Essay on the Theory of the Earth

by baron Georges Cuvier

The banks of coral which the sea has left exposed in the land, as it retired, are remarkable for their uncommon magnitude. The whole shores of Coupang are formed of them, and the low hills in its vicinity are enveloped in them; but a few hundred yards from the town, they disappear, when distinct strata of slate make their appearance. The corals form a bed over the subjacent rocks from 25 to 80 feet thick.

Every thing announces that, in the Island of Timor, there exist no mountains exclusively formed of corals. As in all extensive countries, they are composed of various substances. Quoy and Gaimard having coasted it for about fifty leagues, sufficiently near to enable them to form an idea of its geography, were able to see that it exhibited volcanic appearances in several parts. Besides it abounds in mines of gold and copper, which, in conjunction with what we have already mentioned, shews in a general way the nature of the rocks of which it is composed.

Perhaps, remarks Quoy and Gaimard, the Bald-Head, a mountain of King George’s harbour in New Holland, which Vancouver has described in passing, and on the summit of which he saw perfectly preserved branches of coral, might be adduced as a fact in opposition to the opinion here advanced. Yet the phenomenon exhibited there, is still precisely the same as at Timor, and in a thousand other places[382]. The zoophytes have built upon a basis previously existing, and they occupy only the surface of it. For why should this Bald-Head differ from Mount Gardner, which, although close by it, is formed of primitive rocks? Besides, Peron says, that it has the same geological constitution. (T. ii. p. 133.)

At Rota, one of the Marian Isles, M. Gaudichaud, detached from the limestone rock, at about a hundred toises above the level of the sea, branches of true madrepores, in perfect preservation. Here are, then, three localities in which they are found at great heights. We have observed them, say the French naturalists, at infinitely lower elevations in several other places, as at the Isle of France, where they form a bed more than six feet thick, between two streams of lava; at Wahou, one of the Sandwich Islands, where they have not a greater elevation, but extend for several hundred toises over the surface of the island. In all these cases, it is necessary to distinguish between the lithophytes, which have, by their living powers, formed continuous masses, from those which, after having been rolled about, broken down by the water, and mixed with sea shells, contribute to form those deposits known by the name of madrepore limestone. The latter sort is nothing but the debris of the former. Deposits of this description occur in the Marian Isles, and in those of the Papous; they occur also on the coasts of France, and in several other places.

It would appear from observations made in Timor and other places, that the species of the genus Astræa which are the only ones capable of covering immense extents of surface, do not commence their operations at a greater depth than twenty-five or thirty feet, in order to raise their habitations to near the surface of the sea. Fragments of these species are never obtained, either with the sounding line, or upon the anchors; nor do we ever see them, unless in places where the water is shallow; while the branched madrepores, which do not form thick and continuous beds, either on the elevated places which the ocean has left, or on the shores where they still exist, live at considerable depths.

It is evident, then, that these corals have erected their fabrics on the summits of submarine hills and mountains; and that all those reefs of Taiti, the Dangerous Archipelago, Navigators’ Islands, the Friendly Islands, &c. are composed of madrepores only at the surface.

We thus consider it demonstrated, that the rocks of the solid zoophytes or coral, are not capable of forming the immense bases on which the greater number of the islands that occur in the Pacific Ocean rest.

There now remains for us to state how these animals, by their union, are capable of raising small islets. Forster, as already stated, has given a very good description of the manner in which this is effected. In fact, when these animalcules have raised their habitations to the surface of the water, under the shelter of the land, and they remain uncovered during the reflux of the tide, the hurricanes which sometimes supervene, by the agitation which they produce in those shallow waters, throw up from the bottom sand and mud. These substances are detained in the sinuosities and cavities formed between the corals, and thus serve to fix them together, and connect them into masses. Whenever the summit of this new island can remain constantly uncovered by the sea, and the waves can no longer destroy what they themselves have contributed to form, then its extent is enlarged, and its edges are gradually raised by the successive addition of sand. According to the direction of the winds and currents they may long remain sterile; but if the seeds of vegetables be transported to them from the neighbouring shores, by the action of these two causes, then, in latitudes favourable to their development, we presently see these islands becoming covered with verdure, the successively accumulated remains of which form layers of soil, which contribute to the elevation of the surface.

But, in order that this phenomenon of growth be accomplished, the distance from land must not be too great, because then the vegetables cannot get so easily to the islets in question, which then remain almost always bare and sterile. And for this reason what navigators report of those madrepore Islands of the Great Ocean, which are covered with verdure, and are yet at a great distance from any known land, has always appeared to us extraordinary; and that so much the more, that, in those vast spaces, the violence of the waves, which nothing can break there, must disturb the operations of the zoophytes. We do not, however, deny the existence of these islands, which it would be interesting carefully to examine anew; for, whenever navigators meet with low islands between the Tropics, they do not hesitate, in compliance with the generally received opinion, to say that they consist of madrepores. Yet how many islands, which scarcely rise above the surface of the water, recognise no such origin? We may mention, as an example, the Island of Boni, situated under the equator, the beautiful vegetation of which rises upon limestone. Cocoa Island, near Guam, is in the same condition, being also composed of limestone. In general, if they are inhabited, consequently they have springs or lakes of fresh water, we may almost be certain that they are not composed of lithophytes, or are only so in part, because springs could not be formed in their porous substances. Some of the Caroline Isles are excessively low; we supposed them encrusted with madrepores; and as they have inhabitants there must be somewhere in them a soil favourable to the accumulation of fresh water[383].

In restraining the power of these animalcules, concludes Quoy and Gaimard, and in pointing out the limits which nature has prescribed them, we have no other object than to furnish more correct data to the naturalists who aspire to great hypothetical considerations, regarding the conformation of the globe. On reconsidering these zoophytes with greater attention, they will no longer be seen filling up the basins of the seas, raising islands, increasing the size of the continents, threatening future generations with a solid equatorial circle formed of their spoils. Their influence, with regard to the road-steads or harbours, in which they multiply, is already great enough, without adding more to it. But, compared with the masses on which they rest, what are their layers, often interrupted, and which must be searched for with care, before they can be recognised, to the enormous volcanic peaks of the Sandwich Islands, the Island of Bourbon, the Moluccas, the Marian Islands, the mountains of Timor, New Guinea, &c. &c.? Nothing, certainly; and the solid zoophytes are in no degree capable of being compared with the testaceous mollusca, with reference to the materials which they have furnished, and still continue to furnish to the crust of the Globe.