354 A NATURALIST IN THE PACIFIC chap. 



are of all sizes up to 3 or 4 inches across. Some of them are 

 hollow and lined with clear quartz-crystals, whilst with others the 

 cavity may be completely filled by interlocking quartz-crystals. 

 The outer surface of one of the agates displays markings showing 

 in relief casts of the " cups " of a minute-celled coral. 



Mingled with the other siliceous materials on the surface of the 

 Kalikoso and Lambasa plains are found fragments of a whitish 

 quartz-rock, having a specific gravity of 2'53— 2'57, being therefore 

 markedly lighter than quartzite (2'63 — 2-67) which it somewhat 

 resembles. It usually occurs as small hand-specimens ; but in the 

 vicinity of Mbati-ni-kama I found blocks, 12 to 15 inches across, 

 lying in the river-bed. Under the microscope it displays a fine 

 radio-globular aggregate of chalcedonic quartz. 



Mention has already been made of the siliceous concretions, 

 composed mainly of hydrous crypto-crystalline silica, which are 

 associated with the silicified coral fragments formed of the same 

 kind of silica on the surface of the plains of Mbua, Lekutu, and 

 Sarawanga. They also occur in the Ndranimako lowlands on the 

 right side of the Yanawai estuary, and in the more elevated inland 

 districts of the Wainunu and Na Savu table-lands at elevations of 

 650 to 770 feet above the sea. They take the form of irregular 

 nodules, or of fliat uneven " cakes," usually two or three inches in 

 size. They are as a rule reddish, but sometimes pink and white. 

 Their hardness is only 3 to 4, and they are easily scratched with a 

 knife ; and when powdered and heated in a closed tube, they lose 

 about one fourth of their weight of water. Under the microscope 

 they exhibit a grey crypto-crystalline groundmass showing very 

 finely granular crystalline silica with the cracks and small cavities 

 filled with more brightly polarising chalcedonic quartz. But they 

 differ as regards their other components and also in their mode of 

 occurrence ; and it is highly probable that the history of their 

 origin is not always the same. 



Those associated with the silicified corals on the Sarawanga 

 and Lekutu lowlands show no structure in the slide that gives me 

 a clue as to their origin ; but they may perhaps represent old 

 Nullipore nodules. Those around Ndranimako are coloured deep 

 red ; and whilst some give no indication as to their source, others 

 are transitional in character, and display in the sections traces of 

 the vacuolar semi-vitreous basic rock of which the original frag- 

 ment was composed. The same red siliceous concretions form the 

 pebbles and gravel in the stream-beds on the surface of the Na 

 Savu table-land, 700 feet above the sea. These red flint-like 



