(_a_) Porphyritic, where the average siz_Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 Vanua Levu, Fiji

phenocrysts

is 3 mm. and over.

(_b_) Non-porphyritic, where the size is less than 3 mm.

A. PORPHYRITIC SUB-GENUS

DESCRIPTION.—Coarse-looking brownish-black porphyritic rocks displaying

large plagioclase crystals that often show a play of colours. Their sp.

gr. is about 2·8. None of the rocks in my collection are vesicular. On

account of the considerable porphyritic development of the plagioclase,

the groundmass is relatively diminished, the large phenocrysts occupying

about a third of the mass. They form ancient basaltic flows more

especially in the vicinity of the isolated hills and mountains of acid

andesite, as around Vatu Kaisia; whilst they may enter into the

formation of the low basaltic plains as in the region west and south of

the Ndreketi River. They are, however, limited in their extent and

occurrence. From the large amount of glass in the groundmass, they may

be inferred to belong to flows formed under different conditions from

those under which the great basaltic plateaux were formed, where the

rock contains but scanty interstitial glass.

In the slide they show the large plagioclase phenocrysts together with a

few small plates of ophitic augite in a groundmass displaying in an

abundant smoky glass a loose plexus of long stout lathe-like plagioclase

prisms partly wrapped around by lesser augites.... The plagioclase

phenocrysts, which attain a size of 4 to 6 mm., give lamellar

extinctions of basic andesine (20°-27°) and show concentric zone-lines

with transmitted light. They often polarise in brilliant colours and are

extensively cross-macled. They contain usually abundant inclusions of

the magma sometimes arranged zone-wise, and are frequently eroded....

Non-ophitic pyroxene phenocrysts are uncommon. In the slide occur one or

two small “plates,” 1 to 2 mm. in size, of ophitic pale-brown augite,

and a number of lesser augites, ·2 to ·3 mm. in size, which in part wrap

around the felspar-lathes and by their aggregation form imperfect

ophitic “plates.”... The long stout felspar-lathes, which are on the

average ·3 to ·45 mm. in length, give lamellar extinctions of 15° to 20°

(medium andesine).... The copious smoky glass is rendered partially

opaque by the abundant development of rods and skeletal crystals of

magnetite, and shows the fibrous devitrification arising from the

formation of incipient microliths. In some rocks there appear in the

smoky glass brownish-yellow patches of the residual magma which under

the microscope cannot be distinguished from palagonite.

All but one of the specimens belong to the species where the

felspar-lathes average over ·3 mm. in length.

B. NON-PORPHYRITIC SUB-GENUS

DESCRIPTION.—Blackish-brown semi-ophitic rocks, sp. gr. 2·74-2·77,

frequently of doleritic texture and showing a few small macroscopic

plagioclase phenocrysts. They are sometimes vesicular, and form old

flows in a few localities, as in the vicinity of Natua in the eastern

part of the Ndreketi plains and in the coast district between Lekutu and

Wailea Bay.

They display in the slide small plagioclase phenocrysts, often abundant,

in a groundmass exhibiting a loose plexus of large lathe-shaped felspar

prisms, together with occasional small ophitic “plates” of augite and

numerous smaller semi-ophitic augites, whilst there is much interstitial

smoky glass.... The plagioclase phenocrysts are as a rule 1 to 2 mm. in

size and do not exceed 3 mm. They often contain abundant inclusions of

the magma sometimes arranged schiller-fashion, and are frequently

eroded. Their lamellar extinctions (15°-30°)indicate andesine

labradorite.... The stout plagioclase lathes, which in most of my

specimens range between ·2 and ·3 mm., and contain at times

magma-inclusions, give the rocks their doleritic texture.... The

occasional small ophitic “plates” of pale augite are not over 1 mm. in

size and give extinctions of +30° from the single cleavage-lines. The

lesser augites, ·2 mm. in size, are several times larger than typical

granular pyroxenes (·02-·03 mm.), and adapt their form to the

interspaces of the felspar-lathes which they in part invest.... The

copious interstitial glass is generally smoky and sometimes quite opaque

through the deposition of magnetite. It is never clear and isotropic,

but displays fibrous devitrification and is usually a little altered.

These rocks come near to those of the previous sub-genus in several

respects, but they differ conspicuously in their non-porphyritic

character, in being sometimes vesicular, and in their general

appearance. All the four species indicated by the length of the

felspar-lathes are here represented; but the doleritic types with the

lathes exceeding ·2 mm. are the most frequent.

10. GENUS OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, non-flu, oph, phen, opac._

CHARACTERS.—In the groundmass the felspar-lathes and prisms are not in

flow-arrangement and the augite is ophitic or semi-ophitic. The

plagioclase phenocrysts are opaque.

This genus may be divided into two sub-genera, porphyritic and

non-porphyritic, according to the average size of the plagioclase

phenocrysts, whether above or below 3 mm.

A. PORPHYRITIC SUB-GENUS.—This is represented by a light grey

porphyritic rock, with sp. gr. 2·75, from the lower part of Mount

Freeland. It comes near to the porphyrites, and displays large opaque

white phenocrysts of plagioclase 5 or 6 mm. long. It is a somewhat

altered rock.

In the thin section it displays the plagioclase phenocrysts in a ground

mass of doleritic and semi-ophitic texture showing a plexus of long

felspar-lathes partly invested by small augites with a fair amount of

altered greenish opaque interstitial glass.... The plagioclase

phenocrysts, which give extinctions of 11° to 15° (acid andesine), are

traversed by a network of fine cracks and contain a quantity of

colourless dust-like inclusions and alteration-products. They are long

and rectilinear in outline and are not much affected by the magma....

The felspar-lathes, which average ·26 mm. in length, are occasionally

lamellar when the angle of extinction indicates acid andesine. Like the

phenocrysts they contain many dust-like inclusions.... The augite may at

times form an aggregate phenocryst of small size; but usually it occurs

as semi-ophitic masses ·1 mm. in diameter.

B. NON-PORPHYRITIC SUB-GENUS.—The specimen representing this group is a

coarse-grained greyish altered rock, sp. gr. 2·81, found in blocks near

Waikatakata (p. 204), showing small somewhat opaque plagioclase

phenocrysts. In the section these phenocrysts are displayed in numbers

together with a few of augite. The groundmass, doleritic in texture in

this species, displays a plexus of long stout felspar-lathes with

numerous semi-ophitic lesser augites, chloritic pseudomorphs after

pyroxene, and scanty interstitial altered glass.... The plagioclase

phenocrysts, 2 to 3 mm. in size, give extinctions (22° to 28°) of basic

andesine. They are traversed by many fine cracks and contain an

abundance of colourless dust-like inclusions, apparently altered magma

products.... The small pyroxene phenocrysts consist of aggregates of

smaller crystals of pale augite. The lesser augites, ·1 mm. in size,

partly invest the felspars.... The broad felspar-lathes, which average

·25 mm. in length and give lamellar extinctions of medium andesine

(15°), contain inclusions similar to those of the phenocrysts.... The

scanty interstitial glass is converted into viriditic and chloritic

materials. Secondary calcite also occurs here and in the chloritic

pseudomorphs.

The only species represented is that with felspar-lathes ·2 to ·3 mm. in

length.

12. GENUS OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, non-flu, oph, non-phen, parv._

CHARACTERS.—In the groundmass the felspar lathes and prisms are not in

flow-arrangement and the augite is ophitic or semi-ophitic. No

plagioclase phenocrysts. Augite phenocrysts when present less than 2 mm.

in size.

DESCRIPTION.—These rocks come near to the non-porphyritic group of genus

9; but differ in the absence or rarity of plagioclase phenocrysts, in

their more frequently vesicular and scoriaceous character, and in the

fresher condition of the rock. Their sp. gr. is about 2·77. They present

themselves usually as blackish-brown doleritic rocks and form ancient

flows in the coast-plains, sometimes exhibiting a columnar structure as

in the crossing of the Ndreketi above Mbatiri. They are, however, of

limited occurrence and are mostly represented in the Ndreketi plains and

in the district between the Lekutu River and Wailea Bay.

Typically they display in thin sections no phenocrysts either of

plagioclase or of pyroxene, and exhibit a plexus of usually long stout

felspar-lathes partly invested by the lesser augites in a copious

smoky glass.... The felspar-lathes, ·25 to ·4 mm. in length, give

lamellar extinctions of 10° to 20° (andesine) and contain a few

magma-inclusions.... The semi-ophitic augites, ·1 to ·2 mm. in size,

are sometimes twinned.... The smoky glass polarises feebly and

displays dark feathery aggregates of microliths. Within it are

brownish-yellow semi-opaque patches of residual glass, which polarise

faintly and behave like palagonite.

The two species with felspar-lathes ·2 to ·3 mm. and ·3 to ·5 mm. are

represented in my collection.

4. GRANULAR SUB-ORDER OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, flu, gran._

13. GENUS

FORMULA.—_Aug, matr, flu, gran, phen, vitr._

CHARACTERS.—In the groundmass the felspar-lathes or prisms are in

flow-arrangement and the augite is granular. The plagioclase phenocrysts

are glassy.

DESCRIPTION.—This genus readily splits up into two sub-genera, the

non-porphyritic, where the plagioclase phenocrysts are less than 3 mm.

in size, and the porphyritic where they are larger.

1. NON-PORPHYRITIC SUB-GENUS.—Dark-brown or blackish rocks displaying

small plagioclase phenocrysts, usually only 1 or 2 mm. in size. Three of

the four species defined by the length of the felspar-lathes are

represented in my collection.

SPECIES A.—Felspar-lathes .02-.1 mm. in average length. This may again

be sub-divided according to the degree of basicity of the rocks:—

(_a_) _Sub-species of greater basicity._—Sp. gr. 2·76 to 2·82.... Such

rocks are represented in dykes and in the prevailing basic agglomerates.

They are at times scoriaceous. The small plagioclase phenocrysts, which

are fairly numerous, give lamellar extinctions of andesine labradorite

(20 to 30°). Two kinds occur which may or may not be represented in the

same slide. In the one the crystal is much corroded and contains

abundant magma-inclusions. It belongs in such a case to an earlier

period. In the other the outlines are clean and regular, and the crystal

is often cross-macled to such an extent that it may be inferred from its

unbroken condition to have been formed _in situ_. Augite phenocrysts

when present are small and scanty, pale-yellow, idiomorphic, and giving

extinctions of +30°. The felspar-lathes, which average ·06-·08 mm. in

length, give extinctions indicating andesine labradorite. The augite

granules are small (·01-·02 mm.). Interstitial glass, generally scanty,

is sometimes abundant when it is smoky, showing fibrous devitrification,

with irregular “lacunæ” filled with a brownish yellow opaque glass like

palagonite.

(_b_) _Sub-species of lesser basicity._—Sp. gr. 2·65-2·70.... The

remarks on the plagioclase phenocrysts of the more basic sub-species

here apply, except that the lamellar extinctions indicate medium

andesine (12°-20°). The characters of the augite phenocrysts and

granules are in both groups the same; but in this case there is more

frequently a suspicion of intergrowth with rhombic pyroxene. The

felspar-lathes are very small, ·04 or ·05 mm, and give simple

extinctions of acid andesine (5-10°). Interstitial glass exists in

moderate amount.

SPECIES B.—Felspar-lathes ·1-·2 mm. in average length.

Blackish or dark-grey rather compact rocks, sp. gr. 2·75-2·79, that

cannot be readily divided into groups according to their basicity. They

form dykes and volcanic “necks” and are sometimes scoriaceous. The small

plagioclase phenocrysts, which are most evident in the slide, present

the two kinds above described under Species A. They give lamellar

extinctions varying from those of medium andesine to acid labradorite

(15-30°). The augite phenocrysts, which are small and scanty,

occasionally show intergrowths of rhombic pyroxene. The augite granules

are generally ·02 to ·03 mm. in size, and here and there a prism form

gives extinctions of +25°. The felspar-lathes which average ·11 to

·15 mm. long, are often rather stout, showing a few lamellæ that give

extinctions of medium and basic andesine. Interstitial glass occurs in

fair amount.

SPECIES C.—Felspar-lathes ·2-·3 mm. in average length.

Blackish rocks with sp. gr. 2·75-2·84. The description of Species B

applies here. The plagioclase phenocrysts are for the most part

microporphyritic. The size of the augite granules is as above given.

2. PORPHYRITIC SUB-GENUS.—This group of rocks is mostly confined to the

slopes and vicinity of Mount Seatura in the western part of the island,

being prevalent in the Mbua and Ndama plains, and occurring also as

dyke-rocks in the Nandi Gorge leading into the Ndriti Basin, and at and

near the coasts of Wainunu Bay between the Tongalevu and Wainunu rivers.

They come near in appearance to the porphyritic forms of the blackish

olivine-basalts belonging to genera 13, 25, and 37 of the olivine rocks;

but they differ in the absence of that mineral, in their lower density,

and in other characters. They are the type to which the term

“porphyritic basaltic andesite” is most frequently applied in the text

when the ophitic structure is not displayed.

They are blackish rocks having a specific gravity of 2·71 to 2·81 and

exhibiting large porphyritic crystals of plagioclase, but they vary in

their minute structure on account of the different size of the felspars

of the groundmass. Those forming dykes in the Nandi Gorge are often more

or less propylitic in character. The felspar-lathes, which have an

average length of ·2 to ·3 mm., sometimes show a few lamellæ giving

extinctions of medium andesine (12°-20°). The plagioclase phenocrysts of

the same andesine are 3 to 5 mm. in size. They are eroded and contain

abundant magma inclusions. There are a few small phenocrysts of pale

brown augite. The augite granules are ·03 or ·04 mm. in diameter, and

there is a little dark opaque residual glass.

