344 



Mr Fryer, Aldahra and neighbouring Islands. 



Next passing to the other islands visited, a brief summary of 

 the chief structural characters of each may be given. 



Assumption is not an atoll but is an elevated reef composed 

 of the same classes of rock as constitute the land-rim of Aldabra. 

 The rock is extremely cavernous ; water can penetrate completely 

 through the land and there is no doubt that, as the caverns 

 inter-connect, the island will be broken up into a number of 

 rocks. 



Astove is an elevated atoll with a very perfect land-rim which 

 is divided by only one pass. It is composed of the same three 

 classes of rock as were found on the land-rim of Aldabra, though 

 coral in position of growth is even more common. The lagoon 

 is very shallow and probably existed almost entire at the time of 

 elevation. It is getting larger ; fresh passes are forming and the 

 atoll will be speedily broken up. 



Gosmoledo is also an elevated atoll but differs from Aldabra 

 and Astove in having but half its circumference capped with land. 

 The remainder, the reef, is elevated coral rock cut down to low 

 tide level and piled with coral debris while the edge is protected 

 by growing lithothamnia. There is no doubt however that once 

 the whole circumference of the rim was land. Sand derived from 

 disintegrated rock is abundant and hides the rock of the islands 

 so that there is a tendency as the rock land is removed to replace 

 it by sand cays. 



The state of Cosmoledo must be considered as forecasting in 

 part the future condition of Aldabra, and in conclusion an hypo- 

 thetical biography of the latter atoll may be given, basing the 

 account on the facts obtained from the various coral islands in the 

 region. 



The geology of Aldabra shows that it is entirely of oceanic 

 origin and has been built up by active coral growth, and therefore 

 our history of the atoll must begin with its formation as a reef 

 beneath the sea. There must first have been, however, some 

 base or mountain-top which reached to within 40 fathoms of the 

 surface, for it is well known that reef-building corals cannot 

 live at greater depths. Of the nature of this base we have no 

 evidence, though it may be taken as probable that it is of the 

 nature of a volcanic mound. We know of volcanoes in the 

 Comoros to the S.W. and the Sealark dredged volcanic mud near 

 Providence to the E., and there is thus no inherent improbability 

 in the existence of volcanic action in between. However this may 

 be, it is necessary to assume the presence of some base, colonised 

 by corals, forming a reef which gradually grew up until it nearly 

 reached the surface. This may be deduced from the fact that 

 the fossil corals are all of a shallow-water facies. As to the form 

 of the reef we suppose both from the geology and from theoretical 



