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.
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Theargument 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 satisfactoryargument. 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 exampleof 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.44It 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 chieflyto 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 ofthe 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.
“Per difender lor ville e lor castelli,Anzi che Chiarentana il caldo senta.”
“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 atpresent 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 movingwater. 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 twohundred 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 greatFather of Riversa 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
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.
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TheAqueous 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 proportionof 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 bedissolved, and the water in which it is dissolved is called asolutionof 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 amixturebut not asolution. 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 asaturated solutionof salt. If, in this case, a portion of the water were to pass away by evaporation, it is clear, weshould have the same quantity of salt as before, in a smaller quantity of water. The consequence would be thatallthe 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 itslength 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 ceaseat 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. Itswalls 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 unharmedthe 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 untilthe following April, when he found that it was covered with an incrustation of limestone several inches thick.46In 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.47Here, 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 rockthirty feet thickin 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. Asthe 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 ithasbeen 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
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.
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Wenow pass to the third division of Aqueous Rocks, those, namely, which are believed to have come into existence chiefly through the agency of animal and vegetable life, and are therefore called Organic. The study of these rocks has been prosecuted with no inconsiderable ardor during the last thirty years; and the facts which have been brought to light are certainly amongst the most curious and interesting in the whole range of physical science. Indeed we are convinced that a simple narrative of the researches which have recently been made upon this subject, and the discoveries to which these researches have led, would be no less attractive, and scarcely less wonderful, than a fairy tale. But it is not for us to wander at large over this vast and tempting field of inquiry. Wemust be content with one or two examples, which may help to illustrate the process of inductive reasoning upon which the general principles of geological science are founded.
It is argued, then, that the present operations of Nature afford the best key for the interpretation of her works in bygone times. We observe various beds of rocks now in course of formation on the surface of the Earth; and within the Crust of the Earth we discover corresponding strata of the self-same rock already complete, and laid by, as it were, in Nature’s storehouse. Side by side, therefore, we may study and compare the finished work and the work that is yet in progress; and if, on a close examination, they are found to agree in all essential characters, we have doubtless a strong presumption, that the same causes which are now producing the one, must in former times have produced the other. This line of argument we have already considered in reference to those two classes of Aqueous Rocks, which are said to be respectively of Mechanical and of Chemical origin. We now proceed to show that it is no less applicable to those which are called Organic. And although we may not hope to unfold all the secret wonders of Nature’s laboratory, that have come to light in recent times, yet we may afford a passing glimpse at her operations, which can scarcely fail to be interesting and instructive.
We have shown how strata of solid rock are sometimes formed in lakes by the precipitation of lime from a state of solution. Now this process cannot take place in the sea; for though lime is present in the sea, the quantity of carbonic acid with which it is there associated, is far more than sufficient to render its precipitation impossible.49But Nature has another contrivance for gathering together the solid elements of her building. The depths of the ocean are teeming with life; and countless tribes of minuteanimals are furnished with the power of extracting the lime from the waters they inhabit, and of reproducing it under a new form. Sometimes, through this mysterious operation of organic life, the lime is converted into a calcareous shell, like that of the oyster; sometimes into a stony skeleton, as in the case of the numerous families of coral-producing animalcules. After death the soft, fleshy substance of these animals melts away and disappears; but the limestone shells and skeletons remain, accumulating during the long course of ages to an almost incredible extent. And, if we are to believe Geologists, out of these accumulated materials, sometimes preserving their original form and structure, sometimes altered more or less by chemical action, sometimes broken up into fragments by mechanical force, has been produced a very large proportion of the limestone rocks which occur so abundantly in the Crust of the Earth.
