THESE extensive inland plains occupy a c_Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 Vanua Levu, Fiji

the island. I estimate that there is an area of about 20 square miles

that does not exceed an elevation of 100 feet above the sea and is often

much less. Of the two villages situated in the midst of these plains,

about 5 miles inland, Kalikoso is about 30 feet and Wainikoro is

scarcely 20 feet above the sea; whilst much of the surrounding district

is similarly low. Taking the 300-feet contour-line as a guide, this

low-lying region, as shown in the map, would be much larger. The plains

are backed on the south by the high mountain-range of Vungalei and

Nailotha. On the north the coast-ranges, which attain a height of 1,100

or 1,200 feet, separate them from the sea-border; whilst on the west

they are shut off from the Mbuthai-sau valley by a line of hills, of

which the minimum elevation is about 700 feet. This region is occupied

by the scanty open vegetation characteristic of the “talasinga” or

“sun-burnt” districts. The tall “Ngasau” reed is common; the Pandanus

trees are frequent; and amongst the bushes and small trees are

represented “Dodonæa viscosa,” “Morinda citrifolia,” and a species of

“Hibbertia.”

Different rivers and streams, rising in the mountains on the south side

of the plains, traverse this area; and after breaking through the

coast-ranges reach the sea. They acquire an exaggerated size from the

circumstance that they are in great part tidal estuaries. The tide

ascends them for several miles reaching behind the coast-ranges into the

plains; whilst the dense mangroves, which line their lower courses

amongst the hills, extend beyond into the low-lying level districts

farther inland. Boats can follow up the winding course of the Wainikoro

River until they reach the village of that name, which lies about five

and a half miles in a direct line from the coast and nearly in the

centre of this part of the island. The mangroves extend up to the

village. The Langa-langa River, which is of much smaller size, is

similarly navigable for three or four miles. The mangroves that line its

banks spread out in broad tracts when in ascending the river we emerge

from the hills into the plains. Above the influence of the tide it

dwindles into a stream ridiculously small. The same remarks apply to the

river two miles to the eastward. The Vui-na-savu River near the eastern

margin of this low-lying region can be ascended by boats into the heart

of the island.

From the foregoing remarks it will be expected that some portion of this

low-lying inland region will be occupied by swamps. This is in truth the

case. About one and a half miles north-east from the village of Kalikoso

is a small fresh-water lake, about 100 yards across, which lies in a

slight depression in the plains and is surrounded by a wide margin of

swampy ground, occupied by reeds and sedges, in which one sinks

knee-deep when approaching the banks. The level of the surface of the

lake is not over 25 feet above the sea, and only a foot or so below that

of the plains around. Since the depth, as I ascertained it, is 15 to 18

feet, it follows that the bottom of the lake is only a few feet above

the high-tide level. The mangroves extend to within a mile of its

border; and it is possible that though lying about five miles inland, it

may be indirectly affected by unusually high tides. It would be

interesting to determine whether the water is ever brackish.

This small lake is, or was, regarded with superstitious awe by the

natives on account of the floating islands that it contains. Different

legends are connected with it, and the Fijians have given it the name of

“Vaka-lalatha,” in allusion to the drifting of the islands from one side

of the lake to the other, the small trees growing upon them acting “in

the manner of sails.” Mr. Horne, who was in this locality in 1878,

refers to the lake in his book, _A Year in Fiji_ (p. 24); but does not

appear to have seen it. Mr. Thomson[102] visited it in 1880, and

describes it as containing a single floating island, a quarter to a half

an acre in extent. The island was in existence, the natives said, in the

days of their great-grandfathers, a statement indicating that the people

of the district had no reason to doubt its antiquity. A chief, who

formed one of the party, dived under the island.

When I visited this lake in 1899 there were three floating islands,

named by the natives “Wanga levu” and “Wanga lailai,” that is, “Large

canoe” and “Small canoe.” The largest was 90 or 100 feet long, whilst

the two smaller were about 50 feet long, the breadth being less than

half the length. They are composed of a dense growth of reeds and sedges

supporting small living trees 10 to 17 feet in height, swamp ferns, and

other smaller vegetation, the whole forming fairly solid

standing-ground, and doubtless possessing considerable thickness.

The origin of these floating islands is probably to be found in the

circumstance that the dense mass of swamp-vegetation lies on a rocky

substratum, and that during some unusually heavy flood large portions of

the swampy soil-cap became detached and floated up. A floating island

occurs near the sea in the Lauthala district on the Lower Rewa in Viti

Levu. The floating island in Derwentwater in the north of England is

said to be “a blister of sublacustrine turf.” Those in Russia and

Hungary, according to Mr. Hanusz, appear to be felted masses of

tree-trunks, branches, and marsh-plants thinly covered with soil.[103]

I will preface my remarks on the geology of the Wainikoro and Kalikoso

plains by observing that this region displays three conspicuous

features. In the first place, it is a region of acid rocks, mostly

altered tuffs, derived from quartz-porphyries, the alteration consisting

in the deposition of quartz often of the chalcedonic type. In the second

place, a silicifying process has been in operation here on an extensive

scale, as evidenced by the abundance of silicified corals lying on the

surface, especially in the district around the lake and by the abundance

of concretions of chalcedony, of fragments of chalcedonic flints, and of

portions of white chalcedonic quartz-rock that in places strew the

ground. In the third place, earthy limonite, or bog iron ore, has been

produced in quantity, particularly around the lake; and in places small

round concretions of impure carbonate of iron cover the soil. Before

referring more in detail to the different parts of this region, it may

be remarked that the silicified corals, flints, iron-ore deposits, &c.,

of this and other parts of the island are dealt with in Chapter XXV.

This region of acid rocks extends about two miles to the west and

south-west of the village of Wainikoro. Here prevail white acid tuffs

often decomposing and altered by the formation of secondary quartz. They

are derived from the degradation of quartz-porphyries or rhyolitic

rocks. The surface is irregular but the elevation is small, the 100-feet

level lying about one mile and the 200-feet level about two miles west

of Wainikoro.

The villages of Wainikoro and Kalikoso are from two to three miles

apart, the intervening district not attaining a greater elevation than

70 feet above the sea. Decomposing altered white acid tuffs here occur

with occasional large blocks, a couple of tons in weight, of apparently

a quartz-porphyry or trachyte with its structure disguised by

silicification. Fragments of siliceous concretions, as chalcedony,

chalcedonic flints, &c., lie on the surface, the soil being friable and

of a deep ochreous red; whilst in places the ground is covered with

round marble-sized concretions composed of a mixture of carbonate of

iron and limonite which I have described on page 356. A few hundred

yards to the north-west of Kalikoso, where there is a little rise, a

decomposed quartz-porphyry is exposed displaying rounded crystals of

quartz fractured in position, the matrix of the rock being impregnated

with chalcedony. In one part of this mound there is a friable white

rock, composed of a crumbling mass of small irregular quartz-grains,

rarely showing crystal-faces. It appears to be disintegrated

quartz-felsite.

In taking the track from Kalikoso to the village of Vungalei, which is

distant about 2½ miles to the south-east, one traverses after the first

mile, where the acid rocks terminate, a rather more elevated region

which rises to a maximum height of 180 feet above the sea. Though the

acid rocks give place to a semi-vitreous pyroxene-andesite as described

on page 212, small fragments of chalcedonic flints are frequent on the

surface over both areas. The district that intervenes between Kalikoso

and the landing-place on the Langa-langa River, about two miles in

length, is not more than 30 feet above the sea, and is crossed by small

sluggish streams, in the beds of which occur numerous fragments of

silicified corals, up to 3 or 4 inches in size, belonging to the Porites

and Astræan type. In these stream-beds also occur bits of mamillated

chalcedony and of onyx, flattish agates, 3 or 4 inches across, and

pebbles of the compact pyroxene-andesite above alluded to, the last

probably derived from the south side of the plains.

The plains extend in a north-east direction as far as Numbu, which lies

between 2½ and 3 miles north-east of Kalikoso. The country between these

two places is usually elevated between 60 and 100 feet and is never more

than 130 feet above the sea. It is known as the Kuru-kuru district. The

surface is strewn with the fragments of flints and of a white

chalcedonic quartz-rock; whilst the soil is friable and deep-red in

colour, limonite in abundant fragments occurring on the ground. Here and

there one passes slabs of a hard white silicified tuff, small portions

of which are frequent on the surface.

Silicified corals and earthy limonite are to be found in abundance

scattered over the surface of the plains immediately surrounding the

small lake of Vakalalatha. We also find lying on the ground in this

district bits of chalcedony and onyx, portions of chalcedonic flints,

and nodules of the size of the fist, which when fractured either

disclose the regular layers of the agate or radiating crystals of

quartz. The silicified corals occur usually in fragments not over 4

inches across, and include portions of branching corals of the Madrepore

habit, bits of massive corals of the Astræan type, small rounded nobs of

“Porites” just as one commonly observes on a reef-flat, &c. They have an

ancient weathered look, and in some cases it is evident from the

existence of boring-shells in the fractured end of a branching coral

that it once lay as a dead fragment on the surface of a reef-flat.

In Chapter XXV. the characters of the silicified corals of the island

are discussed in detail; and I have there advanced the view that the

extensive silicification of the Kalikoso and Wainikoro plains took place

during the consolidation of the calcareous muds of a reef-flat whilst

the land was emerging. I have already alluded on page 222 to an area of

silicification on the neighbouring sea-border between Visongo and Tutu.

There can be no doubt that during the last stage of the emergence the

present situation of the fresh-water lake near Kalikoso was occupied by

the sea, which also extended far over the surrounding plains. The

process of silicification would of necessity be restricted to the period

that has since elapsed; and the discussion is therefore confined to the

nature of the conditions under which this change was induced. As a

factor in the process we cannot disregard the acid character of the

rocks of the district.

THE UNDU PROMONTORY

The north-east portion of the island terminates in a long narrow

promontory which I have named after Undu Point at its extremity.

Commencing at Thawaro Bay and near Tawaki it runs in a straight line for

a distance of between 13 and 14 miles, its breadth varying between 1½

and 2½ miles. Its greatest elevation of nearly 1,600 feet is attained at

its western end; and it diminishes irregularly in height as one proceeds

towards Undu Point, where a height of 400 feet is maintained about a

mile from the cape. It is bordered by reefs sometimes a mile in breadth,

and the reefs prolong the promontory for another three miles beyond Undu

Point. As indicated by the 100-fathom line, the submarine contour

corresponds to that of the land, and the extent of marine erosion during

the existing relations of land and sea is evidently displayed in the

breadth of the reefs. I found no sign of upraised reefs; and although

diligent inquiries were made nothing could be learned of any hot

springs.

It will be seen from the following remarks that pumice-tuffs,

quartz-porphyries, and oligoclase-trachytes, are the prevailing rocks.

On the north side they may be regarded as continuous with the acid rocks

of the region extending from near Lambasa to Thawaro Bay. On the south

side they commence in the vicinity of Tawaki.

(1) THE DISTRICT EXTENDING TWO AND A HALF MILES WEST OF TAWAKI

When proceeding eastward along the north coast of Natewa Bay one enters

the region of acid rocks between 2 and 2½ miles west of Tawaki. Here the

country is much broken, picturesque hills with bare precipitous faces

rising up near the coast, one of which named Natoto has a rudely conical

and truncated form. Grey oligoclase-trachytes having a specific gravity

of 2·4 and possessing the characters described on page 308, prevail in

the district extending west of Tawaki. Sometimes they occur in mass; but

they often form agglomerates. A singular pitchstone-agglomerate occurs

at the coast at the foot of Natoto. The pitchstone, which has a specific

gravity of 2·48, is a semi-vitreous form of a hypersthene-augite

andesite. It shows abundant small pyroxene prisms in its glassy

groundmass and is referred to the prismatic sub-order (5) described on

page 289.

(2) NAITHOMBOTHOMBO RANGE

A high range of hills, forming the backbone of this part of the island,

extends eastward for about five miles from Thawaro and Tawaki. It is

named “Naithombothombo” in the Admiralty chart. From it rise two

conspicuous peaks, Thawaro Peak (1,573 feet) at its western end, where

it overlooks the village of that name, and Mount Thuku (1,288 feet) near

its eastern end. West of Thawaro Peak this range is connected with the

hills beyond by a saddle 600 feet in height, which is ascended when

crossing the promontory from Thawaro Bay to Tawaki.

