These tuffs occur at all elevations from the sea-border, where they may

form the shore-cliffs, to the upper slopes and summits of the

mountain-ranges, where they are found at elevations between 2,000 and

2,500 feet above the sea. In the interior of the island they are

generally to be observed underlying the basic agglomerates. Wherever an

extensive exposure of the agglomerates exists in the mountainous

districts, these tuffs are as a rule to be found at the base of the

cliffs. The precipitous bluff of agglomerate, that so often gives a

character to the mountainous interior, and the line of cliff of the same

formation that runs along the slopes, represent the work of landslips,

as is shown by the huge masses of agglomerate lying on the ground below.

These “slips” are not uncommon, and are due to the undermining influence

of the springs that percolate through the tuffs and clays underlying the

agglomerates.

When the tuffs are well displayed they as a rule show stratification.

The bedding may be indicated either by distinct parting-lines or by

alternating bands of varying degrees of coarseness. That these deposits,

when occurring in mass in the upland regions, are often horizontal or

but slightly inclined, is evidenced by the Nganga-turuturu Cliffs, 1,200

feet above the sea, which are described in Chapter VIII., in the line of

cliffs behind Sealevu (Chapter XI.), and high up the slopes of Mount

Thambeyu (Chapter XII.) as high as 1,500 feet. This is also shown in the

circumstance that the line of junction with the overlying agglomerate,

except in rare cases, as in that of the Mbenutha Cliffs, is generally

horizontal. It is, however, not uncommon to find the beds exposed on the

mountain-flanks dipping away from the axis of the range at a small

angle, as on the slopes behind Mbale-mbale and in the Sokena Cliffs. In

the lower regions, where these deposits are associated with the volcanic

mud-rocks on the basaltic plains, they are but slightly inclined. On the

other hand, as in the Kumbulau district, the sea-cliffs for some miles

may be composed of tuffs more or less steeply tilted.

These tuffs are generally more or less compacted and have a greyish

colour; but as usually exposed in a weathered condition they are often

pale brown or yellowish and are more friable. They may be uniform in

structure, or they may display thin seams of a marl-like clay, or they

may contain numerous lapilli of vesicular basic glass extensively

palagonitised. Not uncommonly they contain larger fragments of basic

rocks, and when these are at all frequent the terms “agglomerate-tuff”

or “tuff-agglomerate” have been employed according to the preponderance

of either material.

Many of these tuffs show no effervescence with an acid; and this is

especially the case with specimens at all weathered. On the other hand

there are just as many that contain a little carbonate of lime, not

usually more than 3 or 4 per cent., but sometimes amounting to 12 or 13

per cent. It often happens in the case of a series of tuffs, apparently

non-calcareous, that an occasional thin band of a fine clay-like rock

contains a good percentage of lime. It is pointed out below, however,

that the absence of effervescence does not necessarily imply the absence

of foraminiferous tests.

Tests of foraminifera, often macroscopic bottom forms, together with

shells of small gasteropods, are displayed at times; but they are as a

rule in such cases not frequent. I found foraminiferous tuffs at

considerable heights in some localities, as for instance between 2,000

and 2,500 feet on the slopes of Mount Thambeyu (page 178), at an

elevation of 1,850 feet on the south slope of the Korotini Range above

Vatu-kawa (page 158), and between 2,000 and 2,400 feet on the summit of

the range between Waisali and Sealevu (page 154). In the last-named

locality, where the tuffs are coarse and often of the nature of

agglomerate-tuffs, they are highly fossiliferous; but such a character

is exceptional.

The submarine origin of the tuffs can often be demonstrated in the

absence of evidence of organic remains, as by their interstratification

with foraminiferous clay rocks, such as we find at an elevation of 1,000

to 1,100 feet on the top of the “divide” between the Ndreketi and

Lambasa basins. A single seam of marl-like rock displaying only a

solitary test of a foraminifer in the slide may throw light on the

origin of the coarser tuffs with which it is associated. The use of the

microscope is essential in the case of some of the harder tuffs, where

there has been a little alteration. Here casts of foraminifera may be

observed, although no carbonate of lime is indicated by an acid. In some

localities where no organic remains are evident in the tuff, fine

waterworn gravel is to be noticed.

These deposits are composed as a rule of sub-angular fragments of

semi-vitreous basic or basaltic rocks and of palagonite, together with

fragments of plagioclase and pyroxene, the interspaces being filled with

fine debris of the same materials. The relative proportion, however, of

the three principal constituents varies considerably, the palagonite,

for instance, being sometimes scanty and sometimes abundant. The size of

the larger fragments in a tuff of the most common kind is about a

millimetre; but deposits rather finer and rather coarser are also

frequent. In the very coarse tuffs and in the breccia-tuffs, where the

larger materials are mostly of palagonite, the larger fragments may be a

centimetre in size and even more, the interspaces being filled with fine

debris of the same character cemented together often by carbonate of

lime.

The fragments of semi-vitreous basic rocks forming a regular constituent

of these tuffs are usually dark and opaque and display a few plagioclase

lathes. They correspond with the type of the semi-vitreous basalt or

basaltic andesite, of which the blocks of the overlying agglomerates are

as a rule composed and are doubtless derived from the same source.

Fragments of unaltered basic glass are rarely to be observed in these

tuffs. It is as a rule all converted into palagonite. This material

presents itself in various stages of secondary alteration, from the

compact greenish or yellowish waxy mass to a white friable pulverulent

substance, which represents the last stage of degradation. These changes

will be found described on page 348. It can, however, be stated here

that they are mainly concerned with hydration. In the case of the

lapilli of finely vesicular basic glass, that is, of basic pumice, which

are inclosed in some of the tuffs, all stages of the secondary

alteration of palagonite are often exhibited, and the last stage of the

change is merely indicated by a white powdery patch containing a few

minute siliceous amygdules. The puzzling little white patches so common

in basic tuffs merely represent lapilli of basic pumice that have been

palagonitised, and then bleached and disintegrated by hydration.

The minerals are more or less abundant and may constitute a third of the

whole mass. They include plagioclase, augite, rhombic pyroxene, and

magnetite, olivine being rare and scanty. Entire crystals of any size

are infrequent. However, crystals of augite, 5 or 6 mm. in length, are

found in the tuffs at Naivaka and of the coast cliffs near the Salt Lake

Passage.... It may be observed that zeolitic minerals which are

frequently developed in the tuffs consisting almost entirely of

palagonite are not typical of the tuffs of mixed composition.

There is nothing suggestive of recent eruptions in any of these

formations. They were formed ages since on the sea-floor at varying

depths around volcanic vents. Sometimes a cone was able to rear itself

above the level of the sea; but in most cases it rapidly succumbed to

breaker-action. Three agencies, concerned with sub-aerial eruptions,

submarine eruptions, and marine denudation, have co-operated in the

production of these deposits, but their parts in the process have varied

greatly. The last is indicated when the tuff is formed of a variety of

basic rocks with but little palagonite. The tuffs containing much

palagonite representing an original vacuolar basic glass are regarded as

mainly the products of submarine-eruptions. In those cases where lapilli

of altered basic pumice occur in the deposit, sub-aerial eruptions are

directly indicated. When an extensive exposure of these tuffs occurs, as

in the case of the Nganga-turuturu Cliffs and in that of the section

displayed near the hill of Korolevu (page 48), all three agencies are

often illustrated.

SAMPLES OF THE SUBMARINE BASIC TUFFS OF MIXED COMPOSITION

A. As examples of the non-fossiliferous tuffs where the palagonite

constituents do not predominate, I will take those exposed in beds in

the coast cliffs and in the low hills in the vicinity of Na Tokalau in

the Kumbulau peninsula (page 90). They are grey in colour and have the

texture of a sandstone, being more or less compacted and showing no

effervescence with an acid. They are composed of fragments of basic

rocks and of minerals, varying in size from ·5 to 1 mm., in a scanty

matrix made up of fine detritus of the same materials and of palagonitic

debris. The fragments of volcanic rocks are rounded and sub-angular, and

formed mainly of a basaltic rock, with a black opaque groundmass showing

some small plagioclase crystals and in places more or less

palagonitised. There are also portions of a hemi-crystalline basic rock

showing small augite crystals. The minerals entire and in fragments,

which make up quite a third of the mass, are mostly of plagioclase, but

monoclinic and rhombic pyroxene are also well represented. It is evident

that through the alteration of the palagonitic constituents, which were

probably more frequent when the tuffs were deposited, the structure of

the matrix is somewhat disguised.

B. A yellowish grey tuff composing the cliffs on the north coast of

Naivaka affords a good example of a tuff where the palagonitic materials

predominate. It is somewhat fine-textured and displays a tendency to

lamination. The powdered material effervesces slightly with an acid. In

the slide it is exhibited as composed mainly of palagonite and of

fragments of minerals, the latter making up about one-third of the whole

and ranging in size usually between ·2 and ·5 mm. A number of more or

less parallel fine cracks, filled with calcite and traversing also the

inclosed crystals of plagioclase, together with small fragments of basic

rocks are displayed in the section. There are a few fragments of

semi-vitreous basic rocks, as just indicated, but the palagonite is the

principal constituent. It shows numerous minute amygdules occupying the

original vacuoles of the basic glass; and in its substance occur

irregular patches formed of a colourless semi-isotropic mineral which is

either zeolitic or a form of opal. Plagioclase and augite compose the

mineral fragments, the former prevailing. Although these tuffs are

derived from a vent that was probably the last in eruption in this

island, they display considerable alteration which is mainly connected

with the secondary changes affecting the palagonite since the deposition

of these materials.

C. As an example of the banded tuffs composed of coarse and fine

materials I will take a compact grey rock forming one of the horizontal

beds in the natural section exposed near the hill of Korolevu (p. 49).

It is also an example of those tuffs which whilst not effervescing with

an acid display a few casts of foraminifera in the slide. The

alternating bands which are about a centimetre in thickness pass

gradually into each other. The bands of finer materials are made up of

sub-angular fragments, ·1 to ·2 mm. in size, of the dark opaque

groundmass of a semi-vitreous basic rock and of a grey hemi-crystalline

groundmass of an augite-andesite, together with palagonite more or less

decomposed, and fragments of plagioclase and augite, whilst the

interspaces are filled with the finer debris. The layers of coarse

materials have much the same composition, the fragments varying usually

between ·5 and 1 mm. in diameter, with occasional larger pieces of

palagonite 2 to 5 mm. in size representing original lapilli of a

vacuolar basic glass. The tests of the foraminifera, which occur

scantily in the layers of fine material, are all minute. They are filled

with palagonitic material.

D. The tuffs prevailing on the higher flanks of the mountainous backbone

of the island are well represented by those exposed at an elevation of

1,200 feet above the sea on the south slope of the Korotini Range behind

Mbale-mbale. It is a somewhat coarse-grained rather hard grey rock

effervescing feebly with an acid. It is composed of sub-angular or

partly rounded fragments, 1 to 4 mm. in size, of various basic rocks,

and of rather smaller fragments of plagioclase and augite, the

interstices being filled up with fine debris of the same materials, in

which a few minute tests of foraminifera of the “Globigerina” type may

be observed. The basic rocks of which the fragments are formed comprise

the following: (_a_) a grey aphanitic augite-andesite, with but little

interstitial glass, presenting a parallel arrangement of the minute

felspar-lathes which have an average length of ·05 mm.; (_b_) a grey

augite-andesite of coarser texture but in other respects similar, the

felspar-lathes being about ·1 mm. in length, whilst there is a little

microporphyritic plagioclase; (_c_) a semi-vitreous basaltic rock

showing small porphyritic crystals of plagioclase and augite in a

groundmass usually black and opaque, but sometimes smoky and displaying

felspar microliths; (_d_) a vacuolar basic glass more or less

palagonitised.

E. As a specimen of the calcareous tuffs those exposed on the south

slope of the Korotini Range at an elevation of 1,850 feet may be given.

They contain about 11 per cent. of carbonate of lime and inclose a few

tests of foraminifera 1 to 2 mm. in diameter. The other constituents are

fragments of semi-vitreous basic rocks and of palagonite, together with

fragments of plagioclase and pyroxene crystals and of an amorphous

siliceous mineral which behaves optically like chalcedonic silica. When

the rock is gently rubbed down, minute fragments of this white mineral

can be picked out. They have a wrinkled surface and an irregular form

and are not affected by acids. In polarised light they display a rude

mosaic or an imperfect radiate structure.

F. As specimens of the fossiliferous agglomerate-tuffs composed mainly

of palagonite, those exposed on the high mountain slopes of the Korotini

Range at heights of 2,000 feet may be here cited. They are described on

p. 154.

_Note._—The examples of mixed tuffs above given represent only some of

the principal types of these deposits. Short descriptions of others will

be found in the detailed account of the geology of the island.

ALTERED BASIC TUFFS OF MIXED COMPOSITION

These form a group of hard compact rocks, the fragmental character of

which is not always apparent in hand-specimens, microscopical

examination of thin sections being usually required for the

determination of their true nature. They are commonly exposed on the

southern flanks of the Korotini Range at the back of Vatu-kawa and

Nukumbolo. They are composed of compacted fragments, varying in

different localities from 1 to 5 or 6 mm. in size, of a variety of

semi-vitreous basic rocks, the matrix being scanty but often containing

zeolites and secondary silica, whilst occasionally secondary calcite is

developed. They contain no organic remains, and palagonite when present

is usually scanty, whilst viridite and similar materials represent the

decomposition of the pyroxene.... A hard breccia-tuff found on the

flanks of Mariko and in one or two other localities contains vesicular

fragments, where the steam-holes are filled with opal or chalcedony, and

the cracks traversing the matrix are also filled with this mineral.

The alteration of these tuffs has evidently arisen from a variety of

causes. In some cases the change appears to be purely interstitial. In

other times it has arisen from contact-metamorphism, or from

hydro-thermal agencies, as in the case of the altered tuffs near the hot

springs at Nukumbolo (p. 161).

