Geology. 295 



will be seen (Pig. 2 A, p. 272) that the basalt marked A, which runs 

 down beneath the sea-level, has its upper surface covered with an 

 old talus consisting of blocks derived from the cliffs of Miocene 

 limestone behind, lying in a matrix of smaller limestone fragments 

 mingled with the debris of the basalts and palagonite tuffs which 

 occur in the neighbourhood, and often including the shells of 

 foraminifera. A similar talus forms the foot of the cliff (see 

 Pig. 2 A, above the letters S.E.-N.W.), where, being very hard, 

 it forms a projecting ledge which is some 9 or 10 feet high. 

 Upon it is about 50 feet of very fresh-looking coral limestone, in 

 which the individual coral stocks, in the position of growth, may 

 often be distinguished. One mass of Porites is between 20 and 

 30 feet high and nearly as broad (see Pig. 4, p. 274). A fine section 

 of the sea cliff is exposed where it is cut through at the mouth of 

 Sidney's Dale (Fig. 7, p. 284). Here also it is seen to consist very 

 largely of recent species of corals, some of which, both from this 

 and other localities, have been noticed by Dr. J. W. Gregory 

 (pp. 206-225). 



If the terrace at the top of the sea cliff be examined it will be 

 found in many places to consist of two parts, an outer broader 

 zone, which is the summit of the raised reef, and wherever the 

 rock is free from soil and vegetation can be seen to consist mainly 

 of fresh-looking corals, and an inner, much narrower zone, running 

 along the foot of the inland cliff and consisting of the same material, 

 being, in fact, the base of the portion which has been cut back by 

 the waves. 



The present fringing reef forms a narrow shelf round the island, 

 being only interrupted where deep water occurs close to the foot 

 of the sea cliff ; and, if a further negative movement of some fifty 

 feet were to take place, it would form a cliff and terrace much 

 like that just described. 



SUMMAEY AND CONCLUSION'S. 



From the foregoing account it will be seen that Christmas Island 

 presents some important peculiarities which differentiate it from 

 other oceanic islands, and are difficult to explain. The island is, 

 in fact, the flat summit of a submarine mountain more than 15,000 

 feet high, the depth of the platform from which it rises being about 

 14,400 feet, and its height above the sea being upwards of 

 1,000 feet. The submarine slopes are steep, for depths of 1,100 

 fathoms occur less than four (in one case less than three) miles, and 

 the foot of the mountain (about 2,400 fathoms) within twenty 

 miles of the coast of the island. As far as the soundings go, they 

 show that the slopes of the upper part of the peak are nearly alike 

 on all sides, being about 2 in 5. Soundings made in the neighbour- 

 hood of the island at depths of more than 1,000 fathoms usually 

 show the bottom to be covered with Glohigerina ooze ; off the 

 south-east corner of the island coral-sand was met with in depths 



