The common foundation of the clusters of islands discovered by modern navigators in the Pacific ocean, as well as of those belonging to New South Wales, is evidently of coral structure, immense reefs of which shoot out in all directions. There is every reason to believe that the islands which are occasionally raised by the tremendous agency of subterraneous volcanoes, do not bear any proportion to those which are perpetually forming, by the silent but persevering efforts of the sea worms by which coral is produced. Banks of coral are found at all depths, and at all distances from the shore, entirely unconnected with the land, and detached from each other. By a quick progression, they grow up toward the surface; while the winds, heaping up the coral from deeper water, rapidly accelerate the formation of these banks into shoals and islands. They become gradually shallower; and when once the sea meets with resistance, the coral is quickly thrown up by the force of the waves breaking against the bank. These coral banks have been seen in all their stages: some in deep water; others with a few rocks appearing above the surface, just formed into islands without the least appearance of vegetation; and, lastly, others covered with soil and weeds.
The loose corals, rolled inward by the billows in large pieces, strike upon the grounds, and, the reflux being unable to carry them away, become a bar to the coagulated sand with which they are always intermixed. This sand, being easiest raised, is lodged at top; and when its accumulated mass is elevated by violent storms, and no longer within the reach of common waves, it becomes a resting-place to birds whom the search of prey draws thither. Their excrement, feathers, &c., augment the soil, and prepare it for the reception of accidental roots, branches and seeds cast up by the waves, or brought thither by birds. Thus islands are formed: the leaves and rotten branches, intermixing with the sand, produce in time a light black mold, in which trees and shrubs vegetate and thrive. Cocoa-nuts, which continue long in the sea without losing their vegetative powers, having been thrown on such islands, produce trees which are particularly adapted to all soils, whether sandy, rich or rocky. The violence of the waves, within the tropics, must generally be directed to two points, according to the monsoons. Hence the islands formed from coral banks must be long and narrow, and lie nearly in a meridional direction. Even supposing the banks to be round, as they seldom are when large, the sea meeting most resistance in the middle, must heave up the matter in greater quantities there, than toward the extremities; and, by the same rule, the ends will generally be open, or at least lowest. They will also commonly have soundings there, as the remains of the banks, not accumulated, will be under water. Where the coral banks are not exposed to the common monsoon, they will alter their direction, and become either round, or extended in the parallel, or of irregular forms, according to accidental circumstances.
Captain Flinders, in his voyage to the South Pacific, gives an account of an unbroken reef of coral, three hundred and fifty miles long, on the coast of New Holland; and he states, that between that country and New Guinea, the coral formations extend through a distance of seven hundred miles, interrupted by no intervals of more than thirty miles in length. He also gives us a lively and interesting description of a coral reef on the southern coast of New South Wales. On this reef he landed, and the water being very clear round the edges, a new creation, as it were, but imitative of the old, was presented to the view. Wheat sheaves, mushrooms, stags’ horns, cabbage leaves, and a variety of other forms, were glowing under water with vivid tints of every shade betwixt green, purple, brown and white; equaling in beauty, and excelling in grandeur, the most favorite parterre of the curious florist. These were different species of coral and fungus, growing, as it were, out of the solid rock, and each had its peculiar form and shade of coloring; but, whilst contemplating the richness of the scene, the destruction with which it was pregnant could not be forgotten. Different corals in a dead state, concreted into a solid mass of a dull white color, composed the stone of the reef. The negro heads were lumps which stood higher than the rest; and being generally dry, were blackened by the weather; but even in these the forms of the different corals and some shells were distinguishable. The edges of the reef, but particularly on the outside where the sea broke, were the lightest parts; within these were pools and holes containing live corals, sponges, sea-eggs and cucumbers; and many enormous cockles were scattered upon different parts of the reef. At low water, these cockles seem most commonly to lie half open, but frequently close with much noise, and the water within the shells then spouts up in a stream, three or four feet high: it is from this noise and the spouting of the water that they are discovered, for, in other respects, they are scarcely to be distinguished from the coral rock.
His description of a coral island which he afterward visited on the same coast, is truly philosophical and throws great light on these surprising productions of nature.
“This little island, or rather the surrounding reef, which is three or four miles long, affords shelter from the south-east winds. It is scarcely more than a mile in circumference, but appears to be increasing both in elevation and extent. At no very distant period of time, it was one of those banks produced by the washing up of sand and broken coral, of which most reefs afford instances, and those of Torres’ strait a great many. These banks are in different stages of progress: some, like this, are become islands, but not yet habitable; some are above high-water mark, but destitute of vegetation; whilst others are overflowed with every returning tide. It seems to me, that, when the animalcules which form the corals 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, but principally to elevate, this monument of their wonderful labors. The care taken to work perpendicularly in the early stages, marks a surprising instinct in these diminutive creatures. Their wall of coral is for the most part in situations where the winds are constant, and when it reaches the surface, it affords a shelter, to the 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. To be constantly covered with water, seems necessary to the existence of the animalcules, for they do not work, except in holes upon the reef, beyond low-water mark; but the coral sand and other broken remnants thrown up by the sea adhere to the rock, and form a solid mass with it, as high as the common tides reach. That elevation surpassed, the future remnants, being rarely covered, lose their adhesive property; and remaining in a loose state, form what is usually called a key upon the tops of the reef. The new bank is not long in being visited by sea-birds; salt plants take root upon it, and a soil begins to be formed; a cocoa-nut is thrown on shore; land birds visit it and deposit the seeds of shrubs and trees; every high tide, and still more every gale, adds something to the bank; the form of an island is gradually assumed, and last of all comes man to take possession. This island is well advanced in the above progressive state; having been many years, probably some ages, above the reach of the highest spring-tides, or the wash of the surf in the heaviest gales. I distinguished, however, in the rock which forms its basis, the sand, coral and shells formerly thrown up, in a more or less perfect state of cohesion; small pieces of wood, 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. The upper part of the island is a mixture of the same substances in a loose state, with a little vegetable soil, and is covered with the casuarina and a variety of other trees and shrubs, which give food to paroquets, pigeons, and some other birds; to whose ancestors it is probable the island was originally indebted for this vegetation.”
WIDE AND INHOSPITABLE DESERTS.
ASIATIC DESERTS.
In Africa, as well as Asia, there are immense tracts of land called deserts, which consist of vast plains composed of loose sand. Large portions of these are utterly destitute of vegetation, and sometimes, in crossing them, the traveler sees not a hill or mountain, or human dwelling, or even a tree or shrub, or blade of grass. All around is a sea of sand; and far as the eye can reach, it is one scene of lifeless solitude and desolation. These trackless wastes are traversed by caravans, which are companies of travelers usually mounted upon camels. Horses travel in these sands with difficulty. Their feet sink in the soil; they are overcome with heat, and parched with drought. The camel, on the contrary, has a large, spongy foot, which does not sink in the sand; he can bear excessive heat, and by a curious contrivance of nature, is enabled to go without water for five or six days. This valuable creature is called the ship of the desert, because it enables the merchants of Asia and Africa to transport their merchandise over the sea of sand, just as a ship carries goods from one part of the world to another, across the briny ocean. It seems really as if Providence had provided this singular animal on purpose to enable mankind to traverse the great deserts which are spread out upon the eastern continent.