Chapter 21

[324]"Memoir on the Siliceous Bodies in the Chalk, Greensands, and Oolite," by J. S. Bowerbank, Esq. F.R.S. &c. Geol. Trans, vol. vi. p. 181.

The assertion that the chalk every where consists almost wholly of organic bodies must likewise be accepted with some limitation. The assiduous observer who searches for hours chalk and flint carefully prepared, and with the aid of an excellent microscope, though he will meet with immense numbers of organisms, will often find a great proportion of atoms without traces of structure. Neither is there much variety in the easily recognizable forms of the English chalk (I write from my own limited experience); many of the species described by M. Ehrenberg, and others, are few and far between; and I have not detected a single example of diatomaceæ. The student therefore must not be discouraged,if, after perusing the glowing accounts of the discoveries of M. Ehrenberg, he should not be more successful than myself. It must, however, be borne in mind, that as the fossil remains of the larger animals and plants are commonly associated together in particular localities, while in similar rocks in other districts they are altogether wanting; in like manner, some strata of the same series may be made up of organic bodies, while others are destitute of them. In fact, such is the case with our English Chalk: some layers in the cliffs at Dover are literally an aggregation of foraminifera and corals, while other beds have but few vestiges of organic remains.

CHALK FORAMINIFERA.

Lign. 115. Chalk-dust; chiefly composed of Foraminifera;highly magnified.(Seen by reflected light.)a, a.Rotaliæ.b.Textulariæ.

Foraminifera of the Chalk and Flint.Lign.115.—If a few grains of soft white chalk from Gravesend or Dover be examined under a high power (1/4inch object-glass of Ross), groups of foraminifera will be perceived, chiefly of Rotaliæ, Rosalinæ, and Textulariæ, as shown inLign.115. If some of the powder be immersed in Canada balsam, (as directed in the instructions at the close of this chapter for preparing chalk for microscopical examination,) the outline of the shells, and the cavities of, the cells, will be apparent;as in the sketchesLign.114,antep. 351. If a chip or slice of flint, rendered transparent by immersion in oil of turpentine or Canada balsam, be viewed first with a low power, (1 inch object-glass,) to discover a good specimen, and afterwards under a high magnifier, (a1/4or an1/8object-glass,) the form of the shell and of the cells will be distinctly seen, as inLign.116. In this beautiful fossil Rotalia, the segments are as sharply defined as in a recent example: and one of the cells (a) is seen to be lined with quartz crystals.

Lign. 116. Section of a Rotalia in flint.(Seen by transmitted light, and showing the chambers partially filled with mineral matter.)a.A cell lined with spar.(× 250diameters.)

At first sight this fossil might be mistaken for a nautilus partially filled with spar; but the reader will remark that the septa, or partitions, have their convex surface towards the aperture; whereas in the shells of the Cephalopoda (Nautilus, Ammonite, &c.) the septa are concave anteriorly. InLign.114,fig.4, a series of casts in flint of the septa of a young Nautilus is represented; by comparing it with the Rotaliæ in the same lignograph,figs.2, 3, this distinction will be obvious. And here it may be necessary again to point out the essential character of the animal of the foraminifera, as distinguished from that of the cephalopoda with chambered shells. In the latter, the body of the mollusk only occupies the large outer chamber; the internal compartments are empty dwellings, which the animal has successively quitted in the progress of its growth, and with which it has no connexion except by the siphunculus. Inthe Rotalia, and allied forms, the body of the animal is inclosed within the shells, and occupies every chamber contemporaneously at every stage: the cells are always filled by the segments of the body. Hence when the shell, which is calcareous, is dissolved in weak hydrochloric acid, the soft body is exposed, and seen to extend to the innermost chamber. The segments are connected by a membranous tube, which some naturalists regard as a common channel of communication between the several digestive sacs of which the body consists; for minute diatomaceæ which the animal has swallowed, (according to Ehrenberg,) are seen within the membranous sacs; as shown inLign.113, which represents the body of a Nonionina, deprived of its shell. The importance of obtaining a correct idea of this structure will presently appear.

When a recent Rotalia is immersed in dilute acid, the soft parts of the body, deprived of the shell, may be obtained entire; they consist of a series of little bags or sacs, united by a tube. The constituent substance appears to be a tough membrane, and is generally of a rich brown or amber colour. The sacs are sometimes full of a granular substance, but are often empty and collapsed.

