536

ANCIENT CORAL-FORMATIONS.

trace very distinctly the mode in which three principal kinds of limestone rocks may have taken their origin from Coral formations.

1109. Now, the Mountain, or Carboniferous Limestone,—a rock very abundant in Britain, extending over large areas beneath the coal-fields, and sometimes exhibiting a thickness of nearly 3000 feet, though in some parts evidently composed of accumulations of Shells, Encrinite stems, &c., exhibits the Coral structure very distinctly in many situations; and these parts are so blended with the neighbouring rock, as to make it appear probable that the latter also was once in the state of coral, but was gradually changed by the process just described. Further, the collections of other animal remains are such, as we should expect to find on the margin of a coral reef or island existing at that epoch; and a similar process of fossilization is taking place at this very time, on the shores of the islands now being built up,— the species of animals imbedded being, however, not the same. The great thickness of the beds of this rock may be very well accounted for, in the same manner as the depth of the coralline masses of recent formation (§ 1077). The Oolite had its origin in the wearing-down of the older limestone beds, with additional matter derived from the skeletons of the races of animals, which existed during the period of its formation. And there can be little doubt that the Chalk-formation owes a considerable part of its substance to the same sources; though part was doubtless derived from the decomposition of shells, and a large proportion in some situations from the remains of animals of extreme minuteness, presently to be described (§ 1107).—There are observed, in rocks of more recent formation,.appearances which still more clearly indicate, that they too were originally formed by Coral-polypes. These are often found within narrow limits, as if they had been reefs or islands of small size. Thus we find a stone, called Coralrag, in Oxfordshire; and very distinct Coral-beds in the Crag of the eastern coast of England. It is interesting to remark, that the remains of Coral, which are found in the older limestones, all correspond with those at present abounding near the equator, and exhibit the Lamelliform structure (§ 1069); whilst they are

gradually replaced in the newer strata, by species more allied to those at present existing in temperate climates. This is one of the many facts, which tend to prove that this part of the earth had, at some former period, a much higher temperature than at present. 1110. Under the class Polypifera, we are probably to include a large and very important group of minute calcareous structures, which, from their strong resemblance to the spiral-chambered shells of some Cephalopoda (§ 894), have been supposed to be formed by animals of that class. Many kinds of them exist on almost every sandy shore; but their extreme minuteness causes them to be usually overlooked, and is also an obstacle to the satisfactory determination of the character of the animals which construct them. By D'Orbigny, who first attracted the attention of Naturalists to the details of this curious group, it was regarded as an order of Cephalopoda; and he gave to it the name of Foraminifera, on account of the communication between the chambers not being established by one principal tubular aperture, the siphuncle, but by numerous minute foramina or porous orifices. The species which exist on the shores of Northern Europe are so minute, as not to be easily examined, even with the microscope; but those which are found in the Adriatic Sea, are sufficiently large to be recognised with the naked eye. From an examination of recent specimens, it has been ascertained that the animals by which these shells are constructed resemble polypes more than any other group, although allied also to the Polygastric Animalcules; and that we are thus to regard these little isolated masses as coralline structures, each being formed by the continual growth of one individual. The character of the animal appears very simple. No part of it is seen externally, except when it is preparing to add a new chamber to its shell. It seems to consist of a series of membranous bags, united together like a string of beads, in the outer layer of which is deposited the calcareous matter that gives the shell-like character. If this latter be removed by a weak acid, the animal membrane will be left; and the spiral may then be unrolled. Whether any of the species possess a regular mouth and tentacula, is yet uncertain.

538

FORAMINIFERA ;-NUMMULITE; MILIOLA.

1111. If we merely consider this group as it presents itself to our notice in the existing epoch, we should have but a very imperfect idea of its importance. The forms under which it once existed in much greater abundance, have been preserved to