(_a_) _Thawaro Peak._—This represents an old “volcanic neck” of

agglomerate rising out of the tuffs that are exposed on its slopes to an

elevation of about 600 feet. As viewed from the saddle above mentioned,

the upper part of the hill presents bare precipitous sides, several

hundred feet in height, of agglomerate, the blocks of which are composed

of a compact hypersthene-augite andesite (sp. gr. 2·48). It displays a

few small phenocrysts of medium andesine and of rhombic and monoclinic

pyroxene; and is referred to the prismatic sub-order (5) described on

page 289, characterised by prismatic pyroxene in the groundmass.

(_b_) _South coast between Tawaki and the foot of Mount Thuku._—The tall

cliffs that rise to a height of from 200 to 300 feet behind Tawaki are

composed of white tuffs and agglomerate-tuffs derived from the acid

rocks of the district. Eastward from Tawaki to the base of Mount Thuku

the coast scenery is particularly fine. A little inland a line of hills,

named “Na Kula,” rises precipitously to a height of 700 or 800 feet, and

in the vertical sides are displayed tuffs and agglomerates probably of

the character of those above noticed. Light-coloured tuffs are sometimes

exposed at the coast in which are inclosed fragments varying in size of

a pitchstone[104] (sp. gr. 2·36) approaching in structure a trachytic

glass. At one place the tuffs were evidently sedimentary and bedded, the

dip being about 15° N.W.

The massive rock most frequently exposed at the coast and on the

hill-slopes between Tawaki and Mount Thuku is a quartz porphyry[105]

displaying abundant porphyritic crystals of quartz and felspar in a

groundmass originally semi-vitreous but now obscurely felsitic in

character. The shore-flat for more than half a mile west of Mount Thuku

is strewn with great numbers of detached columnar blocks, 12 to 15

inches across, of a slightly vesicular oligoclase-trachyte of the type

described on page 308.

(_c_) _North coast between Thawaro Bay and the foot of Mount

Thuku._—Coarse and fine tuffs prevail at the coast and on the

neighbouring hill-slopes; and in the bare rocky faces of the hills

inland they are also to be observed. The streams have worn deep gorges

into their mass. Towards Thuku they are acid and pumiceous, and are

evidently the products of eruption. Towards Thawaro, they are more basic

and darker, and are in part at least to be attributed to marine

degradation.

(_d_) _Mount Thuku._—My ascent of this hill, which is 1,288 feet in

height, was made from the north coast. I found it to be composed from

the foot to the summit of white pumiceous tuffs without any evident

arrangement. It has a narrow top and shows no sign of a crateral

cavity. The hills east and west, as viewed from its summit, are

ridge-shaped and display nothing in their configuration at all

suggestive of craters. The pumice-tuffs of Mount Thuku are

non-calcareous, and exhibit greyish pumiceous lapilli in an abundant

white matrix formed of fine pumice-debris. Under the microscope it

shows the characteristic vacuolar and fibrillar structure; but the

material has not the fresh appearance of ordinary pumice and the

minute cavities are often filled with alteration products. The two

rocky points on the north coast opposite the hill are formed in one

case of a somewhat altered oligoclase-trachyte and in the other of a

quartz-porphyry. Both no doubt represent intrusive masses, the almost

horizontal columns, 12 to 15 inches in diameter, of the former

indicating a nearly vertical dyke.

(3) THE UNDU PROMONTORY EAST OF MOUNT THUKU

East of Mount Thuku the hilly backbone of the promontory is of much less

elevation. About three miles to the eastward the highest hill is 630

feet, and thence to within a mile or two of Undu Point the hills retain

a height of 400 to 500 feet.

(_a_) _The north coast between Mount Thuku and the coast village of

Nuku-ndamu._—On this stretch of coast, about five miles in length, the

shore-cliffs are composed of white and pale-yellow, coarse and fine

stratified pumice-tuffs, the beds being either horizontal or with a

gentle dip northward. They are as a rule non-calcareous, and contain

some quartz grains and small bits, 1 to 3 millimetres in size, of bottle

green compact obsidian, much as one finds in Lipari pumice-tuffs. In

general character, both naked-eye and microscopic, they correspond to

the Mount Thuku pumice-tuffs above described. Large blocks of basic

rocks are occasionally to be observed on this coast, sometimes probably

indicating dykes, but in one place near Mount Thuku forming an

agglomerate. The rock is a dark-grey augite-andesite with a specific

gravity of 2·77. It is compact and has a hemi-crystalline

groundmass.[106]

(_b_) _The south coast between Mount Thuku and Moala, a distance of

about five miles._—Pumice-tuffs and agglomerates are displayed at the

coast, the former often bedded and in one place having a dip of 35 or 40

degrees to the north. A quartz-porphyry, somewhat banded and a little

altered, and displaying rounded quartz crystals 3 or 4 millimetres in

size, is the prevailing massive rock exposed on the hill-slopes and

occasionally at the coast. It is well exhibited about a mile east of

Mount Thuku. Blocks of a grey oligoclase-trachyte also occur. These

rocks are described on pages 308, 309.

(_c_) _The terminal portion of the promontory from Nuku-ndamu and Moala

to Undu Point._—The same pumiceous tuffs, usually non-calcareous, form

the shore-cliffs on the south coast from Moala to Mr. Bulling’s station

at Ndothiu, which lies about 2 miles from the point. On the

corresponding part of the north coast between Nuku-ndamu and Ndothiu

these tuffs are often calcareous; and near the first-named place they

contain sub-angular bits of coral of the size of a walnut. On the beach

at Vunikondi in this locality they are overlain by nearly horizontal

beds of basic lava, the upper surface of which when exposed displays the

smooth, “ropy” crust of a lava flow. The rock is a dark slightly

vesicular augite-andesite, hemi-crystalline in structure, and containing

a fair amount of residual glass.[107] Since the underlying tuffs were

evidently deposited on a sea-bottom, it follows that this is a submarine

flow. I intended to revisit this locality, but was prevented. A detailed

examination of it would be worth undertaking.

From Ndothiu to Undu Point, about 2 miles distant, the interior of the

promontory has an undulating surface, the elevation being usually 200 or

300 feet and rising to 400 feet. On the coasts are exposed bedded

pumiceous tuffs, steeply inclined and usually calcareous. As displayed

in the hill-slopes of the interior they are horizontally stratified and

as a rule non-calcareous. These deposits sometimes exhibit a spheroidal

arrangement indicative of the proximity of a dyke. In one or two places

at the coasts occur basic agglomerates, formed of the same

augite-andesite lava-rock of which the Vunikondi beds are composed, but

scoriaceous and containing more glass in the groundmass. In the

hand-specimen beside me, the steam cavities are of all sizes, from that

of a pin-prick to a third of an inch (8 mm.) and are generally

elongated.

A careful search of the tuff-deposits in this part of the Undu

promontory ought to result in the discovery of remains, both of plants

and of marine mollusks. Mr. Chalmers informed me that fossilised

tree-trunks occur on the coast near Vunikondi; but I was unfortunately

not able to discover them.

* * * * *

BRIEF SUMMARY OF THE GENERAL CHARACTERS OF THE UNDU PROMONTORY FROM

THAWARO AND TAWAKI TO UNDU POINT.—One suggestive negative feature of

this region is to be found in the absence, as far as I could ascertain,

of any trace of a crater. Here also, as in the area of acid rocks

extending westward along the north side of the island to near Lambasa,

hot springs are not to be found. The prevailing rocks are in the first

place the pumice-tuffs, which not only as a rule form the coast-cliffs,

but occur inland as high as the summit of Mount Thuku almost 1,300 feet

above the sea. They were probably in great part ejected from sub-aërial

vents, though no doubt, as in the vicinity of Undu Point, they were

often deposited under the sea. The acid and basic tuffs in the vicinity

of Tawaki and Thawaro are, as I imagine, largely derived from marine

degradation. Next to the pumice-tuffs, massive quartz-porphyries and

oligoclase-trachytes are the characteristic rocks. They are probably in

most cases intrusive, and present themselves sometimes as vertical

columnar dykes, evidently of considerable dimensions.

The basic rocks, however, are not unrepresented. They occur in one or

two places as agglomerates, as in the vicinity of Mount Thuku on the

north coast and near Undu Point; whilst they form “flows” overlying the

pumiceous tuffs at Vunikondi. Occasional blocks lying on the surface on

the north coast are indications of dykes. The basic rocks, nevertheless,

take a very secondary part in the composition of the Undu Promontory.

They are in most cases to be referred to the augite-andesites with a

specific gravity 2·6 to 2·77; but some, as in the case of those forming

the agglomerates of Thawaro Peak, are hypersthene-augite andesites with

specific gravity of about 2·5. Olivine-basalts are not represented.

The vents, from which the materials forming this promontory were

ejected, were arranged in a single straight line for a length of 14

miles. Along this linear fissure, which was probably submarine, vents

were at different times formed; and though owing to sub-aerial and

marine degradation the present surface has been since shaped and

reshaped, their situation may still be recognised in the “necks” of tuff

and agglomerate that form the peaks, and in the large dykes or sills of

quartz-porphyry and oligoclase-trachyte.

CHAPTER XVII

THE VOLCANIC ROCKS OF VANUA LEVU

THE varied character of the volcanic rocks in my collection is brought

out in the following Table, where I have grouped about 400

rock-sections, excluding those of the tuffs and finer detrital deposits.

The small proportion of plutonic rocks should be noted.

Olivine-basalts 23 per cent.

Augite-andesites 40 " "

Hypersthene-augite-andesites 17 " "

Acid andesites, including hornblende

and quartz-andesites, &c. 12 " "

Oligoclase-trachytes, quartz-porphyries

or rhyolites 6 " "

Hypersthene-gabbros and diorites 2 " "

----

100

In order to avoid the necessity of frequently describing rocks of the

same type it has been found requisite to devise a method of

classification. In carrying out this somewhat laborious task I have

often been surprised at the readiness with which rocks, the relations of

which had been previously very difficult to ascertain, fell into their

place in the scheme. Although many of my uncertainties have been thus

removed, a large number of doubtful points remain. I venture to think,

however, that others may be able to employ and also to extend the method

of classification here employed.

The general plan followed has been worked out in detail for the

olivine-basalts and the pyroxene-andesites of the more basic type;

whilst lack of materials has prevented its further elaboration in the

case of the acid andesites, oligoclase-trachytes, quartz-porphyries, &c.

The treatment of all the classes has been uniform, the scheme being the

same whether applied to a basalt or to a dacite.

In describing the general method I will take the Augite class. As will

be seen in the Synopsis that follows, this class is first divided

according to the absence or presence of a groundmass into two

sub-classes, the first referring to the plutonic rocks, which, however,

are not represented in my collection, the second comprising the

augite-andesites which make up 40 per cent. of the total. These

andesites are again divided into four orders, according as the

groundmass presents parallel or non-parallel felspar-lathes, or short

and stout felspars (orthophyric), or displays a felsitic character. The

last two orders are practically unrepresented here, though many examples

of them are found amongst the more acid andesites. Each order is then

split up into three sub-orders depending on the nature of the pyroxene

of the groundmass, whether granular, prismatic, or ophitic.

Each sub-order is broken up into two sections, one displaying

plagioclase-phenocrysts, the other without them, or possessing very few

of them. The first section is divided into two genera, according to the

character of the plagioclase-phenocrysts, whether glassy or opaque, the

second genus often comprising rocks allied to the porphyrites. The

second or aphanitic section is subdivided into two genera according to

the character of the pyroxene-phenocrysts; in the one case they are

macroporphyritic; in the other they are either small or absent. The

genera are split into four species according to the length of the

felspar-lathes, a method which readily separates out the doleritic

rocks. In cases where the materials are abundant, the genera have been

first divided into porphyritic and non-porphyritic sub-genera, based on

the macroporphyritic or the micro-porphyritic character of the

plagioclase-phenocrysts, when present. The species can be also split

into sub-species, according to the degree of basicity of the rocks, as

indicated by the specific gravity.

This method is fully worked out in the later pages and need not be

further described here. With abundant material from different regions it

appears to me that a ready mode is here afforded of assigning to a rock

its place in the scheme. In this way it would be possible to follow the

systematist in his method of comparing plants and animals from different

localities. To facilitate this end, I have suggested in the synopsis the

employment of abbreviations, so that the description of the critical

characters of a rock can be condensed into a formula capable of easy

interpretation.

