The Natewa Peninsula
This remarkable peninsula is connected with the rest of the island by the low-lying Salt Lake district, a narrow isthmus, described in the preceding chapter, which one can cross without rising 50 feet above the sea. My acquaintance with this region is far from complete; but from the following notes a fair general idea of its geological characters may be gathered.
By referring to the map it will be seen that there are three groups of mountains. The north-eastern culminates in Mount Freeland or Ngala, 2,740 feet; the southern is formed by the rugged Waikawa Range, 1,540 feet; whilst the Lea Range to the west attains in Ngalau-levu a height of 1,960 feet. They are much cut up by the denuding agencies, and all bear the stamp of an ancient land-surface. Though hot springs are not infrequent, as at Ndreke-ni-wai, Waikatakata, Ndevo, and Navuni, no evidence of recent volcanic action came under my notice. Submarine deposits occur at intervals on the surface up to elevations of 1,000 feet and over; but with the exception of the comparatively recent upheaval or emergence of some 20 or 30 feet, indicated by the raised reef-masses and foraminiferous tuffs and clays in different parts of the coast, there is nothing to suggest that these changes did not occur ages since. In the frequent alteration of its andesitic rocks, and in the occasional occurrence of porphyrites, we have sufficient indication of the antiquity of this part of the island as far as its volcanic history is concerned.
I will commence the description of this peninsula at its western end.
The broken elevated district that extends eastward from the Salt Lake to Fawn Harbour on the south coast, and to the mouth of the Ndreke-ni-wai River on the north coast, is divided into two principal masses, which are connected by a ridge or col about 400 feet above the sea, which is situated a little east of Viene. The western portion, which may be named the Viene sub-district, attains a maximum height of 1,000 feet. The eastern portion reaches in the peak of Ngalau-levu, a height of 1,960 feet, and may be termed the Lea sub-district.
The Viene Sub-District.—The cliffs on the north coast between Muanaira and a little east of Viene are mainly formed of basic tuffs, often calcareous. At a place about 1½ miles east of Viene, these tuffs as exposed in a coast spur display large flat spiral tests of shallow-water foraminifera 4 or 5 millimetres across. They may be described in this locality as palagonitic calcareous tuff-sandstones, more or less compacted, and containing fragments of palagonitised basic rocks. When crossing the col above referred to one finds similar palagonitic calcareous sandstones and clays exposed on its slopes up to its summit (400 feet).
On the south side, in the vicinity of Vunilangi Inlet, foraminiferous clays and reef-limestones are displayed at the foot of the slopes; and the coast between this place and Tathelevu to the westward is bordered by low cliffs of reef-limestone raised 6 to 8 feet above the high-water mark and displaying massive corals in their position of growth. Near Tathelevu there occur raised reefs 10 to 15 feet above the sea; whilst the hills, 250 to 300 feet in height, at the back of this place are composed of fine and coarse tuffs and tuff-sandstones containing little or no lime, and apparently no organic remains. They are sedimentary tuffs of mixed composition, made up of fragments of plagioclase, rhombic and monoclinic pyroxene, brown hornblende, portions of semi-vitreous basic andesite, and palagonitic debris. In the lower levels they are fine textured with a grain of ·2 to ·3 mm. In the higher part their grain is ·5 to 1 mm., and they are more basic in character and come near to the palagonite-tuffs. At an elevation of 200 feet they form inland cliffs, 50 feet high, in which are imbedded blocks, 2 feet across, of a blackish pyroxene-andesite with a specific gravity of 2·73, and belonging to the prismatic sub-order of the orthophyric order of the hypersthene-augite andesites. It is remarkable for the pyroxene prisms of the groundmass, and shows a little interstitial glass. These cliffs are well displayed behind Navelatha, about half a mile from Tathelevu. Between this locality and the Salt Lake Passage, elevated reef-limestones, forming low cliffs 6 to 8 feet high, together with occasional tuff-agglomerates, occur at the coast.
