Popular Science Monthly/Volume 8/December 1875/On a Piece of Limestone
| ON A PIECE OF LIMESTONE.[1] |
By WILLIAM B. CARPENTER, LL. D., F. R. S.
IN selecting a subject for the lecture which, at the request of the council of the British Association, I undertook to give you during its present meeting, I have been guided by the desire to tell you something that would be new to you in regard to matters with which you are already familiar, and to connect this with the results of my own deep-sea researches, in which I might hope that my own local connection with Bristol would lead you to feel somewhat of a personal interest.
In the rocks that border the Avon on either side, the Bristolian has one of the most characteristic examples of limestone that can be anywhere found; and he has only to go as far as the deep gorge of Cheddar, in the Mendip hills, to find limestone cliffs yet more imposing in height than St. Vincent's rocks; or as far as Chepstow, to see, along the Wye to Tintern Abbey, a still more varied and picturesque display of the same great rock-formation. Its material is sometimes distinguished as the mountain limestone, on account of the rugged character it imparts to the districts in which it prevails; while it is now more commonly known as the carboniferous (coal-bearing), because it forms the basins or troughs in which the "coal-measures" lie. Now, if you look at a geological map of England, you will trace this lime-stone as a band lying obliquely northeast and southwest; beginning in Northumberland, passing through Durham and Yorkshire, through Derbyshire (where it forms the romantic scenery about Matlock), then through the midland counties (where, however, it is generally covered up by later formations), and then into Gloucestershire and South Wales, where its relation to the coal-basins is most distinctly marked. Speaking generally, this oblique band divides England into two great areas: one to the northwest, in which the strata that have been brought to the surface, by the crumpling action that has disturbed the crust of the earth during its cooling, are older than the carboniferous lime-stone; the other to the southeast, in which the strata are newer. You have not to go far from Bristol to see examples of both. As you pass down the Avon, you observe a succession of limestone-strata lying obliquely one beneath another; and at last you come to an end of these, and find that the next underlying rock is that Old Red Sand-stone, of which the massive pier on the Somersetshire side of the suspension bridge is built. And Dundry Hill, which is everywhere so conspicuous, is formed at its lower part of Lias, and at its upper part of Oolite, two later formations which were not deposited until after the carboniferous limestone had been uplifted to something near its present position. By measuring the whole length of the succession of limestone-strata that presents itself along the gorge of the Avon, and making the requisite allowance for their slope, the geologist has no difficulty in determining their thickness; and he can say with certainty that, if these successive beds of limestone were piled horizontally upon one another, in the same manner as when they were first formed, their total thickness would exceed 2,000 feet.
Further, you must think of these strata, not only as they present themselves at the surface, but as underlying all our coal-fields, and as probably extending very far beneath the newer strata to the southeast of the dividing band I have just spoken of. Thus, if you look again at the geological map, and notice how the great South Wales coalfield is surrounded by the blue band that indicates the carboniferous limestone, you must think of this limestone as really continuous over the whole of the included area, since it is met with at all points in which the coal-pits are sunk deep enough to reach it. And so in the midland counties, where the map indicates New Red Sandstone and later formations as the surface-strata, these, on being bored through, are found to have coal beneath them; and if we continue the boring downward through the coal-measures, we everywhere come to the limestone-base of this great and important carboniferous series. How far this series extends beneath the newer deposits which form the land of the southeastern portion of England, no geologist can at present say with certainty. If it really underlies them, it must be at an enormous depth, as the results of the Sub-Wealden boring have clearly proved.
Although we are accustomed to speak of the coal-basins of Northumberland, Durham, Yorkshire, Staffordshire, Gloucestershire, Somersetshire, and South Wales, as distinct and separate, it is important to bear in mind that they were probably continuous when the coal-measures were first formed, the "basins" not having then taken shape. This shape was given them by the great disturbance of the older crust of the earth which marked the close of the Palæozoic period, and which brought up the carboniferous limestone into the ridges that now constitute the borders of the basins.
