CORAL REEFS OF THE INDIAN OCEAN. 15 



Malha Bank, and between Faiqiihar atoll and Madagascar. These channels divide the coral- 

 reef areas into four sections, which may be termed respectively the Malagasy, Seychelles, 

 Chagos, and Maldive'. 



1. Malagasy Section. 



Madagascar itself displays a nucleus of granite running from end to end of the island, 

 broken only by patches of recently active volcanoes. No raised rock of coralline origin is 

 known, but nummulitic limestones of the eocene period form a belt along the western side. 

 The reefs round the island are of inconsiderable extent, mostly mere isolated fringing banks. 

 The former connection of this island with Africa is indubitable, and its structure indicates 

 that it has experienced several changes of level since that connection was broken down. 

 The Comoro islands, a volcanic group N.W. of Madagascar, have only fringing reefs except 

 Mayotta, which possesses a barrier, 3 to 6 miles distant, with upwards of 35 / of water in 

 its lagoon. Two small high volcanic islands arise in the barrier itself, indicating that its 

 formation was probably sui generis, on a bank formed of volcanic debris from the central 

 island. 



Of isolated coral formations there are to the south about lat. 22° in the centre of 

 the Mozambique channel Europa island, 50 to 80 feet high, with low cliffs and sandy hom- 

 mocks, and a small fringing reef, and the Bassas da India bank, a round atoll-reef 9 miles 

 in diameter-, completely enclosing a central lagoon of shallow depth. Between the Comoro 

 and Cape Amber (N. Madagascar) are a series of banks, of which the Geyser is to all 

 intents an atoll with a wide passage to the S.E. ; Glorioso has two islands, for which the 

 descriptions do not indicate any elevation. 



To the north of Madagascar, about lat. 9° 50', are a series of banks and islands, 

 which show an unmistakeable and very uniform elevation of something over 30 feet. Of 

 these two banks to the east Providence and Wizard never probably had an atoll form, 

 they have now merely sand cays (Providence two small i.slands as well) and isolated rocks, 

 while St Pierre in their vicinity is a raised island, one mile in diameter; its elevation 

 would seem to have been but very recent, as it is described as having no fringing reef, 

 the sea directly breaking on its coast. In addition there are three indubitably raised 

 atolls, Cosmoledo, Farquhar and Aldabra. They differ from most similarly elevated atolls, 

 in that their rock still retains even at the surface its organic structure. The lagoons of 

 the original atolls can in each be traced, and the greater part of their basins appears to 

 be covered with water at high tide, although their mud-banks increase in extent in the 

 order named. It is remarkable that the lagoons of all have maintained, or obtained pass- 

 ages with a few fathoms of water to the sea, and they would appear to be regaining 

 their former extent. Aldabra, 19 miles long by 7 broad, has save for two small passages 

 a perfect rim of land, covered by jagged coral, while its lagoon is surrounded by mangroves, 

 which are said to be everywhere " eating the coral away^" 



1 The southern islands of tlie Indian Ocean (Prince them by over 2000 /. They must be regarded rather as 



Edward, Crozet, Heard, Kergneleu, Amsterdam and St Paul) outliers of the great Antarctic Continent, since they all rest 



are separated by at least 1200 miles of ocean from the nearest on the somewhat shallower bank, which extends northwards 



islands of coral formation, or with coral reefs of any sort. from its shores. 



They are all volcanic, and can have no conceivable con- - "Islands in the Southern Indian Ocean." Adminilti/, 



nection with the islands to the north, being separated from p. 366, 1891, from which also much of the information in 



