The Southern Sea-border between Vuinandi and the Vicinity of Tawaki
The basic rocks, which characterise this long extent of coast, give place about two miles west of Tawaki to the acid rocks. I will proceed methodically with the description from Vuinandi eastward.
(a) The coast between Vuinandi and Nakarambo.—Along this coast, spurs from the Mount Thurston Range reach the borders of Natewa Bay, forming a succession of small bays a mile or so across, on the shores of which are situated the villages and towns of Vuinandi, Ndaku-ndaku, Korotasere, and Nakarambo. The basaltic rocks characteristic of the vicinity of Vuinandi are predominant here; but agglomerates and tuffs are of rare occurrence, the prevailing rocks exposed in the spurs being fine textured basaltic andesites (sp. gr. 2·82). A calcareous tuff-clay is, however, to be observed in the east point of Vuinandi Bay, where it is apparently penetrated by a dyke of the above mentioned rock. In Ndaku-ndaku Bay there issue from the shore-flat between the tide-marks some hot springs which are referred to on page [34].
(b) The coast between Nakarambo and Waimotu.—In this tract we meet with rocks of a somewhat different character, though still basic in their type. Through the agglomerates and agglomerate-tuffs of the spurs protrude coarsely crystalline grey pyroxene-andesites.[[91]] They are to some degree altered, and are characterised by their abundant phenocrysts of plagioclase and of rhombic and monoclinic pyroxene, the groundmass being relatively scanty. The agglomerates are formed of similar materials. There is an interesting exposure in the point east of Nakarambo. My specimens from this locality were unfortunately lost; but in my notes reference is made to the coarsely crystalline grey andesite above noticed, to the later intrusion of a dark amygdaloidal rock, and to an altered calcareous tuff.
(c) The coast between Waimotu and Natasa Bay.—A low belt of land often forms the sea-border. Between these two localities there is a broad estuary, the village of Vanuavou being situated on the right side and Malati on the left side. Tuffs usually calcareous and probably submarine are here displayed together with basaltic andesites.
(d) The coast between Natasa Bay and the vicinity of Tawaki.—This extensive stretch of sea-border, nearly 15 miles in length, is characterised by hilly spurs and long intervening low-lying tracts. The prevailing rocks exposed are tuff-clays, somewhat hard and altered, and coarser basic tuffs sometimes calcareous and overlying the former. These sedimentary deposits, which are evidently submarine, are often bedded and have a fairly constant dip over most of this region of 10 to 15 degrees to N.W. or W.N.W. They are frequently pierced by dykes, 6 to 10 feet thick, of compact basic rocks. A specimen of one of these dyke-rocks from between Natasa and Vatu-karoa is a doleritic basalt with scanty olivine (sp. gr. 2·81)[[92]]; but the rock of the dykes east of Vatu-karoa is less basic (sp. gr. 2·72), and may be described as an augite-andesite[[93]] with a doleritic structure in part disguised by alteration. Between Natasa and Sangani there is a remarkable exposure of an intrusive opaque white rhyolitic rock associated with altered tuffs. This rock, which has undergone some degree of alteration, is described on page [311]. It is the only indication of the vicinity of a region of acid rocks that I came upon on this tract of coast. It probably, if traced inland, would be found connected with the district of acid rocks. In following along the coast, however, towards Undu Point, the region of acid rocks is not reached until within 2 or 3 miles west of Tawaki. Reference should be made here to two hot springs that, as described on page [33], rise up on the coast at Natuvu, a little east of Lakemba.
The Inland Mountainous Region between the Eastern
Foot of the Mount Thurston Range and the Vicinity of Tawaki
This inland region corresponds for the most part to the tract of coast before described between Vuinandi and the vicinity of Tawaki. We have here a very much broken area traversed in a northerly and southerly direction by mountain-ridges and penetrated in places on the north side by prolongations of the Wainikoro and Kalikoso plains. The two loftiest summits, the Ndoendamu and Savu-riti peaks of the chart, rise respectively to heights of 2,481 and 2,238 feet. The former is the Hale Peak of the Wilkes’ Expedition; but as regards the native names there is some confusion and I have not been able to clear it up. Several of the lesser peaks rise to over a 1,000 feet, and four or five of them to rather over 1,500 feet.