It is this upheaval which has given us access to a vast storehouse of a material of the greatest value to man. Every Bristolian knows the use of this limestone, alike for building and for the making of roads; and the demand for it in the midland counties, to which the Severn affords an easy water-carriage, hastens the already too rapid demolition of his beautiful cliffs. When "burned," i. e., reduced by heat to the condition of "quicklime," it becomes—in virtue of its peculiar power of combining with water—the basis of all mortars and cements. It is as indispensable to the iron-smelter as the coal by which his furnaces are heated, since without its presence he could not reduce the metal from its ores. It is of no less importance in our great chemical manufactures; such, for example, as that of alkali and bleaching-powder. And the agriculturist makes large use of lime in increasing the productiveness of many soils which would be otherwise comparatively barren.
Now, let us inquire by what agency, and under what circumstances, these vast limestone formations were produced.
You all know that, in particular beds of your Avonside rocks, fossils are met with in great abundance, so that any one who looks for them may find stones that seem almost made up of shells, corals, etc.; but in other beds, some of them of great thickness, scarcely any traces of fossils are found, the whole rock having a uniform sub-crystalline texture. Now, in regard to the first, it is easy to show that the fossils are not merely imbedded in the rock, as they are in a sandstone or a clay, but that the rock is really made up of them; for, when we cut thin slices of such specimens, and examine them with the microscope, we find that the "matrix," or uniting material by which the fossils are held together, is itself composed of minute fragments of the same organic forms, mingled, it may be, with entire specimens of minuter forms. But what are we to say of the massive beds of sub-crystalline stone, in which no trace of fossils is to be found? This question we shall be better able to answer, when we have taken a glance at the other limestones which present themselves in different parts of the great geological succession.
The oldest stratified rocks of which we have any knowledge are those which make up the great Laurentian formation, first investigated by the late Sir William Logan, the distinguished geologist who was employed by the Government of Canada to examine the geological structure of that country. This formation chiefly consists of quartz, hornblende, felspar, and other mineral constituents, without any admixture of lime; but near its base is a very remarkable stratum of "serpentine limestone," extending over hundreds of square miles, which has a distinctly organic structure. It is composed of a series of layers, usually very thin, of carbonate of lime alternating with serpentine (magnesian silicate); and the microscopic examination of the calcareous layers first made by Principal Dawson, of Montreal, and afterward extended by myself, has satisfied us that the calcareous layers form a composite fabric of shelly substance, having a regular chambered arrangement, and that the serpentine takes the place of the original animal which occupied these chambers and formed the shell. The animal resembled, in its extreme simplicity of structure, the minute "jelly-specks" by which the Globigerina-shells that cover the Atlantic sea-bed are even now being formed; and differed from it only as the animal of a large composite coral mass differs from that of a simple coral, in extending itself indefinitely by budding; so that a large continuous zoöphytic growth was produced, bearing a strong resemblance to a coral-reef, instead of the aggregate of minute and separate shells which formed the old Chalk, and which is even now continuing the like formation. I do not know and more remarkable result of microscopic inquiry, than the very distinct evidence it has afforded, in well-preserved specimens of this Eozoön Canadense, of a minutely tubular structure, which my own researches into the structure of the Foraminifera enable me to identify with certainty as belonging to that type. For we are thus carried back in geological time to a period so extremely remote, that (as Sir William Logan remarked) the oldest fossils previously known are modern in comparison. The investigations of Sir Roderick Murchison have shown that the equivalent of the Laurentian in this country is the "fundamental gneiss" of Scotland, which (as I was shown a few days ago by my friend Mr. Symonds, of Pendock) crops up in the Malvern Hills. Now, in Central Europe this fundamental gneiss has a thickness of 90,000 feet; and near its base Prof. Gümbel has recognized the equivalent of the Canadian Eozoön, which must have thus preceded the life of what has been called the "primordial zone," corresponding to our Cambrian rocks, by an interval of time so great that no geologist would venture to assign a limit to it.
