20 PERCY SLADEN TRUST EXPEDITION 



III. THE FORMATION OF BIRD AND DENNIS ISLANDS. 



An examination of the sand of which Bird and Dennis islands are formed, shows 

 that it is composed of broken coral and Lithothamnia in about equal parts, with a small 

 proportion of foraminiferal (Orbitolites, etc.) and molluscan shells. The surface of the 

 sand grains is fairly smooth but not so much so as in those of dunes of wind formation: 

 it is also not so rough and jagged as in similarly constituted sand obtained from various 

 depths below low water mark. From these facts it can be reasonably deduced that the 

 sand (1) came from a reef on which Lithothamnia was very prevalent, and (2) was piled 

 up mainly by sea agency, but that wind also assisted after the surface had risen above 

 low water mark. 



The ocean currents in this area of the Indian ocean are variable both in direction 

 and force and naturally break up still more irregularly over the shallow Seychelles bank. 

 While these might have been directly or indirectly the cause of the accumulation of 

 the material which formed the foundation of these islands even at some considerable 

 depth, the land can only have been affected by their surface movements and by the 

 wind. From the fact that Dennis island is washing away it appears that these surface 

 currents or the winds must have altered considerably since the piling up occurred. 



The topography and surroundings of both islands are similar and suggestive : 

 (1) each island has a fringing reef only from the north-east southwards to the south- 

 west, (2) the unprotected (north-west) side of each island consists of a long slope of 

 sand, (3) the islands are situated on a rather deep (25 — 44 fathoms) part of the edge 

 of the bank. 



The former two facts seem to suggest that the islands have been piled up from 

 the north-west and consist of debris washed up from the edge of the Seychelles bank, 

 together with pelagic foraminifera. On the other hand it is possible that the islands 

 were once larger and that erosion is taking place, chiefly from the south-east, the 

 material washed away forming the long sand slope on the north-west and swamping 

 any reef which formerly existed there. The reef, which, as we have pointed out, is 

 peculiar, possibly then represents the former contour of the land though it may itself 

 be washing away also. There is no evidence, indeed, from present facts, as to whether 

 the islands form part of a land which extended along the northern reefs of the Seychelles 

 bank, or whether they were separate formations. They may have had, in the past, a 

 basis on a reef of coral rock, or on a ledge of granite, and themselves have consisted of a 

 line or area of rock masses against which sand piled up. The rock was eroded away 

 perchance and now the sandy land is itself disappearing. It is idle, however, to 

 speculate ; all we really know is that two sandy islands, which are built on sand- 

 covered banks, exist on the northern edge of the Seychelles plateau and that they are 

 the only surface banks which exist on the 650 miles or so of edge possessed by this 

 large shoal. 