With the exception of the fixing of the positions of the more conspicuous peaks, the interior of this part of the island is unsurveyed. I was able to lay down my positions approximately; but before a systematic examination can be made of the geology of this region a survey is necessary. Several traverses and ascents were made by me; but much more in the way of exploration remains to be done. I venture, however, to think that from the data below given a fairly accurate notion of some of the leading geological features of this region may be formed. The districts visited will be described in their order from east to west.
(a) The mountainous district east of the mountains of Vungalei and Nailotha, extending to the vicinity of Tawaki.—Mr. Thomson[[94]] in his map of the sea-border of this district shows a continuous coast range from near Tawaki to Natasa lying about two miles inland. This is the appearance when the region is seen in profile from a distance; but when viewed from an elevation in its interior its surface is seen to be for the most part traversed by a series of mountainous and hilly ridges trending roughly north and south, the greatest height not exceeding 1,500 or 1,600 feet.
The geological character of the sea-border of this district has already been described on page [208]. It is there shown that fine and coarse sedimentary tuffs, sometimes calcareous, and often penetrated by basic dykes, here prevail. Their general story is one of great denudation, and we have the same testimony impressed on us when examining one of the inland mountain-ridges.
I followed the crest of one of these ridges from the hamlet of Nawi, on the headwaters of the Vui-na-savu River, in a south-easterly direction for about 4 miles to the peak of Uthulanga, which overlooks Lakemba on the shore of Natewa Bay. During the walk my level rose fairly gradually from that of Nawi, less than 100 feet above the sea, until near Lakemba, where a height of 1,400 feet was attained. Here the ridge abruptly terminates in the round-topped peak of Uthulanga, which rises steeply from its crest for another 150 feet or rather more, and has a precipitous face on the southern side descending far down the mountain-slope. This peak, which is about 1,550 feet above the sea, when seen from the opposite shores of Natewa Bay is very conspicuous and looks like a thumb or a nose. The first part of the Fijian name of this peak signifies a nose.
The hamlet of Nawi, from whence the ridge begins, is built on a low mound-like “rise,” which is composed of a dark-grey hypersthene-gabbro. Since plutonic rocks are of very rare occurrence in this island, the Nawi gabbro has a particular interest. It belongs to the group of plutonic rocks described on page [250]; and is to be referred to the more basic kinds, its specific gravity being 2·84. In appearance it is like a diallage-gabbro, and in the slide displays monoclinic and rhombic pyroxene filling the interspaces between the large plagioclase crystals and undergoing respectively the diallage and bronzite stages of schillerisation. This rock forms the type of the group and need not be referred to in more detail here. It should be added that in the bed of the neighbouring river occur blocks of a basaltic andesite (sp. gr. 2·86) referred to genus 1 of the hypersthene-augite andesites. It is a less crystalline form of the gabbro just mentioned.
The prevailing rock exposed for the first 2 miles on the crest of the ridge (by starting from Nawi) was a greenish porphyrite displaying large opaque crystals of oligoclase, but showing much alteration of the propylitic kind, its specific gravity being 2·5, but the structure of the groundmass is much disguised. During the next mile, basic agglomerates and a massive hypersthene-augite andesite were exposed. The last-named is semi-vitreous, and on account of the prismatic pyroxene of the groundmass is placed in genus 5 of its sub-class. Its specific gravity is 2·7. In the last and fourth mile of the ridge was exposed a dark grey doleritic basalt (sp. gr. 2·85), which rings like clinkstone under the hammer, and weathers in a honeycombed fashion. The felspar-lathes average ·3 mm. in length, the residual glass being scanty. The pyroxene phenocrysts include some of rhombic pyroxene; and the rock is therefore referred to genus 4 of the hypersthene-augite andesites. The actual peak of Uthulanga, as it rises abruptly about 150 feet from the end of the ridge, is of agglomerate, the blocks being composed of a compact grey andesite.
The peculiar succession of rocks displayed in this mountain-ridge along its length of four miles is worthy of notice. Neither vesicular nor scoriaceous rocks came under observation; and it is to be assumed that there existed originally a line of submarine vents, some of them ejecting acid and others basic materials. Mainly on account of the great marine erosion during the period of emergence, but partly also on account of the subsequent sub-aerial denudation, a plain ridge now represents this line of vents. Probably the peak of Uthulanga, which is evidently an old volcanic neck, represents the last of the stages in the volcanic history of this ridge; but a very long period must have since elapsed. When, however, we look at the exposed gabbro at the other end of the ridge, we have to carry the period much farther back, since here the superjacent surface volcanic rocks have been stripped off completely. On page [2] I have referred to an island in the Solomon Group where we have such a chain of ancient vents of acid and basic rocks. In that case the forms of the separate hills indicating the original vents are still to be recognised. In this old mountain-ridge of Vanua Levu no such outlines remain except in the instance of the terminal peak.
