In the lower part of the mountain the blocks, usually sub-angular, are

about a foot across; but they become smaller as one ascends towards the

summit, where they are 3 or 4 inches in diameter. At the top there are

extensive exposures in cliff-faces of the agglomerate-tuffs; and here

the finer materials of the matrix include a few rounded pebbles not

exceeding half an inch in size. This is a fact of importance in

connection with the submarine origin of these formations.

As regards their composition, the blocks of the agglomerates have not

the uniform character we would expect to find in the case of materials

directly ejected from a volcanic vent. The most frequent type of rock

represented is a grey hypersthene-augite-andesite, having a specific

gravity of 2·72-2·78. It displays small phenocrysts of plagioclase and

of rhombic and monoclinic pyroxene, but in other respects it exhibits

much variety, not only in the arrangement and average length of the

felspar-lathes (·08 to ·18 mm.) but in the form of the pyroxene of the

groundmass (either granular or prismatic) and in the amount of residual

glass, sometimes abundant, sometimes scanty. Two distinct genera (1 and

5) of the sub-class are therefore represented.

Other rocks found in these agglomerates contain no rhombic pyroxene, and

are referred to genera 13 and 16 of the augite-andesites according to

the presence or absence of plagioclase phenocrysts. In the last case we

have a dark aphanitic rock (sp. gr. 2·74), sometimes scoriaceous, where

the average length of the felspar-lathes may be as little as ·04 mm. One

of the blocks was composed of a highly scoriaceous semi-vitreous rock,

the cavities being filled with a zeolite. Another was composed of a

black porphyritic augite-andesite, showing large crystals of

plagioclase.... The matrix of the agglomerate-tuff is formed of

sub-angular and rounded fragments, up to a centimetre in size, of the

same andesites, the interstitial material being formed of fine detritus

and palagonitic debris.

Though the agglomerates of the peak of Koro-mbasanga are composed of a

variety of rocks, all the rocks are to be referred to the

pyroxene-andesites with specific gravity below 2·8 but above 2·7. They

are therefore less basic than the olivine-basalts and basaltic andesites

of the Koro-tini range, where the density is usually 2·8 and over. Their

variation, however, is more consistent with the characters of an

agglomerate formed by marine erosion. The same may be said of the

sorting of the blocks according to their size and of the occasional

occurrence in the matrix of small rounded pebbles. That these deposits

of agglomerates were formed under the sea is indicated also by their

overlying submarine sedimentary tuffs near the base of the mountain.

(2) THE SOKENA RIDGE.—To the west of Koro-mbasanga, and forming a spur

of the same range, is the flat-topped hill of Sokena, which rises about

1,100 feet above the country at its base and about 1,600 feet above the

sea. From a distance it has the appearance of being formed in its higher

portion of nearly horizontal strata dipping gently northward. In its

upper part it terminates in a line of cliffs about 200 feet in height,

and there is a similar line of cliffs lower down the slopes. These

cliffs are composed of bedded fine and coarse non-calcareous tuffs,

dipping about 10° N.N.W., in which are imbedded without any arrangement

blocks, ranging in size from 2 or 3 inches to 3 or 4 feet, of a

remarkable blackish pitchstone-like rock displaying opaque plagioclase

phenocrysts. It is referred to genus 18 (see page 289) of the

hypersthene-augite andesites, both rhombic and monoclinic pyroxene being

represented in the phenocrysts and in the groundmass where they take the

form of minute prisms (·03 mm.). There is a considerable amount of pale

brown glass. A rock very similar occurs in the Thambeyu agglomerates

(see page 178). The tuffs are formed largely of palagonitic materials,

the angular fragments in the coarser beds being ½ to 2 centimetres in

size, the palagonite being often vacuolar but much affected by

hydration.

These tuffs and agglomerates of the Sokena cliffs apparently contain no

organic remains. They appear to have accumulated under water as the

result of the eruptions of a neighbouring vent without the intermediate

agency of marine erosion.

(3) THE ASCENT OF THE LOVO VALLEY.—About two miles to the east of the

peak of Koro-mbasanga the picturesque Lovo valley cuts deeply in a

southerly direction into the mountainous backbone of the island. The

site of the old town of Lovo lies within the valley about two miles from

its mouth. “Lovo” is the Fijian word for a cannibal-oven; and I gathered

from my natives that in the old times this vale was noted for its

cannibal orgies. It is occupied by the Nasawana tributary of the Lambasa

River, and often becomes so narrow that it may be described as a gorge.

I followed the valley from its mouth, where it is elevated about 300

feet above the sea, for some miles in a southerly direction up to a

height of 1,000 feet, where the northern slope of the great

mountain-mass of Mbatini commences.

On either side of the Lovo valley rise precipitous mountain-slopes,

displaying in their cliff-faces and in the large detached rock-masses

basic agglomerates. The same formation is also usually displayed in the

sides of the river. The blocks composing the agglomerates are formed of

the usual type of hemi-crystalline or semi-vitreous blackish basaltic

andesite so characteristic of these deposits. It is generally compact,

but is at times amygdaloidal. Some distance below the old site of Lovo,

and at an elevation of about 500 feet above the sea, there is an

interesting exposure in the river-side, where the agglomerates overlie

bedded coarse calcareous basic tuffs containing large flat tests of

foraminifera with pieces of molluscan shells, and dipping about 15° S.W.

These tuffs can be traced up the valley towards Lovo.

Displayed in mass in the bed of the river in the same locality, and

beneath the submarine tuff just referred to, is a porphyritic basaltic

andesite (sp. gr. 2·79) containing but scanty interstitial glass, the

felspar-lathes being ·15 mm. in average length. It is referred to genus

1 of the augite-andesites. The same rock is exposed at intervals in the

river-bed as far as Lovo, which is about 850 feet above the sea. At one

place it exhibits a rudely columnar structure, the columns being

horizontal and 2 to 2½ feet in diameter, the trend of the dyke-like mass

being W. by S. and E. by N. Near Lovo a small dyke, 6 feet thick and

trending N.N.W. and S.S.E., pierces the agglomerate. It is composed of a

somewhat aphanitic augite-andesite closely resembling the rocks exposed

in the river-course for a mile or so above Lovo up to an elevation of

1,000 feet. In this upper part of the valley whilst agglomerates are

exposed in the cliffs and precipitous mountain-slopes on either side,

pyroxene-andesites, somewhat aphanitic in texture and with a specific

gravity of 2·68 to 2·7, are displayed in mass in the river bed. These

last-named rocks, which are closely similar to those found on the lower

slopes of Mount Mbatini (see page 173) are, as I should have also

remarked in the case of the basaltic andesite above mentioned, a little

altered, as is indicated by the existence of calcite and viridite in the

groundmass.[79]

From this instructive ascent of the Lovo valley we may learn that whilst

the mountain mass is formed, to a considerable depth, of agglomerates

with underlying submarine tuffs, the deeper seated rocks exposed in the

river-beds are massive intrusive rocks. The overlying agglomerates have

preserved the submarine tuffs from destruction, and there is no

difficulty in assuming that they also were accumulated under the sea,

but in shallow water, as evidenced by the character of the tuffs. I

found no signs of alteration in these tuffs, and except in the case of

the small dyke above noticed there is no sign of the dykes penetrating

the agglomerates. We have here a section into the heart of the

mountain-range; and assuming that the large intrusive masses of basic

andesites had penetrated these deposits, there would certainly have been

some evidence of this in the extensive exposures of agglomerates far up

the mountain-sides. As it is, however, we find such rocks only in the

deeply excavated river-bed. If we imagine a submarine volcanic mountain,

or one but slightly raised above the surface of the sea, to be subjected

during a long period of emergence to marine erosion, the “basal wreck”

of the mountain would ultimately be covered over by submarine tuffs and

agglomerates. This is the condition that seems to be presented here.

I did not make the ascent of Koro-tambu, the other principal peak of the

Koro-mbasanga Range. This round-topped mountain is well seen from the

summit of Mbatini from which it bears N. 30° W. by compass. It is

probably the peak marked 2,753 feet in the Admiralty chart, and is

connected with Mbatini by a saddle not under 1,500 feet in elevation.

Some of the most important features in the above account of this

district may here be emphasised. We have seen that in the peak of

Koro-mbasanga and in the Lovo valley agglomerates and agglomerate-tuffs,

several hundred feet thick, overlie sedimentary submarine tuffs. In the

last-named locality the deeper massive basic rocks are also exposed; and

we may infer in both instances that the agglomerate-formation is a

submarine deposit. On the other hand, in the Sokena Ridge, which is a

spur of the main range, we have apparently the accumulation of materials

on a sea-bottom, directly ejected from a vent without the intervention

of the agency of marine-erosion. In regard to this and other districts

in this part of the island it should be remembered that east and west

occur undoubted evidences of extensive submergence. It has already been

shown that submarine tuffs containing tests of foraminifera and other

organic remains occur at heights of 2,000 feet and over on the summit of

the Koro-tini Range, and it will be subsequently shown that similar

deposits are to be found on the neighbouring slopes of Thambeyu as high

as 2,100 feet.

MOUNT MBATINI

According to the Admiralty chart this is the highest mountain in Vanua

Levu, its elevation being 3,437 feet. It has twin peaks which lie either

N.W. and S.E. or W.N.W. and E.S.E. with each other. The northerly or

westerly peak is pointed and tooth-like. Hence probably arises its name

of Mbatini (mbati-tooth). The southerly or easterly peak is known as

Soro-levu. It has a broadly conical outline with a truncated summit. The

mountain is named Koro-mbasanga in the Admiralty chart, a name that

really belongs to a peak lying about 3 miles nearly due north (N. 5°

W.). The natives are very clear in this matter; but it must be remarked

in this connection that Koro-mbasanga, which signifies “a forked

eminence,” would be a very suitable appellation for the double-peaked

summit of Mbatini.[80] By the natives of the surrounding district the

whole mountain is known as Mbatini; but by the natives of the eastern

shores of Natewa Bay, it is usually known as Soro-levu, since the

western peak is often more or less hidden from view or is less

conspicuous. The profile of this mountain and of the neighbouring region

is shown in the accompanying profile-sketches and also in one of those

illustrating the Koro-mbasanga range on page 167.

As viewed from the top of Mariko to the southward, Mbatini presents

itself as a long mountain-ridge, trending W.N.W. and E.S.E., which is

connected on the north with Koro-tambu, the highest peak of the

Koro-mbasanga Range, by a saddle probably not over 1,500 feet above the

sea, and on the south with the mountain-ridge of Mariko by a _col_ which

appears not to be under 1,000 feet in elevation.

[Illustration: Mount Mbatini from the top of Koro-mbasanga. The distant

peak on the right is one of the summits of the Mariko ridge.]

[Illustration: View from Muanaira on the south coast of Natewa Bay.]

My ascent of this mountain was made from the north by the way of the

Lovo valley. In ascending the Lovo valley one reaches, at an elevation

of about 1,000 feet, the foot of the north slope of Mbatini. The slope

is somewhat steep up to 2,000 feet, the rocks exposed on the surface

being closely similar in the groundmass to those displayed in the upper

part of the Lovo valley. They are compact-looking blackish

augite-andesites (sp. gr. 2·7), the very small felspar-lathes of the

groundmass, which are in flow arrangement, averaging only ·05 mm. in

length. Like the rocks below, they are a little altered; and here the

interstitial glass is also scanty. But they differ in the absence of

rhombic pyroxene and are therefore referred to the augite-andesites

(genus 13).

At 2,000 feet, where one crosses the foot-track from Nukumbolo to

Korolau, the ascent of the true Mbatini ridge begins, the summit lying

nearly two miles to the south-east. Whilst following along this lofty

mountain-ridge we were for the greater part of the time in the

rain-clouds, so that very little was seen of our surroundings. The crest

is densely wooded so that our progress was very slow. The rocks are but

sparingly exposed. At the commencement of the ridge (2,100 feet) is

displayed an altered hypersthene-augite andesite, rudely columnar blocks

of which, up to 2 feet in diameter, were lying about. It belongs to

genus 1 of this sub-class (see page 286) which also includes the rocks

exposed farther along the ridge. In these rocks the felspar-lathes are

small (·05-·07 mm. long) and are not in flow arrangement. The

interstitial glass varies in amount, and the specific gravity is about

2·7.

The ascent is very gradual for the first one and a half miles, when an

elevation of 2,600 feet is attained. From here one ascends the

steep-sided peak of Mbatini, which rises some 700 or 800 feet from the

ridge. As one nears the highest point the crest becomes very narrow,

between 15 and 20 feet across; and on either side there is apparently a

drop of several hundred feet. The actual peak, which is bare and rocky,

is yet narrower; and when it is enveloped in dense mist as it was in my

instance, it is not a very secure situation for a geologist. It is

highly magnetic, as is the case with most of the other bare peaks of the

island. The rocks exposed in the upper 500 feet, that is, in the peak

proper, are highly altered semi-vitreous, but extensively weathered,

hypersthene-augite-andesites which are referred to genus 1 of that

sub-class. Much of the glassy groundmass is replaced by viridite,

silica, calcite, &c. Less altered specimens display in a brown opaque

glass small felspar-lathes averaging less than ·1 mm. in length. They

exhibit phenocrysts of rhombic pyroxene and augite, the first

prevailing.

I did not climb Soro-levu, the other of the twin-peaks. Its ascent

should be made either from Nukumbolo or from one of the villages on the

neighbouring shore of Natewa Bay. My acquaintance with Mbatini, although

very incomplete, enables me however to point out a few of its general

features. As remarked before, there is a general uniformity in the type

of its rocks. The olivine-basalts and basaltic andesites, prevailing in

the Koro-tini Range, are not here represented, nor are the dacites or

acid andesites to be found. The characteristic rocks are more or less

altered hypersthene-augite-andesites having a specific gravity in the

least altered and least vitreous condition of about 2·7; whilst the

average length of the felspar-lathes is always less than ·1 mm. The same

type prevails from the upper part of the Lovo valley to the summit of

Mbatini; but it is only in the actual peak that these rocks show much

glass in the groundmass, though extensively affected by alteration.

Neither tuffs nor agglomerates came under my notice; but they might be

expected to occur on the other slopes. I am inclined to regard this

mountain-ridge as a huge dyke-like mass or sill, representing the

remains of a volcanic vent that has been subjected at different periods

to marine-erosion and in later ages to sub-aerial denudation.

THE VUINANDI GAP

I have given this name to the break between the Thambeyu (Mount

Thurston) and Koro-mbasanga ranges, where the level of the mountainous

backbone of the island descends to about 1,200 feet above the sea. This

is the route taken by the track from Vuinandi on the shores of Natewa

Bay across the island to Lambasa.

