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 376 marine diatoms exist, or convey any adequate idea of the amount of nutriment thus elaborated for the sustenance of animal life. Perhaps an extract from one of Dr. Wallich’s descriptions may help to set the facts in a striking light. Speaking of the Atlantic between the Cape and St. Helena, he says, “It was here that for many degrees, and in bright weather, the ship passed through vast layers of sea-water so thronged with the bodies of a species of Salpa, as to present the consistence of jelly. What their vertical limits were it was impossible to discover, owing to the speed at which the ship was moving. They appeared to extend deep, however, and in all probability were of a similar character to what is called ‛whale food’ in higher latitudes. Each of these Salpæ measured about half an inch in length, but so close was their aggregation, that by a sudden plunge of an iron-rimmed towing-net, half the cubic contents, from which the water had percolated, generally consisted of nothing but one thick glutinous mass of pulp. Each individual presented a minute yellow digestive cavity the size of a millet-seed, which contained Diatomaceæ, Foraminiferæ, and other organic particles.” Another equally good instance of the enormous production and consumption of siliceous matter daily and hourly going on in the ocean is furnished by the guanos which we import for use as manures. The birds whose excreta compose this useful material live chiefly if not entirely on fish; the fish in their turn have fattened upon smaller marine fry, many of whom are wholly microphagous, or in other words, depend on the Diatomaceæ for their subsistence, and the imperishable flinty valves of these minute organisms, therefore, after passing safely through the stomach first of the mollusc and then of the fish, are found at last forming no inconsiderable percentage in the constituents of almost all the guanos. It has been calculated that the ships visiting the Cincha islands remove no less than 500 tons of pure Diatomaceæ yearly in their cargoes of manure, and the estimate is probably within the mark. With these aids we shall be able to credit this apparently insignificant family with a fecundity and indestructibility sufficient for the production of very great geological results, perhaps also to conceive of their having been able, so far as quantity is concerned, to furnish all the siliceous matter for the manufacture of the chalk flints; but we have still to learn how the small and separate organisms become compacted into a homogeneous and apparently structureless mineral mass.

We have already discovered in the stomachs of the Salpæ and their relatives, precisely similar collections of organisms as those which the flint exhibits in a fossil condition, and if we could find a plastic siliceous cement, or some similar agent present in the molluscoid digestive organs, we could readily imagine a flint factory at work on a small scale in the intestines of every “school” of whale food in the ocean. Dr. Wallich believes he has detected such a cementing power in the well-known tendency of the diatom valves to cohere when in a state of comparative purity, and in the solvent action of animal alkalies upon the silex which composes them. Both in the guanos and in all other rich earths, the diatomist frequently meets with little masses consisting entirely of closely aggregated valves, sometimes so perfectly combined as to have lost almost all their original external appearances, and only to be distinguished from amorphous bits of silex by some few ill-preserved markings and sculpturings which have not altogether disappeared. The purer the state of the valves, the stronger this affinity is developed, and the contents of the salpean stomachs, therefore, are singularly well prepared for the aggregative action by reason of the perfect separation of mineral and organic matter brought about in the process of digestion.

Now we know from microscopic examination of the chalk itself, that the ocean in which it was deposited swarmed with Diatomaceæ, and these doubtless formed the food, then as now, of microphagous mollusca, similar in their general characteristics to the Salpæ and their congeners. By this means the supplies of silex which the sea was ever producing, became collected, and separated from the organic matter with which it was associated, thus acquiring a strong tendency to coherence, and becoming fully prepared for the next great change in its circumstances and condition. The mollusc, like the diatom, was destined not merely to eat, but to be eaten. The great whale family, living solely on such dainty fare as jelly-fish and Salpæ, destroy daily hosts of these organisms far more numerous than it is possible for us to realize, and it was in the complex stomachs of these marine monsters that the aggregation of siliceous matter was finally completed. Here it was first accumulated in large quantities, and became subjected to the solvent agency of alkaline animal fluids. These combining chemically with the silex would form soluble hydrates, which, while they effectually obliterated all the delicate sculpturings of the valves, bound the whole closely together by a natural cement till the masses voided at intervals, but still in a viscous state, sank to their resting place to harden and consolidate slowly upon the white and living carpet spread for their reception by the busy Foraminiferæ below. Here a new chapter in flint history opens, and new conditions supervene, to add fresh complexities to this already intricate biography. The floor of the chalk sea boasted other and higher forms of life than the humble foraminifer, and was thickly scattered with sponges, zoophytes, sea-urchins, and starfishes, not very different in form from those of recent oceans. On this surface, strewn with various animals, the plastic silex fell, enclosing here a sponge and there an echinite, and burying each in a flinty sarcophagus, over which it was destined that wise men should speculate and wonder when the long geologic æons were past away, and the white bed was lifted within the reach of quick eyes, diligent hands, and reasoning brains.

Such is the last published life-history of the chalk flints. The theory is certainly startling, but it meets the difficulties which beset the question more fully than any previous hypothesis. By referring the substance of the nodules to aggregated