CHAGOS, a group of atolls in the Indian Ocean, belonging to Britain, disposed in circular form round the Chagos bank, in 4° 44' t o 7° 39' S., and 70 55' to 72° 52' E.
It comprises a large number of low coralline islands and atolls, which are disposed in nine clusters extending over a distance of about 400 m.
It consists of seventeen atolls with an immense number of islands, of which some three hundred are inhabited.
In the extreme south are the isolated atolls of Addu and Fua-Mulaku, separated from Suvadiva by the Equatorial Channel, which is itself separated from the main chain of atolls by One-and-ahalf-degree Channel.'
Following the chain northward from this channel we have Haddumati and Kolumadulu, after which the chain becomes double: to the east the chief atolls are Mulaku, Felidu, South Male, North Male, Kardiva (where the channel of the same name, 35 m.
(1) Those of the atolls north of the Kardiva Channel.
(2) Those of the central division, comprising the atolls between North Male and Haddumati, who are under the direct rule of the sultan, and have been more exposed to Arab influences.
" Sealark " and the German surveying-ship " Planet " to have a somewhat complicated configuration, the island groups and banks of atolls which occur there rising abruptly as a rule from depths of about 2000 fathoms or more.
They are ridges of aeolian limestone plastered over by a thin layer of corals and other calcareous organisms. The very remarkable "serpuline atolls" are covered by a solid crust made of the convoluted tubes of serpulae and Vermetus, together with barnacles, mussels, nullipores, corallines and some true incrusting corals.
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.
The average depth varies from 1500 to 2500 fathoms, and from this level innumerable volcanic ridges and peaks rise almost or quite to the surface, their summits for the most part occupied by atolls and reefs of coral formation, while interspersed with these are depressions, mostly of small area, among which the deepest soundings recorded have been obtained.
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.
Besides the atolls there is a type of island which has been called the elevated coral island.
For the much debated question as to the conditions under which atolls and reefs are formed, see Coral Reefs.
But if this is true of the land fauna as a whole, especially on the atolls, where it consists mainly of a few birds, lizards and insects, the opposite is the case with the marine fauna.
Fish are exceedingly abundant, especially in the lagoons of atolls, and form an important article of food supply for the natives, who are generally expert fishermen.
Moss, Through Atolls and Islands (London, 1889); W.
Barrier and fringing reefs, as well as atolls, occur in the group, but the channels between the islands are dangerous chiefly from the strong currents which set through them.
Their colour is a brown, lighter or darker generally according to the amount of their exposure to the sun - being darker on some of the atolls where the people spend much time in fishing, and among fishermen on the volcanic islands, and lighter among women, chiefs and others less exposed than the bulk of the people.
If these junks could cross the Pacific in the latitude of Hawaii it is not at all unlikely that others running in a south-easterly direction would reach some of the many atolls which stretch over about 35° of longitude, forming the Caroline and Marshall archipelagoes.
The group consists of a number of atolls ranged in two almost parallel lines, which run from N.W.
These atolls are of coralline formation and of irregular shape.
The surface of the atolls is covered with sand, except in a few places where it has been turned into soil through the admixture of decayed vegetation.
TOKELAU (or Union Islands), a group of three atolls in the Pacific Ocean, about 350 m.
The archipelago consists of a number of coral islets and atolls comprising the African Islands (4), the St Joseph group (8), the Poivre Islands (9) and the Alphonso group (3).
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.