The rocks of the Mbua and Ndama plains have a specific gravity of 2·81.

The plagioclase phenocrysts, which yield extinctions of basic andesine

(21-27°), are sometimes a centimetre in length. They are traversed by

cracks filled with dark altered glass or occupied by brownish films. The

felspar-lathes, which average ·11 mm. in length, are often stout and

lamellar and give extinctions like the phenocrysts. Augite phenocrysts

are either absent or scanty; whilst the granules average ·02-·03 mm. in

size. There is usually a little interstitial glass.

A rock, almost holocrystalline and 2·74 in density, which was obtained

from the Tongalevu district in Wainunu Bay, approaches the orthophyric

type in the character of the ground mass. The felspars are short

(·06 mm.) and stout, and yield lamellar extinctions of oligoclase

(5°-10°). The plagioclase phenocrysts are of basic andesine. Amongst the

granules (·025 mm.) of pale brown augite occur prismatic forms giving

oblique extinctions of +30°.

14. GENUS OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, flu, gran, phen, opac_.

CHARACTERS.—In the groundmass the felspar-lathes are in flow-arrangement

and the augite is granular. The plagioclase phenocrysts are opaque.

DESCRIPTION.—Only two rocks are referred to this genus. One which is

dark grey with a specific gravity of 2·72 is exposed in the gorge of the

Mbutu-mbutu River below the falls of Na Savu. Flow-arrangement is

displayed both by the felspar phenocrysts and lathes. The phenocrysts, 2

to 3 mm. in size, owe their opacity to the abundance of inclusions of

brown glass. They are corroded and give extinctions of acid labradorite

(26-32°). Pyroxene phenocrysts are scanty and small. The groundmass has

a characteristic “pilotaxitic” appearance, the densely packed

felspar-lathes averaging only ·05 mm. in length, whilst the pyroxene

granules are ·01 mm. in size. Residual glass scanty.

The other rock is from the range behind Sueni. It shows large

porphyritic crystals (5 or 6 mm.) of medium andesine which contain

magma-inclusions in abundance. The average length of the felspar-lathe

is ·08 mm. and the size of the augite granules is ·02 mm. There is but

little glass. The rock is somewhat altered.

16. GENUS OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, flu, gran, non-phen, parv_.

CHARACTERS.—In the groundmass the felspar-lathes are in flow-arrangement

and the augite is granular. Plagioclase phenocrysts are absent, or if

present very scanty and not usually over 1 mm. in size. When present the

augite phenocrysts are under 2 mm.

DESCRIPTION.—This is a very extensive genus, admitting considerable

variation and including most of the aphanitic augite-andesites, where

the felspar-lathes are as a rule very small (under ·1 mm. in length), as

well as some of the doleritic types where they are very large (·2 to

·4 mm. long). In assigning a rock a place in this genus some regard must

be paid to its macroscopic aspect as well as to the presence or absence

of plagioclase phenocrysts. In many cases two or three small phenocrysts

may be observed in a slide, under a millimetre in size; but they do not

give a character to the naked-eye appearance of the rock, and such rocks

cannot be distinguished from others that do not display them.

These rocks range in specific gravity from 2·55 to 2·85. This large

range is in the main concerned with different degrees of basicity

depending on the character of the plagioclase, the relative abundance of

the augite granules, &c.; but it is also connected with the amount of

interstitial glass. The variety of plagioclase ranges between oligoclase

and andesine labradorite. The fluidal structure is nearly always

well-marked, and the closely packed felspar-lathes have often the

peculiar “felted” appearance of many andesites. A little interstitial

glass is present in most rocks.

Many, perhaps nearly all, of the rocks belong either to dykes or to

larger intrusive masses. All the four species indicated by the length of

the felspar-lathes are represented in my collection, especially the two

with smallest felspars. They may again be split up into two sub-species

according to the degree of basicity of the rocks.

SPECIES A.—Felspar-lathes between ·02 mm. and ·1 mm. in average length.

(1) _Most basic sub-species_.... Sp. gr. 2·75-2·85. Dark-brown and

dark-grey compact aphanitic rocks showing no plagioclase phenocrysts to

the eye. When a few of these phenocrysts are present in a slide they are

not usually much over 1 mm. in size, and give extinctions of andesine

labradorite (20° to 30°). Augite phenocrysts are often absent, and when

present are not over 1 mm. in size and are as a rule scanty,

occasionally affording a suspicion of inter-growths with rhombic

pyroxene. The felspar-lathes which display marked flow-structure, vary

in average length in different rocks from ·05-·08 mm. Lamellar twinning

is rare, the extinctions being those of oligoclase andesine (10° to

20°). The usual extinction, as measured from the long axis of the lathe,

is 10° to 15° (medium andesine). The augite granules are small

(·01-·02 mm.) and abundant. There is generally a little interstitial

glass with small magnetite.

(2) _Least basic sub-species_.... Sp. gr. 2·55-2·75. Dark compact

aphanitic rocks especially characteristic of the Ndrawa district. When

plagioclase phenocrysts are present, they are very scanty and not

generally over a millimetre in size, possessing rectangular clean

outlines and showing but few inclusions. They may display carlsbad

twinning and zoning, or albite twinning, when they give extinctions of

oligoclase andesine (10°-15°). Pyroxene phenocrysts are either absent,

or scanty and small, being usually of pale yellow augite with occasional

indications of intergrowth with rhombic pyroxene. The felspar-lathes as

a rule average ·07 or ·08 mm. and present a dense fluidal arrangement.

They rarely display lamellar twinning and give extinctions measured from

the long axis of oligoclase and oligoclase andesine (2°-8°). The

pyroxene granules are very small, averaging ·01 mm. and less. There is

also fine magnetite. A little interstitial glass is usually present.

When abundant it is not generally smoky but shows clear fibrous

devitrification.... One of the specimens, which is semi-vitreous,

exhibits tube-like steam-pores drawn out to a length of 1-1½

centimetres. The felspar microliths are only ·02 mm. in length. The

copious glass has the character above described.

SPECIES B.—Felspar-lathes ·1-·2 mm. in average length.

This species may also be sub-divided into two sub-species (more basic

and less basic). Since, however, all but one of the fifteen rocks

belonging to the species are of the more basic kind my remarks will

mainly apply to them. They are dark-brown or dark-grey compact aphanitic

rocks, occasionally banded or streaky, in appearance, and ranging in

specific gravity from 2·75 to 2·84. They occur in several districts, but

are especially characteristic of the Ndrawa district. The plagioclase

phenocrysts, if present, are very scanty and small (1 or 1½ mm.). They

contain inclusions of the magma and give lamellar extinctions of

andesine labradorite (20°-30°). Pyroxene phenocrysts do not generally

occur. When present, they are small and of pale yellow augite yielding

large extinctions. Occasionally micro-porphyritic augite is well

represented. The felspar-lathes, which exhibit a well-marked

flow-arrangement, are generally ·13 to ·15 mm. long. Lamellar twinning

is uncommon, the extinctions measured from the long axis indicating

basic andesine (10°-20°). The augite granules are abundant and small

(·01-·02 mm.). Occasional prism-forms yield large extinctions. Magnetite

is abundant, its grains corresponding in size to the augite granules.

There is as a rule a little residual glass, which shows fibrous

devitrification and is not smoky. The banded appearance of some of the

rocks arises from the glass collecting in streaks rudely parallel and

running in the direction of the “flow” of the felspar-lathes.

The only specimen in my collection of “sliced” rocks belonging to the

less basic sub-species is an altered bluish-grey rock (sp. gr. 2·7) from

the range between the Mbuthai-sau valley and the Wainikoro plains. Its

long parallel untwinned felspar-lathes give the nearly straight

extinctions of oligoclase. Fine cracks in the rock are filled with

crystalline silica.

SPECIES C, felspar-lathes ·2-·3 mm. long.

SPECIES D, felspar-lathes ·3-·5 mm. long.

The rocks of these species in the collection are for the most part

dyke-rocks of the more basic kind. They are blackish or dark-brown,

almost doleritic in texture, and range in specific gravity from 2·77 to

2·87. At times they are vesicular or scoriaceous, as in the specimens

from an agglomerate at Undu Point and from a flow or dyke at Vunikondi.

The most typical of these rocks are those of some of the dykes of the

Ndriti basin, which, however, display propylitic alteration in a varying

degree. They would be described as semi-doleritic basalts without

olivine or as non-porphyritic basaltic andesites. Plagioclase

phenocrysts are typically absent, or they are scanty and not over 1 mm.

in size. Augite phenocrysts are usually scanty and small. The

felspar-lathes, which are more or less in flow-arrangement, are rather

stout, and range in average length in different rocks from ·23 to

·35 mm. They often show a few twin-lamellæ which yield extinctions of

medium and basic andesine (15-28°). The augite granules are large

(·03 mm.) in the Ndriti rocks. Magnetites, usually corresponding in size

to the augite granules, are abundant. Interstitial glass occurs often in

fair quantity and is dark and semi-opaque.

At times there can be recognised a later generation of minute felspar

microliths between the much larger lathes. They display a plexus rather

than a flow-arrangement. Whilst the larger parallel lathes of the

Vunikondi rock, above referred to, average ·23 mm. long, the felspar

microliths of the interspaces average only ·03 mm. The significance of

these two crops of felspars in the groundmass is discussed on page 237.

The only rock of the less basic sub-species in my collection is from a

dyke near Vatua-karoa. It shows secondary calcite and viridite and other

evidences of the propylitic change. The felspar-lathes, which average

·3 mm. in length, give extinctions of oligoclase (0-5°). The specific

gravity is 2·72.

5. SUB-ORDER, PRISMATIC, OF THE AUGITE-ANDESITES

(Felspar-lathes in flow-arrangement. _Aug, matr, flu, prism_.)

This sub-order includes dark-brown or blackish semi-vitreous rocks, all

but one of which belong to the genus below described. Since the

exception (which belongs to genus 17 of the synopsis) differs only in

the presence of plagioclase phenocrysts, its separate description is not

needed.

20. GENUS OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, flu, prism, non-phen, parv._

CHARACTERS.—In the groundmass the felspar-lathes are in

flow-arrangement, and the augite is prismatic. Plagioclase phenocrysts

are absent or are very small and scanty, and pyroxene phenocrysts when

present do not exceed 2 mm. in size.

DESCRIPTION.—These dark semi-vitreous rocks occur in agglomerates and as

“necks” and dykes, and are at times scoriaceous. They are usually

compact and aphanitic, showing few if any plagioclase phenocrysts and

having a semi-conchoidal fracture. The specific gravity varies, being

generally 2·6-2·65, but according to the degree of basicity and amount

of glass it may be as low as 2·5 or as high as 2·77. In the less glassy

condition, as in the case of a rock from the ridge east of Na Raro, the

felspar-lathes are relatively scanty and the groundmass is mainly formed

of augite prisms in flow-arrangement. The lathes are generally small,

less than ·1 mm., and rarely over ·2 mm. Their extinctions are those of

oligoclase and acid andesine. The pyroxene prisms, which give the large

extinctions of augite, have a pale muddy-brown hue and are as a rule

·03-·07 mm. long. Granular pyroxene, if present, is subordinate in

amount. The glass, which is always in good quantity and is sometimes

abundant, displays fibrous devitrification. In a rock from the vicinity

of Narengali a variolitic structure is exhibited in the form of

sheaf-like aggregates of fibre-like felspars and skeleton prisms of

pyroxene.

6. SUB-ORDER, OPHITIC, OF THE AUGITE-ANDESITES

(Felspar-lathes in flow-arrangement. _Aug, matr, flu, oph._)

21. GENUS OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, flu, oph, phen, vitr._

CHARACTERS.—In the groundmass the felspar-lathes are in flow-arrangement

and the augite is ophitic or semi-ophitic. Glassy plagioclase

phenocrysts.

DESCRIPTION.—Dark rocks, sp. gr. 2·76-2·8, forming ancient flows and

displaying at times a columnar structure as at Yanutha Point (page 123).

The ophitic character is only in part developed, which may be connected

with the flow-arrangement of the felspars. These rocks come near to the

blackish ophitic basalts with scanty olivine (genus 33 of the olivine

sub-class).

They all belong to the non-porphyritic sub-genus where the plagioclase

phenocrysts are less than 3 mm. in size. These phenocrysts, which often

contain abundant magma-inclusions, give extinctions of andesine

labradorite (20°-30°). The augite phenocrysts are small and composite in

character as often happens with these ophitic rocks. They sometimes

invest the smallest felspar phenocrysts, and occasionally display

intergrowths of rhombic pyroxene. The felspar lathes are ·1 to ·14 mm.

in length, and give extinctions of medium and basic andesine. The augite

granules are large (·02-·06 mm.), and tend to wrap around the lathes.

Interstitial glass exists in fair amount.

CHAPTER XX

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

HYPERSTHENE-AUGITE CLASS

II. SUB-CLASS. HYPERSTHENE-AUGITE-ANDESITES

FORMULA.—_Plag, hypersth-aug, matr._

CHARACTERS.—The pyroxene phenocrysts usually are represented by separate

crystals of the monoclinic as well as the rhombic type, and the two

forms are often associated in the same crystal. The monoclinic form

prevails in the groundmass in most cases.