and recent inquiries have added to our knowledge of these in a most astonishing degree. The fossils termed Nummulites have long been observed to bear a large share in the formation of extensive beds of limestone rock. They were remarked by Strabo in the stones of the Pyramids; and he informs us that they were commonly reputed to be the petrified impressions of the lentils, which had been used as daily food by the workmen engaged in building them, and takes some pains to refute this idea. By subsequent authors, these Nummulites have been supposed to be the opercula (§ 902) of the Ammonite (§ 897), which, it was imagined, might probably form a new one, every time that it added a chamber to its shell. The discovery, however, of minute shells of a similar character at present existing, and evidently belonging to distinct animals, established their claim to a similar position. Nummulites are probably the largest forms of this group, of which D'Orbigny has described fifty-two genera, and above 600 species; some of them measure about an inch and a half in diameter, whilst a great proportion of the rest are microscopic. Many limestone strata of the tertiary period, are almost entirely composed of the larger Nummulites; and these strata constitute the principal part of several mountain ranges in Southern Europe, such as the Alps, Carpathians, and Pyrenees. The Sphinx, as well as the Pyramids, are composed of a limestone loaded with Nummulites.-A more minute shell, the Miliola, of the same description, but no larger than a milletseed, bears an equal proportion in the mass of limestone strata which are quarried near Paris. "We scarcely condescend," observes Lamarck in reference to this fact, "to examine microscopic shells, from their insignificant size; but we cease to think them insignificant when we reflect, that it is by means of the smallest objects that Nature everywhere produces her most remarkable and astonishing phenomena. Whatever she may seem to lose in point of volume in the production of living bodies,

FORAMINIFERA OF CHALK.

539

is amply made up by the number of the individuals, which she multiplies with admirable promptitude to infinity. The remains of such minute animals have added much more to the mass of materials which compose the exterior crust of the globe, than the bones of Elephants, Hippopotami, and Whales."

1112. But these facts sink into insignificance, when compared with those lately revealed by the researches of Ehrenberg, in regard to the existence of a race still more minute, and its multiplication to a still more inconceivable extent. In examining, with a high magnifying power, Chalk or Whiting in a state of extremely minute division, two sets of particles may be distinctly seen; one set having a crystalline structure, and the other presenting some indications of an organic aspect. If these be rendered more transparent by the use of Canada balsam, it is seen that they are polythalamous (many-chambered) shells of great minuteness; some of them in fragments, and others nearly or quite perfect. The size of these varies from to of a line; and when, as in the Chalk of Southern Europe, they constitute nearly the whole mass, there must usually be above a million in every cubic inch. In the fourth part of a cubic line, or of a grain, from 150 to 200 can be ascertained to exist; and thus there would be from 1800 to 2400 in each grain, and far above ten millions in every pound of chalk. In the chalk of this country, and of other parts of the north of Europe, there is a predominance of particles of a somewhat crystalline character; but even these have in all probability once been combined in organic structures. In pursuing his researches on this subject, it occurred to Ehrenberg to examine the finer particles of calcareous matter, which have been artificially separated from chalk, and are employed for various purposes. Of such, the glazing of the papers with which the walls of apartments are sometimes covered, and that of visiting cards, are partly composed; and on scraping a little of this, and subjecting it to microscopic examination, he was delighted to find the organic structure almost everywhere apparent.

CHAPTER XXV.

OF THE CLASS OF POLYGASTRICA.

1113. THE next class we shall consider is that of Polygastrica; which includes the simpler kinds of Infusorial Animalcules. Its true place in the Animal scale cannot be satisfactorily assigned, until the internal organisation of the beings composing it has been more thoroughly examined. At present, the general simplicity of their character, and the absence of any other decided type, leads us to rank them among the Radiata; although few of them present any distinct indications of the radiated conformation. This class was formerly supposed to contain the simplest members of the Animal Kingdom; many tribes belonging to it having been imagined to obtain their nutriment, by direct absorption from the surrounding fluid. It is now known, however, through the researches of Ehrenberg and others, that most of these possess an organisation of much greater complexity, having a distinct mouth (usually surrounded by a fringe of cilia), and internal cavities for the reception of food.

1114. Wherever any decaying organised matter exists in a fluid state, and is exposed to air and warmth, it will speedily be found peopled with minute inhabitants, of the most varied forms and diversified movements, possessed of considerable activity, and evidently endowed with an energetic system of nutrition. They are, therefore, by no means so nearly allied to Vegetables, as are those inactive and simple creatures-the Sponges and their allies. The cause of the spontaneous appearance of these Animalcules, where no germs were previously suspected to exist, and where it was not easy to suppose that they had been conveyed, has been a matter of much speculation. Many have had recourse to the supposition that the germs formed part, in a latent state, of the living tissues of the animal and vegetable structures, from