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



vious day, clearing away an amount of soil which, was almost incre- 

 dible. My companion M. Casimii- Leconte told me that the sea had 

 not been known to wash over this place before. It was apparent 

 that after a few more of such high tides as I had witnessed, a perma- 

 nent breach would be made at this spot, and another lagoon outlet 

 would be formed, which would be continually deepened as the tide 

 set through it. At the south-eastern side of the island I noticed that 

 the land was being rapidly destroyed on the outer shores just opposite 

 to a half-formed barachois, whose margins are situated not 60 yards 

 from the outer shore. If the same process of external destruction 

 continues, whilst the barachois is deepened and scooped out from 

 wdthin, it will not be many years before the ocean makes a new channel 

 into the lagoon at this point. Thus the continuous strip of land which 

 now nearly encircles the lagoon of Diego Garcia is tending to be 

 split up again into a series of islets. At the points where the breaches 

 are made the tides and ocean currents will rush with great force into 

 the lagoon and will scour out deep channels similar to that now exist- 

 ing between Middle and East Islets. 



These facts taken together show how the normal action of tides, 

 winds, and waves is constantly tending to lower to the sea level any 

 dry land that may have been formed by elevation or otherwise. It 

 does not seem to me to be surprising that the majority of atolls and 

 barrier reefs are, under such circumstances, only just able to maintain 

 their surfaces above the sea level. 



No explanation of atoll formation would be complete if it did not 

 include an explanation of the great Maldive atolls. Without 

 attempting to enter into a lengthy discussion of Darwin's views, 

 I will give my own explanation of the atoll. Tilla-dou-Matte atoll 

 is, as is well known, a huge atoll composed of atolls. The islets 

 forming the rim of the main atoll are themselves atolls with their 

 own lagoons ; the main lagoon contains a few secondary atolls 

 corresponding to the coral patches in an ordinary atoll. It will 

 be generally admitted that coral reefs are constantly increasing to 

 seaward because of the excessive growth of coral on their external 

 slopes* As the inward shores of an atoll are constantly being re- 

 moved, and an atoll if completely formed tends to be broken up again 

 into small islets when it has reached a certain size, and as the chan- 

 nels between the islets must be continually deepened by the scour of 

 the tides until deep passages are formed, an atoll like Diego Garcia 



* This statement may at first sight seem at variance with what I have just said 

 about the rapid destruction of land on the outer and inner shores of an atoll ; but 

 in the latter case it is land above water that is destroyed. Coincidently with this 

 process the reef-rock below water is constantly tending to raise itself and to spread 

 in all directions, owing ^o the perpetual growth ox corals and the accumulation of 

 their skeletons. 