REMARKS.—It is not possible to draw a sharp line between the augite and

the hypersthene-augite-andesites; but where two or three phenocrysts of

the rhombic type occur in a slide the rock may be placed in this

division. Between this variety and that where rhombic pyroxene prevails,

both among the phenocrysts and in the groundmass, numerous intermediate

kinds exist. These rocks mostly occur in agglomerates and form small and

large dykes or sills, but rarely are found in flows. They are

distributed over most of the island except in the western portion (the

basaltic districts of Wainunu, Seatura, and Solevu), but reappear again

in the Mbua peninsula in places, as at Mount Koroma.

The pale yellow rhombic pyroxene is uniform in its optical behaviour.

The prisms are noticeably pleochroic, being nearly colourless when lying

across the long axis of the lower nicol and pale yellow when parallel

with it. The intergrowths with monoclinic pyroxene often take the form

of lamellar bands, whilst in some cases a nucleus of the one (usually

rhombic) is invested by a growth of the other.

1 SUB-ORDER, GRANULAR, OF THE HYPERSTHENE-AUGITE-ANDESITES

(_Felspar-lathes not in flow-arrangement._)

FORMULA.—_Hypersth-aug, matr, non-flu, gran._

1 genus (_Vitr._) }

2 " (_Opac._) } See Synopsis, p. 247.

3 " (_Magn._) }

4 " (_Parv._) }

Nearly all of the rocks of this sub-order that are represented in my

collection belong to the genus (1) with phenocrysts of glassy

plagioclase. They vary considerably in appearance and in colour (black

to grey), and occur under very different conditions, as in “necks,” old

flows, large intrusive masses, dykes, agglomerates, &c. Their specific

gravity has rather a wide range according to the degree of basicity. In

the heavier rocks where the rhombic pyroxene is scanty, it is usually

2·7 to 2·8. In the others, where rhombic pyroxene is more predominant

and where the felspar is less basic, it is 2·6 to 2·7.

In the slide small phenocrysts of plagioclase and pyroxene occur in a

groundmass of felspar-lathes and pyroxene granules, whilst there is as a

rule a fair amount of residual glass. The plagioclase phenocrysts, which

give extinctions in different rocks of acid and basic andesine and

contain abundant magma-inclusions, are generally one to two mm. in size.

The pyroxene phenocrysts are small, and may be represented by separate

crystals of the monoclinic and rhombic kinds, or by crystals displaying

intergrowths of the two sorts. The pyroxene granules vary much in size

and are evidently in great part of augite. In most of the rocks the

felspar-lathes are less than ·1 mm. in length. In those where the length

is ·1 to ·2 mm. they are sometimes stout and display a few lamellæ,

yielding extinctions corresponding to those of the phenocrysts.

A singular dark grey almost holocrystalline doleritic rock (sp. gr.

2·85) is exposed in the Thulanga Ridge (p. 211). It shows no plagioclase

phenocrysts, but those of pyroxene are numerous, which, however, do not

exceed 2 mm. in size, so that the rock would be referred to genus 4 of

this sub-order. It appears to be a doleritic form of the plutonic rock

found at Nawi in this neighbourhood (p. 211). The pyroxene phenocrysts

are mostly of brownish-yellow augite, but rhombic pyroxene, either as

separate crystals or as intergrowths, is not uncommon. The plagioclase

lathes are long and fairly stout, giving at times lamellar extinctions

of 20°. Their average length is ·3 mm., and it is to their large size

that the doleritic texture is due. The pyroxenes of the groundmass are

similarly coarse (·2 mm.), and include both monoclinic and rhombic

forms, the latter infrequent. There is a slight tendency to semi-ophitic

behaviour in places; but generally these pyroxenes are irregular in

shape or rudely prismatic.

2. SUB-ORDER, PRISMATIC, OF THE HYPERSTHENE-AUGITE-ANDESITES

(_Felspar-lathes not in flow-arrangement._)

FORMULA.—_Hypersth-aug, matr, non-flu, prism._

5 genus (_Vitr._) }

6 " (_Opac._) } See Synopsis.

7 " (_Magn._) }

8 " (_Parv._) }

This sub-order includes rocks varying much in appearance, but all alike

in the presence of prismatic pyroxene in the groundmass and in the

absence of flow-arrangement of the felspar-lathes. They belong to the

two genera, with glassy and opaque plagioclase phenocrysts. These

crystals are not usually over 2 mm. in size and are of medium andesine

(15°-20°). The pyroxene phenocrysts are small and may be entirely of

monoclinic or of rhombic pyroxene, or the two may be associated either

as lamellar intergrowths, or by displaying an eroded nucleus (generally

rhombic) around which a regular crystal of monoclinic pyroxene has

grown. The felspar-lathes are in some cases less than ·1 mm. long, and

in others ·1-·2 mm. The pyroxene prisms of the groundmass average

·01-·04 mm. in length, and give both straight and oblique extinctions,

the last prevailing. The specific gravity ranges from 2·55 to 2·75

according to the degree of basicity and amount of interstitial glass,

which is usually in fair quantity.

4. SUB-ORDER, GRANULAR, OF THE HYPERSTHENE-AUGITE-ANDESITES

(_Felspar-lathes in flow-arrangement._)

13. GENUS

FORMULA.—_Hypersth-aug, matr, flu, gran, phen, vitr._

CHARACTERS.—Glassy plagioclase phenocrysts.

DESCRIPTION.—This is a group of rocks that comes near the basaltic

andesites represented in genera 1 and 13 of the augite sub-class; and to

the more basic kinds the terms of basaltic andesite is equally

applicable. These rocks, however, differ in the prevalence of rhombic

pyroxene, which occurs as phenocrysts, but always accompanied by

monoclinic pyroxene, whether as separate crystals or as inter-growths.

Such rocks are intermediate between those of the augite and rhombic

pyroxene classes. They are particularly characteristic of the Savu-savu

peninsula, but they are also found in other scattered localities.

Sometimes they appear to form ancient flows, and at other times

intrusive masses and dykes; but they are rarely scoriaceous.

Almost all the rocks in my collection referred to this genus belong to

the species where the felspar-lathes of the groundmass are ·02-·1 mm.

long. They are generally blackish or dark-brown, and the specific

gravity ranges usually from 2·72 to 2·83. They display in the slide a

fair number of plagioclase and pyroxene phenocrysts in a groundmass of

felspar-lathes, pyroxene granules, and magnetite, the interstitial glass

being scanty or moderate in amount.... The plagioclase phenocrysts are

rarely as large as 3 mm., so that most of the rocks belong to the

non-porphyritic group of the genus. These phenocrysts, which are often

zoned, give extinctions of basic andesine (15°-25°). They contain

magma-inclusions, sometimes in abundance, which are arranged in

zone-lines.... The pyroxene phenocrysts are small, the two kinds being

always represented in the same slide. In some cases separate crystals

occur, and in others the two are associated as intergrowths, but in most

cases separate and compound crystals occur in the same section. Not

infrequently the phenocryst is an aggregate of lesser crystals of the

two pyroxenes. The monoclinic form is a brownish yellow augite with

large extinctions and often twinned. The felspar-lathes of the

groundmass, which usually average ·05 or ·06 mm. in length, are either

narrow and untwinned, or they may be stouter and display simple and at

times lamellar twinning, giving extinctions of medium andesine.... The

granules of pyroxene are generally ·01-·02 mm. in size; but occasional

prism-forms occur which give sometimes the straight extinction of

rhombic pyroxene and at other times the large oblique extinctions of the

augite type.

5. SUB-ORDER, PRISMATIC, OF THE HYPERSTHENE-AUGITE-ANDESITES

(_Felspar-lathes in flow-arrangement_)

FORMULA._—Hypersth-aug, matr, flu, prism._

17 genus (_Vitr._) }

18 " (_Opac._) } See Synopsis.

19 " (_Magn._) }

20 " (_Parv._) }

The rocks of this sub-order that are represented in my collection admit

easily of a general description, and since the diagnoses of the genera

are given in the Synopsis, there will be no need to separately describe

each genus.

Almost without exception these rocks form a constituent of agglomerates

in various parts of the island; and they occur in this condition in some

of the highest mountains, as Mariko, Thambeyu, and Koro-mbasanga. The

exception refers to a low mound-like hill, apparently a “volcanic neck,”

that rises from the basaltic plains west of Mbua (see page 58).

In about half of the specimens the rocks are referred to section 10,

where the plagioclase phenocrysts are either small and very scanty or

are absent altogether. In a fair number these phenocrysts are opaque

(genus 18); whilst in a few they are glassy (genus 17). The rocks are

typically blackish or dark grey, and often have a pitchstone-like

appearance, the groundmass being frequently semi-vitreous in character.

Vesicular and scoriaceous rocks occur at times.

In all cases the felspar-lathes and pyroxene prisms are more or less in

flow-arrangement; whilst pyroxene granules, if present, are subordinate.

The felspar-lathes, which are either simple or once-twinned and give

extinctions of acid and medium andesine, are usually small, and average

in different sections ·05-·08 mm. in length. The pyroxene prisms are

pale brown and are ·03 or ·04 mm. long. Most of them give oblique

extinctions of over 25; but in the same slide some give straight

extinctions; the proportion varies in different rocks. The pyroxene

phenocrysts in all the specimens are small (not over 2 mm.), and are

rhombic and monoclinic. In most sections the two forms are represented

by separate crystals and are also associated in the same phenocryst.

Those of rhombic pyroxene have often dark borders. There is a

considerable amount of a pale brown glass in the groundmass, more or

less devitrified. The specific gravity varies considerably, but is as a

rule between 2·55 and 2·75, the more basic rocks containing augite in

preponderance and basic andesine, whilst the less basic possess a large

proportion of rhombic pyroxene and display oligoclase-andesine.

Sometimes, as in the case of a rock composing an agglomerate east of

Nanduri, where the porphyritic plagioclase is opaque and there is some

degree of alteration, the rock looks very like a porphyrite.

THIRD ORDER, ORTHOPHYRIC, OF THE HYPERSTHENE-AUGITE-ANDESITES

FORMULA.—_Hypersth-aug, matr, orth._

CHARACTERS.—Felspars of the groundmass short and broad.

Since the material is insufficient for the separate description of each

genus, a general account of the order is alone given. These rocks are

often represented in agglomerates or they occur as large blocks, either

lying on the surface or imbedded in tuffs. Many of them are somewhat

altered.

They are for the most part dark grey dull-looking rocks, with a specific

gravity of 2·7 to 2·8, showing macroscopic plagioclase together with

conspicuous pyroxene phenocrysts. The plagioclase phenocrysts are

usually small (1 to 3 mm.), and give extinctions of medium andesine

(15°-20°) and in some rocks of acid labradorite (30°). They are as a

rule corroded and are penetrated by numerous fissures, whilst they

contain a considerable amount of altered magma-inclusions with sometimes

other alteration products. The pyroxene phenocrysts are from 2 to 4 mm.

in size. Brownish-yellow augite, giving extinctions of over 30°, and

pale-yellow rhombic pyroxene of the type before described occur

generally in the same slide, and are frequently associated as

intergrowths in the same crystal. They may have regular outlines or dark

eroded borders, and at times they exhibit abundant dark opaque

inclusions. The broad felspars of the groundmass are sometimes

rectangular and give lamellar extinctions of medium and acid andesine

(12°-17°). They vary in length in different rocks from ·05 to ·2 mm. and

more. The pyroxene of the groundmass is generally granular and coarse

(·02-·05 mm.). As indicated by the extinctions of occasional prism-forms

it is composed of both augite and rhombic pyroxene, the former

prevailing. The prismatic sub-order is also represented, and here the

pyroxene of the groundmass is in great part prismatic, the length of the

prisms not often exceeding ·05 mm., whilst both the monoclinic and

rhombic kinds occur. Interstitial glass varies in amount, sometimes

absent, sometimes scanty and viriditic, at other times abundant and

opaque. Magnetite abounds in the groundmass much of it often being of

secondary origin.

FOURTH ORDER, FELSITIC, OF THE HYPERSTHENE-AUGITE ANDESITES

FORMULA.—_Hypersth-aug, matr, fels._

CHARACTERS.—The groundmass presents a rudely granular appearance or a

blurred mosaic.

This order is capable of subdivision, as in the other orders of the

hypersthene-augite-andesites (see Synopsis, page 247); but since it is

only represented by six of my rock-sections, I will confine the

description to the general characters.

These rocks are dark-grey, sometimes granitoid in appearance, with

specific gravity 2·65 to 2·75. They usually show some alteration,

arising from secondary changes within the rock-mass; and probably the

felsitic or semi-mosaic appearance of the scanty groundmass is the

result of such a secondary change. Such rocks in some respects approach

the type of the gabbros. They are frequent on the north coast of Natewa

Bay in the vicinity of Waimotu and also occur in the Valanga Ridge. They

generally present themselves as deeper-seated massive rocks exposed by

the stripping off of the superficial deposits.

There are as a rule more or less conspicuous phenocrysts, up to 3 mm. in

size, of plagioclase and pyroxene, in a relatively scanty micro-felsitic

groundmass, displaying a blurred mosaic, in which a few stout

felspar-lathes can still be recognised, and composed apparently of

felspar and crystalline silica. The “grain” of the mosaic may range in

different rocks from ·005 mm. to ·02 mm. The pyroxene of the groundmass

is largely decomposed, and the scanty residual glass is represented by

viriditic materials.