The Lea Sub-District.—This region, which includes the mountain-range of Ngalau-levu at the back of Lea, is limited by Fawn Harbour and Vunilangi Inlet on the south coast, and by the Ndreke-ni-Wai River and a point between Viene and Lea on the north coast. Its structure, as is shown below, is very complicated, acid and basic rocks being associated in a remarkable manner; whilst over all lie the submarine tuffs. Marine and sub-aerial denuding agencies have shaped and re-shaped the surface to such a degree that it is now impossible to restore it in imagination.
On the north coast of this sub-district, about two miles east of Viene, is exposed an altered darkish porphyrite displaying large opaque crystals of plagioclase, 5 to 7 mm. long, the rock-mass being penetrated by fine veins of chalcedonic quartz, which also traverse the phenocrysts. Its specific gravity is 2·6; but on account of the imperfect development of the felspar-lathes and the amount of altered glass in the groundmass, which also contains a little calcite, it can be only generally referred to the augite-andesites. A greenish altered foraminiferous tuff showing fine cracks filled with chalcedony composes a spur in this locality. A propylitic or highly altered dolerite is exposed half-way between Viene and Lea.
As one nears Lea from the west the lofty spurs of the mountain of Ngalau-levu reach the coast, and basic tuffs and agglomerates prevail. The blocks in the agglomerate are composed of a vesicular semi-vitreous hypersthene-augite andesite, which is assigned to the second prismatic sub-order, since it carries prismatic pyroxene in the groundmass. The town of Lea is picturesquely situated on the coast at the foot of the steep mountain-slopes, being closed in on the east and west by elevated spurs descending to the sea. Fragments of jasper and chalcedony occur in the beds of the streams that here drain the precipitous sides of the range. Two dykes of dark basic rocks protrude through the beach in Lea Bay. They are composed of augite-andesites referred to genus 13 of the augite-class; but the two rocks belong to different species of that genus. In the one the felspar-lathes are only ·04 mm. in length, and there is a little altered glass in the groundmass, the specific gravity being 2·63. In the other the felspar-lathes average ·2 mm. in length, and the rock has a coarser texture, whilst the specific gravity is 2·7. The augite granules are large (·03 mm.), and there are irregular lacunar spaces filled with calcite and lined by a brown palagonite-like material.
I ascended the second highest peak of the Ngalau-levu mountain, which rises to a height of 1,680 feet behind the town of Lea, the highest summit lying to the eastward. Ngalau-lailai, which I also ascended, is a lesser peak, 1,400 feet in height, situated yet nearer to the town. Basic tuffs and agglomerates similar to those exposed in the western spur of the bay occurred all the way up to the bare rocky pinnacles forming the summits. The blocks in the agglomerates are made up of a semi-vitreous augite-andesite, which is sometimes scoriaceous or amygdaloidal, and at other times pseudo-vesicular. Augite crystals, 5 or 6 millimetres in length, are inclosed in the tuffs which contain palagonitised materials, but apparently no organic remains.
In the spur on the east side of Lea Bay occurs a light-coloured altered hornblende andesite. The brown hornblende is mostly represented by black pseudomorphs. Such a rock appears in strange contrast with its basic surroundings. This is followed, as one proceeds eastward along the coast, by basic tuffs and agglomerates. It should have been before observed that blocks of a blackish-brown olivine basalt (sp. gr. 2·89), referred to genus 13 of the olivine class, occur at intervals on the coast between Viene and Ndreke; but the rock never presented itself in position. The tiny felspar-lathes (·03 mm. long) are in flow arrangement; but there is little or no residual glass, and the augite granules (·01 mm.) occur in great abundance.
