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ANALYSES.

I.—The Structure and Distribution of Coral Reefs: being the First Part of the Geology of the Voyage of the 'Beagle,' under the command of Capt. Fitzroy, R.N., during the Years 1832 to 1836. By CHARLES DARWIN, M.A., F.R.S., F.G.S., Naturalist to the Expedition.—Communicated by Colonel JACKSON.

IF, on the one hand, we have too frequently to deprecate the precipitancy with which theories and systems are raised upon the insufficient foundation of a few isolated facts; so, on the other, we have often occasion to regret that an immense number of valuable observations on the most interesting and important subjects remain dispersed, and therefore almost useless, long after there is more than enough from which to deduce some satisfactory conclusion. Ever prone to extremes, we either begin to build without sufficient materials, or go on collecting long after we have an abundant supply to complete the structure. Of the former of these errors, the cause and the consequences are alike evident—it springs from vanity, and ends in disappointment: as to the latter, it is to be attributed to ignorance, indolence, or incapacity—ignorance of what has been done, and indolence or incapacity to make a proper use of what is already collected to our hands; the consequence of which is, to keep us still unacquainted with what we should long since have known.

In effect, what an immense addition to our knowledge of the laws of nature should we possess if a tithe of the facts dispersed in the Journals of observant travellers, in the Transactions of academies and learned societies, were collected together and judiciously arranged! From their very juxta-position, plan, co-relation, and harmony, before unsuspected, would become instantly visible, or the causes of anomaly be rendered apparent; erroneous opinions would at once be detected; and new truths—satisfactory as such alone, or supplying corollaries of practical utility—be added to the mass of human knowledge. A better testimony to the justice of this remark can hardly be afforded than in the work before us—Mr. Darwin's 'Structure and Distribution of Coral Reefs.' To indefatigable research—and, there-

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fore, to a perfect acquaintance with what has been observed and written by others on coral reefs—Mr. Darwin has added his own personal examination of a great many of those interesting structures; and, from the manner in which he has grouped the facts, and then reasoned upon them, the mind remains satisfied that he has detected the law, or rather the process, of nature in their formation. We can do little more, however, in this place than give an outline of the author's arrangement in this highly interesting monograph.

The work is divided into six chapters, preceded by an Introduction, and followed by a copious Appendix; there are also three charts and a few illustrative woodcuts.

In the Introduction, Mr. Darwin adopts the classification of coral reefs into "lagoon islands," or "atolls,"—"barrier" or "encircling reefs,"—and "fringing" or "shore reefs:" though, as he subsequently shows in Chapter V., and declares at p. 102, "the three classes graduate into each other."

Chapter I. treats of "atolls," or "lagoon islands," and is divided into three sections, of which the first is especially devoted to a description of Keeling Atoll, "its structure being in most respects characteristic of the class to which it belongs." The second section contains a "general description of atolls;" and in the third and last section of this chapter the author enters more minutely into the consideration of that remarkable group the "atolls of the Maldiva Archipelago," and of the great "Chagos Bank."

Chapter II. treats of "barrier reefs." In it the author shows the general resemblance in form and structure between "true atolls" and "barrier reefs;" the only characteristic difference being, that in the latter there rises from the central lagoon one or more islands. The remarkable fact of the breaches in the encircling reef being immediately opposite the main valleys of the enclosed land, and the general prevalence of gaps on the leeward side, are pointed out, and satisfactorily accounted for. The probability that barrier reefs are of great thickness is clearly shown; and though, from the known fact that the reef-building polypifers cannot live at great depths, the fact would seem doubtful, yet Mr. Darwin's theory, developed in a subsequent chapter, satisfactorily clears up the apparent difficulty. In the present chapter, however, he confines himself to explaining why the opinions hitherto entertained on the formation of these reefs are quite inadmissible.

Chapter III. is on "fringing" or "shore reefs." These are minutely described, particularly those of Mauritius; and, from the various facts regarding them, it appears that "the dimensions and structure of fringing reefs depend entirely on the greater or

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less inclination of the submarine slope, conjoined with the fact that reef-building polypifers can exist only at limited depths." It would further appear that "in some cases fringing reefs are considerably modified in outline by the course of the prevailing currents."

