Geology, 291 



1 per cent.). Under the microscope it can be seen that the rock, 

 "which has been phosphatized, was a fragmental one ; obscure traces 

 of contained organisms (foraminifera) may be observed occasionally, 

 and in some specimens fragments of bone occur. There are numerous 

 irregular cracks and cavities, which are usually lined with nearly 

 transparent phosphate of lime, showing a beautifully banded agate- 

 like structure. The colour of the rock is as a rule a brownish 

 white, but some specimens are of a darker tint. In some cases the 

 rock is found to consist of fragments of dark and light coloured 

 phosphatized rock, embedded in a cement of transparent yellowish 

 phosphate of lime, apparently deposited from solution in concentric 

 layers round the fragments. Under crossed nicols this banded 

 substance is found to be doubly refractive, polarizes feebly in 

 tints of grey and shows traces of the black cross, the banded 

 structure being rendered very prominent. In many respects this 

 phosphatic rock closely resembles in its structure some of the 

 phosphatic nodules from deep water described by Murray & Eenard 

 in the '^ Challenger" Report on Deep-sea Deposits (p. 391 et seq.). 

 These nodules are described as having been formed as segregations 

 of phosphate of lime replacing carbonate in certain marine deposits. 

 This mode of origin may perhaps account for the presence of some 

 of the small nodules found scattered over the island, but cannot be 

 responsible for the formation of the great masses occurring at 

 Phosphate Hill and elsewhere. The average composition of the 

 phosphate is about 39 per cent, phosphoric acid ; 51*5 per cent. 

 lime; 3*5 per cent, carbonic acid; 2 percent, iron and alumina; 

 the remaining 4 per cent, being made of magnesia, water, fluorine, 

 and other substances ; there is only about '2 per cent, of silica. 

 The brown soil in the neighbourhood of these deposits also contains 

 a considerable amount of phosphoric acid, often amounting to more 

 than 30 per cent. 



On Murray Hill is a bed of .a remarkable rock which seems to 

 have been produced by the phosphatization of a volcanic rock, 

 probably a bed of tuif. It consists of small brown spherules of 

 phosphatic matter cemented by doubly refractive phosphate of 

 •(?) lime : under the lens it presents somewhat the appearance of an 

 oolite. It contains 39 per cent, of phosphoric acid, only 2*5 per cent, 

 of lime, 32'5 per cent, of iron and alumina, nearly 5 per cent, of 

 silica, the remainder being almost all water, either free or combined. 



The rock upon which the phosphate beds rest is, in most places, 

 a dolomitic limestone, which, in the specimens examined, curiously 

 enough seems to contain no trace of phosphate of lime. Descriptions 

 and analyses of some of these dolomitic rocks from Phosphate Hill 

 (Nos. 800, 804, 811) are given on p. 267. 



The Upper Cliffs and Tereaces. 



The rocks composing the upper inland clifFs have not been fully 

 examined, but enough has been done to show that in different 

 places rocks of very different characters occur. These upper cliffs 



