or 16 per cent. of lime. They inclose pteropod shells in places and show
many minute foraminiferous tests of the pelagic type. Their composition
is given on page 323; but it may be here remarked that the residue is
made up mainly of palagonitic debris, fine clayey material and minerals.
The mineral fragments form about 20 per cent. of the mass, and consist
principally of glassy plagioclase, with some rhombic pyroxene, and
magnetite, their size averaging ·1 mm. The tuff-sandstones
interstratified with the clays contain only 2 or 3 per cent. of lime,
and show only a few scattered microscopic tests of foraminifera. About
two-thirds of the rock consist of fragments of a bottle green basic
glass, vacuolar and but little altered, the rest being composed chiefly
of glass debris, plagioclase, and a little pyroxene, the larger mineral
and glass fragments averaging ·3 to ·5 mm. in size. They are in fact
submarine hyalomelane tuffs very similar to those first met with at the
foot of the mountain, which are referred to on page 108. (They are
described on page 333.)
These interbedded clays and tufaceous sandstones of the Mbenu-tha cliffs
were deposited under somewhat different conditions. The clays represent
the quiet deposition in fairly deep water of fine materials derived from
the degradation of acid andesites as well as of basic rocks. The
hyalomelane tuff-sandstones were formed more rapidly by the accumulation
of fine volcanic ash consisting of fragments of a basic glass ejected
from some neighbouring volcano that rose above the sea-surface.
Submarine hyalomelane-tuffs with basic agglomerates appear to be of
common occurrence around the base of the Navuningumu mountain. As we
leave the range behind and begin to descend the long spur that slopes
northward to Ndreketi, we find for the first mile or two these
agglomerates. But where the deeper rocks are exposed at an elevation of
600 feet, near the village of Singa-singa, there are displayed fine
basic pumiceous tuffs and compact palagonitised clays containing little
if any lime, the last, however, containing a few casts of microscopic
foraminifera. The tuff is made up of minute fragments, the largest less
than ·1 mm. in size, of a basic hyalomelane glass, which is vacuolar,
and often fibrillar like ordinary pumice, and in places shows the early
stage of alteration into palagonite. The clay principally consists of
more or less palagonitised debris of the same basic glass, together with
minute fragments of plagioclase and rhombic pyroxene. These tuffs and
clays represent the two conditions of deposition above referred to, the
last indicating a period of quiescence when the fine materials resulting
from the degradation of both acid and basic andesites were slowly
accumulating in deep water, the first denoting the activity of a
neighbouring supra-marine vent from which fine dust and ash formed of
basic pumice were ejected.
[Illustration: MBENUTHA Cliffs showing volcanic agglomerates overlying
tuffs and clays, containing shells of pteropods and foraminifera, which
are raised 1,100 feet above the sea.]
[_Face p. 111._
The bed of agglomerate, 60 to 70 feet thick, which overlies the
foraminiferous tuffs and clays exposed in the line of cliff extending
from Mbenu-tha to Mumu, is made up of subangular blocks, not usually
over 6 inches in diameter, of an acid andesite of the general type found
in the Ndrandramea region, but possessing a semi-vitreous
groundmass.[52] By clambering up the steep slope on the south side of
these cliffs, it will be observed that this thick bed of agglomerate is
covered by bedded foraminiferous clays and tuffs similar to those that
underlie it. It is therefore without doubt submarine, and presents the
result of the more violent outbursts of some neighbouring vent. That
this vent is now represented by the “plug” of basic lava forming the
peak of Navuningumu is highly probable. It is, however, noteworthy that
these beds of agglomerates, tuffs, and clays, as shown in the photograph
of the cliffs, are all inclined at an angle of 20° towards the axis of
eruption or to the westward. The tuffs and clays underlying the
agglomerates are, as already remarked, much disturbed in places. It
would seem that all the beds here exposed were originally horizontal,
and were tilted up during the disturbances accompanying the outbursts of
volcanic activity.
The natural section, which the Mbenu-tha cliffs present, is doubtless
due to landslips. Similar exposures, displayed by cliffs of basic
agglomerate with submarine tuffs and clays at their base, are common on
the mountain-slopes of other parts of the island. Water oozes through
the underlying soft deposits, and the result is seen in the occurrence
of huge masses of agglomerate on the slopes below.
From the details here given respecting Navuningumu and its surroundings,
it is apparent that there have been two stages in the history of this
volcanic mountain. The first was submarine and was characterised by the
discharge of acid lavas which consolidated around the vent and were
afterwards covered over with deposits of foraminiferous clays. The
second was in the last part supra-marine. With the renewal of activity,
the overlying acid andesites were broken through and basic materials
were discharged from the new vent. The bed of acid agglomerates exposed
in the Mbenu-tha cliff belongs to that period of the second stage when
the explosive agencies were most violent. It represents the extensive
destruction of the overlying rocks. The foraminiferous tuff-sandstones
are submarine accumulations of the finely comminuted fragments of basic
pumice that constituted the dust and fine ash discharged from a
supra-marine vent. The scoriaceous and amygdaloidal blocks of the basic
agglomerates overlying these tuffs around the base of the mountain have
had a similar origin. The original ash-cone that at one time rose above
the surface of the sea has long since been destroyed by the denuding
agencies; and its situation is alone indicated by the “neck” of basic
lava-rock that forms the peak of Navuningumu.
A very long period must have elapsed since this last stage in the
activity of the vent. The clays containing pteropod-shells and tests of
foraminifera, with which the basic pumice tuffs and the acid
agglomerates were interstratified, are now about 1,100 feet above the
sea, and are situated in the centre of the island. During the emergence
the denudation of the new land-surface was no doubt very great; and
these submarine clays and tuffs, as displayed in the cliffs, owe their
preservation in great part to the protection of the overlying mass of
agglomerate.
Much light is thrown on the history of the whole Ndrandramea region of
acid andesites by the examination of this old volcano of Navuningumu.
Some of the hills, as in the case of Ngaingai and Wawa Levu, seem to
have been stripped of everything that could give information to the
geologist. Others again, like those of Thoka-singa and Ndrandramea,
display here and there on their slopes agglomerates of the same
materials, the rounded forms of some of the blocks being in part
indicative of marine erosion during the emergence of this region from
the sea. In Soloa Levu, however, we have one of these hills partially
surrounded by later basaltic flows and covered in places on its lower
slopes by basic tuffs and agglomerates, probably submarine. In
Navuningumu the original mass of acid andesite is only scantily exposed.
It is for the most part buried beneath submarine clays which are in
their turn covered by the tuffs and agglomerates of later basic
eruptions.
CHAPTER VIII
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
MOUNT VATU KAISIA AND DISTRICT
THIS peak, 1,880 feet in height, starts up suddenly in the mountainous
interior of the island. Being situated in the valley of the Yanawai
river, which opens to the south, it forms a conspicuous landmark for
vessels off the south coast; but from most other points of view, on
account of its peculiar situation, it is usually difficult and often
impossible to obtain even a glimpse of it.[53] From its remarkable
blunt-topped conical shape it has received the not very appropriate name
of Marling Spike in the Admiralty charts. The natives name it Vatu
Kaisia, the first word signifying “rock,” whilst the second is the name
of a demon.
[Illustration: Profile-sketch of the Vatu Kaisia district from S.S.E.]
Some idea may be formed of its situation and of the character of the
neighbouring country from the profile-sketch and photograph here
produced. I was unable for reasons given below to take a photograph of
the mountain itself, as it was either too near or too far away. Vatu
Kaisia is approached either from Ndrawa on the north or from Ndawara on
the south, the ascent being best made from the west side. The regions
traversed on the way are so densely wooded that the mountain does not
become visible until the traveller is right upon it. He becomes suddenly
aware that there is some huge mass close to him looming above his head
through the trees; and it is with a feeling of awe that he first looks
upon a mountain that although only a few hundred yards away nearly
escaped his search. He is startled by its proximity, and wonders what
strange forces have been at work to place it there; but his view is
transitory, and whether proceeding north or south he sees it no more,
unless he essays to climb its slopes.
Vatu Kaisia lies not in the centre but towards the west side of the
Yanawai valley the river flowing as an impetuous stream around the foot
of its eastern slope. In the profile-sketch the mountain itself conceals
the peculiar feature of its position, which is, however, shown in an
exaggerated form in the geological section below. On its west side rises
a broad ridge running south which in places is not much higher than the
basaltic plateau of Wainunu to the west of it. This ridge is only
separated from Vatu Kaisia by a dark narrow gorge not many hundred yards
in width, across which my natives were able to make themselves heard
when near the summit. The mountain rises 1,100 or 1,200 feet above the
gorge on its west side, which is 700 feet above the sea, and some 1,400
or 1,500 feet above the Yanawai river on the east, which is 300 or 400
feet above the sea. It possesses two peaks, of which the western one is
smaller and lateral and has a height of 1,600 or 1,650 feet, whilst the
eastern is the main peak and rises to 1,880 feet. The saddle between the
peaks has an elevation of about 1,500 feet. It is very difficult to
obtain a distant view of the two peaks, which lie about N.W. and S.E.
with each other. They are either merged into one as in the view from the
south, or else the highest portion of the main peak is alone visible.
On the lower slopes of the mountain as high as 1,100 or 1,200 feet is
exposed a porphyritic doleritic basalt showing semi-ophitic augite and
abundant interstitial glass. Its specific gravity is 2·8, but there is
no olivine. It belongs to a type of basalt described under genus 9,
sub-genus A, of the augite-andesites. The upper double-peaked portion
rises precipitously, displaying bare rocky cliff-faces with a drop of
100 or 150 feet, and formed in mass of a grey andesitic rock with a
specific gravity of 2·71 and showing abundant small porphyritic crystals
of hornblende and rhombic pyroxene. It represents a type of the
hornblende-hypersthene-andesites described on page 301. I was unable,
through want of a rope-ladder, to accomplish the last hundred feet of
the summit; but the general uniformity of structure was evident. No
detrital rocks came under my observation.
That the porphyritic basalt represents a later flow around this old
andesitic mountain is indicated amongst other things by this absence of
tuffs and agglomerates. Vatu Kaisia is undoubtedly the core of an
ancient cone of hornblende-andesite, and as in the case of Mount Soloa
Levu, which is formed of somewhat similar andesites (see page 103), it
has been more or less completely surrounded by later basaltic flows.
Vatu Kaisia and Soloa Levu occupy similar positions with respect to the
great basaltic table-land of Wainunu, the first lying just within its
eastern border, the second lying partly within its western margin.
The structure of the ridge immediately west of Vatu Kaisia lends support
to this view of the formation of this region. The ridge is here, it is
true, elevated a hundred feet or so above the table-land which is about
1,000 feet above the sea; but whilst on its slopes facing the mountain
the same porphyritic basalt prevails, there is a limited exposure on its
top of the same rock (sp. gr. 2·68), differing only in the larger size
of its porphyritic crystals of hornblende and rhombic pyroxene.
[Illustration: Vatu Kaisia]
The narrow gorge isolating the mountain on the west is occupied by a
tributary of the Yanawai River. It has a depth of 400 feet below the
ridge; and as illustrated in the section below it has evidently been
largely formed by the eroding agency of the stream. However, at the
bottom of the gorge there is exposed a heavy aphanitic basalt showing no
olivine and having a specific gravity of 2·85. Though of much finer
texture, the felspar microliths only measuring ·05 mm. in length, it
differs conspicuously from the overlying porphyritic basalt in
possessing little or no interstitial glass. It is referred to genus 16,
species A, sub-species 1, of the augite-andesites (page 280).
The probable structure of this district is shown in the geological
section here given. It is assumed from the limited exposure of the same
rock on the top of the ridge that the basaltic flows which surrounded
the lower portion of Vatu Kaisia at the same time covered over another
similar peak lying immediately west of it. Through stream-erosion Vatu
Kaisia has now been isolated on its west side; and since the basaltic
rocks rise to about the same height on both sides of the gorge thus
produced, the original surface was probably as indicated by the dotted
line in the diagram.
By following the summit of the ridge, as it runs south on the right side
of the Yanawai valley towards Ndawara, some interesting rocks are
observed. For the first mile from the camping-place opposite Vatu Kaisia
the elevation increased from 1,100 to 1,300 feet, and blocks of a
blackish basaltic andesite (sp. gr. 2·76) lay on the ground. About a
mile further on fragments of white quartz-rock appeared on the surface
having been thrown out of a shaft close to the track which had been sunk
to a depth of 15 or 20 feet by a gold miner[54] a few years before. I
could not descend the shaft to examine it: but the specimens picked up
are evidently a white vein-quartz, some of them having a striated
“slickenside” surface on one side.[55] There is evidently a “contact” in
this locality, probably of a basaltic rock with an acid andesite.
Leaving the shaft, the track proceeds southward and eastward, and one
descends gradually from a height of 1,100 feet down to the Yanawai river
where the elevation is only about 150 feet above the sea. Occasional
blocks of basaltic rocks lie on the surface of the ridge, and in one
locality there is exposed a curious-looking agglomerate formed of
fragments of a greenish altered augite-andesite, somewhat scoriaceous,
the cavities being filled with a zeolite. At the crossing of the river a
black basalt (sp. gr. 2·82) occurs _in situ_; whilst loose blocks of
basalt and of an acid andesite occur in the river-bed. Continuing the
journey from the Yanawai crossing to Ndawara near the mouth of the
river, one follows the track across a range of hills, 500 to 600 feet in
height, basaltic rocks prevailing on the surface.
THE NANDRONANDRANU DISTRICT.
Lying north-west of Vatu Kaisia is an elevated district which I have
named after its highest summit, a square-topped peak rather higher than
Vatu Kaisia and probably about 2,100 feet above the sea. Koro-ni-yalewa,
which signifies “town of the women,” is another name of this peak. It is
shown in the sketch given on page 113, and is situated about two miles
north-west of Vatu Kaisia. I did not ascend this mountain, which from
its form would seem to be made of an acid andesite like the Ndrandramea
peaks. Much of this elevated region varies between 1,000 and 1,500 feet
in elevation. It is connected with the Ndrandramea district by somewhat
broken country not much over 1,000 feet in height, which is the “divide”
between the river systems of the Ndreketi and Wainunu. A long
tongue-like extension of similar elevation projects to the north-west
between the Ndrawa and Navuningumu branches of the Ndreketi. This
elevated region is continuous to the eastward with the Tavia Range which
is described below. For convenience the valleys of the upper course of
the Ndrawa river have been included in this district as their geological
features can in this connection be best explained.
