xxv JASPER AND CHALCEDONY 355 



nodules of Ndranimako and Na Savu somewhat resemble the 

 jasper of the island ; but they are sharply distinguished by their 

 microscopic characters, by being easily scratched with a knife, and 

 by the large amount of water which they contain. Rolled stones, 

 which were found in the shallow stream-courses on the surface of 

 the Wainunu table-land 750 to 800 feet above the sea, exhibit in 

 the sections, in spite of the general silicification of the ground- 

 mass, the outlines of the original phenocrysts of felspar, and 

 abundant skeletal magnetite rods, such as would characterise a 

 semi-vitreous basic rock. It is evident that in the basaltic districts 

 of the Na Savu and Wainunu table-lands these concretions have 

 been formed under certain conditions by the decomposition of the 

 silicates of basic rocks. But these conditions do not exist now ; 

 and I infer that the silicified rocks, which occur only in fragments 

 on the surface, represent the silicification that occurred during the 

 emergence of the land ages since. 



Occasionally one comes upon in the mountain districts, as in 

 the vicinity of Ndrawa, large solitary blocks 2 to 4 feet across of 

 a whitish chert-like rock which has a hardness of 5 or 6, the harder 

 variety having a specific gravity of about 2*58 and the softer, 

 which yields a fair amount of water, a specific gravity of about 

 2*46. I noticed such solitary masses also on the Mbua plains. 

 The first-named locality is dacitic and the last basaltic. They 

 exhibit in the slides a patchy appearance, showing in some places 

 finely granular crypto-crystalline silica and in others a coarser 

 mosaic of chalcedonic quartz. Apart from the absence of any 

 definite coral structure, I can only surmise that they were originally 

 masses of reef-limestone. Their elevation even in the mountainous 

 districts was not over 400 or 500 feet above the sea. 



Fragments of jasper, which are associated with nodules of 

 chalcedony and silicified corals in the Lambasa plains, are also to 

 be found as pebbles and small blocks in the mountain streams of 

 the Ndrawa, Ndrandramea, and Lea districts, together with bits of 

 chalcedony and quartz-crystals. They do not occur, or are of rare 

 occurrence, in the recently emerged Kalikoso district and probably 

 belong to an earlier stage in the history of the island's emergence 

 from the sea. They have a hardness of 6 to 7, not being scratched 

 by a knife, and a specific gravity of 2-65 to 270 ; whilst but little 

 water is given off in the closed tube. They are a variety of 

 chalcedony, rendered opaque by the large quantity of red oxide of 

 iron that it contains, and are really, therefore, iron-flints. The 

 microscopical section in one case displays in the clear spaces a 



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