(3) Sea-border between the Langa-langa River and Thawaro Bay
The next part of the coast which I visited was that opposite Tutu Island. Here a range of high hills, 1,100 or 1,200 feet in height, sends a lofty spur to the sea, in the precipitous faces of which are exposed breccia-tuffs and agglomerates derived from acid rocks. A specimen of the tuffs shows, besides fragments of altered rhyolitic or trachytic rocks, portions of decomposing pumice, the vacuoles and tubular cavities of which are filled with alteration products. The blocks in the agglomerates are altered oligoclase-trachytes, both compact and vesicular. These deposits are non-calcareous and rarely display bedding; but in one place there was a rude arrangement of the materials, the dip being to the north-west. I crossed this coast range where it is only 600 feet above the sea, and descended into the Kalikoso plains in the vicinity of Numbu.
In the coast district between Tutu Island and the village of Naua, 3½ miles to the east, the same altered coarse pumiceous and trachytic tuffs, occasionally bedded and dipping W.N.W., form the low hills near the sea. In one place, where the elevation was less than 200 feet, I noticed on the surface small fragments of branching corals. They had perhaps weathered out of the tuffs, and though in part silicified still effervesced freely in an acid.
Between Naua and the town of Visongo occur low hills formed of acid tuffs. At an elevation of 200 feet the tuffs display a remarkable character. When examined with the lens they are seen to be crowded with the minute tests of foraminifera of the “Globigerina” type. The deposit forms a fairly hard light-grey clay-rock which according to the usual acid-test has no lime. Under the microscope it is seen to be composed nearly in equal proportion of the tests of foraminifera and of very fine detritus apparently derived from acid rocks, the materials being generally not over ·01 mm. in diameter, though some of the felspar fragments measure ·05 mm. across. A secondary process of silicification, as exhibited in the occurrence of minutely granular quartz, has affected the matrix of the clay as well as the tests of the foraminifera. The deposit is of 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 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.