This region is well distinguished from most of the other districts of
the island by the prevalence of aphanitic augite-andesites. These rocks
have also supplied the agglomerates of the locality, and the
palagonite-tuffs which are in places extensively represented are
evidently in great part derived from vitreous forms of the same rocks.
We seem to get nearer to supra-marine eruptions in this region than in
most others. The palagonitic-tuffs and agglomerates appear to have
rapidly accumulated in shallow water, and there is reason for regarding
one exposure of the aphanitic augite-andesites as at all events a
shallow-water lava-flow. The aphanitic character of the massive rocks,
however it may have arisen, is here, as I take it, associated with the
shallow-water habit of the tuffs and agglomerates.
(1) EAST SIDE OF THE NANDRONANDRANU DISTRICT.—By following the track
leading from the ridge on the west side of Vatu Kaisia northward to
Ndrawa one rises gradually to a more elevated region. The rocks exposed
on the surface for the first mile are for the most part altered
hypersthene-augite-andesites possessing a micro-felsitic groundmass.
When a height of about 1,400 feet was attained, the track could not have
been far from the peak of Nandronandranu, but on account of the wood no
view was obtainable. In this locality between 1,300 and 1,400 feet soapy
palagonitic clay-rocks and coarser palagonite-tuffs are displayed on the
surface. No organic remains are to be noticed in the specimens collected
here, but they are much affected by hydration. Judging from the
fossiliferous character of similar deposits over a large part of the
island, it is highly probable that these tuffs and clays are also
submarine.
Afterwards a descent was made to an undulating region about 1½ miles
across and elevated between 750 and 850 feet. The blocks there displayed
on the surface are composed of a dark rather compact augite-andesite
with a specific gravity of 2·75 (see genus 13) and of an altered
greenish aphanitic augite-andesite with a specific gravity of 2·59 in
which calcite occurs as an alteration product (genus 16). Aphanitic
rocks of this character as shown below, are very prevalent in the
north-west and north parts of the Nandronandranu district, but are not
usually altered.
(2) THE NORTH-WEST PART OF THE NANDRONANDRANU DISTRICT.—The best route
to follow here is to take the track from Nambuna to Ndrawa. After
crossing the upper portion of the Wainunu table-land one reaches the
headwaters of the Ndavutu River and then ascends the watershed between
the Ndreketi and Wainunu river-systems, reaching Savulu, about 1,050
feet above the sea, where a solitary house marks the site of an old
mountain town. This region is much cut up in deep valleys usually 200 to
300 feet deep, which are occupied by affluents of the Ndrawa branch of
the Ndreketi, flowing north. The valley of the main affluents is from
400 to 500 feet in depth; and this constant ascent and descent of steep
and often slippery valley sides makes the journey very tedious.
At Savulu one stands within the Nandronandranu district. Behind lies the
Wainunu table-land with its olivine basalts; but here aphanitic
augite-andesites prevail and extend to Ndrawa and beyond. They are
exposed in position in the stream-courses and furnish most of the blocks
and pebbles found in the bed of the main Ndrawa River for miles down its
course towards the sea. They are dark, compact, and non-porphyritic
rocks and are all referred to genus 16 of the augite-andesites as
described on page 279. They vary, however, in certain features, as in
the specific gravity, the amount of glass, &c. The residual glass is,
however, usually small; but in a stream-course east of Savulu I found in
position at an elevation of 750 feet a semi-vitreous scoriaceous variety
of these rocks, in which the steam-pores had been drawn out into long
tubular cavities half an inch and more in length. The scoriaceous
character is infrequent; but reference should here be made to another
exposure of a slaggy semi-vitreous rock showing abundant steam-pores in
the tuff-district of the river valley above Ravuka. It differs in some
respects from the prevailing rock, since it displays prismatic augite as
well as felspar microliths in its glassy groundmass, and is for this
reason referred to genus 20 of the augite-andesites. In the elevated
region east of Savulu the aphanitic augite-andesites are in places
overlain by tuffs and agglomerates formed of the same materials. There
is a very good exposure of the tuffs in the Nganga-turuturu cliffs about
2 miles west of Savulu.
(3) THE NGANGA-TURUTURU CLIFFS.—These picturesque cliffs, 50 to 70 feet
in height, rise up at the head of the Liwa-liwa valley between Savulu
and Ndrawa. They are elevated about 1,200 feet above the sea; and
probably derive their name from a small waterfall which, after
descending over their face, drops into the valley below. At its bottom
is situated the hamlet of Liwa-liwa, which is about 600 feet above the
sea. This is the Fijian word for “cold,” and doubtless it has allusion
to the coolness of the valley. On account of the more rapid weathering
of the tuffs in the lower part of the cliffs, there is a rude shelter
afforded by the overhanging portion which is the main feature of
interest that the cliffs present from a native’s point of view.
The tuffs composing the cliffs are horizontally bedded and overlie the
prevailing aphanitic augite-andesite exposed on the valley-slopes below.
Originally grey in colour, they have been largely affected by the
hydration accompanying the weathering process. They are fine in texture
and somewhat friable, but contain no lime, and are chiefly made up of
the palagonitised fine detritus of vitreous varieties of the aphanitic
augite-andesites of the district. No organic remains came under my
notice. Some of the beds contain a number of lapilli of basic pumice, 1
to 3 centimetres in size, which are often in the last stage of the
disintegration produced during palagonitisation. It would seem probable
that these lapilli, after having been ejected from some supra-marine
vent, were deposited with the tuffs in the sea around. It should,
however, be not forgotten that vesicular and pumiceous materials may be
discharged during a submarine eruption. When I visited the museum at
Catania, Prof. Platania showed me portions of a bomb, highly vesicular,
that had been thrown up in a submarine eruption off Vulcano in the
Lipari Islands.
(4) THE UPPER VALLEYS OF THE NDRAWA RIVER.—The two valleys of Liwa-liwa
and Ndrawa meet at Ravuka, where their two streams unite to form the
main Ndrawa River. The former is the largest; and its large impetuous
stream, during its descent of about two miles from Liwa-liwa past
Lutu-kina to Ravuka, which is between 200 and 250 feet above the sea,
has a drop of 300 or 350 feet. The main stream flows with a gentle
gradient to the coast about ten miles away. I did not descend its course
for more than two miles below Ravuka, where some hot springs well up
through the gravel on the left bank (see page 31.)
This is a region of palagonite-tuffs which like those of the
Nganga-turuturu cliffs are mainly derived from vitreous and
semi-vitreous aphanitic augite-andesites. They do not effervesce with an
acid, and neither foraminiferous tests nor other organic remains occur.
The palagonitic material is usually vacuolar, the vacuoles being filled
with palagonitic glass or with a zeolite as in the more altered rocks.
Where bedding is shown, the beds are generally horizontal. These tuffs
are extensively displayed in the sides and beds of the rivers from
Liwa-liwa and Ndrawa to Ravuka and as far as I went down the main river,
namely to the hot springs. They are associated with agglomerates, formed
of the aphanitic augite-andesites, below Ravuka and in the Ndrawa
valley.
(5) THE VICINITY OF NDRAWA.—The village of Ndrawa, which is not elevated
more than 300 feet above the sea, is situated in the heart of the island
in a deep valley more or less hemmed in by the mountains. This is one of
the wettest localities in Vanua Levu, and probably, as in the case of
that of Ndriti in the Seatura basin, the rainfall is not far under 300
inches in the year. In the river-gorge descending westward to Ravuka are
displayed horizontally bedded palagonite-tuffs and agglomerates above
referred to in the description of the Ravuka district, and the same
rocks are exposed on the mountain-slopes to the south of the
village.[56]
Immediately to the north lies a broken hilly country, about 800 feet
above the sea, which has to be crossed on the way to Mbatiri and is much
cut up by streams descending from the vicinity of Na Raro to join the
Ndrawa River below Ravuka. The prevailing rocks are tuff-breccias and
agglomerates. The first are made up chiefly of angular fragments, less
than an inch in size, of aphanitic augite-andesites, some of them being
more or less vitreous and in different stages of palagonitisation,
whilst the finer material derived from the same rocks contains some
carbonate of lime. The agglomerates are composed of the same type of
these augite-andesites, with however but little interstitial glass. It
should be added that pebbles of a kind of jasper or iron-flint occur in
the stream-beds in this locality. (The microscopical characters are
described on page 355.)
By following up the valley that extends to the east from Ndrawa, one
enters after about a mile into the region of Na Raro, which is described
on page 123.
THE TAVIA RANGES.
North of Vatu Kaisia the elevated Nandronadranu district divides into
two ranges, one of which stretches eastward to the south of Na Raro as
far as the gap of that name, whilst the other extends southward on the
east side of the Yanawai valley. Near the angle of bifurcation is
situated Mount Tavia, a remarkable pyramidal peak marked 2,210 feet in
the Admiralty chart and lying 1½ miles north-east (N33°E) of Vatu
Kaisia. It is shown in the view facing page 108. All this region is
densely wooded, and I had chiefly to rely on “course-and-distance,” and
on my aneroid, to determine the surface-configuration.
(1) RANGE ON THE EAST SIDE OF THE YANAWAI VALLEY.—No ascent of these
hills was made. They vary from 1,500 to 1,800 feet in height, and
judging from the loose blocks and gravel in the bed of the Yanawai River
below Vatu Kaisia they would seem to be mainly formed of basaltic rocks,
acid andesites being also represented. However, I crossed the southern
end of the range, where it is 500 to 600 feet in height, to the north of
Ndawara, and found basaltic andesites prevailing at the surface.
(2) RANGE EXTENDING EASTWARD FROM MOUNT TAVIA ON THE SOUTH SIDE OF NA
RARO.—Mount Tavia, which has the appearance of a dacitic peak, was not
ascended; but the range was crossed in two places in going from Ndrawa
to Vatu-vono and from Valeni to Nareilangi, its usual height varying
between 1,200 and 1,500 feet, the extreme height being about 1,700 feet.
In making the traverse from Ndrawa to Vatu-vono, one first passes
through a part of the hornblende-andesite region of Na Raro, which is
described in a later page. Afterwards while ascending the north slopes
of the range, basaltic andesites, often doleritic in texture and
referred to genus 1 of the augite-andesites, are usually found as far as
the summit 1,200 to 1,300 feet above the sea. On descending the south
slopes one finds coarse and fine palagonite-tuffs and clays at 900 to
1,100 feet up, similar to those prevailing near the sea-border. They are
probably submarine, but my specimens are weathered and give no
effervescence with an acid. In the bed of the river above Vatu-vono,
about 400 feet above the sea, there occurs in position an aphanitic
augite-andesite (spec. grav. 2·77), referred to genus 16, species A;
whilst blocks of a coarser grained basaltic andesite lie loose in the
stream.
In my traverse across the range from Valeni to Nareilangi I noticed
about a mile from Valeni and not much over 100 feet above the sea an
agglomerate formed of blocks of an altered acid andesite possessing a
micro-felsitic groundmass and showing microporphyritic rhombic pyroxene
with dark alteration borders (spec. grav. 2·5). It is distinct from the
Na Raro rocks; and its presence in an agglomerate seems to indicate the
vicinity of some old acid andesite peak buried beneath later basic
eruptive products. Ascending the south slopes of the range, I found
decomposing basaltic andesites and basic tuffs, the prevailing rocks up
to an elevation of 1,300 feet; but in one locality (800 feet) occurred
large masses of what seemed to be a disintegrating dacitic rock
penetrated by quartz veins less than an inch thick. An aphanitic
augite-andesite, of a somewhat exceptional character (spec. grav. 2·63),
was displayed at the top of the ridge, 1,500 feet above the sea.[57]
Basic rocks were exposed in the spur running northward on the east side
of Na Raro.
THE SEA-BORDER EXTENDING EAST FROM THE YANAWAI RIVER TO THE LANGO-LANGO
RIVER.—In this district is included the area between the foot of the
slopes of the Tavia Ranges and the shores of Savu-savu Bay. This
undulating country, two to three miles in breadth, does not attain a
greater elevation inland than 300 or 400 feet. Fine and coarse
palagonite-tuffs, some of them with the texture of sandstone, are the
characteristic rocks. They at times contain a little lime and probably a
few tests of foraminifera. The palagonitised glass is often vacuolar,
the vacuoles being filled with the same material. In places where they
are well displayed these tuffs generally show bedding, as in a
hill-slope just east of Vuni-evu-evu, where there are fine and coarse
tuffs interstratified and dipping gently W. by S. Basic agglomerates
also occur in this district.
In the promontory named Yanutha Point in the map there is displayed an
old flow of basaltic lava, showing a columnar structure at the end of
the point. The columns are 20 inches in diameter, and are inclined about
20 degrees from the vertical in such a direction that it may be inferred
that the original flow, doubtless submarine, descended at that angle
from N.N.W. The dark grey rock of the columns (spec. grav. 2·76) has a
fair amount of interstitial glass, whilst a blackish compact rock (spec.
grav. 2·78) that represents apparently a more superficial part of the
flow has an abundance of smoky glass in the groundmass. These rocks are
basaltic andesites and are neither vesicular nor scoriaceous, and come
near the basalts of the Kiombo flow which, however, contain some olivine
(see page 92). They are semi-ophitic and are referred to genus 21 of the
augite-andesites which is described on page 283.
NA RARO.
In Na Raro we have one of the most interesting of the isolated
hornblende-andesite mountains of Vanua Levu. Unlike Vatu Kaisia, which
often eludes the observation, Na Raro is visible from most points of
view. It is double-peaked, the two peaks lying in a north and south line
and rising precipitously. It is this feature that gives the mountain
such a variety in its profile. From the north and south it appears as
shown in the accompanying sketch as a sharp conical peak. From the
north-east and south-east, as illustrated in the two other sketches, it
has the form of a blunt or square-topped mountain; and its true shape is
only shown when it is seen from the east or west. In the photograph here
reproduced which was taken about 1½ miles to the south-west, the two
peaks are with difficulty distinguished. (See frontispiece.)
Not many ascents have been made of these precipitous peaks. Mr. A.
Barrack, who kindly supplied me with some information about it, made the
ascent some years ago; and Mr. Blyth (?), a magistrate, also reached the
top. There are stories of some big officials being hauled up in baskets;
and the natives told me of a white man who was seized with a
shivering-fit when he arrived at the summit. It is certainly a rather
hazardous climb; but the safest plan is to resign oneself into the hands
of the natives, who “bundle” one up in an expeditious, if not in a very
ceremonious, fashion. Nareilangi, near the foot of the mountain on the
north side, is a convenient starting-point, and half a dozen stout
Fijians will not prove too many to assist the climber in the difficult
parts of the ascent. Since the top usually becomes clouded as the day
progresses, it is best to spend a night in a cave about 1,400 feet above
the sea from which the ascent can be made in the early morning. The view
from Na Raro is panoramic and extends over a large part of the island
from Naivaka to Savu-savu.
