CHAPTER I. PHYSICAL AND GEOLOGICAL FEATURES OF POLYNESIA.
[Image of page 1]
PHYSICAL AND GEOLOGICAL FEATURES OF POLYNESIA. -- VOLCANIC, CRYSTALLINE, AND CORAL ISLANDS; ATOLLS, BARRIER REEFS, ETC.
CORAL ISLAND, WITH CENTRAL LAGOON.
THE countless pelagic islands that lie scattered, separately or in clusters, over the bosom of the
[Image of page 2]
Pacific, are, almost invariably, composed of either coral or volcanic rocks, or of both; the latter often forming the basis on which, the coral structure has been reared. They have been divided into three distinct groups or classes, namely, High, Median, and Low Polynesia. The first of those, or "High Polynesia," includes the lofty and mountainous islands that tower above the height of 6000 feet; such as the Sandwich Islands, the Marquesas. Tahiti, and New Zealand. These are all of volcanic origin, consisting of basalt, as well as other igneous formations; and contain, in many instances, craters at present in action. They possess a rich and fertile soil, and are eminently beautiful and picturesque; their highest peaks are clad with snow, whilst their valleys display all the luxuriant verdure of tropical vegetation.
"Median Polynesia" comprehends those islands of a moderate altitude, and which are more than 100 feet above the ocean level. These are mostly composed of carbonate of lime, probably coralline rock crystallized, and elevated at some former period by plutonic agency; they are eminently fertile, clothed with evergreen forests, and producing a variety of delicious fruits.
"Low Polynesia" comprises all the countless groups of low coral islands of recent formation, and in the course of formation, many of which are raised only a few feet above the waves, and at times are washed and flooded by occasional storms. They most frequently occur in the form of an oval or horseshoe, wooded with cocoa-nut trees, and enclosing a
[Image of page 3]
smooth central lagoon, around which the outer ring or belt of coral reef acts as a breakwater from the ocean waves. This ring-like sea-wall has generally one, often many, openings, and is always highest on its windward side. These islands are called "atolls," or lagoon islands. The soil upon the coralline islands is often thin and poor, so that little vegetation is produced upon them, if we except the widely-spread cocoa-nut palm (which always flourishes best in close proximity to the ocean), the pandanus, and a few stunted bushes of hibiscus and other dwarf shrubs.
The entire basin of the Pacific is known to be one vast theatre of volcanic action; and every island yet examined in Polynesia consists either of volcanic rooks or coral limestone, and in many instances of basalt and lava, having a girdle or fringe of coral reefs. It has been conjectured that the "atolls," from their generally circular shape, their central lagoons, and the shelving conical form of the submarine mountains, are nothing more than the crests of extinct volcanoes rising to the ocean's surface, and having the rims and bottoms of their craters overgrown with coral.
It is generally supposed that every "atoll" marks the site, and traces the outline of sunken land, on which the corals have found a base to build; and, as no corals have been found living and working at a greater depth than from twenty to thirty fathoms, the old notion that coral-polyps were able to build up their steep walls from great depths in the sea is now no longer regarded as tenable.
[Image of page 4]
Amongst the many remarkable structures reared by madrepores on the summits of marine mountains in the Pacific and Indian Oceans, the Keeling Atoll, or South Keeling Island, as described by Darwin, is peculiarly characteristic of this class of formations. The Keeling Atoll, like other coral islands, has been entirely formed by the growth of organic beings, and the accumulation of their detritus. The reef, which essentially forms the atoll, is described as a ring encircling the lagoon on all sides except at the northern end, where there are two open spaces, through one of which ships can enter. This reef varies in width from 250 to 500 yards; its surface is level, or very slightly inclined towards the lagoon, and at high tide the sea breaks entirely over it. The most frequent coral in the hollows on the reef is pocillopora verrucosa, and is of a beautiful pale lake-red. As soon as an islet is formed, and the waves are prevented from breaking entirely over the reef, the channels and hollows in it become filled up with cemented fragments, and its surface is converted into a hard, smooth floor, like an artificial one of freestone. This flat surface varies in width from 100 to 300 yards, and is strewed with a few large fragments of coral torn up during gales; it is uncovered only during low water. Nothing can be more singular than the appearance at low tide of this "flat" of naked stone, especially where it is externally bounded by the smooth convex mound of nulliporae, appearing like a breakwater built to resist the waves, which are constantly throwing
[Image of page 5]
over it sheets of foaming water. The highest part of the island is close to the outer beach, and averages from six to ten feet above ordinary high-water mark. At a distance of 2200 yards from the breakers. Captain Fitzroy found no bottom with a line of 7200 feet in length; hence the submarine slope of this coral formation is steeper than that of any volcanic cone; and Mr. Darwin is of opinion that submarine cliffs must exist, from the sudden increase of depth at some places, and from the sounding-linE having been cut as if rubbed at certain depths.
Barrier reefs are found either running parallel to the coasts of large masses of land, such as that of New Guinea, or enclosing one or more islands at a greater or less distance from the shore. The Great Barrier Reef, which guards the north-east coast of Australia, and extends to New Guinea, is upwards of 1000 miles in length, and varies in distance from the shore from ten or fifteen to one hundred miles, and even more, though its average distance is about thirty miles. Many of the islands thus protected by barrier reefs consist of lofty volcanic peaks towering to the clouds; their fertile shores are washed by the placid waters of the surrounding lagoons, whilst, at various distances of from a few yards to two or three miles, the sheltering coral ring shields lake and island from the angry ocean. Against this wonderful barrier the long-rolling surges of the Pacific are driven with terrific violence; towering in one wide sheet of water to an immense height, they roll over their majestic crests
[Image of page 6]
of foam, and, with a noise like thunder, dash against this rocky bulwark, spending their harmless vengeance upon its rugged surface: whilst thousands of glittering rainbows are produced by the sun shining through the spray that rises from the breaking billows.