The Cambrian series, consisting of the grits, sandstones, and slates, that form the mountains of North Wales, scarcely contain any limestone; and we may pass from this to the Silurian, or Mid-Wales, series in which we have the well-known Dudley limestone, as well as other less important seams. A slab of Dudley limestone usually shows an extraordinary variety of fossils, among which the most conspicuous are generally the beaded stems of Encrinites; the joints of these stems, when separated by the weathering of the rock, being known in the north as "St. Cuthbert's beads." The whole of this limestone is obviously made up of the corals, shells, crinoids, etc., which we, find imbedded in it, and of a matrix formed by comminuted fragments of the like types. A much greater development of these calcareous beds presents itself in North America, the Trenton limestone occurring in the lower Silurians, and the Niagara limestone in the upper; and these rocks have obviously been formed by the same agency as the Dudley limestone.
Passing on now to the Devonian series, we find beds of limestone interposed among the sandstones, shales, and' conglomerates, of which it is chiefly composed; and these, like the Silurian limestones, are made up of the fossilized remains of corals, shells, crinoids, etc., more or less resembling those of earlier age. It is on the Old Red Sandstone, which is here the uppermost member of the Devonian formation, that, as I have already pointed out, our Carboniferous series immediately rests; its lower beds being distinguished as "limestone shales," on account of the interposition of seams of shale (formed of a mixture of sand and clay) between the layers of limestone.
Postponing for the present the more detailed inquiry into the origin of our own Limestone, of which this general survey is the prelude, I pass on to the Permian formation, which rests upon the Carboniferous, and has been upheaved with it, having been deposited previously to the general disturbance that closed the Palæozoic (ancient life) period. Of this Permian formation there are few traces in our part of England; but it has a much greater development in the north, and to it belongs that remarkable bed of Magnesian limestone which comes to the surface in Northumberland and Durham. It is of this stone (selected on account of the durability it has shown in York Minster and other old buildings) that the Houses of Parliament are built. Now, although very few fossils are found in this rock, yet I believe that most geologists would agree that it was originally formed, like limestones generally, by the growth of corals, shells, etc., which separated the carbonate of lime from the sea-water they inhabited; its subsequent conversion into magnesian limestone having been probably effected by the infiltration of water in which magnesia was dissolved. In the Eozoic limestone of Canada, I have myself frequently met with veins of dolomite (magnesian limestone), which retain the general arrangement characteristic of the original shell, although its minute structure has been obliterated by this metamorphic action.
Passing on now to the Secondary or Mesozoic (middle life) series, we find that although the Trias, which is the oldest member of it, is represented in England by sandstones alone, there is an important bed of limestone in Germany called the Muschelkalk (shell-limestone), which is interposed between the lower and the upper New Red Sandstones. This bed derives its name from the fact that it is obviously formed by an aggregation of shells, mingled with other fossils, among which the beautiful Lily Encrinite is one of the most abundant. In the Lias, which overlies the New Red Sandstone, a considerable portion of lime is generally mingled with the clay deposits of which this formation is principally composed; and some of its beds, especially on the northeast of Yorkshire, are almost entirely calcareous. If you walk along the shore between Saltburn and Whitby, and examine the blocks which have fallen from the lias cliffs above, you will find them to be almost entirely made up of fossils; among which Belemnites—conical chambered shells, with solid calcareous "guards," which belonged to animals resembling cuttle-fishes—are specially abundant. And here, as elsewhere, the calcareous matrix in which the fossils are imbedded, though sub-crystalline in some parts, is obviously made up in others of fragments of shell, etc., ground down by the action of the sea in which the deposit was formed. The Lias abounds in the neighborhood of Bristol, and is exposed in many railway-cuttings. These, when in progress some forty years ago, yielded many valuable fossils, especially skeletons of the great Fish-Lizards, which you will see in the Museum of the Bristol Institution. In this neighborhood, also, you have a splendid illustration of the great Oolitic formation, which is almost entirely made up of calcareous deposits that can be clearly traced to an animal origin, although their condition is now very different. The Coral Rag of Oxfordshire is an old coral-reef that has undergone very little change, consisting