Often, as has been said, the atoll is divided into a number of islets, but in some smaller atolls the ring is complete, and the sea-water gains access beneath the surface of the reef to the lagoon within, where it is sometimes seen to spout up at the rise of the tide.
The group furnished Charles Darwin with the typical example of an atoll or lagoon island.
Christmas Island is probably the largest atoll in the Pacific (it is about 90 m.
Such material, it is suspected, may form the massive bases on which barrier or fringing or atoll reefs are built up.
The flat summit is formed by a succession of limestones - all deposited in shallow water - from the Eocene (or Oligocene) up to recent deposits in the above-mentioned atoll with islands on its reef.
See Geographical Journal, passim; and Atoll of Funafuti: Borings into a Coral Reef (Report of Coral Reef Committee of Royal Society, London, 1904).
The geological sequence of events appears to have been the following: - After the deposition of the Eocene (or Oligocene) limestone - which reposes upon a floor of basalts and trachytes - basalts and basic tuffs were ejected, over which, during a period of very slow depression, orbitoidal limestones of Miocene age - which seem to make up the great mass of the island - were deposited; then elapsed a long period of rest, during which the atoll condition existed and the guano deposit was formed; from then down to the present time there has succeeded a series of sea-level subsidences, resulting in the formation of the terraces and the accummulation of the detritus now seen on the first inland cliff, the old submarine slope of the island.
The coral limestone of the atoll has a peculiar vitrified appearance and gives out a ringing sound when struck or simply walked on.
The Aldabra Islands constitute an atoll consisting of an oval ring of land, some 40 m.
The atoll has an exceedingly healthy climate, and might well be used as a sanatorium for phthisical patients, the temperature never reaching extremes.