At Vuinandi the mountains recede from the coast leaving a broad level

plain extending about two miles inland to the village of Tarawau without

rising over 60 feet above the sea. Basaltic rocks are exposed in the

spurs that descend from the mountains to the coast on each side of the

plain. After traversing the low-lying region that lies between Vuinandi

and the main range, one finds on ascending the eastern slopes, _en

route_ to Lambasa, basaltic andesites of the usual type prevailing up to

1,000 feet. The upper portion of the dividing range, 1,000 to 1,200

feet, is composed of a more compact basaltic andesite which is often

rubbly and in this condition is penetrated by fine cracks, 1/8 of an

inch broad, filled with chalcedony. This rock, which has a specific

gravity of 2·85, has a very fresh-looking appearance in the slide, and

the segregation of silica does not therefore appear to arise from an

alterative change. The felspar-lathes, which are in flow-arrangement,

average ·11 mm. in length, and there is a little residual glass.

The mountains rise on either side of the Vuinandi Gap to about 2,000

feet. Descending on the west side of the range one follows a

stream-course down to a level of 400 feet above the sea, agglomerates

and coarse basic tuffs being exposed on the way. The rocks forming the

agglomerates are for the most part to be referred to genus 1 of the

hypersthene-augite andesites. They are sometimes compact and sometimes

amygdaloidal, the amygdules being formed of chalcedony and other

minerals, whilst the glass of the groundmass is often altered.

The track then lay across a spur, 800 feet in height, principally

composed of a greyish porphyrite, exhibiting large opaque crystals of

plagioclase, 4 to 7 mm. long, in an almost holo-crystalline groundmass

formed of stout lamellar felspars with large augite granules. It is

described on page 268 under the porphyritic sub-genus of genus 2 of the

augite-andesites, and is an unusual type of rock for this island. After

this I descended into the picturesque gorge of the Satulaki River, which

is only elevated about 200 feet above the sea, agglomerates prevailing.

In the vicinity of Satulaki a rather compact basaltic andesite (sp. gr.

2·82) is commonly exposed in position. It is referred to genus 13 of the

augite andesites and belongs to the species with felspar-lathes less

than ·1 mm. in average length. It occurs both north and south of this

place and in the hill-spurs on either side. This is the bed-rock of the

Lambasa plains which here begin and extend to the north coast, being

usually covered with submarine tuffs and clays.

THE THAMBEYU OR MOUNT THURSTON RANGE

Mount Thurston is the name given in the Admiralty charts to the highest

peak (3,124 feet) of this range. There does not appear to be any general

native name. The highest peak visible from the Lambasa side is known as

“Thambeyu.” The lofty mountain-mass, as it is viewed from Vuinandi, is

known as Ulu-i-ndiri-ndiri.[81] The whole mountain-range has yet to be

properly explored. It is a much more complicated system of

mountain-ridges than is indicated in the chart, my acquaintance with it

being restricted to the Thambeyu ridge, the elevation of which is 2,600

feet above the sea. It trends N.N.W. and S.S.E.; but its relation to the

highest peak of the range could not be ascertained, as we were in the

rain-clouds during the two days we were on the mountain.

I made the ascent from the village of Numbu-ni-a-vula about three miles

to the westward, which is only 200 feet above the sea. In the

intervening low district a basaltic andesite is exposed in the

stream-courses. The structure of the ridge, as indicated by the ascent

of its western slope, is shown in the accompanying diagram. The core or

central axis is formed of massive basic rocks which protrude at the

summit and in one or two of the crests of the spurs. The flanks are

composed of submarine tuffs and clays overlaid by agglomerates of

considerable thickness. The tuffs reach to within 50 feet of the top,

whilst the agglomerates extend to within 400 feet of the summit. The

results obtained from this ascent are specially interesting, since it

afforded me the opportunity of studying in a satisfactory manner the

junction of the agglomerates with the tuffs.

There are two caves on the mountain-side which can be used for

night-shelter by those exploring the range. The lowest, 1,500 feet above

the sea, is the Taloko Cave (na-ngara-taloko). The highest is the Ndromo

Cave,[82] 2,100 feet, known to the natives as “na-ngara-vatu-ni-ndromo.”

Like most of the caves all over the island they occur at the junction of

the agglomerates and tuffs, and are to be attributed to the more rapid

weathering of the underlying tuffs.... In describing the results of my

examination of this mountain-ridge, I will deal in succession with the

tuffs, the agglomerates, the junction between these two deposits, and

the axis or core of basic rocks.

[Illustration: [Image: Ideal Section of Thombeyu]]

(1) _The submarine tuffs and tuff-clays._—As exposed in the

stream-courses near and at the foot of the mountain and as high as the

Taloko Cave, these deposits are bedded horizontally. At higher levels,

owing to insufficient exposure the bedding is not so clear. Up to 700 or

800 feet coarse palagonite-tuffs prevail; but they do not effervesce

with an acid, and apparently contain but scanty organic remains. At 950

feet coarse and fine sedimentary tuffs alternate, the last being

greenish foraminiferous tuff-clay rocks, somewhat compacted and

containing 10 per cent. of carbonate of lime. The tests of the

foraminifera, which are abundant and of the Globigerina type, are filled

with calcite. Several fragments, of a semi-vitreous basic rock, not

however exceeding ·2 mm. in size, are inclosed in the deposit; but the

mass of it is made up of yet finer materials of the same rock,

palagonitic detritus, plagioclase fragments, fine calcitic debris, tests

of foraminifera, &c. These fine tuff-clays were evidently formed in

relatively deep-water.

At the Taloko Cave (1,500 feet), where there are exposed rather coarse

tuffs containing bands about a centimetre thick of a fine clay-tuff, the

last-named effervesce freely with an acid, whilst the first contain only

a little carbonate of lime. No sections have been made of these

deposits; but when powdered and examined under the microscope they

appear to have the same general composition as the deposit described

above from an elevation of 950 feet. They are probably foraminiferous

though scantily. The tuffs found at the Ndromo Cave (2,100 feet) contain

4 per cent. of carbonate of lime and small tests of foraminifera are

visible with a lens. The mineral fragments include plagioclase and

rhombic pyroxene, and there are inclosed rounded gravel-fragments, 5 mm.

in size, of a semi-vitreous rock. Palagonitic debris make up the mass of

these tuffs. A coarse deposit from 2,500 feet is non-calcareous, but has

the same general composition.

(2) _The agglomerates._—These deposits are best represented in the upper

part of the mountain, between 1,500 and 2,200 feet above the sea. Here

they often present vertical precipices having a drop varying between 100

and 400 feet, with the submarine tuffs exposed at their base. Such

cliffs, however, display no structure. Their vertical faces are to be

attributed to joints and to the extensive “slips” that frequently occur

on these slopes, when large masses of agglomerate, undermined by the

percolation of springs through the tuffs beneath them, roll far down the

mountain-sides. The blocks of the agglomerates are fairly uniform in

size, being usually 4 or 5 inches across. They are composed of a

semi-vitreous hypersthene-augite andesite, containing both augite and

rhombic pyroxene, but of an unusual type. It is a blackish rock carrying

opaque phenocrysts of plagioclase, and is characterised by the prismatic

form of the pyroxene (monoclinic) of the groundmass. A very similar rock

from the Sokena agglomerates has been before described. It is referred

to genus 18 of the class, and the prismatic sub-order to which that

genus belongs is described on page 289.

(3) _The junction of the agglomerates and submarine tuffs._—This is well

displayed at the Taloko Cave. Here the agglomerates lie conformably on

the sedimentary tuffs; but the line of junction is sharply defined and

the only evidence of transition is afforded by the great diminution in

the size of the blocks of the agglomerates, which are 1 to 2 inches

across. Immediately beneath the agglomerate is a layer 2½ centimetres

thick of a rather coarse sedimentary palagonite-tuff having the

composition of the deposits above described, but not effervescing with

an acid, and showing no foraminiferous tests. The size of its “grain” is

about a millimetre. This passes downward rather abruptly into a

chocolate-coloured marl-like rock, a centimetre thick, which is formed

of the same materials but in a clayey condition. Beneath this is the

calcareous foraminiferous palagonite-tuff referred to in the first

paragraph.

It is apparent that for some time before the agglomerates began to

accumulate on the sea-bottom there had been a fairly uniform deposit of

submarine tuffs, evidently in rather deep water. Then followed a period

during which the finest mud was deposited which is represented by the

thin layer of chocolate-coloured clay. This was succeeded by the

deposition of coarser sedimentary tuffs forming a layer about an inch in

thickness. Then commenced the accumulation of the agglomerates, of which

the materials were at first small and afterwards larger in size.

(4) _The core or axis of volcanic rocks._—This is represented on the

summit by masses, 2 to 5 feet across, of two kinds of hypersthene-augite

andesite, which are referred to genus 1 of that sub-class. One is a

compact grey rock (sp. gr. 2·72) carrying phenocrysts both of rhombic

and monoclinic pyroxene, the former prevailing, and displaying a small

amount of interstitial glass. It is magnetic and exhibits marked

polarity, as noticed in Chapter XXVI. The other is a scoriaceous rock

containing numerous round steam-pores, ranging up to 5 millimetres in

diameter and generally filled with clear quartz-crystals and lined by

chalcedony. It contains semi-opaque glass in abundance, and is

apparently a semi-vitreous form of the rock just described. Both rocks

are to some extent altered.... On the crest of a spur, 500 feet below

the summit, is exposed in position an augite-andesite, assigned to genus

13, sub-genus 1, species B, of that sub-class. It is non-scoriaceous and

exhibits a considerable amount of greenish alteration products. (Sp. gr.

2·79.)

THE AVUKA RANGE

This high range, which lies immediately to the east of Lambasa, attains

its greatest elevation in Mount Avuka, which is 1,976 feet above the

sea. It represents the extension northward to the coast of the inland

Thambeyu mountains that culminate in Mount Thurston. In its upper

portion Mount Avuka presents bare precipitous faces apparently of

agglomerates and some hundreds of feet in height. My acquaintance with

this range is scanty. In a traverse from Lambasa to Ngele-mumu I crossed

it a mile or more south of Mount Avuka, where it is only 700 feet in

elevation. I also rounded the end of the range where it reaches the

coast between Lambasa and the valley of Mbuthai-sau. This last locality,

which is described on page 218, derives especial interest from the

circumstance that here the regions of basic and acid rocks meet. The

basic rocks that occupy nearly all the sea-border from Naivaka to

Lambasa here become mingled with, and finally give place to, the acid

rocks which prevail in all the region eastward as far as Undu Point.

In crossing the range on the way from Lambasa to Ngelemumu, I noticed as

high as 450 feet basic non-calcareous tuffs displaying a concretionary

arrangement suggestive of the proximity of an intrusive igneous rock.

Further up the western slope occur basic agglomerates, whilst at and

near the top (700 feet) there lie on the surface large boulders of a

dark grey hypersthene-gabbro having a specific gravity of 2·7 and

belonging to the type of plutonic rocks described on page 249. It is

very probable that this gabbro forms the axis of the range; and we have

here no doubt one of the oldest of the mountain-ridges in the island.

CHAPTER XIII

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES

(_continued_)

THE VALANGA RANGE

THIS range, which trends N.W. and S.E. between the Mariko mountain-ridge

and the head of the valley of Na Kula, attains a height of 1,880 feet at

its N.W. and of 1,710 feet at its S.E. end. The average elevation,

however, is probably not over 1,300 or 1,400 feet. My acquaintance with

the range is only partial, but it is sufficient to bring to light some

of its leading structural features. Those who follow me will find in

these mountains a very interesting region for their geological

explorations.

(1) TRAVERSE OF THE VALANGA RANGE.—In making the journey from Valanga to

Vunimbua, I crossed the range where its elevation was about 1,300 feet.

Basic agglomerates, containing sometimes amygdaloidal blocks, are

displayed in the low district between Valanga and the foot of the range.

In the stream-course at the base of the slope the deeper seated rocks of

the range are at once exposed. Large masses, 5 or 6 feet across, of

altered grey pyroxene andesites lie in the bed of the stream. Some of

them show opaque porphyritic felspar and have the appearance of

porphyrites (sp. gr. 2·67). They belong to the type described on page

271 under genus 6 of the augite-andesites. Others are grey propylitic

varieties of a basic semi-doleritic andesite penetrated by cracks

containing calcite, and displaying in a groundmass, exhibiting much

viridite and a little pyrites, calcitic pseudomorphs of the felspar

phenocrysts and more or less parallel felspar-lathes, ·15 mm. long and

somewhat altered. Another of the deeper-seated rocks commonly exposed on

the upper west slopes of the range is a dark grey rock showing much

porphyritic pyroxene (sp. gr. 2·72). It has a micro-felsitic groundmass

and is referred to the fourth order of the hypersthene-augite-andesites

described on page 291.

About two-thirds of the way up the western slope of the range, there is

exposed a coarse palagonite-tuff, evidently an incrusting deposit. Stout

crystals of augite can be picked out of it, and it contains also lapilli

up to an inch in size of a basic vesicular semi-vitreous basalt.

Descending the eastern slopes one observes between 1,200 and 1,000 feet

large blocks of the same grey hypersthene-augite-andesite above

mentioned and of a grey granitoid rock of the gabbro type. This last is

a hypersthene-gabbro with specific gravity of 2·75, and belongs to the

group of plutonic rocks described on page 250. Its pyroxene phenocrysts

are often represented by fibrous bastite. One can scarcely doubt that

this gabbro is the plutonic equivalent of the prevailing grey

pyroxene-andesites.

Lower down the slope only small fragments of rocks were exposed,

probably derived from an agglomerate. One of the specimens here obtained

is a doleritic basaltic andesite (sp. gr. 2·77). Another is a very

interesting rock displaying large porphyritic crystals of a mineral like

bronzite in a groundmass originally to a large degree vitreous; but the

glass is now replaced by viridite and secondary crystalline silica. The

“bronzite” is the result of the conversion of associated rhombic and

monoclinic pyroxene into fibrous bastite.

From the results of the traverse across this part of the Valanga Range

it may be inferred that more or less altered grey basic andesites

passing into gabbros chiefly compose it. No doubt at one time it was

largely covered with basic tuffs and agglomerates, but these deposits

have been almost completely stripped off by the denuding agencies, and

were only noticed in one place on the western flank.

That the northern part of the range towards the Mariko ridge has a

similar structure is shown by the character of the loose blocks in the

upper course of the Vunimbua River, which takes its rise on these

slopes. Amongst those in the river above the village I noticed a

solitary block of a coarsely crystalline diorite containing prisms of

brown hornblende a centimetre in length.[83] But the rocks most

frequently represented were propylitic grey hypersthene-andesites, in

which the pyroxene is mostly changed into bastite, whilst the surface

often sparkles with pyrites (see page 297).

