Geology. 271 



last of the eruptions was accompanied by the formation of thick 

 beds of volcanic ash (palagonite tuffs), and it is upon these that the 

 great mass of the Miocene (Orbitoidal) limestones rests. 



The later deposits which have accumulated on the basis thus 

 constituted are very difficult to describe. They may be said to 

 consist of reef-limestones and detrital limestones formed on the 

 submarine slopes of the island, but since after each movement 

 of elevation fresh reefs seem to have grown round more or less 

 of the new shore-lines, the whole structure is very complex. 

 Another source of difficulty is the almost complete obliteration of 

 all organisms in many of the limestones, chiefly in consequence 

 of dolomitization. 



Perhaps the most remarkable of the rocks of Christmas Island 

 are the thick deposits of nearly pure phosphate of lime which cap 

 several of the higher hills. This substance is probably derived 

 from ancient ( ? Pliocene) guano beds formed on the low islets 

 which existed before the first elevation of the island, and is 

 the insoluble residue of beds of limestone altered by this guano. 

 On MuiTay Hill there is a bed of rock, consisting largely of phos- 

 phates of alumina and iron, which has perhaps been produced by 

 the alteration of a bed of volcanic ash by an overlying mass of 

 guano, since completely disappeared. 



It will be convenient to describe the geology of the island under 

 the following heads : — 



1. The central nucleus. 



2. The plateau and hills. 



3. The inland cliffs. 



4. The sea cHff. 



At the same time it will be impossible to keep these sections 

 entirely distinct from one another, e.g. in cases where the inland 

 cliffs have been cut back into the central nucleus and consist of 

 tertiary rocks. 



TJie Central Nucleus. — By far the most important exposure of 

 the central core of the island is at Flying Fish Cove (see Map). 

 In this locality, behind the platform upon which the settlement 

 is situated, a steep slope rises to a height of from 400 to 500 feet. 

 In its northern portion it consists of a succession of cliffs separated 

 by steep talus slopes, while towards the southern end there is 

 a talus slope up to about 250 feet, and above this a vertical or 

 even overhanging cliff of from 200 to 250 feet. At the northern 

 end of the cove the cliffs run nearly ]^. to S., but as they are 

 followed southward their direction changes first J^T.N.E.-S.S.W., 

 then :N'.E. to S.W., E.N.E. to W.S.W., and finally S.E.-KW. 

 (see Eigs. 2 and 6). 



The general structure of the cliff is shown on the sections in 

 Eigs. 2 and 5 (pp. 272 and 276). The oldest limestone visible 

 (A in sections) is exposed at the southern end of the cove, at the 



