438 PERCY SLADEN TRUST EXPEDITION 



lagoon continued to be the lowest area. When it had decreased to some 20 feet or less 

 above sea level, the formation of the secondary lagoon could begin, and this is supposed to 

 have taken place as follows. The pits in the rock both at Aldabra and Assumption have 

 been mentioned as also the fact that they are usually filled with tidal water and are 

 continually increasing in size owing to erosion. On Assumption it was also easy to see 

 that they had been underground caverns, which owing to erosion and denudation had 

 become open to the surface, while in several cases pits were seen which had evidently 

 been formed by the interconnection of two or more smaller concavities. It is supposed 

 that at some point in the primary lagoon of Aldabra several such pits by coalescing 

 formed a small secondary lagoon, filled with salt water and in subterranean connection 

 with the sea. Gradually this increased in size by erosion and ultimately obtained a free 

 channel to the sea probably on the site of Grande Passe (the oldest pass). Continued 

 decrease in level by denudation made the centre of the atoll progressively more subject to 

 erosion and in time a second opening to the exterior was gained in the neighbourhood of 

 Passe Houareau. There was thus formed a small lagoon with two openings to the 

 exterior. From this point onward it is easy to imagine a gradual increase in the area of 

 the lagoon, the formation of more passes and a decrease in level of the land until the 

 present stage is reached. 



A similar history perhaps applies to Cosmoledo and Farquhar ; Astove however, 

 if my deductions are correct, always had a lagoon (the primary lagoon) though it is only 

 recently that it has obtained a free channel to the sea. St Pierre is very small and the 

 primary lagoon is not deep enough to convert it into an atoll, while Assumption has no 

 marked depression in the centre, but on account of the interconnection of pits is gradually 

 being split up into a number of rocks. 



Sea erosion appears to have been of small importance when compared with lagoon 

 erosion, but it must not be forgotten that it is responsible for the fringing reef, and in 

 connection with the ocean current for the shallow area, the site of lost land, which occurs 

 to the east and south-east of Aldabra and Assumption. 



Enough has now been said to indicate the early history, which is supposed to be 

 common to the atolls and islands of the Aldabra series, and it remains to point out their 

 probable future which is more easy than the discovery of their past, for various members 

 of the series illustrate each probable stage. These may be briefly tabulated as follows, 

 though the atolls for convenience are considered apart from the islands without lagoons. 



Stage 1. This is represented by two atolls, Aldabra and Astove. They are 

 characterised by a very perfect land-rim, which is composed almost entirely of coral 

 rock, though a tendency to sand piling is noticeable, locally, on each. In both sea erosion 

 has formed a fringing reef, which is very barren and is not extending seawards, while 

 lagoon erosion is forming new passes and reducing the area of land. The lagoon, very 

 shallow in both atolls, is increasing in depth and Aldabra, with the greatest number 

 of passes, has the deepest lagoon. The latter atoll is also somewhat in advance of 

 Astove in that process of subdivision of the islands by stretches of reef is already 

 noticeable in the formation of the reefs by the sides of Grande Passe and Passe 

 Houareau. 



