t) THE PHOSPHATE DEPOSIT OF OCEAN ISLAND. [vol. lxxix. 



(c) The translucent (subvitreous) variety of phos- 

 phate occurs sometimes in finely laminated masses, sometimes as- 

 a cementing ingredient, and often as an outside coating to 

 phosphatized coral or as a lining to cavities within the unphos- 

 phatized coral. . Its habitat, chemical composition, and structure 

 all point to its rapid deposition from solution. As proof thereof 

 may be cited the fact that this material occurs always in thin 

 lamina?, the composition of which often exhibits a marked differ- 

 ence from that of the contiguous material, and contains a much 

 smaller percentage of minor constituents than the other varieties. 

 Specimens of translucent phosphate have been found in which the 

 bubbles given off by the solution on evaporation have been 

 perfectly preserved : in one case, the whole of the surface of a 

 specimen consisted of the cast of unbroken bubbles, the escape of 

 which had been prevented by a skin of phosphate formed on the 

 surface of the solution. Under the microscope translucent phos- 

 phate is completely isotropic, and it is highly probable that the 

 substance is in a colloidal form. 



This translucent phosphate may be regarded as the ultimate 

 product of the solutions leached from the guano. At the point 

 where it is formed the mother-solution had no further action on 

 coral, but consisted principally of an ecpiilibrium mixture of 

 calcium, hydrogen, phosphoric acid, and carbon dioxide ions and 

 their corresponding salts. Loss of carbon dioxide caused a dis- 

 turbance of this equilibrmm, and tricalcium phosphate, together 

 with a small amount of calcium carbonate, was deposited, consti- 

 tuting the translucent form. 



Y. Distribution within the Phosphate Deposit. 



During my three 3 r ears' residence on Ocean Island, I made a 

 very extensive series of analyses from the soil, the natural surface 

 of the phosphate deposit, and the deposit as exposed in excavations 

 made either for the purpose of working or test. Many of the 

 samples were taken from areas completely outside the then 

 recognized phosphate ' fields,' and in the worked areas the deposit 

 was tested by means of samples, taken at short distances apart,, 

 from the surface of the deposit to its total depth. 



For the first few inches the deposit is black : this, in fact,, 

 constitutes the soil of the island. The tricalcium phosphate 

 content of this soil is often more than 20 per cent, lower than the 

 average for the area ; its calcium carbonate percentage is below 

 the average also, while its organic content is correspondingly high. 

 The total lime percentage is but slightly below the average for the 

 field, proving that the organic matter is present largely as calcium, 

 salts, the phosphoric acid having been released to act on the 

 underlying material. In the excavations the black soil merges 

 quickly (though imperceptibly) into the main mass of phosphate, 

 and, when this is reached, the tricalcium phosphate content 

 remains almost constant as one descends to a jDoint within a few 