As an example of the use of these abbreviations I will take the instance

of a common form of augite-andesite which is represented by the

formula:—“_Plag, aug, matr, flu, gran, non-phen, parv, ·1-·2 mm._” This

is the formula for an aphanitic augite-andesite, and it signifies that

it is a rock of the plagioclase-augite class possessing a groundmass

showing parallel felspar-lathes (between ·1 and ·2 mm, in average

length) and granular pyroxene, and displaying no phenocrysts of

plagioclase or only a very few of small size, whilst pyroxene

phenocrysts if present are micro-porphyritic.

As another example the following formula for a type of porphyrite found

in this island may be given:—_Plag, hypersth-aug, matr, orth, prism,

phen, opac_. This is an andesite in which rhombic and monoclinic

pyroxene are associated both in the phenocrysts and in the groundmass

where the pyroxene is prismatic and not granular. The plagioclase

phenocrysts are opaque and the felspars of the groundmass are of the

orthophyric type.

There are one or two points that require further reference. In the first

place the early employment in the scheme of the characters of the

felspars of the groundmass for distinguishing the orders scarcely seems

needed in the cases where they take the lathe-form; but the importance

of its early use is shown in the acid andesites where it is certainly of

prime importance in an early stage of the classification to distinguish

the rocks by the character of the felspars of the groundmass, whether

lathe-like, orthophyric, or felsitic. It may also be objected that the

two orders obtained by dividing the lathe group into “parallel” and

“non-parallel” divisions are not equivalents of the two other orders,

the orthophyric and the felsitic. The distinction, however, between the

flow or non-flow arrangement, though in practice not always readily

established, is a far-reaching one. On _à priori_ grounds the first

division might be expected to have no plutonic equivalent; whilst in the

second division, it is easy to trace the gradations through the

doleritic stage, where the felspar-lathes are very large, to the

granitoid condition. Then, again, the ophitic habit is as a general rule

confined to rocks with a doleritic or semi-doleritic groundmass, where

the felspar-lathes are coarse and form a mesh-work. Two quite distinct

lines of development unite in the felspar-lathes and begin to diverge

with the difference in their arrangement in the groundmass.

The nature of the difference between the flow and non-flow arrangement

of the felspar-lathes is well brought out in some dykes of basalt and

augite-andesite that I examined in this island and also in the Valle del

Bove on the Etna slopes. In the outer vitreous portion the

felspar-lathes, which are fairly well represented, are all about the

same length and are more or less parallel with the sides of the dyke. In

the central more crystalline portion two sets of lathes can be

distinguished, one (_A_) corresponding in the average length and in the

flow-arrangement with the lathes of the borders, the other (_B_) being

about half the length and forming a plexus between the larger parallel

lathes. Those of the _A_ set, which are those that usually catch the eye

in a section, are contemporaneous in their origin with those in the

margins of the dyke; whilst those of the _B_ set have been subsequently

formed. In the preliminary “stiffening” of the first stage of

consolidation, the whole mass of the dyke would be affected. To this

stage the lathes of the _A_ set belong; whilst to the later stage of

consolidation which would proceed much more slowly in the interior than

at the margins of the dyke, the lathes of the _B_ set are to be

referred. This distinction so plainly illustrated in a dyke must be

postulated for all intrusive masses; but I have not yet found it

possible to make much use of it. Much ground will have first to be

cleared before it can be safely employed, since it is apparent, for

instance, that there are often all gradations in a slide between a lathe

and a phenocryst, and that the term “phenocryst” is applied to crystals

having very different histories.

[Illustration: Diagram illustrating the two sets of felspar-lathes in a

dyke: _A_, long and parallel, dispersed through the mass. _B_, short and

non-parallel and found only in the centre.]

With regard to the ophitic habit of some of the basaltic rocks the

following conclusions may be drawn:

(_a_) Typical ophitic “plates” are not very common in the slides. More

frequently the habit of the pyroxene is semi-ophitic.

(_b_) This character is as a general rule associated with the plexus or

non-fluidal arrangement of the felspar lathes.

(_c_) The felspar-lathes are nearly always large, frequently averaging

more than ·2 mm. in length. This coarse doleritic groundmass is almost

diagnostic of an ophitic rock.

* * * * *

It is not always possible to allow for the influence of locality in

drawing up such a classification as this, since it is well known that in

each volcanic region the rocks have a particular facies recognisable in

hand-specimens as well as in the slide, though it is not easy to express

such a distinction in a definition. Perhaps this is represented in

“adaptation” as we find it in the organic world, and the question arises

as to the value of such characters for critical purposes. Regional

variation plays such an important part that it cannot be ignored in

rock-classification.

SYNOPSIS

I. CLASS.—OLIVINE ROCKS (_Plag., oliv._).

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

| SUB-CLASS. | DIVISION. | ORDER.[108] |

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

| 1. | | |

| No groundmass | | |

| (_non-matr._) | | |

| LIMBURGITES, | |(Not represented in collection.) |

| PERIDOTITES, | | |

| OLIVINE | | |

| GABBROS, | | |

| etc. | | { 1. |

| | | { Felspar lathes not |

| |{ 1. | { in flow-arrangement |

| |{Olivine abundant | { (_non-flu._). |

| |{ (_cop._) | { 2. |

| |{ | { Felspar lathes |

| 2. |{ | { in flow-arrangement |

| Groundmass |{ | { (_flu._). |

| (_matr._) |{ | |

| OLIVINE-BASALTS|{ | { 3. |

| |{ | { Felspar lathes not |

| |{ 2. | { in flow-arrangement |

| |{Olivine | { (_non-flu._). |

| |{relatively | { 4. |

| |{scanty | { Felspar lathes |

| |{(_pauc._). | { in flow-arrangement |

| | | { (_flu._). |

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

II. CLASS.—AUGITE ROCKS (_Plag., aug._).

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

| SUB-CLASS. | ORDER. |

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

| | |

| 1. | |

| No groundmass | Not represented in collection. |

| (_non-matr._) | |

| GABBROS, in part. | |

| |{ 1. |

| |{ Felspar lathes not in flow-arrangement |

| |{ (_non-flu._). |

| 2. |{ |

| Groundmass (_matr._) |{ 2. |

| BASALTIC ANDESITES; |{ Felspar lathes in flow-arrangement |

| AUGITE-ANDESITES; |{ (_flu._). |

| PYROXENE-ANDESITES |{ 3. |

| in part. |{ Felspars short and broad (orthophyric, |

| |{ _orth._).[109] |

| |{ 4. |

| |{ Groundmass felsitic (_fels._)[109] |

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

III. CLASS.—HYPERSTHENE-AUGITE ROCKS (_Plag., hypersth.-aug._).

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

| SUB-CLASS. | ORDER. |

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

| 1. | |

|No groundmass (_non-matr._) |Represented by the Hypersthene-gabbros |

| GABBROS, in part. | in collection. |

| | |

| |{ 1. |

| |{Felspar lathes not in flow-arrangement|

| |{ (_non-flu._). |

| |{ |

| |{ 2. |

| 2. |{Felspar lathes in flow-arrangement |

| Groundmass (_matr._) |{ (_flu._). |

|PYROXENE-ANDESITES, in part.|{ |

| |{ 3. |

| |{Felspars of the groundmass short and |

| |{ broad (orthophyric, _orth._). |

| |{ |

| |{ 4. |

| |{Groundmass granular or presenting a |

| |{ mosaic (felsitic, _fels._). |

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

IV. CLASS.—HYPERSTHENE ROCKS.

V. CLASS.—HORNBLENDE-HYPERSTHENE ROCKS.

VI. CLASS.—QUARTZ-HORNBLENDE-HYPERSTHENE ROCKS OR DACITES.

These last three classes are merely provisional. They include the Acid

Andesites of Vanua Levu, which are all characterised by the prevalence

of rhombic pyroxene amongst the phenocrysts and by its predominance or

rather by its usually exclusive occurrence in the groundmass. All the

classes are capable of being split up into two sub-classes and four

orders as in the case of the third class. The characters of these rocks

are given in Chapter XXI.

VII. CLASS.—HORNBLENDE ROCKS.

In this class are included those rocks where hornblende is the only

ferro-magnesian mineral. It is only represented by two diorites

described on page 251.

VIII. CLASS.—OLIGOCLASE-TRACHYTES. }

IX. CLASS.—QUARTZ-PORPHYRIES AND RHYOLITES } Described in Chapter XXI.

FIRST ORDER OF THE OLIVINE BASALTS.

CHARACTERS.—Olivine in abundance. Felspars of the groundmass

not in flow-arrangement.

FORMULA.—_Oliv., matr., cop., non-flu._

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

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

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

| | |{ 1. |{·02-·1 mm. | 1 |

| | |{ Glassy |{ ·1-·2 " | 2 |

| |{ 1. |{plagioclase|{ ·2-·3 " | |

| |{ Plagioclase |{phenocrysts|{ ·3-·5 " | |

| |{ phenocrysts |{ (_vitr._) | | |

| |{ (_phen._) |{ | | |

| |{ |{ 2. |{·02-·1 mm. | |

| |{ |{ Opaque |{ ·1-·2 " | 1 |

| |{ |{plagioclase|{ ·2-·3 " | 2 |

| |{ |{phenocrysts|{ ·3-·5 " | |

| |{ |{ (_opac._) | | |

| 1. |{ | | | |

| Granular |{ |{ 3. | | |

| (_gran._) |{ |{ Large |{·02-·1 mm. | |

| |{ |{phenocrysts|{ ·1-·2 " | |

| |{ |{of olivine |{ ·2-·3 " | |

| |{ 2. |{ and |{ ·3-·5 " | 1 |

| |{ No |{ pyroxene, | | |

| |{ plagioclase |{ over 2 mm.| | |

| |{ phenocrysts |{ (_magn._) | | |

| | |{ | | |

| |{ (_non-phen._)|{ 4. | | |

| | |{ Small |{·02-·1 mm. | |

| | |{phenocrysts|{ ·1-·2 " | |

| | |{of olivine |{ ·2-·3 " | |

| | |{ and |{ ·3-·5 " | 1 |

| | |{ pyroxene | | |

| | |{under 2 mm.| | |

| | |{ Pyroxene | | |

| | |{scanty and | | |

| | |{ often | | |

| | |{ absent | | |

| | |{ (_parv._) | | |

| | | | | |

| | |{5. _Vitr._ |} | |

| |{ 3. _Phen._ |{ |} | |

| 2. |{ |{6. _Opac._ |} | |

| Prismatic |{ | |} |{ Not |

| (_prism._) |{ |{7. _Magn._ |} |{ represented.|

| |{4. _Non-phen._|{ |} | |

| | |{8. _Parv._ |} | |

| | | | | |

| | |{9. _Vitr._ |} | |

| |{ 5. _Phen._ |{ |} |{ Not |

| 3. |{ |{10. _Opac._|} |{ represented.|

| Ophitic |{ | | | |

| (_oph._) |{ |{11. _Magn._| |{ Not |

| |{6. _Non-phen._|{ | |{ represented.|

| | |{12. _Parv._| ·1-·2 mm. | 1 |

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

SECOND ORDER OF THE OLIVINE-BASALTS.

CHARACTERS.—Olivine in abundance. Felspars of the groundmass

in flow-arrangement.

FORMULA.—_Oliv., matr., cop., flu._

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

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

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

| | |{ 13. |{·02-·1 mm. | 3 |

| | |{ Glassy |{ ·1-·2 " | 2 |

| | |{plagioclase|{ ·2-·3 " | 1 |

| |{ 7. |{phenocrysts|{ ·3-·5 " | |

| |{ Plagioclase |{ (_vitr._) | | |

| |{ phenocrysts |{ | | |

| |{ (_phen._) |{ 14. |{·02-·1 mm. | |

| |{ |{ Opaque |{ ·1-·2 " | 3 |

| |{ |{plagioclase|{ ·2-·3 " | |

| |{ |{phenocrysts|{ ·3-·5 " | |

| |{ |{ (_opac._) | | |

| 4. |{ | | | |

| Granular |{ |{ 15. | | |

| (_gran._) |{ |{ Large |{·02-·1 mm. | 2 |

| |{ |{phenocrysts|{ ·1-·2 " | |

| |{ |{of olivine |{ ·2-·3 " | |

| |{ |{ and |{ ·3-·5 " | |

| |{ |{ pyroxene, | | |

| |{ |{ over 2 mm.| | |

| | |{ (_magn._) | | |

| |{ 8. |{ | | |

| |{ No |{ 16. |} | |

| |{ plagioclase |{ Small |} | |

| |{ phenocrysts |{phenocrysts|} | |

| |{ (_non-phen._)|{of olivine |}·02-·1 mm. | 1 |

| | |{ and |} ·1-·2 " | 5 |

| | |{ pyroxene, |} ·2-·3 " | 2 |

| | |{under 2 mm.|} ·3-·5 " | 1 |

| | |{ Pyroxene |} | |

| | |{ scanty and|} | |

| | |{ often |} | |

| | |{ absent | | |

| | |{ (_parv._) | | |

| | | | | |

| | |{17. _Vitr._|} | |

| |{ 9. _Phen._ |{ |} | |

| 5. |{ |{18. _Opac._|} |{ Not |

| Prismatic |{ | |} |{ represented.|

| (_prism_.) |{ |{19. _Magn._|} | |

| |{ 10. |{ |} | |

| |{ _Non-phen._ |{20. _Parv._|} | |

| | | | | |

| | |{21. _Vitr._|} | |

| |{ 11. _Phen._ |{ |} | |

| 6. |{ |{22. _Opac._|} |{ Not |

| Ophitic |{ | |} |{ represented.|

| (_oph._) |{ |{23. _Magn._|} | |

| |{ 12. |{ |} | |

| |{ _Non-phen._ |{24. _Parv._|} | |

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

THIRD ORDER OF THE OLIVINE-BASALTS.