About two-thirds of the way between Lea and Ndreke-ni-wai there lie close to the shore two islets, 20 to 25 feet high, of reef-limestone, in which massive corals may be observed in their position of growth. Further east, about half a mile west of Ndreke-ni-wai, there is exposed at the coast a bedded light-coloured non-calcareous compacted tuff-rock, dipping 12° to 15° to the southward. It contains pebbles and blocks of acid and basic andesitic rocks, and may be described as an altered hornblende-andesite tuff. Basic agglomerates occur as one approaches Ndreke-ni-wai. This town lies at the mouth of the river of that name, the first river that one meets on the north side of this peninsula. There exist here between the tide-marks some hot springs, to which reference is made on page [34].
When crossing the Natewa peninsula from Ndreke-ni-wai to the head of Fawn Harbour, one reaches a height of 660 feet above the sea. This ridge represents the “divide” between the Lea and Waikawa mountain-ranges. In the lower part of the northern slopes of this ridge occur basic tuffs and agglomerates; but between 200 and 400 feet a light-coloured acid rock of the hornblende-andesite type prevails, both in the form of agglomerate and of loose blocks. This rock is described under the second order of the hornblende-hypersthene andesites on page [299].
Descending the south slope, I found at an elevation of 500 feet a single large mass, about 4 feet across, of quite another type of volcanic rock, which is referred to the orthophyric order of the hypersthene-augite andesites (page [290]). It is a dark grey almost holo-crystalline rock (sp. gr. 2·69) showing porphyritic pyroxene to the eye and displaying in its relatively scanty groundmass short stout felspars, ·05 mm. in length. On the surface of the lower two-thirds of this southern slope occur basic tuffs and agglomerates, basaltic blocks being found in the streams. The tuffs are palagonitic and contain a few calcareous particles. They apparently contain some foraminiferous shells and are doubtless of submarine origin.
It would seem that the axis or deeper portion of this ridge is composed of the hornblende and hypersthene-augite andesites, whilst the basic tuffs and agglomerates form the slopes.
With reference to the south side of the Lea sub-district, it may be observed that whilst on the north side the mountains rise up close to the sea-border, here they are separated from the coast by a broad tract of lowland, where bedded pteropod and foraminiferous clay-rocks are exposed, dipping gently to the south-east. Usually between Fawn Harbour and Vunilangi Inlet the coast is margined by low cliffs of coral-limestone, showing the massive corals in position; but sometimes the deposits above noticed compose the low cliffs and even the islets close by.
The Waikawa Mountains.—This range occupies nearly the whole area of the broad and elevated promontory that is only separated from Taviuni by the narrow straits of Somo-somo, which, however, have a minimum depth of 120 fathoms. These mountains extend to the vicinity of Mbutha Bay on one side and to near Fawn Harbour on the other. Several of the peaks reach to over 1,000 feet, the greatest height given in the Admiralty chart being 1,540 feet. The whole region has a very rugged aspect, the mountains rising up near the coast, whilst the surface is much cut up into ridges and valleys.
A single traverse across the range was alone made, but the results obtained are very suggestive and may doubtless be applied to much of this rugged promontory. I crossed the mountains from Loa to Waikawa. The summit was about a mile broad and undulating, the level varying between 900 and 1,150 feet. At one place on the top there was a deep hollow, some 300 or 400 yards across, perhaps the remains of an old crater-cavity; but the higher slopes were so densely wooded that it was not possible to get a clear view of my surroundings. Basic tuffs and agglomerates prevailed on either side from the foot to the top of the range. Specimens obtained from between 800 and 900 feet above the sea are characteristic palagonitic tuffs of varying degrees of coarseness containing 5 to 10 per cent. of carbonate of lime and a few tests of foraminifera. At 1,100 feet I obtained a specimen which on account of the large proportion of carbonate of lime (35 per cent.) and the abundance of foraminiferal tests may be termed an impure foraminiferal limestone belonging to the group of these rocks described on page [319]. The tests range up to a millimetre in size, and there are also inclosed a few large fragments, 1 to 2 centimetres in size, of shells and crystalline limestone. The residue is made of the detritus of semi-vitreous basic rocks, palagonitic debris, fine clayey material and minerals (15 per cent.), the last including beside plagioclase abundant more or less perfect pyroxene prisms, mostly of the rhombic type.
