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



observer as M. Spurs for those of another species, and since they are 

 in the present day only found between tide marks, these observations 

 afford a farther presumption in favour of a slight elevation having 

 recently taken place. In any case they preclude the idea of any sub- 

 sidence being in progress, as Mr. Darwin fancied to be the case in the 

 Keeling atoll. M. Spurs further informs me that during the time 

 that he was superintendent of the oil company's estate, he caused 

 more than 30,000 pits to be dug on the main island for the purpose of 

 planting coco-nat palms, and that he frequently observed in different 

 localities the same alternate layers of sand and rock that I have 

 described as existing on East Island. These alternations of sand 

 and rock would suggest alternations of very slight subsidence 

 with very slight elevation, rather than a single movement of 

 upheaval ; yet on the supposition that all the layers were formed 

 beneath the water, as their horizontal stratification leads me to 

 believe, I can venture on the following explanation. The mass 

 of rock which forms the base upon which the islets and other dry 

 land rest is solid reef rock, and the whole floor of the lagoon is 

 similarly formed. The latter is covered at depths of 3 or 4 fathoms 

 and upwards by a layer of fine sand, which may attain a thickness 

 of 2 or 3 feet. In protected parts of the lagoon and in spots w r here 

 the changeable currents have ceased to deposit any quantity of sand, 

 corals will grow in considerable quantities, chiefly those wide-spread- 

 ing species of Madrepora which cannot find a lodging on the exterior 

 of the reef, where they would be dashed to pieces by the waves. By 

 the continual growth of new colonies on the top of the old ones 

 which have died, a layer of solid rock of considerable thickness 

 may be formed. Whilst diving for corals at the lower part of the 

 lagoon, I often noticed such layers of half-formed rock on which 

 living coral was growing or not, according as the constantly 

 changing currents were at that time throwing up sand in the locality 

 or not. Thus on the west side of the lagoon, off Point Marianne, 

 there are large tracts of recently formed coral rock, on which no 

 living corals are to be seen, whilst on the east side of the lagoon, 

 exactly opposite to Point Marianne, a similar basis of rock is 

 luxuriantly covered with growing coral. 



Now as the currents are constantly changing, and as the changes 

 may, as I saw, affect an area some miles in extent, one may suppose 

 that an area was first covered with corals growing on the sand, which 

 everywhere covers the reef rock, when the latter lies more than a 

 fathom below the surface. A change in the currents brought 

 abundant sand to the spot, killed the corals, and deposited an even 

 layer of sand of some little thickness over the rock formed by the 

 skeletons of the dead corals. A further change in the currents would 

 again render the spot suitable for coral growth, and a new layer of 



