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 I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS FAVOURABLE TO THEIR INCREASE. _Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS. _Section 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. GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS. CRITICAL INTRODUCTION Chapter I—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.