NA TURE 



[May 13, 1909 



found in pits in the rock, fluctuating, apparently, with the 

 lagoon tides. The cliffs on this north coast are 4 feet or 

 5 feet higher than those before described, and are always 

 much overhanging. Caves penetrate far into their faces, 

 large portions of which have at intervals fallen on to the 

 reef ; this fallen rock appears to become disintegrated 

 quickly, as small pieces are uncommon, the action of the 

 sea being assisted by . boring animals (small Gephyreans, 

 boring molluscs, &c.). As elsewhere round the coast, the 

 rock shows its component corals in a way which leaves 

 no doubt as to their being absolutely in the same position 

 as previous to their elevation. On the reef here there are 

 three or four distinct regions ; close to the cliff there is 

 a small belt of bare rock, often worn into hollows con- 

 taining 3 feet or 4 feet of water ; then a large area, mainly 

 of broken coral fragments covered with Lithothamnia, and 

 edged outside with a small boulder zone ; and outside this, 

 again, buttresses with a few colonies of living corals in 

 the channels. Such are the usual features of the fringing 

 reef at Aldabra, the appearance in the Takamaka section 

 being quite exceptional. 



After the previous descriptions it is possible to speak 

 more generally of Aldabra. The cliffs, as stated, show 

 their structure wondcrfullv clearly ; except in the Southern 

 Bight, where they are sloping and buttressed, they are 

 much overhung, and are crimibling fast. The general 



which lias been worn away in places so as to make clear 

 its section. The ridge itself is formed mainly of a sort 

 of brown conglomerate or pudding stone, which in one 

 place is capped by a challiy deposit, in which there is a 

 certain amount of flinty rock. On the south side, towards 

 the east end of the ridge, are some large masses of dark- 

 brown rock at sea-level. The greater part of the body of 

 the ridge is formed of the conglomerate, but this darker 

 rock seems to enter more into its composition at lower 

 levels. On the sides of the ridge are pinnacles of a 

 rough and pointed rock, which is apparently a calcareous 

 deposit, perhaps a mollusc bed, as it is largely formed bv 

 shells. The pinnacles are only a few feet high on the 

 outer side of the ridge, but towards the centre of the 

 island form a series of grotesque, upstanding pillars and 

 walls, varying up to 15 feet in height, and standing on 

 brown conglomerate rocl-c. 



Picard Island is mainly of the typical coral-rock struc- 

 tuie. To the south-west there is a plain of Platin 

 country, on the east side of which is a large basin in 

 subterranean connection with the lagoon. On the floor 

 of this basin I found several small linnps of dark brown 

 rock, apparently the same as on Esprit, and on its north 

 side there is a certain amount of the same conglomerate, 

 though the main portion of the rock is apparently 

 calcareous ; much of it is highly crystalline calcite. Some 



MjcheL,!, , 





3J. 



variations in level across the land are similar ; the highest 

 point is near the sea, and there is a steady decrease in 

 level to the lagoon. 



As regards the nature of the land, all the northern 

 portion of the atoll — Polymnie Island, Malabar Island, and 

 (he north-east part of IVIain Island — consists of Cham- 

 pignon. The south-east portion of Main Island is chiefly 

 Platin. In the centre of the south of Main Island there 

 is a wide shore zone, and then a belt of Champignon. To 

 the cast and west of this portion are large mounds (65 feet 

 in height) near the shore. They are obviously wind 

 dunes, the seaward slope being gradual with little vegeta- 

 tion, the landward very steep and covered with bush. 

 Opposite each dune the cliffs have almost entirely vanished, 

 a direct slope of sand leading up to the dune. It is 

 noticeable that in Aldabra, as in the other islands of this 

 part of the ocean, dunes are only formed on the coast 

 facing the strong south-east trades. The west portion of 

 Main Island is chiefly Champignon. At Couroupa there 

 is a dip in the rock which appears to extend from 

 the shore right across to the mangrove swamp ; it is 

 filled with sand, and attains a ma.ximum depth of 

 8 feet. 



There are two islands now left which are of peculiar 

 importance, i.e. Esprit and Picard. Round the south and 

 west of Esprit there is a ridge of rock about 30 feet high, 



NO. 2063, VOL. 80] 



extraordinary pieces of rock were fcund, apparently a 

 calcareous matri.x thinly enamelled with a transparent 

 brown substance. Bones, the teeth of sharks and rays, 

 and the remains of other organisms at present un- 

 identified, were numerous in the calcareous rock, and also 

 in a rock which appears to be conglomerate of a fine 

 texture. As at Esprit, there is also a considerable 

 quantity of the pinnacled shell rock. 



It is, of course, quite unnecessary to emphasise the 

 interest of the above formations. It appeared to me that 

 the whole must have been volcanic in origin, and it is 

 to be hoped that the examination of the rocks from Esprit 

 and Picard may furnish a clue as to the nature of the 

 base on which Aldabra is built. 



The four Passes into the lagoon are also interesting, 

 and perhaps give a clue to its formation. They have 

 usually deep central channels, with reefs on either side. 

 Small rock islets are present on these reefs, and it appears 

 certain from their existence that the Passes are steadily 

 increasing in size, and that their reefs are really the re- 

 mains of the land kept up to low-tide level by growing 

 coral. Live coral extends for some distance into the 

 lagoon, there being in all cases a luxuriant bed just inside 

 the Pass. At the mouth of the Pass all corals are largely 

 encrusted with Lithothamnia, and further seawards many 

 are completely killed by these algae. 



