Coral Reefs

Coral Reefs

CORAL REEFS CHARLES DARWIN TABLE OF CONTENTS. CRITICAL INTRODUCTION. INTRODUCTION. CHAPTER I.--ATOLLS OR LAGOON-ISLANDS. SECTION 1.I.--DESCRIPTION OF KEELING ATOLL. Corals on the outer margin.--Zone of Nulliporae.--Exterior reef.--Islets.-- Coral-conglomerate.--Lagoon.--Calcareous sediment.--Scari and Holuthuriae subsisting on corals.--Changes in the condition of the reefs and islets.-- Probable subsidence of the atoll.--Future state of the lagoon. SECTION 1.II.--GENERAL DESCRIPTION OF ATOLLS. General form and size of atolls their reefs and islets.--External slope.-- Zone of Nulliporae.--Conglomerate.--Depth of lagoons.--Sediment.--Reefs submerged wholly or in part.--Breaches in the reef.--Ledge-formed shores round certain lagoons.--Conversion of lagoons into land. SECTION 1.III.--ATOLLS OF THE MALDIVA ARCHIPELAGO--GREAT CHAGOS BANK. Maldiva Archipelago.--Ring-formed reefs marginal and central.--Great depths in the lagoons of the southern atolls.--Reefs in the lagoons all rising to the surface.--Position of islets and breaches in the reefs with respect to the prevalent winds and action of the waves.--Destruction of islets.--Connection in the position and submarine foundation of distinct atolls.--The apparent disseverment of large atolls.--The Great Chagos Bank.--Its submerged condition and extraordinary structure. CHAPTER II.--BARRIER REEFS. Closely resemble in general form and structure atoll-reefs.--Width and depth of the lagoon-channels.--Breaches through the reef in front of valleys and generally on the leeward side.--Checks to the filling up of the lagoon-channels.--Size and constitution of the encircled islands.-- Number of islands within the same reef.--Barrier-reefs of New Caledonia and Australia.--Position of the reef relative to the slope of the adjoining land.--Probable great thickness of barrier-reefs. CHAPTER III.--FRINGING OR SHORE-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. Charles Darwin was never tired of acknowledging his indebtedness to Lyell. In dedicating to his friend the second edition of his "Naturalist's Voyage Round the World" Darwin writes that he does so "with grateful pleasure as an acknowledgment that the chief part of whatever scientific merit this journal and the other works of the author may possess has been derived from studying the well-known and admirable 'Principles of Geology.'" The second volume of Lyell's "Principles" appeared after Darwin had left England; but it was doubtless sent on to him without delay by his faithful friend and correspondent Professor Henslow. It appears to have reached Darwin at a most opportune moment while in fact he was studying the striking evidences of slow and long-continued but often interrupted movement on the west coast of South America. Darwin's acute mind could not fail to detect the weakness of the then prevalent theory concerning the origin of the ring-shaped atolls--and the difficulty which he found in accepting the volcanic theory as an explanation of the phenomena of coral-reefs is well set forth in his book. In an interesting fragment of autobiography Darwin has given us a very clear account of the way in which the leading idea of the theory of coral-reefs originated in his mind; he writes "No other work of mine was begun in so deductive a spirit as this for the whole theory was thought out on the west coast of South America before I had seen a true coral-reef. I had therefore only to verify and extend my views by a careful examination of living reefs. But it should be observed that I had during the two previous years been incessantly attending to the effects on the shores of South America of the intermittent elevation of the land together with the denudation and deposition of sediment. This necessarily led me to reflect much on the effects of subsidence and it was easy to replace in imagination the continued deposition of sediment by the upward growth of corals. To do this was to form my theory of the formation of barrier-reefs and atolls." On her homeward voyage the "Beagle" visited Tahiti Australia and some of the coral-islands in the Indian Ocean and Darwin had an opportunity of testing and verifying the conclusion at which he had arrived by studying the statements of other observers. ...