lower slopes of Mount Koroma than I found in any other part of the
island. It should have been before remarked that one of these flows lies
upon a bed of a hard reddish compact tuff, which appears in the thin
section as an altered palagonite-tuff, containing fragments of minerals
including both rhombic and monoclinic pyroxene, but showing neither lime
nor organic remains. The larger fragments are 2 mm. in size. It seems
likely that this flow ran into the sea during the emergence of this part
of the island.
The prevalence of rocks of the hypersthene-augite-andesite type in Mount
Koroma distinguishes this range from the surrounding regions of
olivine-basalts and basaltic andesites. This district is well worth a
detailed examination, and perhaps the remains of a crateral cavity may
yet be found.
THE COAST BETWEEN NAIVAKA AND KORO-NI-SOLO AT THE FOOT OF THE NORTH
SLOPE OF THE SESALEKA RANGE.—Basaltic andesites, and olivine-basalts of
the Naivaka type occur on this coast. A rock of more acid character,
light grey and much altered, is exposed at the surface where the track
crosses the headland projecting into Ruku-ruku Bay. It is one of the
propylites referred to in my description of the second genus of the
augite-andesites (p. 269). The felspars of the groundmass give the small
extinctions of oligoclase; and in this respect it differs from the other
augite-andesites. Besides the altered plagioclase phenocrysts there is
much microporphyritic augite but slightly changed. Calcitic and other
alteration products occur in the interstitial glass.
MOUNT SESALEKA.—This is the name of the highest peak, 1,370 feet, of a
remarkable ridge-shaped range, which is very precipitous on the east and
north-east sides, where there is a sheer drop apparently of 500 or 600
feet, whilst on the other sides the slope is more gradual, especially on
the north where there is a gentle descent to the sea. The actual summit
is bare, rocky, and narrow. There is a curious native legend relating to
a pond on the top of this hill. From what Mr. Wittstock tells me, it
seems probable that there is a spring near the summit. Close to the top
are the remains of an old “koro-ni-valu” or war-town; whilst numbers of
shells of species of Cardium, Cypræa, and Strombus, such as would be
used for food, lie about. Many years ago there was a prolonged siege of
this stronghold, which is referred to here as indicating that the
defenders had some independent water-supply.
In ascending from Koro-vatu on the west side basic agglomerates and
agglomerate-tuffs were found exposed as far as half-way up. In the upper
half occurred at first fine-grained calcareous tuffs, bedded and dipping
gently down the slope, composed of palagonite-debris, mineral fragments
and calcitic material and displaying a few macroscopic tests of
foraminifera. These tuffs became non-calcareous and coarser as one
approached the summit. A specimen obtained from the top is
coarse-grained, being composed of fragments of basic glass, usually
palagonitised, much augite, a little plagioclase and fresh olivine, but
no tests of foraminifera, the size of the fragments being usually
·5-1·5 mm. Massive rocks were rarely exposed on this side; but half-way
up in a stream course I came upon an exposure of a porphyritic
olivine-basalt containing a fair amount of devitrified interstitial
glass. Its specific gravity is 2·85 and it is referred to genus 25 of
the olivine basalts (page 259). I descended by a gentle slope to the
north, coarse basic tuffs and agglomerates containing amygdaloidal
fragments being displayed on the surface. In a stream at the foot, close
to Koro-ni-solo, were blocks of a heavy compact olivine-basalt with
specific gravity 2·96.
DISTRICT BETWEEN MOUNT SESALEKA, THOMBO-THOMBO POINT, AND VATU-KAROKARO
HILL.—This is a broken country with several abruptly rising lesser
hills. Starting from Koro-vatu and crossing the Thombo-thombo
promontory, I reached the coast of Mbua Bay near Navunievu. Basic tuffs
and agglomerates prevailed on the way, the last containing blocks of a
scoriaceous basaltic lava bearing olivine. The massive rocks exposed
belong in some cases to genus 13 of the olivine-basalts as described on
page 256, being dark grey and having a specific gravity of 2·88, and in
other cases to genus 16, species B, of the augite-andesites when they
are lighter in colour and have a specific gravity of 2·77. In both cases
the interstitial glass is scanty.
I ascended Vatui, one of the numerous small hills of the district. It is
450 feet high and is capped by a bare mass of tuff-agglomerate, 40 to 50
feet high and containing fragments of vesicular basic lava. This mass is
pierced by a dyke, 18 inches thick, which is inclined to the N.NE. at a
high angle of 60 or 65 degrees with the horizon. This dyke is composed
of a compact olivine-basalt which is remarkable for the prevalence of
small augite prisms in the groundmass. It is described on page 265 under
genus 44 of the olivine-basalts. Hand-specimens are magnetic and display
polarity, which is due, as pointed out in Chapter XXVI., to the exposed
situation of the peak.
Vatui in its characters is evidently typical of the other lesser
hills around, which, as viewed from below, possess bare tops and
precipitous declivities of the same formation. All the hills in the
district including Sesaleka are capped by these basic tuffs and
tuff-agglomerates; and doubtless as in the case of Sesaleka these
deposits are all submarine. This is true also of Vatu-karokaro, a
hill 600 feet high, overlooking Mbua Bay and about two miles east of
Sesaleka. In the lower part of this hill is exposed a dark compact
basaltic andesite, referred to genus 13, species B, of the
augite-andesites (sp. gr. 2·83), whilst blocks of a black
olivine-basalt (sp. gr. 2·91) occur in the agglomerate of the
summit. These hills may all be regarded as “volcanic necks” or the
stumps of volcanic cones, probably submarine.
THE DIVIDING RIDGE BETWEEN THE MBUA AND LEKUTU PLAINS.—A level rolling
“talasinga” district intervenes between Mbua Bay and the dividing ridge.
The upper part of this ridge, which attains a height of about 500 feet
above the sea, is composed of a hard grey sandstone-like tuff,
effervescing feebly with an acid, which on examination proves to be
formed in great part of fragments, ·07-·1 mm. in size, of a dark basic
glass occasionally vacuolar. The rest of the deposit consists of
similar-sized fragments of plagioclase and other minerals, and includes
also a few tests of foraminifera of the “Globigerina” type.
The mass of the ridge, however, is composed of coarse tuffs and
agglomerates of a different kind which have been covered over by the
foraminiferous deposit just described. Thus there are exposed on the
lower slopes, tuffs and agglomerates of a basic pitchstone formed of a
brown glass containing a few felspar and pyroxene microliths. In the
tuff the fragments are three to six mm. in size and show evidence of
crushing _in situ_, the interstices being filled with debris of the same
material more or less palagonitised,[34] but there is no carbonate of
lime. Large masses of an agglomerate made up of blocks of an acid
andesite occur higher up the slopes. The component rock belongs to an
unusual type of hypersthene-andesite, specially noticed on page 297.
The interesting feature in this ridge lies in the testimony it affords
that the extensive Mbua and Ndama basaltic plains, on which I was unable
to discover any submarine deposits, were at one time submerged.
THE MBUA AND NDAMA PLAINS.—These rolling plains are a striking feature
in the western end of Vanua Levu. They have an arid barren look, are
clothed with a scanty and peculiar vegetation, possess a dry crumbling
soil often deeply stained by iron oxide, are traversed by rivers without
tributaries descending from the wooded uplands of the interior, and in
fact have well earned the name given to them by the natives of
“talasinga” or sun-burnt land. Both Seemann and Horne have remarked on
the South Australian aspect of these regions, which are characteristic
of the lee and drier sides of the larger islands of the group. Covered
for the most part with grass, ferns and reeds, these low-lying districts
are dotted here and there with Casuarinas, Pandanus trees and Cycads,
whilst such other trees and shrubs as Acacia Richii and Dodonæa viscosa,
add to the variety and peculiarity of the vegetation. The origin of
these “talasinga” districts is discussed in the last chapter.
The Mbua and Ndama plains form a continuous region extending three to
five miles inland to the foot of the great mountain of Seatura, to the
watershed between Mbua and Lekutu, and to the base of Mount Koroma;
whilst it reaches along the sea border from the vicinity of Navunievu
about four miles west of the Mbua River to beyond Seatovo a few miles
south of the Ndama River. Their extent is defined in a general sense by
the 300 feet contour line in the map. Their elevation, however, above
the sea does not generally exceed 200 feet and is usually only 50 or 100
feet; but at the foot of Seatura they rise to between 300 and 400 feet.
Whilst on the one side these plains form a continuation of the lower
slopes of the great Seatura mountain, on the other side they are
extended under the sea as the broad submarine platform, the edge of
which, as defined by the 100-fathom line, lies eight to ten miles off
the coast. It is pointed out on page 372 that this continuity of
surface, both _supra_-marine and submarine, extends probably to the
geological structure and that the submarine platform represents the
extension under the sea of the basaltic flows of the plains.
The whole region of the plains is occupied by olivine-basalts and
basaltic andesites, such as are found on the neighbouring lower slopes
of the Seatura mountain. They are as a rule much decomposed, even at a
depth of several feet below the surface. Typically, they are neither
vesicular nor scoriaceous, and in this respect they possess the
character of submarine lava-flows. The rolling surface of the plain is
varied occasionally by small “rises” or hillocks marking apparently some
secondary cone, of which the much degraded “wreck” alone remains. Here
and there fragments of limonite, approaching hæmatite in its compact
texture, lie in profusion on the soil, representing doubtless small
swamps long since dried up, some of which still occur in the hollows of
the plain. Mingled with these fragments are often pieces of siliceous
rocks and concretions, such as are found in the other “talasinga”
districts of the island, the description of which is given on pages 128,
132, &c.
I will now refer more in detail to some of the points alluded to in this
short description of these plains. With reference first to the compact
limonite, it should be remarked that it occurs on the surface either as
fragments of hollow nodules two or three inches across, or as portions
of flat “cakes” half to one inch thick. It is especially abundant in the
district lying a mile or two on either side of the Navutua stream-course
between Ndama and Mbua. Here the subsoil is charged with ferruginous
matter, and the water of the series of stagnant pools in the bed of the
stream is stained blood-red by iron-oxide, a circumstance that has
naturally given rise to native legends of a corresponding hue. These
fragments of iron ore, which lie between 100 and 150 feet above the sea,
represent the final stage of a process which is now no doubt in
operation on the bottom of the neighbouring pools and small swamps.
Their presence on the surface goes to indicate that this open country
has been for ages a land-surface free from forest, as it is in our own
time.
In a similar manner, the extensive disintegration of the basaltic rocks
that form these plains affords evidence of the great antiquity of these
“talasinga” plains in their present unforested condition. The extent to
which these rocks have weathered downwards is very remarkable. Between
Ndama and Mbua they are decomposed to a depth often of eight or ten feet
below the surface. This is well exhibited in the sides of deep channels
excavated by the torrents during the rains. Here the spheroidal
structure is well brought out in the disintegrating mass, all stages
being displayed in the formation of the boulders that are scattered all
over these plains.
In one locality, near the lower course of the Ndama river, a thickness
of 25 feet of decomposed rock was exposed in a cliff-face. In this case
the rock was a porphyritic basaltic andesite, the disintegrating process
having affected the whole thickness of the large spheroidal masses with
the exception of a hard central nucleus of the size of the fist. In one
of these nuclei by my side it is apparent that during the extension of
the weathering process the phenocrysts of glassy plagioclase become
opaque long before the groundmass is affected. In this specimen the
stage of disintegration as affecting the felspar phenocrysts is at least
one and a half inches in advance of that affecting the groundmass.
This great disintegration of the basaltic rocks, which as pointed out on
page 64 is also in progress on the slopes of the adjacent spurs of Mount
Seatura, is more characteristic of the porphyritic basaltic andesites
than of the olivine-basalts. It is to the spheroidal weathering that we
must look for an explanation of the rounded boulders so frequent in
these districts. It may also be inferred that the soil produced from
this extensive decomposition of the rocks is often very deep. At the
Wesleyan Mission Station at Mbua, on level ground nearly a hundred feet
above the river, a well has been sunk to a depth of 20 feet in soil of
this description; and away to the westward a similar thickness of soil
produced by the same cause is in places to be observed.
Coming to the characters of the basaltic rocks of the Mbua and Ndama
plains, it may be remarked that the prevailing rocks are the porphyritic
basaltic andesites, having a specific gravity of 2·77 to 2·81, which are
in most cases to be referred to genus 13 (porphyritic sub-genus) of the
augite-andesites described on page 278. They possess large phenocrysts
of plagioclase and but little interstitial glass. The other rocks are
olivine-basalts with specific gravity 2·88 to 2·90 and showing only a
few small plagioclase-phenocrysts. They display a little residual glass
and belong for the most part to genus 37 of the olivine basalts
described on page 262. In both these basaltic rocks the felspar-lathes
are in flow-arrangement; but in the basaltic andesites they average
·11 mm. in length, whilst in the olivine-basalts they average ·18 mm.
The low mound-like “rises” in these plains, to which previous reference
has been made, are not usually elevated more than 50 feet above the
general surface. One of these hillocks that lies near the track from
Mbua to Navunievu, about two miles from the Wesleyan Station, is
composed of a remarkable semi-vitreous pyroxene-andesite passing upward
into a rubbly rock of the same nature. The rock of this old volcanic
neck is of an unusual type and is referred to the prismatic order of the
hypersthene-augite andesites described on page 289. Both the felspar and
pyroxene prisms of the groundmass are in flow-arrangement. One of these
mounds near the Mbua Wesleyan Station is apparently formed of the
decomposing basaltic andesite of the district. On its surface are
fragments of earthy limonite and siliceous rocks.
The rarity of submarine tuffs and clays on these plains is somewhat
singular; but in the occurrence of foraminiferous tuffs high up the
slopes of Sesaleka and on the crest of the Mbua-Lekutu dividing ridge we
have evidence of the original submergence of all these lower regions. It
is probable enough that the ages of exposure that have since witnessed
the reduction of the solid basaltic rock to a crumbling mass several
feet in depth were more than sufficient for the stripping off of most of
the overlying submarine deposits. Such deposits are, however, common on
the surface of the extensive “talasinga” plains that constitute much of
the north side of the island.
THE SHELL-BED OF THE MBUA RIVER.—Rather curious evidence of an emergence
of a few feet and of a considerable advance of the delta of the Mbua
river in comparatively recent times is afforded by a bed of marine
shells exposed in the right bank of this river, about 200 yards below
the boat-shed of the Wesleyan Mission Station and about two miles in a
straight line from the sea. This bed, which is about a foot in
thickness, is exposed for a distance of 70 or 80 yards. It slopes
gradually seaward as one descends the river, being raised two or two and
a half feet at its upper end above the river level at low tide, whilst
at its lower end it is at about the water-level. The river-bank is here
15 or 16 feet high, and is composed in its upper half of a fine gravel
of volcanic rocks mixed with earth, which below passes abruptly into a
friable non-calcareous black mud-rock (not bedded and looking like
consolidated swamp mud), in which the layer of shells is contained.