No better illustration can be found than the white earthy limestone, familiar to every one under the name of chalk. An undulating stratum of Chalk Rock, attaining not unfrequently a thickness of one thousand feet, may be said, speaking roughly, to underlie the southeastern half of England. Sometimes it appears at the surface: sometimes it dips downward, and forms a kind of great basin, over which are regularly spread out various other groups of Stratified Rocks. On the southern coast it rises to a height of several hundred feet above the level of the sea in a line of perpendicular cliffs, conspicuous from a distance by their dazzling whiteness. But the White Chalk of England is only an insignificant part of a great rock-formation, which may be traced over extensive areas throughout all Europe, from Ireland to the Crimea, from the Baltic Sea to the Bay of Biscay; and which everywhere preserves in a remarkable degree the same mineral character, and presents to the eye the same general appearance.
Now it had often been suggested by Geologists that thiswide-spread formation derived its existence chiefly from the accumulated remains of organic life. For in many instances the broken shells of minute animalcules could be distinctly observed to constitute a part of the rock. And even where the organic structure could not be so clearly traced, the carbonate of lime composing the Chalk presented just that appearance which would naturally result from the decomposition of such shells. This theory, however, was long put forward with diffidence and received with incredulity. Even scientific men found it hard to persuade themselves that a solid rock of such great extent and thickness could have been the work of agents apparently so insignificant. But it has been confirmed and illustrated in a very interesting and unexpected manner within the last few years.
When the project of connecting Europe and America by a telegraph cable was first set on foot, it became necessary to ascertain, as far as possible, the general configuration of the ocean bottom and the exact nature of the bed on which the cable was to lie. Accordingly in the year 1857 an expedition was fitted out for this purpose under the command of Captain Dayman; and a careful series of soundings was taken between Valentia, on the West Coast of Kerry, and Trinity Bay on the shores of Newfoundland. It was found that the floor of the ocean between Ireland and America is a vast irregular plain, and that by far the greater part is covered over with a kind of soft mud or ooze. Samples of this ooze were scooped up, even at the most profound depths, by means of an ingenious apparatus attached to the sounding-lines, and brought undisturbed to the surface. Afterward they were carried home to England and submitted for examination to Professor Huxley. The result has been to show that the materials of a limestone rock, resembling in every essential feature the White Chalk of Europe, are being spread out at the present day over an area of immense extent on the floor of the Atlantic Ocean.
With the permission of our readers we shall allow Professor Huxley, as far as may be, to tell his own story.50As to the ocean floor itself, “It is,” he says, “a prodigious plain—one of the widest and most even plains in the world. If the sea were drained off, you might drive a wagon all the way from Valentia to Trinity Bay. And, except upon one sharp incline about two hundred miles from Valentia, I am not quite sure that it would even be necessary to put the skid on, so gentle are the ascents and descents upon that long route. From Valentia the road would lie down hill for about two hundred miles to the point at which the bottom is now covered by 1700 fathoms of sea-water. Then would come the central plain, more than a thousand miles wide, the inequalities of the surface of which would be hardly perceptible, though the depth of water upon it now varies from 10,000 to 15,000 feet; and there are places in which Mont Blanc might be sunk without showing its peak above water. Beyond this the ascent on the American side commences, and gradually leads for about three hundred miles, to the Newfoundland shore.”
The central plain here described, which has been since found to extend many hundred miles north and south of the cable line, is covered almost everywhere by that soft, mealy sort of mud of which we have already spoken; and this, it is now confidently believed, is nothing else than a stratum of Chalk Rock in an early stage of formation. When thoroughly dried it assumes a whitish color, and exhibits a texture which even to the superficial observer appears closely to resemble fine chalk. Nay, we are told that if so disposed, one may take a bit of it in his fingers and write with it upon a blackboard. Like chalk, too, when chemically analyzed it is found to be almost pure carbonate of lime.
But there is a yet more striking analogy between the mud of the Atlantic and the White Chalk of Europe. Both have been submitted to the magnifying power of the Microscope; and, after an examination conducted with scrupulous care, a wonderful and almost startling identity of mineral, or rather we should say of organic, composition has been established between them. To the naked eye Chalk is simply a soft, earthy sort of stone. But when a thin transparent slice is placed under the Microscope, the general mass is found to be made up of very minute particles, in which are embedded a vast number of other bodies possessing a well-defined form and structure. These are of various sizes, but on a rough average may be said not to exceed a hundredth of an inch in diameter. Hundreds of thousands of them are sometimes contained in a cubic inch of Chalk, together with countless millions of the more minute granules.
