4 ME. L. OWEN ON THE [vol. lxxix, 



usually less than 50 feet. This covering is known to he lacking 

 only where a thin line of raised heach occurs at the extreme north 

 of the island ; hut at the exti*eme southern point the phosphate, 

 although there is little douht of its occurrence, is masked hy a 

 modern beach-formation. The topography of the coral basis of 

 the island is almost entirely hidden by the capping of phosphate j 

 but there is sufficient evidence available, particularly now that 

 part of the phosphate has been removed, to form some idea of the 

 shape of the underlying mass. This appears to consist of a 

 number of roughly circular platforms bounded towards the sea by 

 almost vertical cliffs, a section of the coral below the phosphate 

 displaying a series of steps. These platforms have been much 

 affected by subaerial erosion (prior to the deposition of the guano),, 

 and by the solutions leached from the guano ; but at least three of 

 them can be traced round the island, and it is found that thej dip. 

 at an angle of rather less than a third of a degree south-south- 

 eastwards. 



Outside these tilted platforms occurs an almost horizontal 

 platform covered with recent beach-material, and beyond this 

 again is the platform of marine erosion at sea-level to which 

 reference has already been made. At the present time this 

 platform is being extended inland by the sea. 1 



IV. The Capping op Phosphate. 



Mr. Danvers Power divides the capping of phosphate in Ocean 

 Island into two main types : ' alluvial ' phosphate and ' rock ' 

 phosphate. Both of these terms I consider misleading, since the 

 ' alluvial ' phosphate has no connexion with alluvial deposits as the 

 term is understood b} r geologists, and the word ' rock ' is used in 

 too limited a sense. I propose, therefore, to replace these terms 

 by using ' incoherent phosphate rock ' for what has been known as 

 ' alluvial ' phosphate, and ' coherent phosphate rock ' for ' rock " 

 phosphate. 



Considering the Ocean Island deposit as a whole, rather more 

 than half is made up of incoherent phosphate, and rather less 

 than half of coherent phosphate. 



Incoherent phosphate consists of 'pebbles,' sometimes sub- 

 angular and measuring 2 inches or more in diameter, together with 

 smaller ' fragments ' and pisolitic and oolitic grains grading down 

 to dust. When wet it forms, in most grades, a clay-like material 

 in which the larger ' pebbles ' are embedded. In colour it varies, 

 according to the percentage of contained tricalcium phosphate, 

 from leather-brown in the lower grades to pale buff in the highest 



1 That the reef at present surrounding' the island is a plane of marine 

 denudation, cut out of an older reef, is shown by the fact that stumps of 

 denuded pinnacles can be observed on the present surface. To-day this 

 marine plane is being- extended inland, the honeycombed coral of the land- 

 mass as it is encroached upon by the sea being filled in with calcium carbonate, 

 both by simple precipitation and by the agency of algEe. 