These shells are, therefore, covered by deposits, 13 or 14 feet in
thickness, of which the upper eight feet are formed of gravel and earth,
and the rest of mud-rock. They are evidently gathered together on the
slope of an old mud-flat.
The shells are all large marine bivalves, belonging to the genera
Ostræa, Meleagrina, Cardium, Arca, &c., no freshwater shells occurring.
They are often much decayed and have lost the ligaments. The valves are
generally separate; but in some cases they are still in apposition, the
cavity being then filled with the same black mud in which the shells are
embedded. They lie about in all positions, some vertical, some
horizontal, and are often piled on each other. In some cases large
borers have perforated one or both of the valves; and here and there
valves may be noticed with smaller oyster-shells attached to the inner
surface. No vegetable remains were discovered with the exception of a
single “stone” of the fruit of the Sea tree,[35] which is common in
these islands, its empty almost indestructible stones occurring
frequently in the drift stranded at the mouths of rivers.
At first sight one would look to human agency for the explanation of
this shell-bed; but many of its features are inconsistent with such a
view. If the shells had been originally collected by the aborigines for
food, the absence of those of marine univalves of the genera Turbo,
Strombus, Cypræa, &c., such as are much appreciated as food by natives,
is inexplicable. The extent of the bed and its uniform thickness are
characters that give no support to such an explanation. It represents,
as I apprehend, an ancient shell-bank formed on a muddy bottom in
comparatively shallow water near the mouth of a river. Since that time
the Mbua River has cut through its old deposits, and the margin of its
delta is now two miles to seaward, the intervening new land being formed
of extensive mangrove-swamps in its lower part, whilst nearer the
shell-bed there is much level land raised a few feet above the sea, on
which the native town and different villages now stand. The amount of
emergence here indicated since the time when this bank of shells was
forming under the sea does not probably exceed a couple of fathoms.
LEKUMBI POINT.—This singular long and low promontory is between three
and three-and-a-half miles in length and rather less than a mile in
average width. It is monopolised by mangroves, except at the extremity
where the swampy ground passes into the dry sandy soil occupied by the
characteristic vegetation of coral beaches. This terminal portion, which
is about a third of a mile in length and raised a couple of feet above
high-water mark, was originally a reef-islet. The outer third of the
cape, however, is cut off from the remainder by a narrow winding passage
in the mangroves, which being 25 or 30 feet wide can be traversed by
boats at and near high-water, and is often used to shorten the journey
down the coast. The flowing tide rushes in at both entrances, and when
the tide is ebbing it finds its way out at both exits, the passage
presenting the readiest way of the filling and emptying of the interior
swamps with the flow and ebb of the tide.
Before explaining the origin of this low tongue-shaped promontory of
Lekumbi, it should be observed that it lies on a long projecting patch
of coral reef which is continuous with the neighbouring shore-reefs.
Depths of seven and eight fathoms are found off the sides and of 11 and
12 fathoms off the end of the reef-patch. This reef in its turn must
have been built up on a submarine bank protruding from the coast. Such a
bank may have originally been produced by the deposits brought down by
the Ndama River which finds an exit through the mangroves near the base
of the cape. With the exception, however, of the Lekutu River, none of
the other Vanua Levu rivers have given rise to such tongues of land at
their mouths. I am more inclined to hold that the submarine shoal, which
underlies the present low cape of Lekumbi, indicates an old lava-flow
from the great crateral valley of Seatura, opposite the mouth of which
it lies. Traces of such flows are still to be found in that locality.
CHAPTER V
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
THE SEATURA MOUNTAIN.—In my description of the profile of this part of
Vanua Levu, reference has already been made (p. 3) to the great mass of
this mountain which occupies five-sixths of the breadth of the island.
Viewed from seaward it looks like a huge table-topped mountain-ridge,
and as such it is represented in the Admiralty charts; but when its true
contours are distinguished it appears, when defined by the 300-feet
level in the map, as a somewhat rounded mass, measuring 12 miles in
length and 10 miles in breadth and attaining a maximum height of 2,812
feet. Seen from the deck of a passing ship it displays more or less
regular volcanic slopes, especially on the east, where there is a
gradual descent at an angle of 3 or 4 degrees for some 10 miles, and on
the north towards the Lekutu lowlands. It also shows a fairly regular
descent towards Mbua Bay on the west. (See profile, p. 62.)
On the west side, however, there is a great gap in the mountain-mass
(the Ndriti Gap), marking, as I hold, an old crateral cavity of large
dimensions, and now occupied by the Ndama River and its tributaries.
The adjacent Seatovo Range to the southward obscures the profile of the
mountain on the south; and it is in fact not at all easy for this reason
to get a view with all the slopes displayed. It is only at times, when
viewed in its complete mass with uninterrupted outlines, as from off the
mouth of the Ndreketi River to the north-east, or when the symmetry of
its long eastern slope is observed from Wainunu Bay that Seatura
displays itself as a gentle-sloped mountain-mass of the Mauna Loa type.
Dense forest clothes the greater part of it, except on the north and
north-west, where it lies within the limits of the scantily vegetated
“talasinga” region.
[Illustration: Profile and Geological Section of the western end of
Vanua Levu from the Wainunu estuary across the summit of the basaltic
mountain of Seatura to the edge of the submarine platform off the Ndama
coast as limited by the 100-fathom line.]
The slopes of this mountain are deeply furrowed by river-valleys which
radiate like the spokes of a wheel from its central elevated mass. Down
its northern slopes flow the Lekutu River and its tributaries and the
principal tributaries of the Sarawanga River. The large western
affluents of the Wainunu River descend from its eastern side, whilst the
Korolevu, Tongalevu, and other small rivers flow south into Wainunu Bay,
and the Ndama River drains its western slopes. In all these cases,
excepting that of the Ndama River, the rivers have worn deep valleys
into the mountain-mass, valleys of denudation that represent the work of
ages. That of the Lekutu is a deep cut almost into the heart of the
mountain; at Nandroro in this valley, which lies 6 to 7 miles inland and
800 feet above the sea, the hills rise steeply on either side of the
river to an elevation of 1,100 and 1,200 feet and more. Some of the
large tributaries of the Sarawanga and the Wainunu flow through
gorge-like valleys 200 to 300 feet in depth. On the western slopes north
of the Ndama river, the mountain-side presents an alternating series of
lofty spurs and deep broad valleys. In fact, all around Seatura its
slopes are deeply furrowed through the denudation and erosion of ages.
The rocks of this ancient volcanic mountain are almost all of the
massive basic type, and except at the mouth of the Ndriti Gap hardly
ever display a scoriaceous character. It is also noteworthy that no
detrital rock, whether agglomerate, tuff, or tuff-clay came under my
observation. The rocks exposed on the surface are mostly blackish brown
olivine-basalts and porphyritic basaltic andesites, the former much
prevailing. In the northern portion, however, grey olivine basalts of a
different type occur. In the great crateral hollow, which I have named
the Ndriti Gap, are displayed numerous dykes formed of highly altered
basaltic rocks that may be classed among the propylites.
The dense forest that clothes the greater part of this mountain offers
many serious hindrances to geological exploration. Except in the
northern portion, views of the surroundings are very limited, and one
has often to rely mainly on the aneroid and the compass to obtain
correct ideas of the contours and general configuration. During most of
the time spent in the southern part of the mountain, my work was greatly
impeded by heavy rains, and from this cause and from the frequent
necessity of following up the stream-courses and of crossing rivers in
flood, I was usually wet through all the day.
_(a) The Eastern Slopes of Seatura._—The basaltic flows, of which this
mountain is principally composed, are best observed on the eastern side
where the original volcanic slopes are preserved. Although the rivers
have worn such deep valleys into the mountain sides, it is however not
often that any great exposure of rock occurs, on account of the dense
forest-growth over much of this region. It is only occasionally that the
columnar structure of these old basaltic flows is displayed. It is
especially well exhibited in the face of a waterfall, distant about two
miles in a straight line from Tembenindio and elevated about 700 feet
above the sea. Here there is an exposure to the extent of 25 feet of
huge basaltic vertical columns, four to five feet across, and pentagonal
in form. The rock is a blackish basalt with scanty olivine and a
specific gravity of 2·87. It is referred to genus 25 of the
olivine-basalts which is described on page 259. Micro-phenocrysts of
plagioclase and a few of augite occur, the olivine being mostly replaced
by pseudomorphs. The felspar-lathes of the groundmass average ·18 mm. in
length, and there is a little brown opaque interstitial glass. Boulders
and fragments of a closely similar basalt, with a specific gravity of
2·9, lie about on the surface in this region. The Seatura slopes here
abut on the plateau of Na Savu, formed largely of volcanic agglomerates,
to be subsequently described.
On the south-eastern slopes of the mountain between Ndawathumi (inland)
and Korolevu (at the coast), somewhat similar basalts with scanty
olivine are exposed (sp. gr. 2·86-2·91). Some of them show the
felspar-lathes of the groundmass arranged in a plexus (genus 25), whilst
others exhibit flow-structure (genus 37), the average length of the
lathes varying in different localities between ·15 and ·21 mm. All
display scanty residual glass. On the shores of Wainunu Bay between the
Wainunu and Korolevu rivers occur porphyritic basaltic andesites with a
considerable amount of glass in the groundmass. There is exposed on the
right side of the mouth of the last-named river a highly basic variety
of olivine-basalt with a specific gravity of 3·07. It is referred to
genus 15 (described on page 258), which includes the most basic rocks in
my collection. There are in this rock no plagioclase phenocrysts and the
felspar-lathes of the groundmass are relatively infrequent, whilst
olivine and augite occur in abundance. There is little or no residual
glass. In the district of Tongalevu blackish olivine-basalts and
basaltic andesites of the usual character are found. In the Na Suva
range, which lies two miles inland from the shores of Nasawana Bay and
forms the southerly extension of the mountain, a somewhat compact
variety of olivine-basalt (sp. gr. 2·92) prevails up to the summit,
1,550 feet above the sea. It is included in genus 37 of the
olivine-basalts. In the length of the felspar-lathes (·15 mm.) it
belongs to the Seatura type of these dark basalts.
_(b) The Western Slopes of Seatura._—Here overlooking the plains north
of the Ndama River the same olivine-basalts and porphyritic basaltic
andesites occur. The vegetation is of the scanty “talasinga” character,
and since there is little or no soil-cap the disintegration of the rocks
has been very great, often extending to a depth of 10 or 12 feet. It is
remarkable that this disintegration is most marked in the “talasinga”
and similar scantily wooded districts of the mountain. On the densely
wooded eastern and southern sides where there is a thick soil-cap, it is
by no means so evident. Here on the western slopes have been carved out
deep broad valleys and lofty spurs, the last in their turn furrowed on
their flanks, without any apparent sufficient cause. The shallow streams
at the bottom of the valleys appear quite incompetent to produce such
great erosion; and doubtless these results are partly due to the action
on the crumbling rock-surface of temporary torrents formed during the
rains.
_(c) The Northern Slopes of Seatura._—Here within the scantily vegetated
“talasinga” region the conformation of the land is well displayed.
Broad, deep and nearly parallel valleys, separated by level-topped spurs
and occupied by the Lekutu and its tributaries, score the mountain’s
slopes. The prevailing rocks are blackish-brown olivine-basalts and
porphyritic basaltic andesites, such as occur around the other parts of
Seatura; but grey olivine-basalts also occur, possessing opaque
plagioclase-phenocrysts and looking like porphyrites. They are
essentially holocrystalline and are probably more deeply situated than
the other basaltic rocks. They are referred to genera 26 and 38
described on pages 261, 263, and have a specific gravity of 2·75-2·83.
Dark doleritic basalts distinct from all the others are exposed in
places.
A good idea of this region may be obtained by following the road
westward from Tavua on the head-waters of the Sarawanga River to Wailevu
on the westernmost tributary of the Lekutu River, a distance of about 6
miles. Leaving Tavua one at once begins to ascend and cross the long
spur that descends from Seatura and divides the valleys of these two
river-systems. On its slopes are exposed much decomposed blackish
basalts possessing scanty olivine and showing large porphyritic crystals
of plagioclase. They have a specific gravity of 2·84 and are assigned to
the porphyritic sub-genus of genus 25 (page 259). At the summit, 800
feet above the sea, occur blocks of a grey holocrystalline basalt with
scanty olivine and semi-opaque plagioclase-phenocrysts referred to genus
26 and having a specific gravity of 2·76. It appears to form the axis of
the spur. Descending to the main Lekutu River, just below Kavula, where
the elevation is about 300 feet above the sea, one observes exposed in
mass in the river-bed a dark semi-ophitic doleritic basalt similar to
the doleritic rocks without olivine prevailing on the coast between
Wailea Bay and Lekutu (see page 50), but differing in the absence of
felspar-phenocrysts. It displays a considerable amount of opaque
interstitial glass and is assigned to genus 12 of the augite-andesites
(page 275). The specific gravity is 2·78, but there are a few minute
irregular cavities in its substance.
On leaving Kavula one crosses another of the Seatura spurs at a level of
650 feet, descending then into a smaller river-valley occupied by a
tributary of the Lekutu, on the banks of which lies the village of
Nawai, 350 feet above the sea. Then another spur is crossed at an
elevation of 450 feet and the descent is made into the valley of the
Wailevu tributary of the Lekutu. Crossing the valley, which at the town
of Wailevu is elevated 300 feet, one rises to a height of 700 feet and
then descends into the Mbua plains. These three almost parallel valleys
of the Lekutu and its two tributaries are worthy of a detailed
examination.
The rocks on the surface between Kavula and Wailevu vary in character.
Nearer Kavula there appears a blackish compact olivine-basalt (spec.
grav. 2·88), showing a little microporphyritic plagioclase and
belonging to genus 37 of the olivine rocks. Further on is exposed one
of the holocrystalline grey olivine-basalts with porphyritic
plagioclase-phenocrysts and specific gravity 2·83. It belongs to the
type described in genus 38 of the rocks on page 263. Nearer Wailevu
there occurs a blackish porphyritic basalt with scanty olivine and
specific gravity 2·81. It contains but little residual glass and is
referred to the porphyritic sub-genus of genus 25. In some cliffs at
the river-side close to Wailevu, there is displayed a semi-vitreous
basaltic andesite, showing large porphyritic plagioclase crystals, 3
to 8 mm. Its low specific gravity (2·68) is to be attributed to the
large amount of glass in the groundmass. There is a loose mesh-work of
felspar-lathes, but the augite is not differentiated. Westward of
Wailevu commence the decomposing basaltic rocks of the Mbua plains.