(2) NGONE HILL.—This is a curious conical hill, about 700 feet in

height, that rises up on the right side of the Vunimbua River about 1½

miles above the village of that name and near the foot of the range. It

evidently represents a “volcanic neck,” and doubtless this vent was the

source of the large blocks forming the basic agglomerate that occurs in

huge masses in the river-course in the vicinity of this hill. On its

lower flanks is exposed a hard compacted tuff, showing pyroxene

crystals, which is composed principally of fragments of a palagonitised

vacuolar basic glass, the minute cavities being often filled with opal.

In the upper part of the hill is displayed a massive altered

augite-andesite penetrated by fine veins of chalcedony. Numerous

irregular cavities filled with the same material occur in its dark

opaque glassy groundmass.

The blocks of the agglomerates found in the vicinity of the hill vary in

size from 4 to 18 inches. They are composed of a compact blackish

semi-vitreous basic andesite (sp. gr. 2·73) of the type characteristic

of the basic agglomerates over most of the island. The matrix of the

agglomerate is hard and somewhat altered, and is chiefly made up of

fragments, ranging up to 5 mm. in size, of a vacuolar basic glass,

sometimes but slightly changed, though usually converted into

palagonite, the vacuoles being filled with chalcedonic opal. The large

masses of coarse tuffs displayed in the bed of a stream-course close to

Ngone Hill are non-calcareous and composed of palagonitic materials.

Palagonite-tuff clays are also exposed in the river-course a little

above Vunimbua. About half-way between the village and the hill there

occurs in position at the river-side an amygdaloidal basic rock, the

amygdules being formed of chalcedonic opal.

It is apparent that this hill represents a lesser vent which probably

dates back to the period before the emergence. All the products of its

eruption are, however, more or less altered. From the absence of sorting

in the blocks of the agglomerates, and from the character of the matrix,

it may be inferred that these deposits have been accumulated directly

from the ejected materials without the intervention of the agency of

marine erosion.

(3) THE WESTERN FLANK OF THE VALANGA RANGE.—One of the boldest pieces of

coast in the island lies on the eastern side of Savu-savu Bay, between

the mouth of the Ndreke-ni-wai River and Valanga Harbour. Here a number

of lofty headlands separated by broad valleys descend with precipitous

fronts to the shore, some of them, as in the case of the Nambathi

promontory on the north side of Valanga Harbour, retaining an elevation

of 1,000 feet within a few hundred yards of the coast.

By following the coast-track from the Ndreke-ni-wai River to Valanga one

crosses some of these headlands. As far as Vatu-lele altered red tuffs,

basic agglomerates, and massive basaltic andesites are the prevailing

rocks. The red tuffs exhibit a double alteration. They were originally

composed of finely pulverised basic vacuolar glass, which subsequently

became palagonitised, and afterwards there was an extensive deposition

of chalcedonic silica and of red iron oxide. No organic remains appear

to exist; whilst the scanty calcite present is evidently an alteration

product. Where the road “tops” the headland on the north side of

Vatu-lele Bay, there is exposed a dyke-like mass of a rubbly

semi-vitreous basaltic rock penetrated in all directions by veins, 1 to

3 inches thick, of a tachylytic glass, splinters of which fuse readily

in the ordinary spirit-lamp flame. The numerous fissures were doubtless

produced during the consolidation of the rock; and subsequently they

were filled with the still fluid residual portion of the magma, which

would be composed of the most fusible constituents. This subject, which

bears on the origin of palagonite, is discussed in Chapter XXIV.

Between Vatu-lele and Urata, palagonite-tuffs and basic agglomerates are

chiefly displayed. On the north slope descending to Urata there is

exposed in the foot-path a dyke-like mass of a dark-grey

hornblende-pyroxene-andesite, an unusual type of rock which is described

on page 298. Just south of Urata I observed an agglomerate containing

large blocks, 3 or 4 feet across, of the deeper-seated altered grey

pyroxene-andesites that with the gabbros and diorites form the axis of

the range.

(4) THE VALLEY OF NA KULA.—In crossing from Sava-reka-reka to Natewa

Bay, one ascends the remarkable valley of the Kula and traverses the

ridge at its head. This ridge, which is about 700 feet in height and

forms the termination of the Valanga Range, is composed of altered grey

hornblende-pyroxene-andesites and of similar holo-crystalline rocks

representing the gabbro or plutonic type of the same. One of these rocks

is described on page 250, under the head of hornblende-gabbro. Another

is referred provisionally to the hypersthene-gabbros (page 249); but it

is extensively occupied by chlorite, viridite, and other alteration

products. Here, as with the other rocks of the Na Kula Ridge, the

plagioclase phenocrysts are opaque, the result of the numerous fine

cracks with decomposition products in the interior of the crystals....

It is thus seen that in general structure the Na Kula Ridge represents

the main axis of the Valanga Range to the north.

The valley of Na Kula is occupied by a river which does not empty

itself, as one would expect, into Savu-savu Bay, but turns off sharply

to the south at right-angles to its previous course, and after breaking

through the coast range, opens into Naindi Bay. This peculiarity has

attracted the attention of the natives. The village of Sawa-Ndrondro,

which lies about 1½ miles up this valley, is not elevated more than 50

feet above the sea. The gradient is evidently not only very slight but

is also irregular, so that in their upper course about 3 miles inland,

where the elevation is only 130 feet, the waters of the river are

partially checked in their flow and form extensive swamps where the

“vitho” or wild sugar-cane flourishes.

(5) CONCLUDING REMARKS ON THE VALANGA RANGE.—It may be inferred from the

geological structure of the range that it is one of the oldest in the

island. The agglomerates and tuffs that enter so largely into the

formation of most of the other mountain-ridges are here to a great

extent absent, except in the lower flanks; and we have exposed the axis

of the range composed of more or less altered grey pyroxene and

hornblende-pyroxene andesites passing, as appears to be the case, into

gabbros and diorites. It is true that the exposure of the gabbros is

limited and that only a single block of diorite came under my notice;

but this might be looked for where the plutonic rocks are deeply seated.

Although far overtopped by the neighbouring agglomerate mountain-ridge

of Mariko, the Valanga Range would seem to date back to a much earlier

stage in the history of the island.

THE MOUNTAIN-RIDGE OF MARIKO

This mountain-ridge, which trends nearly east and west and joins the

Valanga Range, rises in mass to a height of rather over 2,000 feet.

Above this elevation it terminates in several short conical peaks, of

which the highest, 2,890 feet, is named Mariko, the Drayton Peak of the

chart. One of the peaks, lying a little to the east of the summit, and

apparently between 100 and 200 feet lower, is called the Vatu-mbutho or

White Rock. In the profile of the range, as seen nearly “end-on” from

the distant south shore of Natewa Bay, it would appear to be rounded in

its upper part. Its true outline, however, when viewed in length, is, as

described above, namely, a massive ridge with various peaks.

When viewed from the top of the hills behind Valanga, this mountainous

range has a very imposing appearance. On the south side it rises

precipitously to the summit, but the northern slopes below an elevation

of 1,800 or 1,900 feet descend with a very easy gradient for 1½ or 2

miles into the valley of the river Ndreke-ni-wai. In the first case the

average angle of the slope would be from 15 to 20 degrees and in places

often more; whilst in the second case the average inclination would be

about 7 degrees. The contrast between the two sides of the range is very

striking and one ought, I think, to find a parallel in the broken-down

rim of a large crater with a gentle outer slope and a precipitous inner

face. When descending recently the outer slope of Monte Somma, the

ancient Vesuvian vent, I found reproduced some of the features of the

northern slope of Mariko. The tuffs and agglomerate-tuffs that cover

their outer flanks are in both mountains deeply scored by the gorges and

ravines worn by the torrents. After the description of the geological

structure of the Mariko Range, we shall perhaps be in a better position

to consider this question; but until a proper survey of the region has

been made it will not be possible to give a final answer. There are also

many other uncertainties which would be removed by the accurate mapping

of the district, such for instance as the mode of connection between the

Mariko and Valanga Ranges.

The highest peak of the Mariko Range is irregularly square-topped and is

only a few paces across. It has a soil-cap and supports small trees and

shrubs, whilst there is a precipitous rocky face on the east and south.

Like most of the other lofty peaks of the island it is magnetic, and as

remarked on page 368, it markedly deflects the compass-needle.

I made two ascents of this mountain from Vunimbua, one to the highest

peak (2,890 feet), and the other across the range to Nukumbolo at a

point half a mile or more to the west of the summit, where its elevation

is 2,200 feet. Basic agglomerates and agglomerate-tuffs prevail on both

the slopes up to 1,800 or 2,000 feet, the blocks being composed of a

dark semi-vitreous basic andesite referred to the hypersthene-augite

sub-class with specific gravity 2·75. It contains much glass in the

groundmass, and since the pyroxene of the groundmass is prismatic, this

rock belongs to the prismatic sub-order described on page 289. Ordinary

basic tuffs are also well represented on the north flank. On the south

or precipitous side they are usually more or less altered. Here, for

instance, they may take the form of a hard breccia-tuff containing

vesicular lapilli, up to half-an-inch in size, of a semi-vitreous basic

rock, the small steam-holes being either empty or filled with opal or

chalcedony. The matrix of the rock is made up of finer fragments of a

basic vacuolar glass, showing a few felspar microliths, but often more

or less palagonitised. Evidence of further alteration is afforded by the

small cracks and crevices filled with chalcedony.

Other altered tuff-rocks are exposed on the south slope. At an elevation

of 400-450 feet above the sea, and underlying the agglomerates and

breccia-tuffs, I found exposed in a stream-course a hard dark rock

looking like a compact andesite. Under the microscope, however, it is

shown to be an altered palagonite-tuff composed in part of angular

fragments of plagioclase and of rhombic and monoclinic pyroxene, not

exceeding ·15 mm. in size, and containing also similar-sized fragments

of a basic hemicrystalline rock. The base is made up of palagonitic

material and contains a few “Globigerina” tests sometimes displaying

calcite in their interior. Fine cracks filled with chalcedonic silica

testify to a subsequent alteration of the deposit. At 1,500 feet occurs

a hard red altered palagonite-tuff, having a similar composition and

being altered in like fashion, but not displaying tests of foraminifera

in the slide.

The foregoing remarks refer to the main undivided mass of the range,

that is, up to 2,000 feet. The highest peak of Mariko probably

represents in structure the other peaks rising to various heights on

either side of it. Here, at elevations between 2,000 feet and the

summit, a rubbly agglomerate prevails of a somewhat different character

from that occurring at lower levels. It is well exposed in some

cave-cliffs at a height of 2,500 feet and also in the rocky face of the

peak. The rock composing the blocks is a dark-grey aphanitic

augite-andesite (sp. gr. 2·65), referred to genus 20 of that sub-class

and displaying prismatic pyroxene in the groundmass. Smoky residual

glass exists usually in fair amount; whilst in the blocks of the

cave-cliffs it is so abundant that the rock may be termed semi-vitreous.

In the locality just named the blocks are scoriaceous, the steam-pores,

which are drawn out to a length of 5 or 6 mm. and more, being partially

or completely filled with calcite and occasionally with opal. At times

the steam cavities are much larger. In one of my specimens there is an

elongated cavity 5 cm. (2 inches) in length, which has a thin lining of

chalcedony, from the surface of which pyramidal crystals of calcite

project into the interior. (I found the same grey andesite exposed _in

situ_ lower down the south slope at an elevation of 1,800 feet, but

non-scoriaceous.) The matrix of the agglomerate principally consists of

fine palagonitic material with small fragments of plagioclase and

pyroxene but apparently no lime.

At heights of about 2,800 feet on the south side of the peak, and of

1,600 feet on the north flank of the range, are exposed non-calcareous

greyish tuffs remarkable for the quantity of crystals of rhombic

pyroxene, entire and in fragments, that they contain. This is a

characteristic feature of the more acid andesitic tuffs of the island,

and it is to these deposits that the Mariko tuffs in question make a

near approach. They contain at times subangular fragments of more basic

rocks; and are true tuffs in the sense that although perhaps deposited

on a sea-bottom they represent the ejected materials of a subaerial

vent.

The crest of the range, where it is crossed by the road from Vunimbua to

Nukumbolo and for 200 feet below, is formed of a decomposed rock,

perhaps a breccia. A fragment of the rock obtained from the crest is a

grey somewhat altered hypersthene-augite andesite (sp. gr. 2·75) with an

orthophyric groundmass, and referred to the order described on page 290.

This rock may be connected with the tuffs above alluded to.... Reference

may here be made to a black basaltic rock (sp. gr. 2·88) of which, at an

elevation of 2,500 feet at the foot of the peak, I found a portion of a

columnar block about 18 inches across. It may prove to be an

olivine-basalt; but no section has been made of it.

It is apparent from the foregoing description of the Mariko Range that

in general structure it does not differ materially from the other

mountain-ridges of the island, although in the types of the rocks it

presents some variety. Here also we have agglomerates prevailing on the

flanks and forming the summit. As far as the characters of the rocks can

guide us, we cannot determine whether the range has been built up by a

number of vents on a great fissure, or whether it represents the remains

of a huge crater. In this uncertainty we can only appeal to the contrast

between the gentle gradient of the north slopes and the precipitous

descent of the south slopes as favouring the last supposition. We

cannot, however, doubt that the agglomerates of the upper portion of the

range are the products of an eruptive vent or of vents that rose above

the surface of the sea, since the blocks are all of one kind of andesite

and are often scoriaceous. We can be fairly certain that at such a time

the lower slopes were in part submerged, seeing that foraminiferous

tuffs underlying the agglomerates are now exposed. But we have to

distinguish between these submarine basic tuffs of the lower slopes

which may in part be the result of marine-erosion and the grey

rhombic-pyroxene-tuffs of the upper levels which are probably derived

from subaerial eruptions.

THE SAVU-SAVU PENINSULA

I include in this district the promontory west of Naindi Bay and

Sava-reka-reka Bay. Although its surface is much cut up, it has, when

viewed from a distance, a fairly even profile and attains a maximum

height of rather over 800 feet. From the region east of it, it is

separated by the Naindi Gap. Here one can cross the peninsula between

the two bays above named without rising more than 50 feet above the sea.