Professor Huxley succeeded in separating these bodies from the mass of granules in which they were embedded, and by examining them apart, he has ascertained still more fully their exact structure and composition. “Each one of them,” he says, “is a beautifully constructed calcareous fabric, made up of a number of chambers communicating freely with one another. They are of various forms. One of the commonest is something like a badly-grown raspberry, being formed of a number of nearly globular chambers of different sizes congregated together. It is called Globigerina; and some specimens of Chalk consist of little else than Globigerinæ and granules.”
Previous to 1857 the Globigerinæ of the Chalk were a matter of no small controversy among Geologists and Naturalists. Some contended that they were the organic remains—the shells or skeletons—of ancient animalcules. Others were disposed to regard them simply as aggregations of lime, which, so to speak, chanced to assume the form ofthese little chambered bodies; though it was not easy to explain, on this hypothesis, how these chance concretions, however much they varied in size, preserved over the whole of Europe the same exact form and structure. But the controversy is now at an end. The specimens of the Atlantic ooze brought home by Captain Dayman, when examined under the higher powers of the Microscope, are found, like Chalk, to be composed almost entirely of Globigerinæ. And that no doubt may remain as to their organic origin, a portion of the fleshy integument of the little animalcules is seen, in many cases, still adhering to the calcareous skeleton.
“Globigerinæ of every size,” we are told, “from the smallest to the largest, are associated together in the Atlantic mud, and the chambers of many are filled by a soft animal matter. This soft substance is, in fact, the remains of the creature to which the Globigerina shell, or rather skeleton, owes its existence—and which is an animal of the simplest imaginable description. It is, in fact, a mere particle of living jelly, without defined parts of any kind—without a mouth, nerves, muscles, or distinct organs; and only manifesting its vitality to ordinary observation by thrusting out and retracting, from all parts of its surface, long filamentous processes which serve for arms and legs. Yet this amorphous particle, devoid of everything which, in the higher animals we call organs, is capable of feeding, growing, and multiplying; of separating from the ocean the small proportion of carbonate of lime which is dissolved in sea-water; and of building up that substance into a skeleton for itself, according to a pattern which can be imitated by no other known agency.”
That the same process is going on in other parts of the ocean appears by observations made by Sir Leopold M’Clintock during the cruise of the Bulldog in 1860. He discovered that a calcareous ooze having the consistencyof putty is spread out over extensive areas between the Faroe Islands and Iceland, and also between Iceland and Greenland. Of this mud about ninety-five per cent. is composed of Globigerinæ, which in some instances were brought up actually living to the surface, and busily engaged in secreting, by their vital powers, carbonate of lime from the waters of the sea.51
Professor Huxley goes yet one step further in following out the resemblance between the Chalk Rock that exists in the Crust of the Earth and the stratum of Chalk that is now growing up in the depths of the Atlantic. Not only are the Globigerinæ, of which the one is in great part composed, identical with the animalcules that make up about nine-tenths of the other, but even the minute granules that constitute the residue of each formation, correspond in a very remarkable manner. “In working over the soundings collected by Captain Dayman, I was surprised to find that many of what I have called the Granules of that mud were not, as one might have been tempted to think at first, the mere powder and waste of Globigerinæ, but they had a definite form and size. I termed these bodies Coccoliths, and doubted their organic nature. Doctor Wallich verified my observation, and added the interesting discovery that, not unfrequently, bodies similar to these Coccoliths were aggregated together into spheroids, which he termed Coccospheres. So far as we knew, these bodies, the nature of which is extremely puzzling and problematical, were peculiar to the Atlantic soundings.
