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rocks in question ascend, like perpendicular ramparts, from a very great depth. Such structures, it is obvious, must derive their origin from the animals themselves, unless we attempt to account for them on the grounds of a new hypothesis, which intimates that they may grow in a manner similar to the common seaweed, and that the insects found on them are analogous to those which take up their abode on trees and herbs ; a conclusion to which the arborescent appearance of some corals, and the fungous forms of others, are supposed to give a certain degree of countenance.
By some authors these animalcules are called saxigenous, or rock-making, polypes. They are supposed to begin their operations by selecting a suitable spot, such as the summit of a volcano, or the top of a submarine mountain. Having chosen their site, with a reference, it should seem, to an ultimate object, they work with incredible diligence until they reach the surface, above which, as we have already stated, their nature and habits do not permit them to proceed. Mr Lyell remarks, that the circular or oval forms of the numerous coral isles of the Pacific, with the lakes in their centre, naturally suggest the idea that they are nothing more than the crests of submarine volcanoes, having the rims and bottoms of their craters overgrown with coral. This opinion is strengthened by the conical shape of the islands, and the acute angle at which they plunge on all sides into the surrounding ocean. It has also been observed, that although within the circular reefs there is usually nothing discernible but a lagoon, the bottom of which is covered with coral, yet within some of these basins, rocks, composed of porous lava and other volcanic substances, rise up, resembling eminences of igneous origin which have been formed in an epoch not beyond the limits of human observation.*
It is stated by Mr Forster that the polypes raise their
• Lyell's Principles of Geology (4 vols 12mo, 5th edition, Lond. 1837), vol. iii. pp. 287, 288. Dalrymple's Historical Collection of Voyages and Discovery in the South Pacific Oce (4to, Lond. 1770), p. 22.
habitation gradually from a small base, always spreading more and more in proportion as the structure grows higher. The materials, he adds, are a kind of lime mixed with some animal substance. A few miles to the leeward of Turtle Island there was observed a considerable reef, over which the sea broke at all points, and no part of it was entirely under water. East and north-east of the Society group are numerous islets which are only partially raised above the surface; while in some clusters the elevated portions are connected by reefs occasionally dry at ebb-tide. The insects inhabiting these dikes appear as if desirous to shelter their dwelling from the impetuosity of the winds and the rage of the ocean ; but as within the tropics the current of air commonly proceeds from one quarter, they, by instinct, endeavour to extend a ledge enclosing a lagoon, which is thereby entirely screened against the power both of the waves and of the breeze. This process, which is not inconsistent with the opinion entertained by Mr Lyell, is generally admitted to be the most probable cause whence have originated those low islands which stud the intertropical latitudes of the Great South Sea.*
But the High Islands, such as appear decidedly to have had an igneous origin, are more important, viewed as the habitations of man, than the coral ridges just described. It has been already remarked that nearly all the regions of the Pacific bear the most unequivocal marks of having been the scene of volcanic action on a vast scale, and that the prevailing features of the larger masses of land confirm the inference which may still be drawn from the imperfect traditions of the people. Some of the isles present volcanoes in a state of activity; others exhibit only the form and altitude which denote a similar origin in remote ages; while a third class display undoubted tokens of having been violently changed by the force of subterranean fire, if not by the more
* Forster's Observations, p. 149-151.
sudden shock of an earthquake. Tanna and Pico belong to the first class ; Otaheite, Huaheine, and Bolabola, are distinct specimens of the second ; and Easter Island may be adduced as an example of the third. In this last all the rocks are black, burnt, and honeycombed ; some have the appearance of slag; nay, even the soil, which is but thinly spread over the calcined masses, bears a close resemblance to dark-yellow ochre.*
Mr Williams, the author of an interesting work on the South Sea, divides the islands of Plutonic origin into two orders, the Mountainous and the Hilly. In the former, the height of the land varies from 2000 to 15,000 feet above the level of the sea, the towering summits gradually rising from their base till they are lost in the clouds. The sides of these magnificent elevations are clothed with bright verdure of various shades, blending together in a very striking manner the elements of grandeur, wildness, sublimity, and beauty. All the islands of this class exhibit indubitable marks of volcanic eruption. In many of them the rocks are composed of a fine-grained basalt ; in others pumice is found, together with stones of varied appearance, which have evidently undergone the action of fire. It is clear, moreover, that all these islands have at one time been under water; for at the top of the highest peaks, coral, shells, and other marine substances, are seen in great abundance. The savage and romantic appearance of the rocks, their broken, abrupt, and irregular forms, also indicate that at some remote era they have been subjected to disruption by the power of some mighty agent affecting their interior.