[Illustration: PROFILES OF NA RARO.
From the south off Kumbulau Point.
From the north in the Ndreketi Plains.
From the North-east.
From the East-south-east.]
Na Raro rises up to a height of 2,420 feet in the midst of a region of
basic rocks. Agglomerates and coarse tuffs formed of aphanitic
augite-andesites prevail in the broken country on the north and west
sides towards Nareilangi and Ndrawa. Immediately south rises the Tavia
Range with its basaltic andesites and overlying palagonite-tuffs; whilst
on the east lies a spur of this range.
Nareilangi, the village from which the start is made, is about 2½ miles
distant from Na Raro, and though situated in the heart of the island it
is only about 100 feet above the sea. The track first passes through a
district of foraminiferous tuffs and clays reaching up to 200 or 250
feet. Afterwards a broken country extending up to 800 feet is traversed.
Here prevail agglomerates and tuff-agglomerates derived from aphanitic
augite-andesites.[58] One then descends into a valley about 600 feet
above the sea, and from this place the ascent of the mountain proper
begins.
The ascent at first is fairly steep, dacitic tuffs prevailing up to
1,000 or 1,100 feet above the sea and forming in places precipitous
cliff-faces. Large masses of hornblende-andesite lie on the slopes. The
dacitic tuffs distinguish Na Raro from all the other peaks of
hornblende-hypersthene-andesite rocks that I examined. They seem
generally to have been stripped off by the denuding agencies; and only
at times, as around the slopes of Ndrandramea and Thokasinga, are to be
found the remains of agglomerates of the same formation. In the case of
Na Raro, however, the tuffs differ somewhat in their components from the
rocks forming the mountain mass. The tuffs are derived from a
hornblende-andesite of dacitic type; whilst the massive rocks of the
mountain are of hornblende-hypersthene-andesites, without porphyritic
quartz, but approaching the dacitic habit.
The tuffs of Na Raro, which are sometimes compacted and at other times
rather friable, do not display bedding. They contain a little lime; but
I found no tests of foraminifera. They are composed of fragments, up to
a centimetre in size, of a dacite displaying brown hornblende,
plagioclase, and quartz in a microfelsitic groundmass, together with a
few fragments of a semi-vitreous basic andesite.
Above 1,100 feet the tuffs give place to the massive
hornblende-hypersthene-andesite. At an elevation of 1,450 feet, a
shoulder of the mountain is reached, near the top of which is the cave
above mentioned. Crossing the shoulder one descends for 100 or 150 feet
into a gap, thus reaching the foot of the precipitous northern peak,
which rises up like a wall for a height of from 900 to 1000 feet
overhead. It is in mass of the andesite just mentioned, many of its
faces presenting inaccessible cliffs displaying seemingly no structure.
This peak is somewhat lower than the southern peak. I placed its height
at 2,270 feet, which, taking the total elevation of the mountain at
2,420 feet, as given in the chart, makes the difference 150 feet. A deep
and broad cleft, that goes half-way down the mountain, separates the two
peaks. The southern one, which appears to be inaccessible, is evidently
formed of the same acid andesite.
These hornblende-andesites, with or without porphyritic quartz, appear
to be for the most part restricted to the immediate vicinity of Na Raro,
except to the south-west, where at a distance of about a mile and a half
from the mountain at an elevation of 500 feet occur a rubbly
hornblende-andesite and agglomerates of the same materials. Though the
rock is of the Na Raro type, its presence here is suggestive of a
distinct vent of small size, of which most of the traces have been swept
away during the emergence of the island. About half a mile south-east of
this locality at an elevation of 450 feet occur some singular banded
palagonite-tuffs which, although they do not show foraminifera in the
section examined, contain a little calcite and are probably of submarine
origin.... In this locality I found a large white mass, measuring 4 × 4
× 5 feet, formed of a siliceous rock appearing in thin sections as
granular chalcedonic quartz (see page 355).
The hornblende-andesite of Na Raro, as in the case of the rocks of most
of the other peaks of acid andesites, has its peculiar characters. It
differs, for instance, from that of Vatu Kaisia in the larger grain of
the felsitic groundmass (N. R. ·021 mm.; V. K. ·013 mm.), in the absence
or rarity of phenocrysts of rhombic pyroxene, in its lower specific
gravity (2·6 N. R.: 2·7 V. K.), in the presence of a little interstitial
glass, and in other particulars. Both, however, belong to the sub-class
of hornblende-hypersthene-andesites, and are described on page 301. In
the Na Raro rock the rhombic pyroxene is represented in the groundmass.
With regard to the relative age of Na Raro I am inclined to think that
it is the most recent of the acid andesite peaks of the island. Neither
vitreous nor vesicular rocks came under my notice in its vicinity;
whilst the tuffs that clothe its lower slopes are non-pumiceous, though
of dacitic origin, but containing also a few fragments of a semivitreous
basic andesite showing tiny felspar lathes and augite-granules. Since
the everywhere prevailing submarine palagonite-tuffs and foraminiferous
clays do not extend over its area, we may assign to it a later date. It
is evidently also posterior in time to the basaltic andesites and
aphanitic augite-andesites around, which are covered by these submarine
deposits. Relatively recent as it apparently is, this mountain bears the
impress of a high antiquity. There is nothing to indicate that this
“core” of a volcanic mountain belonged to a subaerial vent. Na Raro has
shared in all the later stages of the submergence and emergence of the
island. Though it presents the final page in the history of the
hornblende-andesite volcanoes, that chapter has been for unknown ages
closed.
THE NA RARO GAP.—Between the Tavia and Va-lili Ranges there is a break
in the mountainous backbone of the island, to which I have given this
name. The greatest elevation is probably not over 800 feet. It is from
the south side of this watershed that the Lango-lango river takes its
rise.
CHAPTER IX
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
THE BASALTIC LOWLANDS OF SARAWANGA AND NDREKETI.
ONE of the most striking features of the north side of the island is the
extensive undulating plain that stretches from the Lekutu river to near
Sealevu on the head-waters of the Ndreketi, a distance of almost 30
miles. In its western half this plain slopes gradually to the sea-coast,
where it is bordered by a broad belt of mangroves. In its eastern half,
from the mouth of the Ndreketi eastward, the lofty Nawavi coast range
intervenes between it and the sea-shore. Its breadth varies usually
between 4 and 6 miles, and its elevation, though it reaches a maximum of
about 300 feet, is as a rule between 100 and 200 feet above the sea.
Over nearly all its area it presents the dried-up and scantily vegetated
appearance of the “talasinga” regions. It is an open country mostly
clear of forest; and it is to this character as well as to its peculiar
vegetation that it in some measure owes its barren look. Amongst the
bracken, grass, and tall reeds (Eulalia japonica) that clothe much of
its surface flourish the Pandanus, the Casuarina, and the Cycad, which
give a special physiognomy to the whole area; whilst several sea-side
plants, as Ipomea pes capræ, Morinda citrifolia, Cerbera odollam, &c.,
have spread themselves far and wide over its extent. It is traversed by
the rivers Ndreketi, Sarawanga, and Lekutu, the two first named being
navigable for several miles, as the tide ascends a long way from the
coast.
In its essential characters this region corresponds with the Mbua and
Ndama plains at the west end of the island, which have been previously
described. Wherever the rivers have worn channels of any depth, basaltic
rocks, sometimes columnar in structure, are exposed; and over most of
its surface the same rocks are displayed, often much decomposed and
developing a spheroidal character, or lying in large blocks all around.
Overlying the basaltic rocks in various localities occur foraminiferous
clays and other submarine deposits. This great region of plains is
partially divided into two by the projecting mass of the dacitic
district of Ndrandramea, the slopes of which descend to within 3 or 4
miles of the coast between the Sarawanga and Ndreketi rivers. For
convenience of description I will deal with these two sub-regions
separately under the names of the Sarawanga and Ndreketi plains.
THE BASALTIC PLAINS OF SARAWANGA.— These plains extend about 6 miles
inland to the village of Tembe-ni-ndio on the head-waters of the
Sarawanga river. The prevailing type of basalt in this region is a
porphyritic olivine-basalt showing a few large crystals of glassy
plagioclase and having a specific gravity of 2·84 to 2·9. They are
neither vesicular nor scoriaceous and are referred to genera 25 and 37
of the olivine class. The felspar-lathes of the groundmass average
·2 mm. in length, and there is a little interstitial glass. They cannot
often be distinguished in their characters from the olivine-basalt
displayed in vertical columns, 4 to 5 feet in diameter, on the lower
slopes of Seatura at the back of Tembe-ni-ndio (page 63). It is highly
probable that most of the basalts of these plains belong to lava-flows
that descended from the great Seatura vent. In the lowlands it is much
decomposed, and a spheroidal structure is frequently developed during
the disintegrating process, just as has been noticed in the case of the
Mbua and Ndama plains on the west side of Seatura. The rounded blocks
that commonly occur on the surface may be regarded in each instance as
the nucleus of a weathering spheroidal mass. When this rock is exposed
unaltered in the streams it is usually massive or non-columnar.
There is a less common type of basalt in this region which perhaps may
represent the upper portion of these basaltic flows. I found it exposed
in the bed of the Selesele river about half-way between Lekutu and
Sarawanga and about 2 miles inland, where it formed vertical columns 1½
feet across. It differs principally in the presence of a few small
amygdules and in the greater amount of interstitial glass. The columnar
basalt that Dana in the “Geology of the United States Exploring
Expedition” describes as occurring at the mouth of the same river
probably belongs to the same flow. He remarks that a few hundred yards
back from the “Watering-place” there is an exposure of columnar basalt,
the columns being vertical, 1 to 2½ feet in diameter, and usually
six-sided.
The incrusting submarine deposits found in patches over these plains are
generally calcareous clay-rocks containing tests of formaminifera and
often also univalve, bivalve, and pteropod shells. They are referred to
the foraminiferous mud-rocks described on page 321. Such deposits are
properly dark-coloured; but as exposed at and near the surface they have
often lost by hydration most of their lime, and have acquired by the
removal of the iron oxides a whitish or pale-yellow appearance, whilst
they have a peculiar soapy “feel,” on account of which they are
generally known as “soapstone” amongst the whites. Streams flowing
through such districts have a somewhat milky colour. These deposits are
extensively represented on the slopes of the Sarawanga valley, and
especially to the east of the town of that name. They are well displayed
on the way from Sarawanga to Tembe-ni-ndio, and are also to be seen on
the surface of the plains between Lekutu and the Mbua-Lekutu watershed
to the southward.
In the vicinity of Sarawanga they attain an elevation of 200 feet above
the sea; but they may be traced in patches up to 500 feet on the
adjacent slopes of the acid andesite region of Ndrandramea. Near the
river, and less than 100 feet above the sea, these deposits are in one
place overlain by an agglomerate formed of large blocks, 1 to 2 feet
across, of these Ndrandramea andesites and dacites. In another place,
near the town of Sarawanga, I found them exposed in the river-bank,
where they were covered over by a coarse palagonitic bedded tuff,
dipping gently eastward and somewhat calcareous. From the character of
the shells of marine univalves inclosed in this tuff, it appears to have
been formed in shallow water.
A very interesting display of these surface marine deposits occurs in
the upper part of the Sarawanga valley in the vicinity of Tembe-ni-ndio.
Here we have fine and coarse calcareous palagonitic tuffs, containing
tests of foraminifera, associated with impure foraminiferal limestones.
They occur up to elevations of 300 feet above the sea on either side of
the Sarawanga valley above this town, incrusting on the north side the
lower dacitic slopes of the Ndrandramea district, and on the south side
the lower basaltic slopes of Seatura. At the bottom of the valley, as in
the rising ground between Tavua and Tembe-ni-ndio, they conceal in part
the basaltic rocks of the district.
Near the last-named place, on the right bank of the Tembe-ni-ndio branch
of the Sarawanga river, the foraminiferal limestones are displayed in
low cliffs 15 to 20 feet in height. They are sometimes earthy when they
contain about 25 per cent. of lime, and at other times more compact with
about 45 per cent. of lime, the residue being composed of palagonitic
materials, tiny fragments of minerals and of a basic rock, &c.[59] Large
shells, of Ostræa and Cardium are also contained in these limestones,
the valves being detached from each other. The oyster shells project
from the weathered surface; and it is probable that the name of
Tembe-ni-ndio, which signifies “the shell of the oyster,” may be thus
explained. Underneath the foraminiferal limestones in this locality
occur bedded coarse tufaceous sandstones, slightly inclined E.N.E., and
inclosing waterworn gravel and pebbles. These low limestone cliffs,
although about six miles inland, are not more than 120 or 130 feet above
the sea. In their face there is evidence of an old erosion-line of the
river 10 or 11 feet above its present level.
By following up this branch of the river for a little distance I came
upon an exposure of nearly horizontal bedded palagonitic tuffs on its
floor and sides. Here a coarse tuff, of which the larger fragments
composing it range between 3 and 5 mm. in size, passes upward into a
chocolate-coloured compact tuff-clay formed of the same materials, the
larger averaging ·2 or ·3 mm. in size. These tuffs are made up chiefly
of a palagonitised vacuolar basic glass, the vacuoles being filled with
the alteration products. The lower coarse tuffs contain very little
lime, probably not over 1 per cent., and exhibit no organic remains in
the slide. The upper fine tuffs have 3 or 4 per cent. of lime, and
inclose numerous minute tests of foraminifera of the globigerina type,
their cavities being generally filled with palagonitic material.