The elevated interior is divided into two parts, which are divided by a

_col_, about 250 feet in elevation, which is ascended in crossing from

Naithekoro on the south coast to Na Kama on the north coast. Much of the

surface is clothed with the usual “talasinga” vegetation. Close to the

north shore, with which it is connected by the reef-flat, rises the

small island of Na-Wi, and off the extremity of the peninsula, which is

known as Harman’s Point, is the islet of Naviavia, formed of raised

reef-limestone as described on p. 8. The celebrated boiling springs

known as Na Kama are situated on the north coast opposite Na-Wi. It may

be remarked in passing that besides finding an exit in the springs, the

hot water oozes through the beach and below the tide-marks for several

hundred yards along the shore. These springs are described in detail on

p. 25.

This is one of the few districts of the island in which elevated

reef-masses occur at the sea-border. These old reefs, which attain a

maximum elevation of 250 feet above the sea, are principally restricted

to the neighbourhood of Naindi Bay. (They are referred to in detail in

Chapter II.) But they indicate only a part of the submergence which this

region has experienced. There is an exposure of a very interesting rock

in a stream-course that is crossed on the road from Yaroi to Naindi,

less than a mile from the first-named place, and about 30 feet above the

sea. Here we find a dark, impure “Globigerina” limestone, or, as it

might be also designated, an altered calcareous palagonitic

clay-tuff.[84] The larger fragments in it average only ·2 mm., and it

affords evidence of a period of submergence during which the hill-tops

of the Savu-savu Peninsula were below the sea-level.

We get the same indication, but in a more pronounced degree, in the

stratified sedimentary clay-tuffs which are exposed on the shore-flat of

the south side of the neighbouring Sava-reka-reka Bay. These beds, which

within a distance of fifty paces are inclined 10-15° to the south-west

and the same amount to the north-west, have apparently a quâquâversal

dip. In places they exhibit a spheroidal and concentric structure, and

are penetrated by cracks containing some calcite, but mostly filled with

a white zeolitic mineral.[85] One of these rocks is a bright green, hard

and compact deposit, containing but little lime, and evidently an

altered palagonitic clay-tuff. It contains a few minute tests of the

“Globigerina” type; and on account of the small size of its fragments of

minerals, which range from .01 to .04 mm., it may be regarded as a

relatively deep-water sediment.[86] It is interstratified with a

coarser, somewhat altered palagonite-tuff, which shows but little lime

and only a suspicion of tests of foraminifera. The size of the larger

included fragments does not exceed half a millimetre.... The low hill,

near Yaroi, on which the magistrate’s house is built, is composed of

fine and coarse tuffs, probably submarine. It is doubtful whether any

but sedimentary tuffs occur in this peninsula.

In the hills of the western part of the peninsula, that is, west of Na

Kama and Naithekoro, a particular type of basaltic andesite prevails,

characterised by rhombic pyroxene as well as augite phenocrysts, and

referred for the most part to genus 13 of the hypersthene-augite

andesites. Their specific gravity ranges from 2·76 to 2·83, and the

interstitial glass may be fair or scanty in amount. The average length

of the felspar-lathes is unusually small, ·04-·06 mm. In these respects

the basaltic andesites of the Savu-savu Peninsula differ from the

basaltic andesites found in most other parts of the island, where, as

exemplified by those of the Wainunu, Solevu, and Seatura regions, the

felspar-lathes average between ·1 and ·2 mm. in length, and there is

practically no rhombic pyroxene. A somewhat scoriaceous semi-vitreous

form of pyroxene andesite is exposed on the south slopes above

Nukumbalavu, where it is covered by basic agglomerates. The pyroxene in

the groundmass is here prismatic, and not granular, and for the most

part rhombic; and the rock is referred to the prismatic sub-order of the

hypersthene-augite andesites described on p. 287.

The basaltic andesites of the peninsula are often extensively decomposed

through the weathering process, a spheroidal structure being then

displayed. It rarely happens that the basaltic rocks of this locality

assume a propylitic character. Yet, if this change is due to

hydrothermal metamorphism, we ought to find altered rocks of this kind

in the vicinity of the boiling-springs. Such rocks did not come under my

notice at the surface; but this only indicates that if this alteration

has taken place here, it has been effected at some depth; and, indeed,

it would seem probable that the alteration known as “propylitic” is a

change produced generally in deep-seated rocks.

A semi-ophitic basaltic andesite that is exposed in the small

stream-course at the back of the springs, and not 100 yards distant,

displays no propylitic change, and is only affected by hydration. The

basaltic andesites found on the hill-slopes further inland from the

springs exhibit no change of such a nature. However, rocks of this

description occur at and near the coast about a mile to the westward.

One of them, which is light green in colour, might be taken for a

limestone, since it effervesces with an acid. When examined in the slide

it is shown to be the prevailing basaltic andesite greatly altered. The

porphyritic rhombic pyroxene is replaced by viriditic material; the

plagioclase phenocrysts are replaced by calcite, secondary silica, and

other alteration products; and the structure of the groundmass is

disguised by chalcedony, calcite, viridite, &c. Another rock from this

locality displays great alteration. The structure of the groundmass is

obscured by secondary silica, and is traversed by fine cracks passing

through the felspar phenocrysts and filled with blood-red films of

hematite.

On the hill-slopes behind Harman’s Point, at an elevation of 300 to 400

feet, blocks of a reddish, volcanic rock, greatly altered by the

deposition of silica, were displayed on the surface. The ground was here

strewn in places with beautiful pyramidal prisms of clear quartz,

ranging up to an inch in length. They contain numerous inclusions, their

faces being sometimes deeply etched or eroded. These crystals appear to

have been formed rather rapidly in some highly siliceous thermal

underground waters.

I did not ascend the hills of the portion of the peninsula lying east of

Na Kama and Naithekoro. But whilst crossing the saddle between these two

places, I perceived that the prevailing basaltic andesites extended up

the slopes to the east. The neighbourhood of Naindi Bay offers several

features of interest. The bay, which is circular in shape, is closed in

on the east and west by projecting points, where we find elevated

reef-limestone, 40 or 50 feet above the sea, displaying massive corals

and large “Tridacna” shells in their natural position, and overlaying a

cement-stone composed of blocks of volcanic rocks in a calcareous

matrix. On the beach on the west side of the bay there is exposed a

reddish-grey altered pyroxene-andesite, which, as regards the size of

the felspars of the groundmass and other characters, appears to be an

altered form of the prevailing basaltic andesites of the peninsula. In

the midst of the low passage that isolates the peninsula, which I have

termed the Naindi Gap, there is displayed a highly altered basic

andesite which contains a white, zeolitic mineral in its numerous

cracks.

The small island of Na-Wi consists of two low hills, the highest 130

feet in height, connected by a mangrove swamp and a sandy beach. There

is no trace of a crateral cavity. The prevailing rock is a porphyritic,

compact, basic andesite, differing from the other rocks of the

neighbourhood in the greater amount of glass it contains. Though it is

not easy to find a good, unweathered specimen of the rock, it would

appear that Na-Wi represents an old volcanic neck.

We may infer from the above description of this peninsula that it has a

history similar to that of most other parts of the island. There is

evidence in the upraised reefs and in the “Globigerina” clays and

limestones of considerable submergence at one period; and it is highly

probable that the prevailing basaltic andesites are the products of

submarine eruptions. In my account of the hot springs given on page 26,

reference is made to the absence of any trace of a crateral cavity in

that locality. The same is true, as far as my observation goes, of the

whole peninsula. Altered rocks do not occur in the vicinity of the

springs, but they are to be found at distances a mile and more away. It

does not seem possible to restore in imagination the original form of

this part of the island. The present contours are the results of more

than one reshaping of the surface through the agencies of marine erosion

and sub-aerial denudation.

THE DISTRICT BETWEEN NAINDI BAY AND THE SALT LAKE

Three or four of the peaks of this hilly district rise to about 1,000

feet or rather over, the highest being that of Na Suva-suva, which

attains a height of 1,110 feet. Since my acquaintance with this region

is incomplete, I will confine my remarks to the localities actually

examined.

Through the kindness of Mr. F. Spence, I was able to make use of a track

cleared to the top of Na Suva-suva. This eminence, which forms a

conspicuous landmark for many miles, both landward and seaward, has a

rounded summit and is to all appearance an old volcanic neck. It is

composed in mass in its upper half of a heavy dark olivine-basalt (sp.

gr. 3·01), seemingly non-columnar, and referred to the highly basic

rocks forming genus 16 of the olivine-basalts. There is such a thick

soil-cap on the lower slopes that I was unable to ascertain the

character of the rocks there. It is, however, noteworthy that a very

similar olivine-basalt (sp. gr. 2·99) crops out on the coast south of

this hill and to the east of Naindi Bay. They both contain abundant

small olivine-phenocrysts and a little residual glass, the

felspar-lathes averaging ·1-·14 mm. in length. Since their localities

are rather more than a mile apart, it is not possible to say without a

further examination of the locality whether or not we have here the same

intrusion.

On the coast between Naindi Bay and Salt Lake Passage, calcareous tuffs,

probably fossiliferous, are occasionally exposed in the low spurs

descending to the sea, whilst islets of elevated reef-rock front the

beach.

The coast immediately west of the Salt Lake Passage is of exceptional

interest. Here the sea-cliffs and the shore-flat are formed of an

agglomerate tuff penetrated in all directions by veins of calcite, an

inch and under in thickness. The matrix of this deposit, which is a

little calcareous, is principally made up of fragments, ranging up to 3

or 4 millimetres in size, of vacuolar palagonite, the minute vesicles

being filled with some alteration product. It also contains large macled

augite crystals 5 or 6 mm. in size, which can be picked out in numbers

by the fingers. The blocks vary from a few inches to two feet across,

and are usually composed of an augite-andesite, containing large

porphyritic crystals of augite, and are often amygdaloidal, the

amygdules, 3 or 4 mm. in size, being formed of a zeolite. But blocks of

very different rocks also occur in this agglomerate tuff. One, about two

feet across, was composed of a coarsely crystalline diorite made up, as

described on page 251, of large crystals of hornblende, 2 to 2·5

centimetres long, and of large opaque crystals of acid labradorite.

Another was made of hornblende-hypersthene andesite belonging to the

ortho-phyric order of that sub-class (see page 299). There is a little

altered glass in the groundmass, and large secretions of brown

hornblende, more than an inch in size, are to be observed in the rock.

It is probable that this singular deposit represents a submarine

accumulation of materials ejected from some neighbouring vent. Organic

remains did not come under my notice; but apart from the palagonitic

character of the matrix and the abundance of veins of calcite, the

submarine origin is indicated by the existence of upraised reefs in the

coast districts east and west of this locality. The block of diorite

affords an important clue as to the character of the deep-seated

plutonic rocks in this part of the island. A similar diorite was found

by me amongst the blocks in the bed of the Vunimbua River; and on page

185, reference is made to the probability of such rocks forming the

nucleus of the Valanga Range.

The hills on the west side of the Salt Lake are worth further

examination. On the coast of the Natewa Bay side of this district,

in the vicinity of Vuni-tangaloa and between that place and

Vuni-sawana, there are displayed agglomerates formed of blocks of

hornblende-andesite, some of the specimens being very similar to

that obtained from the block of hornblende-andesite noticed in the

agglomerate-tuff on the neighbouring south coast.

THE SALT LAKE

The low isthmus, about 2½ miles in breadth, which connects the Natewa

Peninsula with the rest of the island, can be crossed without rising

more than 40 or 50 feet above the sea. From the occurrence of upraised

reefs in the islets and in the low sea-cliffs of the south coast it may

be inferred that at no distant period in the history of Vanua Levu this

isthmus was submerged.

The lake, which is oblong in form, is about four-fifths of a mile long

and about two-fifths broad. Its maximum depth according to the Admiralty

chart is 3 fathoms; but the usual depth in the centre varies, as I

found, between 2 and 2½ fathoms. It communicates with the sea on the

south coast by a long narrow passage, rather over a mile in length,

which for the greater part of its course, excepting near its seaward

mouth, is only between 25 and 30 feet broad. Mangroves flourish around

the lake and also line the passage; whilst elevated reef-rock is to be

observed on the sides of the passage. Mr. Horne was informed that corals

abound in the lake-waters; but I find no reference to this point in my

notes. Judging from the density of the effluent water, the specific

gravity of the lake-water is that of the sea. The “rise and fall,” as

noticed below, is considerably less than in the case of the tides at the

coast.

Near the centre of the lake there is a low islet, some 40 paces across,

and only raised about a foot above the level of the lake at the time of

high-water. It is chiefly made up of coral blocks; but there are a few

fragments of basaltic andesite lying about, which were probably brought

there by natives. This islet is mentioned in Mrs. Smythe’s account[87]

of the visit made by Colonel Smythe to the lake in 1860; and by reason

of its little elevation it may be accepted as a rude datum-mark of the

relative level of land and sea in this region. From this it would appear

that there has been no appreciable change of level in this region for

the last forty years.

Except on the north and north-west sides, the lake is more or less

surrounded by hills reaching up to 400 or 500 feet, the passage

representing a break in the range. On the Natewa Bay side the level of

the surface is much lower. The low strip of land that intervenes between

the north-west corner of the lake and Natewa Bay is about a mile across,

and does not attain a greater elevation than 40 or 50 feet above the

sea. On its surface, fragments of basic volcanic rocks are displayed;

but no reef debris came under my notice. At its north-east side the lake

is only separated from Natewa Bay by a neck of land 300 to 400 yards in

breadth and about 100 feet high. It was across this neck that the

natives in old times used to drag their large canoes.

Mr. Horne[88] who visited this neighbourhood in 1878, suggested that the

Salt Lake occupies a crater-cavity. The hills around are of volcanic

formation, and I am rather inclined to support this view; but certainty

is scarcely possible now, on account of the great degradation which the

surface has evidently experienced during and since the emergence; whilst

subsequent reef-growth has also to some extent masked the original form

of the district. It is noteworthy that a somewhat parallel condition of

things is presented a few miles to the west by the circular Naindi Bay

and the low passage, not more than 50 feet above the sea, that partly

isolate the Savu-savu Peninsula.

The peculiar behaviour of the tides in connection with the Salt Lake and

its passage attracted my attention during two visits to this locality.

On the first occasion I noticed that between two and three hours after

the tide at the coast had commenced to rise there was still a strong

flow through the passage from the lake, and that the current was only

reversed in the latter half of the rising tide. During my second visit

at the end of May, 1899, when I was accompanied by Mr. Smallwood, I

spent a night in observing the behaviour of the spring tides at a spot

below the narrow portion of the passage 600 or 700 yards from the

opening on the coast. Here the breadth was about 100 feet, the depth at

low-water 5 feet, and the rise of the tide 4 feet. The current ran

seaward at a velocity varying from 1,500 to 2,500 yards per hour; and it

continued to flow in this direction for 2½ hours after the tide had

begun to rise on the coast. (In the narrow part of the passage the rate

of the current would probably be not over 3 knots.) It is curious that

at the place of measurement the bottom was formed of mud into which the

pole sank six feet without striking a hard substratum. The observations

on the current were made with a vertical float immersed about 3 feet.

