448 Mr. G. 0. Bourne. The Atoll of Diego Garcia [Mar. 22, 



even to Diego Garcia itself, although it lies somewhat apart from the 

 rest of the group. Again, if atolls and barrier reefs are formed around 

 subsiding peaks, it is at least curious that throughout the Laccadive, 

 Maldive, and Chagos groups there are no instances of high islands 

 surrounded by barrier reefs, marking the last remnants of pre-existing 

 land. In the more western parts of the Indian Ocean, between Mada- 

 gascar and the Seychelles, there are numerous atoll islands, and in 

 long. 60° E. there lie the submerged Saya de Malha Bank and the 

 reef known as Cargados Carajos. Between these two lies the exten- 

 sive Nazareth Bank, having over it depths of from 14 to 45 fathoms. 

 The Saya de Malha Bank appears to haye the characters of a sub- 

 merged atoll, having a central depression of 65 fathoms, surrounded 

 by a rim which has only 8 to 16 fathoms on its eastern side, but 

 22 fathoms on the western. Some of the groups north of Madagascar 

 afford very good evidence of upheaval. Aldabra Island, situated in 

 lat. 9° 22' S., long. 46° 14' E., is a perfect instance of an upraised 

 atoll. Captain Wharton describes the external shores as consisting of 

 low coral cliffs, about 20 feet high, the surface of the land being 

 composed of jagged coral rock. The lagoon is entered by a passage 

 varying from 11 to 5 fathoms in depth, but its internal portions are 

 either very shallow or partly dry at low water. Not far distant is 

 the Cosmo Ledo group, a perfect atoll, with a lagoon some 4 fathoms 

 deep, or less. There are ten islets of various sizes on the reef, and 

 all of them appear to have been elevated some 10 feet. There are 

 some hills 40 and 50 feet high on the two largest islands, but these 

 appear, according to Captain Wharton, to be formed of blown sand. 

 The Farquhar group and Assumption Island, situated within the 

 same area, have been raised, according to the same authority, some 

 10 feet. Providence Island, in lat. 9° 14' S., long. 51° 2' E., appears 

 to be a low island situated upon the edge of the atoll-shaped Provi- 

 dence reef. At a distance of 19 miles from Providence Island is the 

 island of St. Pierre, which has no fringing reef. It is particularly 

 interesting, for although it is in close proximity to the low Providence 

 atoll, it has been raised about 40 feet above high water, and in the 

 absence of a fringing reef the sea breaks with great violence against 

 a low cliffy coast, hollowing out a number of caverns which, from the 

 description given in the sailing directions for Mauritius and its 

 islands, appear to open inshore by " blow-holes."* 



Near and among these raised coral formations are several sub- 

 merged banks, the most important of which is McLeod Bank, situated 

 in lat. 9° 57' S., long. 50° 20' E., between Providence Island and the 

 Cosmo Ledo group. The details show that there is a group of coral 



* For the information on the islands north of Madagascar I am indebted to the 

 courtesy of Captain W. J. L. Wharton, R.jN t , F.E.S. 