The plagioclase phenocrysts, which give extinctions of medium and basic

andesine (15°-25°), are often semi-opaque and corroded. They are

traversed by numerous cracks and often contain many whitish alteration

products, though the lamellar structure is usually well preserved. The

pyroxene phenocrysts are composed of brownish-yellow augite (ext. + 30°)

and pale rhombic pyroxene of the type described before, either as

separate crystals or associated as intergrowths. The rhombic pyroxene

crystals are often sub-rounded with dark borders; and as a rule the

pyroxene phenocrysts are much fresher than the plagioclase phenocrysts.

As far as can be ascertained, most of the original pyroxene of the

groundmass was monoclinic with a little rhombic.

CHAPTER XXI

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

ACID ANDESITES

_Previous observations on the Hornblende-Andesites of Fiji_

THESE rocks were first described by Wichmann[112] from specimens

obtained by Kleinschmidt from the mountain of Mbuke Levu in Kandavu.

These Kandavu rocks had a microfelsitic base, the porphyritic brown

hornblende having usually black borders in which a change into epidote

was observed. Rhombic pyroxene was only noted as an occasional

constituent of a rock from Ono. Renard[113] described these rocks from

the vicinity of Ngaloa Harbour in Kandavu and remarked that bronzite was

of more common occurrence than the monoclinic pyroxene. In the

groundmass were numerous felspar and augite microliths, whilst there was

a porphyritic development of plagioclase, hornblende, biotite, and

pyroxene. The hornblende phenocrysts played an important part in the

rock-composition, being surrounded by a black zone of magnetite or

bordered by a bacillary aggregate of small pyroxene prisms, parallel and

colourless or greenish, with extinction-angles of 40°. There was often

also a development of biotite in the heart of the mineral, the whole

hornblende section being sometimes thus transformed.

Mr. Eakle[114] has more recently described the hornblende-andesites from

Mbuke Levu in Kandavu. As the result of his examination of a collection

of volcanic rocks made by the Agassiz expedition in Viti Levu, Kandavu,

Mbenga, Totoya, Malolo, Yasawa group, and in several other small

islands, he inferred that the hornblende-andesites are much more limited

in their occurrence in Fiji than the augite-andesites; whilst

hypersthene-andesite was only represented in the collection from

Vomo-lailai near Waia in the Yasawas. The specimens from Waia had a

microfelsitic base with pseudomorphs of hornblende and some augite. Mr.

E. C. Andrews[115] in his account of his collection of volcanic rocks,

made mostly in the Lau Group and Taviuni, makes no special reference to

hornblende-andesites, the andesites being mainly augitic, rhombic

pyroxene also occurring as a common porphyritic constituent.

It may be inferred from the above and from my own observations in Vanua

Levu below given that hornblende-andesites have a relatively limited

distribution in Fiji. They are not generally distributed as in the case

of the augitic and basaltic andesites; but are confined to certain

localities in Viti Levu,[116] Vanua Levu, Ovalau, Kandavu, Ono, Malolo,

Yasawa Islands, etc.

_The occurrence of quartz-andesites or dacites in Fiji._—In connection

with the existence of these rocks in Vanua Levu, it is noteworthy that

except in Mr. Eakle’s paper there is no reference in any of these

writings to the occurrence of quartz-andesites in Fiji. Wichmann

expressly states that the rocks he examined were free from quartz, and

that up to his time (1882) no quartz-bearing younger eruptive rocks were

known from the South Seas. Mr. Eakle in 1899 described a

holo-crystalline andesite with a felsitic aspect from Malolo and another

similar looking rock from Vatu Mbulo, in the same sub-group of the

Fijian Islands, showing quartz both in the phenocrysts and in the

microcrystalline groundmass, concerning which he observed that it was

perhaps more of a dacite than an andesite. Dacites were found by me in

1884 in the island of Fauro in the Solomon group,[117] and it is

probable that they are of more frequent occurrence in the Pacific than

has been generally supposed. As shown immediately below, they are

represented in Vanua Levu; and the extent of their distribution in the

island depends on the limits we assign to the definition of the term

“dacite.”

If we restrict the term to a hornblende-andesite carrying porphyritic

quartz and displaying a microfelsitic groundmass, such rocks, though

they form some of the highest peaks in the Ndrandramea district, namely

Ngaingai and Wawa-levu, would not be very frequent in Vanua Levu. If,

however, a microfelsitic groundmass is alone necessary to constitute a

“dacite,” the great majority of the acid andesites of the island would

fall under this designation.[118] This has long been a controverted

point in petrology. If I adopted the last procedure, my general

classification for the andesites would fall into confusion and many

rocks without any quartz would be included in the dacites.

In the Synopsis it will be seen that my classification of the

andesites is as far as concerns the great groups based on

the mineral and not on the structural characters. There

are three sub-classes closely allied to each other, the

hypersthene-andesites, the hornblende-hypersthene-andesites, and the

quartz-hornblende-hypersthene-andesites or dacites, which cannot be

distinguished at their boundaries by their petrological characters or by

their different modes of occurrence. These groups of rocks which include

all the acid andesites of the island will now be dealt with.

THE ACID-ANDESITES OF VANUA LEVU

(_Comprising the hypersthene-andesites;

hornblende-hypersthene-andesites;

and quartz-hornblende-hypersthene-andesites or dacites_)

These rocks compose in mass numerous isolated hills that rise up

abruptly in the interior of the central portion of the island. Such

hills, or mountains, as they might be often termed, usually attain a

height of from 700 to 1200 feet above the surrounding country, and

possess precipitous slopes and frequently perpendicular cliff-faces. In

the geological description of the island, I have referred in detail to

these mountains, when speaking of Na Raro, Vatu Kaisia, Ndrandramea,

Ngaingai, etc.; and illustrations of some of them are included in this

work. It may, however, be here remarked that they are as a rule rudely

conical with rounded or peaked summits. The ground-plan is generally

elliptical in outline; and in consequence the profile often varies from

different points of view, so that as in the case of Na Raro, it is that

of a sharp conical peak when the mountain is viewed “end-on,” or of a

broad truncated mass when seen from the side. A similar change of form

is to be noticed in the illustrations of Ndrandramea. No traces of

crateral cavities came under my notice. The rocks are neither vesicular

nor scoriaceous, and are usually massive; but exhibit at times a rudely

columnar structure.

Each hill or mountain has its peculiar variety of these rocks. This is

well shown in the Ndrandramea district. Thus the rocks of Ngaingai and

of Wawa-levu in carrying porphyritic quartz differ from those of all the

other hills around. Those of Soloa Levu are distinguished by the

orthophyric groundmass and by the absence of hornblende. Those of Mount

Ndrandramea again have no porphyritic quartz, but little hornblende, and

possess a micro-felsitic groundmass. The rocks of Na Raro and Vatu

Kaisia differ as regards specific gravity, the “grain” of the felsitic

groundmass, the presence of phenocrysts of rhombic pyroxene, etc. The

characters of these rocks from various localities are contrasted in the

table given on a later page, whilst the different sub-classes to which

they belong are described in detail below.

SUB-CLASS HYPERSTHENE-ANDESITES

These are dark and light grey rocks, sometimes granitoid in appearance.

They pass on the one hand into the hypersthene-augite-andesites before

described and on the other into the hornblende hypersthene-andesites to

be subsequently dealt with. From the former they are distinguished by

the great predominance of rhombic pyroxene both as phenocrysts and in

the groundmass; whilst from the latter they are separated by the absence

of brown hornblende or its pseudomorphs. These rocks are found in the

Ndrandramea, Valanga, and Vunimbua districts. They may form isolated

dome-shaped hills as in that of Soloa Levu, or they may constitute the

deeper-seated rocks of the region from which these hills arise,

as in the Ndrandramea district. In their general mode of

occurrence, however, they cannot be treated apart from the allied

hornblende-hypersthene-andesites and the dacites.

This sub-class may be divided like the hypersthene-augite-andesites into

four orders according to the character of the groundmass; and these are

enumerated in the Synopsis. Only the orthophyric and felsitic orders are

represented in my collection. Of the former the most typical rocks are

those composing the hill of Soloa Levu which is described on page 103.

These Soloa Levu rocks are lightish grey and granitoid in aspect, with

specific gravity of 2·54-2·62, and displaying abundant porphyritic

crystals of pyroxene, 2-3 mm. in size. In the slides they show a large

number of plagioclase phenocrysts together with those of pyroxene in a

relatively scanty groundmass, for the most part orthophyric in texture

and without residual glass.... The plagioclase phenocrysts, which are

not usually over 2 or 3 mm. in size, are often tabular and show distinct

zone-lines. Though they are traversed by minute cracks and have

frequently a semi-saussuritic appearance arising partly from

change-products and partly from the abundance of colourless inclusions,

they yield clear lamellar extinctions of medium and basic andesine

(15°-25°).... The pyroxene phenocrysts, which are not much altered, are

in most cases long pale-yellow rhombic prisms with rounded ends,

behaving optically as described on page 285; but intergrowths with

monoclinic pyroxene may occur and even separate crystals of augite....

The scanty groundmass, though in the main formed of short and broad

felspars, ·12 mm. long, of the orthophyric type, displays in places a

rude mosaic, apparently of quartz and felspar. It also shows abundant

small pyroxenes in the form of small prisms (·05 mm. in length), giving

extinctions nearly always straight but occasionally oblique (30°-35°).

As examples of the felsitic order of these rocks, most of which are

altered like the propylites, I will first take the case of those

deep-seated rocks that are exposed in the river-bed above Nambuna in the

Ndrandramea district. In the least altered state they are dark grey and

mottled, and have a specific gravity of 2·66-2·69. In section they

display tabular zoned plagioclase phenocrysts, usually more or less

occupied by alteration products, but at times giving lamellar

extinctions of basic andesine (20°-25°). The rhombic pyroxene is more or

less replaced by chloritoid pseudomorphs; whilst the “grain” of the

mosaic is often coarse (·03 mm.), and much of it is evidently quartz.

The more advanced stages of alteration of these rocks are described in

the account of the district given on page 106.... Similar rocks, showing

pyrites, occur amongst the blocks of Vunimbua River; but here the

rhombic pyroxene is mostly converted into bastite, and the groundmass is

in part trachytic as well as felsitic in texture. The specific gravity

is 2·7.

(In the last survey of my collection I have found a solitary specimen

from an agglomerate in the Mbua-Lekutu “divide,” which must be referred

to the order with felspar-lathes in flow-arrangement. It is a pale grey

rock showing abundant macroscopic pyroxene prisms, 2 mm. long, mostly

rhombic but showing also intergrowths with monoclinic pyroxene. The

felspar-lathes do not average more than ·1 mm., and there is a quantity

of small prisms of rhombic pyroxene in the groundmass, which also

contains a little residual glass.)

SUB-CLASS HORNBLENDE-HYPERSTHENE-ANDESITES

This is an extensive group which includes the rocks forming several of

the hills in the Ndrandramea district as well as the isolated peaks of

Na Raro, Vatu Kaisia, etc. It passes on the one side into the

Hypersthene-Andesites before described and on the other into the

Hornblende-Hypersthene-Quartz-Andesites, the Dacites of this island.

Of the four orders established in the Synopsis (page 236) according to

the general method there adopted, the first, where the groundmass

exhibits felspar-lathes not in flow-arrangement, is not represented in

my collection.

SECOND ORDER OF THE HORNBLENDE-HYPERSTHENE-ANDESITES

(_Felspar-lathes in flow-arrangement_)

This order is only represented by three rocks, all of which belong to

the prismatic sub-order where the pyroxene of the groundmass is

prismatic and not granular.

Two of these rocks are very similar in appearance and character, though

coming from different localities on the opposite sides of Savu-savu Bay,

one from the agglomerate of Vatu-ndamu in the Kumbulau peninsula (page

91), the other from an intrusive mass in the vicinity of Urata (page

184). They are dark grey, with specific gravity 2·6 to 2·7, and display

macroscopic crystals of hornblende and pyroxene. In the slide they

exhibit in addition numerous phenocrysts of plagioclase, 1 to 2 mm. in

size, in a groundmass showing small felspar-lathes (less than ·1 mm. in

length) in partial flow-arrangement and numerous pyroxene prisms

(·05 mm. long) giving straight extinctions, together with a little

residual glass.... The plagioclase phenocrysts, which give extinctions

of medium and basic andesine, are often tabular and display zone-lines.

They contain abundant pale inclusions arranged zone-wise.... The

hornblende phenocrysts are dark brown, markedly pleochroic, and give

extinctions up to 12 degrees. They have dark resorption borders and are

sometimes deeply corroded. They show in various stages the remarkable

conversion at the borders into fine pyroxene, which is described on page

306.... The pyroxene phenocrysts are more numerous in the Urata rock.

They are for the most part of the pale yellow feebly pleochroic rhombic

type that prevails in the island. A few phenocrysts of pale augite (ext.

35°) may occur in the same slide; whilst the two pyroxenes may be

associated as intergrowths.

A crypto-crystalline variety of these rocks, where the felspar-lathes

and rhombic pyroxene prisms of the groundmass are only in part

differentiated, is found on the hills of Ndreke-ni-wai on the shores of

Natewa Bay (page 201). It is a pale-grey open-textured rock, displaying

numerous small macroscopic crystals of hornblende.