CHARACTERS.—Olivine scanty. Felspars of the groundmass not

in flow-arrangement.

FORMULA.—_Oliv., matr., pauc., non-flu._

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

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

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

| | |{ 25. |{·02-·1 mm. | 1 |

| | |{ Glassy |{ ·1-·2 " | 13 |

| |{ 13. |{plagioclase|{ ·2-·3 " | 5 |

| |{ Plagioclase |{phenocrysts|{ ·3-·5 " | |

| |{ phenocrysts |{ (_vitr._) | | |

| |{ (_phen._) |{ | | |

| |{ |{ 26. |{·02-·1 mm. | |

| |{ |{ Opaque |{ ·1-·2 " | 4 |

| |{ |{plagioclase|{ ·2-·3 " | |

| |{ |{phenocrysts|{ ·3-·5 " | |

| 7. |{ |{ (_opac._) | | |

| Granular |{ | | | |

| (_gran._) |{ |{ 27. | | |

| |{ |{ Large |{·02-·1 mm. |} |

| |{ |{phenocrysts|{ ·1-·2 " |} Not |

| |{ |{of olivine |{ ·2-·3 " |} represented.|

| |{ |{ and |{ ·3-·5 " |} |

| |{ 14. |{ pyroxene, | | |

| |{ No |{ over 2 mm.| | |

| |{ plagioclase |{ (_magn._) | | |

| |{ phenocrysts |{ | | |

| |{ (_non-phen._)|{ 28. | | |

| | |{ Small | | |

| | |{phenocrysts|{·02-·1 mm. |} |

| | |{of olivine |{ ·1-·2 " |} Not |

| | |{ and |{ ·2-·3 " |} represented.|

| | |{ sometimes |{ ·3-·5 " |} |

| | |{ pyroxene, | | |

| | |{under 2 mm.| | |

| | |{ (_parv._) | | |

| | | | | |

| | |{29. _Vitr._|} | |

| |{ 15. _Phen._ |{ |} | |

| 8. |{ |{30. _Opac._|} |{ Not |

| Prismatic |{ | |} |{ represented.|

| (_prism_.) |{ |{31. _Magn._|} | |

| |{ 16. |{ |} | |

| |{ _Non-phen._ |{32. _Parv._|} | |

| | | | | |

| | | |{·02-·1 mm. | |

| | |{33. _Vitr._|{ ·1-·2 " | 3 |

| |{ 17. _Phen._ |{ |{ ·2-·3 " | 1 |

| 9. |{ |{ |{ ·3-·5 " | 2 |

| Ophitic |{ |{ | |{ Not |

| (_oph_.) |{ |{34. _Opac._| |{ represented.|

| |{ | | | |

| |{ |{35. _Magn._|} |{ Not |

| |{ 18. |{ |} |{ represented.|

| |{ _Non-phen._ |{36. _Parv._|} | |

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

FOURTH ORDER OF THE OLIVINE-BASALTS.

CHARACTERS.—Olivine scanty. Felspar-lathes of the groundmass

in flow-arrangement.

FORMULA.—_Oliv., matr., pauc., flu._

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

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

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

| | |{ 37. |{·02-·1 mm. | 2 |

| | |{ Glassy |{ ·1-·2 " | 12 |

| | |{plagioclase|{ ·2-·3 " | 6 |

| |{ 19. |{phenocrysts|{ ·3-·5 " | |

| |{ Plagioclase |{ (_vitr._) | | |

| |{ phenocrysts |{ | | |

| |{ (_phen._) |{ 38. |{·02-·1 mm. | |

| |{ |{ Opaque |{ ·1-·2 " | 2 |

| |{ |{plagioclase|{ ·2-·3 " | |

| |{ |{phenocrysts|{ ·3-·5 " | |

| 10. |{ |{ (_opac._) | | |

| Granular |{ | | | |

| (_gran._) |{ |{ 39. | | |

| |{ |{ Large |{·02-·1 mm. |} |

| |{ |{phenocrysts|{ ·1-·2 " |} Not |

| |{ |{of olivine |{ ·2-·3 " |} represented.|

| |{ |{ and |{ ·3-·5 " |} |

| |{ |{ pyroxene, | | |

| |{ 20. |{over 2 mm. | | |

| |{ No |{ (_magn._) | | |

| |{ plagioclase |{ | | |

| |{ phenocrysts |{ 40. | | |

| |{ (_non-phen._)|{ Small | | |

| | |{phenocrysts|{·02-·1 mm. | |

| | |{of olivine |{ ·1-·2 " | |

| | |{ and |{ ·2-·3 " | 2 |

| | |{ sometimes|{ ·3-·5 " | 1 |

| | |{ pyroxene, | | |

| | |{under 2 mm.| | |

| | |{ (_parv._) | | |

| | | | | |

| | |{41. _Vitr._|} | |

| |{ 21. _Phen._ |{ |} |{ Not |

| 11. |{ |{42. _Opac._|} |{ represented.|

| Prismatic |{ | | | |

| (_prism._) |{ |{43. _Magn._| |{ Not |

| |{ 22. |{ | |{ represented.|

| |{ _Non-phen._ |{44. _Parv._| ·02-·1 mm. | 1 |

| | | | | |

| 12. | | | | |

| Ophitic | | | |{ Not |

| (_oph._) | | | |{ represented.|

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

FIRST ORDER OF THE AUGITE-ANDESITES.

CHARACTERS.—Felspar lathes or prisms of the groundmass not in

flow-arrangement.

FORMULA.—_Aug., matr., non-flu._

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

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

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

| | |{ 1. |{·02-·1 mm. | 9 |

| | |{ Glassy |{ ·1-·2 " | 5 |

| | |{plagioclase|{ ·2-·3 " | 4 |

| |{ 1. |{phenocrysts|{ ·3-·5 " | |

| |{ Plagioclase |{ (_vitr._) | | |

| |{ phenocrysts |{ | | |

| |{ (_phen._) |{ 2. |{·02-·1 mm. | 6 |

| |{ |{ Opaque |{ ·1-·2 " | 4 |

| |{ |{plagioclase|{ ·2-·3 " | 3 |

| |{ |{phenocrysts|{ ·3-·5 " | 3 |

| 1. |{ |{ (_opac._) | | |

| Granular |{ | | | |

| (_gran._) |{ |{ 3. | | |

| |{ |{ Large |} | |

| |{ |{phenocrysts|} |{ Not |

| |{ |{of augite, |} |{ represented.|

| |{ 2. |{over 2 mm. |} | |

| |{ No |{ (_magn._) |} | |

| |{ plagioclase |{ | | |

| |{ phenocrysts |{ 4. | | |

| |{ (_non-phen._)|{ Small | | |

| | |{phenocrysts|{·02-·1 mm. | |

| | |{of augite, |{ ·1-·2 " | 3 |

| | |{ under |{ ·2-·3 " | 1 |

| | |{ 2 mm., |{ ·3-·5 " | 2 |

| | |{ or none | | |

| | |{ (_parv._) | | |

| | | |{·02-·1 mm. | 1 |

| | |{5. _Vitr._ |{ ·1-·2 " | 2 |

| |{ 3. _Phen._ |{ | | |

| 2. |{ |{6. _Opac._ | ·1-·2 " | 2 |

| Prismatic |{ | | | |

| (_prism._) |{ |{7. _Magn._ |} |{ Not |

| |{ 4. |{ |} |{ represented.|

| |{ _Non-phen._ |{8. _Parv._ |} | |

| | | | | |

| | | |{·02-·1 mm. | 1 |

| | | |{ ·1-·2 " | 3 |

| | |{9. _Vitr._ |{ ·2-·3 " | 4 |

| |{ 5. _Phen._ |{ |{ ·3-·5 " | 7 |

| |{ |{ | | |

| 3. |{ |{ |{ ·2-·3 mm. | 1 |

| Ophitic |{ |{10. _Opac._|{ ·3-·5 " | 1 |

| (_oph._) |{ | | | |

| |{ |{11. _Magn._| | |

| |{ 6. |{ | | |

| |{ _Non-phen._ |{12. _Parv._|{ ·2-·3 mm. | 3 |

| | | |{ ·3-·5 " | 3 |

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

SECOND ORDER OF THE AUGITE-ANDESITES.

CHARACTERS.—Felspar-lathes of the groundmass in flow-arrangement.

FORMULA.—_Aug., matr., flu._

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

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

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

| | |{ 13. |{·02-·1 mm.| 10 |

| | |{ Glassy |{ ·1-·2 " | 10 |

| | |{ plagioclase|{ ·2-·3 " | 4 |

| |{ 7. |{ phenocrysts|{ ·3-·5 " | 1 |

| |{ Plagioclase |{ (_vitr._) | | |

| |{ phenocrysts |{ | | |

| |{ (_phen._) |{ 14. |} | |

| |{ |{ Opaque |} | |

| |{ |{ phenocrysts|}·02-·1 mm.| 2 |

| 4. |{ |{ of |} | |

| Granular |{ |{ plagioclase|} | |

|(_gran._) | |{ (_opac._) | | |

| |{ | | | |

| |{ |{ 15. |} | |

| |{ |{ Large |} |{ Not |

| |{ 8. |{phenocrysts |} |{ represented.|

| |{ No (or very |{ of augite, |} | |

| |{few and small)|{ over 2 mm. |} | |

| |{ plagioclase |{ (_magn._) | | |

| |{ phenocrysts |{ | | |

| |{(_non-phen._) |{ 16. |{·02-·1 mm.| 12 |

| | |{Small augite|{ ·1-·2 " | 14 |

| | |{phenocrysts,|{ ·2-·3 " | 4 |

| | |{ under 2 mm.|{ ·3-·5 " | 3 |

| | |{ or none | | |

| | |{ (_parv._) | | |

| | | | | |

| | |{ 17. _Vitr._| ·1-·2 mm.| 1 |

| |{ 9. _Phen._ |{ | |{ Not |

| |{ |{ 18. _Opac._| |{ represented.|

| 5. |{ | | | |

| Prismatic |{ | | |{ Not |

| (_prism._) |{ |{ 19. _Magn._| |{ represented.|

| |{ 10. |{ |{·02-·1 mm.| 4 |

| |{ _Non-phen._ |{ 20. _Parv._|{ ·1-·2 "| 4 |

| | | | | |

| | |{ 21. _Vitr._| ·1-·2 mm.| 3 |

| |{ 11. _Phen._ |{ | |{ Not |

| 6. |{ |{ 22. _Opac._| |{ represented.|

| Ophitic |{ | | | |

| (_oph._) |{ |{ 23. _Magn._|} | |

| |{ 12. |{ |} |{ Not |

| |{ _Non-phen._ |{ 24. _Parv._|} |{ represented.|

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

FIRST ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Felspar-lathes not in flow-arrangement.