(b) The Nailotha and Vungalei Range.—This lofty range, which towers above all around it, attains a height of 2,481 feet in the peak of Nailotha and of 2,238 feet in that of Ndrukau or Vungalei. The first is, as I infer, the Ndoendamu or Hale Peak of the charts; but it is uncertain whether the name Savu-riti should be applied to the northern peak of the range as it is in the charts, or whether it belongs to an independent peak farther to the east. By the natives in the vicinity the northern peak is known as Ndrukau, and I have added the name of Vungalei from the village at its foot. The southern peak is that of Nailotha. The range runs roughly north and south. It is, however, obvious that the internal topography of this part of the island is but scantily known.
Brief reference will first be made to the country bordering the range on the north-west and west sides in the vicinity of the villages of Vungalei and Tembe. In proceeding south from Kalikoso, which lies in the midst of the low-lying Wainikoro plains, to Vungale one traverses this level district, of which the elevation is never more than 150 feet above the sea and often much less. About a mile south-east of Kalikoso the limit of the region of quartz-porphyries and of acid tuffs is passed and the area of basic rocks is entered, a dark semi-vitreous pyroxene-andesite with a flinty fracture prevailing at the surface as far as Vungalei. This rock displays a few small phenocrysts of oligoclase and pyroxenes in a blurred glassy groundmass exhibiting the felspar and pyroxene microliths in process of development (sp. gr. 2·46).
Vungalei itself is only elevated about 130 feet above the sea. In proceeding from this village to Tembe, lying about two miles S.S.W., one crosses a line of hills, about 600 feet in height, which form a spur of the main range. Basic agglomerates similar to those found on the slopes of Mount Vungalei, as described below, prevail in the district between these two villages up to the top of the intervening hills. In places one notices that they overlie the ordinary sedimentary deposit, known as “soapstone,” a slightly calcareous clay-tuff but showing no organic remains to the eye. The rock exposed in the stream-courses is a semi-ophitic basaltic andesite (sp. gr. 2·74) which contains a considerable amount of interstitial glass.[[95]] When proceeding S.S.W. from Tembe on the way to Nandongo one passes on either hand, as described on page [216], hills about 700 feet high displaying vertical cliffs formed of agglomerates over-lying finer sedimentary beds apparently of the “soapstone” character.
I made the ascent of Mount Vungalei from the village of that name. The peak is also known as Ndrukau. Basic agglomerates were exposed all the way up to an elevation of about 2,000 feet, where a bed of a somewhat scoriaceous basaltic andesite was displayed, the upper 200 feet being inaccessible but of the same agglomerate. At a height of 300 feet was observed another layer of the same basaltic rock. These intercalated beds appeared to be lava-flows rather than horizontal dykes. The agglomerates become less coarse in the upper part of the mountain where they take the character of agglomerate-tuffs. The blocks are composed, like those in many other parts of the island, of a dark semi-vitreous basaltic andesite, but often scoriaceous. But the most remarkable features of these agglomerates are the indications of two distinct pauses in their deposition afforded by the occurrence at elevations of 900 and 1,700 feet of a single horizontal bed, two to three feet thick, of coarse palagonite-tuffs. Each bed is exposed at the foot of a tall cliff of agglomerate forming a line of escarpment along the mountain-side. The larger fragments making up these tuffs are usually from 2 to 3 millimetres in size; but the process of palagonitisation is not complete; and we find inclosed abundant angular pieces of a dark fresh tachylyte-glass, finely vesicular, and fusing readily in the ordinary spirit-lamp flame. The tuffs contain little or no lime and seemingly no organic remains. In the lowest bed I noticed a little water-worn gravel. Both beds pass gradually into the underlying agglomerate; but their upper limits are well defined and the agglomerate commences abruptly.