_(d) Traverse of the Northern Part of the Summit of Seatura from Kavula
South-West to Narawai._—The track first lay up the picturesque valley of
the Lekutu River to Nandroro, 2½ miles distant and 800 feet above the
sea. On the way blackish basaltic rocks of the prevailing Seatura type,
with or without scanty olivine, were displayed often in a decomposing
condition. At one place a characteristic grey olivine-basalt, showing
opaque porphyritic plagioclase (sp. gr. 2·87), and looking like a
porphyrite, was exposed. On account of the abundance of the olivine, it
is placed in genus 2 of the olivine-rocks. After Nandroro the path lay
up the steep mountain-side to a height of 1,500 feet: and afterwards
across the summit of the northern part of Seatura, which is here about
two miles in breadth. This elevated region is well wooded with here and
there a patch of “talasinga” land; but it is by no means level, its
elevation varying between 1,400 and 1,800 feet, and it soon became
evident that we were crossing the heads of valleys, sometimes 200 or 300
feet in depth, that could only have been excavated by the torrential
rains. These streamless valleys afford another indication of the
denudation to which this ancient mountain has been subjected.
The rocks prevailing in this elevated northern portion of Seatura, at
heights of 1,500 to 1,800 feet above the sea, are: (_a_) blackish
basalts with scanty olivine, a little interstitial glass, and belonging
to the porphyritic and non-porphyritic sub-genera of genus 25 of the
olivine-rocks: (_b_) grey olivine-basalts with porphyritic opaque
plagioclase, containing but little residual glass, but varying greatly
in the amount of olivine and belonging to the genera 2 and 26 of the
olivine-basalts; they would be classed, as far as appearance goes, as
porphyrites; their specific gravity ranges 2·85 to 2·90. The rock
exposures were, however, scanty; and but little information could be
obtained of the mode of occurrence. No scoriaceous rocks were found
except in the instance of a compact dark basalt without plagioclase
phenocrysts, apparently a dyke rock, and belonging to genus 40 of the
olivine-basalts.
_(e) Ascent to the Summit of Seatura from Ndriti._—The town of Ndriti
lies in the great gap in the south-west side of the mountain which has
been previously mentioned as probably an old crateral cavity. After
traversing a district of highly altered basic rocks or propylites, to be
subsequently described, and reaching an elevation of about 400 feet
above the sea, I came to the long slope that leads up to the summit. A
dense forest hid everything from view, so that the compass and aneroid
had alone to be relied on.
At first one traversed a series of step-like alternations of level
ground and steep “rises,” until the old site of the village of Seatura,
about 1,200 feet above the sea, was reached. There are some strange
legends connected with this old mountain-village, which is now only
indicated by little piles of stones and the debris of a wall, and was
evidently abandoned long ago. We finally reached the summit by following
up a spur or ridge in a northerly direction from Seatura. There was a
precipitous descent on either side of the ridge with evidently a broad,
deep valley to the eastward. The summit was rounded; but on account of
the forest no view could be obtained. There was never any extensive
exposure of rock noticed during the ascent; but all the way up
occasional small blocks of a blackish olivine-basalt were observed on
the surface, of the same general type as that found all around the
mountain and referred to genus 37 in the synopsis.
_(f) The Ndriti Basin or Gap._—This great hollow in the side of Seatura,
which I have named after the town in its midst, is apparently a crateral
cavity now drained by the Ndama river, and its tributaries, and covered
with dense forest to such a degree that a general view of the whole is
impracticable. The glimpses, however, that one obtains of the mountain
scenery are very grand, the town of Ndriti lying in the midst of
mountains that rise almost on all sides of it except on the west. This
great cavity is contracted at its mouth a little below the town and
expands in its interior, where it must be two or three miles in width.
Its floor is fairly level and is elevated only about 200 feet above the
sea;[36] whilst its mountainous sides rise to 2,000 feet and over.
As shown in the map there are two breaks in the outline of this ancient
crater, the one on the west through which the Ndama river flows, the
other on the south where the dividing ridge, separating it from the
Nandi Valley is under 700 feet in elevation. The Nandi Gorge, as I will
term the last-named, is a narrow picturesque ravine leading through the
mountains from Nandi to Ndriti. One follows up a rocky stream-course
hemmed in by precipitous sides until the top of the gorge is reached,
when the watershed is crossed, and the descent is then made to Ndriti by
one of the tributary stream-courses of the Ndama river.
Two or three large rapid streams, after draining its mountainous slopes,
unite within the basin to form the Ndama river, which, as it issues from
its mouth, becomes a comparatively placid stream rolling sluggishly
along to the sea, some five or six miles away, with an average drop of
about thirty feet in a mile. In the course of ages the original
configuration of this great hollow has doubtless been extensively
modified by the denuding agencies. The rainfall on the mountain-slopes
must be very great, probably not under 250 inches in the year[37]; and
Ndriti, though only 200 feet above the sea, is in all probability on
account of its situation one of the wettest places in the island. The
rivers have evidently been important factors in reshaping the original
cavity.
Nearly all the rocks exposed _in situ_ in the beds of the rivers and
streams in the floor of the great Ndriti basin, and for 300 or 400 feet
up its sides are more or less highly altered basic rocks, to which the
old and the new names of greenstone and propylite may be fitly applied.
They often sparkle with pyrites, and not uncommonly effervesce with an
acid, so that one is apt to imagine one’s self in a region of limestone.
The degree of alteration varies considerably, those most altered being
light-coloured and greenish, whilst the others are darker, the specific
gravity ranging from 2·69 to 2·79. In spite of these differences almost
all of them appear to belong to the same eruptive series, being as a
rule sharply distinguished from the prevailing unaltered surface
basaltic rocks of the slopes of Seatura by the size of the felspars of
the groundmass, which average about ·3 mm. in length, whilst those of
the basaltic rocks just alluded to average only ·17 or ·18 mm. long.
These rocks are also well displayed in the sides of the Nandi Gorge; and
from their mode of exposure by river-erosion, as well as from their
relatively coarse crystalline texture, and from their alteration, it may
be inferred that they are older and more deeply situated than any of the
Seatura rocks before referred to. Whether these rocks, which extend over
an area of some square miles, have been altered by solfataric action or
contact-metamorphism,[38] I will not now say. The fact remains, however,
that they are best exposed wherever the streams have worn deeply into
the floor, and lower slopes of the great basin, or have cut down into
the mountain-mass as in the case of the Nandi Gorge. The rocks that lie
in loose blocks on the surface either at the bottom of the basin or on
its slopes extending even to the very summit of the mountain (see page
67), are characteristic blackish olivine-basalts of the type prevailing
around the mountain’s slopes. These propylites are most frequently
exposed as dykes in the beds of the rivers at the bottom of the basin.
Such dykes vary from 4 to 6 feet in thickness, and they are very
conspicuous when they stretch across the river’s breadth projecting more
or less above the water. From their frequency it may be inferred that in
many other small exposures, ill suited for displaying the mode of
occurrence of the rock, we have also to deal with dykes. Judging from
four dykes that were particularly examined, they are all vertical or
nearly so, and all run in much the same direction, namely, N.N.W.—S.S.E.
or N.W.—S.E., whether on the north or south side of the great basin. In
one instance, a rudely columnar structure across the thickness of the
dyke was observed. From their exposure in river-beds it was rarely
possible to ascertain much more than is given above. However, in the bed
of a river, a mile above Ndriti, there was an extensive exposure of a
highly altered greenish rock which was crossed by a vertical dyke, 4
feet thick, formed of a dark grey less altered rock. I have referred
these two propylites to two different genera of the augite-andesites,
the dyke-rock to genus 2, and the other to genus 4. In the case of the
dyke the rock is a little vesicular; whilst in the other it is densely
charged with pyrites. Both have been subjected to the same alteration;
but in a different degree; and it would thus seem that solfataric
influences were here in operation before and after the intrusion of the
dyke.
With reference to the characters of the alteration of these rocks of the
Ndriti basin, it may be remarked that where the change is greatest the
felspars of the groundmass are alone recognisable. The plagioclase
phenocrysts are quite disguised by alteration products, and chlorite,
viridite, epidote, calcite, pyrites, &c., occupy much of the groundmass.
Other rocks are less affected and in a few the change is only slight.
With regard to the prevailing types of the propylites of the Ndriti
Basin, it has already been observed that in most of them the
felspar-lathes of the groundmass are unusually large, the average
length being ·3 mm. From the rare occurrence of olivine in some of
the rocks that are but slightly changed, it is to be inferred that
most of them belong to the augite-andesites, and might be termed
doleritic basaltic andesites. But in other respects they differ
considerably, both as regards the presence or absence of
flow-arrangement of the felspar-lathes, and in the occurrence and
size of the plagioclase-phenocrysts, some having large porphyritic
crystals, others small phenocrysts, and others none at all. Many of
them contained a little interstitial glass. In my classification of
the augite-andesites they are assigned to genera 2, 4, 16, &c., and
additional particulars concerning their characters are given in the
description of those genera. Judging from the average large size of
the felspar-lathes it may be held that, although in other features
they often differ, some of the general conditions under which they
were produced were the same.
On the right bank of the Ndama river, opposite Ndriti, there is a
singular association of a vertical dyke of a bluish-grey basic andesite
with a reddish scoriaceous lava, apparently a flow. The dyke is about 4
feet thick and runs N.W. and S.E., like the other dykes of the basin,
exhibiting also a rudely columnar structure across its breadth. Where
the two rocks are in contact, the dyke has a vitreous border half an
inch thick, and an offshoot of the dyke, four inches wide, has
penetrated the lava, acquiring at the same time a more glassy texture.
The small size of the felspar-lathes of both rocks distinguishes them
from the dyke rocks of the basin, where the felspars are twice as long.
Both rocks show some degree of alteration.[39]
In following the valley of the Ndama River from Ndriti to Telana, about
three miles farther down, one traverses a picturesque region. Emerging
from the great basin the river flows through the rolling plains of the
“talasinga” district. Near Ndriti, and occasionally on the way to
Telana, is exposed a scoriaceous grey basaltic rock; and between two and
three miles below Ndriti there is to be observed in the river-bed
evidence of a comparatively recent flow of a highly basic scoriaceous
lava from the ancient crater of the Ndriti basin. The rock, which is
dark and fresh-looking, shows large porphyritic crystals of augite and
olivine but no plagioclase, whilst the groundmass contains a little
brown interstitial glass. Its characters will be found described under
genus 3 of the olivine-basalts (p. 255). Its specific gravity,
notwithstanding its large empty steam-pores, is 2·91. It differs
markedly from the basaltic rocks of the Seatura slopes and the Mbua and
Ndama plains, in the great porphyritic development of augite and
olivine, in the large size of the felspars and augite of the groundmass,
and in its numerous steam-holes. But in the coarseness of its small
felspars it belongs to the same type as the altered or propylitic basic
rocks of the Ndriti basin. It is probably by some such lava flow from
the old Ndriti crater that the submarine bank was formed off the
adjacent coast on which the low Lekumbi promontory has been built up.
In the numerous dykes of the Ndriti basin and in the great alteration
which their rocks have frequently undergone, we have evidence in support
of the view that this is an old crateral cavity, an opinion that is
supported by the indications of lava-flows that have issued, apparently
in later times, from the mouth of the basin. Reference has already been
made to the locality where a dyke-rock and the rock-mass, into which it
has been intruded, are both propylitic; and from this and other facts,
such as the varying degrees of alteration in different parts of the
basin, it is to be inferred that in the last stage of the activity of
this vent its bottom and sides were extensively affected by solfataric
influences. Since that period, the configuration of the crater-basin has
been greatly modified through the denuding agencies.
The absence, or at least the great rarity, of tuffs and agglomerates in
the case of Seatura is remarkable. The mountain has evidently been built
up in the mass by flows of basic lava; and from this source have no
doubt in an important degree been derived the basaltic flows of the
Ndama, Mbua, and Sarawanga plains, great streams of basalt that further
seaward have helped to form the submarine platform extending several
miles from the coast. The submarine tuffs and agglomerates that occur at
various elevations, reaching as high as 1,200 feet above the sea, in the
Sesaleka, Lekutu, Sarawanga, and Ndrandramea districts lying to the
north-west, north, and east, did not come under my notice on the Seatura
slopes. On the other hand, except in the few localities, where
scoriaceous rocks occur, the general type of the basalts is such as we
would expect to find in submarine flows. In no part of the island,
however, is the antiquity of the land-surface so well attested by the
disintegration of the basaltic flows, which extends here to depths of
ten and even twenty feet. This is in favour not only of the sufficiency
of time, but also of the ability of the denuding agencies to strip off
the surface-deposits.
However this may be, it is evident that the mountain of Seatura
possesses a history quite independent of that of the rest of the island.
I have pointed out in Chapter I. that it represents a mountain of the
Tahitian type. In its radiating valleys and in its basaltic character it
much resembles the mountainous island of Tahiti, which Dana describes as
a gently sloping cone of the Hawaiian order that through the erosion of
ages has become a dissected mountain.[40]
THE SEATOVO RANGE.—This remarkably situated mountain-range, which I have
named after a town at the foot of its western slope, extends from the
valley of the Ndama River to Solevu Bay. It attains a maximum height of
about 1,800 feet, and varies between this elevation and 1,500 feet until
in the vicinity of Solevu, where it descends as a mountainous headland
to the coast. Its summit is narrow and ridge-shaped, and although the
whole range is not interrupted by gaps it has a composite origin. At its
north end, where it is cut off from the Seatura Range by the Nandi Gorge
it helps to close in the large Ndriti basin. Towards the south an
offshoot proceeds eastward and shuts in Solevu Bay. But, although
apparently all the rocks are basic, considerable variety prevails, and
there are many puzzling points in the geological structure of this
region.
At the place where this range abuts on the Ndama valley, below Ndriti,
the grey scoriaceous basalt, before referred to, is exposed at its foot.
However, the usual blackish basaltic rocks, often carrying a little
olivine, form in mass the mountainous southern headland that culminates
in Solevu Peak (Ulu-i-matua); and the same rocks prevail in the lower
regions on the west side of the range from Vuia Point to the valley of
the Ndama River. The southern portion will be described in the account
of Solevu Bay; and I will now give the results of my journey across the
summit of the range about half a mile south of the Leading Peak of the
chart.
The eastern slopes are steep and often precipitous, whilst on the
western side there is a more or less gentle descent to the lower levels,
suggestive of a volcanic slope; and it is remarkable that whilst the
rocks exposed on the precipitous eastern side for the lower two-thirds
are sometimes markedly altered, on the western side they are
comparatively unchanged. These facts at once suggest that we have here
the western rim of a large crateral cavity, though the topography of
this district is not sufficiently well shown in the chart to enable one
to define its original limits. This inference is also supported by the
occasional scoriaceous character of the rocks below referred to.
