Chapter 20

"Where is the dust that has not been alive?"Young.

"Where is the dust that has not been alive?"

Young.

That those infinitesimal forms of animal existence which swarm throughout the waters of the ocean, but whose presence can only be made manifest by the aid of the microscope, are preserved in a fossil state,—that their durable remains constitute mountain ranges, and form the subsoil of extensive regions,—and that the most stupendous monuments erected by man are constructed of the petrified relics of beings invisible to the unassisted eye,—are facts not the least astounding of those which modern Geology has revealed.

This interesting field of research, which the labours of that eminent observer M. Ehrenberg first made known, has since been explored by other naturalists, and in every part of the world many of the Tertiary and Secondary deposits have been found to contain microscopic organisms in profusion. At present this branch of palæontology is in its infancy, and it offers to the young student an inexhaustible and most attractive path of scientific investigation; it possesses, too, this great advantage over many others, that it can be pursued at home, and the materials for its prosecution are everywhere at hand. Unlike my explorations in the Wealden, in which a few fragments of bones, or teeth, scattered at wide intervals through the rocks, and in localities many miles apart, wereoften the only reward of a day's labour, here, in the quiet of my study, I may discover in a few atoms of flint, or grains of chalk, picked up by the road-side, the fossil remains of beings as interesting and extraordinary as the extinct colossal reptiles of Tilgate Forest.

MICROPHYTES.

The microphytes, or fossil Diatomaceæ, described in a previous chapter, (ante,p. 93.,) were formerly classed with the organisms that now claim our attention, under the nameInfusoria; from the belief that generally prevailed among naturalists, of their animal origin. In fact, some eminent microscopic observers, while admitting the vegetable character of Xanthidium, Micrasterias, &c. consider the Naviculæ, Ennotiæ, &c. as belonging to the animal kingdom.

Thus Dr. J, W. Bailey, in a late "Memoir on the Microscopic Organisms in Various Localities of the United States," divides these bodies into three groups; viz.Infusoria,Desmidieæ, andDiatomaceæ; with the remark, that he has separated the two latter tribes from the Infusoria, because so many distinguished naturalists consider them decidedly to belong to the vegetable kingdom: "but," he adds, "while I believe that no positive line of separation can be drawn between certain animals and vegetables, I am yet disposed to regard theDesmidieæ, from the sum of all their characters, as most nearly allied to admitted vegetables; while theDiatomaceæ, notwithstanding Mr. Thwaites's interesting observations on their conjunction,[305]still seem to me, as they have always done, to be true animals. There is such apparent volition in their movements, such an abundance of nitrogen in the composition of their soft parts, and suchresemblance between the stipitate Gomphonematæ and some of the Vorticellæ, that I should be still disposed to class them as animals, even if Ehrenberg's observations of the retractile threads and snail-like feet of some of the Naviculæ should not be confirmed."[306]

[305]The mode of fructification, orconjunction, as it is termed, in the Algæ, consists in the adhesion of two cells or frustules together, and their fusion into one; from their united contents a mass of granular substance is produced, that becomes consolidated and forms the spore or fruit, which, when arrived at maturity, is set free by the bursting of the cell. Mr. Thwaites has ascertained that the fructification is similar in the Diatomaceæ.[306]Smithsonian Contributions, vol. ii. p. 34.

[306]Smithsonian Contributions, vol. ii. p. 34.

Thus, whilst referring Closterium, Arthrodesma, Euastrum, Xanthidium, Micrasterium, &c. as vegetables, to the Desmidieæ, Dr. Bailey places,Actinocyclus,Campilodiscus,Coscinodiscus,Ennotia,Navicula,Gomphonema,Pinnularia,Triceratium, &c. among the Diatomaceæ, as animals.

Of the animal nature of the microscopic objects which now require our attention, there is however no question, although the zoological position and affinities of many of the organisms included in this survey are still but imperfectly determined.

