Coral Reefs Page 01
by CHARLES DARWIN
Although in some respects more technical in their subjects and style than Darwin's "Journal," the books here reprinted will never lose their value and interest for the originality of the observations they contain. Many parts of them are admirably adapted for giving an insight into problems regarding the structure and changes of the earth's surface, and in fact they form a charming introduction to physical geology and physiography in their application to special domains. The books themselves cannot be obtained for many times the price of the present volume, and both the general reader, who desires to know more of Darwin's work, and the student of geology, who naturally wishes to know how a master mind reasoned on most important geological subjects, will be glad of the opportunity of possessing them in a convenient and cheap form.
The three introductions, which my friend Professor Judd has kindly furnished, give critical and historical information which makes this edition of special value.
TABLE OF CONTENTS.
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.
Containing a detailed description of the reefs and islands in Plate III.