of fossil corals, and of the shells, crinoids, etc., that lived on the reef. And the "freestones" of Bath and Portland are mainly composed of the fine sand which was formed by the wearing-down of similar reefs, of which the remains are found here and there. The name "oolite" or roe-stone, is given to the whole formation, on account of the resemblance in texture borne by some of its characteristic members to the roe of a fish; but this "oolitic" structure is not peculiar to the Oolitic formation, being found in other limestones, as I shall presently point out to you. A very curious example of the "metamorphic" action by which the texture of a calcareous rock may be so completely altered as to conceal its origin is afforded, by the fact that the beautiful Carrara marble, which is used for statuary, belongs to the Oolitic formation. If this metamorphism, the nature of which I shall presently explain, proceeds further, it will produce large crystals of calc-spar; and I remember that Mr. Baily, the sculptor of the beautiful statue of "Eve at the Fountain," which is in your Fine Arts Gallery, was greatly embarrassed by a vein of calc-spar that ran through the block from which he cut it, and had to let a patch of marble into Eve's back. The next great calcareous formation above the Oolite is the Chalk, the material of which is exactly the same as that of limestone, although its texture is so different. Our deep-sea researches have entirely confirmed the opinion which had been previously formed on the basis of microscopic research, that the whole of the enormous mass of Chalk now raised up into the cliffs and downs of the southern portion of England was formed on the bed of the ocean, by the agency of animals—chiefly the minute Foraminifera, which separate carbonate of lime from the seawater as the material of their shells; just as successive generations of fresh-water mussels living in a lake form a bed of calcareous marl on its bottom by the decay of their shells, which sets free in a solid form the lime they have taken from the water that poured it into the lake in solution. We have brought up by the hundred-weight, from depths of three miles in the Atlantic, a white mud, which, when dried, exactly resembles chalk; and this, when examined with the microscope, is found to consist partly of perfect shells of minute Globigerinæ, of which many hundreds would only weigh a grain, and partly of what we call Globigerina ooze, which is obviously the product of the decay of former generations of similar shells.
In the Tertiary or Neozic (modern life) series, we find many limestone deposits of considerable importance, but none so vast as those to which I have previously drawn your attention. The most extensive is the "nummulitic limestone," which is one of the oldest members of the Eocene formation, the earliest of the tertiaries. We find this limestone forming a heel of considerable thickness on the flanks of the Pyrenees, and extending from the shores of the Atlantic along the south of France to the Alps, in some parts of which it shows a thickness of fifteen hundred feet, thence across to Asia Minor, Northern India, and probably to the Pacific shore; while another division of it ranges along Northern Africa, and is especially noteworthy in Egypt, where it rises into the hills that border the Nile for a loner distance above Cairo, and furnishes the stone of which the Pyramids are built, and out of which the Sphinx is carved. This stone not merely contained nummulites, which are Foraminiferal shells much larger than Globigerinæ (sometimes attaining the size of a half-crown), but is entirely made up of them, and of the fragments of those which have been ground down by the action of the waves, as well as of other shells inhabiting the same sea; all cemented into a solid mass by the process I shall presently describe. Another limestone of more limited extent, belonging to the Eocene age, is found in the neighborhood of Paris, and has furnished the material of which that beautiful city is built. This is entirely made up of the minute Foraminiferal shells termed Miliolæ, from their resemblance in size to grains of millet, and is known as "miliolite limestone." So in Malta and in the neighborhood of Vienna, there are limestones entirely composed of shells, corals, and Foraminifera, which were formed in the Miocene or Middle Tertiary period. And we have on the coast of Suffolk the calcareous bed known as the "coralline crag," to which equivalents are found in various parts of Europe, that belongs to the Pliocene or Later Tertiary period. The material of this bed is chiefly contributed by the calcareous skeletons of composite animals that formerly ranked as zoöphytes, but are now distinguished as Polyzoa. Although individually extremely minute, in fact microscopic, they have a very complicated structure, allied to that of the lower Mollusks; and they extend themselves like trees by continuous budding, so that the fabrics they form often have a stony solidity. They abound in our own seas, and, as we shall presently find, they extend very far back in geological time.
