Gail Gibbons - Coral Reefs

Reefs of Mauritius.--Shallow channel within the reef.--Its slow filling up.--Currents of water formed within it.--Upraised reefs.--Narrow fringing-reefs in deep seas.--Reefs on the coast of E. Africa and of Brazil.--Fringing-reefs in very shallow seas, round banks of sediment and on worn-down islands.--Fringing-reefs affected by currents of the sea.

--Coral coating the bottom of the sea, but not forming reefs.

CHAPTER IV.--ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.

SECTION 4.I.--ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS

FAVOURABLE TO THEIR INCREASE.

SECTION 4.II.--ON THE RATE OF GROWTH OF CORAL-REEFS.

SECTION 4.III.--ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.

CHAPTER V.--THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF

CORAL-REEFS.

The atolls of the larger archipelagoes are not formed on submerged craters, or on banks of sediment.--Immense areas interspersed with atolls.--Recent changes in their state.--The origin of barrier-reefs and of atolls.--Their relative forms.--The step-formed ledges and walls round the shores of some lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged condition of parts or of the whole of some annular reefs.--The disseverment of large atolls.--The union of atolls by linear reefs.--The Great Chagos Bank.--Objections, from the area and amount of subsidence required by the theory, considered.--The probable composition of the lower parts of atolls.

CHAPTER VI.--ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE

THEORY OF THEIR FORMATION.

Description of the coloured map.--Proximity of atolls and barrier-reefs.-

Relation in form and position of atolls with ordinary islands.--Direct evidence of subsidence difficult to be detected.--Proofs of recent elevation where fringing-reefs occur.--Oscillations of level.--Absence of active volcanoes in the areas of subsidence.--Immensity of the areas which have been elevated and have subsided.--Their relation to the present distribution of the land.--Areas of subsidence elongated, their intersection and alternation with those of elevation.--Amount and slow rate of the subsidence.--Recapitulation.

APPENDIX.

Containing a detailed description of the reefs and islands in Plate III.

INDEX.

THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.

CRITICAL INTRODUCTION.

A scientific discovery is the outcome of an interesting process of evolution in the mind of its author. When we are able to detect the germs of thought in which such a discovery has originated, and to trace the successive stages of the reasoning by which the crude idea has developed into an epoch-making book, we have the materials for reconstructing an important chapter of scientific history. Such a contribution to the story of the "making of science" may be furnished in respect to Darwin's famous theory of coral-reefs, and the clearly reasoned treatise in which it was first fully set forth.

The subject of corals and coral-reefs is one concerning which much popular misconception has always prevailed. The misleading comparison of coral-rock with the combs of bees and the nests of wasps is perhaps responsible for much of this misunderstanding; one writer has indeed described a coral-reef as being "built by fishes by means of their teeth." Scarcely less misleading, however, are the references we so frequently meet with, both in prose and verse, to the "skill," "industry," and "perseverance" of the "coral-insect" in "building" his "home." As well might we praise men for their cleverness in making their own skeletons, and laud their assiduity in filling churchyards with the same. The polyps and other organisms, whose remains accumulate to form a coral-reef, simply live and perform their natural functions, and then die, leaving behind them, in the natural course of events, the hard calcareous portions of their structures to add to the growing reef.

While the forms of coral-reefs and coral-islands are sometimes very remarkable and worthy of attentive study, there is no ground, it need scarcely be added, for the suggestion that they afford proofs of design on the part of the living builders, or that, in the words of Flinders, they constitute breastworks, defending the workshops from whence "infant colonies might be safely sent forth."

It was not till the beginning of the present century that travellers like Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others, began to collect accurate details concerning the forms and structure of coral-masses, and to make such observations on the habits of reef-forming polyps, as might serve as a basis for safe reasoning concerning the origin of coral-reefs and islands. In the second volume of Lyell's "Principles of Geology," published in 1832, the final chapter gives an admirable summary of all that was then known on the subject. At that time, the ring-form of the atolls was almost universally regarded as a proof that they had grown up on submerged volcanic craters; and Lyell gave his powerful support to that theory.