THIRD ORDER OF THE HORNBLENDE-HYPERSTHENE-ANDESITES

(_Felspars of the groundmass, short and broad, of the orthophyric type_)

These rocks occur generally as agglomerates and are more particularly

characteristic of the district between the Mariko Range and the Salt

Lake. They belong for the most part to the prismatic sub-order of the

group and to the section with plagioclase phenocrysts, and fall

naturally into two divisions corresponding to the two genera with

_glassy_ and _opaque_ phenocrysts. The last named would be regarded by

some as porphyrites. The specific gravity of the specimens ranges from

2·52 to 2·7.

The plagioclase phenocrysts, 1 to 2 mm. in size, give extinctions

indicating in some rocks oligoclase-andesine (10°-15°) and in others

basic andesine (15°-25°). Their opacity in the porphyrites is sometimes

due to multiple macling, but more usually it arises from the numerous

fine cracks filled with decomposition products that traverse them. The

phenocrysts of dark brown hornblende are generally abundant and give

extinctions of 15 degrees. They as a rule have dark resorption borders

in which the process of conversion into fine pyroxene is in active

operation. The pyroxene phenocrysts are scanty and in most cases

rhombic; but intergrowths with augite and separate crystals of the

last-named may occur. In the altered rocks or porphyrites they are

largely replaced by bastite and viridite. The felspars of the groundmass

are broad and often rectangular and may give lamellar extinctions of

oligoclase-andesine. The pyroxene in the groundmass of the porphyrites

is often partly decomposed. It is as a rule prismatic. A little

interstitial glass, altered in the porphyrites, is generally present.

FOURTH ORDER OF THE HORNBLENDE-HYPERSTHENE-ANDESITES

(_Groundmass felsitic, displaying a granular mosaic structure_)

These are light and dark grey rocks showing usually macroscopic pyroxene

and hornblende. They vary considerably in appearance from the

open-textured rock to that with a granitoid coarsely crystalline aspect.

They generally carry brown hornblende phenocrysts, but frequently these

are represented by pseudomorphs; and they all have a felsitic

groundmass. They are only separated by the absence of porphyritic quartz

from the dacites of Vanua Levu, which are treated in the next sub-class.

They present all stages from the crypto-crystalline to the

holo-crystalline condition, but all show a groundmass which may be

scanty in the more coarsely crystalline rocks.

These rocks are characteristic of some of the hills of the Ndrandramea

district and of the isolated peaks of Vatu Kaisia and Na Raro. They

include a large proportion of the acid andesites of the island, and all

belong to the prismatic sub-order with prismatic pyroxene in the

groundmass, and to the section with plagioclase phenocrysts. Their

specific gravity ranges usually from 2·55 in the more acid and less

crystalline types to 2·74 in the most crystalline and basic kinds.

In the typical slides they display phenocrysts of plagioclase, pyroxene,

and brown hornblende in a microfelsitic groundmass formed evidently of

felspar and quartz together with much prismatic pyroxene. They may be

conveniently divided into three species according to the size of the

“grain” of the groundmass.

Of the first species, where the “grain” is less than ·01 mm., the rocks

of Mount Ndrandramea are typical. They have a crypto-crystalline

groundmass where the felsitic structure is in process of development and

where the pyroxene prisms or microliths are very minute. The plagioclase

phenocrysts (1 mm. in size) give extinctions of acid and medium andesine

(10°-20°), and are tabular, zoned, and contain abundant pale inclusions.

The hornblende phenocrysts, except in the case of the rocks at the foot

of the hill, are represented by pseudomorphs in various stages of

dispersion, so that this character is likely to be overlooked. The

pyroxene phenocrysts are in most cases of the pale yellow feebly

pleochroic rhombic type, but they may present intergrowths of both the

monoclinic and rhombic forms. These are light grey rocks with a specific

gravity increasing as one descends from the summit, where it is about

2·5, to the base where it is 2·7, a change corresponding with increase

of the ferro-magnesian minerals and with the more crystalline structure

of the groundmass.

The second species, where the “grain” of the mosaic is between ·01 and

·02 mm., is represented by the Vatu Kaisia rock and by that exposed in

the opposite side of the gorge. They are granitoid in appearance and

have a specific gravity of 2·68 to 2·71. Large porphyritic crystals of

pyroxene and hornblende, the last sometimes 7 mm. in length, occur in a

dark grey base. In the slide these phenocrysts together with those of

plagioclase are displayed in a somewhat scanty holo-crystalline

groundmass, where the “grain” of the mosaic averages ·012 mm. The

plagioclase phenocrysts are zoned, and give in different crystals

extinctions in some cases of oligoclase-andesine (10°-12°) and in others

of andesine-labradorite (25°-30°). The hornblende phenocrysts in their

pseudomorphism illustrate the various stages of the process of

conversion into fine granular and prismatic pyroxene. The least altered

crystals have dark resorption borders and are at times deeply corroded.

The pyroxene phenocrysts are for the most part rhombic; but intergrowths

with the monoclinic form occur. The pyroxene of the groundmass consists

for the most part of small rhombic prisms averaging ·05 mm. in length.

The third species, where the groundmass may be described as a coarse

mosaic with a “grain” between ·02 and ·03 mm., is represented by the

rocks of the peak of Na Raro and of Mount Thokasinga in the Ndrandramea

district.

The Na Raro rocks are light grey with a specific gravity in the

unweathered state of about 2·6, and display macroscopic crystals of

glassy plagioclase and hornblende. In the slide they exhibit tabular

phenocrysts of the plagioclase, together with dark pseudomorphs after

hornblende, in a coarsely felsitic groundmass (grain ·022 mm.) where a

little very fine prismatic pyroxene, apparently rhombic, occurs. There

is also a little altered interstitial glass. The plagioclase phenocrysts

are zoned and give extinctions of 15° to 25° (acid and basic andesine),

whilst they often show magma inclusions. An interesting feature of these

rocks is concerned with the rarity or absence of phenocrysts of

pyroxene. They are to be seen, however, in the process of being built up

within the substance of the hornblende pseudomorphs, which consist

entirely of minute prisms and granules of pyroxene and of fine

magnetite. The process seems to consist in the formation of parallel

layers of rhombic and monoclinic pyroxene.

The Thoka-singa rocks are more basic (spec. grav. 2·72 to 2·74), and in

the scanty holo-crystalline groundmass approach the plutonic type. They

are dark grey granitoid rocks displaying abundant macroscopic pyroxene

crystals 2 to 3 mm. long. The original hornblende phenocrysts are only

represented by traces of pseudomorphs of fine pyroxene and magnetite,

the process of dispersion, described on page 307, being almost

completed. The pyroxene phenocrysts are mostly rhombic; but intergrowths

with the monoclinic form occur. The “grain” of the mosaic of the

groundmass is coarse (·023 mm.), and there is a fair amount of prismatic

with a little granular pyroxene, the prisms, ·04 mm. long, giving

usually straight extinctions, whilst the granules are apparently

monoclinic.

SUB-CLASS QUARTZ-HORNBLENDE-HYPERSTHENE-ANDESITES OR DACITES

These rocks are infrequent. They compose in mass the adjacent mountains

of Ngaingai and Wawa Levu in the Ndrandramea district, and appear also

on the lower slopes of the neighbouring mountain of Navuningumu. They

differ chiefly from the hornblende-hypersthene-andesites in the presence

of porphyritic quartz, which, however, is not as a rule abundant. In

their general origin and affinities and in their mode of occurrence they

cannot be separated from the two sub-classes of hypersthene-andesites

and hornblende-hypersthene-andesites before described. They all belong

to the felsitic order of the sub-class, and all are referred to the

sub-order with prismatic pyroxene and to the section with plagioclase

phenocrysts.

They are light grey rocks, with a specific gravity of 2·57 to 2·61,

showing usually dark pseudomorphs after hornblende and a little

porphyritic quartz. In the slide they display these pseudomorphs and

quartz crystals, associated with abundant plagioclase phenocrysts, in a

felsitic groundmass, evidently a mixture of felspar and quartz, with

fine pyroxene, mostly prismatic and rhombic. Pyroxene phenocrysts are

absent or rare; but they may be seen in process of formation in the

substance of the hornblende-pseudomorphs. It is only at times, as in the

instance of the Navuningumu rock, that the brown hornblende phenocrysts

are in part unchanged and that complete pyroxene phenocrysts occur. In

such cases the last may be entirely rhombic, or may exhibit at times

intergrowths with the monoclinic form.

The plagioclase phenocrysts, 2 mm. in size, are often tabular and zoned

and give two sets of extinctions, indicating acid and basic andesine.

Their abundant inclusions are arranged in zones. The quartz-crystals, 1

to 2 mm. in size, present hexagonal sections with rounded angles. They

are sometimes traversed by cracks occupied by iron oxide films. The

pseudomorphs after hornblende, which consist of fine pyroxene mixed with

magnetite, exhibit often the building up in their interior of pyroxene

phenocrysts, apparently rhombic, by long parallel rows of stout prisms.

In other cases the pseudomorphs display the different stages of

dispersion. The fine pyroxene of the groundmass consists mostly of

rhombic prisms (·02-·06 mm. long) with some granules. The “grain” of the

groundmass is usually between ·01 and ·02 mm. There is little or no

residual glass.

The rocks of Wawa Levu and Ngaingai are closely similar, but they differ

in the size of the prismatic pyroxene of the groundmass, which is

coarser in the first-named mountain (·055 mm. long) than it is in the

second (·025 mm. long). In both the “grain” of the mosaic is about the

same (·014 mm.).

TABULAR COMPARISON OF THE ACID ANDESITES OF VANUA LEVU.

NOTE.—The figures in the columns headed “Felsitic” and “Rhombic” refer

to the size in millimetres of the felsitic “grain” and the larger

pyroxene prisms.

Column headings:

P: Plagioclase.

Q: Quartz.

H: Hornblende or its pseudomorph.

R: Rhombic Pyroxene.

M: Monoclinic Pyroxene.

O: Orthophyric.

Rh: Rhombic.

Mo: Monoclinic.

+------------+----+---------------------+---------------------------+

| | +-+-+--------+-+------+-------------+-------------+

| | Gr.|P|Q| H |R| M +-----------+-+------+------+

| | | | | | | | Felsitic. |O| Rh | Mo |

+------------+----+-+-+--------+-+------+-----------+-+------+------+

| summit |2·57|+|+| + | | | + ·014 | | + ·03| |

| | | | | | | | | | | |

| lower part |2·57|+|+| + | | | + ·013 | | + ·02| |

| | | | | | | | | | | |

| 1,500 feet |2·61|+|+| + | | | + ·015 | | + ·05| |

| | | | | | | | | | | |

| 1,500 feet |2·61|+|+| + | | | + ·014 | | + ·06| |

| | | | | | | | | | | |

| west side |2·57|+|+| + |+| | + ·022 | | + ·04| |

| | | | | | | | | | | |

| summit |2·44|+| | + |+| + | + ·007 | | + ·01| |

| | | | | of | | | | | | |

| | | | | | | | | | | |

| 1,600 feet |2·58|+| | + |+| + | + ·008 | | + ·01| |

| | | | | of | | | | | | |

| | | | | | | | | | | |

| 1,200 feet |2·68|+| | ? |+| ? | + ·008 | | + ·01| |

| | | | | | | | | | | |

| saddle at |2·71|+| | + |+| ? | + ·009 | | + ·01| |

| | | | | | | | | | | |

|Kalakala, |2·61|+| | + |+| | + ·02 | | + ·01| |

| | | | | | | | | | | |

|Navuningumu{|2·65|+| | + |+| + | + ·02 | | + ·03| ? |

| near base {| | | |in part | |scanty| | | | |

| {| | | |pseudom.| | | | | | |

| {|2·48|+|+| + |+| | + ·018 | | ? | |

| | | | | | | | | | | |

|Thokasinga |2·74|+| | + |+| + | + ·024 | | + ·03| + |

| | | | | | | | | | | |

|Thokasinga |2·72|+| | |+| + | + ·023 | | + ·04| + |

| | | | | | | | | | | |

|Na Raro, |2·58|+| | + | | | + ·02 | | + ·01| |

| | | | | | | | | | | |

|Na Raro |2·58|+| | + | | | + ·022 | | + ·01| |

| | | | | | | | | | | |

|Vatu Kaisia |2·71|+| | + |+| | + ·013 | | + ·05| |

| | | | | | | | | | | |

|Vatu Kaisia,|2·68|+| | + |+| | + ·01 | | + ·05| |

| | | | | | | | | | | |

|Soloa-levu, |2·54|+| | |+| + | |+| + ·06| |

| | | | | | | | | | | |

|Soloa-levu, |2·57|+| | |+| + | |+| + ·05| |

| | | | | | | | | | | |

|Soloa-levu, |2·62|+| | |+| + | |+| + ·06| + |

| | | | | | | | | | | |

|Soloa-levu, |2·61|+| | |+| + | |+| + ·06| + |

+------------+----+-+-+--------+-+------+-----------+-+------+------+

_Note on the Rhombic Pyroxene of the three foregoing sub-classes of the

Acid Andesites._—The term “hypersthene” has been here used as a

convenient expression equivalent to “rhombic pyroxene.” The mineral is

always a little pleochroic and is never colourless, and it is only in

very rare cases that the term “enstatite” could be used. As a matter of

fact there is practically only one form of rhombic pyroxene represented

in my collections whether in acid or basic andesites or in

hemi-crystalline and plutonic rocks. In the acid andesites it occurs not

only as phenocrysts but also as minute prisms forming a constituent of

the groundmass.