FOSSIL BODIES OF ROTALIÆ.

Fossil remains of the soft parts of Foraminifera.Lign.113.—When examining chalk[325]and flint under the microscope with the view of discovering the fossil bodies described by M. Ehrenberg, I observed that the cells of the Rotaliæ in flint were frequently occupied by a substance varying in colour from a light amber to a dark brown, and closely resembling in appearance the body of the recent foraminifer deprived of its shell. Under a high power, the folds of the membranous sacs and the connecting tube were apparent, and I felt convinced that the substance filling the cells was not inorganic, but the original animal tissues inthe state of molluskite.[326]In short, that the animal had become immersed and preserved in the fluid silex like the insects in amber. The appearance of the first discovered example of this kind is represented inLign.117.

[325]In 1845.[326]Molluskite: a name by which I proposed to distinguish the carbonaceous substance resulting from the soft bodies of testaceous mollusca.

[325]In 1845.

[326]Molluskite: a name by which I proposed to distinguish the carbonaceous substance resulting from the soft bodies of testaceous mollusca.

Lign. 117. Rotalia in Flint, with the fossilized body of the animal in the shell:highly magnified.(Seen by transmitted light.)

SOFT BODIES OF FORAMINIFERA.

In a paper read before the Geological Society in 1845,[327]I ventured to affirm the animal nature of the fossils in question; but the supposition was regarded by geologists as very startling and unsatisfactory; and as the specimens were enveloped in flint, the appearance was attributed to the infiltration of mineral matter of a different colour from the surrounding silex, into the empty chambers; a circumstanceof frequent occurrence in Ammonites, Nautili, and even in the foraminifera; for the latter are often filled with chalk, flint, silicate of iron, crystal, &c. as inLign.116. In these instances, I conceive the shells were either empty when immersed in the fluid chalk or flint, or speedily became so by the decomposition of the soft parts of the animal. But in the fossils under consideration, I believe the live animal was suddenly enveloped, and hermetically sealed, as it were, in its shell, and that putrefaction was thus prevented. The uniformity in colour, and the structure of the substance in the cells, appeared to me incompatible with its assumed mineral origin, and I resolved to follow up the inquiry by an examination of Rotaliæ in chalk; in the hope that by dissolving the shell in acid (as in recent foraminifera), the body of the animal might be detected in an unmineralized state. After many fruitless attempts, several examples of the soft bodies of Rotaliæ were obtained from the grey chalk of Dover, in an extraordinary state of preservation.[328]

[327]Notes of a Microscopical Examination of the Chalk and Flint of the South-East of England, with Remarks on the Animalculites of certain Tertiary and Modern Deposits. Published in the Ann. Nat. Hist., Aug. 1845.[328]To Henry Deane, Esq. of Clapham Common, I am indebted for some of the most illustrative specimens hitherto obtained.

[328]To Henry Deane, Esq. of Clapham Common, I am indebted for some of the most illustrative specimens hitherto obtained.

Lign. 118. The Soft Bodies of Foraminifera; extracted from Chalk:highly magnified.(Viewed by transmitted light.)Fig.1.—An exquisite example of the body of aRotalia;the sacs partially collapsed.2.—Body of aRotalia; the sacs distended with a darkgranular substance.

These marvellous relics were obtained by subjecting a few grains of the chalk to the action of weak hydrochloric acid, by which the calcareous earth and the shells it contained were dissolved; the residue, consisting of particles of quartz and green silicate of iron, and remains of the animal tissues, were placed, in the usual manner, in Canada balsam. Two exquisite specimens of the bodies of Rotaliæ thus obtained are figured inLign.118.[329]

[329]I communicated this discovery to the Royal Society. See Philos. Transactions, 1846, p. 465.

In these fossils the sacs are generally more or less distended with a dark substance, as inLign.118,fig.2: but in some, they are empty and collapsed in folds, just as membranous pouches would appear under similar conditions; as in the exquisite fossil,Lign.118,fig.1.

