The ocean was the home of the first living thing, either plant or animal, that appeared on our planet; seaweeds and salt-water animals are found in much older rocks than any that contain the fossils of land life. Moreover, though called a “wide waste of waters,” and seeming a complete desert as we gaze upon its restless surface on a dull morning, there is a greater number of animals and plants by count, and quite as large a variety, under the waves as above them, and the bottom of the sea—at all events near its margin—is more populous than any bit of woods you ever saw. There exists in our ponds and ditches a race of plants so minute that it requires a powerful microscope to examine them. Under this instrument it is seen that they have delicate, flinty shells or armor, which is of a great variety of forms,—coiled, globular, boat-shaped, spindle-like, and so on,—and always beautifully sculptured. These minute and beautiful diatoms, as they are called, move about freely, and were long supposed to be animals; now they are known to be the simplest of seaweeds, consisting of only one cell. Since life first began, these diatoms, and other microscopic plants much like them, have swarmed not only in the fresh waters, but in all the oceans of the globe, furnishing food for mollusks and all the lowly animals whose food is brought into their mouths by the currents. Innumerable, and as wide-spread as the salt water itself, every one of these myriads of minute plants has left a record; for its delicate, glass-like shell was indestructible, and when the bit of life was lost, it sank slowly down to the bottom. What effect toward perceptible sediment could come from a thing so small that it would scarcely be felt in your eye? One or two, or even a million, would go for little; but century after century, through ages too long for us to comprehend, a steady rain of these exquisitely engraved particles of flint showered down upon the still sea-floor, almost as thickly as you have seen motes in a sunbeam, until there was deposited a layer, many feet in thickness, of nothing but diatom-skeletons. Though this went on to a greater or less extent everywhere in the sea, such deposits are not now to be discovered everywhere, because disturbing causes swept the shells away, or broke up the floor after it had been laid down; but in various parts of the world to-day, you may find wide beds of rock made up wholly of such skeletons, soldered together into hard stone; while in some regions the mud of our sea-bottom appears to consist of almost nothing else. The mighty chalk cliffs of Great Britain and the French coast were built up in precisely this way at the bottom of an ancient sea, whence they have been lifted, but they are composed of much besides diatoms.