(c) Traverse of the coast range from Nandongo to Vanuavou on the shore of Natewa Bay.—This route was taken by Mr. Horne, the botanist, in 1878. I approached Nandongo from Tembe to the northward. The road at first lay between hills about 700 feet in height displaying in their precipitous faces agglomerates overlying fine sedimentary tuffs. These deposits in the form of slightly calcareous basic tuff-clays, the so-called “soapstones,” are exposed in the bed of the Wainikoro River as one nears Nandongo. This village, which is situated on the headwaters of the Wainikoro at an elevation of about 180 feet above the sea, lies near the foot of the range. In its vicinity there is a small thermal spring which is referred to on page [33].
Proceeding south from Nandongo one notices in the stream-course at the foot of the slopes the sedimentary tuff deposits above mentioned, bedding and dipping gently to the west. Farther up the slopes, higher than 250 feet above the sea, there are exposed the deeper-seated rocks of the range in the shape of compact reddish rocks (sp. gr. 2·48), which appear under the microscope to be highly altered acid andesites or oligoclase-trachytes originally displaying flow-structure and a fair amount of glass, but now much disguised by the formation of secondary quartz. On this north slope of the range I also found an amygdaloidal variety of the same altered rocks containing irregular amygdules, 5 or 6 millimetres long, of fibrous quartz or chalcedony. Blocks of basaltic andesite were observed on the summit, which has an elevation of 950 to 1,000 feet. On the southern slopes descending towards Natewa Bay coarse basic tuffs together with blocks of basaltic andesite are chiefly exposed. The last-named probably represent dykes both on the south slope and on the summit. The rocks exhibited on the portion of the coast of Natewa Bay corresponding to this range are dark and light-coloured sedimentary tuffs usually calcareous, with occasional basaltic andesites indicating dykes.... From this traverse it would appear that the range has an axis of altered acid rocks overlain by basic sedimentary tuffs and pierced by basaltic dykes.
(d) The mountainous district lying between the head waters of the Wainikoro River and the Mount Thurston Range.—Of this region I know very little. The highest peak according to the chart has an elevation of about 1,600 feet. Some indication of the character of the inland rocks may be obtained from that of those exposed on the coast between Nakarambo and Waimotu where, as observed on page [208], grey pyroxene-andesites, coarse in texture and almost holo-crystalline in structure, protrude through agglomerates of the same materials. When on the way from Ngelemumu to Wainikoro I crossed the extreme northern prolongation of this range where the elevation above the sea is only 700 feet. Non-calcareous basic tuff-clays occur on the slopes; but the deeper-seated rocks, judging from an exposure on the east side, are dark grey altered pyroxene-andesites penetrated by fine cracks filled with a mosaic of quartz and having a specific gravity of 2·7. On the summit I found a gritty sandstone-like rock, of which my specimen has been lost. In a stream at the foot of the east slope occur small blocks of basaltic andesite probably derived from a dyke. The region of acid rocks, such as quartz-porphyries, oligoclase-trachytes, &c., is not entered until about two miles south-west of Wainikoro.
The Coast Ranges and Sea-border between Mbuthai-sau
and Thawaro or Mbekana Bay
We have in this region the mountains and hills at the coast and the low-lying plains inland. This feature of the north side of Vanua Levu is very remarkable. For some sixty miles, that is to say, for more than half the length of the island, between the mouth of the river Ndreketi and Thawaro or Mbekana Bay, Vanua Levu possesses this character. The coast ranges west of Lambasa, where basic rocks evidently prevail, have been referred to on pages [135], [136]. Those east of the Lambasa mountains as far as Thawaro Bay will be dealt with here; and instead of basic we find acid rocks, such as quartz-porphyries akin to the rhyolites, oligoclase-trachytes, pumice-tuffs, &c.
The sea-border is here characterised not by a continuous range running parallel to the coast, as in the case of the district between Nanduri and the Ndreketi River, but by a number of separate groups of hills and lesser mountains, separated by deep gaps or valleys which are occupied by tidal rivers and extensive mangrove swamps. The tide ascends these rivers into the plains behind the coast ranges, so that a depression of only 30 feet would convert these groups of hills into separate islands and would cover much of the inland plains with the sea. The hills attain an elevation of 1,200 or 1,300 feet a mile or two inland and descend often as bold promontories to the coast. I will refer in order to the different parts of this sea-border.
(1) The Sea-border between Lambasa and Mbuthai-sau
In the sea-border between Lambasa and Mbuthai-sau we have the junction of the regions of basic and acid rocks, the former extending westward to Naivaka, the latter reaching to Undu Point. In such a locality the two types of rocks might be expected to be associated, and this is what occurs. Acid pumice-tuffs and basic agglomerates, sometimes associated, are here displayed. In the low hills between Lambasa and Vuni-ika Bay, which lies west of Mbuthai-sau, I found basic agglomerates prevailing, together with some acid pumice-tuffs. The blocks in the agglomerates are composed of a blackish semi-vitreous pyroxene-andesite (sp. gr. 2·68), which is characterised by prismatic pyroxene in the groundmass, and is referred to genus 6 of the second sub-order of the hypersthene-augite andesites described on page [287].
East of Vuni-ika on the way to Mbuthai-sau, at an elevation of about 50 feet above the sea, dark tuffs containing small fragments of reef-limestone are exposed in a cutting. A little farther on there is a considerable deposit of a pale grey rhyolitic pumice-tuff, a soft stone easily worked, and indeed now quarried by the plantation authorities. It contains no lime and in microscopical characters is scarcely distinguishable from a sample of fine pumice debris collected by me in the Chirica district of Lipari Island. It is made of fragments up to a centimetre in size, of ordinary fibrillar pumice in a matrix of much finer material of the same nature. Portions of crystals of glassy felspar (oligoclase and sanidine) also occur in it, together with some quartz and rhombic pyroxene.
The association in this locality of acid and basic eruptive products was observed by Dana in 1840 in the cliffs of “Mali Point.”[[97]] It is not quite clear whether Mali Island, which lies immediately adjacent to the coast, is here alluded to, or whether it is a headland opposite to it. Dana describes the deposits displayed in these cliffs as coarse aggregates of fragments of pumice and decomposing trachyte, which pass on the one side into fine clayey material, and on the other into an agglomerate formed of angular blocks of vesicular and compact basalt with the interstices filled with pale yellow tufaceous material.