Mr Fryer, Aldahra and neighbouring Islands. 341 



Picard is noted as possessing on its westward shore a narrow 

 stretch of sand on which is built the settlement and which also 

 constitutes the only arable soil on the atoll. In the centre of the 

 island is a rocky plain with a flat pavement-like surface : it is 

 known as Plain Cabris and is the locality from which were 

 obtained certain peculiar rocks to be referred to later. The rest 

 of Picard is more typical of the atoll as a whole ; the surface of 

 the ground has been broken up by rain-water denudation into 

 points, pinnacles and sharp ridges, and the whole is covered with 

 a dense scrub of Pemphis acidula. This combination of rock and 

 jungle, known as ' champignon ' country, forms the great bar to 

 exploration in Aldabra. 



Polymnie, Malabar, and the north of Main island consists of 

 this champignon country. In the S.E. of Main island however 

 another plain of flat pavement-like rock exists. It is covered 

 by a somewhat open and varied jungle, and within its boundaries 

 are found the only fresh-water springs on the atoll. These are 

 situated at Takamaka and consist of pits in the rock about 

 5 ft. deep. Sand dunes 60 ft. high occur along the south of Main 

 island and have been formed by the action of the prevalent winds 

 (S.E. Trades) which blow up sand from the reef and pile it in 

 mounds on the shore. The west of Main island is again entirely 

 of the champignon type of country. 



Leaving now the general topography of the atoll a more 

 detailed account of its structure may be given, and this will be 

 dealt with under three heads, (a) structure of land, (6) structure of 

 fringing reef, (c) structure of lagoon. 

 Str^ioGture. 



(a) The land is entirely rocky and the main rock component 

 is in all cases coral though complications have been introduced by 

 the presence of a quantity of calcium phosphate derived from 

 guano. The rock of the land-rim may be divided into three 

 classes. The first forms a zone round the outside of the laud- 

 rim and is marked by the fact that its component corals are 

 very perfect and are almost all in position of growth. This point 

 is most important as it proves that the atoll is an elevated reef 

 and that it has not been formed by wave piling on a subsiding 

 base. The second class is the pavement-like rock before referred 

 to : it is composed of broken and often triturated coral with such 

 debris as mollusc shells and echinoderm spines, and gives evidence 

 that the inner portions of the reef before elevation were either 

 dead or but feebly growing. The last class of rock is the 

 ' champignon ' ; it is a highly metamorphosed coral rock with 

 scattered inclusions of calcium phosphate derived from guano. 

 Metamorphosed coral limestones from all the islands in this region 

 contain inclusions of calcium phosphate, and it is necessary to 