SUBMARINE BASIC PUMICE TUFFS

These deposits, which, however, are not of frequent occurrence, are

interstratified with volcanic mud-rocks in certain localities, as at the

Mbenutha Cliffs (p. 110), and in the vicinity of the hill of Korolevu

(p. 47). They indicate periods of volcanic activity during the

deposition of the foraminiferous muds, with which they are associated,

when the fine materials ejected from sub-aerial vents fell into the seas

around.

Such tuffs are more or less compact and usually fine in texture. When

the glass fragments are but slightly altered, the tuff-rock is dark

grey; but when the palagonitic change is well advanced, it becomes pale

and yellowish. They are made up chiefly of small fragments of a

bottle-green basic glass, which are as a rule vacuolar and sometimes

fibrillar; but it never happens that the pumiceous character is as

pronounced as in acid pumice; and in some cases the vacuoles or

steam-pores are to be observed only in the minority of the fragments

displayed in a slide. The size of the glass fragments is as a rule

small, in some tuffs averaging only ·1 mm. and in others ·5 mm.; but

occasionally they may be 1 or 2 mm. in diameter.

Fragments of minerals (plagioclase and pyroxene) corresponding in size

to the glass fragments are as a rule well represented, forming a fourth

or a third of the mass. A little fine detritus of a semi-vitreous basic

rock also occurs. Tested in an acid several of the tuffs either do not

effervesce or give an indication of a small percentage of carbonate of

lime; whilst others effervesce freely. They usually display a few minute

tests of foraminifera of the “Globigerina” type, the cavities of which

are filled with fine palagonitic debris.

The palagonitic alteration is to be noticed in all cases; but it varies

considerably in its extent. In the dark grey tuffs it affects the

margins only of a few of the glass-fragments. In the paler tuffs it has

extended more into their substance, although the alteration is never

more than partial. The pale greyish material filling up the interspaces

is composed of disintegrated palagonite. The steam-pores or vacuoles are

sometimes empty; and at other times, especially where the palagonitic

change has begun, they are filled with a granular alteration product.

The glass fuses readily; but is not affected by acids. It is clear and

isotropic, showing however a few scattered microliths. These tuffs

correspond with the hyalomelan-tuff from the island of Munia in this

group as described by Wichmann; but in that instance no mention is made

of inclosed tests of foraminifera.

“CRUSH-TUFFS” FORMED OF BASIC GLASS

This is a remarkable group of compacted tuff-like rocks which as

hand-specimens would be generally regarded as pitchstone-tuffs. Their

detrital origin is, however, often very doubtful. They are composed of

fragments of basic glass, carrying plagioclase phenocrysts, with the

interspaces occupied by palagonite and by the finer debris of the glass

and felspar. The larger glass fragments, which vary in different rocks

from 1 or 2 to 4 or 5 mm. in size, have been crushed _in situ_, the

broken portions often remaining more or less in position. These

fragments are invested by palagonite and have eroded borders, as shown

in the figure on page 342. The glass is bottle-green, non-vacuolar,

fuses readily, and only at times displays incipient crystallisation. The

explanation of the origin of these rocks is attempted in Chapter XXIV.

They contain neither carbonate of lime nor organic remains. The most

typical example is present in a bed underlying a pitchstone-agglomerate

near Narengali (see page 149). It is not uncommon to find evidence of

crushing in the glassy matrix of a pitchstone-agglomerate or of rubbly

pitchstone, as in the Va Lili Ridge (142), the Korotini Bluff (157), and

Mount Soloa Levu (313); and here also palagonite has been produced

around the crushed fragments.

COARSE ZEOLITIC PALAGONITE-TUFFS

These deposits represent coarse kinds of the submarine tuffs of basic

glass, in which the palagonitic change is far advanced, and where

zeolites and at times secondary calcite have been produced in abundance

as a result of the alteration. They present themselves in the mass as

mottled grey rocks which when examined in thin sections are seen to be

composed in great part of fragments of more or less palagonitised

vacuolar basic glass, whilst zeolites are extensively developed in

numerous irregular cavities and in the interspaces. Although displaying

no organic remains, their submarine character is indicated as at Nandua

by the circumstance of their occurring as horizontal beds overlaid by

pteropod-ooze deposits, or as at Tembe-ni-ndio by their forming part of

a series of horizontal beds with a shelly limestone and a foraminiferous

palagonite clay overlying them.

The fragments of bottle green basic glass vary usually between 1 and 4

millimetres. They were originally vacuolar and at times fibrillar from

the lengthening out of the minute steam-pores; but through the

palagonitic change these characters have been often disguised, and it is

only at times that the unaltered glass is observed. Plagioclase and

sometimes augite and occasionally olivine formed phenocrysts in the

original glass. The zeolites, which include chabazite and natrolite, may

be so extensively developed that they make up a fourth or a fifth of the

rock mass. One may observe them in cavities where the walls are lined by

fibrous natrolite with the cube-like crystals of chabazite occupying the

interior. The calcite is usually subordinate to the zeolites, but

sometimes the tuff contains as much as 10 per cent. of this mineral,

which is evidently of secondary origin.... The history of these tuffs in

the district of Nandua and Ulu-i-ndali is no doubt applicable to these

deposits in other localities. They are the products of submarine

eruptions which shattered into fragments the extensive palagonite crusts

of flows of basaltic lava. In Chapter XXIV. I have attempted to show how

palagonite is formed on a large scale in the case of such submarine

displays of volcanic activity.

CHOCOLATE-COLOURED FORAMINIFEROUS PALAGONITE-MARLS

We have here hard, somewhat calcareous, clay-rocks which consist in

great part (nine-tenths) of fine palagonite debris with some fragments

of minerals and a little fine detritus of semi-vitreous basic rocks.

Some hand-specimens would be taken for pure palagonite; but the

fragmental nature appears at once in the slide. This is especially the

case with a rock exposed in a stream-course near Rewa on the shores of

Savu-savu Bay (see page 95). The materials composing them are

exceedingly fine, the largest fragments not usually exceeding ·2 mm. As

a rule they contain a little carbonate of lime and sometimes as much as

10 per cent., whilst a few tests of minute foraminifera are to be

noticed in the slide. These deposits are horizontally bedded, and

underlie a pteropod-ooze rock at Nandua and a shelly impure limestone at

Tembe-ni-ndio. They are not very frequent, and sometimes approach in

characters the volcanic-mud rocks, which, however, are much more mixed

in composition. I regard them in the main as sedimentary deposits

derived from the disintegration of the palagonitised vitreous surface of

a submarine basaltic flow. They pass downward at Nandua, as described on

page 345, into a rock of pure palagonite; and they are only to be found

in localities where basaltic plains or plateaux are covered over with

submarine deposits.

ACID PUMICE TUFFS

The general characters of these deposits are described on pages 10, 218,

220, 222, 223, 231, 233, &c. Such tuffs are restricted to the north-east

part of the island east of Lambasa and Tawaki, and are well displayed in

the coast cliffs. They are pale yellow or whitish, and are usually

non-calcareous. They are composed of the debris of a vacuolar and

fibrillar isotropic glass, nearly colourless and in some localities

altered. Small crystals of quartz and of glassy felspar with bits of

obsidian (up to 3 mm.) and lapilli of rhyolitic glass are inclosed in

them. In places inclosed pieces of coral and coral rock indicate

submarine deposition.

CHAPTER XXIV

PALAGONITE

FROM the sea-border to the mountain-top in almost every part of the

island, palagonite occurs in a fragmental condition. It is only where

tuffs are not found, as in the mountainous mass of Seatura, or where

these deposits are formed of acid rocks as in the north-east portion of

the island, that palagonite has not been observed. Perhaps, it is not

too much to say that the later if not all the stages in the history of

Vanua Levu are bound up with the history of this material. In this place

I will only deal with certain features in the problem connected with the

origin of palagonite which seem to receive further elucidation from my

observations in this island. The literature is already extensive, and

those interested in the matter will find in Zirkel’s _Petrographie_ and

in the _Challenger_ Report on Deep-Sea Deposits by Murray and Renard a

good introduction to the subject.

In Vanua Levu we are confronted with the same difficulty that has

perplexed geologists in various parts of the world. If we expected to

find in this island the source of the enormous quantities of the basic

glass that are represented by the palagonite of the tuffs, we should

look in vain. Basic or basaltic glass usually occurs in agglomerates in

the form of tachylytic pitchstones, as described on page 312, and is

also found at times in basic pumiceous tuffs, as described on page 333;

but it is far from frequent. Palagonite-rock, that is to say, a basaltic

glass converted in mass into this substance, never came under my notice.

In order to clear the ground for the discussion of my own observations,

I will quote from the report on deep-sea deposits above named. Fragments

of basic glass undergoing the palagonite change are found everywhere in

these deposits and especially in the red-clay areas. The hydro-chemical

modifications determining the decomposition of these fragments into

palagonite, and at the same time the formation of zeolites, have

likewise resulted in the complete transformation of these lapilli into

ferruginous argillaceous matter (p. 309). The authors, however, of this

report do not attribute the frequent occurrence of fragments of basic

glass on the bottom of the ocean to the buoyant powers of basic pumice.

Unfortunately, the problem does not permit of such a simple solution.

Basic volcanic glass, writes Prof. Renard, though known only from a few

geological formations and from a few eruptions of recent volcanoes at

the surface of the continents, appears in abundance and in most typical

form among the products of submarine eruptions, as if the deep oceans

had been in some way specially favourable to the development of this

lithological type (p. 299).

The palagonite-tuffs of this island are described in detail in Chapter

XXIII., and a few general remarks are alone needed here. This altered

glass enters into the composition, to a greater or less extent and in

varying stages of disintegration, of nearly all the submarine basic

tuffs and clays. In the volcanic muds, however, and in the tuffs of

mixed character, which are the prevailing deposits, it is associated

with other components. Here the question of the origin of palagonite

within the deposit does not as a rule arise, since there is nothing to

indicate that this material was not derived from rocks previously

palagonitised, and the point of main interest is connected with the last

stages in the degradation of this substance. There are not a few cases,

however, where, unless we assume that the lapilli of vesicular basic

glass were ejected in the palagonitic condition from a volcanic vent, we

must apparently regard the alteration as having occurred in the tuff.

But even this will prove to be by no means a necessary consequence if it

can be shown, as I have attempted to do below, that the palagonitic

condition exists potentially in a particular type of basic glass and

that the effect of hydration is not so much to produce but to make

evident a condition that was previously latent.

It will be therefore of interest to determine whether palagonite occurs

in this island independently of the tuff-deposits, and under such

circumstances that it may be regarded as having been produced within the

rock-mass. An example is afforded in the case of a basaltic flow near

Soni-soni Island, which is fully described on page 92. Whilst the lower

part of this flow is composed of a hemicrystalline basalt with scanty

olivine, the upper portion is made of a basaltic glass which has been

broken up or crushed “in situ,” the spaces between the fragments being

filled with palagonite. It would seem from the peculiar erosion of the

glass fragments that after the crushing a liquid magma occupied the

interspaces, and afterwards solidified and underwent the palagonitic

change.

[Illustration: MAGMA-LAKELET, ·25 mm. in size, magnified 290 diameters,

from a basalt at Navingiri. The groundmass, which is a smoky devitrified

glass containing abundant felspar-lathes, is coloured black. The

magma-lakelet is pale yellow in the slide and displays concentric lines

of congelation. It behaves like palagonite.]

In this connection it is noteworthy that in the sections of the lower

hemicrystalline portion of the flow there are shown in the groundmass

collections of a palagonitic material forming, as I have termed them,

“magma lakelets” of microscopic dimensions (·25 mm. in average size).

These “lakelets” are irregular in form, and are not uncommon amongst a

certain type of basaltic rocks. One of them is figured above; and it may

be added that they are best examined when displayed in a groundmass

containing much smoky, partly devitrified, glass. They are usually more

or less opaque and reddish-brown or yellowish in colour, whilst they

have often a marked zoned structure, the concentric bands conforming to

the irregular contours of the lakelet. In the least affected stage the

zones show fibrous devitrification across their breadth, but as the

palagonitic change progresses the material becomes opaque. In the

secondary changes, such as those associated with the early alteration of

the propylites, these “magma lakelets” are the first affected. They then

present alternating layers of calcite and viridite and are often

bordered by magnetite.

If these “lakelets” were to be described as collections of residual

glass, the description would be insufficient, since they may occur in

the midst of a smoky, partially devitrified, glass. During the last

stage in the consolidation of the basaltic mass, the magma-residuum that

still retains its fluidity collects here and there in the crevices of

the groundmass, and forms little pools of usually microscopic dimensions

into which the felspar-lathes often protrude from the sides. These

little pools or lakelets represent that portion of the yet fluid magma

that during the last stage of consolidation is imprisoned in the

stiffening mass—like the whey in a cheese—whilst the greater part of it

has been squeezed into the cracks of the cooling mass, as occurs in a

dyke-like intrusion below described, or has been extruded on its

surface, as in the case of the basaltic flow above referred to.

