[322] Often termed Polythalamia, 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, are Rotaliæ, or Rosalinæ, and Textulariæ. Immense numbers of minuter Foraminifera also occur, and many shells, which are unquestionably the young state of testaceous Cephalopoda (as Nautilus, 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.
[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.