The sacs regularly diminish in size from the innermost to the outermost cell, and vary in number from fourteen to twenty-six; being more numerous than in the recent species of Rotaliæ that have come under my notice. In some instances small papillæ are seen on the outer surface of the integument; apparently the vestiges of the pseudopodia.[330]

[330]Admirably as my excellent engraver, Mr. Lee, (of Prince's Square, Kennington,) has executed the lignograph,118, I would refer the reader to the steel plate in Philos. Trans. 1846, pl. xxi. for figures of these marvellous fossils.

Not only is the form and general character of the animal substance preserved, but even its flexibility; for in one instance, the body, released by the solution of the chalk and shell, was uncoiled and pressed out, as shown inLign.119,fig.4.

In one specimen, (figured inPhilos. Trans.1846, pl. xxi.fig.10,) the membrane of the largest sacs is much corrugated, and disposed in numerous duplications, probably owing to the empty state of these segments, when immersed in the chalk; but the discoidal contour of the original iswell preserved. This fossil so closely resembles the decalcified body of a recent Rotalia or Rosalina, that an eminent observer who saw it under the microscope at the meeting of the Royal Society, without knowing its history, concluded it to be the body of a recent animal. This extraordinary preservation of the soft delicate tissues of an animal of the cretaceous seas, invisible to the unassisted eye, through the incalculable ages that must have elapsed since the deposition of the chalk in which it was enshrined, is a fact as remarkable as the occurrence of the carcass of the Lena Mammoth, in the frozen soil of Siberia.

FORAMINIFERA IN CHALK AND FLINT.

Lign. 119. Remains of Foraminifera; in Chalk and Flint.(Viewed by transmitted light; highly magnified.)Fig.1.—Shell of aRosalina, filled with mineral matter; in flint.2.—Soft parts of aTextularia; in flint.3.—Cells ofTextularia elongata; filled with mineral matter; the shell not visible; in flint.4.—The soft body of aRotalia, deprived of its shell, and partially uncoiled; obtained from Chalk, × 450diameters.

The soft parts of other foraminifera have been discovered in a similar state of preservation. A fine example of the body of a Textularia, in flint, is figured,Lign.119,fig.2.

The form and disposition of the segments inTextulariaelongata, is shown inLign.119,fig.3. These cells are filled with inorganic matter. The shell of a Rosalina filled with an opaque mineral substance, forming casts of the cells, is represented inLign.119,fig.1.

The preservation of the soft parts of foraminifera and of mollusks, in a fossil state, is a phenomenon of frequent occurrence, and no longer questioned by geologists, notwithstanding the scepticism with which my first announcement of the fact was received. Dr. Bailey, of West Point Military Academy, soon after the publication of my first paper, sent me specimens from the marls of New Jersey.

Foraminiferous Limestones of India.—So much doubt was expressed as to the accuracy of my opinion respecting the nature of the fossil Rotaliæ, that its corroboration by observations on certain limestones in India, by H. J. Carter, Esq. Secretary of the Bombay Royal Asiatic Society, was as gratifying as unexpected. According to the researches of that gentleman, the south-east coast of Arabia is chiefly composed of two distinct limestone formations; the one averaging about 4,000 feet above the level of the sea, and the other 600 feet. The latter forms the desert of Akaf, and with the intervention of the mountains of Oman, which belong to the greater formation, passes up into the lower Sindh; while the former constitutes the high land of the coast, which, parting from the western border of the Desert of Akaf, extends nearly to Cape Aden.

The limestones of both these groups, or formations, consist chiefly of foraminifera; the largest forms being visible to the naked eye. Mr. Carter states that his attention was first directed to the organic composition of the Porebunder limestone, which is imported into Bombay for building, by small amber-coloured specks on the stone, that resembled the soft parts of foraminifera figured and described by me in the Philosophical Transactions. "On subjecting a portion of the limestone to the action of acid, I found them to bewhat Dr. Mantell's observations had led me to expect, the actual remains of the animals, of exquisite beauty in form and symmetrical development. The minute kinds in the Porebunder stone do not average more than1/900of an inch in diameter; and the composite forms are held together by thread-like attachments, which indicate the tubular communications that existed between them when living."[331]

[331]"On the Existence of Beds of Foraminifera, Recent and Fossil, on the South-East coast of Arabia," by H. J. Carter, Esq.; Proceedings of the Bombay Royal Asiatic Society, 1848,

FORAMINIFEROUS DEPOSIT AT CHARING.