“But a few years ago Mr. Sorby, in making a careful examination of the Chalk by means of thin sections and otherwise, observed, as Ehrenberg had done before him, that much of its granular basis possesses a definite form. Comparing these formed particles with those in the Atlantic soundings, he found the two to be identical; and thusproved that the Chalk, like the soundings, contains these mysterious Coccoliths and Coccospheres. Here was a further and a most interesting confirmation, from internal evidence, of the essential identity of the Chalk with modern deep-sea mud.”
We may, therefore, set it down as certain, first, that the formation of Chalk Rock is going on very extensively at the present day; and secondly, that the chief agency employed in its production is no other than the vital action of minute animalcules. This is no longer merely a plausible theory or an ingenious hypothesis: it is simply a matter of fact ascertained by direct observation. If then it is just and philosophical to ascribe like effects to like causes, the conclusion is plain that the White Chalk of Europe came into existence in some far distant age by just such a process as that which is now in operation on the bed of the Atlantic Ocean.
From the Chalk mud of the Atlantic we will now pass to the Coral Reefs that are growing up beneath the waters of the Pacific and the Indian Oceans. Every one has heard of Coral Reefs and Coral Islands; yet we fancy many persons have but vague and indefinite notions about them. We shall, therefore, in the first place, give a brief account of their general appearance, their extent, and their geographical distribution. Afterward we shall give some of the evidence which goes to show that these huge masses of rock owe their existence to the organic powers of minute living animalcules.
The Coral Reef is familiar to the navigator of tropical seas under a great variety of forms, and in many different stages of development. In one case it is a chain of hidden rocks rising not quite to the level of the sea; in another it appears just above the waters, but is washed over by each returning tide; while in another it rises up beyond the reach of the waves, is clothed with luxuriant vegetation,and inhabited by various species of animals, even by man himself. Again there is great diversity of outline among these rocks, whether they are sunk beneath the surface of the waters or lifted above them. But all may be reduced to four classes, of which we propose to give a short description.
First is the Atoll, or lagoon island. It is a circular strip of limestone rock enclosing a shallow lake within, and surrounded by a deep and often unfathomable ocean without. The scene presented by some of these circular reefs is described by travellers as equally striking for its singularity and its beauty. “A strip of land a few hundred yards wide is covered by lofty cocoa-nut trees, above which is the blue vault of heaven. This band of verdure is bounded by a beach of glittering white sand, the outer margin of which is encircled with a ring of snow-white breakers, beyond which are the dark heaving waters of the ocean. The inner beach encloses the still clear water of the lagoon, resting in its greater part on white sand, and, when illuminated by a vertical sun, of a most vivid green.”
These lagoon islands are often found in groups stretching, with little interruption, for many hundred miles across the ocean. The Maldives, for example, which lie a little distance to the southwest of Hindostan, form a continuous chain, running due north and south, four hundred and seventy miles in length and fifty miles in breadth. Each successive link in this chain does not consist, as might be supposed, of a single circular reef, but it is rather a ring of small coral islets, sometimes more than a hundred in number, each of which is itself a perfect Atoll or lagoon island such as we have just described. Of these miniature islets many are from three to five miles in diameter; while the larger rings of which they form a part are from thirty to fifty. The Laccadive islands, a little more to the north, exhibit a similar arrangement, and indeed would seem tobe a continuation of the same group. In the Pacific are found some chains of coral islands yet more extensive; as for instance the Dangerous Archipelago, which is upward of eleven hundred miles in length, and from three to four hundred in breadth; but the islands within these spaces are thinly scattered, and insignificant in size.
Sometimes the annular strip of coral rock encloses within itself a lofty island, which rises up from the centre of the lagoon. In this case it is called an Encircling Reef; the lagoon being simply a broad channel surrounding the island in the centre, and encompassed itself by the coral rock. An example occurs in the island of Vanikoro, celebrated for the shipwreck of La Peyrouse, where the Encircling Reef runs at a distance of two or three miles from the shore, the channel between it and the land having a general depth of between two and three hundred feet. The well-known mountainous island of Tahiti in the South Pacific Ocean is also encompassed by an Encircling Reef, from which it is separated by a broad belt of tranquil water.