This mineral, when composing the phenocrysts, presents itself usually as

single untwinned prisms which exhibit the typical octagonal

cross-sections with much reduced prism-faces. The prismatic sections

give straight extinctions; whilst with the cross-sections we obtain

straight extinctions parallel with the pinakoid faces. The colour in

transmitted light is pale brownish yellow. The pleochroism, though

usually feeble, is quite distinct, the colour being pale yellow when the

prism lies parallel with the long axis of the lower nicol, and almost

white when it lies across. Not infrequently these phenocrysts behave

abnormally and give small oblique extinctions. This is often the case

when monoclinic pyroxene occurs in the same section. The association of

the two pyroxenes in one crystal can in some cases be clearly

recognised. At one time a plate of pyroxene exhibits itself as a coarse

aggregate of the two pyroxenes. At other times the two occur as parallel

intergrowths, as in the accompanying figure. But it is rarely that such

intergrowths are so typically displayed, the reason of which has been

supplied by Zirkel in his _Lehrbuch der Petrographie_; 2nd edit.: I.

271.

_Note on the “magmatic paramorphism”[119] of the hornblende

phenocrysts._—Reference has before been made in the general description

of these rocks to the dark alteration margins of the hornblende

phenocrysts. The dark borders display the “bacillary” structure noticed

by Renard in the case of some hornblende andesites from Kandavu in the

same group of islands,[120] being composed of minute granules and

parallel prisms of pyroxene and also of magnetite grains. With the

Kandavu rocks Renard observed that the tiny pyroxenes were colourless or

greenish and had an extinction angle of 40°. In the Vanua Levu rocks,

however, there is a mixture in these dark margins of both monoclinic and

rhombic pyroxene; and the process may be observed in all stages as it

advances into the interior of the crystal, until a dark pseudomorph or

paramorph of pyroxene and magnetite results. When the magnetite

prevails, the pseudomorph may ultimately form a black patch in which the

process is obscured. But when, as is generally the case, the pyroxenes

are more frequent, it occurs as a dark grey mass.

Finally follows the dispersion of the pseudomorph, which first becomes a

loosely arranged aggregate of the two pyroxenes and magnetite, and then

breaks down, and at length is only represented by small pale patches of

its original constituents. These patches are easily recognisable, and in

not a few rock-sections offer the only indication of the previously

existing hornblende phenocryst. There can be little doubt that this is

the source of much at least of the often abundant pyroxenes of the

groundmass, which are usually most frequent in the vicinity of the

patches.

In the earliest stage when the dark border alone exists, it is not easy

to distinguish the one pyroxene from the other, the granules and prisms

being colourless and very minute, less than ·01 in size. But in a far

advanced stage of the paramorphism the granules and prisms become

sometimes much larger, the first attaining a breadth of ·04 and ·05 mm.

and the last a length of ·15 mm. Finally the interior of the paramorph

is seen to be more or less completely composed of very pale brown augite

and pale yellow rhombic pyroxene in coarse grains and prisms, the first

distinguished by its oblique extinction of 30° to 35°, the last

recognised by its straight extinction and feeble though distinct

pleochroism.

Although as a rule the paramorph becomes dispersed and its pyroxene

constituents are added to the groundmass, it sometimes exhibits a change

of another character. In this case the outer portion is alone dispersed,

whilst the growth of a single large crystal of pyroxene proceeds within

the mass. In a later stage, when the dispersion of the outer part is

complete, we have a fresh-looking pyroxene phenocryst with unformed

edges, on the borders of which little granules and prisms of pyroxene

may be seen arranging themselves, as if the crystal-building was still

in progress, or rather as if it had been interrupted and left unfinished

by the too rapid dispersion of the outer portions of the paramorph.

It will be gathered from the above that the source of the pyroxene of

the groundmass is to be found in the magmatic paramorphism of the

porphyritic hornblende. The hornblende is dark-brown, markedly

pleochroic, and extinctions up to 15° are given in prismatic sections.

It is well known that the conversion of a hornblende crystal into an

aggregate of pyroxene prisms and magnetite was long since experimentally

effected by Doelter and Hussak by immersing the hornblende in molten

basalt, andesite, &c.[121] I would imagine that the transformation of

the hornblende and the dispersion of the paramorph occurred under two

conditions; in the first case whilst the “flow” was still in motion when

the resulting pyroxene would be mixed up in the magma; in the second

case after movement had ceased, but before consolidation of the

groundmass, when a paramorph or pseudomorph would be formed.

OLIGOCLASE-TRACHYTES

The term “trachyte” is here applied in a general sense to a group of

light-grey intrusive acid rocks, having a specific gravity when compact

of 2·4 to 2·45 and showing phenocrysts of glassy felspar, but not of

quartz. These rocks, which are especially characteristic of the

districts around Tawaki and Mount Thuku and of the Wainikoro sea-border,

are often open-textured and sometimes a little vesicular, whilst several

of them exhibit some degree of alteration in the groundmass. In all

cases they appear to be intrusions rather than surface-flows; and at

times they display a columnar structure.[122]

The difference between the oligoclase-trachytes in various localities

appears to be mainly concerned with the varying degrees of

crystallisation. There are two principal varieties. In the most

crystalline type there are small phenocrysts of glassy felspar and a few

of pale augite, the angle of extinction of the last being over 30

degrees. The felspar phenocrysts, which contain but few inclusions and

have sharp rectilinear outlines, in most cases show zoning and give

lamellar extinction of 5° to 12° indicating oligoclase; but some of them

have the tabular untwinned or simple twinned form of sanidine. The

groundmass is in the main composed of minute felspar-lathes, less than

·1 mm. in length, arranged in a dense plexus, and giving nearly straight

extinctions. But it also contains a number of scattered larger

felspar-lathes averaging ·2 mm. in length and giving extinctions of 5°

when simple, and of 8° to 10° when lamellar. There is also some small

prismatic augite in the groundmass but often decomposing. The original

interstitial glass is represented by numerous reddish-brown patches of

devitrified glass.

In the second type of these trachytes, the rock is more open in texture

and is at times vesicular, the specific gravity being usually less than

2·4. The general characters are much the same, but sanidine is better

represented among the phenocrysts, and the groundmass is more blurred;

but when the felspar-lathes are distinct they give an extinction either

nearly straight or from 4° to 8°, according as they are simple or

display lamellæ. The augite of the groundmass is scanty and more or less

decomposed; whilst the interstitial glass when unaltered is in fair

quantity and nearly isotropic.

The alteration observed in several of these oligoclase-trachytes is

restricted chiefly to the interstitial glass in which secondary quartz

and at times calcite and viridite are developed. Scarcely any of them

are quite free from these changes.[123]

The pitchstone or vitreous form of these trachytes is displayed in the

blocks of an agglomerate-tuff between Tawaki and Mount Thuku. It has a

specific gravity of 2·36, is dark-brown, and has a conchoidal fracture.

Phenocrysts of felspar, mostly oligoclase, with extinction-angles of 5°

to 11°, and often penetrated by the magma, are inclosed in a

semi-isotropic groundmass showing incipient development of felspar and

other darker microliths. There are also a few small phenocrysts of pale

augite.

QUARTZ PORPHYRIES AND RHYOLITIC ROCKS

Wichmann when he wrote in 1882 that no quartz-bearing younger eruptive

rocks had hitherto been observed either in Fiji or in the South Sea

Islands generally, had apparently overlooked Dana’s observations in the

Fijian group. The American geologist[124] refers to a rock found on the

north-east shores of Vanua Levu which exhibited in a greenish base

thickly disseminated crystals of quartz (bipyramidal dodecahedrons, 1/8

of an inch in diameter) and glassy felspar, together with a few sphene

crystals.

Quartz porphyries, akin to the rhyolites, are especially characteristic

of the north-east part of the island, to which in fact they are entirely

confined. They perhaps are best represented in the vicinity of Mount

Thuku and in the neighbourhood of the mouth of the Wainikoro River. None

of my specimens have the fresh appearance of the Lipari rhyolites and

all are more or less altered. Their specific gravity does not exceed

2·4, and they are for the most part intrusive in character.

A rock frequently exposed between Tawaki and Mount Thuku[125] contains

abundant phenocrysts of glassy felspar (oligoclase and sanidine) and

quartz in a greenish opaque groundmass having a blurred microfelsitic

structure. There appears to have been a secondary devitrification of the

groundmass since consolidation. The porphyritic quartz crystals are

rounded and about 2 mm. in diameter.

Another rock displayed in the coast-cliffs on the north side of Natewa

Bay, a mile east of Mount Thuku, has a somewhat banded appearance. It

shows crystals of quartz, more or less rounded and 3 to 4 mm. in

diameter, together with phenocrysts of glassy felspar (oligoclase with

lamellar extinction of 5° and sanidine). The groundmass displays traces

of spherulites and is in places semi-isotropic; but for the most part it

is microfelsitic.

The type of rock found in the Wainikoro district and in the adjacent

sea-border, where it may be observed forming dykes in the pumice-tuffs,

is light-grey and loose-textured with a specific gravity of 2·1. It

exhibits small phenocrysts of quartz and of glassy felspar (oligoclase

5° to 12°, and sanidine), with, in one locality only, a scanty amount of

dark green hornblende yielding extinctions up to 20°. The quartz

crystals, which are 1 to 2 mm. in size, are sometimes bipyramidal; but

are often rounded and have fused-like outer surfaces. The groundmass is

semi-isotropic with a blurred aspect, and shows traces of spherulites

and numerous crystallites, with occasional felspar-lathes giving a

nearly straight extinction.

An extensively altered quartz porphyry of a different type is associated

with other altered rocks at the base of Mount Nailotha. It has a

specific gravity of 2·54, and displays large opaque crystals of

plagioclase, 2 to 5 mm. in size with small quartz crystals 1 to 2 mm.

across, in a grey compact matrix. The first-named shows the felspar to

be mostly replaced by alteration products; but occasionally a lamellar

extinction of 6° or 7° can be observed. The quartz crystals are rounded

and penetrated by the magma, and contain numerous strings of

fluid-cavities. The groundmass was originally spherulitic; but this

structure is more or less disguised by the development of a mosaic of

chalcedonic quartz. It shows some micro-porphyritic patches of viridite

and calcite. A singular altered white rhyolitic rock is exposed on the

north coast of Natewa Bay between Natasa and Sangani, where it is

associated with altered tuffs. It is compact with a conchoidal fracture

and has a specific gravity of 2·48. The hand-specimen has a banded

appearance. Under the microscope it appears as a rhyolitic glass for the

most part devitrified and rendered opaque by the formation of secondary

silica. Much of it presents a microfelsitic structure, the bands

appearing as semi-opaque streaks.

Glassy forms of the quartz porphyries or intrusive rhyolitic rocks are

extensively represented in the pumice tuffs of the Undu Promontory and

of the coasts between the Langa-langa river and Lambasa. These tuffs

will be found described on page 336. Fragments of a grey rhyolitic glass

looking like perlite are inclosed in the pumice-tuffs near the mouth of

the Wainikoro River. Under the microscope it is displayed as a

colourless glass inclosing phenocrysts of sanidine, oligoclase (ext.

4°), and quartz, the last with rounded outlines and a fused-like outer

surface. The glass shows in places perlitic cracks; but it is mainly

characterised by a vacuolar structure, the minute cavities being

lengthened out in the direction of the flow and displaying eddy-currents

around the phenocrysts. The elongated steam-cavities sometimes contain

water, but are usually more or less filled with granular materials.

CHAPTER XXII

Basic Glasses and Volcanic Agglomerates

BASIC PITCHSTONE AND BASIC GLASS

IT is not possible to draw a sharp distinction between the pitchstone

and the purely vitreous condition of these glasses. The following

remarks will therefore apply to both.

Regarded as components of the pitchstone-tuffs and palagonite-tuffs

these rocks have a very extensive distribution in the island; but in the

massive state they are hardly ever to be found, whilst in the form of

agglomerates they are only frequent in certain localities, as in the

cliffs of the Korotini Bluff, in the vicinity of Mbale-mbale, on the

slopes of Soloa Levu, and in the dividing ridge between the Mbua and

Lekutu plains. On rare occasions they are to be found in a rubbly

condition, as in the upper part of a basaltic flow described on p. 92,

or they may form veins in a more crystalline basaltic rock as at

Vatulele Bay. Their specific gravity ranges from 2·61 to 2·77, and they

fuse readily before the blow-pipe, the melting beginning in the ordinary

flame. Since they are not dissolved under any condition in HCl, they

would be referred to the old hyalomelane group of basic glasses.

One of the most interesting of these rocks occurs on the slopes of Soloa

Levu. As displayed on the south-west slope, it presents itself as a

brownish-black rock with a specific gravity of 2·61 and exhibiting large

porphyritic crystals (6 to 8 mm.) of plagioclase. It is generally

compact, but it is in places a little vesicular, the minute cavities

being often filled with a zeolite. The mode of occurrence of this

pitchstone-porphyry is described on p. 104. In the slide the plagioclase

phenocrysts give lamellar extinctions (21°-27°) of andesine labradorite,

and have regular outlines, with but few inclusions of the glassy magma.

There are also a few small phenocrysts of augite with dark rounded

borders and showing in some cases lamellar twinning. The groundmass is a

brown rather turbid glass in which dark points of devitrification occur.

It is traversed by cracks that also penetrate the felspar phenocrysts.