As a suggestive instance of the formation of palagonite “in situ,” I

will now refer to a basic tuff-agglomerate on the plateau of Na Savu

(see p. 81) which is penetrated by veins, a few inches thick, apparently

composed of a finely brecciated pitchstone-tuff. In the section the

material forming the veins is seen to be composed of fragments of basic

glass (carrying porphyritic plagioclase and augite) which have been

crushed in position, the interspaces being filled up with the finer

debris of the glass and of the minerals together with palagonitic

material. The glass fragments, which have lost their sharp edges and

angles, are often palagonitised at the borders, and we thus get a patch

of isotropic brown glass with a yellowish margin formed of a feebly

refractive turbid substance. Where this border is not so evident, it is

noticed that the edge of the glass is peculiarly eroded. The indication

appears to be that the fissures in this agglomerate were filled with a

basic magma that after its solidification into a glass was subjected to

a crushing process, and that during this process a partial remelting of

the glass took place which resulted in the molecular change

characteristic of palagonite. Since the unaltered glass-fragments fuse

in the ordinary flame, it would seem that the heat developed during the

crushing might be sufficient to partially remelt the glass without

affecting the rock penetrated by the veins.... It is of importance to

note that in the palagonite-tuffs of the Canary Islands the change is

often most complete along fissures, which thus present the appearance of

being occupied by veins of pitchstone.[127]

In this connection allusion may be made to a dyke-like mass of a rubbly

semi-vitreous basaltic rock exposed at Vatu-lele Bay, described on page

184. It is penetrated in all directions by veins, 1 to 3 inches thick,

of a tachylytic glass which begins to fuse in the ordinary flame. The

glass is traversed by cracks which sometimes contain palagonite. The

basalt, penetrated by the veins, has a smoky groundmass displaying

imperfect felspar-lathes in a feebly refractive glassy base and

containing a few small “magma-lakelets” that cannot be distinguished

from palagonite.[128]

Near the mouth of the Narengali valley (see page 149) I found what

appears to be a palagonite-tuff overlain by agglomerates formed of

tachylytic pitchstone and of semi-vitreous amygdaloidal basalts. The

tuff consists of fragments of a brown basic glass, the larger 1 to 2

millimetres in size, carrying porphyritic plagioclase, and fractured in

position, the interspaces being filled with palagonite. The glass

fragments possess the eroded margins indicated in the accompanying

figure. It may be remarked that this type of tuff differs from that of

the prevailing palagonite-tuffs in being rarely vacuolar, in the absence

of marine organic remains, and in its homogeneous composition. It is

described on page 334 under the head of “crush-tuffs.” Whether it is

derived from the destruction of a mass of basic glass that had

previously undergone crushing and partial palagonisation I cannot say;

but its characters point in the direction of this conclusion.[129]

In the foregoing pages it has been attempted to show that

palagonitisation has taken place in the veins of basaltic glass

traversing in one case a basic tuff agglomerate and in another case an

intrusive basaltic mass, and that it has also occurred in the upper

vitreous portion of a basaltic flow and in the materials now composing a

so-called “crush-tuff.” In order to explain this group of facts I

venture to propose this theory.

In certain types of basaltic lava,[130] when cooling and consolidation

take place under peculiar conditions, such as we would expect to find in

submarine eruptions, there is a residuum of the magma with relatively

low fusibility that remains fluid after general solidification of the

mass is well advanced. As the rock continues to consolidate, portions of

this magma residuum become imprisoned in the mass, like whey in a

cheese, giving rise to the “magma lakelets” above described; whilst

other portions, during the contraction and fissuring accompanying the

cooling process, are squeezed out into the cracks thus formed, or are

intruded on the surface of the consolidating mass, as in the case of a

submarine lava-flow. This solidified magma-residuum differs from the

ordinary basic glass not only in its lower degree of fusibility but in

its mineral composition and in its molecular condition. It probably in

the first place does not differ much in appearance from the typical

glass, but it is an unstable substance and is capable under certain

hydro-chemical conditions of developing the characters of palagonite.

[Illustration: Showing fragments of glass with eroded borders and of

plagioclase with more even edges in a matrix of palagonite traversed by

cracks. The length of the largest fragment is half a millimetre. The

glass has been evidently fractured in position and this is true of one

of the felspar fragments. It is also apparent that whatever its cause

the erosion of the margins of the glass has been produced since the

fracture.]

In those cases where the occurrence of palagonite is associated with

evidence of crushing, the process appears to be in a sense reversed,

since partial palagonitisation of an ordinary basic glass takes place as

a result of the elevation of temperature due to the crushing. The heat

thus developed is sufficient to partly fuse the glass; but since it is

not great, it only affects the most fusible constituents, and the

remelted material corresponds therefore to the magma-residuum of the

consolidating mass, which is referred to in the previous paragraph. It

has the same unstable characters and the same tendency to assume the

palagonitic condition.

This theory centres around the relatively low fusibility of the

magma-residuum that gives rise to palagonite. This degree of fusibility

has yet to be ascertained, since according to the views here advanced it

may even be much lower than that of tachylyte. It is, however,

noteworthy that the melting-point of tachylyte is far below that of the

more crystalline basaltic rocks, since it can be readily determined, as

I have done in the instance of a dyke-like mass penetrated by

tachylyte-veins before referred to, that the veins are composed of a

much more fusible material than the rock-mass. From a very crude

experiment I would infer that the melting-point of ordinary tachylyte is

not much above that of lead (335° C). The fusion-point of an ordinary

hemicrystalline basalt, according to the well-known experiments on the

lavas of Vesuvius and Etna, would probably be over 1,000 degrees C.

Two interesting experiments, the one artificial, the other natural, may

be here cited in connection with this view. Bunsen[131] more than half a

century ago, as a result of some experiments in which he produced

palagonite, arrived at the conclusion that the tuffs formed of this

material are submarine deposits derived from the breaking up of

previously formed palagonite-masses. Having obtained this substance by

placing powdered basalt in an excess of melted potash-hydrate

(Kalihydrat) and then adding water to the silicate of potash thus

formed, he concluded that palagonite results from the reaction between

glowing augitic-lavas and rocks rich in lime and other alkalies.

Although Zirkel quotes in this connection the example where this

material has been produced in the Cape de Verde Islands by basic lava

flowing over limestone, he rejects Bunsen’s explanation as inapplicable

to extensive palagonite districts, such as occur in Iceland, though

allowing that it would account for the local production of this

substance.

I venture to think, however, that in these two experiments the general

principles involved in the production of palagonite are partly

illustrated. We may accept the results of an experiment without

acquiescing in its interpretation. As I take it, it is in the partial

wet fusion of the powdered basalt that the secret of this successful

production of palagonite lies. In both these experiments some of the

conditions of a submarine flow have been reproduced.

Whilst Rosenbusch established the true character of palagonite as the

product of a peculiar alteration of a basic glass, Renard pointed out

the conditions under which it was most typically and in greatest

abundance formed. But Bunsen was happy in his suggestion that

palagonite-tuffs are submarine deposits derived from the breaking up of

previously formed palagonite masses. The question thus resolves itself

into one concerning the conditions of submarine eruptions and the

behaviour during consolidation of a submarine basaltic flow. In the

nature of things the field of investigation is mainly restricted to the

examination of ancient submarine basaltic flows that have been raised

above the sea.

A remarkable series of beds exposed in a stream-course below the Nandua

tea-estate may be here described in connection with the question of the

origin of palagonite formations. As observed on page 86, this locality

lies on the flanks of a basaltic plateau, which are incrusted with

recent submarine deposits. A pteropod-ooze, containing also the tests of

large and small foraminifera and the shells of small bivalves, is

displayed on the sides of the stream-course for the first 150 feet of

the descent. Below this, as shown in the diagram, is a declivity with a

drop of 60 or 70 feet where there is a waterfall. Horizontal beds of the

pteropod-ooze rock are exposed in the upper-third of this declivity; but

below, they pass into a chocolate-coloured marl-like deposit also

horizontally bedded, and sometimes having a banded appearance from the

alternation of layers of different degrees of fineness. This rock

contains 5 or 6 per cent. of carbonate of lime and incloses a few

scattered tests of minute foraminifera of the “Globigerina” and

“Nodosaria” types. In the slide the rock appears to be of massive

palagonite inclosing a few felspar-lathes ·1 to ·3 mm. in length, and

exhibiting a zeolite and calcite in the crevices and cracks. But it was

not until I had discovered the tests of the foraminifera and had

observed some fragments of larger crystals of plagioclase and a little

detritus of a semi-vitreous basaltic rock that its clastic character was

disclosed. The palagonite change has here to a great extent disguised

the character of the deposit.

This palagonite-marl formation is 20 or 30 feet in thickness. It passed

downward into a reddish-brown rubbly unstratified rock which falls to

pieces in one’s hands, breaking up into little cube-like masses an inch

or two across. These masses display in their interior a radiate

prismatic structure; but after drying they crumble into small fragments

exhibiting the same minute prismatic structure, the miniature prisms

being about a millimetre in diameter. Fine cracks, filled with calcite

and a zeolite, traverse this rock in all directions, and no doubt this

peculiar structure arises from shrinkage.

My idea that I was dealing with a clay-rock affected by the proximity of

an igneous intrusion was dispelled when the powdered material presented

itself as pure palagonite with scarcely any mineral fragments. Unlike

the marl above, it does not effervesce with an acid; and appears as a

mass of compacted minute fragments of basic glass converted into

palagonite, which is seemingly non-vacuolar, and containing about 15 per

cent. of water.

In connection with the diagram it should be remarked that I did not find

the palagonite-rock actually passing down into the basalt which,

however, is exposed in the river-bed below. The whole district is

characterised by columnar basalt, and the series of deposits here

described have been formed on the flank of the great basaltic table-land

of Wainunu. It is noteworthy that in the uppermost deposits of the

pteropod-ooze palagonite forms a noticeable proportion (10-20 per cent.)

of the residue; and perhaps most of the fine clayey material is thus

derived. As noted on page 321, minute pellets of pure palagonite are not

infrequent in the residue. Probably about 90 per cent. of the underlying

marl consists of palagonite. In the lowest palagonite-rock the

proportion would be quite 98 per cent.

[Illustration: Diagram showing the succession of deposits below the

Nandua tea-estate. The total thickness is about 250 feet. The figures

refer to the proportion of palagonite.]

Whilst it is apparent that we have represented in this series the

covering of a submarine basaltic flow with submarine deposits, it is

also evident that the mode of junction between the flow and the

overlying deposits is of an unexpected nature. Before drawing any

inferences, it is necessary to point out that when we begin on _à

priori_ grounds to frame our notions as to the course of events on the

surface of a submarine basaltic flow, we are entering a little known

region of inquiry. I would, however, suggest in the light of the theory

before advanced, the following explanation of the appearances presented

by this series.

During the consolidation of the flow much of the magma-residuum that

still retained its fluidity was extruded on the surface, where after

solidification it became palagonitised. According to my view this would

be the typical behaviour of submarine basaltic flows; but, owing to the

unstable and perishable nature of the palagonitic crust of the flow, it

would be rarely preserved in upheaved volcanic regions. There would

probably be, as in the case of the Nandua series, no sharp line to be

drawn between the palagonite-crust and the deposits subsequently

covering it, deposits indeed that would derive no inconsiderable

proportion of their materials from the disintegration of the crust

itself. During and after the emergence of such a district of submarine

eruptions the unstable palagonitic crust would be further subjected to

the hydration resulting from weathering and similar agencies; and as a

result of its final degradation there would often alone remain a bed of

reddish argillaceous material.

In concluding these remarks on palagonite the following summary of the

principal points here dwelt upon may be added:

(_a_) The basic glass, that undergoes the palagonitic change, is the

vitreous form of the magma-residuum that in a particular type of basalt

and under certain conditions remains fluid after the mass of the rock

has solidified. During the last stage of the consolidation it is in part

imprisoned in the “magma-lakelets” of the groundmass; whilst the rest of

it is squeezed into cracks and fissures, or extruded on the surface of

the flow.

(_b_) This glass differs from ordinary basic glass in its molecular

condition, its mineral composition, its low degree of fusibility, and in

its unstable character.

(_c_) The formation of palagonite in connection with the crushing of a

basic glass is to be explained by the hypothesis that the heat developed

during the crushing is sufficient to partially re-fuse the glass, the

material thus produced corresponding to the magma-residuum of low degree

of fusibility, which is above referred to.

(_d_) In submarine eruptions are to be found the conditions favouring

the production of palagonite on a large scale. In the case of such

basaltic flows it is probable that their upper portions are formed

entirely of palagonite arising from the alteration of a vitreous

magma-residuum extruded on the surface in the manner above described.

Such a crust, as a result of shrinkage and other processes, would

probably present itself to the geologist as a somewhat friable material,

passing gradually into the overlying submarine deposits.

* * * * *

_Note on the type of basalt found associated with palagonite._

The type is characterised, it would appear, rather by its structural

features than by its mineral composition. It is the basalt of ophitic or

semi-ophitic habit that would seem to be usually associated with

palagonite; and since this habit is as a rule to be found where the

groundmass displays large felspar-lathes in plexus arrangement, coarse

augites, and at least a fair amount of smoky glass, it follows that a

hemi-crystalline, ophitic or semi-ophitic, doleritic basalt is the type

to be associated with palagonite.

This is the type of rock that forms the lower part of the basaltic flow

near Kiombo, the upper part of which is largely palagonitic. To this

structural type also belong most of the basalts in my collection where

palagonite exists in the form of “magma-lakelets” in the groundmass.

These “lakelets” are almost diagnostic of this type of basalt. Here also

belongs the famous globular basalt of Acicastello on the coast of

Sicily.[132] In such rocks the felspar-lathes form a mesh-work and vary

usually in average length between ·1 and ·3 mm. The augites of the

groundmass, typically semi-ophitic, range up to ·1 mm. in size. They are

always large, that is, over ·03 mm., and this coarseness is another

important indication.

NOTE ON THE CHANGES PRODUCED THROUGH THE HYDRATION OF PALAGONITE.

Most of that which is detailed below is not according to my views

palagonitisation, but the effect of hydration in the disintegration of

this material. The initial molecular condition and the other characters

which represent potentially the palagonitic change are not connected

with hydration; but are concerned with the causes before explained that

led to the formation of a basic glass of such an unstable constitution.

Indeed, there is good reason to believe that the changes to be now

described may be observed under the ordinary influences of weathering in

a wet region.

