14 MB. L. OWEN ON THE [vol. lxxix, 



(2) The guano was deposited on a slowly rising reef, both 

 •deposition and emergence appearing to have been regular and 

 without sensible break. 



(3) The deposit, as it exists at the present day, consists of the 

 insoluble parts of the original guano, together with detrital and 

 oolitic limestones and the directly underlying coral-rock, all of 

 which have been changed, by the metasomatic action of solutions 

 leached from the guano, into a rock composed mainly of tricalcium 

 phosphate and containing small percentages of calcium carbonate, 

 ■calcium fluoride, and silica. This rock still bears the characteristic 

 i'orm of the original limestone. 



(4) Subsequently to the deposition and leaching of the guano 

 -and the formation of the phosphate in its present form, the island 

 .has been tilted about a west-south-west and east-north-east axis 

 towards the south-south-east, the axis being about 1/4 mile north 



■ of the southernmost point of the island (Sydney Point). This 

 tilting occurred in comparatively recent times. After the tilting 



„a slight elevation of the island, to the extent of about a couple of 

 feet, is thought to have occurred, and this movement appears to be 

 continuing at the present time. 



(5) The studj r of the variation of the percentage of tricalcium 

 .phosphate throughout the deposit has brought to light its remark - 

 .able regularity, which may be represented by a series of iso- 



phosphatic planes. The direction of these planes confirms the 

 .gentle tilting suggested by geological evidence which, by itself, 

 might have been overlooked or considered unsatisfactory. It is 

 possible that a study, on the lines here suggested, of the other 



■ completely leached phosphate deposits occurring in the Pacific 

 may help to elucidate the post-Tertiary movements of the Pacific 

 floor ; and it is with the object of stimulating such investigation 

 .that this paper has been written. 



The work on which this paper is founded was done prior to the 

 end of 1914, but up to the present I have had no opportunity of 

 publishing the results achieved, first owing to the war, and secondly 

 • owing to absence abroad. 



My sincere thanks are due to the Pacific Phosphate Company 

 .for permission to use results obtained while in their employ ; to 

 Prof. W. W. Watts, to whose untiring aid and helpful criticism 

 the paper owes its present form, as also to other members of the 

 staff of the Geological Department of the Imperial College of 

 Science & Technology for much advice and help. The arduous 

 work of preparing the paper for publication and of seeing it 

 through the press has devolved on Mr. Gr. S. Sweeting, F.G.S., 

 •to whom my best thanks are due. 



Discussion. 



Mr. J. F. N. Green referred to the interest of the differential 

 -movement, proved, he thought, for the first time for one of the 