The most frequent rocks in the upper two-thirds of the range are grey
porphyritic olivine-basalts, displaying opaque plagioclase phenocrysts
and more or less hematised olivine, the specific gravity being about
2·9. They approach in characters the grey porphyritic olivine-basalts of
the northern part of Seatura (pages 65, 66); but differ amongst other
features in the greater abundance of the olivine and in exhibiting
flow-structure. They are usually almost holocrystalline, and are
assigned for the most part to genus 14 of the olivine-basalts. They are
extensively exposed in the stream-courses on the west side between 500
and 900 feet; and huge masses of the same rocks, but containing less
olivine and more glass, and displaying much calcite, viridite, and other
alteration products, are found near the base of the eastern slopes. The
semi-vitreous condition of these rocks is represented in the large
masses of a dark very scoriaceous porphyritic lava, possessing quite a
cindery appearance, that occur on the narrow ridge-shaped summit. The
groundmass shows a few scattered felspar microliths; but it is in the
main composed of a dark opaque glass. Small cube-like crystals of
chabazite line some of the cavities.
Other basic rocks are not infrequent and apparently represent dykes.
Thus on the eastern side at 800 feet is exposed a dark-grey semi-ophitic
doleritic rock (sp. gr. 2·77) assigned to genus 12 of the
augite-andesites (page 275). The felspar-lathes average ·3 mm. in
length, and there is a little interstitial glass containing viriditic
and calcitic alteration products, the same materials filling small
rounded vesicular cavities. On the same slope between 1,000 and 1,200
feet, there are displayed fresh-looking compact non-porphyritic basaltic
andesites (sp. gr. 2·84), where the felspar-lathes average ·2 mm. and
the interstitial glass is scanty. They are referred to genus 16, species
C, of the augite-andesites. On this side also between 600 and 800 feet
occur blocks of a highly altered slightly vesicular augite-andesite
showing a little microporphyritic plagioclase. It is assigned to genus
13, species B, of the augite-andesites. In one place where it is in
position it is scoriaceous, the steam-holes being round, empty and one
to five mm. in size. In the less glassy rock it displays numerous small
irregular cavities either filled with fibrous viridite or calcite or
showing concentric zones of the two minerals. The felspar-lathes are
·15-·2 mm. in length. In blocks near the foot of the eastern slope occur
a blackish olivine-basalt (sp. gr. 2·88) of the prevailing Seatura type,
possessing a little interstitial glass and felspar-lathes with an
average length of ·2 mm. It belongs to genus 25 of the olivine-rocks....
On the western slopes at a height of 500 feet occurs a dark compact rock
(sp. gr. 2·89) with abundant olivine which is referred to genus 1 of the
olivine basalts. There is a little residual glass, the felspar-lathes
averaging only ·08 mm. in length. A similar-looking rock is exposed at
1,400 feet, which displays felspar-lathes averaging ·2 mm. long (sp. gr.
2·9). It belongs to genus 37 of the same olivine class. Here also is
assigned an aphanitic basalt, with a few scattered large plagioclase
phenocrysts and felspar-lathes averaging ·15 mm. long, which is
displayed near the base of the slope.
I could not satisfy myself as to the presence of tuffs on the slopes of
this range. Some fine argillaceous rocks exposed half-way up on either
side show no lime and contain no organic remains. One specimen beside me
is certainly a disintegrated basic rock. No agglomerates came under my
notice. In the absence or rarity of detrital rocks this part of the
range resembles the adjacent mountain of Seatura.
Although olivine-basalts prevail in this part of the Seatovo Range there
is great variety in their characters; and it does not appear possible to
explain such a diversity except to assume that we have here an old
crateral ridge which has again and again been penetrated by dykes and
has since been greatly denuded. We have here one of those singular
mountain-ridges that characterise the central portion of the island, but
differing in this respect that the submarine tuffs and agglomerates,
which there occur on the surface, even in the higher levels, are here
absent.
SOLEVU BAY.—There are few localities in the island where so many kinds
of basic rocks are displayed as around Solevu Bay. In addition to the
prevailing blackish porphyritic basalts and basaltic andesites, there
are grey porphyritic basalts, grey non-porphyritic basalts, black
basalts with abundant large crystals of olivine, &c., all of which have
their distinctive characters.
This picturesque bay is surrounded by hills. On the west side it is
inclosed by the promontory forming the southern extension of the Seatovo
range which, culminating in Ulu-i-matua, or the “Head-of-the-Strong”
peak, descends at first steeply and then gradually to the coast, where
it projects as Vulavulandre Point. On the east side is a broken line of
hills, of which Koro-i-rea, the hill known to the natives as the “Town
of the Albinos,” is the most conspicuous. Beyond it stretches the
eastern point of the bay, which the Fijians call “Ua-nguru,” that is,
“the noise of the waves.” On the shores lie the village of Nawaindo,
“the running-stream,” and the once populous town of Solevu, which has
given its name to the bay. Solevu, as its name indicates, is the place
of the “great assembly.” In the background rises the three-peaked
mountain of Koro-tolutolu, “the three towns,” which forms a continuation
inland of the eastern arm of the bay, and joins the Seatovo Range at the
head of it. Between these two ranges inclosing the bay lies the valley
of Solevu, down which descends the Solevu River to the sea. In ascending
this valley from the shore, one rises only about 100 feet above the sea
for the first mile or two.
The promontory, which in the even-topped Ulu-i-matua or Solevu Peak,
attains a height of 1,100 feet above the sea, displays on its summit and
on its eastern slopes descending to the Solevu river, and on its western
slopes reaching down to the coast at Vuia, more or less porphyritic
blackish olivine-basalts of the usual type with specific gravity
2·88-2·90. These basaltic rocks contain scanty olivine and only a little
interstitial glass. The felspars of the groundmass vary in different
localities from ·11 to ·15 mm. in average length. The rocks belong to
genus 37 of the olivine class which is described on page 262.
They are in the lower regions often decomposed to a considerable depth,
the spheroidal structure being well displayed during the weathering
process. Where this promontory terminates in the low Vulavulandre point,
these rocks give place in part to grey porphyritic olivine-basalts, with
specific gravity 2·79-2·83, which from the abundance of the macroscopic
opaque felspar look like porphyrites. They come near to the rocks
exposed on the north slopes of Seatura and in the Seatovo Range. At the
end of the point they become scoriaceous and more vitreous; but with
this exception they contain but little glass. They vary somewhat in
character and are referred to genera 2 and 38 of the olivine-class.
The prevailing rock in the interior of the Ua-nguru promontory to the
south of Koro-i-rea is the blackish porphyritic basalt, containing a
little olivine, and often much decomposed; but at the point and on the
east shores of Solevu Bay, there is a considerable variation in the
character of the basic rocks, of which the two following are the most
conspicuous. Near the village of Nawaindo, there is an apparent
intrusion of a black lava-like basalt of high basicity (specific gravity
3·01) showing abundant large olivine crystals, five or six mm. across,
with some porphyritic augite, but no macroscopic felspar. At the point
the rock is somewhat scoriaceous, with calcite occasionally filling the
cavities, whilst the olivine is so thoroughly hæmatised that it glistens
like brown mica. The compact rock contains a little devitrified
interstitial glass, the felspar-lathes being unusually small, their
average length being only ·07 mm. It belongs to genus 15, the most basic
of the genera of the olivine class represented in the island. The second
rock to be noticed is a slightly altered compact basalt without olivine
forming apparently a dyke near the coast about half way between the
village of Solevu and Ua-nguru Point. It has a specific gravity of 2·84,
the felspar lathes (·15 mm.) presenting a marked flow-arrangement,
whilst there is a fair amount of altered residual glass in irregular
spaces, a millimetre in size. The rock, on account of its
joint-structure, could be easily worked as a building-stone. It is
referred to genus 16, species B, of the augite andesites.
The hill of Koro-i-rea, which rises on the east side of the bay to a
height of 850 feet, has a ridge-shaped summit. Its upper half is
composed of a bluish-grey rock looking like a phonolite and usually
compact, except at the top of the hill, where it is a little
scoriaceous. It has, however, a specific gravity of 2·91 or 2·92, and is
in fact a pretty grey olivine-basalt studded with small olivine crystals
about a millimetre in size and showing no other phenocrysts. This type
of olivine-basalt occurs also at Ulu-i-ndali on the east side of Wainunu
Bay, but is rare in the island. It differs amongst other features from
the porphyritic olivine-basalts of the northern part of Seatura and of
the Seatovo range in the absence of plagioclase phenocrysts. There is
apparently no interstitial glass, whilst the average length of the more
or less parallel felspar-lathes is ·13 mm.[41] On the lower slopes of
the hill the common blackish porphyritic basalt or basaltic andesite is
exposed. In the grey-basaltic upper portion of this hill we have
probably an old volcanic “neck.”
Following the line of hills inland from Koro-i-rea, we cross the
intervening saddle 450 feet above the sea, and ascend the slopes of
Koro-tolutolu, a ridge-shaped mountain backing Solevu Bay, and having,
as its name indicates, three peaks, of which the highest is 1,280 feet
above the sea. My observations indicate that this mountain is formed in
mass of the common blackish-basalts described under genus 37, their
specific gravity being 2·88 to 2·94. But Koro-tolutolu has also the
peculiarity that it appears to be in mass magnetic. The rocks obtained
from its summit, half-way up its western slopes, and near its foot on
the same side, all display polarity, a character also of the rocks of
the neighbouring hills of Ulu-i-matua and Koro-i-rea, but in their cases
seemingly confined to the higher levels.[42]
Neither tuffs nor agglomerates came under my notice at Solevu Bay. This
appears to be an ancient corner of the island, from which denudation has
stripped off nearly everything that could guide us in speculating as to
its past. Although the hills of Koro-i-rea and Koro-tolutolu doubtless
represent old volcanic necks, the relation of Ulu-i-matua to the very
differently composed northern part of the same range, as described on
page 73, is extremely puzzling. Then again in the opposite sides of
Solevu Bay we see exposed the remains of lava-flows that bear no
relation to the present configuration of the surface. We may suspect,
however, that most of the volcanic energy was displayed under the sea.
NANDI BAY.—Lying north of Solevu Bay, this bay is situated between
spurs, descending to the coast from the mountainous interior. The valley
extends a long distance inland without much change of level, the
elevation 1½ miles from the coast being not over 100 feet above the sea.
At its head is the Nandi Gorge, which leads into the Ndriti Basin, the
great crateral cavity of Seatura. There are some remarkable lofty,
isolated hills in this valley that would be well worth examining.
That the bay represents the site of an old volcanic centre is indicated
by the occurrence on the shore of two basaltic dykes, one on either side
of the village of Na Savu and 300 to 400 yards apart. The eastern dyke
is perhaps 30 feet thick, whilst that to the west is scarcely half this
thickness. They exhibit an imperfect columnar structure, the columns,
which are 6 to 12 inches across, being inclined at an angle of 15° or
20° from the vertical in such a way that it may be inferred that the
molten material was ejected from some subterranean focus lying to the
northward (or inland) at an angle of 15° or 20° above the horizon. The
basalt is a compact bluish-black rock with specific gravity 2·95-2·99.
It contains abundant olivine but no other phenocrysts and very scanty
interstitial glass, whilst the felspar-lathes average ·1 mm. in length.
It is referred to genus 16 of the olivine basalts, and is remarkable for
the flow arrangement not only of the felspar-lathes but also of the
smaller olivine crystals.
Blackish basaltic rocks of the prevailing type are exposed on the
surface of the broad spur, not over 500 feet in height, that divides the
Nandi and Nasawana valleys and descends to the coast between the two
bays thus named. They belong to genus 37 of the olivine-basalts and
display a few small plagioclase phenocrysts. The felspar-lathes average
·2 mm. in length, and there is a little interstitial glass. Entering
Nasawana Bay we find ourselves on the southern slopes of Seatura, of
which the high Na Suva range that backs the bay is the southern
extension.
THE TABLE-LAND OF NA SAVU.—This remarkable plateau has an elevation
varying usually between 700 and 800 feet above the sea and a maximum
breadth of four or five miles. It is an area of basic agglomerates and
basic tuffs and lies in the hollow between the basaltic mountain of
Seatura and the acid andesitic hilly region of Ndrandramea. For the
convenience of description I have named it after the picturesque falls
of Na Savu[43] at its southern edge. These falls are celebrated in
Fijian tradition; and from the brink in old time the native desirous of
ending his life leapt into the gorge below.
After flowing sluggishly along on the surface of the table-land, the
Mbutu-mbutu River arrives suddenly at the edge of a line of cliffs of
volcanic agglomerate, that here form the southern border of the plateau,
and with a volume 30 to 40 feet across, it plunges down into the ravine
150 feet below. As shown in the view from the gorge below, there is a
break in the middle of the descent. These falls, however, are not easily
accessible. They are best approached by proceeding from Wainunu to
Ndawathumi and thence up the gorge of the Mbutu-mbutu River.
The surface of the plateau of Na Savu is densely wooded. In places it is
marshy, and here thrives the Giant Sedge (Scirpodendron costatum). The
Makita tree (Parinarium laurinum) also flourishes in the wet districts;
and in the drier localities occur the Ndakua (Dammara vitiensis) and the
Ndamanu (Calophyllum-burmanni) together with a palm of the genus
Veitchia. Here on this level watershed between the basins of the Wainunu
and Sarawanga rivers, the sluggish streams flow aimlessly along in but
slightly eroded channels; and it is not always possible to determine the
side of the island to which they ultimately direct their course. In
their beds are pebbles and irregularly formed concretions of an impure
reddish flint which I have described on page 354. On the north and south
sides the table-land is much excavated by the tributaries of the
Sarawanga and Wainunu rivers. On the west where it meets the foot of the
Seatura slope portions of columns of basaltic rocks appear on the
surface, and deep gorges are worn by the large streams descending from
the mountain. On the east towards Nuku-ni-tambua and Tambu-lotu, the
surface is also much cut up. The preservation of this table-land in a
region, where the denuding agencies are very active in their operations
all around it, is to be attributed to its being a level watershed, where
the head-waters of the Wainunu and Sarawanga rivers in part take their
rise but have little or no eroding power.
It is not easy to obtain a good general view of the district of the
falls on account of the dense forest-growth. When making the traverse
from Tambu-lotu to Ndawa-thumi, it is observed that there is here a
singular hollow, about half a mile in length, which receives the falls
at the western end. The river crosses this hollow and is at once
received into the gorge below, but there is no stream to explain the
origin of the cavity. On its north side the cliffs of agglomerate rise
to a height of 150 to 200 feet from their base, but on the south the
sides are much lower. Here there seem to be the remains of the crater of
the ancient vent from which all the tuffs and agglomerates of the
district were derived. We must look for their origin in the vicinity,
and the only evidence of a crateral cavity is this streamless hollow
extending east from the falls of Na Savu.