Chapter IV. is devoted to the growth of coral reefs, and is divided into three sections: first, "on the distribution of coral reefs, and on the conditions favourable to their increase;" second, "on the rate of growth of coral reefs;" third, "on the depths at which reef-building polypifers can live." With regard to the limits of latitude over which coral reefs extend, they seem to be confined to within 30 degrees of the equator, N. and S.; but their distribution within this zone appears to be so capricious, that it cannot be explained by any cause hitherto assigned. As for the rate of growth, it would appear from some facts that it is extremely slow, and from others again almost as quick, geologically speaking. The depth at which corals and corallines can live is very various, and depends chiefly upon the kind of coral and on local circumstances; the reef-building polypifers, it would seem, "do not flourish at greater depths than between 20 and 30 fathoms."

In the four chapters just mentioned, Mr. Darwin has confined himself generally to the arrangement and detail of facts.

Chapter V. is on the theory of the formation of the different classes of coral reefs; and, with Chapter VI., constitutes the most interesting part of the work. In Chapter V. are discussed the various opinions that have been entertained on the formation of coral reefs, none of which satisfactorily accounts for all the phenomena observed: the author therefore propounds his own theory; and we are free to confess it satisfies our own mind completely, and will, we have little doubt, be generally received as the only plausible explanation of all the facts. It applies equally well to every appearance; and we know of nothing more ingenious than the manner in which the apparently compound arrangement, if we may so term it, of the atolls of the Maldiva Archipelago is accounted for, and the extraordinary structure of the Great Chagos Bank explained. Subsidence, either gradual or sudden, and as by paroxysms, is, according to Mr. Darwin, the sufficient cause of all the phenomena presented by coral reefs, however anomalous they may at first appear.

Chapter VI. is appropriated to the "distribution of coral reefs with reference to the theory of their formation;" and is, like Chapter V., of the greatest interest to all who would study the details of the earth's superficial structure.* The subject of this

* We use the term superficial designedly, considering, as we do, all comprised between the limits of our highest mountains and our deepest soundings in the ocean as forming but a thin pellicle of our globe, compared with its diameter and mass.

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chapter is elucidated by a chart, which, as Mr. Darwin says, and we readily believe him, is "the result of many months' labour." In this part of his work, Mr. Darwin considers the relation which active volcanoes and areas of elevation bear to areas of subsidence; and though in some cases the two phenomena appear to approach nearer than they should do for the sake of the theory, still they nowise contradict it; for we hold that whenever there is subsidence there will probably be, at a nearer or greater distance, a corresponding elevation—the result of oscillation, or what the French geologists term mouvement de bascule. Moreover, the direction of the spaces coloured red on the map, and which represent the areas raised, is such relatively to the spaces coloured blue and indicating the depressed areas, that their co-relation of effect seems evident on simple inspection, though their synchronism of action cannot in all cases be fully established.*

In concluding this sixth chapter of his work, Mr. Darwin recapitulates all that has preceded it, and we cannot give a better summary than in his own words:—

"In the three first chapters the principal kinds of coral reefs have been described in detail, and found to differ little as far as relates to the actual surface of the reef. An atoll differs from an encircling barrier reef only in the absence of land within its central expanse; and a barrier differs from a fringing reef in being placed at a much greater distance from the land, with reference to the probable inclination of its submarine foundation, and in the presence of a deep-water lagoon-like space or moat within the reef. In the fourth chapter the growing powers of the reef-constructing polypifers were discussed; and it was shown that they cannot flourish beneath a very limited depth. In accordance with this limit, there is no difficulty respecting the foundations on which fringing reefs are based; whereas with barrier reefs and atolls there is a great apparent difficulty on this head—in barrier reefs, from the improbability of the rock of the coast or of banks of sediment extending in every instance so far seaward within the required depth; and, in atolls, from the immensity of the spaces over which they are interspersed, and the apparent necessity for believing that they are all supported on mountain-summits, which, although rising very near to the surface-level of the sea, in no one instance emerge above it. To escape this latter most improbable admission, which implies the existence of submarine chains of mountains, of almost the same height, extending over areas of many thousand square miles there is but one alternative,—namely, the prolonged subsidence of the foundations on which the atolls were primarily based, together with the upward growth * Though perfect co-relation of direction between the lines of elevation and subsidence would seem to argue synchronism of action, the want of this co-relation of direction is no proof that the different effects did not take place simultaneously: for if, as is probable, the double action of subsidence and depression supposes rupture of the mass or crust of the earth, this may be effected in various directions at the same time, according as the lines of least resistance run in relation to the disturbing force.