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

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

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

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

| | |{ 1. |} | |

| | |{ Glassy |}·02-·1 mm.| 12 |

| | |{plagioclase |} ·1-·2 " | 4 |

| | |{phenocrysts |} | |

| |{ 1. |{ (_vitr._) | | |

| |{ Plagioclase |{ | | |

| |{ phenocrysts |{ 2. |} | |

| |{ (_phen._) |{ Opaque |} |{ Not |

| |{ |{phenocrysts |} |{ represented.|

| |{ |{ of |} | |

| 1. |{ |{plagioclase |} | |

| Granular | |{ (_opac._) | | |

| (_gran._) |{ | | | |

| |{ |{ 3. |} | |

| |{ |{ Large |} | |

| |{ |{phenocrysts |} |{ Not |

| |{ |{ of |} |{ represented.|

| |{ |{ pyroxene, |} | |

| |{ 2. |{ over 2 mm. |} | |

| |{No plagioclase|{ (_magn._) | | |

| |{ phenocrysts |{ | | |

| |{(_non phen._) |{ 4. | | |

| | |{ Small | | |

| | |{ pyroxene | ·3-·5 mm.| 1 |

| | |{phenocrysts,| | |

| | |{under 2 mm.,| | |

| | |{ or none | | |

| | |{ (_parv._) | | |

| | | |{·02-·1 mm.| 2 |

| | |{5. _Vitr._ |{ ·1-·2 " | 2 |

| |{ 3. _Phen._ |{ | | |

| 2. |{ |{ |{·02-·1 mm.| 1 |

| Prismatic |{ |{6. _Opac._ |{ ·1-·2 " | 1 |

| (_prism._) |{ | | | |

| |{ |{7. _Magn._ |} | |

| |{4. _Non phen._|{ |} |{ Not |

| | |{8. _Parv._ |} |{ represented.|

| | | | | |

| | |{ 9 } | | |

| 3. | |{ 10 } | |{ Not |

| Ophitic | |{ 11 } | |{ represented.|

| (_oph._) | |{ 12 } | | |

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

SECOND ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Felspar-lathes in flow-arrangement.

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

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

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

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

| | |{ 13. |} | |

| | |{ Glassy |}·02-·1 mm.| 9 |

| | |{plagioclase |} ·1-·2 " | 2 |

| |{ 7. |{phenocrysts |} | |

| |{ Plagioclase |{ (_vitr._) |} | |

| |{ phenocrysts |{ | | |

| |{ (_phen._) |{ 14. |} | |

| |{ |{ Opaque |} |{ Not |

| |{ |{phenocrysts |} |{ represented.|

| |{ |{ of |} | |

| 4. |{ |{ plagioclase|} | |

| Granular |{ |{ (_opac._) | | |

| (_gran._) |{ | | | |

| |{ |{ 15. |} | |

| |{ |{ Large |} | |

| |{ |{phenocrysts |} |{ Not |

| |{ |{of pyroxene,|} |{ represented.|

| |{ |{ over 2 mm. |} | |

| |{ 8. |{ (_magn._) |} | |

| |{No plagioclase|{ | | |

| |{ phenocrysts |{ 16. |} | |

| |{(_non-phen._) |{ Small |} | |

| | |{ pyroxene |} |{ Not |

| | |{phenocrysts,|} |{ represented.|

| | |{under 2 mm.,|} | |

| | |{ or none |} | |

| | |{ (_parv._) | | |

| | | | | |

| | |{ 17. _Vitr._| ·02-·1 mm.| 2 |

| |{ 9. _Phen._ |{ | | |

| 5. |{ |{ 18. _Opac._| ·02-·1 mm.| 4 |

| Prismatic |{ | | | |

| (_prism._) |{ |{ 19. _Magn._| |{ Not |

| |{ 10. |{ | |{ represented.|

| |{ _Non-phen._ |{ 20. _Parv._|{·02-·1 mm.| 4 |

| | | |{ ·1-·2 " | 2 |

| | | | | |

| 6. | |{ 21 } | | |

| Ophitic | |{ 22 } | |{ Not |

| (_oph._) | |{ 23 } | |{ represented.|

| | |{ 24 } | | |

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

THIRD ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Felspars of the groundmass short and stout

(orthophyric).

FORMULA.—_Hypersth.-aug., matr., orth._

REMARKS.—The same classification is to be employed here as in the case

of the two previous orders, but as the rocks in my collection that

belong to this order are not numerous (nine sections), it will be

sufficient to refer to the general remarks on the order on p. 290.

FOURTH ORDER OF THE HYPERSTHENE-AUGITE ANDESITES.

CHARACTERS.—Groundmass presenting a rudely granular

appearance or blurred mosaic (microfelsitic).

FORMULA.—_Hypersth.-aug., matr., fels._

REMARKS.—My sliced specimens (five) are too few for the elaboration of

this order to which the classification employed for the other orders is

scarcely applicable. This is due to the partial decomposition or

imperfect development of the pyroxene of the groundmass. The general

characters of the order are given on p. 291.

THE PLUTONIC ROCKS.

These rocks are very infrequent and are for the most part

hypersthene-gabbros or norites, with a few representatives of

diorites[110] without pyroxene. True plutonic rocks did not come under

my observation in the western half of the island (west of Lambasa and

Savu-savu), those of Mount Thoka-singa in the Ndrandramea district

making the nearest approach (see p. 302). The localities in which they

were found are below enumerated:—

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

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

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

| | | | | |

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

THE HYPERSTHENE-GABBROS.—These rocks also contain monoclinic pyroxene,

and are the plutonic equivalents of the hypersthene-augite-andesites

which as a rule prevail in the localities where these rocks occur. They

are usually dark grey or steel-grey in colour with a specific gravity

ranging from 2·7 to 2·84 and have a granitoid aspect. The following

characters are common to all the specimens.

They display a mixture of plagioclase and pyroxene, the last filling up

the spaces between the felspars and apparently of later formation. The

plagioclase crystals, which are 1 to 2 mm. in size, are opaque; and

since they are traversed by numerous fine fissures filled with dust-like

decomposition products, their appearance is often semi-saussuritic. They

are much cross-macled, are at times zoned, and give lamellar extinctions

of andesine-labradorite (20°-30°).... The pyroxene includes both the

rhombic and monoclinic forms, the last with extinction angles of over

30°. They may be associated or may occur as separate crystals, the

rhombic prevailing in the less basic and the monoclinic in the more

basic rocks. The rhombic pyroxene is usually more or less converted into

bastite which by further change passes into a chloritic material; whilst

the augite sometimes undergoes the diallagic change resulting from

schillerisation.

Some special features are presented by rocks from different localities.

That from Nawi is most basic and looks like a diallage-gabbro. That from

the Valanga Range (sp. gr. 2·75) contains some quartz, apparently

secondary and filling up the interspaces. The rock from the Na Kula

Ridge shows traces of a groundmass; but it comes near the plutonic type.

A HORNBLENDE-GABBRO.—This granitoid rock, which is from the Na Kula

Ridge, has a specific gravity of 2·72. Hand-specimens display large

porphyritic crystals of hornblende (7 mm. long) in a base of opaque

felspar and smaller hornblende. In the slide we observe besides the

large crystals of plagioclase and hornblende a little pyroxene; but the

mass of the rock consists of greenish-brown hornblende, plagioclase, and

some secondary quartz, forming a coarse mosaic with a “grain” of about a

millimetre. The hornblende is displayed in regular hexagonal sections,

markedly pleochroic, and gives extinctions up to 12°. It shows no dark

resorption borders; and the larger porphyritic crystals have the same

characters. Almost all the plagioclase of the rock is traversed by

numerous fine fissures, and often acquires a semi-saussuritic appearance

from the presence of dust-like decomposition products. The lamellar

extinctions indicate andesine-labradorite. The quartz occurs mostly in

nests. The pyroxene is formed of large grains of both the monoclinic and

rhombic types.

THE DIORITES.—The rock forming blocks in the Vunimbua River has a

specific gravity of 2·78 to 2·8. It is a pretty rock showing long black

blades of hornblende, 10 mm. in length, in an opaque felspar base. In

the slide the hornblende, which is dark brown and markedly pleochroic,

shows six-sided sections with characteristic prismatic cleavage lines,

the longitudinal sections giving extinctions up to 15°. The borders in

some cases display traces of resorption. The felspar (plagioclase) is in

the form usually of broad regular crystals, 3 to 4 mm. in size, and

giving extinctions of andesine-labradorite (28°); they are “clouded”

through the presence of fine alteration products associated with

numerous fissures. The relation between the hornblende and the

plagioclase is not constant. This appears to be partially due to the

occurrence of traces of a groundmass.

The diorite forming blocks in the agglomerate of the coast cliffs, west

of the Salt Lake Passage, is a remarkable rock showing large blackish

hornblende crystals, in the shape of blades 25 mm. long and 3 or 4 mm.

broad, set in a base of opaque plagioclase felspar which surrounds the

hornblende. The last-named is deep-brown, very pleochroic, yields

extinctions up to 22°, and displays but little evidence of resorption.

The plagioclase is irregular in shape and exhibits broad lamellæ giving

extinctions of acid labradorite (28°-30°). It is traversed by numerous

fine fissures filled with decomposition products and contains abundant

dust-like materials. (Spec. grav. 2·8).

CHAPTER XVIII

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

OLIVINE CLASS

SUB-CLASS II

THE OLIVINE BASALTS (_Plag, oliv, matr._)

THIS sub-class includes the plagioclase-olivine-basalts. Although these

rocks are not the most numerous of the basic rocks, they are well

represented in the island, being in great part confined to the western

half, and being especially characteristic of the districts of Wainunu

and Solevu and of the mountains of Seatura and Naivaka. It will be seen

from the Synopsis that this sub-class is split up into two divisions,

according to the relative abundance of the olivine. Many of the rocks

are grey basalts with the olivine more or less hematised; but the

majority are blackish with the olivine usually more or less

serpentinised. In the typical blackish rocks there is a little dark

opaque interstitial glass. In the grey basalts the groundmass is as a

rule holo-crystalline. The specific gravity ranges generally from 2·8 to

It will be noticed in the scheme that the “prismatic” sub-orders, where

the pyroxene of the groundmass is for the most part in prisms, are

scarcely represented. The “ophitic” sub-orders are poorly represented,

since they only include about 10 per cent. of the total. The ophitic

olivine-basalts are indeed mostly confined to the division where the

olivine is scanty, and the felspars of the groundmass are for the most

part not parallel, the plexus arrangement, as will be often pointed out,

being almost essential for the development of the ophitic structure.

With the basaltic andesites, which cannot always be sharply separated

from the basalts with scanty olivine, the proportion of ophitic rocks is

much higher, probably about 20 per cent. The pyroxene in the

olivine-basalts is nearly always augite, intergrowths with rhombic

pyroxene being only occasionally observed.

I. DIVISION OF THE OLIVINE-BASALTS

CHARACTERS.—Abundant olivine.

FORMULA.—_Oliv, matr, cop._

These rocks are characterised by abundant olivine usually as phenocrysts

but sometimes represented in the groundmass. When a basalt presents much

of this mineral in an ordinary hand-specimen and displays at least five

or six phenocrysts in a slide, it is placed in this division.

Olivine-basalts of this character are well exhibited in the hills around

Solevu Bay and in the neighbouring Seatovo Range. They are also fairly

represented on the northern slopes of Mount Seatura, on the coast

between the Wainunu River and Nandi Bay, and on the Wainunu basaltic

tableland. In the Ulu-i-ndali Range, which lies east of the Wainunu

estuary, they are especially frequent. Whilst confined mostly to the

portion of the island west of Savu-savu Bay, they occur sporadically in

other localities to the eastward, as in Na Suva-suva Hill and in some

parts of the Natewa Peninsula. The grey basalts, which form one-third of

the total, are chiefly characteristic of the hill of Ulu-i-ndali, of the

Solevu district, and of the northern slopes of Seatura. Whilst the

blackish basalts usually compose the flows, the grey basalts form dykes

and volcanic necks.

Two-thirds of these olivine-basalts belong to the order presenting

flow-structure and almost all (28 out of 29) are included in the

sub-order exhibiting granular augite in the groundmass. The ophitic

structure is displayed in only one case; and the prismatic form of the

augites is never a predominant feature.

1. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, non-flu, gran, phen, vitr._

CHARACTERS.—Abundant olivine. Felspar-lathes of the groundmass not in

flow arrangement. Pyroxene of the groundmass granular. Phenocrysts of

glassy plagioclase.

DESCRIPTION.—Dark-brown or blackish rocks. Sp. gr. 2·88 to 2·93.

Phenocrysts of pyroxene occur in fair quantity in addition to those of

the olivine and plagioclase. The groundmass displays a plexus of

felspars and augite-granules with much magnetite in grains and

irregular patches. The interstitial glass is scanty or almost absent.