With reference to the basic tuffs and agglomerates of this plateau it
may be observed that they cover the massive basic rocks and are probably
not over 100 or 150 feet in maximum thickness. They are well exposed
where the streams cut into the borders of the plateau. The tuffs are
sometimes bedded and slightly inclined, and they may be fine or coarse
grained. They are more or less palagonitised hyalomelane-tuffs, being
composed mainly of fragments of a basic glass, often finely vesicular
and even fibrillar, the vacuoles being filled with different materials,
whilst the palagonitisation is well advanced. Sometimes they have a
brecciated appearance, and in that case when the alteration of the basic
glass is very extensive we find angular fragments, 1 to 2 inches across,
of a greenish palagonite imbedded in a pale matrix of palagonitic
debris, the whole rock having a soapy feel and a steatitic appearance.
This is well shown on the sides of the stream-course at Ndawathumi which
lies at the border of the table-land. These tuffs effervesce but
slightly with an acid.
The basic agglomerate is displayed in the face of the falls and in the
gorges. The blocks are as a rule composed of semi-vitreous basaltic
andesites of varying type, showing no olivine and containing a fair
amount of smoky glass in the groundmass. At times they are scoriaceous
and display amygdules of calcite or a zeolite. In places the rock shows
large phenocrysts of plagioclase and a semi-ophitic groundmass, when it
is referred to the porphyritic group of genus 9 of the augite-class. In
a few of the scoriaceous blocks the augite of the groundmass is for the
most part prismatic and rarely granular (genus 5).
The massive rocks underlying the agglomerates in the vicinity of Na Savu
are aphanitic augite-andesites, differing in important characters from
the rocks of the agglomerates. They probably represent ancient lava
flows of the Na Savu vent. They are compact (sp. gr. 2·72-2·76), and
display a groundmass formed of a felt of felspar-lathes, averaging ·05
or ·06 mm. only in length, and in flow-arrangement. That occurring just
below the falls is almost aphanitic, but is referred to genus 13,
species A, sub-species _a_, of the augite-andesites. The rock from the
gorge below is of the same character, but on account of its opaque
plagioclase phenocrysts it is referred to genus 14, and is described on
p. 279.
In one place on the plateau a tuff-agglomerate is penetrated by veins, a
few inches thick, formed apparently of a finely brecciated tuff of basic
glass fragments in a palagonitic matrix. It is, however, pointed out on
p. 340 that they were originally veins of basaltic glass which have been
subjected to crushing, and that the palagonite has since been produced.
In concluding this description of the table-land of Na Savu, it may be
inferred that the source of its basic tuffs and agglomerates is to be
found in the same locality; and probably the original vent is now
represented by the hollow extending eastward from the falls. With the
exception of a large block of silicified coral found in the vicinity of
Ndawathumi and of the impure flints of the surface of the plateau, which
are described on pages 354, &c., no direct testimony of its submarine
origin offered itself to me. The palagonitic characters of the tuffs
afford, however, indirect evidence in this connection; and indeed the
occurrence of submarine tuffs and limestones in the vicinity of
Tembenindio on its lower northern slopes (see page 131), and the
existence at elevations of several hundred feet above the sea of
fossiliferous tuffs and clays in the Wainunu and Ndrandramea districts
to the eastward, afford strong presumptive evidence that the tuffs and
agglomerates of the table-land were deposited under the sea, and I may
add in a period subsequent to that of the formation of the great
basaltic flows of Seatura and Wainunu.
CHAPTER VI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
THE BASALTIC PLATEAU OF WAINUNU.—This table-land extends for a distance
of seven miles from the base of the Ndrandramea mountains in the heart
of the island, where it is elevated 1,100 to 1,200 feet above the sea,
to the valley immediately north of the hill of Ulu-i-ndali, where within
a short distance of its termination it still retains a height of 700 to
800 feet. Limited on the west by the valley of the Wainunu River and on
the east by that of the Yanawai River, its breadth varies usually
between four or five miles. It is best seen in profile when viewed from
the south-west on the western shores of Wainunu Bay, between Korolevu
and Nasawana, when it presents itself to the eye as a table-land,
descending with a very gradual slope from the interior towards the
coast. From such a point of view the two great basaltic slopes of
Seatura and Wainunu may be seen together, the former descending eastward
to the Wainunu valley at an angle of 3 or 4 degrees, the latter
descending at right angles to it to the southward with a similar small
gradient of 2 or 3 degrees.
In the profile of the island attached to this work the Seatura slope is
well shown; but that of the Wainunu table-land being seen from the south
is represented only by a level contour-line at the base of the
Ndrandramea mountains. The two great series of basaltic flows, though
closely approaching in a direction at right angles to each other, do not
come into actual contact, and the intervening space is now occupied by
the valley of the Wainunu River. In the accompanying rude outline-sketch
of this region, as seen from off the mouth of the Wainunu estuary, the
relation of this valley to the two great series of basaltic flows is
clearly shown. On the left is the foot of the Seatura basaltic slope; on
the right is the Wainunu basaltic table-land; and between them lie the
estuary and valley of the Wainunu, at the back of which appears the “Na
Savu” table-land, formed of basic tuffs and agglomerates. Behind all
there rise up suddenly the Ndrandramea mountains formed of acid
andesites; whilst in the foreground to the right is the hill of
Ulu-i-ndali, which is composed in the mass of a grey basalt of a type
quite different from the blackish basaltic rocks of the Seatura slope
and of the Wainunu table-land. It was from this view off the mouth of
the estuary that I received my first lesson in studying the structural
formation of the island. I kept it always in my mind’s eye, and for
months in an almost unmapped region it was my only guide.
[Illustration: Profile, looking north from off the mouth of the Wainunu
River.]
The gradual slope of the Wainunu table-land from an elevation of 1,100
or 1,200 feet in the interior to 700 or 800 feet near the coast has
already been referred to. Beyond this lower limit it descends much more
rapidly and within less than a mile it terminates at Masusu in a
steep-sided declivity 300 feet high opposite Ulu-i-ndali, and in a
gentler slope on the eastern side in the Ndranimako district. Its
somewhat undulating surface is well wooded; but on account of the small
gradient the small streams on the table-land do not excavate deep
channels, but flow slowly along in shallow courses and often stagnate in
swampy land where the interesting “Scirpodendron costatum,” the
giant-sedge, flourishes. In their beds occur reddish flinty concretions,
up to 3 inches across in size, and magnetic iron sand in great
abundance. A sample of this sand roughly washed on the spot contains 77
per cent. of magnetic iron.[44]
Basaltic rocks, often exhibiting a columnar structure, are exposed at
intervals on the surface and slopes of this table-land all over its
area. Now and then when traversing this region one comes upon a tract
strewn with large blocks, amongst which occur fragments of huge columns
3 to 4 feet in diameter; but it is on the steep southern slopes of the
plateau in the vicinity of Ndavutu and Masusu that the most extensive
exposures of columnar basalt are to be found. Here there have been large
clearings made for the tea-plantations, and portions of columns 2 to 3
feet in thickness are scattered all over the slopes and surface of
Masusu.
A very interesting exposure occurs on the southern edge of the Masusu
flat facing Ulu-i-ndali. Here there is displayed in the face of a
waterfall a mass of basalt about 40 feet deep, formed of regular
cross-jointed columns, 3 to 4 feet in diameter and often pentagonal in
shape, which are almost perpendicular, being inclined about five degrees
from the vertical. But in the upper portion of the fall the columns are
smaller (2 to 3 feet across) and become arched and nearly horizontal.
This was the only section of the inner mass of the basaltic flows that I
found, and here the columns are almost vertical. In this locality
several other exposures of the columnar basalt occur; but they are all
at the surface and the columns are nearly horizontal or very much
inclined from the vertical, being often pentagonal in form, 2 to 3 feet
across, and sometimes curved with joints 10 to 20 feet in length.
Neither vesicular nor scoriaceous rocks came under my notice in this
region, and the presence of pteropod-ooze deposits and of foraminiferous
clays and tuffs on the slopes of the basaltic tableland indicates that
the flows were submarine. The common character of a sub-aërial basaltic
flow, where there are large vertical columns below and smaller radiating
columns above, did not present itself; and it is probable that the
singular arrangement of the columns in the upper portion of these flows
may be connected with the conditions of depth under which the flows took
place.
It is apparent from the description given by Dana of the columnar basalt
of Tahiti[45] that it was formed under different conditions from those
under which the basaltic flows of Wainunu and Seatura were formed. The
columns composing a cliff 500 feet high in the Matavai valley were 10 to
20 inches across. A bluff, 200 to 300 feet high, in another part of the
valley, was made up of columns 5 to 8 inches in width. The tallest cliff
displayed in places converging and curved columns, which is attributed
to the unequal cooling of the interior of the mass; but it is evident
from a diagram given by the author that the columns were not inclined at
a large angle from the perpendicular.[46] He also refers to some prisms
of a grey basalt exposed just below the Wailuku Falls near Hilo in the
large island of Hawaii which were 8 feet in diameter and were surmounted
by others only 1 to 4 feet across.
The basalts of the Wainunu table-land are blackish and non-vesicular,
with a density of 2·87 to 2·90. They all carry olivine and
microporphyritic plagioclase, and display a little interstitial glass,
and the felspar-lathes are usually in plexus-arrangement, being stout
and often showing twin lamellæ. But the rocks exhibit important
variations in different localities as regards the amount of olivine, the
length of the felspar-lathes, the presence or absence of the ophitic
character, &c., and they are grouped in different genera of the olivine
class (1, 13, 25, 33). Probably the type of genus 25, with scanty
olivine and granular augite, would prevail.
From the varying size of the felspars of the groundmass it is apparent
that the flows are not all of the same character. At Masusu, where the
rock is doleritic in texture, they average from ·25 to ·3 mm. in length.
A mile further north, they are about ·17 mm. long, and two miles more to
the north they average only ·1 mm. in length. It is probable that a
semi-vitreous basaltic andesite (spec. grav. 2·73), that shows no
olivine and is referred to the porphyritic sub-genus of genus 9 of the
augite-andesites, which is exposed in the stream-courses near the base
of the dacitic mountains of the interior, is the product of a later
eruption. Occasionally one finds, as at Thongea in the Wainunu valley, a
basalt rich in olivine (spec. grav. 2·95), the felspars of the base
averaging ·1 mm. in length. It may be remarked here that one cannot draw
a sharp distinction between the basalts of this region and those of the
adjacent eastern slope of Seatura. Their specific gravity is about the
same (2·87 to 2·90); but the coarse texture of the Masusu basalts did
not come under my notice in the last locality, where the felspars of the
groundmass average ·18 mm. in length or about two-thirds the length of
those of the Masusu rocks.
By referring to the section across this part of the island, it will be
observed that the basaltic lavas of this table-land must have issued
from some fissure near the south side of the base of the Ndrandramea
mountains. In crossing the head of this plateau on the way from Nambuna
to Ndrawa one passes from the region of the acid andesites into that of
the basalts. The track first skirts the base of Mount Wawa-Levu, where
the prevailing altered dacitic rocks are exposed in a much decomposed
condition in the stream-courses. Then there is a gradual ascent through
somewhat broken country to reach the western slope of the table-land,
and here are at first displayed the semi-vitreous basaltic andesites
just referred to.
The Wainunu table-land is bisected in a singular fashion by the Ndavutu
River. Since, however, the deep and often gorge-like channel of the
river displays submarine deposits incrusting the basaltic slopes on its
sides, it is evident that the break in the basaltic table-land existed
in part at least before the emergence.
With regard to the total thickness of the basaltic flows of this plateau
I have only a few data. In the bed of the Ndavutu River opposite
Vunivuvundi, and about 400 feet above the sea, there is exposed a
greyish porphyritic rock showing pyrites, apparently an altered
andesite. If this is the bed-rock, the basaltic plateau in that locality
would be 300 to 400 feet in thickness. This is rather over the thickness
of the end of the table-land at Masusu.
I pass on now to consider briefly the submarine deposits that overlie
the marginal slopes of this basaltic table-land in places. They are for
the most part pteropod and foraminiferous ooze-rocks and are extensively
represented on the surface and slopes of the Nandua flat to the north of
Ndavutu, where they occur at all elevations up to 500 feet above the
sea. They are also displayed on the eastern slopes overlooking the
Yanawai but at rather lower heights; and little patches of them occur
here and there in different places but not exceeding 500 feet in
elevation. These friable clayey rocks, which contain from 30 to 40 per
cent. of carbonate of lime, are described in detail on page 320. It may
however be remarked here that these deposits are but partly derived from
the degradation of the submerged basaltic table-land or from the
washings of a basaltic coast. They were formed in a clear sea-way, but
probably at no great depth, at a time when the basaltic plateau was
submerged below the level of breaker-action.
It is remarkable that these deposits do not repose directly on the
basaltic rock. In one place below the Nandua tea-plantation, where there
is a steep descent to the river of about 250 feet, the pteropod
ooze-rock, which is exposed in the upper half, passes down into a
chocolate-coloured marl that contains 5 per cent. of carbonate of lime
and is horizontally bedded. It is composed in the main of fine
palagonitic debris, with some fragments of minerals, &c., and contains a
few microscopic tests of foraminifera. This deposit passes down into
apparently a rock of pure palagonite. The succession of these beds and
their characters are described more in detail on page 344; and as
indicated in the diagram there given it is to be inferred that a very
extensive formation of palagonite has taken place on the surface of a
submarine basaltic flow.
On a similar slope of the Nandua district, and about half a mile nearer
Ndavutu, the pteropod ooze-rock overlies a coarse zeolitic
palagonite-tuff composed in great part of fragments of a highly altered
vacuolar basic glass, but without organic remains. These tuffs are
horizontally stratified. Tuffs precisely similar occur on the northern
slopes of Ulu-i-ndali three miles to the south. They are all described
in detail on page 335.
Some miles up the valley of the Ndavutu River on the steep slope
descending from Vunivuvundi to the river, and on the sides of the river
lower down, are exposed dark palagonitic and sometimes calcareous clays
and tuffs. I traced them as high as 450 feet above the sea where they
were bedded and dipped gently to the west. In the river-channel they
were mostly confined to the right bank, the slope on the other side
being strewn with large fragments of columnar basalt. At the mouth of
the Ndavutu River, there are exposed tufaceous sandstones and a
tuff-conglomerate, probably in great part formed of palagonitic
materials, but I have kept no specimens.