The animals designatedForaminifera,[307]or Rhizopoda,[308]are of a more simple structure than the Polypifera and Echinodermata described in the previous chapters; yet as their relics are for the most part presented to the notice of the geologist as aggregations of shells, forming extensive beds of limestone, it will be convenient to treat of them in this place.

[307]Foraminifera,i.e.bearing foramina—a name derived from the minute openings in the shells and their septa.[308]Rhizopoda; root-like feet; from the long fibrous processes, or pseudopodia.

[307]Foraminifera,i.e.bearing foramina—a name derived from the minute openings in the shells and their septa.

[308]Rhizopoda; root-like feet; from the long fibrous processes, or pseudopodia.

The fact that the fossil remains of Foraminifera, and ofMollusca, alike consist of shells, and constitute strata identical in mineral characters, and deposited under like physical conditions, renders the examination of these Microzoa[309]a fit introduction to the study of the durable remains of the higher order—the Mollusca.

[309]Microzoa; from μικρος,mikros, small, and ζωον,zoon, animal; a convenient term to denote minute animal organisms whose forms can only be defined by the aid of the microscope.

It is scarcely more than a hundred and twenty years ago, that the existence of this numerous order of microzoa was first made known to naturalists byBeccarius, who detected a considerable number of species in the sand on the shores of the Adriatic. But the structure of the animals that secreted these shells is a discovery of comparatively modern date. The early collectors classed these microscopic bodies with the shells of true mollusca; and even M. D'Orbigny, whose elaborate researches justly constitute him a high authority in this branch of natural history, in his first memoir, in 1825, described the involuted discoidal forms as Cephalopoda. This error was corrected by the investigations of M. Dujardin, who in 1835 satisfactorily demonstrated that the Foraminifera are animals of the most simple structure, and entirely separated by their organization from the Mollusca.

But the true nature of this class is so little understood by British collectors of fossil shells,—of course I mean the uninitiated, and the amateur naturalist, for whose use these unpretending pages are designed,—that in order to invest the study of the fossil species with the interest which a knowledge of the structure and economy of the living originals can alone impart, I must give a history of the recent forms somewhat in detail, taking M. D'Orbigny as my chief authority.[310]

[310]The best work for the student to consult is M. D'Orbigny's "Foraminifères Fossiles du Basse Tertiaire de Vienne, Autriche." Paris, 1846. 1 vol. 4to. with plates. I rejoice to learn that a Monograph on the British Foraminifera is in preparation by Dr. Carpenter and Professor Williamson: than whom there are none more competent.

FORAMINIFERA.

TheForaminiferaare marine animals of low organization, and, with but few exceptions, extremely minute: in an ounce of sea-sand between three and four millions have been distinctly enumerated. When living, they are not aggregated, but always individually distinct; they are composed ofa body, or vital mass, of a gelatinous consistence, which is either entire and round, or divided into segments, placed either on a simple or alternate line, or coiled in a spiral, or involuted round an axis. This body is covered with an envelope or shell, which is generally testaceous, rarely cartilaginous, is modelled on the segments, and follows all the modifications of form and contour of the body. From the extremity of the last segment there issue sometimes from one, sometimes from several openings of the shell, or through the numerous pores or foramina, very elongated, slender, contractile, colourless, filaments, more or less divided and ramified, serving for prehension, and capable of entirely investing the shell.

The body varies in colour, but is always identical in individuals of the same species; it is yellow, ochreous, red, violet, blue, &c.

Its consistence is variable; it is composed of minute globules, the aggregation of which determines the general tint. It is sometimes entire, round, and without segments, as inGromia,Orbulina, &c., which represent at all ages the embryonic state of all the other genera. The animal increases by gemmation, each segment being essentially distinct, but connected with the preceding one by a tube or neck. When the body is divided by lobes or segments, the primary lobe, as in the permanent condition of the Gromia, is at first round or oval, according to the genus; once formed it never enlarges, but is enveloped externally by testaceous matter.