Further up the valley about a mile above Tembe-ni-ndio, and about 250
feet above the sea, the impure foraminiferal limestones again appear;
but they here exhibit an important difference in texture. In the
groundmass of those of the lower locality, the calcite is granular and
loosely arranged, or displays in an obscurely indicated mosaic the
commencement of recrystallization. In the case of those of the upper
locality the calcitic material of the groundmass has more completely
recrystallized, and shows a fairly clear mosaic; whilst in one place the
rock was overlain or rather incrusted above by a layer, 3 inches thick,
of a white crystalline limestone, looking like statuary marble, and
inclosing portions of a material like that of the rock beneath it. This
last, when examined in the slide, exhibits itself as formed in mass of
crystalline calcite, displaying a regular mosaic, and inclosing small
fragments of palagonitised materials and of minerals (pyroxene) such as
are abundant in the rock below. In places the grains of the mosaic are
bordered by brown and black iron oxide. It would, therefore, appear that
a metamorphism has been in operation here, and that the process which
began with the recrystallization of the matrix in the lower rock is
almost completed in the overlying thin layer where even most of the
non-calcareous materials have disappeared. No evidence suggestive of
contact-metamorphism came under my notice in this locality. These
foraminiferal limestones are surface formations, and it was in the
uppermost portion that the metamorphism was most complete. We here
witness in operation the transformation of a rock containing 46 per
cent. of carbonate of lime (the residue of minerals, palagonite, &c.),
into a marble or crystalline limestone. I gather that as in the instance
of several of our old British limestones the change is a purely
interstitial one, and is not connected with thermal metamorphism.
These remarks on the basaltic plains of Sarawanga and on their
incrusting submarine deposits may be concluded with a brief reference to
the siliceous concretions, 2 or 3 inches across, the silicified portions
of corals, and the fragments of clay iron-stone and limonite resembling
hæmatite, that occur frequently on the surface. They are common on the
plains south of Lekutu and between Lekutu and Sarawanga, and up to
elevations of 200 feet in the foraminiferous clay district east of
Sarawanga, where fragments looking like portions of the silicified
branches of Madrepores are to be found; but they are not limited to such
localities, and may occur also where the surface is formed of decomposed
basaltic rock. (These matters are generally discussed in Chapter XXV.)
THE BASALTIC PLAINS OF THE NDREKETI.—This low-lying region of rolling
“talasinga” country now serves as the basin of the Ndreketi river, the
largest of the rivers of Vanua Levu. It is usually elevated between 100
and 300 feet above the sea, and its limits are well defined by the 300
feet contour line in the map of the island. On the east it is separated
from the basin common to the Wailevu and Lambasa rivers by the Sealevu
Divide, which is described on p. 136. On the west, as before observed,
it is only in part distinguished from the basin of the Sarawanga by the
spur descending from the dacitic mountains of Ndrandramea. It meets the
coast in the vicinity of the mouth of the Ndreketi; but for two-thirds
of its length it is cut off from the sea by the great Nawavi range. It
supports the characteristic vegetation of the “talasinga” or sun-burnt
land. Whilst the Pandanus and the Casuarina are most conspicuous amongst
the trees, bushes, herbs, grasses and ferns predominate. Here the native
Ginger and the native Turmeric with species of Tacca are frequently to
be recognised, and the waste-land bushes of Dodonæa viscosa and Mussænda
frondosa are abundantly to be found.
As in the Sarawanga plains, the basaltic rocks are here often overlain
or incrusted by submarine deposits, the former exposed in all the deeper
river-beds, the latter frequently displayed in the sides of their
tributaries.
I will deal first with the basaltic rocks. In the places where the
surface deposits have been stripped off, these rocks are generally
exposed as decomposing boulders, the spheroidal structure being well
developed in the weathering process. Not infrequently, however, a rudely
columnar structure is exhibited where the rivers have cut deeply into
the basalt. The columns that I observed were usually vertical. In the
river-bed at the landing-place at Mbatiri, for instance, the columns are
from 2½ to 3 feet across and vertical. As exposed in the river-crossing
about a mile above this town they are 12 to 15 inches in diameter and
also vertical. However, at Na Kalou, a coast village about 1½ miles east
of the mouth of the Ndreketi, where there is an unexpected exposure of
basalt, the columns, about a foot in diameter, are inclined at an angle
of about 20° from the vertical and face to the north.
These rocks are, as a rule, compact, only showing a typical scoriaceous
structure in the case of specimens obtained near the foot of Nakambuta,
an isolated hill about three miles to the southward of Natua, which
probably represents a vent of more recent times. Often, however, they
have a pseudo-vesicular appearance, from the occurrence in the midst of
the patches of interstitial glass of minute irregular cavities that seem
to have been formed during the last stage of consolidation of the magma.
The prevailing type of basalts is a blackish, doleritic, semi-ophitic
rock without olivine, with specific gravity 2·78 to 2·80. They are
characterised by the length of the felspars of the groundmass
(·22-·35 mm.), by the large size of the augite granules (·1-·3 mm.), and
by the quantity of dark interstitial glass. They present two forms, one
with and the other without plagioclase phenocrysts. The first kind is
referred to genus 9 of the augite-andesites (page 272), some of the
specimens being referred to the porphyritic sub-genus, and others to the
non-porphyritic sub-genus, according to the size of the plagioclase
phenocrysts. The second kind, without felspar phenocrysts, belongs to
genus 12 of the same class (page 275). A good example of the porphyritic
rocks is afforded in the large blocks lying in the stream-beds during
the first half of the way from Ndreketi to Sarawanga.
It may be pointed out here that these doleritic, semi-ophitic basaltic
andesites of the Ndreketi plains differ conspicuously from the
prevailing type found on the slopes of Seatura, on the Sarawanga and
Mbua plains, and on the Wainunu table-land. There we have, as a rule,
olivine-basalts, having a specific gravity of 2·86 to 2·90, and showing
but scanty interstitial glass, the felspars of the groundmass being on
the average not over ·2 mm. in length, whilst the augite granules are,
as a rule, only ·02-·03 mm. in diameter, and the ophitic structure is
infrequent.
The submarine deposits, consisting of foraminiferous clays and coarser
tuff-sandstones, the former being usually beneath, are found at
intervals all over this area. They occur inland as far as Vuinasanga and
Nareilangi, near the base of the mountains of Va Lili and Na Raro,
reaching as high as 300 feet, their place being taken on the mountain
slopes by coarser tuffs and agglomerates. When not weathered they are
more or less calcareous, and contain occasionally marine molluscan
shells, whilst palagonitic debris enter largely into their composition.
The foraminiferous clays, often much bleached by hydration, are well
represented around Mbatiri and in the districts between that town and
Natua and Nareilangi. They are relatively deep-water deposits, and
belong to the type described on page 323. Others, again, as exposed in
the banks of the river at Natua, are chocolate coloured and of the kind
referred to in detail on page 335. These foraminiferous clays in the
region between Natua and Mbatiri are overlain in places by coarse,
almost brecciated, tuffs, formed in part of the debris of acid
andesites, such as compose the not far distant mountain of Na Raro.
Since the massive basaltic rocks are exposed in all the deeper rock
channels of these plains, it is apparent that the overlying submarine
deposits can possess no great thickness. Probably they are never 100
feet thick, and usually far less. In many places, through their
denudation, the underlying basaltic rocks are exposed, and in a
decomposing condition largely form the surface. These deposits as a rule
display bedding, the beds being horizontal or at least only inclined 2
or 3 degrees. This horizontality is a nearly constant feature of these
submarine beds, as they overlie the basaltic rocks of the plains; and it
is a feature we should expect to find where there has been emergence
rather than upheaval.
Siliceous concretions and silicified coral fragments, so characteristic
of the surface of some of these plains of Vanua Levu, did not frequently
come under my notice here. They, however, occur occasionally, as in the
district between Nanduri and Natua.
THE NAWAVI RANGE.
With this remarkable coast range, which fronts the Mathuata sea-border
for a distance of 12 or 13 miles between Ravi-ravi Point and Nanduri, I
have unfortunately but scant acquaintance. It attains its maximum
elevation in Mount Nawavi of 2,238 feet, and is described by Mr. J. P.
Thomson,[60] who surveyed this coast, as broken in two nearly opposite
Niurua, the pyramidal mountain of Koro Navuta rising in the gap. Various
other peaks, besides that of Nawavi, are marked in the latest Admiralty
chart; they vary in height from 1,000 to 1,700 feet. As this range lies
only a mile or less back from the beach, it gives to the sea-border a
bold and often precipitous appearance, which is well shown in an
illustration in Wilkes’ _Narrative of the United States Exploring
Expedition_ (iii. 226).
Basic rocks probably prevail in this range. When I ascended its eastern
spurs from Nanduri, and reached a height of 800 feet, only basic tuffs
and agglomerates came under my notice. From Dana’s remarks[61] it is to
be inferred that the “frowning bluffs” opposite Mathuata Island are of
similar formation; and it would seem that the rugged black stones,
described in the _Admiralty Sailing Directions_[62] as topping the hills
behind Ravi-ravi Point, are of the same basic character. From its
contour and profile I would gather that, as in the great mountainous
ridges that constitute the backbone of the island between Va Lili and
Mount Thurston, palagonitic tuffs and clays of submarine origin will,
together with volcanic agglomerates, be found far up the slopes of this
range, and that the axis will prove to be largely composed of massive
basic rocks.
The hot springs referred to by Thomson and others as occurring at the
foot of the north and south slopes, namely at Vatuloaloa, Nambuonu, and
in another unnamed inland locality, are briefly mentioned on page 31.
THE SEALEVU DIVIDE.—This broad range which separates the Ndreketi and
Lambasa basins is an offshoot from the central mountains at Sealevu and
reaches the coast just east of Nanduri. Its highest part according to
the elevation given in the Admiralty chart is 1,437 feet. The road from
Sealevu to Nanduri, which crosses its broad level summit for a distance
of about three miles, does not rise over 1,100 feet. Between 800 and
1,100 feet are exposed calcareous tuffs and clays all largely made up of
palagonitic materials. The coarser might be described as sandstones. The
clays have 12 per cent. of lime and are foraminiferous and are of the
type described on page 321. The rocks displayed on the lower northern
slopes on the way to Nanduri are at first the same submarine deposits,
and afterwards decomposing basaltic andesites. It is apparent that in
the central elevated part of this range there are hills of volcanic
formation more or less completely buried beneath these deposits.
THE DISTRICT BETWEEN NANDURI BAY AND WAILEVU RIVER
The sea-border between Nanduri Bay and Middle Point, nearly four miles
to the east, consists of a fringe of lowland margined by the
mangrove-belts and banked by a line of hills between a quarter and
two-thirds of a mile inland. These hills form a continuation of the
Nawavi coast range of mountains extending from Ravi-ravi Point to
Nanduri. They attain their greatest height in the case of Ulu-i-sori, a
cockscomb-like peak 1,141 feet above the sea. Another of these hills,
Vatu-tangiri, is capped by a remarkable obelisk-like rock. Behind this
coast range lies a hill with an elevation of nearly 1,400 feet.
The rocks exposed for the first mile or two along the coast east of
Nanduri are agglomerates and basic tuffs. The blocks of the
agglomerates, however, are made of an altered grey porphyritic rock
which has the characters of a porphyrite of a rather acid type.[63] This
composition of the agglomerate is quite exceptional and indicates the
antiquity of the volcanic rocks in this locality. Farther along the
coast the typical agglomerates occur, where the blocks, 3 to 10 inches
across, are composed of the usual semi-vitreous black basaltic rock
showing plagioclase phenocrysts. Nearer Middle Point a decaying
doleritic basalt is displayed at the surface. It is similar to the
prevailing rock of the Ndreketi plains, and is referred to the ophitic
rocks forming genus 9 of the augite-andesites.
The elevated promontory of Middle Point is a prolongation of a spur of
Ulu-i-sori. Where it is crossed by the road it is about 350 feet above
the sea. On its west slopes are exposed yellowish-white tuff-like rocks,
evidently the prevailing basic clay-tuffs which have become bleached
through the hydration accompanying the weathering process. Beneath these
deposits lies an amygdaloidal augite-andesite which is bared in places.
The rock is semi-vitreous and the amygdules it contains are often a
centimetre long. They are composed of a white mineral with fibro-radiate
structure and made up of needle-like prisms. It gives off water, but it
is not easily fused, and does not gelatinise in HCl.
From the top of the promontory the road strikes inland in an
east-south-east direction for Tambia, passing inside the coast range,
which is here 600 feet in height, and descending gradually through a
region of basaltic andesite into the valley of the Tambia river. (This
rock, which has a specific gravity of 2·84, displays more or less
parallel stout felspar-lathes, ·23 mm. in length, and has a little
interstitial glass. It belongs to genus 13 of the augite-andesites.) Low
hills shut in the little valley on all sides except where the river
breaks through the coast range. The town of Tambia is not over 100 feet
above the sea. About a mile to the north exist hot springs of
considerable extent which are described on page 32.
The road from Tambia to the Wailevu River traverses an undulating
district varying from 100 to 300 feet above the sea. A basalt containing
a little olivine, with a specific gravity of 2·91, is commonly exposed
at the surface in a disintegrating condition. Here and there occur basic
tuffs. In one locality, there is displayed a dyke-like mass in a small
stream course, 200 feet above the sea, of an altered grey and compact
andesite marked with parallel red streaks or bands. It is an aphanitic
augite-andesite; and is to be referred to genus 13 of the augite
sub-class. It displays closely crowded felspar-lathes, ·07 mm. in
length, in flow-arrangement. The bands are due to the gathering of the
residual glass in streaks parallel to the flow. Chalcedonic flints, some
of them showing the agate-structure, together with fragments of
silicified corals, are found occasionally on the surface in this
district.
THE LAMBASA PLAINS
These remarkable inland plains, about ten miles long and three to five
miles broad, are well described in the Admiralty chart as a low
undulating country covered with grass, screw-pines, and Casuarina trees.
They are backed by the mountains forming the central axis of the island,
whilst broken groups of hills, not usually more than 500 or 600 feet in
height and attaining in Ulu-i-Mbau an elevation of 1,160 feet, intervene
between them and the sea-border. They are traversed by the Wailevu,
Lambasa, and Ngawa rivers which after breaking through the seaward
hill-ranges pass through broad mangrove-belts to reach the coast. The
tide ascends these river-courses for several miles; and in the case of
the Lambasa river boats can follow its winding course for ten miles
penetrating into the heart of the plains. Much of this level inland
region is less than 100 feet above the sea; whilst the contour line of
300 feet by which the region is defined in the map attached to this book
fairly well indicates the higher levels.
The features which we have described in the instances of the Sarawanga
and Ndreketi plains are in the main reproduced in the Lambasa region;
but in the last-named each of the three rivers has a system of hot
springs along its course, namely (as described in Chapter III.), at Na
Kama on the Wailevu River, at Vuni-moli on the Lambasa River, and at
Mbati-ni-kama on the Ngawa River. Basaltic andesites, often exposed at
the surface in a decomposing condition, form the foundation of the
plains. They are overlain by submarine clays containing pteropod shells
and tests of foraminifera; and over these in their turn coarse
palagonitic tuffs and agglomerate-tuffs are found in places. Formations
still more recent are represented by elevated reef-rock on the seaward
side of the hills that bound the plains. Nodules of chalcedony,
silicified corals, and other siliceous rocks, together with fragments of
impure limonite, lie on the surface over much of this region.