There is much that is puzzling about the tuffs of the region between
Ndavutu and Vunivuvundi. The surface pteropod and foraminiferous
ooze-rocks, that are found here and on the Yanawai or eastern border of
the basaltic plateau and in other localities, offer no difficulties; but
the origin of the palagonitic tuffs that in places lie beneath them is
not so easy to explain. At Mr. Simpson’s old estate on the Nandua flat
one finds numbers of huge blocks of columnar basalt scattered about on
the slope descending to the river; and in places there is exposed in a
small stream, up to a height of 500 feet, a fossiliferous ooze-rock
containing marine shells. The ooze-rock is evidently an incrusting
deposit; but when one goes down to the river-side, which is there about
200 feet above the sea, one finds displayed _in situ_ in the river-bed
an amygdaloidal basic lava with coarse tuffs and agglomerates a little
lower down.
THE HILL OF ULU-I-NDALI.—The meaning of the name of this hill is “Head
of the rope.” It is noted on account of the dense growth of tall forest
trees that clothes its surface, such as the Vesi (Afzelia bijuga), the
Ndamanu (Calophyllum burmanni), the Ndakua (Dammara vitiensis), the
Wathi-wathi (Sterculia sp.) &c.; and it may be that its name is
connected with the launching of the large canoes that were at one time
constructed on its slopes.
Ulu-i-ndali, which has a broad level summit 1,100 to 1,150 feet in
height, rises on the left side of the mouth of the Wainunu estuary. Its
relation to the surrounding region is partly shown in the rough sketch
given on page 83. It is separated from the basaltic table-land to the
north by a deep and wide valley, the bottom of which is raised only a
few feet above the sea; the small stream known as Ndawa-ndingo, that
apparently flows through it, is merely a branch of the Wainunu estuary,
the tide ascending it for some distance. This singular valley, like the
main valley of the Wainunu, dates back in great part to the period
preceding the emergence of this region. The steep basaltic slopes of
Masusu, strewn with fragments of large columns, bound it on the north.
On its south side are the lower slopes of Ulu-i-ndali which are composed
of volcanic tuffs.
A long spur descends to the south from Ulu-i-ndali to form the rocky
promontory of Vatu Vono or “Stone turtle,” so-named from the fanciful
resemblance of the large rounded blocks of basalt on the shore to the
backs of turtles. To the south-east extend the low tuff-formed Ravi-ravi
plains which are but slightly elevated above the sea. The Ulu-i-ndali
range is apparently connected by a “col” with a range of similar height
to the eastward, the highest peak of which is about 3 miles distant.
A more or less coarse doleritic grey olivine-basalt forms the mass of
this hill and is chiefly exposed in its upper portion. Around its
slopes, extending from the coast usually halfway up the hill, are
blackish-brown olivine-basalts; they differ amongst other points from
the grey basalts—which are practically holocrystalline, in their greater
amount of interstitial glass, to which, doubtless, is due their dark
colour. These dark basalts also occur scantily on the summit; but from
their greater prevalence on the lower slopes and from some other of
their characters, it may be inferred that they are in the main formed at
the surface. Outside all, on the north and south sides of the hill, are
exposed coarse tuffs composed of fragments of palagonitised vacuolar
basic glass and containing much secondary zeolitic and calcitic
materials. They are purely of eruptive origin, and although containing
no organic remains were doubtless, as in the case of the precisely
similar tuffs of the neighbouring district of Nandua, deposited under
the sea. A description of their characters is given on page 335. Such
tuffs extend as high as 300 feet above the sea on the north-west slopes,
where there are exposures, 10 to 12 feet in thickness, in the dry stream
courses; and here they may be seen overlying the basalt and rudely
bedded, dipping away from the summit at an angle of 15 degrees.
The grey olivine-basalts of Ulu-i-ndali, which often look like
clinkstone, range generally in specific gravity from 2·9 to 2·95. They
contain microporphyritic olivine in abundance, which is usually more or
less hæmatised and in extreme cases of the change looks like brown mica.
Most of them are referred to genus 16 of the olivine class and their
characters will be found described on page 258. The felspar-lathes are
stout and show sometimes lamellar twinning, and on account of their
large size (·2 to ·5 mm in average length) the rock acquires a doleritic
texture. They display as a rule a flow arrangement around the olivine
crystals. Augite granules occur in great abundance, and there is rarely
any interstitial glass.
These grey olivine-basalts are as a rule non-vesicular, but rocks with
minute irregular cavities, though without glass, occur scantily on the
upper slopes. They come near to the grey olivine-basalts of the hill of
Koro-i-rea in the Solevu district, as described on page 77; but they
differ in their doleritic or coarser texture, the felspar-lathes in the
last-named locality being much smaller, their average length being
·12 mm.
The blackish basalts, mostly characteristic of the lower slopes of
Ulu-i-ndali, vary somewhat in character; but they may on the whole be
regarded as surface forms of the more deeply situated grey basalts which
are practically holocrystalline. The rock of this kind that prevails on
the south and west sides has a specific gravity of 2·96. It is referred
to the same genus (16) as the grey basalts, but differs from them in the
circumstance that the microporphyritic olivine is serpentinised and not
hæmatised, and in the occurrence of a fair amount of devitrified
interstitial glass, to which probably the dark colour of the rock is
due.... The dark aphanitic basalt, with flinty fracture and a specific
gravity of 3·00, that is displayed in Vatu Vono Point, is merely a
compact surface variety of the more coarse-textured grey basalts, being
referred to the same genus. Here there is a great abundance of
microporphyritic olivine in a groundmass of parallel felspar-lathes and
augite grains; but the felspars are unusually small, averaging ·1 mm. in
length; and there is a much larger amount of fine magnetite than in the
grey basalts. There seems to be no interstitial glass; and the olivine
when not fresh is usually serpentinised but occasionally hæmatised.
The dark basalts of Ulu-i-ndali when they occur on its upper slopes
become ophitic. A specimen lying beside me has a specific gravity of
2·91. Allowing for the structural differences, it appears as an ophitic
surface variety of the deeper seated grey basalts. A description of it
is given under genus 12 on page 256, of which it forms the type.
From the data above given, the hill of Ulu-i-ndali is to be regarded as
the basal portion of a submarine volcano still retaining part of its
ash-coverings. The grey doleritic basalts probably represent the core
and the dark fine-grained basalts represent the flows of this ancient
vent.
THE KUMBULAU PENINSULA.—South-east of Ulu-i-ndali stretches a remarkable
“talasinga” district which for convenience I will call the peninsula of
Kumbulau. Its south or seaward border is broken and hilly, and presents
an irregular line of hills 300 to 470 feet in height, extending from
Kumbulau Point to Soni-soni Island, which is almost connected with the
coast. The rest of the peninsula is a low-lying and often marshy plain,
which, though elevated in some places 20 to 25 feet above the sea, is
usually much lower. On the north-east side of the isthmus is the narrow
Nandi inlet, bordered by low mangrove-belts, which represents the broad
channel that in a very recent period of the island’s history cut through
the present neck of the peninsula between the head of the Nandi inlet
and Ravi-ravi.
Stratified and often steeply inclined tuff-sandstones and clays, more or
less basic and palagonitic in character, form together with basaltic
agglomerates the prevailing rocks of the peninsula, whether in the hilly
portion or in the plains. They belong to the basic tuffs of mixed
composition described on page 330; and though the agency of eruptions
can be recognised in their components they are also the products of
marine erosion.
Some of the hills represent volcanic “necks”; whilst the low narrow
promontory between Kiombo and Soni-soni Island has been formed by an old
basaltic flow.
I will begin the description of this peninsula with the eastern
extremity north of Kumbulau Point, the interior of which is cut up into
ridgy hills 300 to 350 feet in height. On its eastern coast are exposed
volcanic agglomerates, composed of large blocks, which from their
dimensions given below would weigh between one-third and two-thirds of a
ton, a size indicating the immediate vicinity of the vent, now
obliterated, from which they were originally ejected. Near Kumbulau
Point the blocks, which are made of basaltic andesite, measure five or
six cubic feet. Further north in the vicinity of Vatu-Ndamu, the
precipitous coast cliffs are composed of agglomerates, the large blocks
of which, often ten cubic feet in dimension, are formed, not of the
prevailing basaltic andesites, as in other parts of the peninsula, but
of a grey hornblende-andesite. This singular appearance of an acid
andesite in a region of basic rocks has no doubt given rise to the
native name of Vatu-Ndamu, “the red or brown stone.” It belongs to the
second order of the hornblende-hypersthene-andesites, and is described
on page 298.
Proceeding along the south coast westward from Kumbulau Point, before
arriving at the village of Na Tokalau we pass from the district of
agglomerates into that of the bedded tufaceous sandstones and clays
which are exposed all along the coast to Kiombo about three miles away.
The transition is indicated by the agglomerates becoming interstratified
with the tuff-beds. These sedimentary tuffs are as a rule steeply
inclined at angles of 20 to 40 degrees, the prevailing direction of the
dip being to the north-east, its uniformity for such a length of coast
being noteworthy. These beds however are occasionally “crumpled”; and
here and there a globular structure is developed.
The hills of this region of sedimentary tuffs between Na Tokalau and
Kiombo are the highest of the peninsula. They usually attain a height of
400 feet, but do not reach 500 feet. From each of them descends to the
coast a spur terminating in a rocky point; whilst between these points
lie low sandy flats, where the native villages of Levuka, Kiombo, &c.,
are situated. The tuff-rocks extend to the top of the hills behind Na
Tokalau, and probably this will be found true of most of the other
hills. Agglomerates are not common in the district. In the point west of
Na Tokalau, however, they are overlaid by basaltic agglomerates, some of
the blocks being scoriaceous. In the point east of Levuka, a
chocolate-coloured somewhat calcareous tuff-clay occurs interstratified
in thin beds with the coarser deposits.
The general characters of these tuff-sandstones and tuff-clays have
already been briefly referred to. The former are much more prevalent and
non-calcareous; the latter are sometimes a little calcareous and look
like marl, and may perhaps contain a few tests of foraminifera. Both are
formed of the debris of basic rocks and are more or less palagonitic.
The coarser deposits are described as sample A on page 330. At times
these tuffs are composed of much coarser fragments of the same
materials, some of them a centimetre in size. A type of tuff
intermediate in character is not uncommon.
The promontory that lies between Kiombo and Soni-soni Island has been
formed by a remarkable basaltic flow. The low tongue, about 50 feet high
and 200 to 300 yards across, in which it terminates, was originally
severed by a passage worn by the sea from the main portion; but it is
now joined by a low tract only 2 or 3 feet above the beach and partly
occupied by mangroves.
The structure of the flow is well exhibited in the shore-flat and
coast-cliffs west of Kiombo, and extending to the end of the point. The
waves have here cut into its mass and exposed its structure. Its lower
part, as exposed in the shore-flat, is made of a compact hemicrystalline
basalt; whilst its upper portion, as displayed in the cliffs, 30 or 35
feet in height, is composed of vitreous and semi-vitreous forms of the
same rock looking like pitchstone. The upper vitreous part is sometimes
massive; but usually it is rubbly, with a tendency to form spheroidal
masses. All transitions can there be traced between the hemicrystalline
rock of the shore-flat and the vitreous rock of the cliffs.
The rock of the shore-flat, which has a specific gravity of 2·83, is a
blackish porphyritic basalt with scanty olivine, and on account of the
semi-ophitic character of the augites of the groundmass it is placed in
genus 33 of the olivine class. The plagioclase phenocrysts are 3 to
5 mm. in size. About half of the groundmass is made up of felspar-lathes
(·17 mm. long) and large augites (·11 mm.), the rest consisting of a
smoky devitrified glass containing a few irregular “lacunæ” filled with
the residual magma in the form of a reddish-brown opaque palagonite-like
material. The rock intermediate between the lower and upper portions of
the flow is also intermediate in character, having a specific gravity of
2·77, whilst quite three-fourths of the groundmass are of smoky glass.
The vitreous rocks of the cliffs, though usually rubbly in appearance,
have also the aspect in places of brecciated pitchstone tuffs with the
interstices filled with waxy palagonite; but the microscopical
examination shows that we have not to deal with a rock of detrital
origin. We have here the effects of the breaking up and crushing _in
situ_ of a dark-brown isotropic basic glass[47] carrying porphyritic
plagioclase. The interspaces then became partially filled with the finer
fragments of the glass and of the crushed felspar; but they were in the
main occupied by a still liquid magma which penetrated into the cracks
of the glass-fragments and into those of the felspars, where the
fractured portions in some cases remained _in position_. There it has
become devitrified and often palagonitised. Whether this liquid magma
was produced by a partial remelting resulting from the heat developed
during the crushing of the glassy upper portion of the flow during the
contracting process, or whether it was squeezed upwards from the less
consolidated lower portion, I cannot determine, although the last
supposition seems more probable. At all events the edges of the
glass-fragments are peculiarly eroded as if by the magma. (The bearing
of these facts on the origin of palagonite is discussed in Chapter
XXIV.)
I infer that this flow has descended from the hills west of Kiombo. Huge
masses of agglomerate are exposed in the lower third of the hill marked
“470 feet” in the chart, and immediately north of the town. Fine clayey
tuffs are exposed in the hill at the back and to the westward of this
place; but the locality requires a more detailed examination. The
absence to all appearance of vesicular and scoriaceous rocks in the case
of this basaltic flow is remarkable. This would not have been expected
in the case of a supra-marine flow; and indeed the testimony of the
tuffs of this peninsula sufficiently indicates that during their
deposition the whole district was submerged.
The future inquirer will doubtless discover some old volcanic “necks” in
the hills of this peninsula. One such hill overlooks the Soni-soni inlet
about a mile west of Kiombo. It is a singular isolated hill which I have
named Bare-poll Peak for descriptive purposes. In my notes its height is
stated as 120 feet, but it appeared to me to be rather higher than this.
It is capped by two huge masses, 14 or 15 feet high, of a dark grey
slightly scoriaceous augite-andesite with a cryptocrystalline
groundmass, which apparently form the uppermost portion of a volcanic
“neck” or pipe. According to the size of these rock-masses the “neck”
would have a circumference of 80 or 90 feet. These masses are in part
incrusted with agglomerate.
The adjacent island of Soni-soni, which is almost joined by the
mangrove-belt to the adjoining coast, probably represents one of the
numerous small vents that were once active in this region. Its single
peak is 460 feet in height. As there did not seem much prospect of
finding rocks exposed on its upper part, its slopes being densely
covered with tall reeds, my examination was confined to the lower
portion during a walk around the island. On its east and north sides
occur rocks of much the same character as those exposed in the
neighbouring low promontory to the east of it. In addition to
agglomerates and basaltic andesites occurred a rubbly pitchstone
composed of fragments, up to a centimetre in size, of an opaque brown
glass displaying a few phenocrysts of plagioclase and pyroxene, the
interstices being filled with crushed fragments of the phenocrysts and
finer glass debris. This rock is allied to the “crush-tuffs” described
on page 334. It may be added that the basic tuffs are more frequent on
the west and south sides of the island.