The islands which fall under the denomination of Hilly, vary in height from 100 to 500 feet, and are, in a great degree, destitute of the volcanic phenomena which abound in the others. The rocks, which are said to resemble the aragonite of the Giant's Causeway, are supposed to have been originally coral, and to owe
* Forster's Observations, p. 153.
their present hardness to the action of the atmosphere, as well as to that of water percolating from above through the mass while in a porous state.*
All the Society Islands and many others in the Pacific are surrounded by a belt of secreted rock, from two to twenty yards in width, and situated at a distance which varies from a few feet to more than a mile. Against this barrier the long rolling waves of the ocean are driven with a terrific violence, and towering in one sheet to an immense height, dash themselves upon it with majestic power, though without producing any perceptible effect. The water between the reef and the shore is placid and transparent, at the bottom of which, and in the sloping sides of the banks, an enchanting picture presents itself. Coral of every shape and of every hue, intermingled in the richest profusion, suggests to the imagination the idea of a submarine flower-garden; while among the branches of the madrepore, and the spreading leaves of other varieties, fish of every colour gambol about in conscious security.
But, in point of fact, the distinction now stated between mountainous and hilly does not apply to the physical principles according to which the several islands have been formed; for, as is well known, every one of them hitherto examined consists either of volcanic rocks or of limestone. With regard to the thickness of the coral masses, it has been thought that the species of polypus which contributes most to their formation, does not live where the water is deeper than twentyfive or thirty feet. But it is not improbable that the branched madrepores, which exist at very considerable depths, may lay the foundation of a reef, and raise the platform on which the others are built.
These conjectures, however, do not possess any strong claim upon our confidence, and are only entitled to be ranked with those other opinions which have been brought forward relative to the rate at which coral is supposed to grow in the vaults of the great deep. A
• Williams' Narrative of Missionary Enterprises in the South Sea Islands (8vo, Lond. 1837), p. 21.
modern author observes that the tendency of polypes to multiply in the seas of warm climates is so great, that the bottom of the tropical ocean swarms with countless myriads of them, ever actively employed in constructing their small but enduring habitations. Almost every volcanic cone and ridge, under the surface of the ocean, is made the nucleus of a colony. The calcareous secretions are accumulated into enormous banks or reefs, sometimes stretching to a length of many hundred miles; and these continually rising to view in spots where they were unknown before, endanger the navigation of many parts within the torrid zone.*
There is reason to doubt whether the process be quite so rapid as these remarks might seem to establish. The period of observation has not yet been sufficiently extended to afford ground for any conclusions as to the rate or the precise mode in which such additions to the crust of the earth are effected. The latest surveys, indeed, appear to warrant the opinion that the growth of coral is not so quick as has been commonly imagined. During the late expedition to the Pacific, directed by Captain Beechey, no positive information could be obtained of any channel having been filled up within a given period; and it seems placed beyond doubt, that several reefs had remained more than half a century at nearly the same elevation, at least if measured by the flow of the tide. It is admitted, nevertheless, that the increase of coral limestone may vary greatly according to the situation of mineral springs ; for, in volcanic countries, these are known to issue in considerable abundance at the bottom of the ocean. Examples occur even in the Mediterranean, where they sometimes cause the sea at great depths to be fresher than at the surface; a phenomenon which is said to be very common near some of the islands in the Pacific.t
* Buckland's Geology and Mineralogy considered with Reference to Natural Theology (2 vols 8vo, Lond. 1836), vol. i. pp. 443, 444. Williams' Narrative, p. 29.
*+ Lyell, vol. iii. p. 282. Beechey's Narrative of a Voyage to the Pacific (2 vols 8vo, Lond. 1831', vol. i. p. 258.