FRYER— FORMATION OF ALDABRA, ETC. 407 



the atoll and always supports the Pemphis type of vegetation. Its surface has already 

 been sufficiently described and it will suffice to point out that it indicates extensive 

 rain-water denudation. An experienced observer would without further data be able 

 to hint at the extent of this, but fortunately I have more definite evidence in the abnormal 

 rocks, with which I am about to deal. 



(b) Abnormal Rocks. Rocks, which cannot be considered as normal forms of coral 

 limestone, were found on Picard Island and on Esprit Island, of which the latter will be 

 considered first. 



Esprit consists of a crescent-shaped ridge of rock about \ mile long and 30 feet high, 

 being thus the highest rocky point on the atoll. The ridge itself at its widest measures 

 about 200 yards and is densely clothed in a varied jungle, containing several small forest 

 trees not found elsewhere. The hollow of the crescent is occupied by mangrove swamp. 



The ridge is composed of a curious conglomerate or pudding-stone, which is yellow 

 or brown at the higher levels, but which contains a quantity of dark brown, almost 

 black substance lower down : this in places along the shore projects as large boulders, 

 which have been greatly wind-polished, giving the island an appearance most foreign 

 to coral islands. On the top of the ridge is a small quantity of rock, which superficially 

 resembles flint, while on the sides are pinnacles of a rock composed largely of mollusc 

 shells, though a few fossil corals were also found. These pinnacles on the south side 

 were but 3 or 4 feet high, but on the north measured 15 or 16 feet and were arranged 

 like some fantastic natural maze, out of which it was difficult to find a path. The strange 

 form of the rock had evidently been produced by great rain-water denudation and a 

 natural chimney in the ridge appeared to prove that the shell rock penetrated the 

 conglomerate, which must have been formed round it. (Plate 27.) 



A rough analysis of specimens from the ridge shows that all (both conglomerate 

 and flint-like rock) are chiefly composed of calcium phosphate, the colouring being 

 probably due to a small percentage of iron. No evidence as to the mode of formation 

 was found on Esprit, but fortunately similar rocks exist on Picard and here a clue was 

 obtained. In the south-west is a small plain of " platin " rock, in which is a large basin 

 (10 feet at deepest) partially filled with salt water, which is tidal, being in subterranean 

 connection with the lagoon. Round the sides of the basin were found masses of phosphatic 

 rock resembling that of Esprit, while in addition there was some quantity of a highly 

 crystalline mineral resembling calcite, veins of which radiated into the surrounding 

 " platin " rock. A conglomerate containing an anomalous assortment of vertebrate 

 remains, such as teeth of Careharias, of Diodon, of Scarus, with bones of giant land- 

 tortoises, was also found, its components being apparently cemented together by the 

 calcium phosphate. 



The general mode of formation of these rocks was fairly obvious ; the basin represents 

 a large cavern, into which various marine deposits and, later, tortoise bones were washed. 

 After the deposition of guano, it also was washed by the rain into the cavern and formed 

 the various phosphatic rocks just described : under the influence of rain-water denudation, 

 assisted by the breaking in of the lagoon, the cavern was thrown open to the surface, and 

 now by erosion is rapidly increasing in size. 



52-2 