Foraminiferous Deposit at Charing.Lign. 109.—The little town of Charing, in Kent, has acquired a celebrity among those naturalists who are interested in the present inquiry, by the researches of William Harris, Esq. F.G.S. who some years since made known the existence of a remarkable deposit of chalk detritus, about one foot in thickness, which extends over the outcrop or exposed surface of the firestone in that locality.

This bed consists of a soft, whitish, tenacious clay, which, when immersed in water, is found to be largely composed of minute grains, that prove to be foraminifera. These shells belong to many species and genera; and are associated with the cases of entomostraceous crustaceans, spicula of sponges, &c. The organisms readily separate from the amorphous particles by washing, and specimens may be easily obtained as distinct and perfect as if recent. SeeLign.109,ante,p. 342. Intermingled with the cretaceous forms, are minute fresh-water shells, apparently derived from a modern source.

The Charing deposit appears to have originated from the action of water on the unconsolidated chalk of the neighbouring Downs, before the surface of the hills was protected by a covering of vegetable soil.

Through the liberality of Mr. Harris, I have been able to examine an extensive series of the Charing Foraminifera; and Prof Williamson has figured and described the principaltypes in an interesting memoir in the Transactions of the Manchester Philos. Soc. vol. viii. 1847. As in most of the cretaceous strata, the prevailing species are referable to the genera Textularia, Rotalia, Rosalina, Cristellaria, Lagena,[332]&c. There are numerous spicula of sponges, and needle-like calcareous prisms, which are the detritus of the fibrous shells, called Inoceramus.

[332]This interesting type of Foraminifera is the subject of a Memoir by Prof. Williamson; Annals, Nat. Hist. 1848, vol. i.

Foraminifera of the Oolite, Lias, &c.—The occurrence of certain genera in particular rocks has been incidentally noticed in the previous descriptions, and it is needless to particularize any localities of the Oolite, Lias, and other secondary deposits. It must suffice to state that Dr. Carpenter, Prof Williamson, Prof Phillips, Mr. Sorby, Mr. Rupert Jones, and other able observers, have figured and described foraminifera from the strata between the Chalk and the Carboniferous formations: the report to be drawn up by the two first-named gentlemen for the British Association, will present a resume of the British fossil genera and species.

It is deserving record, that no vestiges of foraminifera have been found in the Wealden strata; the fluviatile origin of those deposits renders it improbable that the remains of these marine organisms should occur in great numbers, yet from, the estuary character of some of the beds, the presence of foraminifera might be expected.

Foraminiferous Deposits of the United States.—Dr. Bailey has made us familiar with the foraminiferous rocks of North America. The various memoirs on this class of fossil animalculites, and on the diatomaceæ (ante,p. 93.), published in the American Journal of Science, and in the Smithsonian Transactions, attest the acumen, and unwearied spirit of research, of this able observer. Not only from the United States, but from numerous localities inAsia and Arabia, Dr. Bailey has transmitted me specimens of limestones containing foraminifera, chiefly of the genera Rotalia and Textularia.[333]

[333]From Beyrout, Damascus, the Mount of Olives, Anti-Libanus.

In the calcareous marls of the Upper Missouri river, extending nearly to the Rocky Mountains, similar fossils are met with.

In the interior of Florida, the white orbitoidal limestone is traversed by flint; and the calcareous and siliceous masses are full of microscopic foraminifera.[334]

[334]Smithsonian Contributions, vol. ii. p. 161.

FORAMINIFERA OF THE CARBONIFEROUS FORMATIONS.

Foraminifera of the Carboniferous Formations.—In the carboniferous limestones of England, the late Mr. Bowman, Prof. Tennant, and Mr. Darker, detected shells of foraminifera, apparently of the genus Fusulina.[335]Prof. Phillips mentions the occurrence of nautiloid foraminifera in the palæozoic limestones of Carrington Park, South Devon, and Yorkshire.[336]

[335]Edinburgh New Phil. Journal, vol. xxx. p. 44.[336]Proceedings of the Polytechnic Society of the West Riding of Yorkshire, 1845.

[335]Edinburgh New Phil. Journal, vol. xxx. p. 44.

[336]Proceedings of the Polytechnic Society of the West Riding of Yorkshire, 1845.

