346 Mr Fryer, Aldahra and neighbouring Islands. 



result was a gradual decrease in level. In time the subterranean 

 caverns, becoming larger, joined each other and finally a series, 

 probably in the neighbourhood of Grande Passe, obtained open 

 connection with the sea. After this a tidal current swept in and 

 out continually dissolving away rock and forming a lagoon. The 

 transition from this stage to the present one is easy : a further 

 pass, Passe Houareau, next formed and the lagoon gradually 

 obtained its present dimensions, though He Esprit, on account 

 of its being composed of an insoluble rock, was left standing as 

 a clue to former conditions. The level of the land gradually 

 decreased by rain-water denudation and fresh passes continued to 

 form until the present state of things obtained. As to the future 

 we may safely prophesy the formation of more passes : lagoon 

 erosion will proceed with increased speed though it is noticeable 

 that land is only cut down to low tide mark, for below this 

 lithothamnia, corals and the piling of sand form a protection 

 against speedy erosion. Looking further into the future and 

 bearing in mind the present state of Cosmoledo we see that 

 the rock islands will gradually be divided up and be separated 

 from one another by long stretches of reef. The islands may 

 be eaten away until no land remains, though it is more probable 

 that it will continue to exist in the form of sand cays so character- 

 istic of many atolls. This stage may be called the Farquhar* stage. 

 Beyond this it is impossible to foretell ; and in conclusion it may 

 be suggested that the life-history of Aldabra will be found typical 

 of most coral rock islands, and finally may give a clue as to the 

 former history of many of the coral sand islands so common in the 

 tropics. 



* Farquhar atoll, 150 miles east of Astove, is in this condition. Vide Trans. 

 Linn. Soc, Vol. sii. p. 144. 