The low island of Na Vatu in the midst of the Soni-soni inlet is about
250 feet across and only 3 or 4 feet above the ordinary high-tide level.
In 1898, when I visited it, this tiny island possessed about 20 houses
and a population of 60 or 70 persons, and I gather from Hazlewood’s
account of these islands that Na Vatu was crowded with houses more than
half a century ago. It was apparently in the first place a sand-key, and
is protected against the wash of the waves by a low sea-wall formed of
large blocks of stone.
An interesting exposure of bedded tuffs and clays is displayed at
Ravi-ravi on the west side of the peninsula. A broad shore-flat has been
formed by the marine erosion of a line of coast composed of these
deposits. The strike is well exhibited, the dip being about 30 degrees
N. by W. Here there are alternating beds, a few inches thick, of coarse
and fine tufaceous sandstones, sometimes calcareous, with marls or
calcareous clays. The mineral fragments of the coarser rocks are
composed of plagioclase, augite and rhombic pyroxene, the last being
abundant and giving a more acid character to these deposits. The
calcareous fragments appear to be principally shell debris. The marl is
in part composed of much finer detritus of the same minerals. The other
materials of these deposits are derived from the degradation of basic
andesitic rocks, and include also a little palagonite. To the westward
of Ravi-ravi these beds show signs of disturbance, being steeply tilted
to the N.W. Agglomerates also occur in the disturbed area.
The history of the Kumbulau peninsula is evidently the history of the
eruptive phases of a number of more or less submerged small vents and of
the periods of great marine erosion that followed during the emergence
of this part of the island. The absence or rarity of dykes is
remarkable; but most of the hills would represent volcanic “necks”
whether of massive rock, tuff, or agglomerate.
THE DISTRICT BETWEEN THE KUMBULAU PENINSULA AND THE YANAWAI
RIVER.—Between Nandi Inlet and the village of Rewa the sea-border is low
and often swampy, whilst occasional spurs descend from the inland range
into the swamps without reaching the coast. Pebbles of “soapstone”
(foraminiferous mud-rock) occur in streams and are no doubt derived from
the incrusting deposits of the neighbouring hill slopes. In one
stream-bed in the swamps is exposed _in situ_ a remarkable
chocolate-coloured rock that looks like a greasy pitchstone or a
palagonite-rock. It is however of detrital origin, and is composed in
mass of minute fragments of a basic, sometimes vacuolar, glass in great
part converted into palagonite; whilst there are a number of broken
crystals of olivine and plagioclase. Through the palagonitic alteration
the fragmental character is somewhat obscured, zeolites being
extensively developed in the interstices. A little lime occurs and there
is a suspicion of foraminifera. The deposit belongs to the group of
palagonite marls described on page 335. The deeper rocks of the district
are represented in a spur by an altered augite-andesite, originally
hemicrystalline and containing much granular epidote.
Proceeding northward from the village of Rewa, one crosses another spur
descending from the inland range. It is formed in mass of a dark
doleritic olivine-basalt (spec. grav. 2·91) characterised by the length
of the felspar-lathes (·28 mm.), possessing a little interstitial glass,
and referred to genus 25 of the olivine class. It probably represents an
ancient flow. Its surface is incrusted, as high as the road ascends,
nearly 200 feet above the sea, by fine and coarse palagonite-tuffs;
whilst the pebbles of foraminiferous mud-rock in the stream indicate the
existence of incrusting marine deposits further up the slopes. The road
then leads down into a low-lying undulating district that forms the sea
border as far as the mouth of the Yanawai, and reaches about two miles
inland without exceeding an elevation of 100 feet, although low hills
occur here and there. This region is fronted by mangrove swamps and is
traversed by the Matasawalevu and Ndranimako streams. It is a district
of basic tuffs and foraminiferous clays, which, as shown below, extend
up the slopes of the basaltic Wainunu table-land that lies behind. The
soil in all the low country between Rewa and the Yanawai is red, heavy,
wet, and clayey; and affords a contrast to the dry friable soil of the
Kumbulau and Kiombo region to the southward.
The Navakavura plain lying north of Rewa deserves especial mention. It
is a low, swampy district which a mile inland is raised only 20 or 30
feet above the sea, and is mostly occupied by casuarina and pandanus
trees. Red argillaceous rocks, representing more or less decomposed
palagonite coarse and fine tuffs, are exposed in the banks of the
streams. Some of them were originally made up of fragments of basic
glass which after being palagonitised became much disintegrated. A
typical specimen by my side has a soapy feel and looks like a lump of
red clay. Microscopical examination shows that it is composed in mass of
palagonite, but in an extreme stage of the alteration process.
After traversing the Navakavura plain, one crosses a low hill rather
over 100 feet above the sea before descending to Ndranimako. On the hill
are exposed reddish clay-rocks, much weathered, but showing vegetable
remains and a few univalve and bivalve shells. Extensive submarine
deposits occur in the inland district west of Ndranimako. They are the
usual foraminiferous clay-rocks or “soapstones,” and in places they
contain pteropod shells. They are well displayed in river-banks, and in
the hill-slopes on either side; but they are probably of no great
thickness since in one locality named Na Savu, nearly two miles west of
Ndranimako, the underlying basaltic rock is exposed in the bed of a
gully, the sides being of “soapstone.” These deposits were formed in
comparatively deep water.[48] The greatest elevation at which they were
observed was about 100 feet; but this was as high as I reached in the
ascent of the river. According to the natives, who are very observant in
such matters, these submarine deposits extend up the slopes of the
adjacent Wainunu plateau. On page 86 reference is made to their
occurrence on the slopes of this basaltic table-land, 1½ or 2 miles
farther north.
In the district between the Ndranimako and the Yanawai rivers basic
tuffs and “soapstone” prevail. In this locality, and especially in the
vicinity of Ndranimako, siliceous concretions 2 to 3 inches across,
occur in places on the surface. Their nature is described in Chapter
XXV.
From the foregoing remarks it may be inferred that the sea-border
between the Kumbulau Peninsula and the Yanawai River is formed of
submarine deposits overlying basic rocks which probably represent
ancient flows. Some of the deposits are largely formed of glassy erupted
materials, which have been converted into palagonite. Others again are
more characteristic sedimentary formations accumulated in relatively
deep water.
CHAPTER VII
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(_continued_)
THE NDRANDRAMEA DISTRICT
THIS hilly region of acid andesites is a continuation of the mountainous
backbone of the island, being separated from the basaltic mountain of
Seatura by the saddle formed by the Na Savu table-land. These acid
andesites exhibit in nearly all cases a felsitic groundmass and
phenocrysts of plagioclase and rhombic pyroxene; whilst many of them are
characterised by brown hornblende more or less pseudomorphosed in the
manner described on page 306, and a few display porphyritic quartz.
Although these rocks have a common facies, they vary considerably among
themselves; and it is difficult to find a term that would strictly
include them all. A general description of their characters is given in
the chapter on the Acid Andesites.
In this interesting region a number of hills or mountains formed in mass
of acid andesites rise up abruptly without any regular arrangement
within an area measuring 5 by 6 miles, and elevated 600 to 1,000 feet
above the sea. Of these hills, thirteen in all, nine range in height
between 1,600 and 2,500 feet above the sea, none of the others rising
less than 1,000 feet above that level. But the actual height of each
hill above the country at its base is much less than this. The height of
the hill-mass, in five or six of the largest, ranges between 900 and
1,200 feet, whilst in the smaller hills it varies between 400 and 800
feet. (See accompanying plan.)
[Illustration: The NDRANDRAMEA District from the westward. The hills and
mountains are of acid andesites and dacites. The foreground is elevated
about 450 feet above the sea.]
[_Face p. 98._
[Illustration: Rough plan of the Ndrandramea district in Vanua Levu;
made with prismatic compass and aneroid by H. B. Guppy.]
These hills have sometimes a rounded profile, when their summits are
usually wooded. Others again terminate in conical bare rocky peaks,
either pointed or truncated. They have often precipitous slopes and
display vertical cliff-faces high up their sides. Their arrangement is
rather singular. To the south and apart from the others lies Soloa Levu
(1,600 feet). Navuningumu (1,930 feet) is similarly isolated on the
north. On the east rises Ngaingai (2,430 feet), the highest of the
peaks, with Wawa Levu (2,000 feet), Vatu Kerimasi (1,900 feet), Vatu
Vanaya (1,600 feet), and Mbona Lailai (2,100 feet) closely clustered by
its side. On the west there is another group of hills, of which
Ndrandramea (1,800 feet) is the highest and best known. Associated with
it are Kala-Kala (1,600 feet), Mako-mako, Thoka-singa (1,300 feet), Vatu
Mata (1,050 feet), and another unnamed peak (1,400 feet) lying west of
Ndrandramea.
The districts between and among the hills are much cut up into lesser
hills and ridges, the result of the very extensive denudation to which
this region has been subjected. The greater part of this area is drained
by the Tambu-lotu tributary of the Wainunu; but in the northern part we
cross the watershed between the Wainunu and Ndreketi basins, and to
reach Navuningumu we cross the valley of one of the tributaries of the
Ndreketi. To the east of the Ndrandramea region extends a broken
country, elevated rather more than 1,000 feet above the sea, and from it
there rise one or two hills with bare cliff-faces, which are probably
composed of similar acid andesites.
Although for the most part composed of these acid andesites, each hill,
as far as my observations show, has as a rule its own type of the rock,
differing from the others in specific weight, in the texture of the
groundmass, and in the relative proportion of the porphyritic
constituents. The petrological characters will be found more fully
discussed in Chapter XXI.; and only some of the more distinctive
features will be noticed here in the following description of the
district.
THE NGAINGAI GROUP OF HILLS.—Within a space less than a mile square rise
Ngaingai, Wawa Levu, and the other three hills above named, so closely
clustered together that the collective name of “Hen and Chickens” might
be aptly applied to the group.
The peculiar form of Ngaingai is shown in the accompanying
profile-sketch. It is the Nangorongoro of the Admiralty chart. The
height of the mountain from its base is 1,100 to 1,200 feet. Its ascent,
which is not difficult, may be made from the west side. Above its wooded
slopes rises its bare rocky peak, from which a magnificent panoramic
view of the western half of Vanua Levu can be obtained. Characteristic
dacites with porphyritic quartz came under my notice all the way up from
the foot to the summit, being occasionally exposed in perpendicular
cliff-faces. Specimens taken from the upper and lower portions are
uniform in character, and have a specific gravity of 2·57. No other
rocks were observed on its slopes. The whole hill-mass is in great part
if not entirely formed of these acid andesites.
The contrast between the narrow crested peak of Ngaingai and the
dome-shaped summit of Wawa Levu is seen in the sketch; and this is the
more remarkable because it is not associated, as far as I could
ascertain, with any important difference in geological character. Wawa
Levu rises precipitously to a height of 900 or 1,000 feet above its
base, and displays often perpendicular cliff-faces on its sides. Its
broad level soil-covered summit is mostly covered with young wood, few
of the trees having trunks more than 4 inches in diameter, whilst they
are usually clothed with damp moss, and are often decayed and
rotten.[49] True dacites, closely similar to those of the neighbouring
Ngaingai and having a specific gravity of 2·61, were displayed often in
slab-like blocks from the base to near the top. The rudely columnar
structure to be observed in some of the other hills is rarely exhibited.
No other rocks came under my notice. The remains of the stone walls of
two old “war-towns,” one of them named “Ndaku-i-tonga,” occur on its
south and south-east slopes.
[Illustration: Profiles of Ngaingai and Wawa Levu from Nambuna to the
south-west. Both are dacitic mountains.]
The other three hills of the Ngaingai group were not ascended by me.
They show the same bare cliff-faces and have to all appearance the same
geological character. Mbona Lailai and Vatu Kerimasi are two
blunt-topped conical hills with precipitous slopes that rise
respectively about 900 and 700 feet above the country at their base.
Vatu Vanaya, about 500 feet in height, has a rounded summit.
THE NDRANDRAMEA GROUP OF HILLS.—A view of these hills from the westward
is given in the accompanying illustration. They have a lower elevation
than the hills of the Ngaingai group, none of them rising to over 1,800
feet above the sea, whilst their height from the base is also less,
ranging between 400 and 900 feet. They rise, as the illustration shows,
in the midst of a densely-wooded broken country.
Ndrandramea, which is 1,800 feet above the sea, has an individual height
of about 900 feet. Fijians in distant parts of the island are familiar
with the name of this remarkable peak. It has a legendary fame; and like
Wawa Levu in the old time it served as a mountain stronghold in times of
war. The remains of a stonewall of a “koro-ni-valu” or “town of war,”
known as Mata-mei-ndami-ndami, occur on its side, 300 or 350 feet below
its summit; whilst among the wild lemon trees that cover the slopes
below large ovoid sling-stones 4 or 5 inches in length may still be
found. Viewed from the south-east, as shown in the frontispiece,
Ndrandramea has the shape of a woman’s breast; and evidently the origin
of its name is connected with this resemblance. But seen from the west
and south-west, as in the other general view of the district (page 98),
it has a broadly truncated conical outline, its form being indeed
somewhat elongated or elliptical.
This hill presents precipitous slopes, and on the south side it shows
bare rocky faces. As seen in the illustration, it might appear
inaccessible; but the ascent is not difficult on the west side. It is
composed in mass of an acid andesite allied to the dacites of Ngaingai
and Wawa Levu, but differing in the hemicrystalline character of the
groundmass (except at the base), in the porphyritic development of
rhombic pyroxene, and in the absence of porphyritic quartz. As remarked
on page 301, the rock becomes more basic as one descends the hill. At
the top its specific weight is 2·44, about 300 feet below it is 2·58, at
700 feet from the top it is 2·68, and at the base of the hill where it
is holocrystalline and has a dioritic appearance it is 2·71. That it
possesses a rudely columnar structure is shown by the occurrence here
and there on the slopes and at the base of the hill of portions of
prostrate columns, 3 to 4 feet broad and sometimes 20 to 25 feet long,
which have a rounded surface and look like fossil tree-trunks. Masses of
agglomerate of the same andesitic rocks lie about in places on the lower
slopes, the included blocks, which are a few inches across, being
sometimes rounded.
The neighbouring hills lying south and west of Ndrandramea are, as far
as my observations show, of the same acid type of andesite. It is
connected with those nearest by a saddle, 1,100 feet above the sea,
where the same holocrystalline form of the rock occurs, having a
specific gravity of 2·7 and being often rudely columnar in structure.