The olivine phenocrysts, of which the larger are 3 to 4 mm. in size,

are as a rule hematised at the borders and in the fissures, and are

sometimes partially serpentinised. In some cases small crystals of

olivine are enclosed in the pyroxene-phenocrysts. The plagioclase

phenocrysts do not usually exceed 2 mm. in size. They give lamellar

extinctions of 15°-28°, and are often cross-macled. They generally

contain magma-inclusions, which may be arranged in zones, and they

sometimes inclose small pyroxene crystals. Their borders are often

eroded. The pyroxene-phenocrysts, which frequently are 3 to 4 mm. in

size, give extinctions of 30° and over, and may be described as

composed of brown augite. It is only at times that intergrowths of

rhombic pyroxene occur. They are often twinned and are sometimes

eroded and may contain magma and other inclusions. The felspars of the

groundmass, which for the most part form a plexus, are small and

stout, their average length varying from ·08 to ·13 mm., whilst they

frequently display lamellar twinning and give extinctions of about 15°

(oligoclase-andesine). The pyroxene-granules of the groundmass, which

are of brown augite, vary in average size from ·02 to ·04 mm.

Two of the four species, where the felspar-lathes are less than ·1 mm.

and between ·1 and ·2 mm. in length, are represented in this collection.

2. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, non-flu, gran, phen, opac._

CHARACTERS.—Abundant olivine. Felspar-lathes of the groundmass not in

flow-arrangement. Pyroxene of the groundmass granular. Phenocrysts of

plagioclase opaque white.

DESCRIPTION.—Grey compact-looking rocks; sp. gr. 2·83 to 2·9.

Interstitial glass scanty. The olivine phenocrysts, which range up to

5 mm. in size, are more or less hematised; and in extreme cases of this

alteration, where schiller-planes are formed, the hand-specimen appears

to carry brown mica. There are sometimes small grains of olivine

(·1 mm.) in the groundmass. The plagioclase-phenocrysts, varying in size

from 2 to 4 mm., owe their opacity partly to their composite character,

when they present an aggregate of smaller clear crystals, and partly to

multiple macling. They give extinctions of acid labradorite (25°-32°).

Pyroxene-phenocrysts, when present, are scanty, pale brown, not over

2 mm. in size, and give the large extinctions (+30°) of augite. In their

absence small augites (·2 mm.) are frequent. Fine granules (·01-·03 mm.)

of similar augite, together with magnetite, abound in the groundmass.

The felspars of the groundmass are fairly stout and lathe-like and show

at times a few twin-lamellæ which give extinctions of 13° to 20° (medium

andesine).

Species represented:

(_a_) felspar lathes ·1-·2 mm.

(_b_) felspar lathes ·2-·3 mm.

3. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, non-flu, gran, non-phen, magn._

CHARACTERS.—Abundant olivine. Felspar-lathes of the groundmass not in

flow-arrangement. Pyroxene of the groundmass granular. No plagioclase

phenocrysts. Large phenocrysts of olivine and pyroxene over 2 mm. in

size.

DESCRIPTION.—A remarkable blackish-grey rock. Although somewhat

scoriaceous, it has a sp. gr. of 2·91. It displays large phenocrysts of

olivine and pyroxene, 4 to 8 mm. in size, in a coarse-textured

groundmass of stout felspars, augite granules, magnetite and a little

glass. The olivine is extensively hematised. The pyroxene phenocrysts

are of brownish-yellow augite with regular outlines and giving angles of

extinction up to 40°. The broad lamellar felspars of the groundmass,

which are on the average ·3 to ·4 mm. long, give extinctions indicating

both acid labradorite (23° to 28°) and medium andesine (16° or 17°). The

abundant augite-grains average ·05 mm. in size; but the prism form

occurs at times.

The species with felspar-lathes ·3 to ·5 mm. long is alone represented.

4. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, non-flu, gran, non-phen, parv._

CHARACTERS.—Abundant olivine. In the groundmass the felspar-lathes are

not in flow-arrangement and the pyroxene is granular. No plagioclase

phenocrysts. Small phenocrysts of olivine and occasionally pyroxene

under 2 mm.

DESCRIPTION.—A grey coarse-grained rock. Sp. gr. 2·9. It displays

abundant small phenocrysts of olivine, all less than a millimetre in

size (·2-·8 mm.), which are hematised in the fissures and at the

borders. The felspar-lathes, which display a few twin-lamellæ giving

extinctions of 16°-18° (medium andesine), vary greatly in size. The

smaller are ·1 to ·3 mm. and the larger ·3 to ·5 mm. long; but the two

are connected by felspars of intermediate length. The abundant augite

granules average ·037 mm. in breadth. Pyroxene phenocrysts are not

represented in the slide. From its coarsely crystalline texture this

rock merits the field-name of a grey doleritic basalt; and except in the

arrangement of the felspar-lathes it does not differ from the grey

doleritic basalts of genus 16.

12. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine abundant. Felspars of the groundmass not in

flow-arrangement. Pyroxene of the groundmass ophitic or semi-ophitic. No

plagioclase phenocrysts. Small phenocrysts (under 2 mm.) of olivine and

occasionally a few of pyroxene.

DESCRIPTION.—A dark greenish-brown rock, with sp. gr. 2·91, showing

abundant micro-porphyritic olivine in a groundmass consisting of ophitic

pale-brown augite inclosing the felspar-lathes, together with small

olivines, patches of magnetite, and a little altered interstitial glass.

The olivine-phenocrysts are about ·5 mm. in average size and are more or

less hematised. The felspar-lathes, which average ·15 mm. in length,

often show twin-lamellæ that give extinctions of 23°-30°

(andesine-labradorite).

The species with felspar-lathes ·1-·2 mm. long is alone represented.

13. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, flu, gran, phen, vitr._

CHARACTERS.—Olivine abundant. Felspars of the groundmass in

flow-arrangement. Pyroxene of the groundmass granular. Phenocrysts of

glassy plagioclase.

DESCRIPTION.—Dark grey or dark brown rocks with sp. gr. 2·88 to 2·99.

Phenocrysts of olivine, pyroxene, and plagioclase occur in a groundmass

showing partially parallel felspar-lathes, abundant pyroxene grains, and

fine magnetite, residual glass being scanty or absent. The

olivine-phenocrysts do not exceed 3 or 4 mm. in size and in some rocks

are less than 1 mm. They are usually more or less serpentinised and

hematised at the borders and in the cracks; but sometimes they are

almost fresh and present regular outlines. In some rocks the olivine

also occurs as grains (·3 mm.) in the groundmass. When the phenocrysts

of olivine have blackish borders they are surrounded by a halo, as

though the crystal had attracted the magnetite from the groundmass

immediately around. The plagioclase phenocrysts vary from 1 to 3 mm. in

size. They often contain abundant magma-inclusions and give lamellar

extinctions of 15° to 25° (basic andesine). In some rocks they are

rudely parallel. The pyroxene phenocrysts, which are of pale

brownish-yellow augite giving extinctions of over 30°, do not usually

exceed 3 mm. They present regular octagonal cross-sections and sometimes

display lamellar twinning. Occasionally there is a suspicion of

intergrowth with rhombic pyroxene. The felspar-lathes, which according

to the species vary much in length, at times show a few lamellæ. The

augite grains of the groundmass are abundant and are as a rule about

·02 mm. in size; but in some rocks they are larger and in others

smaller.

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

where the felspar phenocrysts are larger than 3 mm., and the

non-porphyritic where they are smaller. All four species, as indicated

by the length of the felspar-lathes, are represented.

14. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, flu, gran, phen, opac._

CHARACTERS.—Olivine abundant. Felspars of the groundmass in

flow-arrangement. Pyroxene of the groundmass granular. Opaque

plagioclase phenocrysts.

DESCRIPTION.—Dark grey rocks, with sp. gr. 2·9 to 2·93, showing

phenocrysts of olivine and pyroxene with opaque whitish phenocrysts of

plagioclase in a groundmass of felspar lathes, pyroxene grains, and

magnetite, with occasional fine olivine. The olivine phenocrysts, which

are sometimes 5 or 6 mm. in size, are often deeply eroded. They are at

times so extensively hematised along the schiller-planes that they

appear like brown mica. The plagioclase phenocrysts owe their opacity in

part to their consisting of an aggregate of lesser crystals which are

clear and glassy and give lamellar extinctions of 20° to 30° (andesine

labradorite). They do not usually exceed 3 mm. and are sometimes scanty.

The pyroxene phenocrysts, which are at times infrequent, may be 5 mm. in

size. They are of pale yellowish-brown augite, giving extinctions of

40°. The felspar-lathes are rarely lamellar; but in one such case the

angle of extinction was 17° (medium andesine). The grains of augite in

the groundmass average ·02 mm. in diameter.

The only species represented is that with the felspar-lathes ·1 to

·2 mm. long.

15. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, flu, gran, non-phen, magn._

CHARACTERS.—Abundant olivine. In the groundmass the felspars are in

flow-arrangement and the pyroxene is granular. No plagioclase

phenocrysts, but large phenocrysts, over 2 mm., of olivine and pyroxene.

DESCRIPTION.—This genus includes the most basic rocks represented in my

collection. They are compact heavy blackish rocks with sp. gr. 3 to 3·1,

and display large porphyritic crystals of olivine and pyroxene often 3

or 4 mm. in size. The olivine phenocrysts may be fairly fresh with clean

outlines, or they may be deeply eroded and stained by iron oxide, or

they may be passing into serpentine. The pyroxene phenocrysts may be

either eroded or possess regular outlines. They are of pale brown augite

and give extinctions over 30°. The pyroxene granules, which average

·02 mm. and are very abundant, are of the same augite. The

felspar-lathes are relatively scanty. In the two rocks here included

they average in length ·06 mm. and ·08 mm. When lamellæ can be

recognised they give extinctions of 30°-40° (labradorite). The untwinned

lathes give extinctions of 20°-28° (labradorite).

The only species represented is that with felspar-lathes less than

·1 mm. in length.

16. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, cop, flu, gran, non-phen, parv._

CHARACTERS.—Abundant olivine. In the groundmass the felspar-lathes are

in flow arrangement and the pyroxene is granular. There are no

plagioclase phenocrysts; but there are numerous small phenocrysts, under

2 mm. in size, of olivine and occasionally a few of pyroxene.

DESCRIPTION.—As a highly basic genus this ranks next to the preceding

one, the specific gravity of the rocks ranging from 2·91 to 3·01. All

the four species indicated by the varying length of the felspar-lathes

are represented in my collection. The rocks of the first two, with the

felspars averaging less than ·1 mm. and between ·1 and ·2 mm., are

compact aphanitic basalts, only displaying an occasional small

phenocryst of augite and blackish-grey or bluish-black in colour. Those

of the last two species, with the average length of the felspar-lathes

·2-·3 mm. and ·3-·5 mm. respectively, are lightish-grey coarse-textured

rocks of the doleritic type. In all the rocks no pyroxene phenocrysts

are displayed in the slide; and the olivine phenocrysts, which are very

numerous, do not usually exceed 1 mm., though occasionally the average

size is 1·3 mm., and not infrequently it is only ·5 mm. In some cases

where the larger olivine phenocrysts lie athwart the current of the

felspar-lathes, the smaller (·5 mm.) lie with their long axes parallel

to the flow. The olivine is either fresh, or it may be beginning to

serpentinise in the cracks, or it may be in part hematised. The crystals

may have regular outlines, or they may be rounded and sometimes deeply

eroded. The pyroxene granules of the groundmass are of pale-brown

augite, and average ·01 to ·03 mm. in size. Occasionally a few prism

forms occur, giving extinctions of 30° to 40°. In the case of the more

compact rocks, with the felspar-lathes averaging less than ·2 mm. in

length, lamellar twinning is but scantily to be noticed in the lathes,

which give extinctions measured from the long axis of 20° to 25° and by

the twin lamellæ of 30° to 35°, indicative of acid labradorite in both

instances. With the coarser doleritic grey basalts, where the

felspar-lathes are stouter and have an average length exceeding ·2 mm.,

lamellar twinning is more frequent; the extinctions afforded by the

lamellæ range between 15° and 25° (medium andesine). Residual glass is

scanty in these rocks, and in the grey dolerites it is often difficult

to recognise any.

2. DIVISION OF THE OLIVINE-BASALTS

CHARACTERS.—Scanty olivine.

FORMULA.—_Oliv, matr, pauc._

25. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, non-flu, gran, phen, vitr._

CHARACTERS.—Olivine scanty. Felspars (lathes and prisms) of the

groundmass not in flow-arrangement. Pyroxene of the groundmass granular.

Glassy plagioclase phenocrysts.

