part 1] PHOSPHATE DEPOSIT OF OCEAN ISLAND. 5 



grade, the former consisting very largely of pisolitie and oolitic 

 grains, and the latter being more clay-like. 



The tine material of the incoherent phosphate is derived partly 

 from the insoluble residue of the original guano, and partly from 

 the product of the rapid interaction of the solutions leached from 

 the guano with the directly underlying coral-sand. The coarser 

 material is the product of the slower action of replacement of 

 •calcium carbonate by tricalcium phosphate. This is accomplished 

 by solutions, derived from the guano, which have lost their power 

 ■of violently attacking coral by passage through the upper layers of 

 the coral-sand. 



The internal structure of the ' pebbles,' to which reference has 

 been made above, proves them to be concretions. The}' are built 

 Tip, like agate, of a series of concentric lamina?, the outermost one 

 being the surface of the 'pebble,' and they imitate exactly the 

 limestone concretions from which they are immediately derived, 

 except for a slight difference in colour and for the fact that their 

 main constituent is tricalcium phosphate. 



Coherent phosphate is, in the main, almost identical in 

 chemical composition with the incoherent phosphate. When any 

 ai - ea is considered, however, the coherent phosphate is found to 

 contain slightly less phosphate than the surrounding incoherent 

 material. The coherent phosphate may be divided into three 

 classes: — (a) f ragmental ; (&) phosphatized coral in situ; and 

 >(c) subvitreous phosphate. 



(a) The fragmental phosphate may be coarse or fine in 

 grain, grading imperceptibly into the incoherent variety. It 

 occurs distributed in an irregular manner throughout the beds of 

 incoherent phosphate. In external features and internal structure 

 it imitates the various detrital limestones which are formed on a 

 coral reef. Where it is laminated, the laminae are but rarely 

 horizontal ; and every fact that I have observed in regard to this 

 phosphate tends to prove its direct derivation from detrital coral- 

 rock, or in rare cases by the secondary cementation of incoherent 

 phosphate, the cementing material being, in this case, either sub- 

 vitreous phosphate or phosphate precipitated from solution in a 

 state of fine division. It may be stated, indeed, that the morpho- 

 logical characters of the phosphates generally are almost entirely 

 derived from those of the pre-existing coral reef, and that all the 

 typical forms in which reef -limestones occur find their counterpart 

 in the phosphate deposit. 



(5) Phosphatized coral in situ. — The coral platform, upon 

 which the other types of phosphate deposit rest, consists of closely- 

 packed masses of pinnacles (Karrenfeld), often 30 to 50 feet high. 

 There is little doubt that these pinnacles owe their form, primarily, 

 to subaerial denudation which occurred before the phosphate was 

 deposited. The solutions leached from the guano have, however, 

 considerably modified their form, rounding their outlines and, in 

 some cases, converting them partly or wholly into a compact mass 

 of phosphate. 