The early stages of alteration are well displayed in some of the tuffs

formed mainly of basic vacuolar glass, the submarine character of which

is often indicated by a few tests of foraminifera. Whilst the glass

retains its original bottle-green colour, it loses the clean sharp

conchoidal edges and displays rough and uneven or granular borders. With

a high power the surface of the fragment is seen to be minutely pitted

or pock-marked in places, the shallow circular pits, less than ·01 mm.

in diameter, being sometimes arranged in a row like a number of

overlapping rain-prints. This process proceeds until all the surface is

affected, and from this cause there is often an appearance of polygonal

markings. The pock-marking, however, continues; and as the pits encroach

more and more on each other an irregularly wrinkled rough surface

results. Up to this time the glass has retained much of its original

colour; but its clearness is replaced by turbidity, and collections of

very minute rounded, rod-shaped, and irregular granules, composed of a

colourless feebly polarising material, are displayed here and there in

its substance, whilst some of the previously empty vacuoles are now

filled with water.

In the next stage the hydration of the iron-oxides begins, and the glass

becomes opaque and yellowish or reddish-brown, and has a more granular

appearance, polarising feebly. Cracks now traverse the substance, and

penetrate into the vacuoles, which, as they become filled with the

alteration products, whether palagonitic, zeolitic, or siliceous, become

ruptured and curiously distorted. The hydration and consequent

disintegration continue until the deep stain of the iron-oxide is

removed, and a semi-pulverulent whitish material remains. This is the

history of the little bleached powdery patches so common in basic tuffs,

each representing originally a lapillus of basic pumice. This powder

when examined with the microscope is shown to be made up of fine

granular and tubercular materials which lose much of their distinctness

when mounted in Canada balsam. It is not affected by boiling in HCl, and

contains usually an abundance of minute siliceous oval amygdules that

have been freed in the last stage of the disintegration of the

palagonite.

Such is the story of the degradation of the palagonite daily in

operation in the basic tuffs of this island. From this source is

doubtless derived much of the finest constituents of the submarine clays

so common over Vanua Levu.

_Supplementary note on the occurrence of palagonite in the glassy matrix

of pitchstone-agglomerates and in rubbly pitchstones._—In my last

revision of the proofs I find that I have not laid sufficient stress on

the production of palagonite under these conditions. The evidence of

crushing is often very evident, and especial references to this point

will be found in the index under “Pitchstone,” and on page 334 under

“Crush-tuffs.”

CHAPTER XXV

SILICIFIED CORALS AND FLINTS

SILICIFIED corals, together with siliceous minerals (quartz, chalcedony,

jasper, &c.) and siliceous concretions are evidently widely distributed

in these islands. Kleinschmidt in his journal refers to large blocks of

flint on the island of Ono, from which the natives used to obtain their

musket-flints,[133] and he collected from this island as well as from

Viti Levu, Ovalau, &c., numerous specimens of these and other siliceous

minerals and rocks, such as hornstone, chalcedony and jasper, which were

examined by Wichmann and described in his paper.[134] Mr. Andrews

observed silicified corals on the summits and higher slopes of Vanua

Mbalavu.[135] The Fijian name for flints, “ngiwa” (thunderbolt) or

“vatu-ngiwa” (stone-thunderbolt), affords a good instance of that

curious superstition connected with the origin of these stones, which

came also under my notice in the Solomon Islands,[136] and in fact is

widely spread.

In Vanua Levu these siliceous rocks and minerals are in places abundant.

They are especially frequent on the surface of the extensive low plains

on the north side of the island which constitute the basins of the

Sarawanga, Ndreketi, Wailevu, and Lambasa rivers; but it is in the

low-lying district of Kalikoso, in the north-eastern part, that they

exist in the greatest quantity. They do not occur usually at greater

elevations than 300 feet, and are found as a rule at much lower levels.

It must be understood that reference is not here made to quartz-veins,

such as are found in certain localities and of which mention is made on

pages 106, 116. It is not with the ordinary products of contact or

general metamorphism that we have here to deal; but with the remarkable

surface-collections of silicified corals, nodules and flints of

chalcedony, fragments of white quartz-rock, bits of jasper, and certain

curious siliceous concretions, that occur often in association with

fragments of limonite in these low-lying regions. All the siliceous

materials above named have, as the microscope indicates, a common

character, chalcedonic silica in a greater or less degree being the

basis of all of them, whether coral, flint, white quartz-rock, or

jasper. It soon became apparent whilst examining these districts that

one general condition prevailed whilst this extensive deposition of

silica and the formation of the beds of limonite were in progress. It

cannot, however, be pretended that these processes are actually in

operation on the plains now. Except in the case of the limonite in a few

localities the processes have been suspended; but they were in active

operation not long ago: and an examination of the general characters of

the districts will probably disclose some of the conditions under which

these products have been formed.

On the surface of the Kalikoso plains, where these materials are most

abundant, we find silicified corals associated with fragments and

nodules of chalcedony, flints, white quartz-rock, limonite, concretions

of carbonate of iron, &c., in the low-lying and often swampy district

around the fresh-water lake, the whole region being only elevated

between 20 and 60 feet above the sea. This is an area of decomposing

acid rocks (quartz-porphyries, trachytes).[137] On the other hand in

most of the regions where these materials occur on the surface we have

areas of basic rocks (basalts and basaltic andesites) incrusted in

places with submarine tuffs and foraminiferous clays, the volcanic rocks

undergoing extensive disintegration. Such for instance are the Lekutu,

Sarawanga, Ndreketi, and Lambasa plains. In the Lambasa plains, which

are described in this connection on page 139, we find besides the corals

and flints and nodules of chalcedony, fragments of jasper. In the

Sarawanga and Lekutu lowlands, we find silicified corals and limonite;

but here the crystallised silica of the corals contains a large quantity

of water, whilst in its lesser degree of hardness and in its low

specific gravity it comes near to semi-opal. In these and other

localities, as in the level country around Ndranimako on the right side

of the Yanawai estuary, we find curious concretions of the same kind of

hydrous silica more or less crystalline. These concretions are described

below.

It may be remarked that nearly all the districts in which the silicified

corals and concretions, siliceous minerals, and limonite occur, are

scantily vegetated “talasinga” lands[138] with reddish soil. Except in

the instance of the Kalikoso plains, the swamps and lakes have as a rule

long since disappeared, their sites being alone indicated by the

limonite on the surface. In the Mbua plains, however, there are

occasional small ponds and swamps, and there is no doubt that the

limonite so bountifully represented on the dry districts is still in

process of formation.

Before drawing some general inferences as to the conditions under which

this deposition of silica and iron took place, I will refer to the

characters of the materials thus produced.

The silicified corals include massive corals of the Astraean and

“Porites” kinds and branching specimens of the Madrepore type or habit.

The former are rarely larger than 7 or 8 inches across and are merely

fragments. The latter are always portions of branches, never exceeding 3

or 4 inches in length. In the last case it is sometimes possible to

show, as in the case of a specimen found on the Kalikoso plains, that

before silicification occurred the dead fragment of branching coral had

been extensively eroded by solvent agencies and had been penetrated by

burrowing molluscs. The larger blocks of massive corals have usually

been extensively chipped by the natives in obtaining flints. In past

times they were carried from one place to another, the result being that

occasionally they were brought to me in the mountain-villages, all

showing evidence of their having supplied flints to a past generation.

These corals are as a rule completely silicified. When a massive

specimen is broken across it is not infrequently found that whilst the

coral structure is preserved in its outer part, the inner portion is

composed of a compact seemingly structureless mass of bluish-white or

pale-grey flint, which has the characteristic microscopical appearance

of chalcedony and a specific gravity of 2·59.[139] It is from the more

compact parts of the silicified massive corals that the “worked” flints

found on the surface were obtained, though in some of them, as in the

case of a “scraper” in my collection, the traces of coral structure are

still apparent to the eye. Wichmann observed in the case of the

silicified corals from Fiji that the whole petrifying process appears to

consist in the saturation of the coral with silica, the coral structure

being usually distinct, whilst the septa, often still calcitic, show the

points of the calcite crystals projecting into the chalcedony which

forms the mass. Lime however rarely occurs in the silicified corals of

Vanua Levu. It was only in the case of one or two localities that the

corals displayed any effervescence with an acid. In the microscope slide

the massive specimens appear to be entirely of chalcedonic silica, the

outlines of the cells and of the septa being indicated by ferruginous

material. In a specimen of Porites by my side the crystallization of the

silica has advanced beyond the chalcedonic stage and the coral is

composed entirely of minute quartz-crystals, ·2 to ·4 mm. in size, often

irregular, but sometimes forming doubly-terminated prisms. This has

produced a somewhat crumbling rock, which is easily powdered by the

finger; and in this case, therefore, the complete crystallization of the

silica is resulting in the disintegration of the silicified coral.

The ordinary silicified massive corals of Vanua Levu, where the

replacement by chalcedonic silica is complete, though the structure is

preserved, have a hardness of about 6 and a specific gravity of 2·54,

and yield but little water in the closed tube. Occasionally, however, as

in the Sarawanga plains and in the Lekutu lowlands we find silicified

fragments of branching corals which are easily scratched with a knife

and have a hardness of 3 to 4 and a specific gravity of 2·3. The

fractured surface is milk-white or reddish, and looks like semi-opal.

When powdered and heated in a closed tube, the material loses one fourth

or one fifth of its weight of water, the finest dust (passing away in

the steam) being deposited on the sides of the glass. In the slide there

is displayed a finely granular crypto-crystalline structure with in

places a somewhat coarser quartz-mosaic, whilst chalcedonic quartz fills

minute cracks in the mass. No coral structure is preserved. Numerous

points coloured by iron oxide occur in the section, and minute dust-like

inclusions abound, which are doubtless water-pores. I have described on

a later page certain concretions found associated with these silicified

corals which though formed of the same crypto-crystalline hydrous

silica, are apparently silicified portions of nullipore-rocks.

The fragments of flint that occur commonly on the surface in these

districts are, as above remarked, derived from the hard silicified

coral-masses. Nodules of chalcedony, having all the appearance of having

originated in cavities, are also very frequent. They may take the

mamillary, agate, or onyx form, some of the agates when polished making

beautiful specimens. These nodules are of all sizes up to 3 or 4 inches

across. Some of them are hollow and lined with clear quartz-crystals,

whilst with others the cavity may be completely filled by interlocking

quartz-crystals. The outer surface of one of the agates displays

markings showing in relief casts of the “cups” of a minute-celled coral.

Mingled with the other siliceous materials on the surface of the

Kalikoso and Lambasa plains are found fragments of a whitish

quartz-rock, having a specific gravity of 2·53-2·57, being therefore

markedly lighter than quartzite (2·63-2·67) which it somewhat resembles.

It usually occurs as small hand-specimens; but in the vicinity of

Mbati-ni-kama I found blocks, 12 to 15 inches across, lying in the

river-bed. Under the microscope it displays a fine radio-globular

aggregate of chalcedonic quartz.

Mention has already been made of the siliceous concretions, composed

mainly of hydrous crypto-crystalline silica, which are associated with

the silicified coral fragments formed of the same kind of silica on the

surface of the plains of Mbua, Lekutu, and Sarawanga. They also occur in

the Ndranimako lowlands on the right side of the Yanawai estuary, and in

the more elevated inland districts of the Wainunu and Na Savu

table-lands at elevations of 650 to 770 feet above the sea. They take

the form of irregular nodules, or of flat uneven “cakes,” usually two or

three inches in size. They are as a rule reddish, but sometimes pink and

white. Their hardness is only 3 to 4, and they are easily scratched with

a knife; and when powdered and heated in a closed tube, they lose about

one fourth of their weight of water. Under the microscope they exhibit a

grey crypto-crystalline groundmass showing very finely granular

crystalline silica with the cracks and small cavities filled with more

brightly polarising chalcedonic quartz. But they differ as regards their

other components and also in their mode of occurrence; and it is highly

probable that the history of their origin is not always the same.

Those associated with the silicified corals on the Sarawanga and Lekutu

lowlands show no structure in the slide that gives me a clue as to their

origin; but they may perhaps represent old Nullipore nodules. Those

around Ndranimako are coloured deep red; and whilst some give no

indication as to their source, others are transitional in character, and

display in the sections traces of the vacuolar semi-vitreous basic rock

of which the original fragment was composed. The same red siliceous

concretions form the pebbles and gravel in the stream-beds on the

surface of the Na Savu table-land, 700 feet above the sea. These red

flint-like nodules of Ndranimako and Na Savu somewhat resemble the

jasper of the island; but they are sharply distinguished by their

microscopic characters, by being easily scratched with a knife, and by

the large amount of water which they contain. Rolled stones, which were

found in the shallow stream-courses on the surface of the Wainunu

table-land 750 to 800 feet above the sea, exhibit in the sections, in

spite of the general silicification of the groundmass, the outlines of

the original phenocrysts of felspar, and abundant skeletal magnetite

rods, such as would characterise a semi-vitreous basic rock. It is

evident that in the basaltic districts of the Na Savu and Wainunu

table-lands these concretions have been formed under certain conditions

by the decomposition of the silicates of basic rocks. But these

conditions do not exist now; and I infer that the silicified rocks,

which occur only in fragments on the surface, represent the

silicification that occurred during the emergence of the land ages

since.

Occasionally one comes upon in the mountain districts, as in the

vicinity of Ndrawa, large solitary blocks 2 to 4 feet across of a

whitish chert-like rock which has a hardness of 5 or 6, the harder

variety having a specific gravity of about 2·58 and the softer, which

yields a fair amount of water, a specific gravity of about 2·46. I

noticed such solitary masses also on the Mbua plains. The first-named

locality is dacitic and the last basaltic. They exhibit in the slides a

patchy appearance, showing in some places finely granular

crypto-crystalline silica and in others a coarser mosaic of chalcedonic

quartz. Apart from the absence of any definite coral structure, I can

only surmise that they were originally masses of reef-limestone. Their

elevation even in the mountainous districts was not over 400 or 500 feet

above the sea.