Near the summit there occurred in one place blocks of a highly basic blackish olivine-basalt (sp. gr. 2·99), marking evidently the situation of a dyke. This rock is referred to genus 13 of the olivine-class. It displays abundant phenocrysts of olivine and augite with but little plagioclase. Interstitial glass is scanty, the groundmass consisting of stoutish felspar-lathes (·06 mm. long), abundant augite granules and prisms, and magnetite.
The Waikawa mountains would thus seem to possess the same general structure that characterises many of the ranges of the island. Submarine basic tuffs and agglomerates cover their sides and their summits, the deeper rocks forming the axis of the range being in this case not so frequently exposed.
On the coast between Waikawa and Navuni, rather over a mile east of Fawn Harbour, basic agglomerates, palagonitic tuff-sandstones, and calcareous clay-rocks containing pteropod and foraminiferous tests, prevail. The tuff-sandstones and clay-rocks are bedded, the stratification being often well shown in the horizontal sections displayed in the shore-flat. In one locality within an area a few hundred yards across, a quaquaversal dip was exhibited. At Navuni, where the hills reach the coast, the same formations occur. I ascended the stream-course there for about a mile, basic tuffs and agglomerates being exposed in its sides, whilst blocks of a heavy dark olivine-basalt[[89]] lay in the bed. The hot springs which issue inland at the side of this stream are described on page [35].
The Basin of the Ndreke-ni-wai River.—With the region, which is bounded on the north by the Mount Freeland Range or the Ngala mountains and on the south by the Waikawa Range, I have but slight acquaintance, except in the case of the coast fronting Natewa Bay. A little way up the course of the river Ndreke-ni-wai, which drains this area, lies the town of Koro-ni-yasatha, where Mr. Horne, the botanist, spent some days in 1878. Probably much of this area is not over 200 feet above the sea, and apparently there is a good deal of talasinga country.
The Coast between the Mouth of the Ndreke-ni-wai River and the Foot of the Ngala or Mount Freeland Range.—Between this estuary and Valavala, two miles to the eastward, occurs a bedded calcareous palagonitic tuff of sedimentary origin, dipping steeply to the north. In one locality there is a rudely columnar dyke of a porphyritic augite-andesite. Coarse basic tuffs exposed in the cliffs and shore-flat of Ko-nandi-nandi Point on the side of Valavala Bay display a spheroidal structure, due probably to the vicinity of some igneous intrusion. The sea-border extending from this bay to Natewa, and farther on to Waikatakata, near the foot of the Ngala mountains, is in most parts a broad low strip of coast-land, where rock-exposures are infrequent. A dark grey andesite forms the blocks of the agglomerate in this locality. It is noticeable on account of the prismatic pyroxene of the groundmass; and it is assigned to genus 5 of the second (prismatic) sub-order of the hypersthene-augite andesites. A blackish semi-vitreous pyroxene-andesite occurs in the vicinity of Natewa.
At Waikatakata (the Fijian word for “hot water”), where an outlying spur of the Mount Freeland or Ngala Range reaches the coast, hot springs issue on the hill-side, as described on page [34]. On the slopes around the springs lie huge masses of an aphanitic basaltic andesite having a specific gravity of 2·81 and referred to genus 16 of the augite sub-class. It displays a characteristic andesitic groundmass, showing crowded felspar-lathes in flow-arrangement with average length of ·17 mm., and containing scarcely any residual glass.
Proceeding along the coast east of Waikatakata, one enters the region of altered pyroxene-andesites, for which Mount Freeland, or the Ngala Range, is remarkable. There are first to be observed on the shore blocks of a grey altered ophitic dolerite, which belongs to the non-porphyritic division of genus 10 of the augite-andesites and is described in detail on page [275]. Afterwards the characteristic rocks of the district occur. The lofty spurs of the Ngala Range here reach the shore; and between them lies the coast village of Ngara-vutu, from which the ascent to the summit is best made.