Dr. Dale Owen is said to have obtained "well characterized polythalamia from the oolitic portion of the carboniferous (Pentremitic,ante,p. 298.,) limestone of Indiana."[337]And M. de Verneuil discovered a species of Fusulina, in the Millstone-grit of the coal formation of the Ohio.

[337]American Journal of Science, vol. xlvi. note to p. 311.

But the most remarkable deposits of foraminifera in the palæozoic rocks, are those of Russia, described by Sir Roderick Murchison.[338]The upper beds of the Mountain limestone in the Lower Volga, consist of laminated calcareous shales, composed of an aggregation of shells of Fusulinæ. Bands of limestone, through a vertical extent of two hundred feet, are loaded with Fusulinæ; layers from five inches to five feetin thickness, consist of a pure white Fusulina limestone; the foraminifera are all of one species, theFusulina cylindrica.

[338]Geology of Russia in Europe, vol. i. p. 86. pl. i. fig. 1.

Foraminiferous Limestone of New Zealand.—"On the eastern coast of the Middle Island of New Zealand, to the north of Otago, strata of yellow and fawn-coloured limestone appear on the surface at Ototara, and continue to Kakaunui. This rock is generally friable and porous; it contains terebratulæ, spines and cases of echinoderms, pseudo-belemnites, teeth of sharks, &c. A microscopical examination shows it to be in a great part composed of an aggregation of very small polythalamia."[339]The specimens of the Ototara limestone received from my son, are very rich in minute corals and shells, and foraminifera of the European cretaceous type: species of Rotalia, Cristellaria, Globigerina, Textularia, Rosalina, Nodosaria, Dentalina, &c. Among them are two forms which occur at Charing: namely,Rosalina Lorneiana,Lign.109,fig.5, and Textularia elongata: of the latter a specimen in flint is figured,Lign.119,fig.3. The soft parts of Rotaliæ are preserved in the Ototara limestone, as in our chalk.

[339]Notes on the Geological Structure of the Middle Island of New Zealand, by Walter Mantell, Esq. of Wellington. 1848.

There are likewise, as at Charing, cases of Entomostracæ of the generaBasidiaandCythereis.[340]

[340]See Geol. Journal, vol. vi. p. 339. pl. xxix.

The assemblage of fossil remains in the Ototara rock has decidedly a cretaceous aspect, but till the geological position of the strata in relation to the other formations of the Island is determined, it would be premature to regard these limestones as the equivalents in time of the Chalk formations of Europe.[341]

[341]A list of the Ototaran fossils collected by Mr. Walter Mantell, is given in Geol. Journal, vol. vi. p. 329.

Tertiary Foraminifera.—The marine tertiary depositswhich contain foraminifera in abundance, are so numerous, that it is unnecessary to particularize any. The sands of the Paris basin in some localities are so full of microscopic forms, that a cubic inch of the mass contains sixty thousand. The friable calcareous strata at Grignon are a loose aggregate of the shells of foraminifera and minute mollusks; and as the fossil shells from that locality are very common, and generally filled with debris, the student will have no difficulty in obtaining specimens for examination. The tertiary argillaceous deposits of England are less rich in foraminifera than the arenaceous; but the usual types occur in the London Clay, at Highgate, Clapham Common, Bracklesham Bay, &c. The Eocene marls of the United States are rich in foraminifera.

FORAMINIFERA OF THE FENS.

Foraminifera of the Fens of Lincolnshire and Cambridgeshire.—Though the alluvial deposits of the fen-districts are comparatively of modern date, yet the rich assemblage of foraminifera contained in the clay of certain districts is so interesting and instructive, that a brief notice of them must not be omitted. The foraminiferous character of the Lincolnshire alluvium was first made known to me by specimens from Bolton, sent me by Professor Williamson; and their extension over a wide area in Cambridgeshire, by a liberal supply from Mr. Smith, of March. The bed that abounds in these shells, is about seven feet beneath the surface, and consists of a fine sea sand combined with carbonaceous and argillaceous matter. By washing about a gallon of this earth in water, an ounce of polythalamia and organic detritus may be obtained. The perfect shells are as fresh as if just dredged up from the sea; the soft parts—the membranous segments held together by their tubular connexion—in many instances remaining in the shell; these parts may be obtained by the solution of the shell in acid. When the Rotaliæ are rendered transparent by immersion in Canada balsam, their appearance, by transmitted light isidentical with that exhibited by the fossil specimens; and if viewed by reflected light, the body may be seen occupying all the cells of the shell; but the segments are somewhat collapsed; evidently from the shrinking of the animal tissues after death.[342]

[342]Two specimens are figured in my notice of fossil Foraminifera. Phil. Trans, p. iv. for 1846, pl. xxi. figs. 13, 14.

