Q PROCEEDINGS OP THE INDIANAPOLIS MEETING. 



It was discovered early in the exploitation of the ground that the most solid lava 

 masses covered abundant phosphate. Captain Harding, the superintendent, received 

 permission to test this view by running a short tunnel from near his office, at the south 

 end of the island, beneath the most compact ledges known to him. Hardly a fathom 

 of excavation was required to develop abundant supplies of the phosphate. At the 

 inner end of this tunnel — say thirty feet long — the air moves from within outwards 

 against the prevailing trade-wind. The rocks are somewhat fragmental and the 

 rubble is coated with a white incrustation which proves to be a phosphate, probably 

 of alumina, as there is no lime present. This discovery led to the conclusion that the 

 abundant thin white coatings between the lava layers all through the island consist 

 of phosphate, instead of gypsum, as hitherto supposed. The phosphate was also dis- 

 covered by us at the extreme southern point of the island, at the water's edge, as well 

 as near the northern end. In fact, there is more or less of this mineral disseminated 

 through the volcanic mass everywhere upon the island except in the ash. 



The phosphatic mineral exists in several distinct compounds, which are readily 

 recognized. The " agate " and " crust" varieties carry about forty per cent, of phos- 

 phoric acid and twenty-four per cent, of water. Red varieties of different shades 

 carry twenty-seven and twenty-four per cent, of phosphoric acid, and the iron oxide 

 increases at the expense of the alumina. With the diminution of the acid there is 

 also a loss of water, ranging down to sixteen per cent. The lowest grade of all is a 

 reddish earth, full of grains or nodules of the redondite, carrying ten or twelve per 

 cent, of phosphoric acid. This may be compared to soil covering the ledges. 



Our researches have not been specially directed towards a study of the peculrarities 

 of the several grades of phosphate, but rather to their origin. The present chemical 

 combination is evidently not the original one. The existence of phosphatic nodules 

 suggests conditions similar to those required in the separation of limonite from in- 

 ferior grades of iron ore. It does not seem possible that these phosphates can have 

 been derived from the droppings of birds, which have saturated the rocks by infiltra- 

 tion ; nor is it easy to see how they can have been derived from phosphate of lime, 

 such as occurs at Sombrero, Navassa, Aruba, and many of the small islands of Central 

 America. Their hydrated character leads to the belief that they were originally an- 

 hydrous, receiving water from the saturation of the rock with rain.- If the mineral 

 was originally anhydrous lime phosphate, one might imagine the absorption of the 

 lime by sulphuric acid produced through oxidation of the volcanic sulphur to con- 

 stitute gypsum, which was leached out subsequently. J5ut the nearly complete absence 

 of lime renders this view of its disappearance problematical. 



Only one or two other similar occurrences of this redondite are known. May it not 

 be possible that there was a volcanic outburst through a bed of the lime phosphate, 

 and that in some way alumina and iron replaced the lime ? Is it possible that the 

 mineral was disseminated through the lava in a gaseous condition ? The scarcity of 

 this form of phosphate and its occurrence in small volcanic islands makes some such 

 view possible. 



There is still another possible view : There is one known phosphuret — Schreiber- 

 site — found in meteors. Originating beyond the earth's atmosphere in the absence of 

 oxygen, this combination is practicable; but if a similar compound were brought to 

 the surface by an igneous ejection it would soon become a hydrous phosphate. The 

 existence of iron not to be distinguished from meteoric masses is well known in the 

 Greenland basalts, and it is generally conceded that it has originated in the earth and 

 not from extra-terrestrial sources. Why may not this redondite have come up from 