The segments, which successively appear, are agglomerated together in seven different ways, and these modifications are the basis of M. D'Orbigny's classification. The discoidal forms, as theRotalia,Rosalina,Cristellaria, &c. are involute, like the Nautilus, and divided by septa or partitions, which, like the enveloping shell, are perforated. The lobes of the bodyoccupy contemporaneously every chamber, and are connected by a tube that extends through the entire series. Inthe spiral form, as the Textularia, &c., the same structure is apparent.

Whatever the form of the body, the filaments always consist of a colourless transparent matter; they are capable of being elongated to six times the diameter of the shell. They often divide and subdivide, so as to appear branched; and though alike in form in the different genera, vary much in their position. In some species they form a bundle which issues from a single aperture, and is withdrawn into the same by contraction; in others, the filaments project only through each of the pores in that portion of the shell which covers the last segment: in many they issue from both the large aperture and the foramina. These filaments or pseudopodia fulfil in these animals the functions of the numerous tentacula in the Star-fishes; serving as instruments of locomotion and attachment.

Neither organs of nutriment nor of reproduction have been discovered. In the genera having one large aperture from which the filaments issue and retract, we can conceive nutriment to be absorbed by that opening; but this cannot be the case in the species which have the last cell closed up; in these the filaments issuing through the foramina are probably also organs of nutrition. M. D'Orbigny considers the Foraminifera as constituting a distinct class in zoology; though less complicated than the Echinoderms and the Polypifera in their internal organization, they have the mode of locomotion of the first; while by their free, individual existence, they are more advanced in the scale of being than the aggregated and immovably fixed animals of the latter class.

But though I consider the animal of the Foraminifera as single, and the additional lobes, or segments, as the continuous growth of the same individual, I must state that some eminent naturalists regard the entire structure as a series of distinct individuals, developed by gemmation from the first formed segment, like the clusters of the compoundTunicata; and not as a single aggregated organism, made up of an assemblage of similar parts indefinitely repeated. In a palæontological point of view, it matters not which opinion is adopted.[311]

[311]See a masterly paper on the structure of Nummulina and Orbitoides, by Dr. Carpenter; Geol. Journal, vol. vi. pp. 21-39, with admirable representations of the structural details.

Lign. 109. Foraminifera.Chalk. Charing.Fig.1.—Globigerina cretacea;the original is1/60of an inch in diameter.2.—Textularia Globulosa;1/40.3.—Verneuilina tricarinata;1/30.4.—Cristellaria rotulata;1/40.5.—Rosalina Lorneiana;1/40.

CLASSIFICATION OF THE FORAMINIFERA.

Classification of the Foraminifera.—The number of genera is so great, that I can only attempt to convey a very general idea of the principles of classification adopted by M. D'Orbigny, and give a few illustrations of some of the most abundant fossil species.

That the reader may be cognizant of the usual aspect of these shells five specimens from the Chalk, belonging to as many genera, are here represented (Lign.109); the deposit whence they were obtained will be described hereafter.

As the mode in which the growth of the body, and consequently of the shell, takes place, differs greatly in certain groups, an obvious and natural arrangement is suggested, by which the class is divided into seven orders:—

1. The primary, or simplest type; one segment or cell; as inOrbulina.

2. The segments arranged in circular lines; as inOrbitolina.

3. Segments disposed in a straight or arched single line; successively increasing from the first to the last cell; as inNodosaria,Lign.111.

4. Segments, spirally and discoidally disposed, on the same plane, like cells of the Nautilus; as inCristellaria,Lign.109.

The same type, but coiled obliquely, and inequilaterally, like the shells of Gasteropoda; as inGlobigerina,Lign.109,Rosalina,Lign.109.

5. Segments developed alternately on the right and left of the first, and successively on each side the longitudinal axis; as inTextularia,Lign.109.

6. Arrangement of the segments combining the modes of 4 and 5; that is, the segments are formed alternately, but the whole are coiled spirally, either obliquely, or on the same plane; as inAmphistigena.