Fragments of jasper, which are associated with nodules of chalcedony and

silicified corals in the Lambasa plains, are also to be found as pebbles

and small blocks in the mountain streams of the Ndrawa, Ndrandramea, and

Lea districts, together with bits of chalcedony and quartz-crystals.

They do not occur, or are of rare occurrence, in the recently emerged

Kalikoso district and probably belong to an earlier stage in the history

of the island’s emergence from the sea. They have a hardness of 6 to 7,

not being scratched by a knife, and a specific gravity of 2·65 to 2·70;

whilst but little water is given off in the closed tube. They are a

variety of chalcedony, rendered opaque by the large quantity of red

oxide of iron that it contains, and are really, therefore, iron-flints.

The microscopical section in one case displays in the clear spaces a

beautiful globular aggregate, each globule having a nucleus of the iron

oxide and giving a black cross in polarised light. In another case the

globular structure is less perfect, and the chalcedonic groundmass is

penetrated by a multitude of fine cracks filled with iron oxide.

The deposits of limonite vary in character in different localities, and

evidently they have not all the same history. The soil of the low-lying

plains around Wai-ni-koro and Kalikoso, and especially in the vicinity

of the fresh-water lake, is often coloured a deep ochreous red. Small

fragments of an earthy yellowish-brown limonite occur on the surface in

quantity and are particularly abundant near the lake. They yield much

water when heated. In some places in this district, as in the country

traversed between Wai-ni-koro and Kalikoso, the surface is strewn with a

number of small round concretions of the size of small marbles (6 to

12 mm.) which are composed of a mixture of carbonate of iron and

limonite, but show no recognisable structures. They are somewhat friable

and give off much water when heated, whilst they effervesce freely in

hot hydrochloric acid. It is probable that some of the earthy limonite

of the Kalikoso district contained originally iron carbonate and has

been produced from concretions such as I have just described.

The variety of limonite found in fragments on the surface of the plains

of Mbua, Lekutu, and Sarawanga, at elevations usually of 100 or 200 feet

above the sea, is a heavy compact kind with a specific gravity of 3 to

3·5, and closely resembling red hematite. Since, however, it is lighter

in weight and still contains a little water, it may be regarded as in

the transition stage. It occurs as portions of cake-like masses varying

usually from a third of an inch to rather over an inch in thickness. As

a rule it is found in localities where no lakes or swamps now exist and

may be associated, as in the Sarawanga and Lekutu plains, with

silicified corals and siliceous concretions; but in some cases, as in

that of the Mbua plains, ponds and swamps are still scantily represented

in the vicinity, and the water of the stagnant streams is deeply

coloured with iron (see page 56).

Ironstone gravel occurs in great quantity strewn over the surface of the

basaltic table-lands, especially in the case of that between the Wainunu

and the Yanawai rivers. The smaller gravel varies usually between one

eighth and one third of an inch in size, the larger fragments being

about an inch. The specific gravity is 3·1 to 3·2. The material forming

the finer gravel dissolves with but little effervescence and scanty

residue in hot hydrochloric acid; it gives off water and is evidently

impure limonite. The larger fragments, 1 to 2 inches in size, represent

the partial conversion into limonite of a basic volcanic rock with much

glass in the groundmass which formed probably the surface of the

basaltic flows of the plateaux. There must be an enormous amount of this

iron-stone in the island. The finer gravel has a concretionary

character, some of the pieces appearing like bits of stick that have

been converted into limonite. It seems to have been formed during the

disintegration of the rock on the moist surface of these densely wooded

basaltic plateaux; the process was not accomplished in ponds or swamps,

but was carried out on ordinary damp ground.

It must be observed in the above connection that the soil in the areas

of basalt and basaltic andesites, which occupy a large portion of the

surface of the island, contains a large amount of fine magnetic

iron-sand. After heavy rains the foot paths glisten with this fine

material which has been washed out on the ground. This is especially the

case in the extensive scantily vegetated “talasinga” regions where the

basaltic rocks are disintegrating for a considerable depth. The

river-sand of these areas, after a little washing, yields about 75 per

cent. of magnetic iron grains which give in some cases a slight titanium

reaction. The amount of magnetic iron-sand in these rivers, as for

instance in the Yanawai and the Wainunu, must be very great. In the beds

of the small sluggish streams on the surface of the Wainunu table-land

the amount is also very large.

Any explanation of the origin of the extensive silicification evidenced

by the occurrence of silicified corals and siliceous concretions on the

surface in various parts of the lower regions of the island will have to

include that of the formation of the limonite fragments so often

accompanying them. The necessary conditions would, I think, be afforded

by an emerging land-surface during the consolidation of the exposed

calcareous muds and the subsequent draining of the new surface. On parts

of the newly formed land, there would follow the successive stages of

sea-water, brackish, and fresh-water swamps, such as are clearly

indicated by the abundance of silicified coral fragments that strew the

surface of the low-lying and often swampy districts around the fresh

water lake of Kalikoso.

In such a locality as that of Kalikoso, there were no doubt at the time

of the emergence large tracts covered with chalky calcareous mud derived

from reef-debris; and it was during the consolidation of this mud in the

recently reclaimed area that the fragments of coral imbedded in it

became silicified. In these cases where the imbedded corals were already

much decayed, it is probable that the empty cavities thus produced were

filled with silica, and that in this manner the nodules of chalcedony

were produced. Here and there a pebble or a larger block of a volcanic

rock would have been inclosed in the mud; and in this case also silica

largely replaced the original material of the stone. I imagine that with

the evaporation of the water in the mud during the drying and

consolidating processes the proportion of silica in solution would

attain a degree of super-saturation and that the silicification would

hence be brought about.

With the consolidation of the mud the deposition of silica ceased; and

in the case of any coral fragments, where the transformation was not

completed, decay would often commence. In the instance of some bits of

coral found imbedded in foraminiferous mud-rock in the Lambasa plains

the process of the change had been suspended, and the fragments were in

a state of decay, and coloured red by iron oxide. If silicification

occurred in a submarine deposit only after it became a portion of an old

land-surface we ought not to find incompletely silicified corals

inclosed in it. For these reasons I do not consider that silicification

would occur in the case of submarine deposits long after they have been

raised above the sea.

On the other hand it would seem that the deposition of silica in the

hard parts of dead organisms does not proceed in the shallow-water

calcareous mud of coral reef coasts previous to emergence. Silicified

corals have never as far as I know been found under such conditions. Nor

could the coral fragments now lying on the Kalikoso plains, often only

elevated some 20 or 30 feet above the sea, have undergone this change

whilst exposed on the land-surface as they now lie. They must have been

inclosed in some material containing abundant free silica; and it is

reasonable to suppose that this material was the chalky mud of the

reef-flats on which they once lived. If this is admitted, then it

follows that since, as above assumed, silicification does not occur in

such a mud either before upheaval or long after it has been raised above

the sea, it must take place in the intermediate period, or in other

words whilst the recently exposed submarine deposits are consolidating

and drying.

Several objections at once occur with reference to this explanation of

the silicification of corals in this island; but much more investigation

is needed to establish any view on the subject. In the Kalikoso plains,

however, we have a critical locality for the pursuit of this inquiry.

Concretions of carbonate of iron and deposits of earthy limonite are

here associated with silicified corals on the surface of a level and

often swampy district around a freshwater lake in a region which is only

elevated 20 to 60 feet above the sea. We are dealing here with an area

of land that has emerged in comparatively recent times as far as the

history of the island is concerned. The element of time is limited, and

the problem is not complicated, as it would be in the case of an old

land-surface, raised some hundreds of feet above the sea, by the

intrusion of many other disturbing agencies. Nature has simplified

matters here for the inquirer.

The evidence of recent emergence with regard to the whole island is

discussed in Chapter II., and need not be again referred to here; whilst

the general description of the Kalikoso district is given in Chapter

XVI. In this connection it may be remarked that before their emergence

the Kalikoso plains were covered by the waters of a large irregular

sea-water lagoon or lake, which though more or less surrounded by hills

had free communication with the sea on the north along the line of the

passages now occupied by the Wai-ni-koro and Langa-langa rivers. Both

massive and branching corals then thrived in the waters of the lagoon.

There is no ground for supposing that during the emergence there was an

intermediate stage characterised by brine-ponds and salt-swamps. The

drainage from the slopes of the mountains to the southward would have

prevented it. Whilst this change of level was in operation, brackish

water collected in the deeper part of the original lagoon, forming a

lake which as evidenced by the present distribution of limonite on the

surface of the plains was then far larger than it is now. As the plains

became exposed large flats covered with chalky mud in which dead corals

were more or less imbedded were bared; and there and then as the drying

and consolidation proceeded silicification took place in the manner

before surmised. This deposit was of no great thickness, and has been

since removed by the denuding agencies, whilst the silicified corals

remain behind.

When in the Solomon Islands I was unable to find the source of the

chalcedonic worked flints of such frequent occurrence in that region. In

my general work on those islands (pp. 77 to 80) reference is made to

this subject. It will probably be shown that there as in Fiji most of

the flints are silicified corals.

In conclusion it may be remarked that those who object to the

explanation of the origin of silicified corals advanced in this chapter

will be able to find support for their alternative hypothesis in many

facts detailed in these pages. Vanua Levu, for instance, abounds in hot

springs; and Mr. Andrews might regard this fact as giving strength to

his view that the silicified corals of Vanua Mbalavu in this group owe

their condition to the agency of superheated water derived from volcanic

rocks, more especially since hot springs are found on the island. Such

an explanation could not, I think, apply to the extensive area of the

Kalikoso plains where the silicified corals are associated with limonite

on the surface of a recently emerged area. If these changes had been

induced by hydrothermal action, one ought to find evidence of this in

those localities in Vanua Levu where the hot springs issue from

foraminiferous clay deposits, as in the vicinity of Vuni-moli; but no

traces of such a transformation came under my notice. Wichmann does not

advance any explanation of the silicification of the corals; but he

considers that the “hornstones,” which he obtained from Fiji, rocks

corresponding to the chert-like rocks described by me on page 355, are

the products of disintegration of the basic andesites. I have already

pointed out that certain siliceous nodules have probably this origin. It

is also likely that some of the jasper of Vanua Levu has been thus

formed.

_Note on a silicified Fern Rhizome._—This is a specimen, about three

inches long, picked up by a native in a stream near Sueni in the centre

of the island. It has the appearance of being a portion of the stem or

rhizome of a tree-fern, and is permeated in its entirety by chalcedonic

quartz, the fibro-cellular structure being still preserved. No other

specimen of the kind came under my notice. The probability of the

occurrence of silicified plant-remains in the pumice-tuffs of the Undu

Promontory is pointed out on page 233.

CHAPTER XXVI

MAGNETIC ROCKS

THE literature on the subject of the magnetism of rocks is very

extensive,[140] and even if I was capable of doing so, any attempt to

deal generally with this complicated phenomenon would be out of place

here. Zirkel in his characteristically thorough fashion has reviewed the

subject in his general work on petrography, but since the date of the

last publication of that book, 1893-94, the literature has been much

increased and the subject has from time to time been opened up in

scientific periodicals, occasionally in ignorance of the labours of

those that have gone before. Here, the local magnetisation of rocks is

alone considered, the general question of earth magnetism not being

entered into.

According to Zirkel one of the earliest known observations of this

phenomenon was made by Bouguer, the French geographer, whilst he was

engaged in the measurement of a degree of the meridian in the vicinity

of Quito in 1742. Alexander von Humboldt, however, was one of the first

to attract general attention to this subject by the announcement of his

discovery in 1796 of a “great magnetic mountain” in the heart of

Germany. He was then director-general of the mines in two Franconian

principalities; and in order to awaken the interest of German physicists

and mineralogists in this matter, he announced his discovery with an air

of mystery, and did not disclose the locality for many months. He then

placed his specimens in the mining-office at Bayreuth to be sold at so

much by weight for the relief of poor miners. His plan succeeded, and

this young savant who had yet before him his great career, had soon

enlisted the interests of several of the noted scientific men in

Germany, including Werner the mineralogist, Voigt the mathematician,

Blumenbach the naturalist, Charpentier, and others. The amount of

attention that this subject then excited can be inferred from the pages

of the “Intelligenzblatt der Allgemeine Literatur-zeitung” for 1796-1797

and from the contemporary publications. It has been almost forgotten

now, and the matter is indeed often approached “de novo.”

However, although by these means the data became largely increased, no

generally accepted explanation resulted. Opposing views continued at

various times to be advanced; and it has only in recent years come to be

recognised that the magnetic polarity[141] of rocks in exposed

situations, as in the mountain-peak or in the crested spur, often arises

from atmospheric electricity independently of the inductive action of

terrestrial magnetism. This is the conclusion to which the later

evidence given by Zirkel is directed and was that which Oddone and Sella

formed from their study of the magnetic rocks of the Central Alps. It is

not, however, always necessary to suppose that the affected rocks have

been struck by lightning, although Sella and Folgheraiter have shown

that this is the result of such a contact. They may be found, as

indicated by Mr. Harker, in mountainous localities where thunder-storms

are remarkably rare, and where the peaks act, it is suggested, as

natural conductors. It is easy to show, remarks the same author, that no

lapse of time is required for rocks in exposed situations to become

magnetised. The stones of cairns erected a few years before on the

mountain-tops of the Isle of Skye become invariably highly magnetic;

whilst the loose stones lying on the ground display this property to a

much less degree. Nor is it requisite that the rocks affected should be

basic volcanic rocks. It has long been known that granites, trachytes,

&c., can possess magnetic polarity[142]; and the existence of this

quality among acid volcanic rocks is well shown in the case of the

dacites in Vanua Levu, rocks which compose some of the isolated

mountain-peaks.