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of the reef-constructing corals. On this view every difficulty vanishes: fringing reefs are thus converted into barrier reefs; and barrier reefs, when encircling islands, are thus converted into atolls the instant the last pinnacle of land sinks beneath the surface of the ocean.

"Thus the ordinary forms and certain peculiarities in the structure of atolls and barrier reefs can be explained; namely, the wall-like structure on their inner sides—the bason or ring-like shape both of the marginal and central reefs in the Maldiva atolls—the union of some atolls, as if by a ribbon—the apparent disseverment of others—and the occurrence, in atolls as well as in barrier reefs, of portions of reef, and of the whole of some reefs, in a dead and submerged state, but retaining the outline of living reefs. Thus can be explained the existence of breaches through barrier reefs in front of valleys, though separated from them by a wide space of deep water; thus, also, the ordinary outline of groups of atolls, and the relative forms of the separate atolls one to another: thus can be explained the proximity of the two kinds of reefs formed during subsidence, and their separation from the spaces where fringing reefs abound. On searching for other evidence of the movements supposed by our theory, we find marks of change in atolls and in barrier reefs, and of subterranean disturbances under them; but, from the nature of things, it is scarcely possible to detect any direct proofs of subsidence, although some appearances are strongly in favour of it. On the fringed coasts, however, the presence of upraised marine bodies of a recent epoch plainly shows that these coasts, instead of having remained stationary—which is all that can be directly inferred from our theory—have generally been elevated.

"Finally, when the two great types of structure—namely, barrier reefs and atolls on the one hand, and fringing reefs on the other—are laid down in colours, as on our map, a magnificent and harmonious picture of the movements which the crust of the earth has within a late period undergone is presented to us. We there see vast areas rising, with volcanic matter every now and then bursting forth through the vents or fissures with which they are traversed. We see other wide spaces slowly sinking without any volcanic outbursts; and we feel sure that this sinking must have been immense in amount as well as in area, thus to have buried over (under?) the broad face of the ocean every one of those mountains above which atolls now stand like monuments, marking the place of their former existence. Reflecting how powerful an agent, with respect to denudation, and consequently to the nature and thickness of the deposits in accumulation, the sea must ever be, when acting for prolonged periods on the land, during either its slow emergence or subsidence; reflecting, also, on the final effects of these movements in the interchange of land and ocean-water—on the climate of the earth—and on the distribution of organic beings; I may be permitted to hope that the conclusions derived from the study of coral formations, originally attempted merely to explain their peculiar forms, may be thought worthy of the attention of geolgists."

The work concludes, as we have said, with an Appendix. In this will be found the detailed description of all the reefs and

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islands in the map; namely, all those of the Pacific Ocean, the East Indian Archipelago, the Indian Ocean, the coast of Africa, the Red Sea, the West Indies, and the Bermuda Islands. There is also a copious index for reference. Upon the whole, we can safely say we have rarely met with a work every way so satisfactory. Industry of research, and consequent abundance of facts, scrupulous attention in acknowledging the sources whence all that has not been observed by himself has been derived, candour and modesty in the discussion of the subject, order in the arrangement, and lucidity in explanation—all combine to render this monograph highly deserving the attention of the geographer, the navigator, and the savant, to whom we cordially recommend it accordingly; and we wait with great impatience the works promised by the same talented naturalist on Volcanic Islands and on South America.