1136 Geological Society. 



Devon in late Miocene times, before early Pliocene submergence 

 set in. The gradual ' swamping ' of sediment-bearing rivers by 

 the advancing Pliocene sea from the south-west is correlated with 

 certain physical features apparent, especially the ' 400-foot 

 plateau.' 



2. 'The Phosphate Deposit of Ocean Island.' By Launcelot 

 Owen, A.E.S.M., A.E.C.S., F.G.S. 



Ocean Island (lat. 0° 52' S. ; long. 169° 32' E.), in the Western 

 Pacific Ocean, consists of a mass of terraced and dolomitized 

 coral-limestone which rises to a height of 300 feet above low water, 

 spring tide. 



Its surface is almost completely covered by a capping of calcium 

 phosphate of exceptional purity. This phosphate can be divided 

 into three varieties: — (a) amorphous calcium phosphate, formed 

 of the insoluble residue of the original guano ; (b) detrital coral- 

 limestone, converted into calcium phosphate by solutions leached 

 from the guano ; and (c) phosphatized coral in situ. 



The dolomitized limestone base shows evidence of having 

 suffered considerable subaerial and marine erosion prior to the 

 deposition of the guano, this deposition having occurred during the 

 final emergence of the island. 



A detailed study of the composition of the deposit shows that 

 the percentage of tricalcium phosphate (Ca 3 P 2 8 ) at any point 

 varies in a remarkably regular manner, according to the position 

 of the point on the island. So regular is this variation, in fact, 

 that, if the position and height above sea-level of any point are 

 known, the percentage of tricalcium phosphate in the deposit at 

 that point can be foretold with considerable accuracy. 



The trend of this variation suggests that: — (a) the original 

 guano was deposited on the coral base while a slow negative move- 

 ment of the strand-line was in progress, and no sensible break 

 occurred either in the deposition of the guano, or in the movement 

 of emergence ; (&) subsequent to the deposition of the guano and 

 its conversion into phosphate, the island was tilted at about a third 

 of a degree south-south-eatwards. This supposition is strengthened 

 by the occurrence of raised beaches in the north of the island, and 

 of phosphate in situ below sea-level on the south. 



A study of similar deposits may help to throw some light on the 

 post-Tertiary movements of the floor of the Pacific Ocean. 



