Mr Fryer, Aldabra and neighbouring Islands. 345 



considerations that the outsides were flourishing, while the inner 

 portions were but feebly growing or dead. This means that the 

 shape of the reef was probably that of a shallow basin. 



The next point in its history was its elevation above the 

 surface of the sea, though how this occurred is somewhat doubtful. 

 It would naturally be supposed that it was accomplished by 

 a slight local elevation of the earth's crust, but against this is the 

 objection that there is evidence of an elevation of equal amount 

 throughout the western Indian ocean, and it is difficult to conceive 

 of an alteration in the level of the earth's crust which would be 

 so uniform over such a large area. The only alternative is an 

 alteration in the level of the sea, and it may be mentioned that 

 there are points in favour of this hypothesis though they are 

 of too indefinite a nature to bring up for discussion in this 

 paper. 



The extent of the elevation may be placed at about 60 ft., 

 for Esprit island is 30 ft. high, and it is probable that the reef 

 hardly reached the surface before elevation, while since then an 

 enormous amount of denudation has occurred. After elevation 

 the whole reef was placed out of water and the depression in 

 the centre was dry. In Astove it will be remembered there was 

 evidence that the present lagoon was largely existing before 

 elevation, and may therefore be called a primary lagoon, while 

 in Aldabra we now see that the present lagoon must be a 

 secondary lagoon developed after elevation. The question of its 

 formation will be dealt with shortly, but first certain deductions 

 must be made from Esprit island which consists almost entirely of 

 calcium phosphate. The phosphoric acid was undoubtedly derived 

 from guano, and to have formed the phosphate rock on Esprit it is 

 necessary to suppose that a solution obtained entrance to a large 

 underground cavern which may have contained a certain amount 

 of sand or mud. The whole space was gradually filled with the 

 deposit, and to have obtained such a large quantity it is necessary 

 to suppose that the top of Esprit was the sink or drain from 

 a large area covered with guano. This area was probably the 

 primary lagoon which after becoming dry was filled with guano 

 by oceanic birds. For the present, therefore, it is supposed that 

 all the peculiar phosphate rocks of Aldabra were formed by a 

 solution of guano obtaining entrance to a subterranean pocket. 



Turning next to the formation of the secondary lagoon, 

 mention was made of the fact that the rock as a whole is still 

 very cavernous so that water can penetrate completely through 

 the land-rim. At the time of elevation this feature must have 

 been more marked, and therefore the sea could at once begin 

 dissolving away rock and enlarging any spaces. Simultaneously 

 rain-water denudation was acting on the surface and the natural 



VOL. XV. PT. IV. 23 



