S42 Mr Fryer, Aldahra and neighbouring Islands. 



credit the latter substance with some power of inducing meta- 

 morphosis though its nature is not understood. In all the three 

 classes of rock pits and subterranean caverns are numerous and 

 usually contain salt-water which fluctuates tidally. They all 

 show signs of erosion and are increasing in size. Evidence is thus 

 given that the reef as a whole before elevation was but badly 

 consolidated and that even now water can percolate completely 

 through the land-rim. 



A few exceptional rocks remain to be dealt with. He Esprit, 

 in the lagoon, consists of a ridge of rock half a mile long and 

 some 30 ft. high, and is thus the highest rocky point on the atoll. 

 The body of the ridge consists largely of calcium phosphate : 

 on its sides are pinnacles and walls of a much denuded rock 

 composed almost entirely of mollusc shells. Plain Cabris on 

 Picard island again shows somewhat similar structures but a 

 crystalline mineral resembling apatite should be noted as also 

 fossil bones of the giant land tortoise. It may be mentioned 

 that Esprit island is of special importance if an attempt is 

 made to reconstruct the early stages of Aldabra after elevation. 



(6) The second section of the atoll consists of the Fringing 

 Reef. Evidence obtained from the amount of wave erosion and 

 from rocks of elevated coral limestone still in situ proves that the 

 fringing reef of Aldabra is a ledge of elevated coral rock cut down 

 to low water mark and preserved at that point by growing 

 lithothamnia and by the piling of sand and coral on the fiat. 

 The fringing reef of Aldabra therefore is not a sign of the 

 increase of the atoll seaward, but on the contrary is evidence 

 of the loss of land owing to wave erosion. 



(c) The last section of the atoll is the lagoon, some of the 

 features of which have already been mentioned. Those to be 

 further treated are the extent of lagoon erosion and the formation 

 of passes. Judging by the maze of small islands and rocks, once 

 part of the land-rim, at least two-thirds of the lagoon were at one 

 time land. It is still increasing for the rock is reduced to mud ; 

 the mud is swept to sea and the land continues to diminish. 

 Contrasting lagoon erosion with sea erosion it will be noticed 

 that while the latter is uniform over any stretch of coast the 

 former is just the reverse, and is extremely irregular. Passes 

 therefore must be formed from the lagoon side of the land-rim 

 and not from the sea. All stages of pass formation are shown 

 on Aldabra. At Camp Frigate the mangrove swamp passes 

 completely through the land-rim to the sea, though but little 

 water flows through yet. The western channels show complete 

 passes through the land-rim but the fringing reef has not yet 

 been divided. Grande Passe finally is a pass with a complete 

 channel, and it further shows that the channels of the passes 