The basaltic rocks of this region rarely show olivine, and belong as a
rule to the basaltic andesites, being referred to genera 13 and 21 of
the augite-andesites, the specific gravity being about 2·8. The
felspar-lathes, ·12 to ·14 mm. in length, are in flow-arrangement, and
the augite is at times semi-ophitic; whilst there is a little
interstitial glass. The basic rocks prevailing between Vatu-levoni and
Vandrani belong to genus 13 of the augite-hypersthene-andesites and have
a groundmass of much finer texture, the felspars only measuring ·05 mm.
Their specific gravity ranges between 2·7 and 2·75.
The overlying foraminiferous and pteropod clay rocks, the so-called
“soapstone,” are exposed over large areas of the surface. A good idea of
the important part they take in the formation of the lower plains may be
formed by visiting the hot springs of Vunimbele, close to Vuni-moli,
which issue from the side of a deep trench cut into these deposits. As
generally displayed at the surface, they have been subjected to so much
hydration in the weathering process that they appear as yellowish-white
clay-rocks deprived of their lime; and it is only now and then that the
remains of foraminifera and pteropods can be detected. They are,
however, fairly well preserved around the base of Ulu-i-mbau in the
vicinity of Koro-wiri, where they contain, besides the shells of
pteropods and foraminifera, portions of decaying coral, and extend to
200 feet and over above the sea. Here they are overlain by rather
coarser basic tuffs of mixed character, containing 5 or 6 per cent. of
carbonate of lime and some palagonite, which I followed as high as the
track lay, rather over 500 feet above the sea.[64] The reef-limestones,
already noticed as exposed in the low hills between Wailevu and Lambasa,
lie a mile or two inland and reach to 100 feet above the sea.
The fragments of siliceous rocks, which with occasional bits of impure
limonite, occur at intervals all over the surface of these plains and
largely form the gravel and pebbles in the river-beds, include nodules
of chalcedony, fragments of jasper or iron-flint, white quartz-rock
formed of chalcedonic silica, silicified corals, &c. They are especially
frequent in the vicinity of Nasawana and Koro-utari, and include fine
specimens of agates and of onyx.
CHAPTER X
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
THE VA-LILI RANGE
THIS range extends from the Na Raro Gap before mentioned to the
Ndreke-ni-wai river. It is partly isolated on the north-east from the
Korotini Range, the extension eastward of the mountainous axis of the
island, by a depression or saddle which at its lowest part is not more
than 1,200 or 1,300 feet above the sea; but there is no real break in
the line of mountains. It is, however, convenient to make this
distinction, and I have named the dip between the two ranges, the
Waisali Saddle. The range now to be described attains its greatest
elevation in the summit of Va-lili, which is 2,930 feet above the sea.
There are two or three other peaks that exceed 2,000 feet, and much of
the range is not under 1,700 feet. My acquaintance with this range is
not extensive; but it will serve to illustrate its general geological
structure.
The summit of Va-lili is very conspicuous from most points of view. From
the north, east, and south-east, it has a remarkable broad and
square-topped profile with a little conical elevation in the centre.
From the south-west, it displays a different outline with a solitary
squarish block on the top, and this is the form most familiar to the
navigator. Unfortunately, for reasons given below, I did not quite reach
the summit, and although I was able to obtain sufficient data for
forming a general idea of the structure of this part of the range, the
structure of the actual summit has yet to be ascertained.
(1) ASCENT OF VA-LILI FROM NARENGALI.—This village, which is elevated
400 feet above the sea, lies about two miles in a direct line, N.N.E.
from the peak. In traversing the intervening country, one crosses the
Loma-loma ridge, elevated 1,000 feet, on the top of which was once
situated the village of Loma-loma visited by Horne in 1878. The rocks
exposed on the surface are scanty, a hard palagonite-tuff, which owes
its induration to a calcitic cement, occurring on the upper part of the
ridge; the original site of the village being marked by a large block of
this stone.[65] The track then descends into the valley of the Loma-loma
river, about 400 feet above the sea, in the bed of which occur blocks of
an amygdaloidal basaltic andesite, containing phenocrysts of both
rhombic and monoclinic pyroxene, and referred to genus 1 of the rhombic
pyroxene andesites. The amygdules are formed of calcite.
PROFILE-SKETCHES OF THE VA-LILI RANGE.
[Illustration: View from the south-east near Savarekareka.]
[Illustration: View from the south-west.]
Beyond the river the ascent of the northern slope of Va-lili begins. As
high as 1,100 feet occur basic agglomerates overlying fine and coarse
palagonite-tuffs, which are at times horizontally bedded, the finer
kinds being sometimes calcareous, and like that of the Loma-loma ridge
above mentioned. At 1,300 feet is a line of tall cliffs which extend for
some distance at intervals along the mountain-slope, and are indicated
by some fine waterfalls. My track struck these cliffs at a place named
“Nangara-ravi” (the leaning cave-rock) where they have a height of 150
feet or more. The tall cliff leans slightly forward, so that it forms a
shelter at its foot, and hence the name. It is composed of a
tuff-agglomerate, the blocks, which are formed of a semi-vitreous
basaltic andesite of the augite class, being not generally more than 3
or 4 inches across. These blocks, which are rounded on the outer exposed
side and angular on the imbedded side, are inclosed in a hard, probably
calcareous matrix. The whole face of the cliff has the appearance of
having been worn smooth by attrition, and there are not to be observed
the projecting blocks from its surface which are so characteristic of
other agglomerate-cliffs. It shows no stratification; but at its base
flush with the cliff-face are large masses of a basic massive rock. But
few portions of rock have been detached from the cliff. However, I found
in the midst of a huge fallen fragment of the agglomerate a dyke-like
mass of a basaltic andesite, which differs chiefly from the rock forming
the blocks of the agglomerate in being more crystalline. This dyke must
have been about 15 feet thick.
Having regard to these various features, I am inclined to consider that
this leaning cliff represents one side of a large fissure in the
agglomerates which was occupied by a dyke. Reference has been above made
to the fact that the agglomerates may be seen overlying the tuffs
farther down the slope, so that the conditions favourable for landslips
exist. I have shown on page 111 that the origin of the Mbenutha cliffs
where agglomerates lie on clayey tuffs may be thus attributed to a
landslip. In the case of the Nangara-ravi cliffs, the occurrence of this
fragment of a large basaltic dyke is of some importance in connection
with the origin of the basic agglomerates of this locality.
The top of the mountain-ridge is about 700 feet above Nangara-ravi, or
2,000 feet above the sea. The tuffs and agglomerates that once existed
here have been stripped off to a great extent and the deeper rocks of
the range are in part exposed. The upper part of this ridge (1,700 to
2,000 feet) is formed of a rubbly pitchstone where a basic glass has
been broken up and then consolidated, the interstices being filled up
with palagonite as described in other cases on page 313. Though
non-vesicular, it is just such a rock as one would expect to find on the
surface of a lava-flow or on the sides of a dyke.
The crest of the range is here only a narrow ridge. I followed it along
in a north-west direction, gradually ascending on the way, and in time
the rubbly pitchstone gave place to a hardened palagonitic clay rock,
which was observed as high as 2,300 feet. It apparently contains a
little lime, and probably was at one time foraminiferous; but it is now
much affected by hydration. Soon after this, we arrived at the foot of
the steep ascent leading to the summit of Va-lili. We were now rather
over 2,400 feet above the sea; but my natives refused to go on, the
heavy rain having made the slope too slippery for a safe ascent. With
much reluctance I retraced my steps; and as the bad weather continued
for several days after, I did not make another attempt. There would,
however, be no difficulty in dry weather.
(2) TRAVERSE OF THE VA-LILI RANGE FROM THE OLD SITE OF LOMA-LOMA TO
WAIWAI ON THE COAST OF SAVU-SAVU BAY.—This route, which was taken by Mr.
Horne, the botanist, in the reverse direction in 1878, is thus described
in his book, _A Year in Fiji_ (p. 19): “The path, rough and apparently
not much used, ran along streams, up steep ascents and down awkward
descents, over slippery boulders and fallen trees, up the sides and
along the crests of densely wooded mountains.”
Ascending the north slope of the range I found at the Tangi-nandreli
cave, which is 1,050 feet above the sea, a palagonite-tuff-sandstone
underlying the basic agglomerate. This tuff, which is of the type
described just below, does not effervesce with an acid, and shows no
tests of foraminifera when examined with a lens. Further up the slopes
large masses of agglomerate occur. At 1,350 feet I came upon a large
mass of a fine-grained compacted palagonite-tuff made up chiefly of
fragments of palagonitized vacuolar basic glass. Among the mineral
fragments occur plagioclase, augite, and rhombic pyroxene, and a little
fresh olivine, which is of very rare occurrence in these
palagonite-tuffs. It contains little or no lime, and shows no tests of
foraminifera in the slide. The summit of the range is here about half a
mile broad, and is relatively level. I placed its elevation at 1,760
feet, which is not far from Horne’s estimate of 1,800 feet. The southern
slope, which is the rainy side of the range, is much cut up into gorges.
In the upper 200 feet palagonite-tuffs, similar to those above referred
to, are displayed, and basic agglomerates occur lower down.... This part
of the range is remarkable through being completely covered over by
palagonite-tuffs and agglomerates. It has been pointed out above that
this is not the case with the range close to the highest peak, where the
underlying rocks are in part exposed at the crest of the range.
(3) THE EASTERN PEAK OF THE VA-LILI RANGE.—This hill, about 1,100 feet
in height, overlooks the Mbale-mbale branch of the Ndreke-ni-wai river.
At its foot near the river there is exposed at the roadside a rubbly
pitchstone formed of a basic glass, inclosing porphyritic crystals of
plagioclase, augite, and olivine, which is described on page 313. Here
also occurs an agglomerate made up of blocks of a semi-vitreous basaltic
andesite (sp. gr. 2·78), showing prismatic pyroxene in the groundmass,
and referred to genus 20 of the augite-andesites.
The upper part of the hill displays the same agglomerate, and a
tuff-agglomerate in which small fragments of the basaltic andesite are
inclosed in a matrix largely made up of fine debris of basic glass.
There protrudes through these detrital rocks at the top of the hill a
broad dyke-like mass of the same basaltic andesite that forms the
agglomerate around; and the structure of the hill is thus displayed as
that of an old volcanic neck. It has evidently an axis of massive
basaltic rocks, more or less covered over with agglomerates and tuffs.
(4) THE NAMBUNI SPUR.—This singular spur runs down to the coast between
Waiwai and Wailevu; but it is partly separated by a deep gap from the
main range. It attains a height of 550 to 600 feet, and has two little
peaks which the natives call Vatu-tolutolu and Vatu-tangitangiri. Its
position is shown in one of the profile-sketches of Va-lili, given on
page 141. The crest of the spur is formed by a dyke-like mass, 25 to 30
feet thick, which is composed of a basic agglomerate passing down into a
palagonite-tuff. The blocks of the agglomerate are composed of a
semi-vitreous basaltic andesite, showing minute felspar-lathes in
flow-arrangement in an abundant smoky glass, the fine pyroxene being not
differentiated. The tuff, into which the agglomerate passes down is
non-calcareous, and displays no organic remains. It is, however,
composed of fragments, which do not generally exceed a millimetre in
size, of palagonitised vacuolar glass, basic andesites, plagioclase,
monoclinic and rhombic pyroxene, &c.
This dyke-like mass forms the axis of the ridge and protrudes vertically
about 100 feet, the bulk of the spur being composed of a compacted
brecciated palagonite-tuff made up mainly of fragments a centimetre in
size, of a basic vacuolar glass, sometimes fibrillar, which is
extensively palagonitised.
The filling up of a fissure in a mass of tuff-breccia by
palagonite-tuffs and agglomerates probably occurred during the
submergence, the original dyke-rock having been removed by marine
erosion. After the emergence the subaerial denuding agencies reshaped
the surface, and as a result of the less yielding character of the
materials filling the fissure, they protrude as a dyke-like mass from
the crest.
In a cliff-face of the adjacent main range there are displayed an
agglomerate of basaltic andesite and a pitchstone-breccia, composed
of fragments of but little altered basic glass, the interstices
being filled up with palagonite. In the case of the Kiombo flow I
have endeavoured to explain the origin of a closely similar
pitch-stone-breccia (page 92).
(5) THE SEA-BORDER AND THE LOW-LYING DISTRICTS AT THE BASE OF THE
VA-LILI RANGE.—It may be generally remarked that palagonite-tuffs and
clays, often foraminiferous, prevail in these localities. Thus in the
sea-border between Waiwai and the mouth of the Ndreke-ni-wai basic
agglomerates are displayed where the mountains approach the coast; but
further west a broad tract of undulating land, elevated usually 100 to
300 feet, intervenes between the range and the sea-border, and here
coarse and fine palagonite-tuffs predominate.... On the north-west the
foraminiferous tuffs and clays of the Ndreketi plains approach the
Va-lili range in the vicinity of Vuinasanga, and extend for at least 200
or 300 feet up its sides.... At the east end of the range, where the
slopes descend to the plains of the Waisali valley, a little west of
Mbale-mbale, there are exposed bedded palagonite-tuffs, tilted up at an
angle of about 20° to the south-west. They contain a little lime and
display microscopic tests of foraminifera, the palagonite being minutely
vacuolar, the cavities also being filled with the altered glass. I
noticed those submarine deposits at an elevation of 100 feet, but
probably they reach much higher.
The inference to be drawn from the data above given concerning the
Va-lili range seems clearly to be this. We have here indicated the
emergence of a submarine mountain-ridge covered over with
palagonite-tuffs and agglomerates, the last being uppermost. These
coverings have been in places stripped off by the denuding agencies and
the underlying massive basic rocks exposed. These rocks, however, vary
much in texture, some being vitreous, as in the case of the pitchstones,
others hemi-crystalline as in the case of the basaltic andesites; and it
is to be gathered from this and other similar indications that different
submarine vents were formed along a fissure or fissures at the
sea-bottom. No evidence of subaerial eruptions came under my notice.
After the vents became extinct they were buried beneath the
palagonite-tuffs and agglomerates. During and after the emergence the
denuding agencies reshaped the surface of the range and left but little
of its original form.
