Figure 4.—MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant Powell; the white spaces in the middle of the separate small reefs, both on the margin and in the middle part, are meant to represent little lagoons; but it was found not possible to distinguish them clearly from the small islets, which have been formed on these same small reefs; many of the smaller reefs could not be introduced; the nautical mark (dot over a dash) over the figures 250 and 200, between Mahlos Mahdoo and Horsburgh atoll and Powell’s island, signifies that soundings were not obtained at these depths. Figure 5.—NEW CALEDONIA, in the western part of the Pacific; from Krusenstern’s “Atlas,” compiled from several surveys; I have slightly altered the northern point of the reef, in accordance with the “Atlas of the Voyage of the _Astrolabe_.” In Krusenstern’s “Atlas,” the reef is represented by a single line with crosses; I have for the sake of uniformity added an interior line. Figure 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by Captain Moresby and Lieutenant Powell. PLATE III.—MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE VOLCANOES. Illustration: Plate 3 Shewing the Distribution of the Different Kinds of Coral Reefs, Together with the Position of the Active Volcanoes in the Map. The principles, on which this map was coloured, are explained in the beginning of Chapter VI.; and the authorities for each particular spot are detailed in the Appendix to “Coral Reefs.” The names not printed in upper case in the Index refer to the Appendix. THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS. INTRODUCTION.