A third class of Coral Reefs consists of those which run parallel to the shores of continents or great islands, from which they are cut off by a broad channel, to which the sea has free access through certain open passages in the rock. They are called Barrier Reefs; and differ from the former only in this, that they do not surround the land, but run parallel to it at a distance of some miles. The Great Barrier Reef of Australia offers a noble example. It has been described as a huge, massive, submarine wall or terrace, fronting the northeastern coast of that continent, varying from ten to ninety miles in breadth, and extending, with some trifling interruptions, to a length of 1250 miles. Another reef of the same kind, 400 miles in length, faces the western coast of the long narrow island of New Caledonia.
When a chain of Coral rocks approaches close to theshore, so as to leave no intervening channel of deep water, they are called Fringing Reefs; and these constitute the fourth and last class of the Coral formation. They prevail everywhere in tropical regions, and appear as banks of Coral encrusting the rocky shores of islands and continents.
As regards the geographical distribution of Coral Reefs, the first circumstance that claims our notice, is that they are exclusively confined to the warmer regions of the globe. They exist in great profusion within the tropics, and are rarely to be found beyond the thirtieth parallels of latitude on each side of the Equator. The only remarkable exception is in the case of the Bermuda Islands in 32° north latitude; but here, it is to be observed, the ocean is warmed by the waters of the Gulf Stream. Another singular fact is the almost total absence of Coral Reefs from the Atlantic Ocean. In fact, the Bermudas, we believe, constitute here again the only exception. The Pacific, on the contrary, is wonderfully productive of coral; also the Indian Ocean, the Persian and Arabian Gulfs, and the Red Sea.
It may gratify, perhaps, the curiosity of some readers, if we add a word on the Red Coral which is now so favorite an ornament in the fashionable world. Though it never attains to the magnitude of those reefs and islands we have been describing, it partakes nevertheless of the same peculiar structure; and no doubt is entertained that, like them, it derives its existence from animal life, in the manner we shall presently explain. It is produced chiefly in the Mediterranean, in the Red Sea, and in the Persian Gulf; and is brought up from the great depths by means of a grappling apparatus attached to boats. The largest pieces have a shrub-like branching form, and are supposed to grow to the height of one foot in about eight years.52
So much for the existence of the Coral Formation. Next comes the question of its origin, with which, of course, we are chiefly concerned. It is now the received belief of all distinguished Naturalists, that these huge and wide-spread masses of limestone rock, against which the breakers of the ocean are ever thundering in vain, are the work of tiny marine animalcules, and chiefly of those seemingly insignificant creatures known by the name of Polyps or Zoophytes. The Zoophyte, they tell us, is a mason who himself produces the stones that he employs in his building. “He has neither plane, nor chisel, nor trowel; there is no sound of hammer in his city. He erects mighty and enduring edifices, yet has no mechanical power by which to raise his rocks to their summits. He can answer thee nothing—no tongue, no eyes, no hands, no brains has he—yet from the caves of old ocean has he raised that which fills you with admiration.”53Surely if all this be true, these countless myriads of animalcules call aloud to us from the depths of the ocean in language that cannot be mistaken: “Know ye that the Lord He is God; it is He that hath made us, and not we ourselves.”54
The Zoophyte belongs to the simplest form of the animal creation. Its body consists merely of a pouch or stomach, with tentacles arranged round the margin, which it can extend at pleasure to supply itself with food. In many species the individuals grow together on a common stem, from which new members are constantly shooting forth like buds from the branches of a tree. Hence the origin of the name Zoophyte, which literally means a plant-like animal. The common stem on which they grow is sometimes composed of a horny substance, but more generally it is pure carbonate of lime, which they secrete by the powers oforganic action from the waters of the sea. It forms, therefore, a kind of internal skeleton or framework, to which the soft, gelatinous parts of the animal adhere, pretty much as, in the case of other animals, the flesh adheres to the bones. Thus we have, as it were, a community of living creatures, growing together upon one common stony framework, called a Polypidom or Polyp edifice, which they themselves build by the very fact of living.