It is approached by a deep channel through the coral reef which fringes the entire eastern side of the island.
Besides the atolls there is a type of island which has been called the elevated coral island.
It is the centre of the Algerian and Tunisian coral fisheries and has an extensive industry in the curing of sardines; but .the harbour is small and exposed to the N.E.
Tetuaroa and Tubai, besides the three western Leeward Isles, are coral atolls.
The town occupies the whole of a small coral island at the mouth of Mossoril Bay.
The volcanic d'Entrecasteaux Islands are mostly larger, more elevated (the highest being 8000 ft.), and stand in deeper water than the Louisiade group. To the east of Kiriwina (Trobriand) lies a small group of uniquely formed islets, each of which is completely surrounded by a steep forest-clad marginal rampart of coral 300 to 400 ft.
The Asiatic and Malayan affinities of many of its animals, as well as the physical conditions of the bed of the Indian Ocean, make it highly probable that Madagascar, while once forming part of Africa, is the chief relic of a considerable archipelago formerly connecting that continent with Asia, its other portions being shown by groups of small islands, and by coral atolls and shoals, which are gradually disappearing beneath the waves.
Rarotonga and Mangaia, in the Cook group, and Niue or Savage Island are the largest of these; Penrhyn and Suwarrow, though but small coral atolls, contain excellent harbours.
There now is your insular city of the Manhattoes, belted round by wharves as Indian isles by coral reefs--commerce surrounds it with her surf.
The soil, usually of a reddish-brown colour, is for the most part disintegrated lava mixed with decayed vegetation; occasionally there is also a mixture of disintegrated coral limestone.