Mr. Jukes, in his narrative of the voyage of H. M. S. "Fly," says, speaking of these lagoon-enclosing reefs, "The water was perfectly clear, and of great and almost unfathomable depth right up to the outer slope or submarine wall of the reef. The long ocean swell being suddenly impeded by this barrier, lifted itself in one great continuous ridge of deep blue water, which, curling over, fell on the edge of the reef in an unbroken cataract of dazzling white foam. Each line of breakers was often one or two miles in length, with not a perceptible gap in its continuity."
The corals, which are the chief agents in reef-making, or, at all events, in continually building up and keeping them together, are many of them of gigantic size. There are massive kinds at work on the outer edges of the reefs that could not exist within the lagoons, where the small and delicately-branching species flourish. It is difficult to get a sight of them alive and working, because of the heavy seas that break outside; but large blocks are sometimes rolled up on the reefs, twelve or fifteen feet in diameter; and crags of porites, twenty feet long by ten feet high, all forming one connected mass of polyp-cells, have been observed detached from their place of growth, and hurled by the waves on to the shore.
[Image of page 7]
BARRIER REEFS; CORALS.
For the non-destruction of these barrier reefs by the unceasing action of the surf breakers, Mr. Darwin thus accounts. He says, "The ocean throwing its breakers on these outer shores appears an invincible enemy, yet we see it resisted and even conquered by means which at first seem most weak and inefficient. No periods of repose are granted, and the long swell caused by the steady action of the trade-wind never ceases. The breakers exceed in violence those of our temperate regions, and it is impossible to behold them without feeling a conviction that rooks of granite or quartz would ultimately yield and be demolished by such irresistible forces. Yet these low, insignificant coral islands stand, and are victorious, for here another power, as antagonist to the former, takes part in the contest. The organic forces separate the atoms of carbonate of lime, one by one, from the foaming breakers, and unite them into a symmetrical structure. Myriads of architects are at work night and day, month after month, and year after year, and we see their soft gelatinous bodies, through the agency of the vital laws, conquering the great mechanical power of the waves of an ocean, which neither the art of man, nor the inanimate works of nature could successfully resist."
The delicate and beautifully-coloured corals which branch out into long fronds and clustering boughs as it were, are inhabitants of the inner edges of the reefs within the calm lagoon. Mrs. Farmer tells us, "It is a pleasant thing to float in a canoe over the shallow parts of these very clear waters on a fine day. Keeping your oars still, you may watch the
[Image of page 8]
busy and beauteous life below; you may see fish of bright hues playing in and out of the coral stems and branches, seeming to be glad of a refuge from their enemies in the open sea; while the gentle ripple of the waves, touched by the light of a brilliant sun, heightens the charm of the scene."
Mr. Jukes thus draws us a graphic picture of a sheltered nook in one of these lagoons--a natural aquarium on a grand and inimitable scale. He says, "Round masses of maeandrina (brain coral) and astraea were contrasted with delicate leaf-like and cup-shaped expansions of explanaria, and with an infinite variety of branching madreporae and seriatoporae; some with mere finger-shaped projections, others with large branching stems, and others again exhibiting an elegant assemblage of interlacing twigs, of the most exquisite workmanship. Their colours were unrivalled, vivid greens, contrasting with more sober browns and yellows, mingled with rich shades of purple, from pale pink to deep blue. Bright red, yellow, and peach-coloured nulliporae clothed those masses that were dead, mingled with beautiful pearly flakes of eschara and retipora; the latter looking like lace-work in ivory. In among the branches of the corals, like birds among trees, floated many beautiful fish, radiant with metallic greens and crimsons, or fantastically banded with black and yellow stripes. Patches of clear white sand were seen here and there for the floor, with dark hollows and recesses beneath overhanging masses and ledges."
It is somewhat as follows that the inner sides of
[Image of page 9]
PROGRESS OF A CORAL ISLAND.
the reefs gradually become islands, on which trees grow, and human beings find a habitation. After the reef rises so high as to be nearly bare at low water the corals cease to build. Large masses of their dead cells are often detached by the combined action of the sun and waves, and thrown by the surf on to the summit of the reef, so as to give it a higher elevation. Then the washing of the waves wears down the more delicate kinds of corals, and rubs them into powder; this powder fills up vacant spaces, whilst unseen chemical agency aids in forming masses of limestone. Wherever, throughout the ocean, rocks are formed, there is also certain to be an abundance of animal life. Sea-weed creeps over them--shell-fish attach themselves in thick layers, whilst the teeth and palates of fishes, and other debris, serve to increase the mass. Drift-timber is constantly cast ashore, and the floating cocoa-nut, and other stray seeds from some neighbouring isle, at length take root in the soil that is soon formed by the deposits of sea-birds and other decaying matter. At length, as vegetation spreads, the soil becomes more productive; and, after the lapse of ages, the feathery cocoa-nut trees wave their lofty heads against the blue sky, whilst the lower forms of animals begin to make the "atoll" their home. Lastly, a canoe is driven ashore, and its shipwrecked occupants become masters of this new creation.