Since it is my object to build up a theory of the origin of the
ridge-mountains as I proceed with the systematic description of the
island, it will be here convenient to follow up the preceding remarks on
the Va-lili Range by a preliminary reference to the great ridge district
lying east of it.
When a panoramic view of this region is obtained, one observes a series
of lofty ridges more or less parallel and running about N.W. and S.E.
There are the Va-lili, Narengali, and Sealevu ridge-mountains with
lesser ridges between. The intervening valleys are elevated about 400
feet above the sea, whilst the mountains rise up to over 2,000 feet. In
many localities this configuration of the surface would be attributed
mainly to subaerial denudation. In this island I will endeavour to show
that these mountain-ridges existed before the emergence. They do not owe
their form to the rivers that flow through the valleys, though no doubt
river-erosion has brought these features into greater relief.
In Vanua Levu, as there will be frequent occasion of showing, rivers
often flow in valleys that they have not made. This is especially
pointed out on page 151; and it is necessary to emphasise it here,
before proceeding farther with the description of the geological
structure of the mountain-ridges.
THE WAISALI SADDLE
This saddle, which connects the Va-lili and the Koro-tini ranges, has
probably a minimum elevation of not over 1,200 or 1,300 feet. To
understand this district thoroughly a regular survey is, however,
necessary. It is only at times in this densely wooded range that a view
of the surrounding country is obtained; but in spite of this drawback I
was able by a diligent use of watch, aneroid, and prismatic compass, to
obtain a fair general notion of the surface-configuration.
The track that proceeds westward from Waisali to Narengali leads also to
the villages of Na Sinu and Sealevu. About 1½ or 2 miles from Waisali,
the track branches off to the westward for Narengali and to the
northward for Na Sinu and Sealevu. After half an hour’s walk along this
last-named path, one comes to a place where at an elevation of about 900
feet it branches off to the left for Na Sinu, crossing the lowest part
of the saddle, and to the right for Sealevu across the Koro-tini Range.
It may here be remarked that since the natives are gradually abandoning
their mountain-villages and are settling at the coast, many of the
mountain-tracks used by me will before long be overgrown and forgotten.
In taking the path from Waisali to Narengali one soon enters the hilly
country where large masses of basic tuffs and basic agglomerates, the
last formed of blocks of a compact basaltic andesite, occur on the
surface up to 700 or 750 feet above the sea. The rock just named has a
specific gravity of 2·84, and since it displays rhombic pyroxene amongst
its phenocrysts, it is placed in genus 1 of the hypersthene-augite
andesites. Above this elevation, and as far as the top of the range,
1,800-1,900 feet above the sea, porphyritic basaltic andesites, having a
specific gravity of 2·8, prevail at the surface. They display small
porphyritic crystals of plagioclase, augite, and rhombic pyroxene in a
groundmass composed of small felspar-lathes, prismatic pyroxene, and
much smoky glass, and are referred to genus 5 of the same pyroxene
andesites. It is probable, judging from one of these exposures, that
such rocks are dyke-like masses: but on account of the thick soil-cap it
is not possible to obtain a good view of them.
In the stream-courses occur large blocks of altered basaltic andesites
of the propylitic type, having a specific gravity of 2·64 to 2·70, and
exhibiting abundant alteration products, such as calcite, viridite, &c.
These propylites, I presume, constitute the deeper portion of the range.
It will often be necessary to distinguish between the altered basaltic
andesites, such as are above referred to, and the relatively fresh rocks
of the same type. The former are light coloured (sp. gr. 2·6 to 2·75),
and are only exposed in gorges and stream-courses that deeply score the
mountain-slopes. The latter are blackish (sp. gr. about 2·8), and at
times penetrate the covering of tuffs and agglomerates.
Descending the opposite or north-west side of the saddle-range, one
finds the same basic andesites, both fresh and altered, down to about
1,100 feet above the sea. Then the track leads one down a precipitous
slope into the picturesque gorge traversed by the head-waters of the
Narengali River. At its lower end the gorge opens out into the broad
Narengali valley, and here the dense forest of the higher districts
gives place to the scanty vegetation of the “talasinga” region.
The rocks exposed in the sides of the gorge are basic agglomerates
overlying palagonitic tuffs of mixed composition and evidently
sedimentary. On the bottom lie huge masses, some of them 70 or 80 tons
in weight, of altered grey aphanitic or non-porphyritic
augite-andesites, penetrated in some cases by thin veins of white
quartz, and at times displaying a rudely columnar structure, the columns
being 12 to 14 inches across. Sometimes the alteration is mainly
confined to the filling of the fissures with chalcedonic quartz, minute
nests of the same material occurring in the groundmass. At other times
the small augite granules are also decomposing. The specific gravity
varies from 2·64 to 2·73; the rocks being referred to genus 16, species
A, of the augite-andesites. Occasional detached masses of a propylitic
basic andesite, displaying porphyritic plagioclase and pyroxene, also
occur in this gorge, the felspar phenocrysts being largely occupied by
calcitic and other alteration products, whilst much viridite occurs in
the groundmass. It exhibits both monoclinic and rhombic pyroxene; and on
account of the prism form of the groundmass pyroxene it is placed in the
2nd sub-order of the hypersthene-augite andesites. These altered rocks
are deep-seated intrusive masses that were originally covered over by
the basic agglomerates and palagonite-tuffs exposed in the sides of the
gorge.
Below the gorge there is an extensive exposure in the sides and bed of
the river of light-coloured calcareous tuffs which were originally
composed of palagonitic materials; but owing partly to hydration, and
partly to other secondary changes, the original structure is much
disguised.
Crossing the river in the midst of these tuffs there is a dyke, 15 feet
thick, formed of a propylitic basaltic andesite, a semi-vitreous rock in
which calcitic and zeolitic materials have been developed in quantity.
The dyke, which is not columnar, is steeply inclined at an angle of 45°
to the north-east.... Further down the river-valley as far as Narengali,
occur basic tuff-agglomerates.
THE TRACT OF NAKAMBUTA
This is a tract of broken country that projects from the mountainous
backbone of the island (between the Va-lili and Koro-tini ranges) into
the heart of the Ndreketi plains in the vicinity of Natua. As limited by
the 300-feet contour line, it is indicated in the map attached to this
work. Its general level varies between 300 and 600 feet in elevation;
but a number of isolated peaks are included within this area. More than
one of these hills attain a height of 1,000 feet, Nakambuta a very
conspicuous hill being as much as 1,500 feet. Basaltic andesites with
basic agglomerates and palagonite-tuffs prevail.
Towards Natua the basaltic andesites, which are often much decomposed,
are of the doleritic type referred to in the account of the Ndreketi
Plains on page 133. Inland, towards Narengali and Va-lili, these rocks
are often more or less glassy and take the form of pitchstones; whilst
the agglomerates have the same character. The first probably represent
submarine flows of basaltic lava which have spread far and wide over the
Ndreketi plains. The inland rocks are, as is pointed out below, the
products of vents that, as in the case no doubt of Mount Nakambuta, rose
out of a shallow sea. The palagonite-tuffs and clays, often
foraminiferous, which cover the Ndreketi plains, are extensively
represented in the lower levels up to 400 feet or more.
Between one and two miles to the westward of the Narengali valley, and
immediately north of Va-lili, the agglomerates, overlying
palagonite-tuffs, form lofty precipices. The agglomerates are composed
of blocks of more or less vitreous basaltic-andesites, some of them
semi-vitreous and amygdaloidal, some in the form of pitchstone, and
others again as tachylyte that fuses in the lamp-flame. The underlying
palagonite-tuffs are bedded, and are composed of fragments of basic
glass that originally inclosed porphyritic crystals of plagioclase. In
the slide it is observed that the glass and mineral fragments have often
been re-fractured as they lie in the tuff and that the former have
rounded angles and eroded edges. The interstices are filled with a more
or less palagonitised magma. Similar rocks occur in other localities,
and they will all be found described on page 334. It may, however, be
remarked here that in all cases these rocks would seem to have undergone
some crushing, the heat developed in the process being sufficient to
partially remelt the glass. A high temperature was not required to
effect this fusion, since splinters of the tachylyte occurring in the
overlying agglomerate fuse in an ordinary lamp-flame. It is pointed out
on page 341 that tuffs of this character differ in origin and in
characters from the prevailing foraminiferous palagonite-tuffs.
The road from Narengali to Natua traverses the length of this district.
At and near the mouth of the Narengali valley there are exposed basic
tuffs and agglomerates, the blocks in the last case being formed of a
semi-vitreous, vesicular or almost scoriaceous basaltic andesite. In
this neighbourhood the track passes across the top of a waterfall which
is the result of the existence of a huge dyke-like mass of a compact
basaltic andesite showing a little interstitial glass and referred to
genus 13 of the augite-andesites. It lies in a district of tuffs and
agglomerates. Farther on, about two miles north-west of Narengali, the
track crosses some rounded hills, elevated about 600 feet, on the top of
which is displayed a concretionary pitchstone, showing little nodular
concretions of the size of filberts, and having the microscopical
characters of “variolite,” as described on page 313. This is the only
locality of this rock that is known to me.
My acquaintance with the tract of Nakambuta is, however, very imperfect.
But it is apparent that in the pitchstones and in the semi-vitreous
basic rocks, sometimes vesicular and amygdaloidal, we get a nearer
approach to the products of subaerial eruptions than is to be observed
in most other portions of the island. The examination of the Nakambuta
peak by some future investigator will bring to light some interesting
facts concerning this region. It is not unlikely that during a late
stage of the emergence of this region Nakambuta and the other peaks
around protruded as active vents above the surface of a shallow sea, at
the bottom of which the products of their eruptions accumulated.
THE VALLEY OF THE NDREKE-NI-WAI AND ITS TRIBUTARIES
Ndreke-ni-wai, which signifies “the hollow of the water,” is the name of
a broad tidal estuary, opening into Savu-savu Bay, which is formed by
the union, about half a mile above its mouth, of two rivers, the
Mbale-mbale River flowing from the north-west past a village of that
name, and the Vatu-kawa River, the largest, flowing from the eastward,
which I have also named after a village on its banks. The valleys of
these two rivers are separated by a mountainous dividing-ridge connected
by a saddle with the main range. Its highest peaks rise to 2,100 feet
above the sea, the elevation of this “divide” rapidly decreasing as it
approaches the coast, where, within a mile of the beach, it terminates
in some low hills 200 or 300 feet in height.
It may be observed here that a mouth of the river was originally
situated 700 or 800 yards to the west of its present site. This old
mouth is now represented by a lagoon communicating with the Mbale-mbale
River above, but closed by the sand-mound of the beach at its lower end,
which, however, is occasionally broken through when the rivers are in
flood. This lagoon is shown in the view facing page 153.
The valley of the Mbale-mbale River, which is much the smaller of the
two rivers, is bounded on the north by the precipitous slopes of the
Koro-tini Range, which rise to over 2,000 feet, and on the south and
west by the lofty Va-lili Range. The valley, above the village of
Mbale-mbale, is broad and low-lying; and one can ascend it to the
vicinity of Waisali, three to four miles from the river’s mouth, without
attaining an elevation of 100 feet above the sea. The main stream, which
flows down from Waisali, is joined near Mbale-mbale by a more impetuous
stream that descends the steep mountain-sides just to the east of the
Koro-tini Bluff.
The valley of the Vatu-kawa River is bounded by lofty mountain-ranges
that rise to elevations varying from 2,000 to 3,500 feet. On the south
side lies the Mariko Range, on the east lies Mount Mbatini, the most
elevated peak of the island, whilst on the north rise up the steep
slopes of the Koro-tini Range and of the mountainous “divide.” The
valley has such a gentle gradient that one can follow it inland for five
or six miles from the estuary to the vicinity of Nukumbolo without
exceeding an elevation of 100 feet above the sea. Below Na Salia the
valley is confined between the hills that approach the river; but above
that village it is very broad; and on account of its slight fall the
river here often changes its course, so that the floor of the valley is
strewn with water-worn blocks and pebbles marking the old channels.
The Vatu-kawa River, which rises on the west slopes of Mbatini, flows
with a placid current past Nukumbolo and Na Salia, until it reaches the
village of Vatu-kawa, where it is joined by its impetuous tributary, the
Wai-ni-ngio, “the river of the shark.” This affluent, after descending
the steep slopes of the Koro-tini mountains, bursts through the dividing
range that separates the Mbale-mbale and the Vatu-kawa valleys. It would
seem that the Wai-ni-ngio without any great effort on its part might
become a tributary of the Mbale-mbale River.
The great character of these two valleys, as shown above, is their
little elevation above the sea. For miles inland the level does not
attain 100 feet, and high ranges rise steeply in each case on either
side to 2,000 feet and over. Here, as in the instance of most of the
large valleys of the island, the original configuration of the surface
was not dependent on river-erosion. Rivers no doubt have done much to
carve out the lesser and to deepen and widen the greater valleys; but,
as is often remarked in this work, the main features of the surface were
in existence before the emergence of the island from the sea.
The geological formation of the slopes of these two valleys is described
in the accounts of the various ascents of the mountains bounding them.
Since foraminiferous tuffs occur high up their sides, up to elevations
of 2,000 feet and over, the valleys themselves were at one time no doubt
also covered with these submarine deposits, which, however, have been in
great part stripped off by the denuding agencies. They are still to be
found, containing large tests of foraminifera, between Mbale-mbale and
Waisali; but the basaltic andesites, originally underlying them, are
more frequently exposed. One of these rocks found a little east of
Waisali, which has an aphanitic appearance and a specific gravity of
2·82, is merely a basic glass in its early stage of crystallisation,
being made up of very minute crystallites 1/5000 of an inch in length.
On the surface in this locality there also occur basic agglomerates
containing scoriaceous rocks, the products of some of the last stages of
volcanic action in this part of the island.... In the case of the broad
part of the Vatu-kawa valley above Na Salia blocks of basic rocks
derived from the mountains around strew the bottom in great abundance.
Lower down, where the valley is confined between the hills, basic
agglomerates and coarse tuffs are displayed in the hill-sides.
Mention should be made here of the various hot springs existing in these
valleys in the low levels near the rivers and stream-courses. In the
Vatu-kawa valley they exist at Nukumbolo, and in the Mbale-mbale valley
at Natoarau, Waitunutunu and other localities. These springs are
described in Chapter III.
[Illustration: DUNIUA LAGOON, representing an old mouth of the
Ndreke-ni-wai. Behind rises the Korotini Tableland (2,000-3,000 feet).
The cliff-like declivity over the head of the lagoon is the Korotini
Bluff.]
