FRYER— FORMATION OF ALDABRA, ETC. 435 



Aldabra series we have no actual knowledge ; but, as there are only a limited number of 

 methods by which such a base could be obtained, the matter can be carried somewhat 

 further. By the most famous theory of coral islands, that of Darwin, it would be 

 supposed that, as the Indo- African bridge disappeared, mountain-tops were left projecting 

 above the sea and in tropical regions would naturally be surrounded by a fringing reef. 

 As the mountains continued to sink, the fringing reef would become a barrier-reef and the 

 barrier-reef an atoll, which by active growth and wave piling could remain at the surface 

 in spite of the subsidence. We have seen however that all evidence gained on Aldabra 

 and its neighbours points to a method of formation by elevation and not by subsidence, 

 and Darwin's theory is therefore untenable. 



In the case of the Maldives, Gardiner showed that the base on which this archipelago 

 is built, is probably the higher part of the Indo-African bridge, cut down below the 

 surface of the sea by current erosion, and at first it was felt probable that the same 

 explanation might apply to the Aldabra series, but, on learning that its members arise 

 abruptly from a depth of 1000 fathoms, this supposition seemed unlikely*. There 

 remains but one alternative. It is known that during submarine volcanic action mounds 

 of ashes and mud are produced, which can even form islands, as in the case of Graham's 

 Island f, and these mounds arise abruptly from the sea-bottom as appears to be the case 

 with the members of the Aldabra series. In addition we know of an active volcano in the 

 Comoros, of volcanoes but recently extinct in Madagascar, and the "Sealark" dredged 

 undoubted volcanic mud from off the slope of Providence, and therefore the whole area 

 of the South-West Indian Ocean appears to be liable to volcanic action and it does not 

 seem too rash a theory to suppose that the bases in the case of the Aldabra series are 

 volcanic mounds. 



These mounds may have reached to within 40 fathoms of the surface when formed ; 

 may have been volcanic islands, which were cut down to that depth by the sea or 

 may have been built up from great depths by the gradual deposition of animal remains 

 from the surface ; these are problems which we cannot decide, but it is quite certain that 

 in the case of each island or atoll some base reached to within 40 fathoms and was 

 colonised by reef-building corals. 



Up to this point I have referred to the members of the series collectively, but 

 henceforward I shall take Aldabra as my type on account of the superior historical 

 evidence it contains. The shape of the Aldabra reef before elevation has already been 

 dealt with in relation to the "platin" and phosphatic rocks}, and without further 

 discussion it will suffice to say that it is supposed to have been that of a shallow 

 basin with the outer edge formed by flourishing corals and perhaps lithothamnia, while 

 the inner portions were covered with rubble and sand with here and there a live colony of 

 corals and in parts extensive mollusc beds. From the facies of the fossil corals we know 

 that this reef had nearly reached the surface (within 10 fathoms) but its exact depth 

 is naturally indeterminable. 



* It is of course just possible that low peaks of the submerged bridge have been built up by deposition 

 of material from the surface, but Gardiner's proof of the existence of deep currents in the South Indian Ocean 

 makes this unlikely. 



t Vide Lyell, Principles of Geology, Vol. ii., p. 59. 



t Ante p. 405. 



