288 Christmas Island. 



considerable thickness, it seems almost certain tliat this is the case. 

 Cliffs composed of these Orbitoidal limestones are easily dis- 

 tinguishable from those formed of later reef-limestones, by their 

 flat smooth faces and the straightness of all their lines, which is the 

 consequence of the system of jointing which splits the mass up 

 into roughly cubical blocks. The magnificent range of cliffs 

 (first inland) which forms the fine headland, Egeria Point, are no 

 doubt of this age, and so perhaps are those of South Point. 



On the plateau and higher cliffs no Orbitoidal limestones at all 

 occur, but according to Dr. Gregory some of the corals (e g. 

 Orhicella herklotsi) collected from the reefs on the central plateau 

 are of Miocene type. If this is so it indicates that in these 

 localities the basis of older rocks has been exposed by denudation ; 

 but since no rocks containing Orhitoides were seen and, on the other 

 hand, traces of volcanic rocks were found, it seems probable that the 

 Orbitoidal limestones never completely covered the volcanic basis, 

 for had they done so traces of them should remain on the plateau. 



The Eocks of the Centeal Plateau and Hills. 



The general characters of the central plateau and of the hills 

 which occur on its borders are described above in the introductoiy 

 section (pp. 11-13). As already mentioned, some of the limestones 

 exposed in the interior contain corals of Miocene type (see p. 208), 

 and therefore most probably form part of the central nucleus 

 which has been exposed by the very extensive denudation that 

 the island must have undergone. Other reasons for supposing 

 that this may be the case are, that in the middle of the island 

 occasional pebbles of volcanic rock may be found, and that, as 

 mentioned on p. 18, the great depth of the reddish-brown soil 

 covering much of the central region has certainly been derived from 

 the decomposition of volcanic rock, at least in great part. It is 

 unfortunate that the Miocene age of some of the rocks of the plateau 

 could not be determined by me while on the island, since a careful 

 examination of their relations, and fuller collections of their fossils, 

 might have thrown much light on the age of the highest limestones 

 and on the date of the first elevation of the island above the sea. 

 This event was no doubt post-Miocene, but that it was not long 

 subsequent to that period seems to be rendered probable by the 

 fact that since it took place a series of negative movements of the 

 shore-line has occurred, a succession of inland cliffs has been cut 

 back, and enormous masses of calcareous rock, both reef-limestones 

 and more especially detrital limestones which cover much of the 

 fianks of the island, have been formed. 



The latest of the limestones covering the highest parts of the 

 island and forming, in my opinion, an atoll reef and island, have 

 been either to a large extent removed by denudation or have 

 undergone groat alteration, which has led to the destruction 

 of the contained organisms. The rocks composing the summit 



