On Coral Reefs and Islands. By JAMES D. DANA.

(Continued from page 62.)

Formation of Reefs, and Causes of their Features and Geographical Distribution.

An inquiry into the causes and origin of the features presented by coral reefs and islands, has led us to glance at the nature of coral zoophytes, and at the effects of various agents upon their development. The way has thus been prepared for considering the bearing of these facts, and of other influencing causes, on the growth of the coral plantation as a whole. While, therefore, the preceding pages treat of zoophytes as individual species, the following will relate to those results which proceed from their accumulation, and the causes which have determined the features and geographical distribution of reefs and islands.

1. Formation of Reefs.

Very erroneous ideas prevail respecting the appearance of a bed or area of growing corals. The submerged reef is often thought of as an extended mass of coral, alive uniformly over its upper surface, and, by this living growth, gradually enlarging upward; and such preconceived views, when ascertained to be erroneous by observation, have sometimes led to scepticism with regard to the zoophytic origin of the reef rock. Nothing is wider from the truth; and this must have been inferred from the descriptions already given. Another glance at the coral plantation should be taken by the reader, before proceeding with the explanations which follow.

Coral plantation and coral field, are more appropriate ap- ^ pellations than coral garden, and convey a juster impression of the surface of a growing reef. Like a spot of wild land, covered in some parts with varied shrubbery, in other parts bearing only occasional tufts of vegetation over barren plains of sand, here a clump of saplings, and there a carpet of variously-coloured flowers-such is the coral plantation. Numerous kinds of zoophytes grow scattered over the surface

like vegetation upon the land: there are large areas that bear nothing, and others that are thickly overgrown. There is, however, no green sward to the landscape; sand and fragments fill up the bare intervals between the flowering tufts; or where the zoophytes are crowded, there are deep holes among the stony stems and folia.

These observations will prepare the mind for some disappointment in a first view of coral reefs. Nature does not make greenhouses, but distributes widely her beauties, and leaves it for man to gather into gardens the choicer varieties. Yet there are scenes in the coral landscape which justify the brightest colouring of the poet: where coral shrubbery and living flowers are mingled in profusion; where Astræa domes appear like the gemmed temples of the coral world, and Madrepore vases, the decorations of the groves; and as the forests and flowers of land have their birds and butterflies, so

Life in rare and beautiful forms

Is sporting amid those bowers of stone;"

for fish of various hues, red, blue, purple, green, and other brilliant shades, keep constant play, appearing and disappearing among the branches.

These fields of growing coral spread over submarine lands, such as the shores of islands and continents, where the depth is not greater than their habits require, just as vegetation extends itself through regions that are congenial. The germ or ovule, which, when first produced, swims free, finds afterwards a point of rock or dead coral to plant itself upon, and thence springs the tree, or some other form of coral growth. The analogy to vegetation does not stop here. It is well known that the debris of the forest, decaying leaves and stems, and animal remains, add to the soil; and that accumulations of this kind are ceaselessly in progress; that by this means, in the luxuriant swamp, deep beds of peaty earth are formed. So it is in the coral mead. Accumulations of fragments and sand from the coral zoophytes, and of shells and other relics of organic life are in constant progress: and thus a bed of coral debris is formed and compacted. There is this difference, that a large part of the vegetable material consists of elements which escape as gases in decomposition,

whereas coral is itself an enduring rock material, undergoing no change except the mechanical one of comminution. The animal portion is but a mere fraction of the whole zoophyte.

In these few hints, we have the whole theory of reef-making: not a speculative opinion, but a legitimate deduction from a few simple facts, and bearing close analogy to operations on land. The coral debris and shells fill up the intervals between the coral patches, and the cavities among the living tufts, and in this manner produce the reef-deposit, which is finally consolidated while still beneath the water.

The coral zoophyte is especially adapted for such a mode of reef accumulation. Were the nourishment drawn from below, as in most plants, the solidifying coral rock would soon destroy all life: instead of this, the tree is gradually dying below while growing above; and the accumulations cover only the dead portions. Moreover, to prevent accident, where these accumulations do not keep pace with the progress of death, organic incrustations cover the lifeless trunk, and protect it from the dissolving waters.