The point of difficulty in the behaviour of the tides is this. The water

is running rapidly out of the lake for nine hours; whilst during the

remaining three hours there is a sluggish return-flow up the passage

into the lake. A far greater quantity of water finds an exit by the

passage than is returned by the same channel; and I can only explain

this by assuming that there is an extensive percolation of water from

Natewa Bay into the lake. It is easy to show that with such a narrow

effluent, which cannot have a sectional area exceeding 180 square feet,

the level of the lake would be only lowered 2 or 3 feet, if the average

velocity during the nine hours was two nautical miles. The great bulk of

the water would thus remain unchanged. The ultimate result of such

conditions would be a lake of brine. Since, however, the sea-water of

the lake possesses the ordinary density, it is apparent for this reason

only that there is some other means of supply than by the present narrow

passage leading to the sea. The mean level of the Salt Lake is evidently

rather above that of the sea, perhaps a foot or two; and the

“rise-and-fall” is probably very small.

CHAPTER XIV

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES

(_continued_)

THE NATEWA PENINSULA

THIS remarkable peninsula is connected with the rest of the island by

the low-lying Salt Lake district, a narrow isthmus, described in the

preceding chapter, which one can cross without rising 50 feet above the

sea. My acquaintance with this region is far from complete; but from the

following notes a fair general idea of its geological characters may be

gathered.

By referring to the map it will be seen that there are three groups of

mountains. The north-eastern culminates in Mount Freeland or Ngala,

2,740 feet; the southern is formed by the rugged Waikawa Range, 1,540

feet; whilst the Lea Range to the west attains in Ngalau-levu a height

of 1,960 feet. They are much cut up by the denuding agencies, and all

bear the stamp of an ancient land-surface. Though hot springs are not

infrequent, as at Ndreke-ni-wai, Waikatakata, Ndevo, and Navuni, no

evidence of recent volcanic action came under my notice. Submarine

deposits occur at intervals on the surface up to elevations of 1,000

feet and over; but with the exception of the comparatively recent

upheaval or emergence of some 20 or 30 feet, indicated by the raised

reef-masses and foraminiferous tuffs and clays in different parts of the

coast, there is nothing to suggest that these changes did not occur ages

since. In the frequent alteration of its andesitic rocks, and in the

occasional occurrence of porphyrites, we have sufficient indication of

the antiquity of this part of the island as far as its volcanic history

is concerned.

I will commence the description of this peninsula at its western end.

The broken elevated district that extends eastward from the Salt Lake to

Fawn Harbour on the south coast, and to the mouth of the Ndreke-ni-wai

River on the north coast, is divided into two principal masses, which

are connected by a ridge or _col_ about 400 feet above the sea, which is

situated a little east of Viene. The western portion, which may be named

the Viene sub-district, attains a maximum height of 1,000 feet. The

eastern portion reaches in the peak of Ngalau-levu, a height of 1,960

feet, and may be termed the Lea sub-district.

THE VIENE SUB-DISTRICT.—The cliffs on the north coast between Muanaira

and a little east of Viene are mainly formed of basic tuffs, often

calcareous. At a place about 1½ miles east of Viene, these tuffs as

exposed in a coast spur display large flat spiral tests of shallow-water

foraminifera 4 or 5 millimetres across. They may be described in this

locality as palagonitic calcareous tuff-sandstones, more or less

compacted, and containing fragments of palagonitised basic rocks. When

crossing the _col_ above referred to one finds similar palagonitic

calcareous sandstones and clays exposed on its slopes up to its summit

(400 feet).

On the south side, in the vicinity of Vunilangi Inlet, foraminiferous

clays and reef-limestones are displayed at the foot of the slopes; and

the coast between this place and Tathelevu to the westward is bordered

by low cliffs of reef-limestone raised 6 to 8 feet above the high-water

mark and displaying massive corals in their position of growth. Near

Tathelevu there occur raised reefs 10 to 15 feet above the sea; whilst

the hills, 250 to 300 feet in height, at the back of this place are

composed of fine and coarse tuffs and tuff-sandstones containing little

or no lime, and apparently no organic remains. They are sedimentary

tuffs of mixed composition, made up of fragments of plagioclase, rhombic

and monoclinic pyroxene, brown hornblende, portions of semi-vitreous

basic andesite, and palagonitic debris. In the lower levels they are

fine textured with a grain of ·2 to ·3 mm. In the higher part their

grain is ·5 to 1 mm., and they are more basic in character and come near

to the palagonite-tuffs. At an elevation of 200 feet they form inland

cliffs, 50 feet high, in which are imbedded blocks, 2 feet across, of a

blackish pyroxene-andesite with a specific gravity of 2·73, and

belonging to the prismatic sub-order of the orthophyric order of the

hypersthene-augite andesites. It is remarkable for the pyroxene prisms

of the groundmass, and shows a little interstitial glass. These cliffs

are well displayed behind Navelatha, about half a mile from Tathelevu.

Between this locality and the Salt Lake Passage, elevated

reef-limestones, forming low cliffs 6 to 8 feet high, together with

occasional tuff-agglomerates, occur at the coast.

THE LEA SUB-DISTRICT.—This region, which includes the mountain-range of

Ngalau-levu at the back of Lea, is limited by Fawn Harbour and Vunilangi

Inlet on the south coast, and by the Ndreke-ni-Wai River and a point

between Viene and Lea on the north coast. Its structure, as is shown

below, is very complicated, acid and basic rocks being associated in a

remarkable manner; whilst over all lie the submarine tuffs. Marine and

sub-aerial denuding agencies have shaped and re-shaped the surface to

such a degree that it is now impossible to restore it in imagination.

On the north coast of this sub-district, about two miles east of Viene,

is exposed an altered darkish porphyrite displaying large opaque

crystals of plagioclase, 5 to 7 mm. long, the rock-mass being penetrated

by fine veins of chalcedonic quartz, which also traverse the

phenocrysts. Its specific gravity is 2·6; but on account of the

imperfect development of the felspar-lathes and the amount of altered

glass in the groundmass, which also contains a little calcite, it can be

only generally referred to the augite-andesites. A greenish altered

foraminiferous tuff showing fine cracks filled with chalcedony composes

a spur in this locality. A propylitic or highly altered dolerite is

exposed half-way between Viene and Lea.

As one nears Lea from the west the lofty spurs of the mountain of

Ngalau-levu reach the coast, and basic tuffs and agglomerates prevail.

The blocks in the agglomerate are composed of a vesicular semi-vitreous

hypersthene-augite andesite, which is assigned to the second prismatic

sub-order, since it carries prismatic pyroxene in the groundmass. The

town of Lea is picturesquely situated on the coast at the foot of the

steep mountain-slopes, being closed in on the east and west by elevated

spurs descending to the sea. Fragments of jasper and chalcedony occur in

the beds of the streams that here drain the precipitous sides of the

range. Two dykes of dark basic rocks protrude through the beach in Lea

Bay. They are composed of augite-andesites referred to genus 13 of the

augite-class; but the two rocks belong to different species of that

genus. In the one the felspar-lathes are only ·04 mm. in length, and

there is a little altered glass in the groundmass, the specific gravity

being 2·63. In the other the felspar-lathes average ·2 mm. in length,

and the rock has a coarser texture, whilst the specific gravity is 2·7.

The augite granules are large (·03 mm.), and there are irregular lacunar

spaces filled with calcite and lined by a brown palagonite-like

material.

I ascended the second highest peak of the Ngalau-levu mountain, which

rises to a height of 1,680 feet behind the town of Lea, the highest

summit lying to the eastward. Ngalau-lailai, which I also ascended, is a

lesser peak, 1,400 feet in height, situated yet nearer to the town.

Basic tuffs and agglomerates similar to those exposed in the western

spur of the bay occurred all the way up to the bare rocky pinnacles

forming the summits. The blocks in the agglomerates are made up of a

semi-vitreous augite-andesite, which is sometimes scoriaceous or

amygdaloidal, and at other times pseudo-vesicular. Augite crystals, 5 or

6 millimetres in length, are inclosed in the tuffs which contain

palagonitised materials, but apparently no organic remains.

In the spur on the east side of Lea Bay occurs a light-coloured altered

hornblende andesite. The brown hornblende is mostly represented by black

pseudomorphs. Such a rock appears in strange contrast with its basic

surroundings. This is followed, as one proceeds eastward along the

coast, by basic tuffs and agglomerates. It should have been before

observed that blocks of a blackish-brown olivine basalt (sp. gr. 2·89),

referred to genus 13 of the olivine class, occur at intervals on the

coast between Viene and Ndreke; but the rock never presented itself in

position. The tiny felspar-lathes (·03 mm. long) are in flow

arrangement; but there is little or no residual glass, and the augite

granules (·01 mm.) occur in great abundance.

About two-thirds of the way between Lea and Ndreke-ni-wai there lie

close to the shore two islets, 20 to 25 feet high, of reef-limestone, in

which massive corals may be observed in their position of growth.

Further east, about half a mile west of Ndreke-ni-wai, there is exposed

at the coast a bedded light-coloured non-calcareous compacted tuff-rock,

dipping 12° to 15° to the southward. It contains pebbles and blocks of

acid and basic andesitic rocks, and may be described as an altered

hornblende-andesite tuff. Basic agglomerates occur as one approaches

Ndreke-ni-wai. This town lies at the mouth of the river of that name,

the first river that one meets on the north side of this peninsula.

There exist here between the tide-marks some hot springs, to which

reference is made on page 34.

When crossing the Natewa peninsula from Ndreke-ni-wai to the head of

Fawn Harbour, one reaches a height of 660 feet above the sea. This ridge

represents the “divide” between the Lea and Waikawa mountain-ranges. In

the lower part of the northern slopes of this ridge occur basic tuffs

and agglomerates; but between 200 and 400 feet a light-coloured acid

rock of the hornblende-andesite type prevails, both in the form of

agglomerate and of loose blocks. This rock is described under the second

order of the hornblende-hypersthene andesites on page 299.

Descending the south slope, I found at an elevation of 500 feet a single

large mass, about 4 feet across, of quite another type of volcanic rock,

which is referred to the orthophyric order of the hypersthene-augite

andesites (page 290). It is a dark grey almost holo-crystalline rock

(sp. gr. 2·69) showing porphyritic pyroxene to the eye and displaying in

its relatively scanty groundmass short stout felspars, ·05 mm. in

length. On the surface of the lower two-thirds of this southern slope

occur basic tuffs and agglomerates, basaltic blocks being found in the

streams. The tuffs are palagonitic and contain a few calcareous

particles. They apparently contain some foraminiferous shells and are

doubtless of submarine origin.

It would seem that the axis or deeper portion of this ridge is composed

of the hornblende and hypersthene-augite andesites, whilst the basic

tuffs and agglomerates form the slopes.

With reference to the south side of the Lea sub-district, it may be

observed that whilst on the north side the mountains rise up close to

the sea-border, here they are separated from the coast by a broad tract

of lowland, where bedded pteropod and foraminiferous clay-rocks are

exposed, dipping gently to the south-east. Usually between Fawn Harbour

and Vunilangi Inlet the coast is margined by low cliffs of

coral-limestone, showing the massive corals in position; but sometimes

the deposits above noticed compose the low cliffs and even the islets

close by.

THE WAIKAWA MOUNTAINS.—This range occupies nearly the whole area of the

broad and elevated promontory that is only separated from Taviuni by the

narrow straits of Somo-somo, which, however, have a minimum depth of 120

fathoms. These mountains extend to the vicinity of Mbutha Bay on one

side and to near Fawn Harbour on the other. Several of the peaks reach

to over 1,000 feet, the greatest height given in the Admiralty chart

being 1,540 feet. The whole region has a very rugged aspect, the

mountains rising up near the coast, whilst the surface is much cut up

into ridges and valleys.

A single traverse across the range was alone made, but the results

obtained are very suggestive and may doubtless be applied to much of

this rugged promontory. I crossed the mountains from Loa to Waikawa. The

summit was about a mile broad and undulating, the level varying between

900 and 1,150 feet. At one place on the top there was a deep hollow,

some 300 or 400 yards across, perhaps the remains of an old

crater-cavity; but the higher slopes were so densely wooded that it was

not possible to get a clear view of my surroundings. Basic tuffs and

agglomerates prevailed on either side from the foot to the top of the

range. Specimens obtained from between 800 and 900 feet above the sea

are characteristic palagonitic tuffs of varying degrees of coarseness

containing 5 to 10 per cent. of carbonate of lime and a few tests of

foraminifera. At 1,100 feet I obtained a specimen which on account of

the large proportion of carbonate of lime (35 per cent.) and the

abundance of foraminiferal tests may be termed an impure foraminiferal

limestone belonging to the group of these rocks described on page 319.

The tests range up to a millimetre in size, and there are also inclosed

a few large fragments, 1 to 2 centimetres in size, of shells and

crystalline limestone. The residue is made of the detritus of

semi-vitreous basic rocks, palagonitic debris, fine clayey material and

minerals (15 per cent.), the last including beside plagioclase abundant

more or less perfect pyroxene prisms, mostly of the rhombic type.

Near the summit there occurred in one place blocks of a highly basic

blackish olivine-basalt (sp. gr. 2·99), marking evidently the situation

of a dyke. This rock is referred to genus 13 of the olivine-class. It

displays abundant phenocrysts of olivine and augite with but little

plagioclase. Interstitial glass is scanty, the groundmass consisting of

stoutish felspar-lathes (·06 mm. long), abundant augite granules and

prisms, and magnetite.

The Waikawa mountains would thus seem to possess the same general

structure that characterises many of the ranges of the island. Submarine

basic tuffs and agglomerates cover their sides and their summits, the

deeper rocks forming the axis of the range being in this case not so

frequently exposed.

On the coast between Waikawa and Navuni, rather over a mile east of Fawn

Harbour, basic agglomerates, palagonitic tuff-sandstones, and calcareous

clay-rocks containing pteropod and foraminiferous tests, prevail. The

tuff-sandstones and clay-rocks are bedded, the stratification being

often well shown in the horizontal sections displayed in the shore-flat.

In one locality within an area a few hundred yards across, a

quaquaversal dip was exhibited. At Navuni, where the hills reach the

coast, the same formations occur. I ascended the stream-course there for

about a mile, basic tuffs and agglomerates being exposed in its sides,

whilst blocks of a heavy dark olivine-basalt[89] lay in the bed. The hot

springs which issue inland at the side of this stream are described on

page 35.