The organisms of the Lincolnshire alluvium have been thoroughly investigated by Prof. Williamson; they comprise many species and genera, of the usual types; as Rotaliæ, Rosalinæ, Polystomellæ, Textulariæ, Lagenæ, Nodosariæ, &c. It is remarkable, that though a marine estuary deposit, no vestiges of diatomaceæ have been observed.

The bed so rich in foraminifera, extends west and south-west of the Wash. Mr. Smith sent me a mass of sandy clay, from a well sunk in the town of March, to the depth of twenty-five feet, that was loaded with these beautiful organisms.[343]

[343]The reader interested in these inquiries should peruse the highly interesting Memoir by Prof. Williamson, "On some Microscopical Objects found in the Mud of the Levant, and other Deposits." Manchester Phil. Trans, vol. viii.

Recent Foraminiferous Deposit at Brighton.—An interesting fact connected with the phenomena under review is deserving record. The presence of the fossils of an older formation, in strata subsequently deposited, and in part composed of the detritus of the rocks whence the organic remains were derived, is not uncommon: such fossils are termed by the French geologists "fossiles remaniés." The nature of these re-deposited fossils is generally obvious; either by the water-worn condition of shells, bones, &c. or from their containing particles of their parent bed; or if casts, from their mineral composition. Thus in the chalk of St. Catherine's Mount, near Rouen, there are numerous casts of Ammonites, Scaphites, and other shells, composedof marl full of particles of greensand. These have evidently been washed out of the preceding cretaceous beds of firestone or glauconite; and re-deposited in the chalk strata in which they are now imbedded.[344]* * * * *

[344]M. D'Orbigny.

Along the sea-shore, to the east of Brighton, there is a bank of sand and calcareous mud, the detritus of the neighbouring cliffs, in the progress of formation; and in this sediment Mr. Reginald Mantell discovered recent Rotaliæ, Nodosariæ, &c., with frustules of Bacillariæ, Coscinodisci, Naviculæ, and other diatomaceæ; associated with cretaceous polythalamia washed out of the chalk,[345]The difference in the aspect of the recent and fossil organisms was so evident, as to leave no doubt of the correctness of this interpretation. Here, then, at the present moment, a deposit is going on, whose organic contents consist of an assemblage of species of living animalcules of our seas, with the fossil forms of the ancient chalk ocean; in like manner as in the bed of the Nile, the Nummulites of the tertiary rocks are being imbedded with the existing mollusks and desmidiæ of that river (ante,p. 345.).

[345]Seeante,p. 99.

GEOLOGICAL DISTRIBUTION OF THE FORAMINIFERA.

Geological distribution of the Foraminifera.—According to the observations of M. D'Orbigny, the first appearance of the tribes of minute beings which have played so important a part in the elaboration of materials for the formation of the sedimentary rocks of the secondary and tertiary ages, and are at this moment invisible but powerful agents in the accumulation of calcareous sediments at the bottom of the sea, was in the Carboniferous epoch, and by a single type, theFusulina(ante,p. 346.). I believe no certain evidence of the occurrence of Foraminifera in Silurian or Devonian deposits has been obtained.

M. D'Orbigny gives the following summary of the distribution of the known fossil and recent species:—

The above statistical view was published six years ago; but the great activity of research that has since prevailed, has largely augmented the known number both of fossil and recent forms. M. D'Orbigny's recent Tables[346]give for the Jurassic or Oolite 10 genera; Cretaceous, 38 genera; Tertiary, 60 genera; but this estimate must be far too low.

[346]Cours Elémentaire de Paléontologie.

I have thus endeavoured to convey a general idea of the highly important results obtained by the microscopical investigation of the minute organisms that enter so largely into the composition of many of the fossiliferous deposits.