One finds occasional reference to the highly magnetic character of the

rocks in oceanic islands of volcanic origin, but the nature of the

property is not always described; and it is sometimes not possible to

gather from the data given whether the magnetism affects the whole

mountain mass, when it would be of the regional kind, due probably to

induction, or whether it is the simple magnetic quality that almost all

basic volcanic rocks possess on account of the fine magnetite

disseminated through the rock, or whether there is evidence of a deposit

of magnetite in the vicinity, or whether it is a mere surface phenomenon

confined to the bare rocks of peaks and ridges, when such rocks, whether

gabbro, granite, basalt, trachyte, or dacite, display magnetic polarity.

Dana, with regard to the basaltic mountain of Tahiti, remarks that the

compass was often rendered useless by the local attraction of the rocks,

bearings taken being found to vary two to three points on changing the

position of the instrument.[143] Major Haig says that the compass

becomes perfectly useless anywhere in the neighbourhood of one of the

mountain-masses or extinct craters in Mauritius, and attributes this

effect to the magnetite in the basalt.[144]

On the summit of Mauna Loa in Hawaii, at the edge of the great crater

and in the vicinity of the site where Commodore Wilkes carried out his

pendulum observations in 1840, I found my compass-needle greatly

affected by local attraction, but I neglected to inquire further into

the matter. Judging from my sojourn of twenty-three days on this

mountain-top, thunder-storms are of very rare occurrence there; but the

electric condition of the air is at times very evident, and its

physiological effects are somewhat distressing. My blanket at night

crackled in my hands and emitted sparks, so that I could trace with my

finger the letter A in phosphorescent hues on its surface.

That lightning is directly responsible in some instances for the

magnetic polarity of rocks in mountain-peaks is also well established.

It has been illustrated in an indirect fashion only last year in the

disaster on the Wetterhorn. Rocks partially fused by thunderbolts and

displaying polarity occur on the summit of the Riffelhorn and on one of

the peaks of Monte Rosa, and fulgurites have been also obtained from

Mont Blanc.... It is not always easy to explain, however, isolated cases

of polaric rocks where no signs of fusion occur. Whilst descending into

the Valle del Bove from the Etna Observatory, I picked up four small

volcanic bombs of basic lava, of which one displayed polarity, the poles

being situated at the sides of the bomb. Zirkel quotes the observation

of Naumann on the summit of the volcano of Moryoshi in Japan. Here out

of a number of lava-blocks lying about only one exhibited marked

polarity, whilst the rest showed no signs of it.

Before dealing with the polaric rocks of Vanua Levu, I will refer to two

localities in other parts of the group where magnetic rocks have been

observed. During the Wilkes’ expedition in 1840,[145] Lieutenant

Underwood observed great local attraction at Naikovu, a rock 90 feet

high of volcanic formation lying off the south end of Nairai Island. He

found a “deviation” of 13¼ points (149 degrees) at the top of the rock,

whilst at the foot near the water the needle gave correct bearings. In

the _Sailing Directions for the Pacific Islands_, published in 1900, the

“deflection” at the summit is said to be 87 degrees. It is stated by Mr.

Eakle in his paper (quoted on p. 293) on the rocks collected by the

recent Agassiz expedition that this rock is composed of an

augite-andesite.... I have learned from Mr. Alex. Barrack that there are

some highly magnetic rocks on the west coast of Viti Levu in the

vicinity of Likuri Harbour in the Nandronga district. It is said that

specimens sent down to the colonies were found to contain 95 per cent.

of magnetite.

It is very probable that the results obtained by me for Vanua Levu can

be generally applied to the other large islands of the group. The

observations were made during my various geological journeys and deal

only with certain aspects of this interesting subject.

The first feature in this connection is the frequency with which simple

magnetism is displayed by the acid as well as basic volcanic rocks of

this island. About 95 per cent. of the volcanic rocks collected attract

both ends of the needle.[146] This property of volcanic rocks is well

known, and is to be attributed to the magnetite in the groundmass.[147]

On examining the character of the non-magnetic rocks it appears that

almost all belong to two groups where magnetite might be expected to be

scanty. The first includes the pitchstones or basic glasses, sometimes

fresh, at other times more or less palagonitised. The second comprises

the highly altered basic rocks, where the ferro-magnesian silicates have

been replaced by viridite, calcite, and pyrites. It is not, however, to

be implied that rocks of these two kinds will not sometimes attract the

needle. Many do not, and those in my collection that do so act feebly.

Coming to the magnetic polarity displayed by some of these rocks, when

the ordinary hand-specimen behaves like a magnet in attracting one pole

of the needle and repelling the other, it is to be at first observed

that a rock can become polaric without being previously magnetic. Dr.

Folgheraiter has observed polarity in the case of fragments of ancient

bricks and pottery; and he has described the same effect in the masonry

of a house struck by lightning. In one or two of the Vanua Levu acid

rocks showing polarity this can be also premised since magnetite is

present in very slight degree.

Polarity is very frequent among the volcanic rocks of this island. Out

of 520 specimens in my collection, which was made without any reference

to this matter, 80, or 15 per cent. are polaric. Of these seven-eighths

are basic and the rest are acid rocks; but this proportion is partly

accounted for by the far greater prevalence of basic rocks in the

island. The basic rocks showing polarity include some of the heaviest

olivine-basalts with a specific weight of 3·0, as well as some of the

lighter augite-andesites with specific weight of 2·7. They comprise the

coarse textured dolerite as well as the vitreous pitchstone and include

both scoriaceous and amygdaloidal rocks. The polaric acid rocks are

mostly referable to the dacites, with a specific gravity of 2·5 to 2·6.

Humboldt remarked long ago that there is no direct relation between the

degree of polarity and the specific weight. This is well brought out in

the table subjoined; but it should be at once observed that there is an

indirect relation. Although when we arrange the rocks in a series

according to their specific weight we find no corresponding relation in

the amounts of the polarity, we observe that the extent to which

polarity can be developed is markedly greater in basic than in acid

rocks. From this it may be inferred that the degree of intensity of the

exciting cause required to give polaric powers of a certain value to an

acid rock, like a dacite, would be much greater than that necessary to

endow a basalt with equal powers. We should not expect to find the same

amount of polarity in the bare rocky peaks of two adjacent mountains,

where one was of dacite and the other of basalt; and, other things being

equal, if two mountains had been exposed for ages to the same

conditions, we should regard the polaric powers of the two as nature’s

equivalent values for the work of atmospheric electricity, on the two

rocks in question. We have two such mountains in Vanua Levu in the case

of the adjacent peaks of Ngaingai (2,448 feet) and Navuningumu (1,931

feet) which are about 2¼ miles apart and possess similar bare rocky

pointed summits. I take it that the polaric power of 25° of the dacite

(sp. gr. 2·57) in the first case is equal to the power of 90° of the

basaltic andesite (sp. gr. 2·82) in the other. In the dacitic peak of

Ngaingai and in the basaltic peak of Navuningumu we can measure what

work atmospheric electricity can accomplish in the course of ages in the

magnetisation of rocks. The other conditions being taken as about the

same, the main determining difference is to be found in the

rock-characters.

In the table on the opposite page we have a series of volcanic rocks

placed according to their specific weights, which range from 2·5 to 3·0,

and in the second column are shown their relative polaric powers as

indicated by the number of degrees the north end of the magnetic needle

is repelled by the corresponding pole in the hand-specimen. For this

purpose a magnetic needle 2½ inches in length (strictly speaking 6·5

centimetres) was employed, a card marked in degrees being placed

beneath. The north pole of the stone was placed in contact with the

north end of the needle, and after the needle had become stationary in

its new position a reading was taken.

These polaric rocks came under my notice over most of the island. They

are infrequent in the district between Undu Point and the Wai-ni-koro

River, where, however, acid tuffs are largely exposed; and I did not

find them in the Natewa Peninsula east of Lea, their absence from my

collections made in the Mount Freeland range being remarkable. But it is

probable that this is due to the surface conditions, since dense wood

covers the slopes, and bare rocky peaks are rarely to be seen.

With regard to the influence of locality on the occurrence of polaric

rocks, the results may thus be classified. About one-third are found in

the exposed rocky peaks of hills and mountains. Another third are found

where the rocks are bared in headlands, coast cliffs, inland-bluffs,

ridge-tops, and in the open basaltic plains where trees are scanty. On

the other hand, a third occur in situations, as in wooded districts

where the rock exposure is scanty, when it is not easy to explain the

polarity, unless it was developed in clear districts that have since

become covered with forest.

_Table showing the Relation between the Specific Gravity and the

Polarity of Volcanic Rocks._

+---------------------+--------------------+------------------------+

|Character of rock. | Specific gravity. |Amount of polarity.[148]|

+---------------------+--------------------+------------------------+

| | | |

|Dark olivine-basalt | 3·00 | 10° |

|Grey " " | 2·94 | 29° |

| " " " | 2·92 | 7° |

|Dark " " | 2·90 | 10° |

| " " " | 2·87 | 30° |

|Basaltic andesite | 2·82 | 90° |

| " " | 2·77 | 5° |

|Pyroxene-andesite | 2·72 | 38° |

|Dacite | 2·61 | 17° |

| " | 2·59 | 5° |

| " | 2·57 | 25° |

| " | 2·50 | 14° |

+---------------------+--------------------+------------------------+

In no place did any evidence of the direct action of lightning come

under my notice. Mr. S. Skinner who kindly looked at a few of these

rocks says that he found no trace of fulgurites in them. It is probable

that here as in the mountains of Skye, as described by Mr. Harker, these

effects are the result of the general influence of atmospheric

electricity independently of the direct agency of lightning. The

frequency of polaric rocks in the highest peaks of the island is very

remarkable. Generally speaking, all the bare summits of the mountains

are polaric. In my experience there is no exception. All the rocks

obtained from the actual summits show polarity. The variety of rocks

thus affected is suggestive; and this chapter may be concluded with a

brief reference to their mode of occurrence on some of the

mountain-peaks.

In Mbatini, 3,437 feet in height, which is the highest mountain of Vanua

Levu, the pyroxene-andesite of which the bare rocky peak is composed is

somewhat weathered and has a polaric or repellent power of 28°.

Specimens of rock obtained below the top show no polarity, the mountain

being well wooded except at the summit. In the adjacent mountain of

Koro-mbasanga,[149] the polaric rocks are limited to those exposed in

the peak which is bared of vegetation. The rocks in question are

tuff-agglomerates, the small blocks of pyroxene-andesite standing out

from the tuff having a polaric force of 14° or 15°. This effect has been

produced in greatest intensity in the isolated peak of Navuningumu

(1,931 feet) in the Ndrandramea region. Here the bare summit is formed

of a semi-vitreous, slightly vesicular, basaltic andesite with a

specific gravity of 2·82 in its present condition.[150] This rock is

powerfully polaric, and rendered the compass useless, the deviation

generally to the westward varying from 20° to 50°. I place its repellent

force at about 90°, hand specimens affecting the magnetic needle at a

distance of 13 or 14 inches. None of the various rocks obtained from the

wooded slopes below displayed polarity.

The neighbouring mountain of Ngaingai is composed entirely of dacite

having a specific weight of 2·57. The highest point of the summit, 2,448

feet above the sea, is bare and rocky, and the stone here is markedly

polaric, the repellent force being about 25°. Specimens from the lower

wooded slopes show no polarity. Near by rises the hill of Ndrandramea,

which is composed in mass of acid andesites or dacitic rocks. The summit

(1,800 feet) is scantily vegetated, and here the somewhat weathered rock

which has a specific weight of 2·44 (probably near 2·5 in the fresh

condition) has a polaric force of 14°. Specimens of a more compact rock

taken from the wooded slopes 300 feet below the summit (sp. gr. 2·58)

and from 700 feet below the top (sp. gr. 2·68) showed no such effect;

but a specimen taken from a mass of agglomerate in the last locality

repels the needle 12°. Its specific gravity is 2·61, and no doubt the

mass had been originally a portion of an exposed cliff-face.[151]

The summit of Mariko (2,890 feet), the Drayton Peak of the chart, is

formed of a rubbly agglomerate of a compact basic andesite. Though it

displays bare rock-faces, the actual peak has a soil-cap at least 18

inches deep and supports small trees and shrubs. Notwithstanding this, I

found when standing on the peak that my compass was very noticeably

affected, the pull being to the eastward, whilst the amount of deviation

increased from 11° to 16° when changing from the sitting to the standing

position. Specimens of blocks from the agglomerate forming a rock-face

10 feet below the summit possessed polaric powers of 12° and 5°. Others

of the same rock exposed in a cliff-face 450 feet below had a weak

repellent power of only 4°.... As in the case of Mariko, the top of

Thambeyu (2,700 feet) is vegetated; and beneath the smaller trees blocks

of polaric rocks lie on the surface. One of these, a pyroxene-andesite

(sp. gr. 2·72), from which I obtained a specimen, has a polaric power of

38°. In another case, that of an amygdaloidal rock of the same

character, the repellent power is 14°.

I might mention several other polaric peaks, but it will be sufficient

to refer to one or two other localities. In the mountainous basaltic

district around Solevu Bay the peaks are usually polaric. Specimens from

the top of Uli-i-matua, 1,100 feet, have a repellent power of 15°. The

three-peaked hill of Koro-tolu-tolu appears to be in the mass of polaric

basalt from the foot to the summit, having a repellent power varying

from 4° to 30°, the most active specimens being obtained from the lower

slopes, which, however, are scantily covered with trees. Samples of the

grey basalt from Koro-i-rea show polaric powers of 3° to 7°.

As examples of the numerous lesser hills with bare rocky polaric summits

I will first take Bare-poll Hill facing Soni-soni Island. This hill is

only about 150 feet above the sea, its top being formed by two large

masses of a basic andesite lava with a glassy groundmass, incrusted with

agglomerate, the whole representing a volcanic “neck.” A specimen of the

rock masses has a repellent force of 22°. Another instance is afforded

by Vatui, a hill 450 feet in height situated south of Mount Sesaleka.

