408 PERCY SLADEN TRUST EXPEDITION 



The details of this process of rock formation must be left to the specialist, but 

 there seems no doubt that the general method is correct, and applies not only to Picard 

 but also to Esprit Island. It appears, therefore, that the ridge of the latter island 

 was formed underground in a large fissure or series of caverns in the rock, to which guano 

 or a solution of its main constituents obtained entrance. It follows directly then that 

 the general level of Aldabra was originally over 30 feet high and that it has been reduced 

 to its present level by a process of rain-water denudation and erosion ; the limestone, 

 being soluble, has been washed away while the insoluble phosphatic rock remains. 



Carrying our deductions further, we see that in early times there was no lagoon, 

 for the land round Esprit must have been over 30 feet high. It was probably a low 

 portion of the future atoll, for in order that such a large quantity of phosphate may have 

 collected in one place it is necessary to suppose that the region above Esprit was the 

 lowest part or sink of a large area*. 



Though of less interest a word must be said about the shell rock, which was 

 undoubtedly laid down beneath the sea. I have already shown it to be probable that 

 the inner portions of the reef before elevation were built rather by the accumulation 

 of debris than by active coral growth, and therefore were probably somewhat lower than 

 the edge. This depression in the reef before elevation I call the " primary lagoon." 

 As we see from the shell rock, it was a favourable position for mollusc beds. After 

 elevation it must have become dry and was filled with guano, ultimately forming the 

 collecting area from which the phosphatic material drained to form the present 

 Esprit Island. 



This small island in the Aldabra lagoon therefore plays a most important part in any 

 attempt to reconstruct the past history of the atoll. 



III. ALDABRA. LAGOON AND PASSES!, AND THEIR FAUNA. (Plate 26.) 



The first point to be noticed about the lagoon is its extreme shallowness, for only 

 in the channels radiating from the main passes are more than a few feet of water 

 found. At low spring tides a large area becomes almost dry, and even at low neaps 

 most of its surface is impassable to a large dug-out canoe. In the centre of the lagoon 

 is a slight depression, which at low spring tides contains perhaps 4 feet of water and 

 is known as "Barachois Torti" from the fact that green turtles often become imprisoned 

 there until the tide rises. The channels in the lagoon are 8 or 9 fathoms deep near 

 Grande Passe, but within a mile become very shallow and, except that they float 

 a boat when the rest of the lagoon is impassable, would hardly be noticed by a casual 

 observer. 



A second feature of the lagoon is the great extent to which it is landlocked, the 

 passes being wholly inadequate to allow the tides to flow evenly inside and outside 



* [Mr Hughes suggests that birds may have especially congregated in this area. This seems to me to be 

 not improbable since in my experience some islands and some areas of islands are occupied by " breeding fairs " 

 while others in the neighbourhood, apparently equally suitable, are quite free from breeding birds. J. Stanley 

 Gardiner.] 



t The " passes " are the channels through the land-rim from the outer sea to the lagoon. 