These cracks are filled with a feebly refractive material like

palagonite; and there are traces of the early stage of the palagonitic

change in one or two places. This is of importance, because on the

north-west side of the hill occurs the same rock, in which the basic

glass has been converted into a reddish-brown almost opaque palagonite;

but in this case the porphyritic crystals of plagioclase are more

affected by the magma, being rounded and extensively penetrated

schiller-fashion by this material; whilst the augite phenocrysts are

somewhat similarly affected. The altered glass is also vacuolar, the

cavities being filled with a zeolite. There is an indication of some

degree of crushing in the fracture of some of the felspar phenocrysts

_in situ_. There appears to be a connection, as shown on p. 342, between

the crushing of a basic glass and the formation of palagonite. It is

noteworthy that with this change the specific gravity drops from 2·61 in

the comparatively fresh rock to 2·14 in the palagonitised hydrated

condition.

As another example of these basic pitchstones I will take that forming

an agglomerate near Mbale-mbale. It has a specific gravity of 2·77 and

displays phenocrysts of plagioclase, olivine, and augite. The

first-named, which give the lamellar extinction of acid labradorite,

(22°-28°), are fresh-looking and only affected to a small extent by the

magma. Those of olivine and augite are in much the same condition. The

glass of the groundmass is rather turbid and displays numerous dark

patches of incipient crystallisation, which in some cases prove to be

composed of brush-like crystallites around a clear H-shaped nucleus, and

in other cases have a more prismatic form.

A vitreous rock having some of the characters of a variolite is found

near Narengali (see page 150). It, however, has the low specific gravity

of 2·43 and is not readily fusible with the blow-pipe. It displays an

imperfect spheroidal structure on a small scale, being made up of

nodules, the largest having the size of a filbert. In the slide it

appears as a grey glass made up of sheaf-like aggregates of fibre-like

crystallites, apparently of felspar, with minute skeleton prisms of

pyroxene in parallel arrangement, and is traversed by perlitic cracks.

THE VOLCANIC AGGLOMERATES

In this place my remarks will be chiefly confined to a summary of some

of the leading features of these formations. The agglomerates, which

pass by all gradations through the tuff-agglomerates into the submarine

tuffs, rank amongst the most prevalent and the most conspicuous of the

rocks exposed at the surface in this island. Their lithological

characters vary according to the type of the massive rocks of the

district. Thus in the Ndrandramea district the blocks are composed of

the prevailing acid andesites. In the Koro-mbasanga district they are

formed of hypersthene-augite-andesites. In the Korotini and Va-lili

ranges they are composed of olivine basalts and basaltic andesites. The

agglomerates derived from basaltic rocks and basic andesites are by far

the most frequent, and it is to them that the following general

observations apply.

The basic agglomerates and tuff-agglomerates are found almost everywhere

and at all elevations up to 2,500 feet above the sea and over. They

compose the inland cliffs and the long lines of precipitous declivities

that give character to the valleys and gorges of the mountainous

interior. The blocks are often scoriaceous and semi-vitreous, but the

characters of the rocks will be found described on page 316. They are

generally sub-angular and vary in size from a few inches to one or two

feet; and, though sometimes heaped together in confusion, they will

generally be found in the case of any extensive exposure to be rudely

sorted according to size, or to present a rude horizontal arrangement.

The matrix varies much in amount, being sometimes barely appreciable and

at other times so abundant that the deposit may be termed a

tuff-agglomerate. Typically it has the character of the palagonite-tuffs

of mixed composition described on page 326, being made up of fragments

of palagonitised vacuolar basic glass, portions of crystals of

plagioclase and augite, with the debris of the basic semi-vitreous and

hemi-crystalline rocks forming the blocks. When it is scanty it contains

neither carbonate of lime nor organic remains; but in the

tuff-agglomerates it may be calcareous and may inclose tests of

foraminifera and molluscan shells.

From the circumstance that the basic agglomerates overlie submarine

sedimentary tuffs and clays almost everywhere, their submarine origin

could alone be safely postulated. There are one or two localities that

throw especial light on the conditions under which these accumulations

occurred. They are dealt with at some length in the general description

of each district and only a brief reference can be made to some of their

indications here.

The testimony supplied by the interesting exposures on the slopes of

Mount Thambeyu (page 178) goes to show that after the deposition of the

foraminiferous tuffs and clays the stage of the agglomerates was ushered

in gradually. The tuffs increased in coarseness, and afterwards they

were covered up with an agglomerate formed of blocks at first only one

or two inches in size, but afterwards of larger dimensions.... Curious

evidence is afforded by the agglomerates of Mount Vungalei (page 213),

where two beds of palagonite-tuff, at elevations of 900 and 1,700 feet,

mark two pauses in the accumulation of the agglomerates. In each case

the pause was introduced by the gradual decrease of the agglomerates

which gave place by gradation to the tuffs. In each case also the pause

was followed by a sudden renewal of the deposition of agglomerates.

With reference to the maximum thickness of these deposits, it would

appear that on the slopes of the Korotini Range this amounts to some

hundreds of feet, if we also include the tuff-agglomerates. Their origin

is to be attributed partly to eruptions and partly to marine erosion.

The two agencies although often associated were in their turns

predominant in their different phases, and it is not too much to suppose

that the agglomerates without arrangement, with scanty matrix, and

composed of scoriaceous blocks, belong more to an eruptive period, and

that those with abundant tufaceous matrix and sorted blocks are mainly

the product of marine erosion. In either case the deposition was

submarine.

But the history of these agglomerates and of their associated

foraminiferous tuffs and clays must of necessity be a complicated one,

since they indicate a minimum emergence of 2,500 feet. Their

accumulation first began when a number of vents, in linear arrangement,

were striving to raise their heads above the surface of the sea. It was

continued after the waves had ultimately worn the volcanic islets down

to below the sea-level, and the shoals became covered over with

submarine deposits. Again and again no doubt this struggle between the

eruptive agencies and the waves was renewed, until at length the great

emergence began, and probably from that date the agency of marine

erosion was predominant.

When on the island of Stromboli I had presented for my observation at

least two modes of agglomerate-building under the sea. There was the

ordinary work of the marine erosion of the lava-cliffs, of which the

beach represents but a small part of the result; and there were the

dribbling eruptions of the crater, from which at intervals of only a few

minutes masses of semi-molten lava bounded down the steep slopes into

the sea.

_Note on the general characters of the rocks of the basic

agglomerates._—In appearance the basic rocks forming the blocks are

often very similar. They are usually compact blackish with a

semi-vitreous aspect and display some plagioclase phenocrysts. But to

enumerate the types to which they belong would be to go over much of the

ground traversed in the classification of the basic rocks, whether

olivine basalts, basaltic andesites, ordinary augite-andesites, or

hypersthene-augite-andesites. The groundmass as a rule contains much

smoky glass, but the hemi-crystalline portions of it vary considerably

in character. Whilst fine granular augite prevails, semi-ophitic coarser

augites are not uncommon, and prismatic pyroxene, sometimes of the

rhombic form, is represented in the groundmass of the rocks composing

the agglomerates of Mount Thambeyu and of the Sokena Cliffs. In some

localities, as on the south-west slopes of the Korotini Range, rocks of

the basic pitchstone kind are predominant.

CHAPTER XXIII

CALCAREOUS FORMATIONS, VOLCANIC MUDS, PALAGONITE-TUFFS

THE classification that is adopted in my work on the geology of the

Solomon group with respect to the calcareous formations and volcanic

muds of those islands is only in part applicable to the calcareous rocks

and volcanic deposits of Vanua Levu. Deposits strictly comparable with

those of the Solomon Islands here exist, and have in some places an

extensive distribution; but many others cannot be referred to that

classification. In addition to the calcareous oozes and volcanic muds,

such as are now forming off these reef-bound coasts, the result partly

of marine erosion and partly of sub-aerial denudation, there are many

kinds of submarine deposits in Vanua Levu that have been largely formed

from the materials ejected by volcanic vents. Basic glasses, for

instance, often finely vesicular and usually converted into palagonite,

enter largely into the composition of submarine deposits that frequently

form the surface from the sea-borders to the summits of the

mountain-ranges; and it is by the degradation of a land-surface formed

of such materials that the volcanic muds comparable to those of the

Solomon Islands are mainly produced. It is therefore apparent that we

have to distinguish here between the deposits of sedimentary and

eruptive origin, a distinction, however, which is not always easy to

make, since they are in both cases submarine, and doubtless were often

in process of forming together. The deposits most prevalent in the

island are the submarine tuffs partly sedimentary and partly eruptive in

their origin and the overlying volcanic agglomerates. The first are

usually palagonitic and calcareous and often contain organic remains,

being usually associated with volcanic muds and clays mainly the product

of marine erosion.

In connection with the employment of the terms “upraised” and “elevated”

in the case of the Vanua Levu deposits I will take this opportunity to

remark that I do not thereby commit myself to the view that there has

been an actual upheaval of this region. This is a matter, however, that

will be found discussed in Chapter XXVII.

THE UPRAISED CORAL LIMESTONES

These reef-limestones are scantily represented in the island, though one

can scarcely doubt that they were once far more extensive, having been

largely stripped off by the denuding agencies. They are mostly found on

the south coast between Naindi Bay and Fawn Harbour, and rarely extend

to heights greater than 20 or 30 feet above the sea, usually composing

the sea cliffs and not occurring as a rule inland. Massive corals are

often to be seen imbedded in their position of growth, as described in

Chapter II.; and as far as the absence of signs of disturbance is

concerned, these ancient reefs might owe their present situation, either

to the withdrawal of the sea or to the upheaval of the land. Such

reef-limestones exist over much of the Pacific, and they belong to the

usual type of these rocks.

SHELLY AND FORAMINIFERAL LIMESTONES

These rocks are composed partly of reef-debris, partly of volcanic

detritus, and partly of the tests of foraminifera (usually bottom

forms), fragments of lamellibranchiate and gasteropod shells, together

with those of pteropods, and other organic remains. Occasionally

separate valves of the genera “Cardium” and “Ostraea” are inclosed in

the limestone. These rocks have been evidently formed in rather shallow

water. In places they overlie palagonite-tuffs and clays, also

foraminiferal. Similar limestones are doubtless forming at the present

time off the coast.

They are usually hard in texture and greyish or pale yellow in colour.

They contain between 25 and 45 per cent. of carbonate of lime; whilst

the residue consists of fragments of minerals (10 to 15 per cent.),

including plagioclase, monoclinic and rhombic pyroxene, and occasionally

brown hornblende, with siliceous casts of foraminifera (4 to 20 per

cent.), mostly formed of chalcedonic silica but sometimes black and

glauconitic; the remainder (30 to 40 per cent.) being composed of

rounded and sub-angular portions of palagonite and semi-vitreous basic

rocks, of which the larger vary from ½ to 1 millimetre in diameter. In

some cases the carbonate of lime of the inclosed organic remains has

been mainly replaced by more or less crystalline silica. In others a

recrystallisation of the calcitic material is in progress, as described

on page 131; and the matrix presents in places a mosaic of calcite.

These rocks have therefore been subject to some degree of alteration,

the causes probably lying within the mass. They are, however, far from

frequent. They are best represented in the upper valley of the Sarawanga

River in the vicinity of Tembe-ni-ndio, where they reach to a height of

about 250 feet above the sea. The greatest elevation at which I found

them was in the mountainous interior of the Waikawa Promontory, where

they occur at a height of 1,100 feet above the sea.

As samples, the results of the examination of two rocks from the

Tembe-ni-ndio district are here appended:—

Carbonate of lime 46 per cent.

{Fragments of palagonite and of semi-vitreous

{basic rocks 30 " "

Residue {Minerals 12 " "

{Secondary silica replacing the carbonate of

{lime in the organic remains 12 " "

---

100

It displays to the naked eye fragments of shells including pteropods,

and numerous tests of foraminifera 2 or 3 millimetres in diameter. In

the section it displays in addition coral debris and a considerable

quantity of rounded and sub-angular pieces of palagonite and

semi-vitreous basic rocks, usually less than a millimetre across, with

smaller fragments of minerals (pyroxene and plagioclase), and much

calcitic material in the matrix.

Carbonate of lime 25 per cent.

Debris of palagonite and of basic volcanic rocks 45 " "

Minerals 10 " "

Siliceous casts of foraminifera 20 " "

---

100

The organic remains mainly consist of tests of foraminifera, many of

which occur in the residue as colourless siliceous casts. The fragments,

whether of minerals or of volcanic rocks, are usually less than half a

millimetre across. The foraminifera are mostly small and of the

“Globigerina” type; but there is a cast of a tube of some

boring-mollusc, and fragments of shells also occur.

PTEROPOD-OOZE ROCKS

These rocks are bluish-grey when not exposed; but through the hydration

accompanying exposure they become much lighter in colour. They are

crowded with pteropod shells, and contain also small gasteropod and

lamellibranchiate shells together with tests of foraminifera both

microscopic and macroscopic. They yield between 30 and 40 per cent. of

carbonate of lime, the residue being made up of disintegrated

palagonitic debris and fine clayey material derived from the same

source, together with a fair amount of mineral fragments (10 per cent.)

which include plagioclase, pyroxene, and brown hornblende, and measure

in the case of the larger fragments between ·1 and ·4 mm. in diameter.

