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



wind and waves alone, without any assistance from, the upheaval of 

 the bed of the sea. But I am not satisfied that this has been the 

 case in the Chagos group. Storms are of very infrequent occurrence 

 there, and the horizontal masses of reef rock standing above high 

 water mark cannot be attributed to the normal action of the prevailing 

 winds and currents. 



Tn the Florida reefs the nature of the soil betrays its origin — 

 its strata slope towards the sea on every side, and the lamination of 

 the rocks attests the long- continued action of waves and spray. Bat 

 the alternate horizontal layers of sand and rock occurring so abun- 

 dantly at Diego Garcia are quite different ; they do not dip seawards, 

 their composition differs from the rocks of the Florida reefs, and their 

 edges, instead of showing signs of accumulation of fresh material, 

 are often bluff, and show that the sea is gradually eating them away. 

 It is difficult to explain these appearances except on the hypothesis of 

 slight elevation. It might be objected that if any upheaval had taken 

 place, the banks lying at various depths below the surface wouid 

 have been raised to different heights, and that it would be in 

 the highest degree unlikely that so many would be raised some 

 4 or o feet above high water mark and no more throughout so large 

 areas as the Laccadive, Maldive, and Chagos Islands, and the various 

 low groups in the Pacific. The force of the objection must be 

 admitted, but it may be observed that atolls raised from 10 to 40 feet 

 above the waves are not so uncommon as has been hitherto sup- 

 posed, and that the numerous submerged banks lying at very various 

 depths show that all the reefs have not been raised to one height 

 in a single area of elevation. The uniform level of many atolls and 

 barrier reefs admits of a further explanation. A reef raised some 

 few feet above the sea level is at once attacked by the waves, and as 

 the rim is very narrow, it must soon be worn away till the whole of 

 the land is eaten away, and its surface is brought awash once more. 

 Thus every slight movement of elevation would soon be compensated 

 by the denuding action of the waves. The island of St. Pierre, already 

 described, is a good instance of this process of erosion. It cannot 

 be doubted that this island, which has recently been raised 40 feet, is 

 undergoing rapid waste, and must soon be reduced to the level of the 

 sea. At Diego Garcia I was. astonished at the rapid destruction of 

 dry land which is in progress, on the outside as well as the inside 

 of the lagoon. The destruction is not so great on the outside as on 

 the inside as a rule, for in the former case the rampart of coral 

 boulders thrown up by the waves compensates in many places for 

 their erosive action. But in the bay above Horsburgh Point, exposed 

 to the full strength of the S.E. trades, the destruction is very great. 

 M. Spurs, an old resident on the island, writes to me on this subject : 

 " Cette destruction est tres rapide ; Diego perd en moyenne un pied 



