FRYER— FORMATION OF ALDABRA, ETC. 437 



to a large extent the primary lagoon of the reef when under the sea ; from the luxuriance 

 and distribution of the fossil corals it is probable that it was not so near the surface before 

 elevation as that of Aldabra, and consequently did not rise to such a height. 



The next event of importance which happened to Aldabra (and also to the neigh- 

 bouring islands) was the deposition of a large quantity of guano, and though we do 

 not know exactly when this took place, yet it is fairly certain that it occurred before 

 the secondary lagoon was formed. The guano was deposited solely by marine birds, 

 which at the present day, though very plentiful in the Indian Ocean, do not exist in 

 sufficient numbers to form guano at all rapidly on more than one or two islands (cp. Bird 

 Island, Seychelles *, Cargados Carajos t). So great is the quantity of guano on the various 

 islands of the South Indian Ocean, that it probably amounts even now to over a million 

 tons, and it is a matter of interest as to why there were formerly such vast numbers of 

 birds where now there are relatively few. Two explanations have been offered ; firstly, 

 that the birds were driven from their breeding places by man, and secondly, that the 

 increasing vegetation gradually reduced and obliterated the open spaces necessary for 

 ground-breeding birds. Against the former is the fact that many of the guano islands 

 have only just been colonised and have never been known to be "bird islands," while 

 against the latter I might mention that on Bird Island, Seychelles, on which I spent 

 a fortnight, the birds are quite capable of preventing the continuous growth of any form 

 of vegetation on their breeding places by means of fresh guano, which is quite unsuitable 

 to plant life. To provide an alternative theory I would question the effects of the last 

 glacial period (post-pliocene) on the bird-life of the northern hemisphere. The birds must 

 have been driven south, and marine birds would find it necessary to breed on oceanic 

 islands, for the shores of continental lands for many reasons could not be suitable. 

 A temporary congestion might thus have been produced which, with the retreat of the ice 

 in the north, would have been relieved by a return wave of migration. 



Whatever may be the true explanation of this enormous quantity of guano, there 

 is no doubt that the derived phosphoric acid has played a most important part in the 

 post-elevation history of the Aldabra series ; its universal inclusion in the metamorphosed 

 limestones has already been mentioned ; in addition it shows that the reef when first 

 elevated must have been very porous and slightly consolidated, for the guano has been 

 washed into the greater part of the rock on the atoll. At the present day we know that 

 the land-rim can be completely penetrated by salt water, and from the phosphatic 

 inclusions it appears certain that at first the island was equally, if not more, porous ; 

 from the beginning therefore sea water could penetrate far into the land, enlarging 

 caverns and crevices by solution and erosion, though at the same time, this process, 

 by allowing concavities to collapse, must have resulted ultimately in better consolidation 

 at the expense of a reduction in surface level. Rain water denudation has probably been 

 an even more potent factor in reducing the level of the land from its original 50 feet 

 or more above sea level to its present average of about 12 feet. This gradual surface 

 sinkage was apparently more or less equal over the whole island and thus the primary 



* Percy Sladen Trust Expedition, Vol. xiv., p. 15 (Trans. Linn. Soc). 

 t Op. etC, Vol. i., p. 125. 

 SECOND SERIES— ZOOLOGY, VOL. XIV. 56 



