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The formation of Coral Reefs is partly a vital and partly a chemical process. No more wonderful instance of animal agency is to be found than that which is exhibited in the work of the coral zoophyte. In the warmer latitudes of every sea, where the water is sufficiently clear and tranquil, we find the homes of the polype, which, endowed with the power of secreting lime from the waters of the sea, rears with it its polypidom, and forms those mountains commonly known as coral reefs and islands. According to Cuvier's arrangement of animals, the builders of these reefs belong to the fourth, or lowest division of animal life, which consists of zoophytes, or radiated animals. These zoophytes are subdivided into five classes, the fourth of which contains the polypes, of which there are three orders; viz., fleshy polypes, gelatinous polypes, and polypifera polypes, the last of which are the coral-builders. These tiny architects of mighty works begin life in the shape of a gemmnule, which, dropping from a pile of coral into the sea, swims about for some hours or sometimes days, until it finally settles on another portion of old coral, or other hard substance which is to be found at the bottom of the sea. I then gradually lifts itself into the shape of a tube, round the edge of which there soon appears a delicate, thick-looking rim, the beginning of the house in which it is to live. This chalk, deposited by the live jelly out of its own substance, looks at first like a milky fluid, but it soon becomes hard, and then the sides begin to rise up, something like a fine pencil-case. Next comes the growth of the mouth and the tentaculæ of the polype, which enable it to catch the food upon which it is to live, and with which it carries on its work of building. After the appearance of the tentacula, the remaining jelly takes the form of a tube polype, and for the rest of its life goes on ever catching food and building therewith new stories to its house. As the outside case grows higher and higher, the polype follows up after the rising walls of its house, always keeping its mouth just below the top of the wall, but high enough to enable it to thrust out its tentacula. The coralline polype does not approve of solitude. Vast numbers, past man's power to count, build their cells side by side. Nor are their chalk pipes dead walls, but rather, so long as the polype lives in them, living bones; for out of the body of the animal run little vessels into the outer case, making their body and their wall one living whole.

Further, all the polypes in one pile are joined together by their vessels into one great living creature; so that what one polype eats passes on to assist in nourishing the rest.

Very numerous are the family and genera of these wonder-working zoophytes; the more abundant, according to Darwin, being the Madrepores, Astræas, Porites, Meandrinæ, and Mallipores, at moderate depths. Millepores, Seriatopores, and other delicate forms, are found at depths varying from 15 to 20 fathoms. Although we are used to speak of coral reefs as rising from "unfathomable depths," in ordinary cases reef-building polypifers do not flourish at greater depths than 20 or 30 fathoms; a certain amount of light and warmth being necessary to their existence. It is true, however, that detached coral and coral drift have been found at the depth of 270 fathoms; but this was not the work of the true reef-building zoophyte. As has been said, not only animal but also chemical agency is at work in the building of coral reeks, which, though chiefly effected by the lime-secreting zoophytes, are in some measure owing to the promiscuous aggregation of marine débris.

Coral, as produced by the polypes, is almost pure carbonate of lime, which, although soft and porous at first, becomes in time so hard and compact as to be used by the South Sea Islanders for architectural purposes. However, in the formation process the pure secretion of the animals envelopes sponges, sea-weeds, star-fishes, sea-urchins, drift-wood, drift-coral, shells, and such-like; the whole mass being consolidated into a compact rocky mass by the growth of the new coral and through chemical action—that action consisting in the transfusion and percolation of carbonated water among the particles of lime of which the pure coral is mainly composed.

All this agency combined with that produced by the infiltration of carbonate of lime from decomposed coral, gives to the rock a brecciated appearance, and renders it extremely analogous to some older limestones of the secondary formations.

On the other hand, coral reefs are sometimes found closely resembling in formation some of the earthy varieties of chalk; those arise from the consolidation, in lagoons and sheltered water-channels, of particles of decomposed coral and other matter. A third species of coral-stone has a sparry crystalline aspect but this is only found when the reefs have been upheaved by subterranean agency; as for instance, on the hills of Tahiti, where a stratum of fossil coral exists.

Thus, in the work of the polype we may find almost every gradation of limestone; varying from the soft chalky mass formed by animals even now alive, to the hard, compact texture of saccharoid marble, which has stood against wind and wave for thousands and thousands of years.

The rapidity of the growth of these rocks is a