THE BASIN OF THE NDREKE-NI-WAI RIVER.—With the region, which is bounded

on the north by the Mount Freeland Range or the Ngala mountains and on

the south by the Waikawa Range, I have but slight acquaintance, except

in the case of the coast fronting Natewa Bay. A little way up the course

of the river Ndreke-ni-wai, which drains this area, lies the town of

Koro-ni-yasatha, where Mr. Horne, the botanist, spent some days in 1878.

Probably much of this area is not over 200 feet above the sea, and

apparently there is a good deal of talasinga country.

THE COAST BETWEEN THE MOUTH OF THE NDREKE-NI-WAI RIVER AND THE FOOT OF

THE NGALA OR MOUNT FREELAND RANGE.—Between this estuary and Valavala,

two miles to the eastward, occurs a bedded calcareous palagonitic tuff

of sedimentary origin, dipping steeply to the north. In one locality

there is a rudely columnar dyke of a porphyritic augite-andesite. Coarse

basic tuffs exposed in the cliffs and shore-flat of Ko-nandi-nandi Point

on the side of Valavala Bay display a spheroidal structure, due probably

to the vicinity of some igneous intrusion. The sea-border extending from

this bay to Natewa, and farther on to Waikatakata, near the foot of the

Ngala mountains, is in most parts a broad low strip of coast-land, where

rock-exposures are infrequent. A dark grey andesite forms the blocks of

the agglomerate in this locality. It is noticeable on account of the

prismatic pyroxene of the groundmass; and it is assigned to genus 5 of

the second (prismatic) sub-order of the hypersthene-augite andesites. A

blackish semi-vitreous pyroxene-andesite occurs in the vicinity of

Natewa.

At Waikatakata (the Fijian word for “hot water”), where an outlying spur

of the Mount Freeland or Ngala Range reaches the coast, hot springs

issue on the hill-side, as described on page 34. On the slopes around

the springs lie huge masses of an aphanitic basaltic andesite having a

specific gravity of 2·81 and referred to genus 16 of the augite

sub-class. It displays a characteristic andesitic groundmass, showing

crowded felspar-lathes in flow-arrangement with average length of

·17 mm., and containing scarcely any residual glass.

Proceeding along the coast east of Waikatakata, one enters the region of

altered pyroxene-andesites, for which Mount Freeland, or the Ngala

Range, is remarkable. There are first to be observed on the shore blocks

of a grey altered ophitic dolerite, which belongs to the non-porphyritic

division of genus 10 of the augite-andesites and is described in detail

on page 275. Afterwards the characteristic rocks of the district occur.

The lofty spurs of the Ngala Range here reach the shore; and between

them lies the coast village of Ngara-vutu, from which the ascent to the

summit is best made.

MOUNT FREELAND OR THE NGALA RANGE

This high range forms a conspicuous object in the profile of this part

of the island. It derives its Fijian name from the old war-town of Ngala

that was situated at an elevation of 1,500 or 1,600 feet overlooking

Ngara-vutu on the west. The main mass of the range takes a crescentic

sweep, 3 or 4 miles in extent and facing Natewa Bay. It incloses the

coast district of Tunuloa. The steep mountain-slopes here rise to 2,000

feet and over, the greatest elevation being 2,740 feet. The densely

wooded spurs of the range occupy most of the area between the town of

Natewa, Kumbulau Point, and Mbutha Bay.

The summit is a long narrow ridge covered with a dense entangled mass of

Freycinetia stems which render progress very difficult. The rocks

exposed all the way up from the stream-courses in the vicinity of

Ngara-vutu to the top, are almost all of the same type, namely altered

augite-andesites. They are dark grey in colour and effervesce slightly

with an acid, whilst occasionally they show a little pyrites.[90]

Agglomerates were not observed and tuffs only in one locality, 1,200

feet above the sea, where a highly altered tuff composed of debris of

the prevailing rocks was exposed. At an elevation of 500 feet there

occurred a grey doleritic porphyritic rock, displaying large opaque

crystals of plagioclase, and looking like a porphyrite. It exhibits a

semi-ophitic groundmass, and is probably an intrusive mass. A

description of it will be found under genus 10 of the augite-andesites

(page 274).

The general petrological characters of the principal summit of Mount

Ngala point to its high antiquity as a volcanic mountain. It probably

ranks among the oldest extinct volcanic vents in the island.

Proceeding along the coast from Ngara-vutu to Navetau, the Buli’s town

of the Tunuloa district, one observes blocks of basic rocks. Farther to

the eastward for a distance of two or three miles basic agglomerates and

basic tuff-agglomerates prevail along the sea-border, being often

extensively exposed in the cliff-faces of the hills. The rock composing

the blocks is a hemi-crystalline pyroxene-andesite remarkable for the

prismatic pyroxene in the groundmass, and referred to genus 17 of the

hypersthene-augite sub-class.

I crossed the range to Ndevo on the other side of the peninsula from a

place three or four miles west of Kumbulau Point, rising on the way to a

height of 930 feet. For the lower 300 feet on the north slope basic

agglomerates and basic tuff-agglomerates are exposed. They are made up

of the same materials as those above described on the coast. Near and at

the summit occur compacted brecciated tuffs made up of palagonitised

basic materials but containing no lime, and these are associated with

light greenish fine-textured hard tuffs of an acid character, but

without lime or organic remains. In this last case the deposit is formed

of mineral fragments (oligoclase, rhombic and monoclinic pyroxene, &c.),

the debris of a hemi-crystalline volcanic rock, and a quantity of

greenish alteration products.

On the south side in the vicinity of Ndevo the sea-border is composed of

calcareous palagonitic clays and tuffs containing pteropod shells and

large tests of foraminifera in abundance. These deposits, which are

horizontally bedded, extend a mile inland and reach to between 250 and

300 feet up the slopes. A hot spring is stated to occur between the

tide-marks near Ndevo.

At intervals all along the coast between Ndevo and Nuku-ndamu, passing

on the way the villages of Koro-i-vonu, Tuvumila and Kanakana, fine and

coarse basic tuffs, often calcareous, are to be seen exposed in the

sea-cliffs and in the low hills behind. They are bedded and dip 5° to

10° W.S.W. near Ndevo and 15° to 20° S. b W. south of Kanakana. The

lower slopes of the Ngala mountains here approach the coast, and it is

highly probable that the submarine tuffs which form the sea-border

extend a considerable distance inland and to some height above the sea.

Between Nuku-ndamu and Tukavesi, where the mountains rise close to the

beach, occur basic agglomerates and agglomerate-tuffs, derived from

basaltic andesites. A specimen of the last named represents an uncommon

type of basaltic rock (sp. gr. 2·85), which on account of the character

of the felspars and of the pyroxene of the groundmass is referred to the

prismatic sub-order of the orthophyric order of the hypersthene-augite

andesites (page 290).

It may be inferred from the foregoing remarks that whilst the main

elevated mass of the Ngala Range is composed of altered basic andesites,

the product of ancient eruptions, the basic agglomerates, tuffs, and

clays, which occur on the lower slopes and in the outlying spurs, are of

later date. These tuffs and clays are evidently of submarine origin, at

least in the lower levels; but although fossiliferous deposits were not

observed at greater elevations than 300 feet above the sea, it is

probable that future investigators will find them at much higher levels.

Their discovery, as before noticed, at an elevation of 1,100 feet in the

adjacent Waikawa mountains, renders it likely that as in the case of the

mountainous districts of the main portion of the island the whole of the

Natewa peninsula was at one time submerged, or at least all the region

excepting the summit of Mount Ngala.

CHAPTER XV

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES

(_continued_)

THE NORTH-EAST PORTION OF THE ISLAND

THIS large area, which extends for a distance of nearly forty miles from

the eastern slopes of the Mount Thurston Range to Undu Point, forms the

region closing in Natewa Bay on the north. It would be difficult to

imagine an area of this size with a greater variety of surface or

showing such a lack of arrangement of its principal features. The hills

and mountains on the north side gather at the coast, and extensive

inland plains, raised but a few feet above the sea and strewn with

silicified corals, occupy a portion of its interior. A long valley with

a very small gradient extends nearly across its breadth; and the rivers

are for the most part tidal estuaries fed, except in one or two cases,

by insignificant streams. There is, however, a lofty range of

ridge-mountains in its broadest part attaining a height of 2,500 feet;

whilst away to the east stretches the great Undu Promontory singular for

the straightness of its form.

Volcanic rocks of acid types, such as oligoclase-trachytes,

quartz-porphyries, and white pumice-tuffs prevail in the northern part

between Undu Point and the promontory opposite Mali Island. In the

southern part, from the foot of Mount Thurston and Vuinandi to the

vicinity of Tawaki, massive rocks and tuffs and agglomerates of basic

characters predominate. Although my acquaintance with this area is

incomplete, the data below given will be sufficient to enable a general

idea to be formed of its structure. The more conspicuous features in its

geology will gradually come into prominence as the various localities

visited are described.

THE SOUTHERN SEA-BORDER BETWEEN VUINANDI AND THE VICINITY OF TAWAKI

The basic rocks, which characterise this long extent of coast, give

place about two miles west of Tawaki to the acid rocks. I will proceed

methodically with the description from Vuinandi eastward.

(_a_) _The coast between Vuinandi and Nakarambo._—Along this coast,

spurs from the Mount Thurston Range reach the borders of Natewa Bay,

forming a succession of small bays a mile or so across, on the shores of

which are situated the villages and towns of Vuinandi, Ndaku-ndaku,

Korotasere, and Nakarambo. The basaltic rocks characteristic of the

vicinity of Vuinandi are predominant here; but agglomerates and tuffs

are of rare occurrence, the prevailing rocks exposed in the spurs being

fine textured basaltic andesites (sp. gr. 2·82). A calcareous tuff-clay

is, however, to be observed in the east point of Vuinandi Bay, where it

is apparently penetrated by a dyke of the above mentioned rock. In

Ndaku-ndaku Bay there issue from the shore-flat between the tide-marks

some hot springs which are referred to on page 34.

(_b_) _The coast between Nakarambo and Waimotu._—In this tract we meet

with rocks of a somewhat different character, though still basic in

their type. Through the agglomerates and agglomerate-tuffs of the spurs

protrude coarsely crystalline grey pyroxene-andesites.[91] They are to

some degree altered, and are characterised by their abundant phenocrysts

of plagioclase and of rhombic and monoclinic pyroxene, the groundmass

being relatively scanty. The agglomerates are formed of similar

materials. There is an interesting exposure in the point east of

Nakarambo. My specimens from this locality were unfortunately lost; but

in my notes reference is made to the coarsely crystalline grey andesite

above noticed, to the later intrusion of a dark amygdaloidal rock, and

to an altered calcareous tuff.

(_c_) _The coast between Waimotu and Natasa Bay._—A low belt of land

often forms the sea-border. Between these two localities there is a

broad estuary, the village of Vanuavou being situated on the right side

and Malati on the left side. Tuffs usually calcareous and probably

submarine are here displayed together with basaltic andesites.

(_d_) _The coast between Natasa Bay and the vicinity of Tawaki._—This

extensive stretch of sea-border, nearly 15 miles in length, is

characterised by hilly spurs and long intervening low-lying tracts. The

prevailing rocks exposed are tuff-clays, somewhat hard and altered, and

coarser basic tuffs sometimes calcareous and overlying the former. These

sedimentary deposits, which are evidently submarine, are often bedded

and have a fairly constant dip over most of this region of 10 to 15

degrees to N.W. or W.N.W. They are frequently pierced by dykes, 6 to 10

feet thick, of compact basic rocks. A specimen of one of these

dyke-rocks from between Natasa and Vatu-karoa is a doleritic basalt with

scanty olivine (sp. gr. 2·81)[92]; but the rock of the dykes east of

Vatu-karoa is less basic (sp. gr. 2·72), and may be described as an

augite-andesite[93] with a doleritic structure in part disguised by

alteration. Between Natasa and Sangani there is a remarkable exposure of

an intrusive opaque white rhyolitic rock associated with altered tuffs.

This rock, which has undergone some degree of alteration, is described

on page 311. It is the only indication of the vicinity of a region of

acid rocks that I came upon on this tract of coast. It probably, if

traced inland, would be found connected with the district of acid rocks.

In following along the coast, however, towards Undu Point, the region of

acid rocks is not reached until within 2 or 3 miles west of Tawaki.

Reference should be made here to two hot springs that, as described on

page 33, rise up on the coast at Natuvu, a little east of Lakemba.

THE INLAND MOUNTAINOUS REGION BETWEEN THE EASTERN

FOOT OF THE MOUNT THURSTON RANGE AND THE VICINITY OF TAWAKI

This inland region corresponds for the most part to the tract of coast

before described between Vuinandi and the vicinity of Tawaki. We have

here a very much broken area traversed in a northerly and southerly

direction by mountain-ridges and penetrated in places on the north side

by prolongations of the Wainikoro and Kalikoso plains. The two loftiest

summits, the Ndoendamu and Savu-riti peaks of the chart, rise

respectively to heights of 2,481 and 2,238 feet. The former is the Hale

Peak of the Wilkes’ Expedition; but as regards the native names there is

some confusion and I have not been able to clear it up. Several of the

lesser peaks rise to over a 1,000 feet, and four or five of them to

rather over 1,500 feet.

With the exception of the fixing of the positions of the more

conspicuous peaks, the interior of this part of the island is

unsurveyed. I was able to lay down my positions approximately; but

before a systematic examination can be made of the geology of this

region a survey is necessary. Several traverses and ascents were made by

me; but much more in the way of exploration remains to be done. I

venture, however, to think that from the data below given a fairly

accurate notion of some of the leading geological features of this

region may be formed. The districts visited will be described in their

order from east to west.

(_a_) _The mountainous district east of the mountains of Vungalei and

Nailotha, extending to the vicinity of Tawaki._—Mr. Thomson[94] in his

map of the sea-border of this district shows a continuous coast range

from near Tawaki to Natasa lying about two miles inland. This is the

appearance when the region is seen in profile from a distance; but when

viewed from an elevation in its interior its surface is seen to be for

the most part traversed by a series of mountainous and hilly ridges

trending roughly north and south, the greatest height not exceeding

1,500 or 1,600 feet.

The geological character of the sea-border of this district has already

been described on page 208. It is there shown that fine and coarse

sedimentary tuffs, sometimes calcareous, and often penetrated by basic

dykes, here prevail. Their general story is one of great denudation, and

we have the same testimony impressed on us when examining one of the

inland mountain-ridges.

