponte Lucano, phenomena of the same kind are exhibited. The waters of the Anio, which are saturated with carbonate of lime, form incrustations of Travertine on the banks of the river; and at the celebrated falls, where the whole volume of the stream leaps at a bound from a height of three hundred and twenty feet, the most beautiful stalactites are formed by the foam.

The formation of Travertine is going on with no less activity in other parts of the Italian Peninsula. At the baths of San Filippo, in Tuscany, there are three warm springs which contain a very large amount of mineral matter in solution. The water which supplies the baths falls into a pond, where it has been known to deposit a solid stratum of rock _thirty feet thick_ in twenty years. In the same neighborhood are the mineral baths of San Vignone. The source from which the water flows is situated on the summit of a hill not more than a few hundred yards from the high road between Sienna and Rome; and so rapid is the formation of stone, that half a foot of solid Travertine is deposited every year in the pipe that conducts the water to the baths. At this spot we have a very good illustration of the argument we are now considering. As the stream of water flows down the slopes of the hill, a thin layer of Travertine rock is produced on the surface of the earth, almost before our eyes; and so it was previous to our own time, and so it has been for ages, as history and tradition testify. The quantity produced in each year and in each century is comparatively small, but we can have no doubt that it _has_ been produced by the means described. Now, beneath the surface of the Earth, immediately below these modern formations, of which we have so clearly ascertained the origin, we find strata of the same kind, composed of the same materials, and arranged in the same way, layer resting upon layer, down to a depth of two hundred feet: and the Geologist accounts for the formation of the one according to the same laws which he has seen at work in the production of the other.[48]

_CHAPTER VIII._

STRATIFIED ROCKS OF ORGANIC ORIGIN--ILLUSTRATIONS FROM ANIMAL LIFE.

_Nature of organic rocks--Carbonate of lime extracted from the
sea by the intervention of minute animalcules--Chalk rock--Its
vast extent--Supposed to be of organic origin--A stratum of
the same kind now growing up on the floor of the Atlantic
ocean--Coral reefs and islands--Their general appearance--Their
geographical distribution--Their organic origin--Structure
of the zoophyte--Various illustrations--Agency of the
zoophyte in the construction of coral rock--How the sunken
reef is converted into an island and peopled with plants and
animals--Difficulty proposed and considered--Hypothesis of Mr.
Darwin--Coral limestone in the solid crust of the earth._