406 PERCY SLADEN TRUST EXPEDITION 



It is felt that this rock satisfactorily determines a most important point in the history 

 of the atoll. 



The second class of rock is that which was described as "platin." It also is obviously 

 composed of corals to a large extent, but differs from the preceding class of rock in that the 

 component corals are much broken and triturated, and are seldom if ever* in the position 

 of growth. With the coral fragments are pieces of lithothamnia, numerous mollusc shells, 

 echinoderm spines and pockets of foraminifera, among which Orbitolites is prominent. 

 After weathering the surface of the rock becomes flat, and pavement-like slabs break away, 

 leaving again a flat surface underneath. By means of the pits and wells it was possible 

 to make observations at some distance from the surface (6 — 8 feet), but the components of 

 the rock seemed to be similar, though in consistency they differed somewhat in being softer 

 and more chalky. It is suggested that the pavement-like surface has been produced by suc- 

 cessive solutions and re-depositions of calcium carbonate by rain-water. (Plate 25, fig. 2.) 



The formation of this rock is to some degree a matter of speculation, but to me 

 it appears to be strictly comparable with that of the rock found on the inner portions 

 of a reef fringing an atoll, which consists in the consolidation of broken fragments washed 

 from the growing edge. The first class of rock shows that Aldabra once lay beneath 

 the sea and was fringed by a flourishing reef; the currents sweeping over it must have 

 carried large quantities of debris and waste material away from the living corals. Now it 

 is known that the latter are very intolerant of such substances and therefore towards 

 the interior of the reef conditions were unsuitable for them. Consequently the inner 

 portions of the reef must have been built up by debris rather than by live corals. 



At present the debris rock or " platin " is found mainly in the east of the atoll, 

 but, as the next class of rock has been much metamorphosed, it is uncertain what its 

 former distribution may have been. Assuming that when Aldabra was under the sea 

 the currents flowed in the same direction as at present (mainly from east to west), it 

 is probable that the east of the reef inside the growing edge was then the most barren, 

 while further west patches of live reef would have existed, becoming more numerous up to 

 the growing edge in the west. 



The third class of rock, the " champignon," is a metamorphosed rock, which must 

 be regarded as a secondary derivative of coral-limestone. It is somewhat crystalline, and 

 is always marked by the possession of inclusions of calcium phosphate, the pure calcium 

 carbonate being white, while the inclusions show on a freshly fractxired surface as sharply 

 defined brown patches. (Plate 25, fig. 1.) 



Metamorphosis is of course frequent in coral rock, but it must be regarded as peculiar 

 that the metamorphosed coral limestones in Aldabra and the neighbouring islands always 

 contain inclusions of calcium phosphate. The origin of the phosphate is certainly to 

 be found in the deposit of guano, portions of which were washed into spaces in the rock 

 and included during metamorphosis. There appears to be some connection between the 

 metamorphosis and the presence of the phosphate. It is hoped that Mr Hughes may throw 

 further light on the question. 



The distribution of this rock is very wide, for it is found over the greater part of 



* One large colony of Cceloria was noticed in position of growth near Takamaka. 



