Thoughts on a Pebble, by Gideon Algernon Mantell, a Project Gutenberg eBook.

In the Red Coral, so largely employed in the manufacture of beads, brooches, and other ornaments, not only the animalcules, but also their receptacles, are composed of a soft perishable substance. When alive, the polypes, as well as the investing fleshy integument, are of a delicate bluish tint; the internal calcareous axis alone possesses the peculiar red colour. Upon being taken out of the sea, vitality quickly ceases, the soft parts decompose, and the beautiful crimson stone commonly known as the true coral, is obtained free from all traces of the soft mass by which it was secreted. Although an actual investigation of the facts described can only be instituted near the seas of warm climates, yet our coasts abound in certain coral-zoophytes in which similar phenomena may readily be observed. Most persons in their rambles by the sea-side must have noticed on the fuci, algæ, shells, pebbles, &c., patches of a white earthy substance, which when closely examined resemble delicate lace-work. These apparently calcareous incrustations are clusters of the zoophytes termed the Flustra, or sea-mat.[AT] When removed from the water, this aggregation of polypes seems coated over with a glossy film or varnish; and with a lens of moderate power the surface is seen to be full of pores, disposed with much regularity. If viewed under the microscope while immersed in sea-water, a very different appearance is presented. Every pore is found to be the opening of a cell whence issues a tube fringed with several long feelers or arms; these expand, then suddenly contract and withdraw into the cell, and again issue forth; the whole surface of the Flustra being covered with these hydra-like animalcules. The Flustra, therefore, like the corals, constitutes an assemblage of polypes, each individual being permanently fixed in a durable cell, and the whole attached to a common integument by which the calcareous frame-work was secreted and maintained.[AU]

Note X. Page 53. Infusorial earth from Richmond in Virginia.

The greatest natural operations are produced by the most simple and apparently inadequate agents: for as the illustrious Galileo emphatically remarked, "La nature fait beaucoup avec peu, et ses opérations sont toutes également merveilleuses." The profound thinker Hobbes, in the same spirit observes, "The majesty of God appeareth no less in small things than in great, and as it exceedeth human sense in the immensity of the universe, so also doth it in the smallness of the parts thereof." This sublime truth is strongly impressed on the mind of the geological inquirer, who perceives that whole countries and mountain ranges of great elevation and extent, are wholly composed of the aggregated remains of beings of such infinite minuteness that but for the powerful optical instruments of modern times, their presence would never have been suspected.

A few years only have elapsed since the sagacious Ehrenberg first drew attention to this subject, and pointed out the proper method of investigation;[AV] and so rapid has been the progress of discovery in this department of science, that infusorial deposits, as these beds of fossil animalcules are designated, have been detected in every quarter of the globe. A fact equally unexpected and remarkable has also been established, namely, that at the present moment similar minute living agents are largely contributing to the increase of the solid materials of the crust of our planet.

The infusorial earth of Virginia, alluded to in the text, is a yellowish siliceous clay, forming a deposit from twelve to fifteen feet in thickness, upon which the towns of Richmond and Petersburgh are built. The surface of the country over which it extends is characterized by a scanty vegetation, owing to the siliceous nature of the soil dependent on the minute organisms of which it almost entirely consists. When a few grains of this earth are properly prepared for microscopic examination, immense numbers of the shields or cases of animalcules are visible under a magnifying power of 300 diameters; in fact, the merest stain left by the evaporation of water in which some of the marl has been mixed, teems with these fossil remains.[AW]

These organisms are of exquisite structure, and comprise many species and genera. The most beautiful and abundant are the circular shields, termed Coscinodisci (sieve-like disks), which are elegant saucer-shaped cases, elaborately ornamented with hexagonal apertures disposed in curves, somewhat resembling the engine-turned sculpturing of a watch; these shells are from 1/1000 to 1/100 of an inch in diameter. A segment of one of these disks, highly magnified, is represented in Lign. 18, fig. 2. The body of the living animalcule was protected and enclosed by a pair of these concave shells, the perforations admitting of the exsertion of filaments or tentacula. This species of Coscinodiscus abounds in the present seas, and constitutes no inconsiderable proportion of the food of Pectens and other testaceous mollusca.[AX]

All the animalcules found in the Richmond earth are marine, and most of them belong to genera, and many to existing species; although the position of the American strata proves that they are referable to a period of immense antiquity.

In Germany, beds of a white infusorial earth, resembling magnesia in appearance, and termed Bergh-mehl, or fossil farina, occur at Bilin, and several other places: at San Fiora in Tuscany, near Egra in Bohemia, in the Bermudas, Barbadoes, &c., similar deposits have been discovered; all being composed of the shields of various kinds of animalcules. But I must not extend these remarks, and will only add a few observations on the infusorial earth of Barbadoes, which has but recently been brought under the notice of geologists by Sir Robert Schomburgk, and is especially interesting for the exquisite beauty and variety of its organisms, and the circumstances under which the deposit occurs.

Barbadoes, an island of the West Indies, is about twelve miles in length from north to south, and consists of coral reefs, capped in one district by tertiary sandstones and limestones, which attain a height of 1200 feet above the sea. Over the rest of the island, coral reefs form the entire surface, which is divided by vertical walls of coral, some of them nearly 200 feet high, into six terraces, indicating as many periods of upheaval. In the lowest reef, Indian hatchets have been found twenty feet above high water mark; shewing that the last movement, at least, took place within the human period. The tertiary strata are more or less inclined, and in many places vertical, and contorted. Strata of marl, several hundred feet thick, predominate; and there are beds of bituminous coal, sandstone, clays, and ferruginous sands. Arenaceous limestone containing teeth of sharks, spines of echini, and shells, forms the summit of a hill nearly 1,000 feet high. The white marls abound in 300 species of the most beautiful siliceous infusoria; many are peculiar, others the same as occur in the Richmond earth, and some belong to recent species.[AY]

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Note X. Page 53. Infusorial earth from Richmond in Virginia.