Central Plateau and Hills. 13 



covered mth a bed of blocks of phosphatic rock, which in a few- 

 places can be seen to rest on a highly dolomitized limestone, 

 showing few or no traces of organic structure. The whole of the 

 slopes and terraces below this hill are thickly strewn with nodules 

 of the phospliate, and fragments of this substance are found enclosed 

 in the more recent limestones forming the lowest cliffs. 



Another form assumed by the raised rim of the plateau occurs on 

 the northern side (Fig. 4 C). Here it forms a flat surface 200 or 

 300 yards broad, on the inner side of which there is a sudden 

 drop of 10-15 feet to the plateau, forming a low cliff of cavernous 

 dolomitic limestone, which can be traced some distance. On the 

 outer side is a steep slope, covered with talus, leading down to the 

 level upper terrace. The flat surface is similar to the reef flat of 

 an atoll, the inner cliff to the sudden drop of two or three fathoms 

 which often occurs on the lagoon side of the reef flat, and the 

 talus slope represents the reef slope on the seaward side. Much, 

 liowever, remains to be done in the examination of the limestones 

 before it can be definitely decided whether this interpretation of 

 the physical features described is the correct one. 



Here and there on the surface of the plateau, particularly on the 

 northern portion, there are groups of limestone pinnacles of fan- 

 tastic shape, often 15 feet or more in height; these do not usually 

 show many traces of coral structure, but are largely composed of 

 fragments of branching Litliothamnion, Halimcda, and many pieces 

 of Carpenteria, all evidences of shallow-water conditions. In other 

 places in the northern angle of the plateau, just beneath the soil, 

 and forming slight ridges, is a chalk -like rock, showing, as a rule, 

 no trace of organic structure, but in the centre of the larger blocks 

 there is sometimes a hard unaltered core, which contains delicate 

 shelled foraminifera and some Lithothamnion. This rock was most 

 probably a chidky mud deposited in a quiet corner of the lagoon. 

 Another characteristic feature of this northern region is the 

 occurrence of rounded hills, often less than 50 feet high : their 

 lower portion is a soil-clad slope, and the flat-topped summit is 

 composed of much fissured limestone, forming lines by pinnacles 

 separated by deep fissures and channels. The whole is covered 

 with tliick forest. 



Fartlier south there are two or three ridges running in an 

 approximately cast-and-west direction and having their longest 

 slope to the south, so that the plateau descends in that direction 

 to about 500 feet. It seems not impossible that these ridges may, 

 in part at least, represent the uppermost cliff of the northern 

 and eastei'n sides, and that when the sea stood at about this level 

 the coast, or more probably the submerged reef of the southern side 

 of the island, formed a wide bay opening towards the south-west. 

 The southern slopes of these ridges are largely formed by bare 

 limestone, often full of coral, and here and there, on the level 

 also, there are considerable areas covered with reefs split up into 

 pinnacles of rock by winding channels and deep fissui'es. 