But on land, there is the decay of the year, and that of old age, producing vegetable debris; and storms prostrate forests. And are there corresponding effects among the groves of the sea? It has been shewn that coral plantations, from which reefs proceed, do not grow in the "calm and still" depths of the ocean. They are to be found amid the very waves, and extend but little below 100 feet, which is far within the reach of the sea's heavier commotions.* Here is an agent which is not without its effects. The enormous masses of uptorn rock found on many of the islands may give some idea of the force of the lifting wave; and there are examples on record,

* During the more violent gales, the bottom of the sea is said, by different authors, to be disturbed to a depth of 300, 350, or even 500 feet; and Sir H. De la Beche remarks, that when the depth is 15 fathoms, the water is very evidently discoloured by the action of the waves on the sand and mud of the bottom. In the Comptes Rendus, t. xii., 774, M. Siau mentions that parallel ridges are formed on the bottom by the motion of the water, which may be readily distinguished at a depth of at least 20 metres. The hollows between such ridges or zones are occupied by the heavsubstances of the bottom. Similar zones were distinguished at a depth of 18 es to the north-west of the St Paul's Roads.

to be found in various treatises on Geology, of still more surprising effects. We must, therefore, allow that some effect will be produced upon the coral groves. There will be trees prostrated by gales, as on land, fragments scattered, and fragmentary and sand accumulations commenced. Besides, masses of the heavier corals will be uptorn, and carried along over the coral plantation, which will destroy and grind down everything in their way. So many are the accidents of this kind to which zoophytes appear to be exposed, that we might believe they would often be exterminated, were they not

* Lyell, vol. ii., p. 38-40. Speaking of the force of the waves on coasts, Lyell mentions the transportation of a block of stone, ninety feet from its bed, which was eight feet two inches, by seven feet, and five feet one inch in its dimensions, and another nine feet two inches, by six-and-a-half feet, by four feet, having been "hurried up an acclivity to a distance of 150 feet."

In an article on the subject, by Thomas Stevenson, civil engineer, of Edinburgh, published in the Transactions of the Royal Society of Edinburgh (vol. xvi., 1845), it is stated, as a deduction from 267 experiments, extending over twenty-three successive months, that the average force for Skerryvore, for five of the summer months, during the years 1843-1844, was 611 lb. per square foot; and for six of the winter months of the same year, it was 2086 lb. per square foot, or three times as great as during the summer months. During a westerly gale, at the same place, in March 1845, a pressure of 6083 lb. was registered by Mr Stevenson's dynamometer (the name of the instrument used). He mentions several remarkable instances of transported blocks. One of gneiss, containing 504 cubic feet, was carried by the waves five feet from the place where it lay, and then became wedged so as no longer to be moved. Of the manner in which it was moved, Mr Reid (as cited by Mr Stevenson) says:-"The sea, when I saw it striking the stone, would wholly immerse or bury it out of sight, and the run extended up to the grass line above it, making a perpendicular rise of from thirty-nine to forty feet above the high-water level. On the incoming waves striking the stone, we could see this monstrous mass, of upwards of forty tons weight, lean landwards, and the back-run would uplift it again with a jerk, leaving it with very little water about it, when the next incoming wave made it recline again."

Mr Stevenson states also, that the Bell Rock Lighthouse, in the German Ocean, though 112 feet in height, is literally buried in foam and spray to the very top, during ground swells, when there is no wind. On the 20th of November 1827, the spray rose to the height of 117 feet above the foundations or low-water mark; and, deducting eleven feet for the tide that day, it leaves 106 feet, which is equivalent to a pressure of nearly three tons per square foot.

With such facts, any incredulity respecting the power of waves should be laid aside. Moreover, it may be remarked, that the Pacific is a much wider Ocean than the Atlantic, with far heavier waves in its ordinary state.

singularly tenacious of life, and ready to sprout anew on any rock where they may find quiet long enough to give themselves again a firm attachment.

But it should be observed, that the sea would have far less effect upon the slender forms characterising many zoophytes, among which the water finds free passage, than on the massive rock, against whose sides a large volume may drive unbroken. Moreover, much the greater part of the strength of the ocean is exerted near tide level, where it rises in breakers, which plunge against the shores. Yet, owing to the many nooks and recesses deep among the corals, the rapidly-moving waters, during the heavier swells, must produce whirling eddies of considerable force, tending to uproot or break the coral clumps. These disrupting and transporting effects will be less and less as we recede from the shores; yet all coral depths must experience them in some degree.

There is another process going on over the coral field, somewhat analogous to vegetable decay, though still very different. Zoophytes have been described as ever dying while living. The dead portions have the surface much smoothed, or deprived of the roughening points which belong to the living coral, and the cells are sometimes half obliterated, or the delicate lamella worn away. This may be viewed as one source of fine coral particles; and as the process is constantly going on, it is not altogether unimportant. This material is in a fit condition to enter into solution, and it cannot be doubted that the water receives lime from this source, which is afterwards yielded to the reef.

In the Alcyonia family, which includes semi-fleshy corals, and the Gorgoniæ, the lime is often scattered through the polyps in granules; and the process of death sets those calcareous grains free, which are constantly added to the coral sands. The same process has been supposed to take place in the more common reef corals, the Madrepores and Astræas, and it is possible that this may be to some extent the case. Yet it would seem, from facts observed, that after the secretion has begun within the polyp, the secretion of lime going on takes place against the portions already formed, and in direct union with them, and not as granules to be afterwards cemented.

VOL. LII. NO. CIV.-APRIL 1852.

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