Mr. Darwin has subsequently produced a work, entitled The Structure and Distribution of Coral-Reefs, being the first part of the Geology of the Voyage of the Beagle,' London, 1842, 8vo, pp. 214, in which he has stated his views in a systematic and comprehensive manner. He observes that, without any distinct intention to classify coral-reefs, most voyagers have spoken of them under the following heads, which are nearly identical with the descriptive terms already employed in this article :—Lagoon Islands, or Atolls ; Barrier, or Encircling Reefs ; and Fringing, or Shore Reefs. With respect to the lagoon islands, he remarks, 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 inland 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 of a bright but pale green colour. The naturalist, he adds, will feel this astonishment more deeply after having examined the soft and almost gelatinous bodies of the apparently insigmficant creatures, the polypifers in NAT. II W. which construct these vast works; 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. 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 ; on which account Mr. Darwin in this work Invariably uses the term "atoll," which is the name given to these circular groups of coral islets by their inhabitants in the Indian Ocean, and is synonymous with lagoon island. Reefs in most respects allied to those of the fringing class also occur around submerged banks of sediment and of worn-down rock ; and others are scattered quits irregularly where the sea is very shallow. Mr. Darwin has given a separate chapter of his work to each of these classes of coral-reefs; but the classification, lie states, admits of a more funda mental division into atoll-formed and barrier-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 elope of the adjoining land, there is no such difficulty. Barrier-reefs, though little less marvellous in
their structure than atolls, had previous to Mr. Darwin's researches received but little attention, and scarcely any attempt bad been made to explain their origin. He concludes, in conformity with the summary statement of,his theory, given above, that in them as well as in atolls, the foundation on which the coral was primarily attached has subsided, and that during this downward movement the reefs have grown upwards. "This conclusion," lie says, " explains most satisfac torily the 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." After three descriptive chapters, Mr. Darwin describes in three others the dis tribution and growth of coral-reefs, gives in detail his theory of the formation of the different classes, and discusses their distribution with reference to the theory of their formation. In an appendix, every existing coral-reef is briefly described in geographical order, as far as the author possessed information ; and their distribution over the equatorial regions of the globe is further illustrated by a map.
Still more recently, since the year 1847, the subject of coral-forma tions has been treated at great length by Dr. James D. Dana, in the ` Geology' of the ` United States Exploring Expedition,' during the years from 1838 to 1842, under Commander Wilkes, U.S.N. This volume was published at New York, but is undated.