Thus, then, we see that, in the case of the Secondary and Tertiary limestones, there can be no question of their production by the agency of animals, which separated carbonate of lime from its solution in sea-water, and formed it into corals, shells, etc., just as similar animals are doing at the present time. And we have in these calcareous deposits many instances of local "metamorphism," which show that the existence of a sub-crystalline, or even of a complete crystalline, arrangement in the particles of carbonate of lime is no proof that the materials of these deposits were not originally drawn from their solution by the agency which formed those whose organic origin is obvious. Thus in the neighborhood of the Giant's Causeway, where volcanic rocks have burst up through the chalk which forms a long succession of fine cliffs on the Antrim coast, this chalk has been so altered in texture as almost to resemble marble, all trace of its original nature being obliterated. Knowing, as we do, how much more extensive and potent must have been the agencies which were at work in metamorphosing the Palæozoic rocks, we have no difficulty in accounting for the fact that vast beds of our Carboniferous Limestone now show little or no trace of the organic texture which we believe them to have originally possessed. That you may better understand the nature of this metamorphosis, I shall now show you some of the chemical properties of carbonate of lime, which is the material of all calcareous rocks alike, whether showing the perfect crystalline form of calc-spar, the close minutely-crystalline arrangement of marble, the sub-crystalline texture of limestone, the "roe-stone" aggregation of oolite, or the fine powdery condition of chalk.
If we treat a piece of any one of these substances with dilute nitric or muriatic acid, an effervescence is immediately produced by the liberation of carbonic acid, while the lime is dissolved; and this gives a ready way of distinguishing a calcareous from any other rock. In "burning" limestone, on the other hand, the union of the carbonic acid and the lime is dissolved by heat; the carbonic acid is driven off, and the lime remains behind in the condition of "quicklime." This is very greedy (so to speak) of carbonic acid, and is always trying to get it back again. We can dissolve a small quantity of quicklime in water; and if we leave this with a large surface exposed to the air, it gradually recombines with the carbonic acid which it draws from the air; and, as the carbonate is nearly insoluble in water, it falls as a fine white powder, making the water turbid. We may do the same in a moment, by blowing through a pipe into a glass of lime-water, our breath containing a considerable quantity of carbonic acid; and we may then clear the liquid again, by a drop or two of nitric or muriatic acid. But, though insoluble in pure water, carbonate of lime is slightly soluble in water which is already charged with carbonic acid; and, as all rain-water brings down carbonic acid from the air, it is capable of taking up carbonate of lime from the soils and rocks through which it filters; and it thus happens that all springs and rivers, that rise in localities in which there is any kind of calcareous rock, become more or less charged with carbonate of lime kept in solution by an excess of carbonic acid. This is what gives the peculiar character lo water which is known as "hardness;" and a water hard enough to curdle soap may be converted into a very "soft" water (as the late Prof Clark, of Aberdeen, showed) by the simple addition of lime-water, which, by combining with the excess of carbonic acid, causes the precipitation of all the lime in solution in the form of insoluble carbonate, which gradually settles to the bottom, leaving the water clear. It is this solvent power of water charged with carbonic acid, which has been the great agent in the metamorphism of many calcareous rocks, whereby their texture has been entirely changed, while their composition remains unaltered; and it acts with augmented potency where heat and pressure concur to increase it. Of this we have an example in the action of hot springs highly charged with carbonic acid, such as we often find in volcanic localities; it is to such that the formation of the "travertine" limestone of Italy is due, the carbonate of lime being slowly deposited almost in the condition of marble, when the excess of carbonic acid is disengaged, and the water is dispersed in vapor, by free exposure to air. We have familiar examples of this, on a more limited scale, in the formation of the "stalactites" which hang from the roofs of caves in limestone rocks (as at Cheddar), and in the "stalagmitic" crust formed by their droppings on the floors.