I followed the crest of one of these ridges from the hamlet of Nawi, on

the headwaters of the Vui-na-savu River, in a south-easterly direction

for about 4 miles to the peak of Uthulanga, which overlooks Lakemba on

the shore of Natewa Bay. During the walk my level rose fairly gradually

from that of Nawi, less than 100 feet above the sea, until near Lakemba,

where a height of 1,400 feet was attained. Here the ridge abruptly

terminates in the round-topped peak of Uthulanga, which rises steeply

from its crest for another 150 feet or rather more, and has a

precipitous face on the southern side descending far down the

mountain-slope. This peak, which is about 1,550 feet above the sea, when

seen from the opposite shores of Natewa Bay is very conspicuous and

looks like a thumb or a nose. The first part of the Fijian name of this

peak signifies a nose.

The hamlet of Nawi, from whence the ridge begins, is built on a low

mound-like “rise,” which is composed of a dark-grey hypersthene-gabbro.

Since plutonic rocks are of very rare occurrence in this island, the

Nawi gabbro has a particular interest. It belongs to the group of

plutonic rocks described on page 250; and is to be referred to the more

basic kinds, its specific gravity being 2·84. In appearance it is like a

diallage-gabbro, and in the slide displays monoclinic and rhombic

pyroxene filling the interspaces between the large plagioclase crystals

and undergoing respectively the diallage and bronzite stages of

schillerisation. This rock forms the type of the group and need not be

referred to in more detail here. It should be added that in the bed of

the neighbouring river occur blocks of a basaltic andesite (sp. gr.

2·86) referred to genus 1 of the hypersthene-augite andesites. It is a

less crystalline form of the gabbro just mentioned.

The prevailing rock exposed for the first 2 miles on the crest of the

ridge (by starting from Nawi) was a greenish porphyrite displaying large

opaque crystals of oligoclase, but showing much alteration of the

propylitic kind, its specific gravity being 2·5, but the structure of

the groundmass is much disguised. During the next mile, basic

agglomerates and a massive hypersthene-augite andesite were exposed. The

last-named is semi-vitreous, and on account of the prismatic pyroxene of

the groundmass is placed in genus 5 of its sub-class. Its specific

gravity is 2·7. In the last and fourth mile of the ridge was exposed a

dark grey doleritic basalt (sp. gr. 2·85), which rings like clinkstone

under the hammer, and weathers in a honeycombed fashion. The

felspar-lathes average ·3 mm. in length, the residual glass being

scanty. The pyroxene phenocrysts include some of rhombic pyroxene; and

the rock is therefore referred to genus 4 of the hypersthene-augite

andesites. The actual peak of Uthulanga, as it rises abruptly about 150

feet from the end of the ridge, is of agglomerate, the blocks being

composed of a compact grey andesite.

The peculiar succession of rocks displayed in this mountain-ridge along

its length of four miles is worthy of notice. Neither vesicular nor

scoriaceous rocks came under observation; and it is to be assumed that

there existed originally a line of submarine vents, some of them

ejecting acid and others basic materials. Mainly on account of the great

marine erosion during the period of emergence, but partly also on

account of the subsequent sub-aerial denudation, a plain ridge now

represents this line of vents. Probably the peak of Uthulanga, which is

evidently an old volcanic neck, represents the last of the stages in the

volcanic history of this ridge; but a very long period must have since

elapsed. When, however, we look at the exposed gabbro at the other end

of the ridge, we have to carry the period much farther back, since here

the superjacent surface volcanic rocks have been stripped off

completely. On page 2 I have referred to an island in the Solomon Group

where we have such a chain of ancient vents of acid and basic rocks. In

that case the forms of the separate hills indicating the original vents

are still to be recognised. In this old mountain-ridge of Vanua Levu no

such outlines remain except in the instance of the terminal peak.

(_b_) _The Nailotha and Vungalei Range._—This lofty range, which towers

above all around it, attains a height of 2,481 feet in the peak of

Nailotha and of 2,238 feet in that of Ndrukau or Vungalei. The first is,

as I infer, the Ndoendamu or Hale Peak of the charts; but it is

uncertain whether the name Savu-riti should be applied to the northern

peak of the range as it is in the charts, or whether it belongs to an

independent peak farther to the east. By the natives in the vicinity the

northern peak is known as Ndrukau, and I have added the name of Vungalei

from the village at its foot. The southern peak is that of Nailotha. The

range runs roughly north and south. It is, however, obvious that the

internal topography of this part of the island is but scantily known.

Brief reference will first be made to the country bordering the range on

the north-west and west sides in the vicinity of the villages of

Vungalei and Tembe. In proceeding south from Kalikoso, which lies in the

midst of the low-lying Wainikoro plains, to Vungale one traverses this

level district, of which the elevation is never more than 150 feet above

the sea and often much less. About a mile south-east of Kalikoso the

limit of the region of quartz-porphyries and of acid tuffs is passed and

the area of basic rocks is entered, a dark semi-vitreous

pyroxene-andesite with a flinty fracture prevailing at the surface as

far as Vungalei. This rock displays a few small phenocrysts of

oligoclase and pyroxenes in a blurred glassy groundmass exhibiting the

felspar and pyroxene microliths in process of development (sp. gr.

2·46).

Vungalei itself is only elevated about 130 feet above the sea. In

proceeding from this village to Tembe, lying about two miles S.S.W., one

crosses a line of hills, about 600 feet in height, which form a spur of

the main range. Basic agglomerates similar to those found on the slopes

of Mount Vungalei, as described below, prevail in the district between

these two villages up to the top of the intervening hills. In places one

notices that they overlie the ordinary sedimentary deposit, known as

“soapstone,” a slightly calcareous clay-tuff but showing no organic

remains to the eye. The rock exposed in the stream-courses is a

semi-ophitic basaltic andesite (sp. gr. 2·74) which contains a

considerable amount of interstitial glass.[95] When proceeding S.S.W.

from Tembe on the way to Nandongo one passes on either hand, as

described on page 216, hills about 700 feet high displaying vertical

cliffs formed of agglomerates over-lying finer sedimentary beds

apparently of the “soapstone” character.

I made the ascent of Mount Vungalei from the village of that name. The

peak is also known as Ndrukau. Basic agglomerates were exposed all the

way up to an elevation of about 2,000 feet, where a bed of a somewhat

scoriaceous basaltic andesite was displayed, the upper 200 feet being

inaccessible but of the same agglomerate. At a height of 300 feet was

observed another layer of the same basaltic rock. These intercalated

beds appeared to be lava-flows rather than horizontal dykes. The

agglomerates become less coarse in the upper part of the mountain where

they take the character of agglomerate-tuffs. The blocks are composed,

like those in many other parts of the island, of a dark semi-vitreous

basaltic andesite, but often scoriaceous. But the most remarkable

features of these agglomerates are the indications of two distinct

pauses in their deposition afforded by the occurrence at elevations of

900 and 1,700 feet of a single horizontal bed, two to three feet thick,

of coarse palagonite-tuffs. Each bed is exposed at the foot of a tall

cliff of agglomerate forming a line of escarpment along the

mountain-side. The larger fragments making up these tuffs are usually

from 2 to 3 millimetres in size; but the process of palagonitisation is

not complete; and we find inclosed abundant angular pieces of a dark

fresh tachylyte-glass, finely vesicular, and fusing readily in the

ordinary spirit-lamp flame. The tuffs contain little or no lime and

seemingly no organic remains. In the lowest bed I noticed a little

water-worn gravel. Both beds pass gradually into the underlying

agglomerate; but their upper limits are well defined and the agglomerate

commences abruptly.

It is thus seen that in the formation of the agglomerates there were two

pauses when they gradually gave place to deposits of fine detritus made

up of a vesicular basic glass that has since been largely converted into

palagonite. Then followed a sudden renewal of the agglomerate-producing

process. We can scarcely doubt that the agglomerates, with their

scoriaceous blocks, and the palagonite-tuffs are in the main the direct

products of volcanic eruptions. The rival claim of marine denudation as

the agency concerned can be mostly but not altogether excluded. The

agglomerates and tuffs of Mount Vungalei cannot be distinguished from

those so often described in the case of the great inland ranges of the

other parts of the island, the submarine origin of which is frequently

demonstrated by the inclosed organic remains. It would seem that in the

instance of Vungalei these deposits took place around the shores of a

volcanic mountain that rose above the sea. On page 315 it is pointed out

that in Stromboli with its dribbling eruptions we have a good

illustration of the manner in which such deposits could be formed.

My examination of the mountain of Nailotha was restricted to the lower

slopes up to an elevation of 600 feet; but the results obtained are

very suggestive. When following up the stream-course on the way from

Tembe to the mountain one notices in its bed blocks of a

light-coloured rock like a compact quartzite. It is, however, a highly

altered oligoclase-trachyte (sp. gr. 2·53) with its structure much

disguised by secondary quartz. This is the first intimation one gets,

on leaving behind the district of basic agglomerates about Tembe, of

the otherwise unexpected character of the acid rocks displayed in the

lower part of Nailotha.[96]

A torrent here cuts deeply into the mountain-side. At the base, between

200 and 300 feet above the sea, a bluish-grey rather scoriaceous rock

with the steam-pores drawn out to a length of from 1 to 2 centimetres,

is first exposed. Its specific gravity after allowing for the cavities

is less than 2·6. It shows a groundmass partly disguised by secondary

quartz and containing numerous small vesicles lined by quartz and filled

with viridite and epidote. Where the alteration is less advanced small

parallel felspar-lathes with fine decomposing pyroxenes are shown. The

lathes give extinctions of two or three degrees and average ·04 mm. in

length. In its somewhat scoriaceous nature, in the absence of

phenocrysts, and in its less altered condition, this rock differs from

those exposed higher up the ravine; but it is evidently to be referred

to the same acid type. At a height of 300 feet a light grey highly

altered oligoclase-trachyte (sp. gr. 2·43) is exhibited. It contained

originally some phenocrysts of felspar and pyroxene, which, however,

have been more or less replaced by calcite, quartz, and other materials;

whilst the groundmass, originally hemi-crystalline but now blurred by

the deposition of silica, shows felspar-lathes in process of

development.

A little farther up the gorge there is displayed another highly altered

rock with a siliceous appearance, such as has been noticed above as

forming blocks in the stream near the foot of the mountain. It is an

oligoclase-trachyte impregnated with crystalline silica and exhibiting a

singular prismatic structure, the small columns or prisms being only 3

or 4 inches in diameter. Between 400 and 500 feet occurs a

light-coloured compact rock sparkling with pyrites and also displaying a

columnar structure, the columns being between 4 and 6 inches across. It

looks like a limestone and effervesces freely with an acid; but it is in

fact a highly altered oligoclase-trachyte (sp. gr. 2·5) of the propylite

type. It seems originally to have inclosed a few phenocrysts of

oligoclase, sanidine, and pyroxene, and here and there a stout

felspar-lathe is to be noticed in the groundmass giving straight

extinction. The whole texture of the rock, however, is more or less

impregnated with secondary quartz, calcite, chlorite, viridite, pyrites,

&c. Farther up the ravine, between 500 and 600 feet in elevation, is

displayed a remarkable quartz-porphyry which exhibits opaque porphyritic

crystals of felspar as well as rounded crystals of quartz in a grey

compact base. It has been subjected to considerable alteration and will

be found described on page 310.

At 600 feet large slabs of the ordinary sedimentary clay-tuffs,

containing but little lime and showing no organic remains to the eye,

lay about on the more level slopes, and evidently formed a surface

deposit incrusting the altered massive rocks. If my ascent had lain up

the mountain-side away from the stream-courses, these sedimentary rocks

would alone have been observed. In the gorge, however, is exposed to

view the deeper-seated rocks that make up the mountain’s mass. We have

here a thickness of about 400 feet of altered acid rocks. All are

doubtless intrusive, and were subjected to alteration after the

development of the columnar structure and before the deposition of the

overlying clay-tuffs or “soapstones,” which are no doubt of submarine

origin. These sedimentary tuffs belong probably to the period when the

submarine agglomerates and palagonite-tuffs of the neighbouring peak of

Vungalei were formed.

(_c_) _Traverse of the coast range from Nandongo to Vanuavou on the

shore of Natewa Bay._—This route was taken by Mr. Horne, the botanist,

in 1878. I approached Nandongo from Tembe to the northward. The road at

first lay between hills about 700 feet in height displaying in their

precipitous faces agglomerates overlying fine sedimentary tuffs. These

deposits in the form of slightly calcareous basic tuff-clays, the

so-called “soapstones,” are exposed in the bed of the Wainikoro River as

one nears Nandongo. This village, which is situated on the headwaters of

the Wainikoro at an elevation of about 180 feet above the sea, lies near

the foot of the range. In its vicinity there is a small thermal spring

which is referred to on page 33.

Proceeding south from Nandongo one notices in the stream-course at the

foot of the slopes the sedimentary tuff deposits above mentioned,

bedding and dipping gently to the west. Farther up the slopes, higher

than 250 feet above the sea, there are exposed the deeper-seated rocks

of the range in the shape of compact reddish rocks (sp. gr. 2·48), which

appear under the microscope to be highly altered acid andesites or

oligoclase-trachytes originally displaying flow-structure and a fair

amount of glass, but now much disguised by the formation of secondary

quartz. On this north slope of the range I also found an amygdaloidal

variety of the same altered rocks containing irregular amygdules, 5 or 6

millimetres long, of fibrous quartz or chalcedony. Blocks of basaltic

andesite were observed on the summit, which has an elevation of 950 to

1,000 feet. On the southern slopes descending towards Natewa Bay coarse

basic tuffs together with blocks of basaltic andesite are chiefly

exposed. The last-named probably represent dykes both on the south slope

and on the summit. The rocks exhibited on the portion of the coast of

Natewa Bay corresponding to this range are dark and light-coloured

sedimentary tuffs usually calcareous, with occasional basaltic andesites

indicating dykes.... From this traverse it would appear that the range

has an axis of altered acid rocks overlain by basic sedimentary tuffs

and pierced by basaltic dykes.

(_d_) _The mountainous district lying between the head waters of the

Wainikoro River and the Mount Thurston Range._—Of this region I know

very little. The highest peak according to the chart has an elevation of

about 1,600 feet. Some indication of the character of the inland rocks

may be obtained from that of those exposed on the coast between

Nakarambo and Waimotu where, as observed on page 208, grey

pyroxene-andesites, coarse in texture and almost holo-crystalline in

structure, protrude through agglomerates of the same materials. When on

the way from Ngelemumu to Wainikoro I crossed the extreme northern

prolongation of this range where the elevation above the sea is only 700

feet. Non-calcareous basic tuff-clays occur on the slopes; but the

deeper-seated rocks, judging from an exposure on the east side, are dark

grey altered pyroxene-andesites penetrated by fine cracks filled with a

mosaic of quartz and having a specific gravity of 2·7. On the summit I

found a gritty sandstone-like rock, of which my specimen has been lost.