7. Segments round a common axis, on two, three, four, or five, opposite faces, returning after each entire revolution; the new cells being placed exactly on the preceding series; as inQuinqueloculina,Pict. Atlas, pl. lxii.fig.12.

The Foraminifera vary considerably in magnitude; by far the greater number of species are invisible to the unassisted eye, and the aid of a lens or microscope is required to define the structure even of the largest; yet many are of sufficient size to be recognized, as for example theSpirolinæ,Lign.112. A few genera are from a quarter of an inch to nearly an inch in diameter, as theOrbitoides Mantelliof thetertiary formations of North America,[312]and theNummulina, commonly termedNummulites, of Europe,Lign.110.

We will now describe the genera selected for illustration, commencing with the large and well-known type, whose aggregated remains form extensive beds of crystalline limestone in the Alps, and in Asia, and Africa.[313]

[312]Dr. Morton's Synopsis of the Organic Remains of the Cretaceous group of the United States. Philadelphia, 1824, p. 45, pl. v. fig. 9.[313]Geol. Journal, vol. v.

[312]Dr. Morton's Synopsis of the Organic Remains of the Cretaceous group of the United States. Philadelphia, 1824, p. 45, pl. v. fig. 9.

[313]Geol. Journal, vol. v.

Lign. 110. Nummulites, or Nummulina;nat.From the Great Pyramid of Egypt.Fig.1.—Transverse section of a Nummulite, showing theform and arrangement of the cells.2, 3.—Specimens with part of the external plate removed.

Nummulina lævigata.Lign.110.—The shell is of a discoidal or lenticular form, composed of numerous cells, concentrically arranged round an axis on the same plane; both sides of the disk covered by a smooth thick plate.

NUMMULITES.

Under the name ofNummulites, from their resemblance to a piece of money, the fossil shells of this genus of Foraminifera have long been known to naturalists, and are figuredin many of the early works on petrifactions. They occur in immense quantities in certain rocks, and are of all sizes, from a mere point, to disks an inch and a half in diameter; thus exceeding in magnitude all other animals of this class.

Perfect specimens appear as a calcareous solid circular body, of a lenticular shape; smooth, and slightly convex on both sides, and without any visible structure. On splitting the fossil transversely, or rubbing down one of the convex planes, a series of minute cells, arranged in a discoidal spire, is brought to view, as shown inLign.110,fig.1. But this description gives a very inadequate idea of the complicated and exquisite structure of the original, which has been admirably worked out by Dr. Carpenter. This eminent physiologist has shown that each chamber was occupied by a living segment, connected with other segments by one or more tubular prolongations, which absorbed nutrition from without, by means of filamentous pseudopodia, that projected through a system of passages leading from the medial plane to the external surface.[314]A figure of the supposed form of a living Nummulina is given in Pict. Atlas, p. 187.

[314]Geol. Journal, vol. vi. p. 21. See also a paper by Prof. Williamson, "On the minute Structure of the Calcareous Shells of some recent species of Foraminifera." Trans. Microscop. Soc. vol. iii. p. 105.

The specimens figured inLign.110are from the limestone that forms the foundation rock of the Great Pyramid of Egypt, and of which that structure is in great part composed. Strabo alludes to the Nummulites of the Pyramids, as lentils which had been scattered about by the artificers employed on those stupendous monuments, and become stone.[315]Silicified masses of Nummulites are occasionally met with; polished slices of such specimens are richly figured by the sections of the inclosed Foraminifera.

[315]An interesting fact was communicated to me by a friend who lately descended the Nile; the Nummulitic limestone rocks are in some parts of the course of the river washed into the stream, and becoming disintegrated, the Nummulites are set free, and re-deposited in the recent mud of the Nile.