Without the aid of the most perfect optical instruments which modern science and art have produced, even the existence of many of these structures could not have been demonstrated; and we cannot doubt, that were the powers of the microscope increased, the fossil remains of beings still more minute would be detected; and that rocks and strata which now appear to consist of amorphous particles of lime, of silex, and of iron, would prove to be the aggregated skeletons of animals, yet more infinitesimal than those which have formed the subject of our contemplations. How strikingly illustrative are these phenomena of the profoundremark of the illustrious Galileo—"La nature fait beaucoup avec peu, et ses opérations sont toutes également merveilleuses."

INSTRUCTIONS FOR THE MICROSCOPICAL EXAMINATION OF CHALK, FLINT, AND OTHER ROCKS.

MICROSCOPICAL EXAMINATION OF CHALK.

Chalk.—The following method is that recommended by M. Ehrenberg, Place a drop of water upon a plate of thin glass, and put into it as much scraped chalk as will cover the fine point of a knife, spreading it out, and leaving it to rest a few seconds; then withdraw the finest particles which are suspended in the water, together with most of the liquid, and let the remainder become perfectly dry. Cover this dried spot of chalk with Canada balsam (the turpentine ofAbies balsamea), and hold the plate of glass over the flame of a lamp, until the balsam becomes slightly fluid, without froth or air-bubbles; it should be maintained in this position (the glass being kept as hot as the finger will bear) for a few minutes, until the balsam is found to have thoroughly permeated the substance to be examined. It is preferable to place a piece of very thin glass upon the balsam, and gently press it down, and allow it to remain. The best flatted crown-glass should be used for placing the chalk or other objects on. It is convenient to have the slips of glass of one size, or the specimens will require different boxes for their reception; three inches by one inch is that usually employed. These objects require to be viewed with a power magnifying three hundred times linear, that is, in diameter; and if the process has been properly conducted, it will be seen that the chalk is chiefly composed of well-preserved organisms. In these preparations the cells of the foraminifera appear at first black, with a white central spot; this is caused by the air contained in those cavities, for air-bubbles always appear as black annular bodies; by degrees, the balsampenetrates into all the single cells, the black rings of the air vesicles disappear, and the structure of the original is beautifully displayed.[347]

[347]Specimens of chalk, flint, and other rocks for examination; or specimens prepared on slides, may be obtained of the microscopic artists, named in the Appendix.

Soft part of Rotaliæ in chalk.—The manner in which I obtained the unmineralized soft bodies of foraminifera from chalk has already been mentioned (ante,p. 360.); but it may be useful to offer a few additional suggestions; for such fossil remains are not easily extracted. Many experienced microscopical observers have not succeeded in obtaining one good specimen; but others have been more fortunate, or persevering.

In several glass test-tubes, (the more the greater chance of success,) put a few grains of chalk powder: pour the tube half full of diluted hydrochloric (muriatic) acid—about ten parts water to one of acid—agitate, and set the mixture by: when all action has ceased add one or two drops of undiluted acid to each tube, and repeat the process at due intervals till all the calcareous matter is dissolved. Pour off the fluid, substitute distilled water, agitate, and then let the sediment subside. The residue will consist of atoms of quartz and other insoluble mineral matter, and animal tissue, if there be any. Then, with a camel-hair pencil, place a small portion of the sediment on a glass slide, and when dry cover it with Canada balsam, and treat it as above directed. Among a dozen slides thus mounted, there will probably be two or more good examples of the body, or detached membranous segments of Rotaliæ or Textulariæ, like those figured inLign.118.

Calcareous Sandstones and Marls.—These substances may be examined by the same process; but if of loose texture, Dr. Bailey recommends that some of the sandypowder should be spread very thinly on a plate of glass, with or without water, and by the aid of a lens of moderate power the roundish grains should be selected and picked up with fine forceps, or the point of a needle, and transferred to another piece of glass, having on one spot a thin coat of Canada balsam. This should be gently heated over a spirit lamp, when the balsam will penetrate the grains, and render them transparent; by this process the minutest shells, &c. may be detected. For a hasty exploration, the dust may be rendered sufficiently transparent by a drop or two of oil of turpentine.

Sandy calcareous marls may be examined by diffusing a few grains in water in a wine-glass, the lighter portions will be suspended in the fluid, and may be placed on glass, and when dry prepared with Canada balsam in the usual manner.

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