FRYER— FORMATION OF ALDABRA, ETC. 401 



of shallow passes broken by small islands, which have obviously been cut away from the 

 adjoining portions of the land rim. 



Of the numerous islands in the lagoon immediate reference need only be made to the 

 two largest, Michel in the east of the lagoon and Esprit in the west, of which the former 

 is unimportant, the latter of great geological interest. 



Other physical features, to which attention should be called, are the fringing reef and 

 the mangrove swamp ; the former extends entirely round the atoll, except that it is 

 divided by the channels (passes) into the lagoon, and is indefinite to the east and south- 

 east, where the sea is shallow for about a mile. The mangrove swamp is found round the 

 shores of the entire lagoon and varies up to a mile in width, occupying all suitable spots 

 between tide marks. 



It will doubtless be noticed, without further reference to topographical features, that 

 the atoll is divided into certain definite physical divisions, and for convenience in discussion 

 it is proposed to deal with each separately. These sub-sections will be : — 



(1) The land. 



(2) The lagoon, comprising the lagoon shores (mangrove swamp) and the passes. 



(3) The fringing reef and also the outer slope, though as to the latter very little 



information can be given. 

 The animals and plants collected are being dealt with in subsequent papers and so 

 lengthy reference would be out of place in this article : a short section will, however, be 

 devoted to those which are not to be treated later, and to other forms of special interest, or 

 from which definite deductions can be made. 



B. Land Topography. (Plates 23 — 25.) 



In this sub-section I propose to give some general description of the superficial features 

 of the land of Aldabra, and then pass on to a more detailed account of its structure. All 

 the various islands of which the atoll is composed are formed entirely of rock, which is 

 usually found quite exposed on the surface, and is rarely covered by soil of any kind : 

 exceptions to this generalisation are found in parts of Picard, Main, Michel and Esprit 

 Islands, where sand has been piled up by the joint forces of wind and wave and forms 

 a thin covering. In addition to this, in former times a large quantity of guano was 

 distributed over the atoll, but now it is only found in holes and crevices, the greater bulk 

 having been washed down by rain-water and to a large extent absorbed into the constitu- 

 tion of the rocks. 



The rock itself comprises a variety of limestones, all of coral reef origin, though many 

 have been much altered and phosphatised by the guano referred to above ; there is no rock 

 on Aldabra which has persisted from any continental connection and the atoll has had an 

 entirely different origin. 



The surface of the rock produces perhaps the most striking impression of anything on 

 the atoll, for, instead of being smooth or slightly rough, it has been attacked by rain- 

 water, which has gradually dissolved much of it away leaving a surface fretted as if by 

 a powerful acid ; sharp points, projecting pinnacles and deep pits are found everywhere, 

 making work exceedingly trying. Under these circumstances it might be expected that 