The Nummulitic limestones are of the Eocene or ancient. Tertiary epoch, as the labours of Sir Roderick Murchison in the Alps, Apennines, and Carpathians first established: Nummulites are unknown in the Secondary formations.[316]

[316]Geol. Journal, vol. v.

Orbitoides.—The fossil bodies thus named are disciform, like the Nummulites; and one species, which forms the constituent substance of ranges of limestone mountains, 300 feet high, near Suggsville, in North America, was first described by my lamented friend, the late Dr. Morton, of Philadelphia, asN. Mantelli, in his work on the Cretaceous Fossils of the United States.

The discovery that the Nummulitic deposits of the Old World were of the tertiary period, directed attention to their supposed geological equivalents in America; and on a careful examination of their fossils, the rocks proved to be tertiary, and the shells true Foraminifera allied to the Nummulites, but generically distinct.[317]A reference to Dr. Carpenters memoir, previously cited, is necessary to comprehend the complicated structure of these fossils.[318]

[317]Manual of Geology, p. 208.[318]Geol. Journal, vol. vi.

[317]Manual of Geology, p. 208.

[318]Geol. Journal, vol. vi.

Siderolina, orSiderolites, is a genus of Foraminifera, which may be described as Nummulites, in which the turns of the spire are intercepted by elongated appendages, that project beyond the periphery of the disk, and produce a stellated figure. These fossils abound in the cretaceous strata of Maestricht.

Fusulina.—The shell is fusiform, being elongated transversely to the axis; the cells are divided internally by constrictions. Only one species is known, (F. cylindrica,) and this is confined to the Carboniferous formation; it is the most ancient or earliest type of the class, according to the present state of our knowledge.

Lign. 111. Foraminifera, &c.Fig.1 and 2.—Campilodiscus; a perfect frustule, and part of another, seen from above, Tertiary; Egra in Bohemia.See ante,p. 96.3a.—Lituola nautilotdea.Chalk, Chichester.(By Mr. Walter Mantell.) Side view, × 8.3b.—Front view of the last cell of 3d, to show the foramina with which it is pierced.3c.—The last cell of 3a.3d.—Side view of a young shell, before the produced, or straight part appears, × 20.4a.—Flabellina Baudonina.Chalk.(M. D'Orbigny.) A young individual seen in profile, × 12.4b.—The same, viewed laterally, shows the oblique arrangement of the cells.5.—Nodosaria.Chalk, Chichester.(By Mr. Walter Mantell.) The line below indicates the natural size.

NODOSARIA. LITUOLA.

Nodosaria.Lign.111,fig.5.—Straight, elongated cells placed end to end, separated by constrictions; the last formed cell has a round central aperture. Several beautiful species abound in the Chalk; specimens often occur adhering to the surface of the flint nodules.

Nodosariæ are abundant in tertiary deposits. Mr. Walter Mantell discovered Foraminifera of this genus in the blue clay of Kakaunui, in New Zealand.

Cristellaria.Lign.109,fig.4.—The shell is in the form of a compressed Nautilus; it has a single aperture, which is situated at the angle of the keel; the cells are oblique.

This genus comprises seventy fossil species, which occur in the Lias, Oolite, and Chalk. Living species are abundant in almost every sea.

Flabellina.Lign.111,fig.4a.—In a young state, this shell, like the preceding, resembles that of a Nautilus, and the cells are oblique; but in the adult, are of a zigzag (chevron) form. It has a single round aperture.Fig.4ashows a young individual, seen in profile;fig.4ba lateral view, exhibiting the obliquity of the cells.

Species of Flabellina are often found in cretaceous strata. The genus is not known in a recent state.

Polystomella.—In its general form this genus resembles the other nautiloid shells above described, but its structure differs essentially; for there are several apertures along the side of the shell, as well as the opening in the last segment. The cells are simple, and each is a single cavity. The figures and details of structure, given by Professor Williamson, must be referred to, for an insight into the organization of this beautiful and complicated type of Foraminifera.[319]One species of Polystomella is said to occur in the Chalk; I have not detected this genus in our cretaceous deposits. Recent species swarm in our seas; and may be easily obtained from the mud and sand on the shores at Brighton.[320]

[319]Trans. Microscop. Society, vol. ii.[320]Mr. Poulton has specimens of the shells, and the bodies of the animals deprived of the shell, mounted for the microscope.

