the causeways on either side of the road, and remembering of what a countless multitude these are the representatives, it seems impossible to make the reader clearly grasp the fact that every pebble among them, together with every larger nodule in cliff or pit, are but masses of the collected valves or coverings of marine plants, each in itself so inconceivably minute that the highest powers of the microscope are taxed to detect and analyse their forms, gathered, aggregated, and moulded into shape by the all-powerful influence of affinity, and the marvellous chemistry of digestion—yes, digestion! For we are about to prove, or at least attempt to prove, that the chalk flints are nothing other than the rejected materials of many a primeval cetacean banquet—perhaps the strangest remains of the hugest meal the world has ever seen.
If we pick up a few flints at random, take them home and examine them carefully, we shall discover several noteworthy things.
Selecting a dark specimen, and breaking it with a smart blow into fragments, we shall readily procure several bits thin and transparent enough to bear investigation under the microscope. From these pieces it will be easy to select many which exhibit white dots; others, dark points, or yellowish streaks and patches. Under the lens these little marks resolve themselves into well-preserved and very beautiful fossils, the white dots will turn out to be Foraminiferæ of precisely similar form and species with the chalk builders; the yellowish discolorations will be manifestly due to the presence of silicified sponge tissue, while the remaining organisms will most likely comprise spicula of various kinds and several Xanthidia. We give below a figure of each of these as they appear when highly magnified.
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a. Xanthidium vestitum. b. Xanthidium. c. Foraminiferæ.
d. Spicules of Sponge, greatly magnified.
But besides these fossiliferous flints which we shall come across in any heap of ballast, a good hunt among the larger nodules is sure to reward us by other curious specimens. If we are lucky we shall find here and there a funnel-shaped stone with a serrated rim, looking something like a mushroom; or another, having the external form of some inhabitant of the cretaceous sea, perhaps a star-fish or sea-urchin printed deep and clear upon its surface, as in wax or clay.
The first of these two specimens is a fossil-zoophyte, originally similar in appearance and family to the sea anemones of our aquaria, but now permeated with silex; the second tells its own tale, and is simply a print or cast of urchin, or star, taken in plastic flint; both, however, point unmistakeably to a time in their history when the hard material of which they are composed was soft and impressible, ready to take the form of any firmer substance with which it might happen to come in contact. Bearing this quality in mind let us return to the microscopic fossils of our illustration. These three forms, Foraminifera, Xanthidia, and spicules of sponge represent the chief varieties of organic remains found in the flint, each of them abounded in the waters of the chalk ocean, and the problem before us therefore narrows itself to the consideration how they were first collected and compacted into the plastic masses of siliceous matter which have hardened into the pebbles we daily tread upon. A little study of the life history of one of these organisms, the Xanthidium vestitum, may help us towards an answer.
There is no doubt now that this peculiar organism belongs to the important family called Diatomaceæ, the most minute and extensive group throughout the whole marine and fluviatile world. The general character of these singular creations was sketched in the paper to which we have already referred, but a few further particulars concerning them will be useful. They are best described as free, single-celled vegetable organisms, each enclosed by a siliceous valvular envelope, whose form may be aptly illustrated by a pill-box or two watch-glasses meeting rim to rim; the surface of both cover and bottom of the box being beautifully sculptured with elaborate network like designs, while the whole is so extremely minute as seldom to measure more than from three to five thousandth parts of an inch in diameter. The valves are filled with a brown endochrome, and endowed while living with a marked though limited power of locomotion. In favourable localities recent Diatomaceæ exist in such profusion as quite to baffle description, while their numbers and geologic influence must once have been far greater than at present, whole strata in various districts, often more than a hundred feet in thickness, being entirely composed of their indestructible remains, sometimes forming a light loose soil, such as is found in parts of Bohemia, America, Australia, and Algeria, sometimes compacted into a homogeneous mass, having all the hardness and coherence of the hardest flint, like the semi opal of Bilin, which is nothing other than a cohering mass of coverings from fossil Diatomaceæ. Now this curious and prolific family to which the Xanthidia of the flint belong, formed almost the sole food of certain molluscoid tribes of the cretaceous ocean. Precisely the same thing occurs in our own seas, but we fear it will be impossible to give the general reader more than the vaguest notion of the enormous numbers in which