In a stream at the foot of the east slope occur small blocks of basaltic

andesite probably derived from a dyke. The region of acid rocks, such as

quartz-porphyries, oligoclase-trachytes, &c., is not entered until about

two miles south-west of Wainikoro.

THE COAST RANGES AND SEA-BORDER BETWEEN MBUTHAI-SAU

AND THAWARO OR MBEKANA BAY

We have in this region the mountains and hills at the coast and the

low-lying plains inland. This feature of the north side of Vanua Levu is

very remarkable. For some sixty miles, that is to say, for more than

half the length of the island, between the mouth of the river Ndreketi

and Thawaro or Mbekana Bay, Vanua Levu possesses this character. The

coast ranges west of Lambasa, where basic rocks evidently prevail, have

been referred to on pages 135, 136. Those east of the Lambasa mountains

as far as Thawaro Bay will be dealt with here; and instead of basic we

find acid rocks, such as quartz-porphyries akin to the rhyolites,

oligoclase-trachytes, pumice-tuffs, &c.

The sea-border is here characterised not by a continuous range running

parallel to the coast, as in the case of the district between Nanduri

and the Ndreketi River, but by a number of separate groups of hills and

lesser mountains, separated by deep gaps or valleys which are occupied

by tidal rivers and extensive mangrove swamps. The tide ascends these

rivers into the plains behind the coast ranges, so that a depression of

only 30 feet would convert these groups of hills into separate islands

and would cover much of the inland plains with the sea. The hills attain

an elevation of 1,200 or 1,300 feet a mile or two inland and descend

often as bold promontories to the coast. I will refer in order to the

different parts of this sea-border.

(1) THE SEA-BORDER BETWEEN LAMBASA AND MBUTHAI-SAU

In the sea-border between Lambasa and Mbuthai-sau we have the junction

of the regions of basic and acid rocks, the former extending westward to

Naivaka, the latter reaching to Undu Point. In such a locality the two

types of rocks might be expected to be associated, and this is what

occurs. Acid pumice-tuffs and basic agglomerates, sometimes associated,

are here displayed. In the low hills between Lambasa and Vuni-ika Bay,

which lies west of Mbuthai-sau, I found basic agglomerates prevailing,

together with some acid pumice-tuffs. The blocks in the agglomerates are

composed of a blackish semi-vitreous pyroxene-andesite (sp. gr. 2·68),

which is characterised by prismatic pyroxene in the groundmass, and is

referred to genus 6 of the second sub-order of the hypersthene-augite

andesites described on page 287.

East of Vuni-ika on the way to Mbuthai-sau, at an elevation of about 50

feet above the sea, dark tuffs containing small fragments of

reef-limestone are exposed in a cutting. A little farther on there is a

considerable deposit of a pale grey rhyolitic pumice-tuff, a soft stone

easily worked, and indeed now quarried by the plantation authorities. It

contains no lime and in microscopical characters is scarcely

distinguishable from a sample of fine pumice debris collected by me in

the Chirica district of Lipari Island. It is made of fragments up to a

centimetre in size, of ordinary fibrillar pumice in a matrix of much

finer material of the same nature. Portions of crystals of glassy

felspar (oligoclase and sanidine) also occur in it, together with some

quartz and rhombic pyroxene.

The association in this locality of acid and basic eruptive products was

observed by Dana in 1840 in the cliffs of “Mali Point.”[97] It is not

quite clear whether Mali Island, which lies immediately adjacent to the

coast, is here alluded to, or whether it is a headland opposite to it.

Dana describes the deposits displayed in these cliffs as coarse

aggregates of fragments of pumice and decomposing trachyte, which pass

on the one side into fine clayey material, and on the other into an

agglomerate formed of angular blocks of vesicular and compact basalt

with the interstices filled with pale yellow tufaceous material.

East of the Avuka Range that limits the Lambasa plains on this side is

the picturesque valley of Mbuthai-sau. This broad valley runs in a

south-east direction into the heart of the island. So small is the

gradient that the river flowing down it can be ascended in boats for

some miles; whilst Ngele-mumu, a village situated between 5 and 6 miles

up the valley, is not much over 50 feet above the sea. This valley in

its lower part roughly divides the regions of acid and basic rocks that

lie east and west of it respectively. It has, however, been above

pointed out that the two regions overlap in the coast region on the west

side of the valley. The two types of rocks are also associated in the

coast hills immediately east of the valley, since when striking inland

from Lloyd Point to the Mbuthai-sau sugar-cane district, one leaves

behind the acid rocks at the coast and traverses a region of basic

agglomerates. With these qualifications, therefore, the above line of

demarcation holds good.

(2) THE SEA-BORDER BETWEEN THE MBUTHAI-SAU AND THE LANGA-LANGA RIVERS

The rocks predominating in this district are white and pale-yellow

pumice-tuffs and pumice agglomerates, with quartz-porphyries and

oligoclase-trachytes as intrusive masses. The light colour of the

sea-cliffs, thus composed, makes them conspicuous from seaward. Their

appearance evidently led Mr. Horne into an error in 1878 when he viewed

this coast from his canoe during his sea-passage from Lambasa to Tutu.

“The coast from Lambasa”—he says—“is a series of bold projecting bluffs

of agglomerate interspersed with seams of coralline sandstone.”[98] The

principal feature of this coast, however, is the prevalence of

light-coloured pumice-tuffs, though it is not improbable that elevated

reef-formations may occur in some localities.

I particularly examined the coast districts in the vicinity of the

Wainikoro and Langa-Langa rivers. In a spur that descends to the right

bank of the river, a little above the mouth of the Wainikoro, is exposed

an open-textured rhyolite or quartz-porphyry containing, as described on

page 310, phenocrysts of glassy felspar (oligoclase and sanidine),

quartz, and a little hornblende. Some interesting exposures occur on the

coasts between Narikosa Point (Nari-Roso Point of the chart) and the

mouth of the Wainikoro. Here the pumice-tuffs and agglomerates are

pierced by dykes, some of them vertical and 6 feet across and composed

of a light-grey rhyolitic rock, similar except in its compact texture to

the rock above mentioned.[99] The small quartz crystals, which are 1 to

2 millimetres in size, are sometimes bipyramidal. Many of them are

rounded and have a fused-like surface. The pumice-tuff, which displays

no effervescence with an acid, is composed mainly of finely pulverised

vacuolar and fibrillar istropic colourless glass, and contains also

small fragments of glassy felspar and small rounded quartz crystals,

such as occur in the rock forming the dykes in these deposits. Imbedded

in the tuffs in places are small masses, up to 4 inches in size, of a

pretty grey vesicular rhyolite-glass exhibiting the intermediate

condition between compact obsidian and pumice which is so characteristic

of the rocks of Vulcano in the Lipari Islands. A more detailed account

of this rock is given on page 311. In the pumice-agglomerates of this

locality occur pale-yellow decayed and altered pumice blocks.

The headland, known as Narikosa Point, which lies between the mouths of

the Wainikoro and Langa-langa rivers, terminates in a rocky spur formed

by a large intrusive mass or dyke of a reddish oligoclase-trachyte

altered in character and displaying in places a rudely columnar

structure.[100] The pumice-tuffs and agglomerates are well exposed at

the coast between Narikosa Point and the Langa-langa River. In the

vicinity of Songombiau, a village here situated, the tuffs are

penetrated by dykes. One dyke at the back of the village is 4 or 5 feet

thick and has vitreous margins. It is composed of a darkish

pyroxene-andesite which in the interior of the intrusion in

hemi-crystalline and a little vesicular, but in the margins it is more

glassy and rather scoriaceous.[101] Though not a basic rock in itself,

its specific gravity in the compact state being only about 2·55, it is

relatively basic when contrasted with the rhyolitic and trachytic rocks

of the district, which have when compact a specific gravity of not over

2·3.

Pumice-tuffs and agglomerates appear in the cliff-faces of the hills on

either side of the lower course of the Langa-langa River from the

vicinity of Kalikoso to the sea. At one place about 3 miles up the river

a dyke of trachyte, much altered and showing columns 12 to 18 inches

across, was exposed at the bank.

(3) SEA-BORDER BETWEEN THE LANGA-LANGA RIVER AND THAWARO BAY

The next part of the coast which I visited was that opposite Tutu

Island. Here a range of high hills, 1,100 or 1,200 feet in height, sends

a lofty spur to the sea, in the precipitous faces of which are exposed

breccia-tuffs and agglomerates derived from acid rocks. A specimen of

the tuffs shows, besides fragments of altered rhyolitic or trachytic

rocks, portions of decomposing pumice, the vacuoles and tubular cavities

of which are filled with alteration products. The blocks in the

agglomerates are altered oligoclase-trachytes, both compact and

vesicular. These deposits are non-calcareous and rarely display bedding;

but in one place there was a rude arrangement of the materials, the dip

being to the north-west. I crossed this coast range where it is only 600

feet above the sea, and descended into the Kalikoso plains in the

vicinity of Numbu.

In the coast district between Tutu Island and the village of Naua, 3½

miles to the east, the same altered coarse pumiceous and trachytic

tuffs, occasionally bedded and dipping W.N.W., form the low hills near

the sea. In one place, where the elevation was less than 200 feet, I

noticed on the surface small fragments of branching corals. They had

perhaps weathered out of the tuffs, and though in part silicified still

effervesced freely in an acid.

Between Naua and the town of Visongo occur low hills formed of acid

tuffs. At an elevation of 200 feet the tuffs display a remarkable

character. When examined with the lens they are seen to be crowded with

the minute tests of foraminifera of the “Globigerina” type. The deposit

forms a fairly hard light-grey clay-rock which according to the usual

acid-test has no lime. Under the microscope it is seen to be composed

nearly in equal proportion of the tests of foraminifera and of very fine

detritus apparently derived from acid rocks, the materials being

generally not over ·01 mm. in diameter, though some of the felspar

fragments measure ·05 mm. across. A secondary process of silicification,

as exhibited in the occurrence of minutely granular quartz, has affected

the matrix of the clay as well as the tests of the foraminifera. The

deposit is of deep-water origin and appears to have been formed from the

fine washings of a coast some distance away.

It is noteworthy that the surface in this locality is in places strewn

with quantities of angular fragments and round pebbles of chalcedonic

flint, together with large nodules which when broken across are found to

be occupied by radiating crystals of quartz and were doubtless formed in

the cavities of some rock. Reference has already been made to the

partially silicified coral fragments lying on the surface of the low

coast hills a few miles to the west of this locality. They indicate a

relatively recent emergence and we get the same indication from the

flints on the surface of this district. These evidences, however, relate

only to the last stage of the emergence. The testimony of the silicified

“Globigerina” clay carries us back to the earlier periods of these

changes of level, and probably dates back to a time when the greater

part of this portion of the island was submerged, with the exception of

the mountain peaks. Not the least interesting feature of the emergence

is the silicifying process that accompanied it. This is illustrated on a

much greater scale in the neighbouring inland plains of Kalikoso and

Wainikoro which are described in Chapter XXV.

Proceeding eastward from Visongo to Namukalau one traverses a coast

district not elevated more than 300 feet above the sea. Here there are

displayed whitish and pale yellow compacted tuffs differing in aspect

from those prevailing to the westward and often bedded, the dip being

about 20° N.W. or N.N.W. They show no lime and apparently inclose no

organic remains. Where the upper surface of a bed is bared, it shows

regular shrinkage-lines inclosing hexagonal spaces 2 to 3 inches across;

but there is no corresponding columnar structure in the bed-mass. The

rock is very light in weight and homogeneous in texture and looks a

little like China-clay. In a section its structure appears obscure; but

it seems to be formed of the finest detritus, derived from some acid

partly devitrified glass, the pumiceous structure being in places

faintly indicated; but the whole mass appears to have been subjected to

a process of alteration perhaps similar to the ultimate palagonitic

change in basic rocks. In the slide a few small felspar fragments, about

·1 mm. in size, are displayed.

Just east of Namukalau is the mouth of the Vui-na-savu River, the Na

Savu River of the chart. This is a tidal river, and is navigable for

boats for several miles. In the lower part of its course agglomerates

and tuffs prevail, probably in part at least derived from acid rocks.

Near Rauriko, which lies about 5 miles up the river, a bare bluff,

overlooking the valley on the east, is formed of a decomposed trachytic

rock remarkable for the fact that it displays, as described on page 370,

faint magnetic polarity. Above Vitina, a mile or two farther up, I found

a similar rock but amygdaloidal in character. On the head-waters of the

river is situated the hamlet of Nawi, where, as mentioned on page 211, a

plutonic rock of the gabbro type occurs. Tuffs and agglomerates appear

to prevail in the low coast tract between the Vui-na-Savu river and

Thawaro Bay.

* * * * *

In drawing some general inferences respecting the acid formations,

mostly fragmental in character, that are displayed in the sea-border

between Lambasa and Thawaro Bay, it is necessary to distinguish between

the deposits west and east of the Langa-langa River. Between Lambasa and

the river just named the tuffs may usually be regarded as the products

of sub-aerial eruptions. Some of the specimens might have been obtained,

as far as their characters go, from the pumice-districts in the island

of Lipari. Their limeless condition and the apparent absence of organic

remains are noteworthy features, though of course the products of

sub-aerial eruptions may be deposited under the sea. It is, however,

remarkable that no compact obsidian came under my notice. The fragments

of rhyolitic glass, intermediate in structure between compact obsidian

and pumice, that were imbedded in the pumice-tuffs in one locality, were

probably ejected from some sub-aerial vent.

In the region between the Langa-langa River and Thawaro Bay acid tuffs

and agglomerates prevail; but they have all been subjected to alteration

by the deposition of secondary silica, and the pumice-structure when

present is largely disguised. They have evidently, in part at least,

been derived from compact rhyolitic and trachytic rocks, and are

probably in some measure the products of marine erosion. Although

neither lime nor organic remains were detected, the presence of the

altered “Globigerina” clay near Visongo is very suggestive and indicates

a considerable submergence of this region at some distant period.

Much, however, remains to be done in the examination of the peaks of the

coast ranges of this part of Vanua Levu; and it is likely that some

interesting results will be obtained from such an exploration.

CHAPTER XVI

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES

(_continued_)

THE NORTH-EAST PORTION OF THE ISLAND (_continued_)

THE WAINIKORO AND KALIKOSO PLAINS