1888.] and the Coral Formations of the Indian Ocean. 441 



In my visit to Diego Garcia in 1885 I took with me, among other 

 works, the splendid papers of Alex. Agassiz on the Tortugas and 

 Florida reefs, and diligently compared all his observations with struc- 

 tures existing at Diego Garcia. I was much struck by the analogy 

 between the formations, but I also found differences in the stratifica- 

 tion of rocks, &c, which led me to the conviction that there has been 

 a very small amount of elevation in the Chagos group, which 

 certainly has not been the case in the Florida reefs. This conviction 

 was shared by an old resident on Diego Garcia, M. Spurs, a naturalist 

 who has devoted many years to the study of coral reefs. 



A reference to the map will show that Diego Garcia is a typical 

 atoll ; a narrow strip of land varying in width from a mile to 30 

 yards, nearly completely encircles a lagoon of irregular shape. The 

 lagoon is open to the ocean towards the north-west, its mouth being 

 divided by three small islets into four channels, of which three are 

 sufficiently deep to allow ships to enter the lagoon. The three islands 

 are known by the names of Bird Island, Middle Island, and East 

 Island (lie des Oiseaux, He du Milieu, and He de l'Est), the last 

 named being the largest of the three, and the one on which I spent 

 much of my time during my visit. The whole of the land composing 

 the atoll is very low ; the highest point in the island is not more than 

 30 feet above the level of high tide, and this height, which is quite 

 exceptional, is due to the accumulation of great heaps of sand 

 through the action of the S.E. trade winds which blow with consider- 

 able strength for more than one-half of the year. Diego Garcia is 

 the southernmost atoll of the Chagos group ; it lies in S. lat. 7° 26', 

 E. long. 72° 23', and forms the last of the great chain of coral forma- 

 tions reaching from the Laccadive Islands through the Maldives to 

 the Chagos group. To its south-west lie the submerged atoll-shaped 

 reefs known as Pitt's Bank and Centurion's Bank, to its north lies 

 the huge submerged atoll known as the Great Chagos Bank. It is an 

 interesting fact that throughout the Laccadive, Maldive, and Chagos 

 groups there is no instance of a fringing or of a barrier reef ; nothing 

 but coral structure rises above the waves, all the islands are atolls ; 

 none of these are upraised, but there are several submerged banks. 

 The existence of this long line of atolls seemed to be one of the 

 strongest arguments in favour of Darwin's theory on the formation of 

 coral reefs. If the depths given in Stieler's hand-atlas are correct, 

 the three groups stand on a submarine bank lying 1000 fathoms 

 below the surface in an ocean of an average depth of 2000 fathoms. 



In Diego Garcia the nature of the soil varies considerably from 

 place to place. In some localities it consists of nothing else than bare 

 coral rock upon the surface of which coral boulders are scattered 

 about ; in other places it is composed wholly of calcareous sand, and 

 one may dig down for 6 or 8 feet without finding coral rock. It is 