Its summit is capped by a naked mass of tuff-agglomerate pierced by a

dyke 18 inches thick of an olivine-basalt, with a specific gravity of

2·90 and a polaric power of 10°.

A somewhat suggestive example is afforded by the hill of Na Suva-suva,

1,110 feet high, which overlooks Naindi Harbour to the east of Savu-savu

Bay. It is only occupied by trees in its upper part, and a specimen of

the olivine-basalt, of which the hill is composed, that was obtained

from the wooded summit, shows no polarity; whilst another from the

slopes, two-thirds of the way up, which had been cleared of trees, has a

repellent force of 10°. The polarity of the olivine-basalt from the

well-wooded slopes of Ulu-i-ndali, a range 1,100 feet in height on the

east side of the Wainunu estuary, is not so easily explained; the

intensity varies from 8° to 28°. Ngalau-levu, a hill 1,650 feet in

height, rising behind Lea on the south coast of Natewa Bay, is polaric

in its upper portion. Specimens of a hemicrystalline basic andesite,

somewhat scoriaceous, which form the agglomerate of the rocky summit,

have a repellent force of 18°, whilst a similar rock from the

agglomerate of an exposed spur two-thirds of the way up the hill has a

force of as much as 38°. A curious case of polarity is exhibited in a

bare tuff overlooking the Vui-na-Savu River between Rauriko and Vitina.

It is composed of a much weathered whitish trachytic rock, which in

appearance affords no promise of polarity, but has the power of

repelling the magnetic needle 2° to 3°.

* * * * *

_Note on the Average Polarity (Repellent Power) of the Volcanic

Rocks of Vanua Levu._

It would appear from the table given below that the difference in the

average polarity of acid and basic rocks is not very great. The average

for rocks with a specific gravity below 2·7 is about 10°; and that for

heavier rocks is about 14°. The difference mainly lies at the maximum

end of each series, the capacity for extreme polarity being, as before

remarked, markedly greater in the basic rocks.

+-----------+------------------------+------------+-------------------+

| | | | Polarity.[152] |

| Specific | Character of rocks. | Number of +--------+----------+

| gravity. | | specimens. | Range. | Average. |

+-----------+------------------------+------------+--------+----------+

| 2·50-2·59 | Dacites | 6 | 2°-25° | 11·6° |

| | | | | |

| 2·60-2·69 | Dacites and | 5 | 3°-16° | 8·6° |

| | augite-andesites | | | |

| | | | | |

| 2·70-2·79 | Augite-andesites | 5 | 5°-38° | 13·2° |

| | | | | |

| 2·80-2·89 | Basaltic andesites and | 18 | 3°-90° | 15·0° |

| | olivine-basalts | | | |

| | | | | |

| 2·90-3·00 | Olivine-basalts | 14 | 3°-29° | 12·7° |

+-----------+------------------------+------------+--------+----------+

It is, however, noteworthy, as indicated by the value of the average in

each series that not one of them is a good series. They form curves

which in each case present an extreme maximum variant which is

suggestive of quite another degree of magnetising agency. This is also

illustrated in the combined curve of all the results given above. The

acid as well as the basic series are thus characterised, and the extreme

maximum variants are in each instance afforded by the highest mountain

peaks. It is probable that there is an accelerating ratio of

magnetisation with increased elevation. However that may be, it appears

evident from my observations on the two adjacent peaks of Ngaingai and

Navuningumu that the limits of polarity acquired through atmospheric

electricity without the direct action of lightning would be, as measured

by the scale here employed, four times as great for a dacite (25°) as

for a basaltic andesite (90°).

CHAPTER XXVII

SOME CONCLUSIONS AND THEIR BEARINGS

VANUA LEVU is a composite island built up during a long period of

emergence, that began probably in the later Tertiary period, by the

union of a number of large and small islands of volcanic formation

representing the products of submarine eruptions. It differs in this

respect from Viti Levu which is much more massive in its profile and

possesses a greater proportion of plutonic rocks. When, however, Viti

Levu comes to be systematically examined, it is likely that traces of

its composite origin will be found. The evidence seems to show that it

is older than Vanua Levu; but they are both situated on the same

submarine platform, the area of which is nearly equal to the combined

areas of the two large islands that rise from it.

This platform, as indicated in the small plan of the group, is limited

by the 100-fathom line in the charts; but since the reefs on their

seaward slopes plunge down precipitously, such a line practically serves

to delineate the margin of the plateau. It has been my object to show on

previous pages[153] that this submarine platform is a basaltic plateau

built up by submarine lava-flows and incrusted with coral reefs and

their deposits. It has been pointed out that this platform passes

gradually, as it proceeds landward, into the low-lying basaltic plains

that constitute the sea-border in the western part of the island, where

the breadth of the submerged plateau is greatest. The basaltic flows of

the plains often display the almost vertical columns of slightly

inclined flows. Their apparent termination at the sea-border, where they

are in places covered over with submarine deposits, cannot, however, be

accepted as their real limits. According to my view they extend several

miles seaward and form the platform, as is shown in the sections on

pages 62 and 107.

[Illustration:

FIJI ISLANDS.

FROM ADMIRALTY CHART 780, CORRECTED TO 1901.

ALL DEPTHS BEYOND 100 FATHOMS ARE COLOURED BLACK.

HEIGHTS IN FEET, DEPTHS IN FATHOMS.

]

We have in the great basaltic mountain of Seatura, which forms the bulk

of the western end of the island, a probable source of many of these

basaltic flows; and the occurrence inland in the western half of Vanua

Levu of elevated table-lands of basalt like that of Wainunu, which

extend from the centre of the island to near the coast, afford testimony

that the formation of these flows extends over a considerable period of

the island’s history.

It is held by Professor Agassiz that these submarine platforms are the

work of erosion into the flanks of the up-heaved islands.[154] In

Chapter II. it has been pointed out that the eroding agencies are not

actively in operation in our own day, and that there is good reason for

the belief that the process of amalgamation by which Vanua Levu has been

built up during a prolonged period of emergence, is not suspended at the

present time. It is assumed that the uniformity in Nature’s methods has

not been broken. If, however, we have here platforms of erosion, the

coasts of Vanua Levu, as far as my interpretation goes, supply no

evidence of it; and we have to imagine that a period of emergence

extending over a geological age has been followed by a similarly vast

period of erosion without much change in level.

Whatever agencies have been at work, the production of submarine

platforms 10 to 20 miles in width must have been a stupendous operation;

and we shall be obliged to inquire whether plateaux, either submarine or

upheaved, occur in association with volcanic islands in other parts of

the world, and under what conditions they have been formed. At least

four hypotheses have been framed with regard to the submarine platforms

of Fiji. There is first the original theory of subsidence of Darwin; but

Vanua Levu, which presents one long story of emergence, offers nothing

to support this view. There is the growth of a reef seaward on its own

talus, as advanced by Murray. There is the theory of erosion of Agassiz.

There is lastly my own idea of basaltic plateaux incrusted by reefs. We

may therefore inquire as to the evidence afforded by Vanua Levu in

favour of these views. Basaltic flows, in places covered by submarine

deposits, form the low plains at its sea-border, where the platforms are

broadest; and there rises a basaltic mountain of the Mauna Loa type,

occupying most of the western end of the island. No one would be bold

enough to place the limit of these basaltic flows at the water’s edge;

and as is indicated in the sections, they probably extend for miles

under the sea.

If we look for an island which in its extensive palagonite-formations,

in its basaltic table-lands and later basaltic flows, in its huge

mountain-ridges, and in its evidence of submergence, most resembles

Vanua Levu, we seem to find it in Iceland. It is in Iceland, I think,

that we must expect an explanation of many of the puzzling features in

the structure of Vanua Levu.

I pass on now to refer to some of the general points in the geology of

this island, which have been dealt with in detail in the earlier

chapters of this work. With regard first to the distribution of the

volcanic rocks, it may be remarked that my materials do not lend

themselves to making a geological map. The most comprehensive idea of

the principal points in the geological structure will be obtained by

reading the description of the profile given in Chapter I. There is,

however, a method in the distribution of the rocks that may be again

noticed here. The plutonic rocks are very scantily exposed, as is shown

on page 249; and they are not displayed at all in the western half of

the island. The more basic eruptive rocks, the olivine-basalts and

basaltic andesites, are mainly confined to the western half, that is,

west of Nanduri on the north and of the Ndreke-ni-wai River on the

south. Ordinary augite-andesites occur also in the western half; and

together with the hypersthene-augite-andesites they are found over most

of the rest of the island, excluding the north-east portion, east of

Lambasa and Tawaki, where quartz-porphyries, oligoclase-trachytes, and

acid pumice tuffs prevail. The acid andesites, including the

hornblende-andesites and the dacites or felsitic andesites, are best

represented in the Ndrandramea district in the midst of the basic rocks.

They occur in the isolated peaks of Na Raro and Vatu Kaisia and in one

or two other localities, as in the Valanga Range and on the shores of

Natewa Bay in the vicinity of the Salt Lake. These peaks of acid

andesites, as in the instances of Vatu Kaisia and Soloa Levu, are at

times in part overwhelmed or surrounded by the basaltic flows. This

singular feature of bosses of acid rocks in the midst of basaltic fields

offers another point of resemblance between Iceland and Vanua Levu.

The mountain-types vary considerably, the ridge-mountains, however,

being most characteristic of the island. The basaltic mountain of

Seatura, though its lava-flows were evidently in the main submarine,

belongs as before observed to the Mauna Loa type. In its radiating

valleys and gorges and in other characters it recalls the description

given by Dana of the island of Tahiti. The peaks of acid andesites,

represented in the isolated hills and mountains of the Ndrandramea

district, and in the solitary mountains of Vatu Kaisia and Na Raro, are

the necks and stumps of submarine volcanoes dating back to the

pre-basaltic period of the island. It is, however, in the great

mountain-ridges of the central portion of the island, those of Va Lili,

Korotini, Nawavi, Thambeyu, Mbatini, Mariko, &c., that we find, as just

remarked, the most typical features of the internal topography of Vanua

Levu.

Agglomerates overlying palagonite-tuffs and clays, that are usually

foraminiferous and sometimes inclose molluscan shells, clothe the slopes

of these mountain-ridges up to elevations of 2,500 feet and over above

the sea. Most of these great ridges, now more or less covered over by

these submarine deposits, represent lines of submerged vents, of which

only a few raised their summits above the sea in the earliest stages in

the history of the island. At this early period there were no coral

reefs. Some of the ridges present a marked parallel arrangement,

recalling the arrangement of the mountain-ridges and lesser chains of

hills as described by Dr. Johnston-Lavis in the account of his visit to

Iceland.[155] The description of Hekla (as given by Thoroddsen) as “an

oblong ridge which has been fissured in the direction of its length and

bears a row of craters along the fissure,”[156] comes very near to my

conception of the original condition of these great mountain-ridges

before the emergence. Dr. Johnston-Lavis sees in Hekla a type of

volcanic mountain very different from that of Vesuvius and Etna. He

regards it as a ridge marked by a number of parallel ridges and furrows,

and built up along a main fissure with a number of subsidiary parallel

fissures.

The part taken by palagonite in the composition of the finer deposits

over the greater portion of Vanua Levu is another prominent

characteristic of the island. Palagonite, as I have suggested in Chapter

XXIV., is formed probably on the surface of submarine flows of an

ophitic basaltic rock.

The age of the more recent of the deposits of this island, the

fossiliferous tuffs, the pteropod-ooze rocks, and the foraminiferous

muds, cannot be far different from that of the same deposits in other

parts of the group, since it is apparent that the same general movement

of emergence has affected both of the two larger islands. Professor

Martin of Leyden informed Dr. Wichmann that the fossil shells found in

the tuffs of Viti Levu, Ovalau, and other islands were Tertiary but not

older than the Miocene.[157] Dr. Dall, after examining the fossil

mollusks collected by Professor Agassiz from the elevated limestones of

Fiji, confirmed the impression formed by the latter as to their late

Tertiary age. None of the genera were extinct, and the fossils were in

his opinion younger than Eocene and either Miocene or Pliocene.[158] The

Rev. J. E. Tenison-Woods described as extinct Tertiary fossils, some

corals and mollusks from the interior of Ovalau.[159] Mr. H. B. Brady,

basing his conclusions on the character of the foraminifera, assigned a

Post-Tertiary date to the Suva “soapstone” taken at elevations up to 100

feet in that neighbourhood.[160] Professor David referring to some

fossil teeth of Carcharodon and to a fossil Tridacna found at Walu Bay

infers that the deposits are at least as old as Pliocene but not as old

as the earlier Tertiaries.[161] Since, as pointed out by Professor

David, the latest movements of emergence have taken place in recent

geological time, these various observations go to show that whilst the

latest exposure of deposits has occurred in recent time the mass of the

fossiliferous deposits date back to the Pliocene and the Miocene

periods.

According to Wichmann these islands were in a continental condition

during the Palæozoic and Mesozoic periods, and it was only in the later

Tertiary age that the movement of subsidence began that prepared the way

for the formation of the more recent deposits. The submergence during

the Tertiary period and the subsequent emergence are facts that cannot

be gainsaid; but we may ask where is the evidence of the continental

condition during the earlier periods. There is little in the results

obtained from Vanua Levu that directly supports such an hypothesis.

Under such circumstances one ought to have discovered in the deposits of

this island some evidence of this early condition, and there should be

found in the fauna and flora some traces of the original organisms.

According to Hedley there is some indication of a continental condition

in the molluscan fauna, and he quotes Fairmaire as regarding the

Coleoptera as of a continental character; but no one, that I am aware

of, has found any direct evidence of the Pre-Tertiary periods in this

group. It is in harmony with the geological characters to assume that

these islands made their first appearance during the Tertiary epoch.

