XXV SILICIFIED CORALS 353 



distinct, whilst the septa, often still calcitic, show the points of the 

 calcite crystals projecting into the chalcedony which forms the 

 mass. Lime however rarely occurs in the silicified corals of Vanua 

 Levu. It was only in the case of one or two localities that the 

 corals displayed any effervescence with an acid. In the microscope 

 slide the massive specimens appear to be entirely of chalcedonic 

 silica, the outlines of the cells and of the septa being indicated by 

 ferruginous material. In a specimen of Porites by my side the 

 crystallization of the silica has advanced beyond the chalcedonic 

 stage and the coral is composed entirely of minute quartz-crystals, 

 •2 to "4 mm. in size, often irregular, but sometimes forming doubly- 

 terminated prisms. This has produced a somewhat crumbling 

 rock, which is easily powdered by the finger ; and in this case, 

 therefore, the complete crystallization of the silica is resulting in 

 the disintegration of the silicified coral. 



The ordinary silicified massive corals of Vanua Levu, where the 

 replacement by chalcedonic silica is complete, though the structure 

 is preserved, have a hardness of about 6 and a specific gravity of 

 2*54, and yield but little water in the closed tube. Occasionally,, 

 however, as in the Sarawanga plains and in the Lekutu lowlands 

 we find silicified fragments of branching corals which are easily 

 scratched with a knife and have a hardness of 3 to 4 and a specific 

 gravity of 2*3. The fractured surface is milk-white or reddish, and 

 looks like semi-opal. When powdered and heated in a closed 

 tube, the material loses one fourth or one fifth of its weight of 

 water, the finest dust (passing away in the steam) being deposited 

 on the sides of the glass. In the slide there is displayed a finely 

 granular crypto -crystalline structure with in places a somewhat 

 coarser quartz-mosaic, whilst chalcedonic quartz fills minute cracks 

 in the mass. No coral structure is preserved. Numerous points 

 coloured by iron oxide occur in the section, and minute dust-like 

 inclusions abound, which are doubtless water-pores. I have des- 

 cribed on a later page certain concretions found associated with 

 these silicified corals which though formed of the same crypto- 

 crystalline hydrous silica, are apparently silicified portions of 

 nullipore-rocks. 



The fragments of flint that occur commonly on the surface in 

 these districts are, as above remarked, derived from the hard 

 silicified coral-masses. Nodules of chalcedony, having all the 

 appearance of having originated in cavities, are also very frequent. 

 They may take the mamillary, agate, or onyx form, some of the 

 agates when polished making beautiful specimens. These nodules 



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