XXV SILICIFIED CORALS 



Ititf c^t the SePt f a ' ° ten S "'" Ca ' Citic - show ' he Points of the 



hde the massive specimens appear to be entirely of cha eZc 



<hca, the outhnes of the cells and of the septa bring ind cate^ I bv 



ferrug,nous material. In a specimen of Pontes by° myside th e 



te find 3 h " ', Si "' Ca haS adVanCed >« *e cha, edoni 

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



2 to ; 4 mm. ,n stze, often irregular, but sometimes formin/doublv 

 terminated prisms. This has produced a somewhat c°rurnblinC 

 rock, which is easily powdered by the finger; and n thTcase 



herefore, the complete crystallization of the s ilea is result n" in 

 the disintegration of the silicified coral. resmtin s in 



The ordinary silicified massive corals of Vanua Levu, where the 

 replacement by chalcedonic silica is complete, though the structure 

 .preserved have a hardness of about 6 and a specific grav ty of 

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



weTnd' *l\: ? 3raWanga P ' ainS and fn the Lekut » '-viand 7 ; 

 we find silicified fragments of branching corals which are easily 



gr^tvtf T" Vw and haTC a hardnesS of 3 to 4 and a sped c 



f„„\ y >L 3 ' The , frac t ured '"rface is milk-white or reddish, and 

 ooks l, ke s , When powdered and heated ^ a ^ 



tube, the material loses one fourth or one fifth of its wei-ht 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 portion of 

 nulhpore-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, havino- 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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