OBSERVATIONS OF CAPTAIN FLINDERS.

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Holland, examined the coral formations in process there; and his remarks seem to me to give the true theory of coral reefs, if there be added the fact of the natural precipitation of carbonate of lime from the sea-water in which it is held in solution, and the formation of the cement by electrical agency and heat.

"It seems to me," he writes, "that when the animalcules, which form the coral at the bottom of the ocean, cease to live, their structures adhere to each other by virtue either of the glutinous remains within, or of some property in salt water; and the interstices being gradually filled up with sand and broken pieces of coral washed by the sea, which also adhere, a mass of rock is at length formed. Future races of these animalcules erect their habitations upon the rising bank, and die in their turn to increase this monument of their wonderful labors.

"The care taken to work perpendicularly in the early stages, would mark a surprising instinct in these discriminative creatures. Their wall of coral, for the most part, in situations where the winds are constant, being arrived at the surface, affords a shelter, to leeward of which their infant colonies may be safely sent forth; and to this, their instinctive foresight, it seems to be owing that the windward side of a reef, exposed to the open sea, is generally, if not always, the highest part, and rises almost perpendicular, sometimes from the depth of two hundred, and perhaps many more fathoms."

Commander Wilkes, of the United States Exploring

Squadron, sounded only one hundred and fifty fathoms off from the perpendicular coral cliff of Aurora Island, but found no bottom with a line of that length.

To be constantly covered with water seems necessary to the continued existence and activity of the coral animalcules. It cannot, indeed, be perceived that they are living at all, except in holes upon the coral reef itself that are below low-water mark, where we have often watched the progress of their rising structures, when we could not detect with the closest inspection the busy little builders themselves; yet imagination has been busy in tracing their work as Eneas was, under the cloud, at young Carthage :

Miratur molem Æneas, magalia quondam;

Miratur portas, strepitumque, et strata viarum
Fervet opus.

Almost as fast as they build, the coral sand, always suspended and washed about in sea-water, fills up the little cells, and pores, and interstices of the minute masonry, while broken remnants of dead coral and other matter thrown up by the sea are caught and cemented to the growing wall, and form a solid mass with it as high as the common tides reach. When that limit is attained, and the surface of the reef is now out of, or even with the water, the labor of the coralligenous zoophyte is over, the sea gradually recedes, the rampart rises, the limed debris or fragments upon it, being now rarely covered with water and dried by the sun, lose their adhesiveness and become brittle remnants, form

GRADUAL FORMATION OF AN ISLAND.

187

ing what is called sometimes a key upon the top of the reef, from the Spanish Cayo.

This new bank is, of course, not long in being visited by sea-birds; salt-plants take root upon it, branches of floating sea-weed are caught and entangled by it; muscles, and crabs, and echinuses, and turtles, and krakens, perhaps crawl upon it and leave their shells, and a soil begins to be formed. By and by a cocoanut, or the drupe of a tropical Pandanus, is thrown ashore; landbirds light on it and deposit the seeds of shrubs and trees, and augment it, perhaps, with a layer of guano. Every high-tide, and still more, every gale, adds something to the bank in the shape of matter-wrecks, organic or inorganic. At length appears the blue hummock of a tropical island, and last of all comes man to take possession, cast there by Providence, and glad not to have the sea his grave, or in quest of discovery and gain.

I have repeatedly seen and stepped upon progressive and unfinished parts of creation like this, where, as traced by a poet-observer of the Processes of Nature—

The atom thrown from the boiling deep,
The palm-tree torn from its distant steep,
The grain by the wandering wild-bird sown,
The seed of flowers by the tempest strown,
The long kelp forced from its rocky bed,
And the cocoanut, on the waters shed,-
These gather around the coral's lee,

And form the isle of the lonely sea.

There is an island in Australia called Half-way Island, from the fact, we believe, that nature does not yet

seem done with it, or to have finished its creation; yet above the reach of the highest spring-tides or the wash of the surf in the heaviest gale. A navigator who has visited it says, that he distinguished in the coral rock which forms its basis, the sand, coral, and shells formerly thrown up and cemented together by the lime always held in solution by sea-water. Small pieces of wood also, pumice-stone, and other extraneous bodies which chance had mixed with the calcareous substances when the cohesion began, were inclosed in the rock, and in some cases were still separable from it without much force.

We have observed the same at the lonely South Pacific island of Rimatara, over whose verdure-clad coral remains we once had a joyous day's ramble. The same is true, also, of other reefs at the Sandwich Islands, where, as at Honolulu for instance, blocks of it are quarried from exposed parts, and used for building purposes, (to which it is well adapted,) besides being burned into lime.

From an admirable work on corals, published in the Scientific and Natural History series of the London Tract Society, and containing a number of very accurate wood-cuts, representing different species of coral polypi and corallines, we learn that coral is found in different parts of the Mediterranean and Red Sea, not only attached to rocks, but also to movable bodies, as stone vases and fragments of lava. It is also discovered at different depths, but thrives best in a warm and sunny

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189

aspect. Light operates powerfully in its growth, and its deposition by the living creature is by no means rapid.

It is thought to require eight years for a stem of Mediterranean or Red Sea coral to obtain the average height of ten or twelve inches, in water from three to ten fathoms deep; ten years if the water is fifteen fathoms; twenty-five or thirty years if the water is a hundred fathoms; and at least forty years if the depth is one hundred and fifty fathoms.

It is more beautiful in shallow water, where the light reaches it, than where an immense body, absorbing most of the luminous rays, deprives it of their curiously modifying influence. Having attained its full growth, it is soon pierced in every part by worms, (which attack even the hardest rocks,) loses its solidity, and but slight shocks detach it from its base. The polypi perish, and the coral stem, by attrition with the sea-worn pebbles, as it rolls along, is soon reduced to powder, or coral sand.

Captain Hall says of the reefs in the seas about Loo Choo, Indian Ocean, what I have often heard American whalemen say of those in the Mozambique channel, which is the region of ocean most prolific in curious shells, that when the sea has left a reef for some time between the tides, it becomes dry, and appears to be a compact rock, exceedingly hard and ragged. But no sooner does the tide rise again, and the waves begin to wash over it, than millions of worms protrude themselves from holes on the surface, which were before quite invisible.