Those who have had opportunities of observing the changes which have taken place in the condition of recent corals that have been upheaved by volcanic action above the level of the sea, in the "area of elevation" to which Mr. Darwin drew attention forty years ago, assure us that their texture is often so changed, that detached pieces of them could not be distinguished from pieces of sub-crystalline limestone. I well remember having first learned this from Mr. S. Stutchbury, who was the curator of the museum here when I was a youth, and who was the first to observe the ring of upraised coral which encircles the cone of the great volcano of Tahiti, and which belongs to the same type as that now forming reefs around the coast of that island. He told me that some specimens of it, which he had collected and brought home, were treated by his brother, a professed mineralogist, as specimens of carboniferous limestone. The formation of oolites, again, may be studied at the present time. When a bed of calcareous sand, formed by the wearing down of shells or corals, is raised above the sea-level, and is penetrated by rain-water charged with carbonic acid, this, dissolving the carbonate of lime of the surface-layer, deposits it again around the grains of the deeper layers, which it invests with concentric coats. Such oolites present themselves in various geological epochs, indicating the similarity of the past and present conditions. There are oolitic beds, for example, in the Clifton rocks; and we thus know that these must have been shore formations; while other beds seem to have been deep-sea deposits, resembling the Globigerina mud of the present Atlantic sea-bottom. For there is in Russia a very extensive bed of limestone belonging to the carboniferous series, which is as completely composed of Fusuinæ (an extinct type of foraminifers about the size of a sugar-plum) as the nummulitic limestone is of nummulites. In the clay-seams, again, which we sometimes find inter posed between beds of pure limestone, numerous Foraminifera are found well preserved, of which some belong to types still living; and my friend Mr. H. B. Brady, of Newcastle, who has been lately making a microscopic study of the Carboniferous Foraminifera, has found their remains abundant in specimens of this limestone which do not show any indications of organic structure that are obvious to the naked eye. If the Globigerina-mud were to be subjected to the pressure of an enormous weight of rock deposited above it, and then to the heat and pressure which we know must have accompanied the great crumpling of the earth's crust that made the marked separation between the Paleozoic and the Secondary epochs, we may well believe that it would have been metamorphosed into a limestone closely resembling the least fossiliferous of the Avonside rocks; and we have no difficulty in accounting for the vast thickness of these beds, if we regard them as having been progressively formed on the bottom of a very deep ocean, through a long succession of ages.
That certain beds of the Avonside rocks are ancient Coral-Reefs, cannot be a matter of question; for we find them to be entirely made up of fossil corals, together with the fossilized shells and crinoids which such reefs would have supported. This was especially the case with what used to be called the "black rock" under the seawall, which has been nearly all quarried away since, when a boy, I brought home a piece of it as large as I could carry, wondering at such an accumulation of fossils, but without any such understanding of their import as that which I am endeavoring to give you. Every one has heard of the coral reefs and islands, which are popularly said to be "built up" in tropical seas by the agency of "insects," as bees build their waxen combs. And I suppose that every one of you is familiar with specimens of some kind of coral brought home by a seafaring friend, or has seen such in your museum. Now, the fact is, that all these corals are the production of animals resembling in essential points the common sea-anemone, but differing from it in depositing a stony skeleton in the fleshy substance which forms its base, and also in the radiating partitions which surround its stomach. We have on our own shores a small type of the coral-forming polyps, in the little Caryophyllla, which, when the animal is expanded, you would take to be a small sea-anemone, but which, when contracted, shrinks down into its stony cup. The Fungia of tropical seas is a much larger solitary polyp of the same kind; and you will often meet with its stony disk, four or five inches in diameter, with beautiful thin vertical plates radiating from the centre to the circumference, very much like the "gills" of the under-side of a mushroom (fungus), whence its name is derived. But all the more massive corals are the skeletons of composite animals; that is, of polyps which bud like plants, and thus grow to large dimensions. In some cases they form tree-like structures, in which you will find a multitude of polyp-cells, sometimes very small, each having its characteristic arrangement of radiating plates. But in the reef-building corals, the polyp-cells are packed closely together; and the older portion becomes so completely solidified by calcareous deposit that, when broken across, it looks like a stone. This is especially the case with the Meandrina, or brain-stone coral, so named from the resemblance which its furrowed surface bears to the convoluted surface of the brain; hemispherical masses of this coral are not unfrequently to be seen in museums having a diameter of from two to three feet; and in the upraised coral-cliffs of Bermuda they are reported to be five or six feet in diameter. The polyps lie in rows along the furrowed surface, and the active life of the composite mass does not extend far down; its stony interior being the product of its earlier life, as the heart-wood of a tree is the product of previous successions of leaf-buds. But it is no more correct to say that the polyps have built up the stony mass, than it would be to say that the leaves of a tree build up its woody stem, or that our own soft parts build up our bony skeleton. The hard parts are formed in each case by a process of growth; soft tissue being first produced as a part of the animal body, and this being subsequently solidified by mineral deposit, the material for which is absorbed by the animal from the sea-water in which it lives.
