pg 3[1/2]INTRODUCTION

The object of this volume is to describe from my own observation and the works of others, the principal kinds of coral-reefs, and to explain the origin of their peculiar forms. I shall not here treat of the polypifers, which construct these vast works, except as to their distribution, and the conditions favourable to their vigorous growth.

Without any distinct intention to classify coral-reefs, most voyagers have spoken of them under the following heads: 'lagoon-islands' or 'atolls', 'barrier' or 'encircling reefs', and 'fringing' or 'shore reefs'. The lagoon-islands have received much the most attention; and it is not surprising, for everyone must be struck with astonishment, when he first beholds one of / these vast rings of coral-rock, often many leagues in diameter, here and there surmounted by a low verdant island with dazzling white shores, bathed on the outside by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of water, which, from reflection, is generally of a bright but pale green colour. The

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pg 4[2/4]naturalist will feel this astonishment more deeply after having examined the soft and almost gelatinous bodies of these apparently insignificant coral-polypifers, and when he knows that the solid reef increases only on the outer edge, which day and night is lashed by the breakers of an ocean never at rest. Well did François Pyrard de Laval, in the year 1605, exclaim, 'C'est une merueille de voir chacun de ces atollons, enuironné d'un grand banc de pierre tout autour, n'y ayant point d'artifice humain.' The above sketch of Whitsunday Island, in the S. Pacific, taken from Captain Beechey's admirable Voyage, although excellent of its kind, gives but a / faint idea of the singular aspect of one of these lagoon-islands. Whitsunday Island is of small size, and the whole circle has been converted into land, which is a comparatively rare circumstance. As the reef of a lagoon-island generally supports many separate small islands, the word 'island', applied to the whole, is often the cause of confusion; hence I have invariably used in this volume the term 'atoll', which is the name given to these circular coral formations by their inhabitants in the Indian Ocean, and is synonymous with 'lagoon-island'.

Barrier-reefs, when encircling small islands, have been comparatively little noticed by voyagers; but they well deserve attention. In their structure they are little less marvellous than atolls, and they give a singular and most picturesque character to the scenery of the islands they surround. In the accompanying sketch, taken from the Voyage of the Coquille, the reef is seen from within, from one of the high peaks of Bolabola,¹ / one of the Society Islands. Here, as in Whitsunday Island, the whole of that part of the reef which is visible is converted into land. This is a circumstance of rare occurrence; more usually a snow-white line of great breakers, with here and there an islet crowned by cocoa-nut trees, separates the smooth waters of the lagoon-like channel from the waves of the open sea. The barrier reefs of Australia and of New Caledonia, owing to their enormous dimensions, have excited much attention: in structure and form they resemble those encircling many of the smaller islands in the Pacific Ocean.

With respect to fringing, or shore reefs, there is little in their structure which needs explanation; and their name expresses their comparatively small extension. They differ from barrier reefs in not lying far from the shore, and in not having within them a broad channel of deep water. Reefs also occur around submerged banks of sediment and of worn-down rock; and others are scattered quite irregularly where the sea is pg 5[4/5]very shallow; these are allied in most respects to fringing reefs, but are of comparatively little interest.

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I have given a separate chapter to each of the above classes, and have described some one reef or island, on which I possessed most information, as typical; and have afterwards compared it with others of a like kind. Although this classification is useful / from being obvious, and from including most of the coral-reefs existing in the open sea, it admits of a more fundamental division into barrier and atoll-formed reefs on the one hand, where there is a great apparent difficulty with respect to the foundation on which they must first have grown; and into fringing reefs on the other, where, owing to the nature of the slope of the adjoining land, there is no such difficulty. The two blue tints and the red colour on the map (Plate III) represent this main division, as explained in the beginning of the last chapter. In the Appendix, every existing coral-reef, except some on the coast of Brazil not included in the map, is briefly described in geographical order, as far as I possessed information; and any particular spot may be found by consulting the Index.

Several theories have been advanced to explain the origin of atolls or lagoon-islands, but scarcely one to account for barrier-reefs. From the limited depths at which reef-building polypifers can flourish, taken into consideration with certain other circumstances, we are compelled to conclude, as it will be seen, that both in atolls and barrier-reefs, the foundation to which the coral was primarily attached, has subsided; and that during this downward movement, the reefs have grown upwards. This conclusion, it will be further seen, explains most satisfactorily, the pg 6[5/6]outline and general form of atolls and barrier-reefs, and likewise certain peculiarities in their structure. The distribution, also, of the different kinds of coral-reefs, and their position / with relation to the areas of recent elevation, and to the points subject to volcanic eruptions, fully accord with this theory of their origin.²