It is thus seen that in the formation of the agglomerates there were two pauses when they gradually gave place to deposits of fine detritus made up of a vesicular basic glass that has since been largely converted into palagonite. Then followed a sudden renewal of the agglomerate-producing process. We can scarcely doubt that the agglomerates, with their scoriaceous blocks, and the palagonite-tuffs are in the main the direct products of volcanic eruptions. The rival claim of marine denudation as the agency concerned can be mostly but not altogether excluded. The agglomerates and tuffs of Mount Vungalei cannot be distinguished from those so often described in the case of the great inland ranges of the other parts of the island, the submarine origin of which is frequently demonstrated by the inclosed organic remains. It would seem that in the instance of Vungalei these deposits took place around the shores of a volcanic mountain that rose above the sea. On page [315] it is pointed out that in Stromboli with its dribbling eruptions we have a good illustration of the manner in which such deposits could be formed.
My examination of the mountain of Nailotha was restricted to the lower slopes up to an elevation of 600 feet; but the results obtained are very suggestive. When following up the stream-course on the way from Tembe to the mountain one notices in its bed blocks of a light-coloured rock like a compact quartzite. It is, however, a highly altered oligoclase-trachyte (sp. gr. 2·53) with its structure much disguised by secondary quartz. This is the first intimation one gets, on leaving behind the district of basic agglomerates about Tembe, of the otherwise unexpected character of the acid rocks displayed in the lower part of Nailotha.[[96]]
A torrent here cuts deeply into the mountain-side. At the base, between 200 and 300 feet above the sea, a bluish-grey rather scoriaceous rock with the steam-pores drawn out to a length of from 1 to 2 centimetres, is first exposed. Its specific gravity after allowing for the cavities is less than 2·6. It shows a groundmass partly disguised by secondary quartz and containing numerous small vesicles lined by quartz and filled with viridite and epidote. Where the alteration is less advanced small parallel felspar-lathes with fine decomposing pyroxenes are shown. The lathes give extinctions of two or three degrees and average ·04 mm. in length. In its somewhat scoriaceous nature, in the absence of phenocrysts, and in its less altered condition, this rock differs from those exposed higher up the ravine; but it is evidently to be referred to the same acid type. At a height of 300 feet a light grey highly altered oligoclase-trachyte (sp. gr. 2·43) is exhibited. It contained originally some phenocrysts of felspar and pyroxene, which, however, have been more or less replaced by calcite, quartz, and other materials; whilst the groundmass, originally hemi-crystalline but now blurred by the deposition of silica, shows felspar-lathes in process of development.
A little farther up the gorge there is displayed another highly altered rock with a siliceous appearance, such as has been noticed above as forming blocks in the stream near the foot of the mountain. It is an oligoclase-trachyte impregnated with crystalline silica and exhibiting a singular prismatic structure, the small columns or prisms being only 3 or 4 inches in diameter. Between 400 and 500 feet occurs a light-coloured compact rock sparkling with pyrites and also displaying a columnar structure, the columns being between 4 and 6 inches across. It looks like a limestone and effervesces freely with an acid; but it is in fact a highly altered oligoclase-trachyte (sp. gr. 2·5) of the propylite type. It seems originally to have inclosed a few phenocrysts of oligoclase, sanidine, and pyroxene, and here and there a stout felspar-lathe is to be noticed in the groundmass giving straight extinction. The whole texture of the rock, however, is more or less impregnated with secondary quartz, calcite, chlorite, viridite, pyrites, &c. Farther up the ravine, between 500 and 600 feet in elevation, is displayed a remarkable quartz-porphyry which exhibits opaque porphyritic crystals of felspar as well as rounded crystals of quartz in a grey compact base. It has been subjected to considerable alteration and will be found described on page [310].
At 600 feet large slabs of the ordinary sedimentary clay-tuffs, containing but little lime and showing no organic remains to the eye, lay about on the more level slopes, and evidently formed a surface deposit incrusting the altered massive rocks. If my ascent had lain up the mountain-side away from the stream-courses, these sedimentary rocks would alone have been observed. In the gorge, however, is exposed to view the deeper-seated rocks that make up the mountain’s mass. We have here a thickness of about 400 feet of altered acid rocks. All are doubtless intrusive, and were subjected to alteration after the development of the columnar structure and before the deposition of the overlying clay-tuffs or “soapstones,” which are no doubt of submarine origin. These sedimentary tuffs belong probably to the period when the submarine agglomerates and palagonite-tuffs of the neighbouring peak of Vungalei were formed.
(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.