CHAPTER XI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
THE KORO-TINI RANGE OR TABLE-LAND
THE level-topped range that forms the mountainous backbone of the island
for a distance of nearly 10 miles is one of the remarkable features of
Vanua Levu.[66] In the general profile of the island it is named the
Koro-tini Table-land on account of the level profile which it presents
whether viewed from the north or from the south. But this is merely its
appearance _en masse_. When it is examined in detail it is found that
although much of the range has an elevation between 2,000 and 2,400 feet
above the sea, it attains an elevation of about 3,000 feet in the case
of two gently sloping peaks. With regard also to its table-top, it is
necessary to remark that whilst in some portions of the range the summit
is broad and level, in others it is much cut up into ridges, and in
others again it presents a single narrow crest. Nor can we realise on
looking at the profile the extent to which its slopes have been carved
out by river-erosion, and we get no indication of the several lofty
spurs that descend north and south far into the plains, as in the case
of the spur west of Sueni and in that terminating in the Koro-tini
Bluff. In the profile the eye ignores the details with which the
investigator during many toilsome ascents has filled the pages of his
note-books. To this extent it is useful in that it enables him to rise a
little above the level of his facts, and permits him (to employ a
figure-of-speech) to regard the style and general character of the
edifice without being exclusively absorbed in the study of the bricks.
This range, which extends from a mile or two west of Sealevu to a couple
of miles east of Sueni, is connected on the west with the Va-lili Range
by the Waisali Saddle before described, and on the east with the
Thambeyu or Mount Thurston Range by a broken chain of mountains, of
which Koro-mbasanga is the most conspicuous. It is connected by an
elevated _col_ with Mount Mbatini and the Mariko Range to the southward.
The name of Koro-tini has been applied to this range because it is
familiar to the natives. It signifies “ten towns,” and was given to a
once populous district on the slopes of the lofty bluff overlooking on
the north the mouth of the Ndreke-ni-wai. I crossed the range in four
places, namely, between Waisali and Sealevu, between Mbale-mbale and
Vandrani, between Vatu-kawa and Vandrani, and between Nukumbolo and
Sueni.
(1) TRAVERSE OF THE KORO-TINI RANGE FROM WAISALI TO SEALEVU.—Starting
from Waisali by the Narengali track, I ascended the east slope of the
Waisali Saddle, as described on page 146, until an elevation of about
750 feet was reached, when my way lay to the northward across the
Koro-tini Range to Sealevu. At 850 feet a singular altered tuff was
displayed in position in a stream-course. It shows calcite and pyrites,
and is interesting from the fact that although it is made up largely of
basic glass the tuff does not seem to have undergone the palagonitic
change.
Afterwards, there was a fairly steep ascent to the summit of the range,
2,400 feet above the sea, which has merely a ridge-like crest. Between
an elevation of 1,400 feet and the top there are exposed at the surface
compacted coarse and fine palagonite-tuffs and agglomerate-tuffs formed
of the same materials. They contain often abundant organic remains, such
as valves of “Cardium” and “Pecten” shells, macroscopic tests of
Foraminifera, and some curious scale-like bodies, showing a concentric
structure and about an inch across, which look like fish-scales. It is
probable that these interesting rocks extend to a greater elevation than
2,400 feet, which was merely the highest level reached in the traverse,
but is not the highest point of the range.
These deposits are made up in mass of a more or less palagonitised basic
glass originally containing phenocrysts of plagioclase and pyroxene. The
palagonitic process is nearly always far advanced; but it is seen in all
its stages, the least altered materials fusing under the blow-pipe into
a black glass. The fragments are usually sub-angular in the case of the
coarse tuffs; but small rounded pebbles up to half an inch in size and
fine water-worn gravel are not infrequent. The matrix is composed of
palagonitic debris, portions of crystals of plagioclase and pyroxene,
fine gravel, occasional tests of foraminifera; and it often contains a
fair amount of carbonate of lime, in one specimen tested as much as 13
per cent. The amount of lime, however, varies, being in some places
scanty.
The term “conglomerate” could not be applied to the coarser deposits,
since the sub-angular and angular fragments always predominate. They
could scarcely be deemed “breccias” on account of the mixture with
pebbles and gravel. Their character is therefore intermediate between
the two. I have used the expression “agglomerate-tuff” because it best
describes their appearance. A specimen of such a rock presents a curious
mixture, in the well-compacted mass, of angular and sub-rounded
fragments of palagonite up to an inch in size, small pebbles and fine
gravel of the same material, and detached valves of “Cardium,” entire
and broken. One is forced to draw the inference that these materials
accumulated in shallow water. They are such as might have been produced
by the marine erosion of an emerging volcanic island endeavouring to
hold its own above the waves. But from the occasional occurrence of
blocks of a scoriaceous basaltic rock it would appear that during the
formation of the deposits there were periods of eruption.
At times massive and comparatively fresh-looking basaltic rocks are
exposed _in situ_ on the mountain sides in the midst of these submarine
deposits. A specimen obtained at 1,800 feet is a semi-ophitic
porphyritic olivine-basalt with a specific gravity of 2·86 and showing a
little interstitial glass. The mode of exposure did not admit of my
ascertaining the exact relation of these rocks to the deposits. They are
no doubt dyke-like masses representing the original fissures of eruption
of a submarine vent; and during the emergence they were covered up with
tuffs and deposits, the work of the marine erosion of the emerging land.
These, however, are points on which light will be thrown when we come to
examine other localities.
Descending the northern slopes of the range from the summit to Sealevu
the general course was N.N.E. Several valleys were crossed, of which
that occupied by the Na Sinu river was 600 feet in depth, the rivers and
streams all flowing to the north-west into the Ndreketi basin. Basic
tuffs and agglomerates were exposed at the surface all the way down to
Sealevu, 400 feet above the sea.
At the head of the Sealevu valley, about a mile or rather more above the
village, and a little east of the track followed in the descent above
described, the mountain-range terminates abruptly in lofty cliffs 400 or
500 feet in height. At their base, which is about 1,000 feet above the
sea, once stood the village of Lovutu. These cliffs are formed of basic
agglomerate-tuffs which display a horizontal arrangement, but there is
no distinct bedding. They have the castellated appearance that often
characterises horizontally bedded sedimentary formations. The inclosed
rock-fragments vary in size from 18 inches to half an inch and smaller.
The larger are angular or sub-angular, and are composed of
hemicrystalline basaltic andesites, scoriaceous and vesicular and
sometimes amygdaloidal. The smaller fragments are more or less rounded
and of the same material. The matrix is made up of fine detritus of the
large fragments and of lapilli of a vacuolar palagonitic basic glass,
whilst small crystals of calcite fill the cavities and line the
fissures. The phenocrysts of plagioclase and augite inclosed in the
altered glass also display extensive alteration, and in the first case
are largely replaced by calcite, secondary quartz, and other products.
No organic remains came under my notice; but on account of the
alteration of the tuff-matrix their preservation could hardly be
expected. Bearing in mind, however, the fossiliferous character of the
tuffs and agglomerates in the higher part of the range, it can scarcely
be doubted that the agglomerate-tuffs of the Sealevu cliffs are also
submarine.
Each traverse of the great Koro-tini Range will provide us with new
facts to aid us in framing an explanation of the origin of this long
mountain-ridge. The principal lesson to be learned from the journey
across the range from Waisali to Sealevu, and from the visit to the
cliffs, is concerned with the great extent and thickness of these
submarine basic tuffs and agglomerates. From 1,000 feet above the sea up
to the summit, 2,400 feet in height, they are almost the only rocks
exposed, excepting the occasional masses of basaltic rocks, which
probably represent dykes. Their maximum thickness must amount to some
hundreds of feet.
(2) TRAVERSE OF THE KORO-TINI RANGE FROM MBALE-MBALE TO VANDRANI.—In
this traverse the track before ascending to the summit crosses a spur of
the Koro-tini Bluff, and then descends into the valley of the Natoarau
river on the east side of it. It will therefore be convenient to
describe the bluff before giving my description of the journey across
the range.
The Koro-tini Bluff is a lofty headland (if I may so term it), lying
about four miles inland from the mouth of the Ndreke-ni-wai. It attains
an elevation of about 2,000 feet, and terminates above in a line of
precipices 300 or 400 feet in height. It represents the southern edge of
the level-topped mountain range behind, and like the Sealevu cliffs on
the north side it affords a natural section of its mass. It is shown in
the plate facing page 153, where it rises at the back of the lagoon.
Approaching the bluff from Mbale-mbale, one crosses a low-lying district
less than 100 feet above the sea before striking the spur. Here and in
the lower few hundred feet of the spur are exposed basic agglomerates,
and occasionally in the mass a semi-vitreous vesicular olivine-basalt,
almost like a pitchstone, and displaying large porphyritic crystals of
plagioclase, 5 or 6 millimetres long, the agglomerates being made up of
the same material. Higher up, at elevations between 1,000 and 1,500
feet, are exposed coarse palagonite-tuffs made up of fragments, usually
1 to 3 mm. in size, of extensively palagonitised basic vitreous rocks,
such as occur in the cliffs above. These tuffs become coarser as one
approaches the precipitous bluff, the base of which lies about 1,650
feet above the sea. Here the cliffs present a bare rocky face, some 200
feet high. The lower portion is composed of an agglomerate-tuff, and the
upper portion mainly of agglomerates. These deposits display no bedding
excepting a single plane of division inclined steeply to the north at an
angle of perhaps 40°.
The blocks in the agglomerate-tuff are either angular or sub-angular,
and are less than a foot across. They are all composed of more or less
vitreous porphyritic olivine-basalts, showing large crystals of
plagioclase a fifth of an inch (5 or 6 mm.) in length. But they vary
somewhat in character. Some of them, that are vesicular and almost
scoriaceous, may be termed from their glassy nature porphyritic
pitchstones. Others again, where the groundmass is hemi-crystalline, may
be designated porphyritic compact basalts, and are referred to genus 37
of the olivine-basalts.
The matrix of the agglomerate-tuff is made up of angular fragments, up
to 5 mm. in size, of singular vitreous and semi-vitreous
olivine-basalts, in part palagonitised. There is evidence of crushing
_in situ_ of some of the porphyritic felspar crystals; but it is not so
marked as elsewhere noticed. The palagonite is also in part
interstitial, a character that goes to support the view advanced on page
342, that the palagonite may be connected in its origin with the heat
developed during crushing, only a moderate temperature being required
for the partial fusion of the glass.
In crossing the range by this route from Mbale-mbale one first ascends,
as above observed, the spur of the Koro-tini Bluff up to a height of
1,200 feet. The track then descends into the valley-gorge of the
Natoarau river on the east, the bottom of which is 750 feet above the
sea, and from here the climb begins. One ascends the bed of the stream
course, clambering over slippery rock surfaces up to 1,200 or 1,300
feet, where the stream is left, and the mountain-slopes, often steep and
precipitous, are then followed to the summit, 2,000 feet in height.
Coarse and fine palagonite-tuffs and agglomerate-tuffs of the same
character are exposed on the surface from the commencement of the ascent
up to 1,850 feet; but they are displayed much more extensively in the
stream-course than in the soil-covered upper slopes.
The tuffs are grey except when hydrated, when they turn yellowish-brown.
Some of them contain lime, as much at times as 10 or 12 per cent.;
whilst others possess little or none. Tests of foraminifera are not
infrequently inclosed, even as high as 1,850 feet. A description of one
of these tuffs containing a few tests of Globigerina, which was obtained
at 1,200 feet, is given on page 331, under sample D. It will be there
seen that they are derived from different basic rocks, some containing
but little glass, others mainly vitreous, only the more glassy
constituents being palagonitised. The palagonite-tuff sandstones exposed
in large blocks on a bare spur at 1,850 feet contain 12 per cent. of
lime, the largest tests of foraminifera being not over half a
millimetre.[67] These tuffs occasionally show bedding. At 1,000 feet
they dip gently to the S.S.W., and at 750 feet they are inclined about
15° in the same direction. In this last locality they consist of
alternating layers, 1 to 4 inches in thickness, of fine and coarse
tuffs, the coarser looking like sandstone.... The blocks in the
agglomerate-tuff are sub-angular, and of an olivine-basalt with
hemi-crystalline groundmass,[68] their size ranging from 2 feet to an
inch. I noticed one large block of this rock, measuring 2 × 1½ × 1 feet,
imbedded alone in the tuffs at 1,200 feet. At one place a tuff
containing small fragments of basalt displayed a concretionary
structure, indicating probably the proximity of a dyke, the globular
masses being 4 feet across. A little lime occurs in the matrix of the
agglomerate-tuff.
The summit of the range, 2,000 feet in height, is “ridgy,” about half a
mile in width, and cannot therefore be described as table-topped. The
rocks exposed in blocks on the surface are composed of a semi-ophitic
olivine-basalt containing a large amount of interstitial glass which
shows the fibrous crystallites of the early stage devitrification. It is
referred to genus 33 of the olivine-basalts.
Descending the northern side of the range I followed the steep slopes
down to 1,000 feet above the sea. A rubbly doleritic olivine-basalt,
semi-ophitic, and assigned to the same genus (33), prevailed on the way;
and it is probable that a waterfall with a drop of 50 feet or more that
is situated on these slopes indicates a large intrusive mass of this
rock. During the rest of the descent to Vandrani, which lies in a valley
at the foot of the range, and is elevated about 300 feet above the sea,
basic agglomerates and palagonite-tuffs, together with deposits
intermediate in character, were exposed at the surface. At times a
semi-ophitic doleritic basalt similar to those displayed above, but
without olivine, occurred in position. The blocks in the agglomerates
are formed of a compact semi-vitreous basaltic rock, and are sometimes
vesicular. At one place the palagonite-tuffs exhibited signs of
alteration, being traversed by small fissures not over a third of an
inch broad (5 to 8 mm.), and filled with a zeolite behaving like
natrolite.
In some cliffs by the river at Vandrani are displayed fine and coarse
non-calcareous palagonite-tuffs, bedded and dipping about 15° N. by W.
They are penetrated by cracks, 5 mm. in breadth, which are filled with
chalcedony. These tuffs are evidently in part derived from acid as well
as from basic rocks, though mainly from the latter; and they show other
alteration-characters. At the mouth of the Vandrani valley there are
exposed in the river-bed coarse palagonite-tuff sandstones containing a
little lime, and probably a few tests of foraminifera.
Reference may here be made to the mountain of Ravi-koro which, when seen
from the north-east, rises up as a partially independent peak, with a
broad base and a conical truncated summit, immediately west of the track
followed in the descent from the summit of the range to Vandrani. It is
probably not much under 2,000 feet in height, and exhibits bare
precipitous cliff-faces on the north side. It would be worthy of the
attention of the future investigator.
