340 Mr Fryer, Aldabra and neighhouring Islands. 



Aldabra and neighhouring Islands. By J. C. F. Fryer, B.A., 

 Gonville and Caius College. 



[Read 22 November 1909.] 

 Plate XII. 



[Absti-act.] 



Introduction. 



The group of islands comprising Aldabra, Assumption, Cosmo- 

 ledo and Astove is situated some 270 miles to the N.W. of 

 Madagascar and thus lies in the extreme S.W. corner of the 

 Indian Ocean. Aldabra has long been known as the last spot 

 in the old world on which indigenous giant land tortoises continue 

 to exist. It was also known to have a peculiar land avifauna; 

 and these facts combined with the discovery of unusual percentages 

 of iron, alumina and silica in guano from the locality pointed 

 to a possible land connection with Madagascar in previous 

 times. 



In 1905 the Sealark expedition carried on investigations in 

 the Indian Ocean relating partly to the former land connection 

 between India and Africa and partly to the formation of the coral 

 islands of this ocean. The Aldabra group however was not 

 visited and therefore from both points of view a further expedition 

 was desirable. Certain of the more general results of this latter 

 expedition are set forth in this paper. 



Topography. 



Aldabra is an atoll 24 miles long and from 4 — 10 miles broad. 

 The land-rim is very perfect and is divided by narrow passes into 

 four large islands called respectively Picard, Polymnie, Malabar, 

 and Main or South islands. Round the outside of the atoll is 

 a narrow fringing reef which is divided by channels from the 

 passes and is indefinite or non-existent on the extreme east. 



To landward the fringing reef is bounded by rocky cliffs 

 10 — 20 ft. in height which are overhanging and show evident 

 signs of wave erosion. 



The highest point on the land-rim (20 ft. above sea-level) 

 is always situated near the sea-cliffs and there is a gradual slope 

 to the lagoon which is fringed with dense mangrove swamps. 

 Small overhanging cliffs are usually found at high tide mark 

 in the swamp and give striking evidence as to the rapidity of 

 lagoon erosion. The lagoon itself is very shallow except near the 

 passes ; it is muddy towards the outsides and sandy in the centre. 

 Small islands and rocks are numerous round its outskirts and all 

 show much loss due to erosion. 



The general topography of the larger islands is as follows : 