[319]Trans. Microscop. Society, vol. ii.

[320]Mr. Poulton has specimens of the shells, and the bodies of the animals deprived of the shell, mounted for the microscope.

Lituola.Lign.111,fig.3a.—In a young state the shell is nautiloid, as infig.3b, 3d; but becomes produced by age, and assumes a crosier-like form, as infig.3a. The cellsare filled with a porous testaceous tissue, as shown infigs.3b, 3c; which also illustrate the foraminiferous character of the shells in this class of animals; for both the external testaceous covering, and the septa of the cells, exhibit perforations.[321]

[321]The perforations are omitted, by mistake, in the figure of the adult shell,fig.3a.

Lign. 112. Spirolinites in Flint.Chalk. Sussex.(By the late Marquis of Northampton.)The specimens are only sections of the shells Imbedded in flint, and seen as opaque objects with a lens of moderate power. The small figures denote the size of the originals.

SPIROLINA.

Spirolina.Lign.112.—The general form resembles that of Lituola: the young shell being a discoidal, involute, andbecoming produced by age; but the internal structure is different; the cells are simple cavities.

The chalk and flints of Sussex abound in these crosier-like shells, whose existence in the cretaceous rocks was first made known by my deeply lamented friend, the late Marquis of Northampton. The annexed lignograph, from drawings by his lordship, shows the form and structure as displayed by sections in fractured flints. Four species were named by Lord Northampton (seeWond.p. 32-5); but it is doubtful whether all the specimens belong to more than one species; the apparent diversity of structure may arise simply from the different planes in which the sections happen to have been made.

Globigerina.Lign.109,fig.1.—The shell is turbinated, the cells are spheroidal, and the last, or terminal one, has a semilunar aperture at the umbilical angle. Several fossil species abound in the Chalk and in the tertiary deposits; and many living species swarm in our seas.

Lign. 113. Nonionina Germanica, × 290.A recent specimen of the body of the animal; the shell removed by acids.From the North Sea, Cuxhaven.(M. Ehrenberg.)a, a.—Naviculæ and other organisms in the segments of the animal.

Nonionina.—Lign.113.—A nautiloid shell, with simple cells; the last cell has a single narrow aperture placed transverselyover the dorsal aspect of the spire. One species occurs in the chalk formation of Germany; several in tertiary deposits, and in the existing seas.

The figure,Lign.113, represents the body of the animal deprived of its shell, to illustrate the nature of certain fossils from the Chalk.

Lign. 114. Foraminifera in Chalk and Flint.× ×.(Seen by transmitted light.)Fig.1, 2, 3, 6.—Different forms ofRotalia.2.—Resembles the recentRotalia stigma; Ehrenberg;> from the North Sea, near Cuxhaven.4.—Portion of aNautilus, showing five chambers, partially separated, each pierced by the siphunculus: in Flint, fromIreland.5.—Thebody of a Rotalia, in Flint; the shell is not apparent.

ROTALIA—ROSALINA.

Rotalia.Lign.114.—The shell, though nautiloid in its contour, is regularly turbinated, the cells not globular; the last cell has a central, semilunar, transverse, aperture. There are fifty fossil species. The Rotaliæ appear in the Lias, Oolite, and Chalk, in immense numbers, and swarm in the present seas.

Rosalina.Lign.109,fig.5.—The shell is depressed; the spire apparent on one side; the aperture is a prolongedslit extending from one cell to another, and opening on the umbilicus; that is, on the side opposite to the spire. There are eighteen fossil, and many recent species of this genus.

