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 corresponding 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 ·1 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 up the river, a bare bluff, overlooking the valley on the east, is formed of a decomposed trachytic rock remarkable for the fact that it displays, as described on page [370], faint magnetic polarity. Above Vitina, a mile or two farther up, I found a similar rock but amygdaloidal in character. On the head-waters of the river is situated the hamlet of Nawi, where, as mentioned on page [211], a plutonic rock of the gabbro type occurs. Tuffs and agglomerates appear to prevail in the low coast tract between the Vui-na-Savu river and Thawaro Bay.
In drawing some general inferences respecting the acid formations, mostly fragmental in character, that are displayed in the sea-border between Lambasa and Thawaro Bay, it is necessary to distinguish between the deposits west and east of the Langa-langa River. Between Lambasa and the river just named the tuffs may usually be regarded as the products of sub-aerial eruptions. Some of the specimens might have been obtained, as far as their characters go, from the pumice-districts in the island of Lipari. Their limeless condition and the apparent absence of organic remains are noteworthy features, though of course the products of sub-aerial eruptions may be deposited under the sea. It is, however, remarkable that no compact obsidian came under my notice. The fragments of rhyolitic glass, intermediate in structure between compact obsidian and pumice, that were imbedded in the pumice-tuffs in one locality, were probably ejected from some sub-aerial vent.
In the region between the Langa-langa River and Thawaro Bay acid tuffs and agglomerates prevail; but they have all been subjected to alteration by the deposition of secondary silica, and the pumice-structure when present is largely disguised. They have evidently, in part at least, been derived from compact rhyolitic and trachytic rocks, and are probably in some measure the products of marine erosion. Although neither lime nor organic remains were detected, the presence of the altered “Globigerina” clay near Visongo is very suggestive and indicates a considerable submergence of this region at some distant period.
Much, however, remains to be done in the examination of the peaks of the coast ranges of this part of Vanua Levu; and it is likely that some interesting results will be obtained from such an exploration.
CHAPTER XVI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
THE NORTH-EAST PORTION OF THE ISLAND (continued)
The Wainikoro and Kalikoso Plains
These extensive inland plains occupy a considerable area in this part of the island. I estimate that there is an area of about 20 square miles that does not exceed an elevation of 100 feet above the sea and is often much less. Of the two villages situated in the midst of these plains, about 5 miles inland, Kalikoso is about 30 feet and Wainikoro is scarcely 20 feet above the sea; whilst much of the surrounding district is similarly low. Taking the 300-feet contour-line as a guide, this low-lying region, as shown in the map, would be much larger. The plains are backed on the south by the high mountain-range of Vungalei and Nailotha. On the north the coast-ranges, which attain a height of 1,100 or 1,200 feet, separate them from the sea-border; whilst on the west they are shut off from the Mbuthai-sau valley by a line of hills, of which the minimum elevation is about 700 feet. This region is occupied by the scanty open vegetation characteristic of the “talasinga” or “sun-burnt” districts. The tall “Ngasau” reed is common; the Pandanus trees are frequent; and amongst the bushes and small trees are represented “Dodonæa viscosa,” “Morinda citrifolia,” and a species of “Hibbertia.”