DESCRIPTION.—About two-thirds of these rocks have a common facies, being

closely similar in appearance, brownish-black in colour, and with spec.

grav. usually between 2·84 and 2·92. They belong to the species with the

felspar-lathes ·1 to ·2 mm. in length. They are essentially the rocks of

the old submarine basaltic flows; and they are often columnar, the

columns being 2 to 4 feet across. My remarks will mainly apply to this

predominant group.

To the eye they are somewhat compact and show scattered porphyritic

crystals of plagioclase. In the slide they display numerous phenocrysts

of plagioclase, with a few of olivine and pyroxene, in a groundmass

formed of stout lamellar felspar-lathes and small prisms forming a

plexus with granular augite in the meshes. There is a good deal of

magnetite and generally scanty residual glass. The plagioclase

phenocrysts are usually 2 to 3 mm. in size, but they may be smaller (1

to 2 mm.) or larger (3 to 5 mm. or more) when the rock has a porphyritic

appearance.[111] They are often cross-macled and at times show zoning.

In many slides two kinds are distinguished by the extinctions which

indicate in one case medium andesine (15° to 22°) and in the other acid

labradorite (27° to 32°). They contain inclusions of the magma and are

often eroded. The pyroxene phenocrysts are of pale brown augite, scanty

and small, and give extinctions of 30° to 40°. They are sometimes

twinned and may be eroded and contain inclusions of the magma. The

olivine phenocrysts, which do not usually exceed 2 or 3 mm., are mostly

rounded, but sometimes have the regular outlines, and are in various

stages of serpentinisation. The felspars of the groundmass, which

average ·17 mm. in length, are mostly stout and lamellar; but they

exhibit all transitions from the lathe-shape with one or two lamellæ to

broad multi-lamellar prisms where the breadth is half the length. They

give lamellar extinctions averaging 15° to 18° (andesine). The augite

granules of the groundmass are pale brown and average ·02 to ·03 mm. in

size. In a few cases they are larger (·05 mm.) which is an indication of

an approach to the ophitic type. In most slides occur a few small

augites of prism-form, two or three times the size of the granules,

which give extinctions of over 30°. Where the phenocrysts of augite are

very scanty or absent, there exist large grains (·1 mm.) of an

intermediate size. The magnetite is often abundant, occurring in

crystals, rods, and irregular masses, the last associated often with the

interstitial glass which is present in small quantities in most rocks,

being greenish or brownish and showing fibrous devitrification.

In some localities semi-vitreous rocks referable to this genus are

frequent. This is especially the case in the Naivaka peninsula, where

the rocks show a fair amount of glass in the groundmass, the porphyritic

augite being well developed, whilst the pyroxene of the groundmass is

only in part differentiated. Three of the four species are here

represented. Those with large felspar-lathes (·2-·3 mm. long) and coarse

augite granules (·05) approach the semi-ophitic rocks included in genus

33.

26. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, non-flu, gran, phen, opac._

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes and

prisms are not in flow-arrangement and the pyroxene is granular. The

plagioclase phenocrysts are opaque.

DESCRIPTION.—Dark grey porphyritic rocks, which, from the opacity of the

felspar phenocrysts, look like porphyrites. They are not very frequent

and occur mostly on the northern slopes of Mount Seatura. Two different

types occur in my collection which may be regarded as sub-genera. In the

most basic kind, where the sp. gr. is 2·86 to 2·89, the plagioclase

phenocrysts, 2 to 3 mm. in size, owe their opacity chiefly to their

aggregate structure. They give lamellar extinctions (15°-30°) of

andesine labradorite. Porphyritic olivine is scanty and more or less

hematised; but a fair amount of olivine grains, less than ·1 mm., occur

in the groundmass. Pyroxene phenocrysts are scanty, but microporphyritic

pale brown augite (·1 mm.) is frequent. In the groundmass are found

stoutish felspar-lathes, averaging ·2 mm. long, together with an

abundance of fine augite granules (·01-·02 mm.) and fine magnetite, the

residual glass being scanty.... In the other type the sp. gr. is 2·75;

and the plagioclase phenocrysts 4 to 6 mm. in size give extinctions of

andesine and acid labradorite (10°-30°). There is an approach to the

orthophyric structure in the groundmass, as is indicated by the number

of short broad felspars, averaging ·2 mm. in length and giving lamellar

extinctions of acid and basic andesine. The granular augite of the

groundmass is coarse (·04 mm.), and occasional prism-forms give

extinctions of 40°.

33. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, non-flu, oph, phen, vitr._

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes are not

in flow-arrangement and the pyroxene is ophitic or semi-ophitic.

Plagioclase phenocrysts glassy.

DESCRIPTION.—These brownish-black rocks are all of the semi-ophitic

type. Although no ophitic “plates” occur in the slide, the augites of

the groundmass have no longer the granular form, but are large,

·08-·1 mm. in size, and tend to invest the felspar-lathes. The specific

gravity ranges from 2·78 to 2·86. As in other of the ophitic and

semi-ophitic rocks of this collection (genera 9, 10, 12, of the

augite-andesites), the large size of the felspar-lathes (·2-·3 mm. long)

of the groundmass gives a doleritic texture in the slide. In most of the

other characters these rocks approach those of genus 25 which possess

felspar-lathes more than ·2 mm. in length. But they are more often

semi-vitreous, and display a considerable amount of dark smoky glass

showing numerous magnetite rods and skeletal crystals with fibrous

devitrification. The plagioclase phenocrysts, which vary much in size in

different rocks (in some 2 or 3 mm., in others 4 or 5 mm.), give

extinctions of andesine labradorite (20°-35°). They are often eroded and

contain numerous large black inclusions of the magma. The pyroxene

phenocrysts, which are of pale-brown augite, often have an

aggregate-structure, having been formed _in situ_. Others again consist

of single crystals and have been much affected by the magma. The olivine

phenocrysts, which are at times deeply eroded, are generally small and

in part serpentinised.

37. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, flu, gran, phen, vitr._

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

flow-arrangement and the pyroxene is granular. Glassy plagioclase

phenocrysts.

DESCRIPTION.—Brownish-black rocks which cannot be distinguished, except

in the flow-arrangement of the felspars of the groundmass, from those

described under genus 25. Like them they enter into the formation of the

basaltic plains of Sarawanga and Mbua and elsewhere. Most of the rocks

of this genus group themselves into one type where the felspar-lathes

average in length ·15-·21 mm. The sp. gr. is usually between 2·87 and

2·91. Though rarely porphyritic, such rocks display to the eye a few

small scattered glassy phenocrysts of plagioclase and an occasional

grain of olivine. It is to this type of the genus that the following

description applies.

In the thin sections they display small plagioclase phenocrysts, with a

few of olivine and occasionally of pyroxene, in a groundmass where the

flow-arrangement of the felspar-lathes is well marked, the rest of the

groundmass being made up of granular augite with magnetite and generally

a little residual glass.... The plagioclase phenocrysts do not usually

exceed 2 mm. in size and contain magma inclusions. Two kinds are often

indicated in the same slide by the extinctions, namely, one of medium

andesine (17°-22°), and the other of acid labradorite (28°-33°).... The

pyroxene phenocrysts are of pale brown augite; but they are small (less

than 2 mm.), scanty, and often absent when their place is taken by

microporphyritic augite, ·2 mm. in size.... The olivine phenocrysts are

generally small. Though sometimes showing the long hexagonal sections,

they are often rounded and more or less serpentinised.... The felspars

of the groundmass present more typical lathes than are to be observed in

the non-parallel felspars of the rocks of genus 25. The twin-lamellæ,

when present, are fewer; but give similar extinctions (15°-21°) of

medium andesine.... The augite granules are, as a rule, ·02 or ·03 mm.

in diameter; but occasional more prismatic forms occur, two or three

times the length, which give extinctions of over 30°. The magnetite is

abundant, and the scanty interstitial glass is green or brown and

displays fibrous devitrification.

The following three species, as indicated by the length of the

felspar-lathes, are represented in my collection:

(_a_) ·02-·1 mm.

(_b_) ·1-·2 mm.

(_c_) ·2-·3 mm.

38. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, flu, gran, phen, opac._

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

flow-arrangement and the pyroxene is granular. Opaque plagioclase

phenocrysts.

DESCRIPTION.—Grey rocks, sp. gr. 2·78 to 2·83, showing small opaque

porphyritic crystals of plagioclase with a few phenocrysts of olivine

and pyroxene in a groundmass of parallel felspar-lathes, augite

granules, and magnetite, with very scanty, if any, interstitial

glass.... The plagioclase phenocrysts, 2 to 3 mm. in size, are often

aggregates of smaller crystals. They contain colourless granular

inclusions and are sometimes zoned, giving extinctions of medium

andesite (15°-18°), and of andesine labradorite (25°-29°).... The

pyroxene phenocrysts are pale-brown, scanty, 2 to 3 mm. in size, often

twinned and give the large extinctions of augite.... The olivine

phenocrysts, which do not exceed 2 or 3 mm., are deeply eroded by the

magma and are hematised and schillerised. Small grains also occur in the

groundmass.... The felspar-lathes, which in the species here represented

average ·15 mm. in length, are stout and give lamellar extinctions of

andesine (18°-22°).... The augite granules are pale-brown and usually

·02-·03 mm. in diameter.

The only species represented in my collection is that with the

felspar-lathes ·1-·2 mm. long.

40. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, flu, gran, non-phen, parv._

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

flow-arrangement and the pyroxene is granular. No plagioclase

phenocrysts; but there are a few small phenocrysts of olivine and

sometimes of pyroxene under 2 mm. in size.

DESCRIPTION.—Compact-looking non-porphyritic blackish-brown rocks, sp.

gr. about 2·9. Occasionally a little vesicular. For the most part

dyke-rocks.

In the slide are displayed a few small phenocrysts of olivine and

pyroxene in a groundmass formed of more or less parallel felspar-lathes,

augite granules, magnetite, sometimes in rods, and a little greenish

devitrified residual glass.... The pyroxene phenocrysts are of

pale-brown augite and are generally less than a millimetre in size. They

may be single crystals or they may be formed of an aggregate of a few

smaller crystals.... The olivine phenocrysts rarely exceed 2 mm. in size

and are in part serpentinised.... The augite granules vary usually from

·01 to ·03 mm. in diameter.... The felspar-lathes of the rocks in this

collection are large, often exceeding ·2 mm. in length, giving the rock

a doleritic texture in the slide. In a single slide they may range from

·1 to ·6 mm. When lamellar they give extinctions of 15° to 25° (basic

andesine).

Two species are represented in this collection:--

(_a_) with felspar-lathes ·2-·3 mm.

(_b_) with felspar-lathes ·3-·5 mm.

44. GENUS OF THE OLIVINE-BASALTS

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

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

flow-arrangement and the pyroxene is in great part prismatic. There are

no phenocrysts of plagioclase; but there are a few small phenocrysts of

olivine and sometimes of pyroxene, less than 2 mm. in size.

DESCRIPTION.—A dark grey compactish rock; sp. gr. 2·9; showing a little

macroscopic olivine; forming a dyke in the tuffs on the summit of the

hill of Vatui (p. 54).

In the section it exhibits a few phenocrysts of olivine (more or less

serpentinised) and of augite in a groundmass formed of stout augite

prisms and small augite granules with felspar-lathes, magnetite, and a

little devitrified yellowish interstitial glass. The augite prisms and

the felspar-lathes are in flow-arrangement.... The pyroxene phenocrysts,

which are pale brown and give extinctions of over 30° from the single

cleavage-lines, may be aggregates of five or six smaller crystals or

single crystals presenting sometimes lamellar twinning. The first are

doubtless formed _in situ_. The second though showing regular outlines

may have a nucleus giving a different extinction and possessing eroded

margins. The stout augite prisms of the groundmass, which are

occasionally twinned, have an average length of ·2 to ·3 mm. and give

angles of extinction with the long axis of 30° to 40°. The

felspar-lathes average only ·07 mm. in length and afford extinctions,

when untwinned, of 18° to 24° (acid labradorite).

The only species represented is that where the average length of the

felspar-lathes is between ·02 and ·1 mm.

