^96 Christmas Island. 



of more than 900 fathoms, and this sand is no doubt derived from 

 the wear of the cliffs of this part of the island, which is exposed to 

 the full force of the swell of the Southern Ocean. Round the rest 

 of the coast fragments of volcanic rocks and pieces of manganese 

 dioxide are recorded from various depths up to 1,100 fathoms : two 

 soundings of 385 and 925 fathoms respectively brought up calcareous 

 algoe. Between the island and Java lies a long narrow trough, 

 which is one of the abysses of the Indian Ocean, being upwards of 

 3,000 fathoms deep in places. Its long axis lies parallel to the 

 south coast of Java, the submarine slopes of which appear to be 

 formed by a great fault and are very steep, the 2,000-fathom line 

 being only a few miles from the land. 



Forming the flat summit of the Christmas Island peak we 

 meet with a succession of limestones ranging from the Eocene 

 (or Oligocene) up to recent reef deposits, and accompanying 

 the older Tertiary deposits are various volcanic rocks, most 

 important of which are basalts and trachytes lying beneath the 

 Eocene (or Oligocene) limestone, while above it are basalts and 

 basic tuffs separating it from the Miocene Orbitoidal limestone 

 which seems to make up the great mass of the island. The total 

 thickness of these older Tertiary and the interstratified volcanic 

 rocks is, as far as can be ascertained, about 600 feet, but it is 

 probable, as above stated, that some of the rocks exposed on the 

 plateau may be Miocene, in which case the series is considerably 

 thicker. The occurrence of such a series of Tertiary deposits on 

 an oceanic island is, I believe, unknown elsewhere, although 

 Wallace mentions that Upper Miocene deposits occur in the Azores. 

 Another point of importance in the case of these Tertiary rocks 

 is that they, especially the Miocene Orbitoidal limestones, end 

 abruptly on the coast in vertical cliffs sometimes 250 feet high, 

 so that it is clear that the area which they originally covered must 

 have been much larger than the present island, and that it has 

 been cut down to its present dimensions by repeated faulting 

 and slipping down of its peripheral region. All these limestones 

 must have been deposited in shallow water, probably less than 

 100 fathoms deep. At present the Eocene limestones are found 

 up to about 250-300 feet, the Miocene Orbitoidal limestones up to 

 about 550 feet, while the summit rises 1,200 feet above the sea. 

 If we suppose the Eocene limestones to have been deposited in 

 100 fathoms (and it was probably much less), the range of the 

 oscillation with reference to the sea-level which the island can be 

 proved to have undergone is between 200 and 300 fathoms, 

 a small proportion of the total depth to the ocean floor. 

 Speaking generally, this oscillation appears to have consisted, 

 first, of a gradual depression, allowing of the accumulation of 

 the Miocene Orbitoidal limestones and those composing the higher 

 land, then a period of rest, followed by a succession of movements 

 of elevation (or better, negative movements of the shore-line), 

 which have given rise to the terraced structure of the island and 