The admirable researches of Mr. Darwin have shown us that, although the reef-building corals seem unable to live and grow at depths greater than twenty fathoms (one hundred and twenty feet), yet that if their base gradually subsides, at a rate not greater than that of coral-growth, the reef or island will be kept up to the surface by such growth; so that, if we could bore down into it, we might find the coral-structure to have a depth of many hundreds or even thousands of feet. The recent soundings of the Challenger around the Bermuda islands, which are entirely composed of coral, indicate that they form the summit of a pillar rising from a depth of twelve thousand feet; and as we have no instance of a mountain having such a shape, it seems probable that the upper part of this pillar, at any rate, must have been formed of coral, which kept growing upward, in the manner indicated by Mr. Darwin, while the bottom was slowly subsiding. It is commonly supposed by geologists that the limestone beds of which I have been speaking are the result of the metamorphosis of ancient coral formations, which attained their great thickness by continuous growth at their living surface, as their base gradually subsided. But it appears to me that all we know of existing coral formations renders it unlikely that there should have been such a continuity of area in ancient coral formations, as would be required to account for the continuity in the area of our great beds of carboniferous limestone; and that this continuity is far better accounted for by supposing them to have been formed in the manner I previously indicated—by the foraminiferal life which recent researches have shown to be even now forming a calcareous deposit over vast areas of the ocean-bottom.
Thus, then, we should regard the beds which show distinct coral-structure as representing reefs or islands of limited extent in the Palæozoic ocean; while the formation of those beds of vast area, in which few or no traces of animal life are found, may be fairly referred to the agency of minute forms, essentially similar to those of the Old Chalk and of its existing representative (Globigerina-mud), whose habitation is the deep sea.
No inconsiderable proportion of the calcareous material of some of the local beds seems to have been furnished by the stems and bodies of the Crinoids (lily-like animals), which abounded in the Palæozoic seas, and of which the representatives at the present time have been proved by recent deep-sea exploration to be much more numerous and widely diffused than was previously supposed. I remember to have seen these very conspicuous in polished sections of the old "black rock;" and certain beds in the limestone of Derbyshire, which are worked for marble chimney-pieces, seem almost entirely composed of their remains. The stems of the Crinoids of the Carboniferous period were not beaded like those of the Dudley (Silurian) limestone, but were cylindrical in form; they had, however, the same jointed structure and central canal; and you will thus readily recognize them when cut either longitudinally, transversely, or obliquely.
It has been further recently shown that Polyzoa essentially resembling those of our modern "coralline crag" existed at this epoch, and had a share in the formation of certain beds of the carboniferous limestone. There is a particular bed in St. Vincent's rocks, which has been found by Mr. Stoddart to be composed of fragments of their delicate calcareous fabrics, with Foraminifera, and other small forms of animal life; and he has appropriately named it the microzoic bed. And Prof. Young, of Glasgow, has been fortunate enough to find, in a clay-seam of the carboniferous limestone in his neighborhood, a collection of these fabrics preserved entire in the fullest perfection.
Thus we seem justified in the conclusion that the vast strata of carboniferous limestone, which in England alone must cover tens of thousands of square miles, and has a thickness of more than two thousand feet, had their sole origin in the continuous life of innumerable generations of humble animals, which, in times long past, did the work that is still being performed in the depths of our own seas by animals of similar types, which we may believe to be their lineal descendants. I have shown you how we are indebted to their agency for the abundant supplies they have provided of a material most useful—I may say indispensable—to us. Let us take care that, with our larger capacities and higher aims, we strive to promote the welfare of those who come after us, by doing well, each in his station, that which our powers and opportunities best fit us to accomplish.—Author's advance-sheets.
- ↑ A Lecture given to the workingmen of Bristol, at the meeting of the British Association, August 28, 1875.