Recurring to the principal features of the range between Mbale-mbale and
Vandrani, one may remark the extensive occurrence of basic agglomerates
and tuffs on both slopes, the prevalence of olivine-basalts, the
frequency of the semi-vitreous and vitreous or rather pitchstone
condition of these rocks, and their semi-ophitic character, especially
on the summit and north slopes. From the vesicular structure of the
rocks of the Koro-tini Bluff and from the character of its tuffs and
agglomerates, it is to be inferred that they are the direct products of
eruptions, probably in shallow seas. On the other hand, the tuffs (often
foraminiferous) as well as the agglomerate-tuffs of the north and south
slopes of the range are in part suggestive of marine erosion. Intrusive
masses of basalt are to be observed occasionally, and doubtless to this
cause may be attributed the concretionary structure of the tuffs in
places, and the alteration of these deposits in one or two localities,
where they are penetrated by cracks filled with chalcedony.
(3) TRAVERSE OF THE KORO-TINI RANGE FROM VATU-KAWA TO VANDRANI.—On
leaving Vatu-kawa[69], which is not more than 50 feet above the sea, the
ascent for the first 600 feet up the steep mountain-side lies along the
rocky bed of the Wai-ni-ngio River, which from its rapid fall has more
the character of a torrent. On its sides are exposed basic agglomerates
and agglomerate-tuffs; whilst the large boulders in its bed are composed
of a somewhat altered olivine-basalt. At 600 feet the track abandons the
stream-course for the steep mountain slopes, and thence up to 1,100 feet
similar agglomerates and tuffs prevail. At this last-named elevation
there are displayed fine and coarse indurated palagonite-tuffs, a little
altered in character and with little or no lime. A specimen of the
former, of which the materials composing it do not exceed ·2 mm. in
size, shows in the slide an occasional “Globigerina” test filled with
palagonitic debris. Such a marine deposit is evidently not of
shallow-water origin. The coarser tuff is made up of compacted
sub-angular fragments, not over 2 mm. in size; but contains no organic
remains. The prevailing rocks exposed between 1,100 and 1,900 feet, a
little below the summit of the range, are somewhat altered compacted
non-calcareous breccia-tuffs, composed of sub-angular fragments 5 or
6 mm. in size, of a more or less glassy and often vacuolar basic or
basaltic andesite, only in part palagonitised, the vacuoles as well as
the interstices between the fragments being sometimes filled with a
zeolite.[70]
The summit of the range may be described as a “ridgy” table-land. Though
about 2 miles in breadth, its level only varies between 2,000 and 2,200
feet, the inequalities being probably the effect of denudation. Here, as
in many other similar localities, on account of the dense forest it was
only possible to determine the surface-configuration by the use of
compass, watch and aneroid. The prevailing rocks displayed in this
region are grey non-calcareous basic tuffs, somewhat altered in
character, and composed of fragments usually not exceeding 1 mm. in size
of a basic glass, the palagonitic process being masked by other changes.
These tuffs often become brownish-yellow through hydration. Tests of
foraminifera are enclosed, but they are very scanty.
On the north slopes basic agglomerates and palagonite-tuffs are the
predominant rocks down to the foot of the range. A specimen of the tuffs
taken at 1,300 feet is calcareous in patches and probably contains tests
of foraminifera; but it is too much weathered to enable one to speak
with certainty on this point. The interesting feature of this slope is
the exposure at 1,600 to 1,700 feet of large blocks of a dark grey
hypersthene-augite andesite referred to the orthophyric order of those
rocks described on page 290. Lower down (1,000-1,300 feet) occasional
solitary blocks of the same rock, but somewhat altered, occur imbedded
in the palagonite-tuff. This type of rock which is characterised by the
orthophyric structure of the groundmass and by other features is rarely
represented in Vanua Levu.
Summing up the general results of this traverse we observe that here, as
in other parts of the range, basic agglomerates, breccias, and tuffs,
the last however scantily foraminiferous, occupy a great extent of the
slopes and summit. The alteration of these deposits on the southern
slopes is noteworthy. The only deeper seated massive rocks observed were
the pyroxene-andesites above alluded to.
(4) TRAVERSE OF THE KORO-TINI RANGE FROM NUKUMBOLO TO SUENI.—The hot
springs at Nukumbolo, which are described on page 24, rise up through
agglomerate-tuffs. Around the bathing pools lie large masses of altered
palagonite-tuffs which give the first indication of the region of
altered rocks that extends from Nukumbolo to the lower slopes of the
range, a distance of about three miles.
For about a mile and a half or two miles from this place the track lies
through a broken country and does not rise to a height more than 300
feet above the sea. A variety of altered rocks are here exposed in
position in the stream courses. Some of them are fine and coarse basic
tuffs showing secondary calcite, quartz and opal, as alteration
products. Others are palagonite-breccias with the vacuoles of the
altered glass filled with opal. Others again are massive basic rocks,
such as fine-textured augite-andesites, or doleritic basaltic andesites,
semi-ophitic in character, the plagioclase phenocrysts being more or
less occupied by calcitic and other products. The alteration is not
always far advanced, but it is sufficiently marked to give a common
character to the rocks of the district.
Ascending the lower slopes of the range up to 800 feet one finds the
altered rocks still exposed in the stream-courses; but the changes
exhibited are not always the same. A specimen from 500 feet looks like a
tuff, but in the slide it appears as a semi-vitreous augite-andesite,
its substance being penetrated by fine veins of chalcedonic quartz and
opal, whilst the same material is developed within the larger
plagioclase crystals. Another specimen from 800 feet, which is
apparently a tuff, contains so much lime that it effervesces freely with
an acid. It was composed originally of fragments of a hemi-crystalline
basic rock, of which the plagioclase phenocrysts have been replaced by
calcite; whilst the augite and interstitial glass is now represented by
viridite and a chloritic mineral. It is to be inferred that at some time
hot springs were very numerous in the district between Nukumbolo and the
lower slopes of the range, those at Nukumbolo, as far as I know, alone
existing in our time.
From a height of 1,100 or 1,200 feet the mountain slopes rise steeply to
the summit rather over 2,000 feet in elevation. At the foot are exposed
_in situ_ aphanitic augite-andesites,[71] which in some specimens show a
little alteration in the chalcedonic quartz filling minute cracks, and
in one case there is an irregular cavity, ¾ inch across, filled with
milk-white opal. Another rock exposed at the foot of the steep ascent is
a semi-vitreous basaltic-andesite, doleritic in texture and ophitic
structure, but apparently not much changed.[72] At 1,700 feet is
displayed a vesicular basic andesite, semi-vitreous in character, and
above this I found a porphyritic basaltic andesite.
The summit of the range is 1½ or 2 miles in breadth and is relatively
level, its undulating surface varying in elevation between 1,900 and
2,200 feet. The prevailing rocks exposed on this elevated plateau are
vitreous pitchstone-like rocks finely vesicular and scoriaceous, the
cavities being filled either with aragonite or with opal. The basic
glass, of which they are formed, shows incipient crystals, and begins to
fuse in an ordinary flame. One specimen obtained here is a doleritic
basaltic andesite, slightly ophitic and containing a fair amount of
residual glass.[73] However, the vitreous and scoriaceous character of
most of the rocks on the summit is very remarkable. (Similar rocks occur
on the top of Mount Thambeyu where the slopes of the mountain are
covered with submarine tuffs and agglomerates.) There is a precipitous
descent on the north side of the range to Sueni at its foot, massive
basaltic andesites being exposed at first, whilst basic tuffs and
agglomerates are displayed lower down.
The special features of this traverse of the range are the alteration of
the tuffs and massive rocks between Nukumbolo and the lower southern
slopes, the variation in character of the basic rocks in the upper
southern slopes, the occurrence of vitreous vesicular and scoriaceous
rocks on the summit, and the restriction of the ordinary basic
agglomerates and tuffs to the northern slopes. Any attempt on my part to
explain the structure of this part of the range from the data here given
would be futile without comparing them with those obtained from other
parts of the range. It will be subsequently pointed out that the
difficulties will be in part removed if it is assumed that the submarine
palagonite-tuffs and agglomerates, that so often cover the flanks of the
mountains to their summits, have been in this case largely stripped off
by the denuding agencies.
(5) THE SUENI VALLEY.—My acquaintance with the extreme eastern part of
the Koro-tini Range is restricted to the descent of the picturesque
valley from Sueni to Koro-utari. It is occupied by a tributary of the
Lambasa River, and is bounded on the east side by the lofty slopes of
the main range, and to the westward by a mountainous spur that projects
far into the Lambasa plains. Sueni lies by the river-side in the midst
of mountains which rise steeply on most sides to heights of 2,000 feet
and over, and often display precipitous bare faces apparently of
volcanic agglomerates. Numerous waterfalls may be observed on their
flanks, which, as in other localities, doubtless indicate the occurrence
of large intrusive dykes. Sueni is situated about 300 feet above the
sea, the descent to Koro-utari at the mouth of the valley, a distance of
3 to 4 miles in a direct line, being about 150 feet.
The river as it flows down the valley from Sueni to Koro-utari traverses
a region of basic agglomerates and agglomerate-tuffs. These deposits, as
they are displayed in the hill-slopes lying W.S.W. and at the back of
Sueni, are composed of blocks of the size of the fist of a vesicular
basaltic andesite; whilst the large masses on the surface are made of
the same, but non-vesicular, rock. The blocks in the agglomerates
between Sueni and Koro-utari range usually from a few inches to a foot
in diameter. A specimen obtained from one of them is made of a partly
vitreous basaltic andesite; whilst in another case the rock is an
altered basic andesite, the glassy groundmass being largely impregnated
with colloid silica looking like opal under the lens.[74]
Nearly a mile below Sueni, within a space of less than 60 yards, there
are exposed at the river-side in the agglomerates three vertical or
nearly vertical dykes, 4 to 6 feet in thickness. They trend roughly N.E.
and S.W., and are non-columnar, except in the case of the one farthest
up the river, which has rude, transverse joints.[75] The rocks composing
these dykes are somewhat doleritic basaltic andesites, olivine being
very rare or absent. The two highest, which are only 15 to 20 feet
apart, are made of similar rocks characterised by abundant interstitial
glass, and having a sp. gr. of 2·78. The rock of the third dyke, about
50 yards farther down the river, has but scanty glass in the groundmass,
the sp. gr. being 2·89. The differences between the two types
represented in the three dykes are mainly concerned with the degree of
crystallisation, and it is probable that though not contemporaneous they
were derived from the same fluid magma which, as we may infer from the
proximity and lie of the dykes, was situated at no great depth.[76]
GENERAL INFERENCE RESPECTING THE KORO-TINI RANGE.—If we can imagine a
line of vents, protruding in some cases above the surface of the sea,
that were ultimately worn down to a common level through marine-erosion,
and were then largely covered over with submarine tuffs and
agglomerates, we should have in our mind’s eye the first and most
important stage in the formation of this range. If we then assume that
there followed a period of emergence characterised by a renewal on a
very extensive scale of marine-erosion, during which the agglomerates
were mainly formed, and that since that period the sub-aerial denuding
agencies have been for ages in operation, we shall, I think, obtain some
idea of the history of the Koro-tini Range.
CHAPTER XII
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
THE KORO-MBASANGA RANGE
AS is illustrated in the accompanying profile-sketch, the relatively
level-topped range of Koro-tini gives place at its eastern end to a
broken line of mountains, of which the round-topped Koro-tambu, 2,753
feet in height, and the pinnacled Koro-mbasanga,[77] 2,537 feet, are the
highest peaks. Further east lies the broad Vuinandi Gap which separates
the Koro-mbasanga and Mount Thurston, or Thambeyu, ranges. The
twin-peaks of Mount Mbatini, the highest mountain of the island, appear
in the background in the sketch, and to the left rises Thambeyu, the
second highest summit.
We enter here another complex region of mountains; and if the character
of the rocks are sometimes different we shall yet have to bear in mind
in our interpretation of its geological features the lesson derived from
the examination of the Koro-tini Range. Before and behind all our facts
of observation lie the two great periods of marine-erosion and the later
ages of sub-aerial denudation.
When approached from the north, the western part of the range has a rude
crescentic form, and looks like the remnant of a gigantic crater-cavity
about two miles across. At the back rise, as shown in the second of the
profile-sketches, the precipitous slopes of Koro-mbasanga proper; whilst
the two spurs descending from it, one on the west, the Sokena spur,
towards Koro-utari, the other on the east, at the back of Nasawana, give
the crescentic figure open to the north. The last-named village lies
nestled in this great hollow, the floor of which, though not in its
lowest part below the level of the Lambasa plains, is not over 200 feet
above the sea. However, the facts adduced in the following description
of this region do not give much support to this view of its
surface-configuration.
For the convenience of description, I will first describe the peak of
Koro-mbasanga, and then the Sokena ridge and, lastly, the Lovo valley
that cuts through the range to the eastward.
[Illustration: These three sections form a continuous profile-sketch of
the mountainous axis of Vanua Levu for a length of 15 or 16 miles and
include the Thambeyu, Koro-mbasanga, and Koro-tini ranges as viewed from
the northward near Na Kama. The eastern section is at the top and the
western section at the bottom. The summit of Thambeyu was covered with
clouds.]
[Illustration: Koro-mbasanga from the north-north-east.]
(1) KORO-MBASANGA.—The ascent of Koro-mbasanga is best made from
Nasawana, a village at its base, elevated rather over 200 feet above the
sea, and distant about a mile and a half north-east from the peak. On
the way to the foot of the mountain we traverse an undulating region of
basaltic andesite,[78] which is merely the extension to the base of the
mountains of the basaltic Lambasa plains. After commencing the ascent of
the steep slopes, we find exposed in a stream-course, 700 to 800 feet
above the sea, a sedimentary basic tuff, presenting layers of coarse and
fine materials, and partly palagonitic in composition. It is a little
calcareous, and apparently incloses tests of foraminifera. These
submarine deposits have evidently been stripped off the basaltic
low-lands beneath. It thus becomes evident that the structural features
of the Lambasa plains (basaltic rocks overlain by submarine deposits)
are preserved to the base of the range.
These submarine deposits, as exposed in the stream-course, lie beneath
agglomerates which repose horizontally upon them; and from this level up
to the bare rocky peak of the mountain, agglomerates and
agglomerate-tuffs are alone displayed either as large detached masses or
in cliff-faces.