Such rocks are somewhat friable and correspond with the pteropod-ooze

rocks of the Solomon Islands; but they are not very frequent, being best

represented on the flanks of the basaltic table-land between the Wainunu

and Yanawai rivers, as in the vicinity of the Nandua tea-plantation

where they extend up to 500 feet above the sea. In this particular

locality (see page 345) they overlie horizontally-stratified tuffs and

clays, composed of the debris of a basic glass usually vacuolar but now

for the most part converted into palagonite, and showing a few small

tests of foraminifera.

These deposits are always either surface or incrusting formations.

The circumstance of their passing down into characteristic

palagonite-formations is repeated in the case of the Tembe-ni-ndio

limestones, as observed on page 131; and there is no doubt that much

of their non-calcareous material is derived from the disintegration

of palagonite.

_Sample of pteropod-ooze rock from below the Nandua tea-plantation._

Carbonate of lime 38 per cent.

{Palagonitic debris and clayey material 51 " "

Residue {Minerals 8 " "

{Casts of foraminifera 3 " "

---

100

Fine clayey material makes up the greater part (72 per cent.) of the

residue. It presents the microscopic characters of material derived from

the degradation of palagonite. Amongst the mineral fragments, of which

the larger are ·2 to ·3 mm. in size, occur brown hornblende, pyroxene,

felspar, magnetite, &c. The casts of foraminifera are usually

glauconitic, but a few are of crystalline silica. A number of curious

little pellets of palagonite, oblong or oval in form, occur in the

residue. Their size is ·3 to ·6 mm., and they apparently represent the

minute amygdules of palagonite that occupy the vacuoles in an altered

basic glass.

FORAMINIFEROUS VOLCANIC MUD-ROCKS

These deposits, which represent the “volcanic muds” forming around the

coasts of volcanic islands, are more or less consolidated clay-rocks.

They contain in varying numbers the tests of foraminifera with

occasionally pteropod-shells. The former are usually minute and of the

“Globigerina” type; but in some rocks larger bottom-forms prevail. The

original colour of these deposits is bluish-grey, but as generally

displayed they are pale-brown and considerably affected by hydration and

are known as “soapstones” in the group. The ultimate effect of exposure

is the production of a whitish or yellowish soapy rock that has lost all

the carbonate of lime and all the organic remains and breaks down easily

in the fingers. Such a crumbling material, when examined with a high

power, cannot be distinguished from the products of the final

disintegration of palagonite as described on page 348.

These deposits are frequently displayed in the lower regions up to

elevations of 300 feet above the sea, incrusting the basaltic plains of

Lekutu, Sarawanga, Ndreketi, and Lambasa, that occupy such a large area

of the north side of the island, and exhibiting there in the prevailing

horizontality of their beds the same indication that is presented by the

vertical position of the columns of the under-lying basalt, namely, the

comparative absence of disturbance during the emergence. At the foot of

the mountains these deposits are interstratified with and finally

overlaid by coarse palagonite-tuffs, also containing marine remains; and

these are in their turn covered over by the agglomerates that often

enter so largely into the composition of the mountainous backbone of the

island. Such beds form apparently the lowest of a series which begins

with the foraminiferous clay and ends with the agglomerate. But in some

places, as has been noticed in the cases of the pteropod-ooze rock of

Nandua, and of the shelly and foraminiferous limestone of Tembe-ni-ndio,

beds composed largely of characteristic palagonite lie beneath.

In the elevated interior of the island these volcanic mud-rocks are

usually concealed by the tuffs and agglomerates. Occasionally, however,

they are to be seen exposed by landslips high up the flanks of the

mountains, as on the slopes of Thambeyu 1,000 feet above the sea. An

interesting exposure of them is displayed in the heart of the island in

the face of the Mbenutha cliffs where the elevation is about 1,100 feet.

Here they are overlaid by a thick bed of agglomerate; and tuff-beds

largely made up of vacuolar basic glass debris and showing a few

foraminifera are interstratified with them; but they exhibit signs of

considerable disturbance (see page 109).

These deposits contain between 5 and 25 per cent. of carbonate of lime,

and as a rule about 90 per cent. of the residue consists of fine clayey

material derived from the final degradation of palagonite and of basic

rocks. The mineral fragments (plagioclase, augite, rhombic pyroxene, and

occasionally hornblende) vary much in amount, their average proportion

being 13 or 14 per cent. of the mass. Their size is usually less than

·2 mm. and does not exceed ·4 mm. Casts of foraminifera are nearly

always present in the residue and form generally 3 or 4 per cent. of the

whole deposit. Sometimes they are black and glauconitic; but more

frequently they are white and composed of chalcedonic silica. Such casts

represent on a small scale the results of the same silicifying operation

to which the flints and silicified corals that occur so frequently on

the surface in some localities owe their origin (see Chapter XXV.).

With regard to the age of these volcanic mud-rocks of Vanua Levu, it is

most likely that as in the case of similar deposits in Viti Levu, which

were examined by Mr. H. B. Brady, they are of post-tertiary origin.

Samples of the Suva “soapstone” containing 5 or 6 per cent. of lime and

displaying shells of foraminifera, pteropods, and other molluscs, were

obtained from different heights up to 100 feet above the sea. Since 87

out of the 92 species of foraminifera represented in the deposits are

known to be living now in the Pacific, Mr. Brady had no hesitation in

assigning the beds to the post-tertiary epoch.[126]

SAMPLES OF THE VOLCANIC MUD-ROCKS

A. _From districts west of Ndranimako, 100 feet above the sea._

Carbonate of lime 20 per cent.

{Fine debris of palagonite and

{ semi-vitreous basic rocks 62 " "

Residue {Minerals 14 " "

{Casts of foraminifera 4 " "

---

100

The organic remains consist mainly of tests of minute foraminifera of

the “Globigerina” type, casts of which, both glauconitic and

chalcedonic, occur in the residue. About 88 per cent. of the residue

consist of fine clayey materials less than ·25 mm. in size. The mineral

fragments, which average about ·1 mm. in diameter, are mostly of felspar

with a little pyroxene and brown hornblende.

B. _From the Mbenutha Cliffs, 1,100 feet above the sea._

Carbonate of lime 15 per cent.

{ Fine material mainly derived from

{ the degradation of palagonite 60 " "

Residue { Minerals 23 " "

{ Casts of foraminifera 2 " "

---

100

This rock is somewhat hard, so that the proportion of fine clayey

material, which is however large, cannot be accurately determined. It

shows in places dark streaks composed of an abundance of minute and

often perfect tabular crystals of zoned plagioclase and prisms of

rhombic pyroxene, the size in neither case exceeding half a millimetre,

both of them being derived from the acid andesites of the neighbourhood.

In the slide it displays minute tests of foraminifera of the

“Globigerina” type in a matrix formed mainly of palagonitic debris,

fragments of minerals and semi-vitreous basic rocks. The larger

fragments of the minerals and of the volcanic rocks do not exceed

·15 mm.; but most of the material is very fine. The tests of the

foraminifera are sometimes filled with the matrix, but often they are

entirely of calcite and exhibit in polarised light a dark cross.

C. _From between Natua and Mbatiri, about 290 feet above the sea._

Carbonate of lime 25 per cent.

{ Fine material derived from the degradation of

{ palagonite and of semi-vitreous basic rocks 62 " "

Residue { Minerals 2 " "

{ Casts of foraminifera 11 " "

---

100

This is a relatively deep-water deposit, the foraminifera being minute

and of the “Globigerina” type. About 90 per cent. of the residue

consists of fine clayey material, with which the calcite is so

intimately mixed that each particle is highly refractive and effervesces

freely in an acid. The mineral fragments (pyroxene and felspar) are very

scanty, the largest being less than ·25 mm. The white casts of

foraminifera, composed of chalcedonic silica, form a conspicuous

elements in the residue.

D. _From the vicinity of Mbatiri, 100 feet above the sea._

Carbonate of lime 4 per cent.

Fine material derived from the degradation of palagonite 90 " "

Minerals 2 " "

Casts of foraminifera 4 " "

---

100

About 94 per cent. of the residue consists of fine clayey material. The

fragments of minerals are very scanty and are all less than ·2 mm. in

size. The casts of foraminifera are white and of chalcedonic silica.

From the fineness of the materials and the small size and pelagic

character of the foraminifera, this deposit may be regarded as formed in

relatively deep water.

E. _From the eastern flank of the Wainunu table-land, 200 feet above the

sea._—This is a shallow-water deposit and contains, besides small

gasteropod shells, large flat tests of foraminifera 5 or 6 mm. in

diameter. It possesses 24 per cent. of carbonate of lime, 62 per cent.

of palagonitic debris, &c., and 14 per cent. of minerals.

ALTERED VOLCANIC MUD-ROCKS

This group includes compact hard foraminiferous usually dark-brown

rocks, which exhibit evidence of alteration in their induration, in the

presence of pyrites, and in the chalcedonic quartz filling fine cracks

in the rock-mass. Occasionally special types of alteration occur, one of

which will be referred to in the description of some of the rocks given

below. The proportion of carbonate of lime is generally small; but

sometimes it amounts to 10 per cent. or more.

They admit of being examined in thin sections; and their true nature is

at times so much disguised that I have taken them at first for aphanitic

basic andesites. In the slide they display a few scattered tests of

foraminifera of pelagic habit in a matrix composed of the fine debris of

palagonite and of basic rocks, together with fragments of plagioclase

and pyroxene. Most of the material is very fine, and the size of the

largest mineral fragments does not exceed ·2 mm.

Such rocks, however, are not very frequent. They may be displayed on the

flanks of mountain-ranges buried beneath basic tuffs and agglomerates,

as in the case of Mount Mariko (page 187) and of the mountain-slope

behind Mbale-mbale (page 158); or they may be found at much lower levels

as at Savarekareka Bay (page 190), where, however, they assume sometimes

a peculiar character. Though in the last-named locality the alteration

is possibly connected with thermal metamorphism, it is probable that in

most instances it is a normal interstitial change occurring in beds of

some antiquity which are covered over by a considerable thickness of

later deposits. In places, where these rocks have been subjected to much

hydration in the weathering process, they become red in colour, as is

found on the flanks of Mount Mariko.

SAMPLES OF THE ALTERED VOLCANIC MUD-ROCKS

A. From between 400 and 500 feet above the sea on the south slope of the

Mariko Range.... The characters and mode of occurrence of this rock are

described on page 187.

B. From an elevation of 1,100 feet on the south slope of the Korotini

Range.... The description of the locality will be found on page 160.

This rock is hard and compact and looks like an altered basic rock

showing a few minute specks of pyrites. It is composed of fine

palagonitic debris, and small fragments of semi-vitreous basic rocks and

of crystals of pyroxene and felspar, none of the fragments exceeding

·2 mm. in diameter. Tests of minute foraminifera, filled with matrix and

of the “Globigerina” type, occur very scantily. There is little or no

carbonate of lime; but secondary silica, both colloid and crystalline,

is present as an alteration product.

C. From the vicinity of Yaroi, 30 feet above the sea.... The locality is

described on page 189.

This is a dark grey hard compact rock, containing probably between 10

and 15 per cent. of carbonate of lime, and looking like an altered

limestone. In the section it displays minute tests of foraminifera of

the “Globigerina” type in a matrix composed of fine disintegrated

palagonitic material, impregnated with calcite and containing also

fragments of minerals (augite and felspar), none of which exceed ·2 mm.

in diameter. There are also a few similar-sized fragments of

semi-vitreous basic rocks. Some fine cracks in the rock-mass are filled

with a quartz mosaic. The tests of the foraminifera remain calcitic; but

their cavities are filled either with the matrix or with calcite or with

a colourless fibro-radiate mineral polarising in blackish-blue hues.

D. From the south shore of Savarekareka Bay.... The locality is

described on page 190.

This is a bright green hard compact rock with flinty fracture and not

effervescing with an acid. In the slide it shows a few casts of

foraminifera of the “Globigerina” type in a matrix composed mainly of

fine debris (·01-·04 mm.) of felspar and pyroxene with much greenish

opaque amorphous alteration products. The abundance of pyroxene is

remarkable. The material of the tests of the foraminifera is altogether

replaced by a greenish yellow mineral, occurring in grains and radiating

prisms, apparently epidote.

E. From an elevation of 950 feet on Mount Thambeyu.... The locality is

described on page 177.

A hard dark grey rock containing 10 or 15 per cent. of carbonate of lime

and showing fine specks of pyrites. In the slide are displayed numerous

tests of foraminifera of varying size up to ·5 mm.; scattered patches of

pyrites; fragments of a semi-vitreous basic rock, not exceeding ·15 mm.

in diameter, and of plagioclase and pyroxene; in a matrix of the finest

debris of the same materials impregnated with granules of calcite. The

tests of the foraminifera are filled either with calcite, showing a

black cross in polarised light, or with a zeolite, or with pyrites, or

with the matrix.

SUBMARINE BASIC TUFFS OF MIXED COMPOSITION

These tuffs, which are composed not only of palagonitic materials but

also of the fine detritus of usually semi-vitreous basic rocks, rank

first in frequency amongst the volcanic sedimentary deposits of the

island. In their character they pass on the one hand into the

foraminiferous volcanic mud-rocks or clay rocks and on the other into

the tuff-breccias and tuff-agglomerates. We have here a series beginning

with the agglomerate and ending with the clay that represents in a

general sense the successive stages of the degradation of the same

materials.

Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 Vanua Levu, Fiji · 第 32 / 34 章

(_a_) Porphyritic, where the average size of the plagioclase