Kala-kala, about 1,600 feet above the sea, is an imposing-looking hill
with perpendicular cliff-faces on some of its sides. I did not ascend
it, but found at its base a rock of the same andesitic type, differing
from that of Ndrandramea in the more crystalline character of the
groundmass, and having a specific gravity of 2·61. West of Kala-kala is
the outlying hill of Vatu Mata with a flat top and rising only about 400
feet from its base. It has all the appearance of being composed of the
same andesitic rocks. It is shown on the left-hand in the illustration.
Lying south of Kala-kala are the two peaks of Mako-mako and Thoka-singa,
rising respectively 1,400 and 1,300 feet above the sea. I ascended the
last-named, which has a rounded summit covered with trees. Approaching
it from Nambuna on the east, I found at its foot a large mass of
pitchstone-agglomerate, formed of fragments of vitreous basic rocks,
such as occurs around the lower part of Soloa Levu on the other side of
the valley. The slopes of Thoka-singa, between 200 and 450 feet below
the summit, are strewn with masses of another kind of agglomerate made
up of blocks 3 to 8 inches across, occasionally rounded, and composed of
the same felsitic andesite, of which the mass of the hill is formed.
This last-named rock is exposed in bulk in the upper part, but on the
summit the agglomerate reappears. It has a granitoid appearance, and is
distinguished from the acid andesites of the other hills of the
Ndrandramea district by its greater specific gravity (2·72 to 2·74), by
its holocrystalline texture, and by the coarse grain of the mosaic of
its felsitic groundmass, which is probably quartz-bearing but is
relatively scanty. It is, however, referable to the same group of
felsitic andesites, but is to be placed at the basic end of the series.
(Its description is given on page 302.) In Thoka-singa we have therefore
a hill which is evidently formed in mass of these holocrystalline
felsitic andesites but covered in places with an agglomerate of the same
materials. I have already referred to this feature in the structure of
Ndrandramea. Since the blocks are sometimes rounded, such agglomerates
may represent the result of marine erosion during the emergence of this
part of the island. In the case of Navuningumu, where they lie abruptly
on calcareous clays containing tests of foraminifera and shells of
pteropods, a different explanation appears to be needed.
THE HILL OF SOLOA LEVU.—This isolated hill, which presents another type
of these acid andesites, has a broad rounded summit; and though elevated
about 1,600 feet above the sea, the hill itself rises only 800 or 900
feet above the country at its base. It is not easy to obtain a view of
the profile of this hill and to ascertain its relation to its
surroundings; and it was only when I viewed it from near the top of Vatu
Kaisia six miles to the eastward that I was able to understand its
position. Looking from that standpoint across the basaltic table-land of
Wainunu one observed Soloa Levu rising as a dome-shaped hill at the
western margin of the table-land and apparently not separated from it.
The examination of the district shows that on the east and south-east
sides this hill was in part surrounded by the great basaltic flows by
which the table-land was built up. Basic tuffs and agglomerates,
however, occur on the lower slopes on the north-west, west, and
south-west sides, so that Soloa Levu in fact lies in the midst of an
area of basic rocks.
The type of acid andesite which is displayed in the upper two-thirds of
the hill is distinguished from those of the other hills of the
Ndrandramea district by its orthophyric groundmass. Instead of a fine
mosaic, the matrix displays as a rule an arrangement of short stout
plagioclase prisms; but in one of my slides the two forms of groundmass
are associated. In their general characters as described on page 296,
they cannot be separated from the acid andesites of the Ndrandramea
district. Their specific weight ranges between 2·54 and 2·62, and like
most of the other acid andesites they contain little, if any,
interstitial glass. Huge blocks of these rocks lie about on the slopes,
often assuming a columnar form, the fragments of such columns being
sometimes 5 or 6 feet in diameter, and 12 to 15 feet in length. I found
one such block standing erect like a solitary obelisk.
The best way to observe the basic rocks that invest the lower slopes of
Soloa Levu is to follow the track that skirts it on the south side on
the way from Tambu-lotu to Vunivuvundi. Palagonitic tuffs containing in
places a little lime[50] and composed of fragments of basic glass of
varying size and more or less palagonitised extend from Tambu-lotu and
Nuku-ni-tambua (two villages lying about a mile to the westward) to the
west and south-west slopes of Soloa Levu. A pitchstone-agglomerate,
formed of fragments of a basic glass inclosing large crystals of
plagioclase felspar one-third of an inch in length, is associated with
these tuffs on the lower north-west, west, and south-west slopes of the
hill. The tuffs are formed of the same materials as the
pitchstone-agglomerates, but differ in their character of being more or
less palagonitised. However, on the north-west side the latter have also
undergone this change. On page 312 will be found a description of the
basic glass of these agglomerates in its fresh and in its altered
condition. Huge blocks of these rocks strew the surface on the
south-west slopes of Soloa Levu, and in one place the underlying acid
andesite that forms the mass of the hill is exposed in a stream-course.
These pitchstone-agglomerates and palagonitic pitchstone-tuffs are
elevated between 600 and 750 feet above the sea. As one proceeds on the
road to Vunivuvundi and skirts the south-east side of the hill one
ascends the western border of the basaltic Wainunu table-land which,
however, is much cut up by rivers in this locality. Here the tuffs and
agglomerates give place to a basaltic andesite, and on reaching an
elevation of 1,000 feet we arrive at the top of the table-land from
which an ascent of Soloa Levu is easily made. The road then lies on, but
parallel to, the border of this plateau for some distance until it
descends into a deep valley worn by one of the tributaries of the
Wainunu River.
This hill of Soloa Levu is in fact a mass of acid andesite situated in
the midst of an area of basic rocks. I found basaltic rocks exposed in
the stream courses to the north and similar rocks prevail on the
north-west on the way between Nambuna and Tambu-lotu. It has been above
remarked that on the east and south it has been in part surrounded by
the basaltic flows of the Wainunu table-land, and that pitchstone-tuffs
and agglomerates cover its lower slopes on the west and south-west, yet
it is not easy to find any trace of the vent from which they flowed or
were ejected.
It may be here remarked that the occurrence here and there of basic
rocks in the midst of this region suggests the vicinity of dykes. For
instance, in a deep gulley about half a mile south-west of Kalakala,
where a dacitic rock was exposed _in situ_, I came upon a single large
mass of an aphanitic augite-andesite of the type described under genus
16, species A, of the augite-andesites.
THE ALTERED ACID ANDESITES OF THE NDRANDRAMEA DISTRICT.—One of the most
important features of the geological structure of this district lies in
the fact that the bed-rock exposed in the lower region between the hills
is a highly altered acid andesite of the type found in the hills around.
By referring to the map of this locality, it will be observed that
between the Ndrandramea hills on the west and the Ngaingai hills on the
east is the valley of the Tambu-lotu river and its tributaries, an open
broken country deeply eroded by the streams, and elevated 600 to 700
feet above the sea. These altered rocks are well exposed in the deep
gorge-like channel of the river between the village of Nambuna and the
foot of Ndrandramea, and in fact in all places in this district where
the streams have worn deeply into the surface.
They have a coarse felsitic groundmass, and are described under the
felsitic order of the hypersthene-andesites on page 297. They present
all degrees of change from the hard dark grey mottled rocks, in which
the phenocrysts of plagioclase and rhombic pyroxene are in part replaced
by calcitic, viriditic, and chloritic materials, to those where the
pseudomorphism and alteration is complete, when the decomposition
products give their character to a pale yellowish rock, which sparkles
with pyrites and often effervesces briskly with an acid. After this
comes the final stage of disintegration, and we get a whitish rotten
stone, often full of pyrites, the last condition of which is shown in a
kaolin-like material exposed in the river-side.
The extensive alteration of these rocks is also indicated by the
occurrence amongst the gravel of the river-bed and small stream courses
near Nambuna of fragments of clear quartz prisms, half an inch across,
and of nodules, three inches in size and sometimes hollow in the centre,
formed of radiating quartz crystals that once filled cavities in the
altered rock. Small masses of vein-quartz also occur in these streams,
formed in a fissure by the growth of the crystals from the sides towards
the centre. I was unable to find the source of the quartz; but it is
probable that it was produced near the line of contact between the
basaltic flows to the eastward and the older felsitic rocks of the
district. The great alteration of the acid andesitic rocks exposed as
the bed-rocks in this region may in all probability be attributed to the
vicinity of these basaltic rocks. The two formations apparently come
into contact about a mile east of Nambuna. In traversing this district
on the road to Ndrawa one first observes _in situ_ in the streams the
decomposed felsitic bed-rock with occasional loose blocks of a
quartzitic rock that displays in the thin section a mosaic of irregular
grains of quartz. Afterwards, as one rises gradually to the top of the
basaltic plateau, basaltic rocks are alone exposed in position.
In the character of the fine river sand a clue may be found to the exact
locality of the contact. In the midst of the andesitic area between
Nambuna and Ndrandramea, the sand, besides containing much magnetic
iron, is also composed to a large extent of rhombic pyroxene prisms,
clear quartz grains, and fragments of plagioclase, all derived from the
porphyritic crystals of the dacites, &c. Near the basaltic district we
find that the quartz and rhombic pyroxene have disappeared, the sand
being largely made up of magnetic-iron grains mixed with fragments of
plagioclase.
[Illustration: Profile and Geological Section of Vanua Levu, across the
island from the Sarawanga (north) coast to the Yanawai (south) coast.]
THE EXTENT OF THE AREA OF ACID ANDESITE ROCKS IN THE NDRANDRAMEA
DISTRICT.—By referring to the map of this locality it will be observed
that this region of andesites extends northward to the Navuningumu
Range, and that on the south it would be separated from the district of
tuffs and agglomerates, named the table-land of Na Savu, by a line
joining the hills of Soloa Levu and Thokasinga. On the east it is
bounded by the basaltic area of the Wainunu table-land. On the west it
extends at the surface, with an occasional overlying patch of submarine
tuffs and clays, for a distance of at least two or three miles from the
base of the hills, and sometimes, as in the direction of Sarawanga, more
than half way to the coast. I have endeavoured to show the relation of
these acid rocks to the basalts and to the sedimentary deposits in the
geological section.
When taking the track from Sarawanga to Nambuna by way of Ndrandramea
one soon enters the region of these acid andesites. The prevailing rock
exposed on the surface, where it is usually much decomposed, is a
bluish-grey hypersthene-andesite with a specific gravity of 2·54, and
displaying in a cryptocrystalline groundmass, where the felsitic texture
can be recognised, abundant phenocrysts of plagioclase and rhombic
pyroxene. As high as 500 feet above the sea it is occasionally capped by
patches of palagonitised clays and tuffs scantily foraminiferous, and at
one place I noticed a patch of agglomerate, the subangular blocks six to
eight inches across being formed of the same acid andesite. In the same
way by taking the road from Tembe-ni-ndio to Nambuna, passing the hill
of Kala-kala on the way, we leave behind the foraminiferous tuffs and
limestones of the lower coast regions; and when about 400 feet above the
sea we enter the inland district of felsitic andesites which begin about
two miles from Tembe-ni-ndio.
THE NAVUNINGUMU RANGE.—By following the track from Nambuna to
Navuningumu one skirts the bases of Wawa Levu and Ngaingai, where
dacitic rocks are exposed. After passing the watershed[51] between the
Wainunu and Ndreketi rivers, the track descends into the deep valley of
one of the western tributaries of the Ndreketi, where a characteristic
holocrystalline type of these felsitic andesites is exposed. Approaching
Navuningumu one finds exposed at its base agglomerates, composed of
scoriaceous and amygdaloidal semi-vitreous basic rocks, overlying a dark
tufaceous sandstone which on examination proves to be a basic pumiceous
tuff of the type described on page 333, and scantily foraminiferous.
We stand now in a region of basic rocks on the south-east side of the
range, and before us rises abruptly the weird-looking magnetic peak of
Navuningumu, which is well represented in the accompanying illustration.
In the wet season its summit is usually enveloped in the thunder-clouds.
Its elevation above the sea is 1,930 feet, but estimated from its base
its height is 1,000 to 1,100 feet. The natives also name this peak Na
Seyanga, after a town that once existed in this locality. It is the
summit of a range that extends a mile or more to the north where it
terminates in a lesser peak known as Mumu.
Ascending the peak of Navuningumu from the south-east one finds
exposed in its lower part, up to 1,200 feet above the sea,
pitchstone-agglomerates (composed of fragments of a vitreous basic
rock) and white tufaceous sandstones (containing a few tests of
foraminifera), such as are described below in the case of the
neighbouring Mbenutha Cliffs. Between 1,300 and 1,500 feet there is
displayed in position a typical dacite of the type described on page
303.
[Illustration: MT. TAVIA (2,210 feet) from VATU KAISIA. It is probably
formed of an acid andesite.]
[Illustration: The magnetic peak of NAVUNINGUMU (1,931 feet) from the
south. The summit represents a basaltic neck.]
[_Face p. 108._
The peak itself is formed of a dark-brown slightly vesicular
semi-vitreous basaltic andesite, of which, in fact, for the upper 200
feet, the summit is composed. The rock is somewhat rubbly; and where it
is exposed on the bare peak it is powerfully magnetic, displaying
polarity in a marked degree, and rendering the compass useless (see page
368). A specimen of the magnetic rock, which is a little vesicular, has
a specific gravity of 2·82. It is referred to genus 1 of the
augite-andesites described on page 267. It displays in the slide
porphyritic plagioclase, with a little augite, in a groundmass formed of
a plexus of minute felspar-lathes (·06 mm. in length), and exhibiting a
large amount of a brown opaque glass in which grains and rods of
magnetite with a few pyroxene granules are developed. The magnetite in
the groundmass, although abundant, is not in greater quantity than is
usually found in semi-vitreous basaltic rocks without polarity.... This
terminal mass of basic lava-rock evidently forms the “plug” of a
volcanic pipe that pierces the acid andesitic rocks of the district; and
from this ancient vent were doubtless ejected the basic tuffs and
agglomerates that now cover the lower slopes of the mountain.
The conditions under which this volcano displayed its activity are
further illustrated in a remarkable section exhibited on the east side
of the mountain half a mile or more north of the summit. Here there is a
line of bold cliffs, in which, as shown in the illustration, a bed of
agglomerate, 60 or 70 feet thick, overlies a series of foraminiferous
clays and tufaceous sandstones, which are elevated about 1,100 feet
above the sea. The locality is named “Mbenu-tha” or “Rubbish-heap.” It
is well known to the natives on account of its caves, which serve as a
half-way resting-place on the road from Nambuna to Ndreketi. These caves
have been produced by the more rapid weathering of the underlying clays
and sandstones. The line of cliff extends northward to Mumu, the peak at
that end of the range, and preserves there the same structure. The clays
and tuff-sandstones are more or less stratified, and dip generally to
the west or south-west at an angle perhaps of 20 degrees; but in more
than one place they show signs of great disturbance, being contorted and
steeply tilted.