44 1 Mr. G. C. Bourne. The Atoll of Diego Garcia [Mar. 22, 



water mark, and could only believe that they were originally formed 

 as reef or shingle rock below high water mark, and had been subse- 

 quently raised to their present position. I was thus led to believe that 

 a slight elevation had taken place, and this belief was strengthened by 

 a study of the formation of East Islet. This islet is about 800 yards 

 long, and nearly 100 yards broad ; its westernmost extremity is com- 

 posed of masses of sand piled up on the underlying reef rock, and in 

 this place there is a clump of high trees (Hernandia peltata). The 

 eastern and by far the larger part of the islet is of different formation. 

 The even surface of the soil is covered with a low scrub, but bears no 

 high trees nor coco-nut palms. It forms a low plateau, the surface of 

 which does not slope down towards the lagoon, but is perfectly hori- 

 zontal, and stands 4 feet above the very highest spring tides. The 

 shore on the lagoonward side shows an abrupt fall of 6 feet to the 

 reef, which in this place extends for a distance of 50 yards towards 

 the lagoon, and is only left uncovered at the lowest spring tides. At 

 the eastern extremity of the island there is no reef, but from 1^ to 

 2 fathoms of water are found within a few yards of the shore. This 

 point is exposed to the ocean, and a strong and constant current sets 

 against it, so that it is undergoing a considerable amount of erosion. 

 On the north or seaward side the reef again extends outwards from 

 the shore, the latter differing from the inner shore in the presence of 

 a talus of large boulders which have been thrown up against it by the 

 waves. Wells have been sunk in various parts of the island, though 

 for some reason which I cannot explain, water is only found in one 

 of them. Numerous pits, some of which are 9 feet deep, have also 

 been dug for the purpose of planting coco-nuts. These pits and wells 

 expose the interesting structure of the superficial part of the island. 

 Beneath a thin surface layer of sand and mould lies a horizontal layer 

 of stratified shingle rock, in which large imbedded coral masses may 

 occasionally be distinguished ; this layer is about 2J feet thick. 

 Beneath it is a layer of loose coral sand about 18 inches thick, and 

 beneath that is another layer of coral rock of the same character as 

 the first, and about 3 feet thick. Beneath this is another layer of 

 friable sand lying on the solid reef rock into which the excavations 

 did not penetrate. These layers lie perfectly horizontally, and do not 

 dip in any direction. They crop out above the reef on the steep 

 eastern and southern shores, and as the loose sand is washed out by 

 the waves the overhanging layer of rock breaks off and falls down in 

 large masses. The central parts of this area are absolutely beyond the 

 reach of any waves at the present time, and as the strata of rock and sand 

 run evenly through it, there is no evidence of its having been formed 

 by successive additions of material through the action of the waves. 

 Nor can it possibly have been formed under the surface of the water 

 unless it has since been raised to its present position, for, as I have 