CHAPTER XIX

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

AUGITE CLASS

SUB-CLASS II

AUGITE-ANDESITES INCLUDING THE BASALTIC ANDESITES

(_Plag, aug, matr._)

THIS sub-class, which comprises 40 per cent. of the volcanic rocks, is

characterised by the absence of olivine on the one hand, and by the

rarity or absence of rhombic pyroxene on the other. On the basic side it

shades into the olivine-basalts through the basaltic andesites, and on

the acid side by intermediate stages into the hypersthene augite

andesites; and for these reasons it is not always possible to draw a

sharp line of distinction. In cases where a hand-specimen displays no

macroscopic olivine and where a solitary small phenocryst of olivine is

alone observed in the slide, it should be referred to this sub-class;

and here also all doubtful specimens as regards the occurrence of

olivine should be placed. When the question of the occurrence of rhombic

pyroxene arises, it should be remembered that the great prevalence of

monoclinic pyroxene amongst the phenocrysts and the practical absence of

rhombic pyroxene from the groundmass are essential characteristics of

this sub-class. Rhombic pyroxene is only indicated at times by

intergrowths in the phenocrysts.

The basaltic andesites enter into the formation of old “flows,” as in

the Mbua and Ndama plains. The other rocks enter principally into the

composition of dykes, necks, and agglomerates.

I. GRANULAR SUB-ORDER (AUGITE-ANDESITES)

FORMULA.—_Aug, matr, non-flu, gran._

1. GENUS OF THE AUGITE-ANDESITES

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

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

flow-arrangement and the augite is granular. Phenocrysts of glassy

plagioclase.

DESCRIPTION.—These rocks frequently form dykes; and it is probable that

most of the instances where the nature of the exposure could not be

ascertained also fall into this category. They are dark-brown or

blackish, and their sp. gr. ranges, except in the semi-vitreous rocks,

from 2·7 to 2·83. They are sometimes vesicular, and rocks with abundant

interstitial glass are common. They admit of grouping into two

sub-genera according to the size of the plagioclase phenocrysts:

(_a_) Porphyritic, where the average size is 3 mm. or over.

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

Nearly all the rocks in my collection belong to the second group.

In the sections they display phenocrysts of plagioclase and occasionally

of pyroxene in a groundmass formed of a plexus of felspar-lathes, augite

granules, magnetite, and usually a fair amount of smoky more or less

opaque interstitial glass.... The felspar phenocrysts, which are

sometimes abundant, give lamellar extinctions of andesine labradorite

(15° to 30°). They are frequently small (1 to 2 mm.) and contain often

many magma-inclusions. Whilst the corroded aspect of some indicate that

they belong to an earlier period, the aggregate character and regular

outlines of others suggest that they have been produced in position....

Pyroxene phenocrysts are absent in half the rocks. When present they are

generally small and of a pale augite which gives extinctions of 30°.

Their size does not usually exceed 2 mm.; and they may consist of single

crystals (sometimes twinned) or of an aggregate of smaller crystals. At

times there is a suspicion of intergrowth with rhombic pyroxene; but no

phenocrysts formed alone of that mineral occur.... The augite granules

of the groundmass as a rule vary from ·02 to ·04 mm. in diameter.

Occasional prism-forms giving large extinctions occur.... The

felspar-lathes vary much in length in different rocks. In some they

average as little as ·05 mm., and in others as much as ·2 mm.; but the

doleritic type with yet longer lathes is not represented in the

collection except among the altered rocks. Most of the lathes show a

single median twin-line, and when broader they display twin-lamellæ. The

angles of extinction indicate acid and basic andesine.

Three out of the eight species distinguished by the length of the

felspar-lathes occur in my collection, that with the longest lathes

(·3-·5 mm.) being not represented.

2. GENUS OF THE AUGITE-ANDESITES

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

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

flow-arrangement and the augite is granular. Opaque plagioclase

phenocrysts.

DESCRIPTION.—This genus may be divided into two groups according to the

size of the plagioclase phenocrysts, the first “porphyritic,” where they

average 3 mm. and over, the second “non-porphyritic,” where they are

smaller than 3 mm., usually not over 2 mm. The former would include some

of the “porphyrites,” and to this only one of the rocks sliced is to be

referred. All the rest belong to the non-porphyritic type; and several

of them are rocks that have undergone the propylitic change, as

indicated by the formation of pyrites, chlorite, calcite, and other

alteration-products.

(_a_) PORPHYRITIC SUB-GENUS.—A greyish rock, with sp. gr. 2·78, showing

abundant porphyritic opaque plagioclase (4 to 7 mm.), from the vicinity

of Satulaki. These phenocrysts are often aggregates of lesser crystals,

or they may be extensively cross-macled. They are traversed by numerous

fine cracks and show much dust-like included material. They are in part

corroded by the magma and give evidence of fracture in their present

position, the re-union being sometimes effected by the growth of new

substance. Their lamellar extinctions (10° to 20°) are those of

andesine. The groundmass displays a plexus of stout felspar-lathes,

averaging ·1 mm. long, with the meshes occupied by coarse augite

granules, ·03 to ·05 mm., with little, if any, interstitial glass. The

felspars are often lamellar and give extinctions like those of the

phenocrysts.

(_b_) NON-PORPHYRITIC SUB-GENUS.—Reference will first be made to some of

the propylitic rocks of the dykes of the Ndriti Basin which belong to

this group (see p. 70). They are greenish or greyish, with sp. gr. 2·76

to 2·8, and often sparkle with pyrites and contain secondary calcite,

sometimes to such an extent that they might be taken at first sight for

impure limestones.

The small opaque plagioclase phenocrysts (under 2 mm.), that they

contain, are more evident in the slide than in the hand-specimen, and

scarcely give a macroscopic character to the rock. They give extinctions

(10° to 30°) ranging from those of acid andesine to acid labradorite,

and are traversed by numerous cracks occupied by calcitic and other

alteration products. The few pyroxene phenocrysts that once existed are

now entirely represented by chloritic pseudomorphs. The groundmass

displays a doleritic texture, exhibiting a plexus of long

felspar-lathes, ·2 to ·4 mm. in average length, which often present a

false resemblance to a flow-arrangement from their aggregation into

bundles. They are often clouded by secondary products, but occasionally

give lamellar extinctions (20° to 30°) indicating andesine labradorite.

The rest of the groundmass is greatly altered, the granular augite and

the interstitial glass, which originally existed in fair amount, being

replaced by calcite, chlorite, pyrites, and occasionally epidote, so

that the rock mass appears largely impregnated with alteration products.

In addition there is much secondary magnetite, and in some cases there

are a few minute cavities filled with chalcedonic silica and zeolites.

Reference may here be made to a singular rock from Ruku-ruku Bay, which

resembles the Ndriti rocks in its propylitic alteration, but the

felspar-lathes of the groundmass, ·21 mm. in length, give the small

extinctions of oligoclase. Spec. grav. 2·61.

Most of the prevailing rocks of Mount Freeland belong to this sub-genus.

They are dark grey and show small opaque plagioclase phenocrysts 1 or

2 mm. in size. They usually, however, are more or less altered, the

change being often of the propylitic type, calcite, chloritic material,

viridite, and occasionally pyrites occurring as alteration products. The

specific gravity of the altered rocks is 2·61-2·69; that of the least

affected is about 2·76. They all, however, belong to the same genus,

displaying small phenocrysts of plagioclase and augite in a groundmass

composed of minute stoutish felspar-lathes (·03-·06 mm.), augite

granules, magnetite, and a little residual glass. The plagioclase

phenocrysts owe their opacity partly to the numerous fine cracks

traversing them and partly to the alteration products. The pyroxene

phenocrysts, which are mostly of pale yellow augite, display at times

intergrowths of rhombic pyroxene.

4. GENUS OF THE AUGITE-ANDESITES

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

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

flow-arrangement and the augite is granular. There are no plagioclase

phenocrysts, and those of augite when present are small (under 2 mm.).

DESCRIPTION.—Two groups of these rocks occur in my collection. In the

one there are vesicular and scoriaceous rocks forming dykes near

Nukunase and near the village of Ndriti. They display a plexus of

felspar-lathes with abundant smoky more or less devitrified glass, the

augite granules not being always differentiated. The felspar-lathes vary

from ·1 to ·2 mm. in average length, and when lamellar give extinctions

of basic andesine (25°). There are no pyroxene phenocrysts, and the

augite granules when present average ·02 mm. in size.

In the other group are included some propylites from the dykes of the

Ndriti Basin. They are greyish or greenish rocks, have a sp. gr. of 2·72

to 2·76, sparkle often with pyrites, and contain so much secondary

calcite that they effervesce freely with an acid. Except in the rarity

or absence of plagioclase phenocrysts, they come near to the propylitic

rocks described under genus 2. They usually display a doleritic

groundmass exhibiting long felspar-lathes, ·2 to ·33 mm. in length,

which present the same pseudo-flow arrangement from their being gathered

into bundles. The alteration corresponds precisely to that previously

described, chlorite, epidote, pyrites, &c., occurring in quantity as

secondary products.

II PRISMATIC SUB-ORDER OF THE AUGITE-ANDESITES WHERE

THE FELSPAR-LATHES ARE NOT IN FLOW-ARRANGEMENT

FORMULA.—_Aug, matr, non-flu, prism._

The augite-andesites, which display in the groundmass a plexus of

felspar-lathes and much prismatic pyroxene, are not frequent in my

collection. About half of the specimens belong to agglomerates, whilst

the rest are of the massive type, none apparently being obtained from

dykes. They admit of the same classification as that generally adopted

for the “granular” sub-orders; and it must be not forgotten that

granular pyroxene also occurs but is not predominant.

5. GENUS OF THE AUGITE-ANDESITES

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

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

flow-arrangement and the augite is for the most part prismatic.

Plagioclase phenocrysts glassy.

DESCRIPTION.—Except as regards the prismatic pyroxene these rocks do not

differ much from the “granular” augite-andesites. Those before me show

phenocrysts of plagioclase and sometimes of augite in a groundmass

displaying a mesh-work of felspar-lathes, prismatic pyroxene, and much

interstitial glass.... The plagioclase phenocrysts, 1 to 3 mm. in size,

show abundant magma-inclusions arranged either zone-wise or parallel to

the twinning-planes. They are often eroded.... The phenocrysts of

augite, which give extinctions of over 30°, are often rounded and

display glass and other inclusions.... The prismatic pyroxenes of the

groundmass vary in average length from ·03 to ·08 mm. They have the

peculiar pale muddy brown hue characteristic of the prismatic augite in

these rocks, and give oblique extinctions up to 30° and over. They may

be short and broad or long and slender, and when there is much glass in

the rock they resemble the felspar-lathes in their forked and imperfect

extremities. Granular pyroxene occurs, but is subordinate.... The

felspar-lathes, ·1 mm. long, are rarely lamellar, and give extinctions

measured from their long axis of 20° (basic andesine).

6. GENUS OF THE AUGITE-ANDESITES

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

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

flow-arrangement and the augite is for the most part prismatic. The

plagioclase phenocrysts are opaque.

DESCRIPTION.—Light and dark grey rocks displaying abundant opaque

plagioclase phenocrysts not exceeding 2·5 mm. They are somewhat altered,

one of the specimens having a sp. gr. of 2·68.

In the section they exhibit phenocrysts of plagioclase, and occasionally

of augite in a groundmass of felspar-lathes, pyroxene prisms and

granules (the former predominating), with a fair amount of altered

interstitial glass.... The plagioclase phenocrysts owe their opacity to

the great number of fine and sometimes parallel cracks filled with

alteration products, that traverse them. Although much of their original

material has often disappeared, they still display the lamellar twinning

of medium andesine (15° to 20°).... The phenocrysts of pale yellowish

augite, which give the large extinctions of that mineral, exhibit but

little alteration, although lying in the same slide with those of the

plagioclase.... The pyroxene prisms of the groundmass are of the same

yellowish augite. They are broad with rounded extremities and are

arranged in a loose plexus.... The felspars of the groundmass, which

average ·1 mm. in length and give extinctions of medium andesine, are

either lathe-shaped or short and broad when they display lamellæ. The

last-named approach the orthophyric type, and such rocks come near the

porphyrites; but I do not feel justified in placing them in a separate

orthophyric order.

III OPHITIC SUB-ORDER OF THE AUGITE-ANDESITES WITH THE

FELSPARS OF THE GROUNDMASS NOT IN FLOW-ARRANGEMENT.

FORMULA.—_Aug, matr, non-flu, oph._

These rocks form generally ancient flows. They are for the most part

semi-ophitic, large ophitic “plates” being uncommon.

9. GENUS OF THE AUGITE-ANDESITES

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

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

flow-arrangement. The augite of the groundmass is ophitic or

semi-ophitic. Glassy plagioclase phenocrysts.

This genus may be divided into two sub-genera,

Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 Vanua Levu, Fiji · 第 31 / 34 章

THESE extensive inland plains occupy a considerable area in this part of