Coming to the subject of the movements whether of land or sea that led

to the appearance of these islands, we shall not be begging the question

if we speak of their “emergence.” There is no doubt as to there having

been during and since the Tertiary epoch an emergence of some thousands

of feet, allowing for the original depth of the foraminiferous deposits

now found at elevations of over 2,000 feet above the sea. In Chapter II.

it is shown that there is good ground for the belief that these changes

of level have not altogether ceased. Of what nature, we may ask, is this

movement. We have before us the grand conception of Suess that the

emergence of the land in the different phases of geological time has

been produced by the general lowering of the level of the ocean arising

from local subsidences of the earth’s crust. This view in the case of

the recent calcareous formations of the Pacific is applied to the

terraces of the Loyalty Islands;[162] and it follows that it is also

applicable to the elevated calcareous deposits of the islands of the

Western Pacific as a whole, as in the case of the Tongan Islands, the

New Hebrides, the Solomon Group, &c. Such a general change of level

ought to be represented in the large island of Hawaii in the North

Pacific, since it could not be confined to one locality in this ocean.

There is no evidence of emergence, as far as I know, presented by this

island. During my sojourn there, I examined much of its coasts. Now the

antiquity of the flora of this group is sufficiently attested by the

circumstance that it ranks first among the oceanic groups of the Pacific

for the number of endemic plants that it possesses; and the same

conclusions may be drawn from the insects and the birds. There is no

evidence in this group, one of the most ancient of the Pacific

archipelagoes, of that great movement of emergence, which is abundantly

demonstrated over the Western Pacific.

The standpoint is therefore taken that the movement of emergence which

began in the Tertiary period and is probably still in operation is

confined to the southern portion of the tropical Pacific. Speaking of

the time of the Fijian emergence, Professor Agassiz observes that “it is

not unnatural to assume that it was coincident with the elevation of

Northern Queensland, and that the area of elevation included New Guinea,

the islands to the east of it as far as New Caledonia, and as far east

as the most distant of the Paumotus, and extended northward of that line

to include the Gilbert, Ellice, Marshall, and Caroline Islands.”[163]

From the report of Mr. Andrews[164] it is evident that in the Lau

Islands of the Fiji Group volcanic outbreaks have taken place since the

last upheaval. He describes in the case of Mango and other islands the

manner in which cliffs of limestone form inliers in flows of andesitic

lava. In the history of these islands he first distinguishes the period

of calcareous deposits, when the bedded limestones forming the submarine

plateau were laid down. Then followed a period of volcanism during which

masses of volcanic materials were erupted along the axis of elevation.

Alternating epochs of upheaval and stable equilibrium ensued, during the

last of which the reefs grew outwards and formed the terraces now so

characteristic of the profiles of the islands. After the last upheaval

the volcanic forces became again active. There is much of special

interest in the account given by Mr. Andrews of the Lau Group. The

blocks of limestone included in the volcanic agglomerates distinguish

the Lau detrital rocks from those of Vanua Levu. There is no evidence

that coral reefs existed during the early stages of the emergence of

Vanua Levu to be obtained from the submarine formations found on the

higher levels, 1,000 to 2,500 feet above the sea.

The period of emergence for this island may be divided into an earlier

and into a later stage, the last corresponding to the age of emergence

of the Lau Islands. The earlier stage, which may be termed the “Pre-Lau”

stage, is represented by the deposits of the higher slopes of Vanua

Levu, that is above 1,000 or 1,200 feet. This is really the critical

epoch in the history of this group, and assuming that the movement of

emergence has been fairly uniform over the archipelago we cannot but be

astonished at the absence of all traces of ancient reefs in the earlier

stage.

We may infer from the observations of Mr. Lister[165] that the islands

of the Tonga Group represent the Lau stage of the emergence. They are

similar in height and in general geological structure to the islands of

Lau, that of Eua, for instance, which has an elevation of 1,100 feet,

being formed of reef-limestones overlying volcanic tuffs. Dykes

penetrate the tuffs but do not enter the incrusting calcareous strata.

Mr. Harker,[166] after examining the collections of Mr. Lister, remarks

that all the rocks excepting those from Falcon Island appear to be of

submarine formation. The volcanic material, he adds, seems to have been

almost exclusively of fragmental character. It would be rash, it is

remarked, to refer all the rocks to a Recent age, and some of them may

be found to go back far into Tertiary times.

My division of the long period occupied by the emergence of the Fiji

Islands into two stages, the Lau stage corresponding to elevations of

less than 1,000 or 1,200 feet, and the Pre-Lau stage which includes the

earlier evidence of emergence found at heights exceeding these

elevations and ranging up to 2,000 or 3,000 feet, may perhaps be

applicable to other regions of emergence.

As bearing on the question of the isolation and antiquity of the Pacific

Islands the following approximate results for the Hawaiian, Fijian, and

Tongan floras may be here quoted.[167] These data are liable to

correction; but they are near enough to the truth to be very suggestive.

Of peculiar genera of flowering plants and ferns the Hawaiian Islands

possess about 40, the Fiji Group about 16, and the Tongan Islands none.

Of endemic species of flowering plants there are about 80 per cent. in

Hawaii, about 50 per cent. in Fiji, and 3 or 4 per cent. in Tonga.

Granting that there is much to be done yet in the investigation of these

floras, it would be underrating the brilliant results of the labours of

Hillebrand and Seemann to characterise their work as sampling. Let us

suppose, however, that the floras of Hawaii, Fiji, and Tonga have been

only sampled, the data above given would be still reliable. It is quite

possible to obtain a botanical equivalent corresponding to the

geological estimates of the relative ages of these islands; and taking

the proportion of endemic plants as our guide, the Lau stage, as

represented by the Tongan Islands, would have a value of 3 or 4, the

Pre-Lau stage now exhibited in the earliest stage of emergence of Vanua

Levu would have a value of 50, and the Hawaiian stage older than all

would have a value of 80. These results are intended as suggestive and I

hope to work out this subject in the second volume. They make the

problem of the relative antiquity of these islands more mysterious than

it even appeared before.

With regard to the vexed question of the light thrown on the past

condition of these islands by the present state of their floras and

faunas, it may be at once observed that my belief in the general

principle that islands have always been islands has not been shaken by

the results of the examination of the geological structure of Vanua

Levu. In a correspondence in _Nature_ about fifteen years ago it was

suggested by me that this is the position we ought to take with regard

to the stocking with plants of the islands of the Southern Ocean, such

as Kerguelen; and I take the same standpoint for the islands of the

Pacific. If the distribution of a particular group of plants or animals

does not seem to accord with the present arrangement of the land, it is

by far the safest plan, even after exhausting all likely modes of

explanation, not to invoke the intervention of geographical changes. New

possibilities of inter-communication, new ways of looking at old facts,

and new discoveries of an unexpected nature come monthly before us in

the progress of scientific research; and I scarcely think that our

knowledge of any one group of organisms is ever sufficiently precise to

justify a recourse to hypothetical alterations in the present relations

of land and sea.

The hypothesis of a Pacific continent,[168] whether it takes a

trans-oceanic form, as advocated by Von Ihering, Hutton, Baur and

others, or whether it is represented by an island-continent isolated in

mesozoic times, as suggested by Pilsbry, receives no support from the

geological characters of Vanua Levu. Nor can I accept as regards Fiji

Mr. Hedley’s theory of the Melanesian Plateau. There is no evidence that

the various islands of the Fiji Group were ever amalgamated and no

indication of a geological nature that they were ever joined to the

Solomon Group. The Fijis, as we see them, have had an independent

history, and the process at work is not one of disruption but of

amalgamation, lesser islands being united to larger islands during the

prolonged period of emergence. Mr. Hedley, however, has some weighty

data on his side more especially zoological; but even here it would be

wise to suspend one’s judgment. Though the great mass of botanical

evidence is as respects Fiji opposed to such connections, the

distribution of Dammara may, however, be fairly claimed on their behalf.

The dilemma into which such discussions lead us is aptly stated by Dr.

Pilsbry. If we do not accept the hypothesis of a Pacific continent, we

have to explain the cessation of the means of transportal in later

geological times, since this is implied in the isolation necessary for

the development of peculiar characters in a fauna or a flora. This was

the dilemma that presented itself to me in studying the origin of the

Fijian plants. Assuming on geological grounds that the insular condition

had been always maintained I had to explain the differentiation in the

inland plants, or in other words to account for the failure of the means

of transportal that once existed. Since this subject bears directly on

the past condition of the Fiji Islands, I may be pardoned for referring

to it here. It belongs properly to the second volume which it is

proposed to devote to the dispersal and distribution of Pacific plants;

but as I contest the pre-existence of a Pacific continent, it is fitting

though not necessary that this difficulty should be faced at once.

If we in imagination combine in a typical island the characters of the

flora presented by islands of different elevation in the Pacific we get

a result of this kind in an island of the height of Hawaii, nearly

14,000 feet. The littoral plants of such an island are found all over

the coasts of the tropical Pacific, and for the explanation of this fact

we look mainly to the agency of the ocean-currents. The plants of the

mountain summit, belonging to the temperate genera of Geranium, Rubus,

Ranunculus, Vaccinium, &c., are represented at least generically on the

tops of the lofty ranges of the Pacific coasts and in the interior of

the continents; and we find the explanation of the wide diffusion of

such plants in the agency of the migrant birds that at no distant time,

if not actually in our own time, were regular visitors to these mountain

regions. The plants of the marsh, of the stream, and of the pond, belong

often to species that occur in similar stations over a great portion of

the world, such as species of Drosera, Ruppia, Potamogeton, &c.; and

here the agency of wild duck and other waterfowl may be observed in

active operation.

But when in such an island we regard the intermediate region between the

uplands and the coast, usually the forest-zone, we find an area of

change not only for the plants but also for the birds. It is here that

the new genera of plants have been developed that distinguish the floras

of the Pacific groups each from the others; and here also the migrant

bird, having from some cause changed its ways, has given rise to new

varieties and to new species. It is with this loss of the migratory

powers of the birds of the forest-zone that I connect many of the

important differences between the forest-floras of the different groups

of the Pacific. At one time, it would seem, birds were far more active

agents in dispersing seeds and fruits over these archipelagoes than they

are at present; but it is not held that this is concerned with the

extermination and extinction of the birds of these islands in the

present day. The change dates far back and is far-reaching in its

effects. It is assumed in this argument that the alpine plant and the

plant of the pond and of the sea-shore preserve their characters by

reason of the means of free dispersal that they still enjoy; and it is

inferred that the plant of the forest-zone has varied more because

opportunities of transportal of its kind no longer are afforded. Many a

line of ancient migration is now broken.

It is suggested that in the past when birds were more generalised in

type they were much more migratory in habits and that limitation of

range has been associated with specialisation. The plants dispersed by

the birds have undergone a parallel series of changes. At first widely

distributed, as in the more generalised types of birds, they became

localised in proportion as the birds to which they owed their means of

dispersal lost their migratory ways; and both plant and bird began to

vary. There is, I am convinced, a profound connection between birds and

plants, of which we now perceive only the last of a long series of

changes. This subject will be dealt with at length in the volume on

plant-dispersal; and it is only referred to here to illustrate my

contention that we have yet much to learn before it would be safe to

look to hypothetical changes of sea and land to explain difficulties in

distribution.

APPENDIX

_Note on the Stone-Axes._—Two of these polished stone-axes from a

collection made in Vanua Levu were selected for sections. One is

light-green and smooth. The other has a very different appearance, being

blackish and rather rough, its smooth surface having been apparently

lost by lying in a stream-course or in wet ground for a long period.

Both, however, are made of the same type of basaltic rock, the specific

gravity in one case being 2·93, in the other 2·97. It is an aphanitic

basalt with scanty olivine containing little or no residual glass and

referred to genus 40 of the olivine-basalts. It is by no means a common

type of basalt in Vanua Levu, and I cannot refer it to any particular

locality on account of the peculiarities it presents when contrasted

with rocks of the same genus. The olivine is very scanty and small, and

in one of the specimens is represented only by pseudomorphs. The

felspar-lathes vary usually from ·05 to ·2 mm. in length, and the augite

granules which are very abundant are ·01 or ·02 mm. in diameter. There

is an occasional small phenocryst of augite. The rock shows little or no

alteration and cannot be characterised as a greenstone. The greenish hue

of one axe is due to weathering; but its extension into the internal

black portion of the tool is not appreciable.

* * * * *

_Note on the ascent of the tide up the Ndreketi River._—On July 20th and

21st, 1899, by observing the surface density it was ascertained that at

high-water the sea-water reached Navundi a mile or two below Mbatiri. At

low-tide it reached about half-way between Kanathangi and Navundi. The

moon was in her quarters.

* * * * *

_Note on the “talasinga” districts._—This subject will be discussed in

the second volume.

INDEX

NOTE.—It has been deemed best to follow the example of the Admiralty

surveys in the spelling of native names. In this book, therefore—

Mb = the Fijian B

Th = the Fijian C

ND = the Fijian D

NG = the Fijian G and Q

Abbreviations, for rock descriptions, 236

Acicastello, Sicily, basalt and palagonite, 347

Acid and basic rocks, regions of, 219, 374

Agassiz, Prof. A., 294, 373, 376, 377

Agates, 138, 139, 227, 353, 354

Agglomerates; _see_ Volcanic agglomerates

Algæ in hot springs, 24, 25, 33, 38

Alps, magnetic rocks of the, 362, 363

Altered rocks; _see_ Propylites, etc.

Andesites, acid, 98-108, 112, 123-127, 193, 194;

relative frequency of, 235;

classification of, 293-306;

distribution of, 374;

peaks in basaltic flows, 104, 115, 116, 374;

altered, 105, 297;

columnar, 102, 104;

pre-basaltic, 375