222 A NATURALIST IN THE PACIFIC chap. 



deep-water origin and appears to have been formed from the fine 

 washings of a coast some distance away. 



It is noteworthy that the surface in this locality is in places 

 strewn with quantities of angular fragments and round pebbles of 

 chalcedonic flint, together with large nodules which when broken 

 across are found to be occupied by radiating crystals of quartz and 

 were doubtless formed in the cavities of some rock. Reference 

 has already been made to the partially silicified coral fragments 

 lying on the surface of the low coast hills a few miles to the west 

 of this locality. They indicate a relatively recent emergence and 

 we get the same indication from the flints on the surface of this 

 district. These evidences, however, relate only to the last stage of 

 the emergence. The testimony of the silicified " Globigerina" clay 

 carries us back to the earlier periods of these changes of level, and 

 probably dates back to a time when the greater part of this portion 

 of the island was submerged, with the exception of the mountain 

 peaks. Not the least interesting feature of the emergence is the 

 silicifying process that accompanied it. This is illustrated on a 

 much greater scale in the neighbouring inland plains of Kalikoso 

 and Wainikoro which are described in Chapter XXV. 



Proceeding eastward from Visongo to Namukalau one traverses 

 a coast district not elevated more than 300 feet above the sea. 

 Here there are displayed whitish and pale yellow compacted tuffs 

 differing in aspect from those prevailing to the westward and often 

 bedded, the dip being about 20 N.W. or N.N.W. They show no 

 lime and apparently inclose no organic remains. Where the upper 

 surface of a bed is bared, it shows regular shrinkage-lines inclosing 

 hexagonal spaces 2 to 3 inches across ; but there is no correspond- 

 ing columnar structure in the bed-mass. The rock is very light in 

 weight and homogeneous in texture and looks a little like China- 

 clay. In a section its structure appears obscure ; but it seems to 

 be formed of the finest detritus, derived from some acid partly 

 devitrified glass, the pumiceous structure being in places faintly 

 indicated ; but the whole mass appears to have been subjected to a 

 process of alteration perhaps similar to the ultimate palagonitic 

 change in basic rocks. In the slide a few small felspar fragments, 

 about "i mm. in size, are displayed. 



Just east of Namukalau is the mouth of the Vui-na-savu River, 

 the Na Savu River of the chart. This is a tidal river, and is 

 navigable for boats for several miles. In the lower part of its 

 course agglomerates and tuffs prevail, probably in part at least 

 derived from acid rocks. Near Rauriko, which lies about 5 miles 