Textularia.Lign.109.—This, and the following genus, belong to that order of Foraminifera in which the segments or cells are arranged in two or three distinct axes (ante,p. 342.), and by their gradual increase give rise to an elongated conical but not spiral shell, which in its general outline resembles that of certain gasteropoda, but is easily distinguished by its internal structure. The shell is conical, compressed, formed of alternate cells, with a transverse aperture placed on the inner side. Upwards of thirty fossil species are known. The Textulariæ are in great abundance in the cretaceous rocks; and, together with Rotaliæ and Rosalinæ, constitute a large proportion of the minute organisms of the secondary formations as well as of the present seas.

Verneuilina.—Lign.109,fig.3.—A turriculated shell, with a slit or aperture transverse to the axis of involution, and placed on the umbilicus. This genus, of which but one species is known, is peculiar to the cretaceous deposits.

CHALK ANIMALCULITES.

Strata composed of Foraminifera.—From this concise exposition of the characters of the genera that most frequently occur in a fossil state, we pass to the examination of the organic composition of those limestones which are in a great measure made up of the debris of Foraminiferæ. We will commence the investigation with that common substance, the white chalk of the South-East of England.

It has long been known that a large proportion of the purest white chalk consists of minute chambered shells,[322]and corals.

[322]Often termedPolythalamia, meaning many chambers or cells.

Mr. Lonsdale, some years since, first showed that by brushing chalk in water, and examining the sediment, shells,corals, and foraminifera might be obtained in abundance; but it was not at that time suspected that the residue of the detritus was almost entirely composed of distinct organic structures, so minute as to require high magnifying powers, and a peculiar mode of manipulation, for their detection and definition.

M. Ehrenberg demonstrated that even the fossils discovered by Mr. Lonsdale are colossal, in comparison with the infinitesimal structures of which the finer particles of the chalk consist; for one cubic inch of the limestone is found to contain upwards of a million of well-preserved animal organisms.

The chalk, therefore, is an aggregation of extremely minute fossils and inorganic particles. The yellow, soft, writing chalk of the North of Europe, according to M. Ehrenberg, is composed of about half its mass of organic remains; but in the chalk of the South of Europe, the fossils predominate. The amorphous atoms of the cretaceous limestone do not, as was formerly believed, arise from a precipitate of lime previously held in solution, but from the disintegration of the assembled organisms into more minute calcareous particles; and these have subsequently been reunited by a crystalline action, into regular, elliptical, granular, bodies.

M. Ehrenberg infers that the compact flint nodules have originated from an aggregation of pulverulent particles of siliceous organisms; and upon this hypothesis explains the absence of flint nodules, and the abundance of siliceous infusoria, in the beds of marl that alternate with the chalk in the south of Europe, and their presence in the chalk of northern Europe, in which the marls are wanting. In other words, he supposes, that in the former case the siliceous shells of the animalcules were spread abroad and deposited in layers or strata; and in the latter were aggregated into nodular masses. This opinion is not, however, supported by facts; for, though the animal origin of lime, flint, and iron,may be admitted to a great extent, yet the deposition of silex and lime from aqueous solutions, is carried on at the present moment upon an enormous scale; and it cannot be doubted that to such a process is attributable the formation of the nodules, layers, dikes, and veins of flint, which traverse the chalk, and other rocks.[323]

[323]See my "Memoir on a Microscopical Examination of Chalk and Flint," Annals of Nat. Hist., Aug. 1845.

The most abundant microscopic organisms in the English chalk and flint which I have examined, areRotaliæ, orRosalinæ, andTextulariæ. Immense numbers of minuter Foraminifera also occur, and many shells, which are unquestionably the young state of testaceous Cephalopoda (asNautilus,Ammonite, &c.).

Spines of Sponges, and of Echinoderms, also frequently appear in the field of the microscope: and a spongeous structure is so common in flint, that an eminent observer conceives that all the flints, both nodular and tabular, have originated from poriferous zoophytes;[324]an hypothesis altogether inadmissible.

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