Chapter II.; and as far as the absence of signs of disturbance is

concerned, these ancient reefs might owe their present situation, either to the withdrawal of the sea or to the upheaval of the land. Such reef-limestones exist over much of the Pacific, and they belong to the usual type of these rocks.

SHELLY AND FORAMINIFERAL LIMESTONES

These rocks are composed partly of reef-debris, partly of volcanic detritus, and partly of the tests of foraminifera (usually bottom forms), fragments of lamellibranchiate and gasteropod shells, together with those of pteropods, and other organic remains. Occasionally separate valves of the genera “Cardium” and “Ostraea” are inclosed in the limestone. These rocks have been evidently formed in rather shallow water. In places they overlie palagonite-tuffs and clays, also foraminiferal. Similar limestones are doubtless forming at the present time off the coast.

They are usually hard in texture and greyish or pale yellow in colour. They contain between 25 and 45 per cent. of carbonate of lime; whilst the residue consists of fragments of minerals (10 to 15 per cent.), including plagioclase, monoclinic and rhombic pyroxene, and occasionally brown hornblende, with siliceous casts of foraminifera (4 to 20 per cent.), mostly formed of chalcedonic silica but sometimes black and glauconitic; the remainder (30 to 40 per cent.) being composed of rounded and sub-angular portions of palagonite and semi-vitreous basic rocks, of which the larger vary from ½ to 1 millimetre in diameter. In some cases the carbonate of lime of the inclosed organic remains has been mainly replaced by more or less crystalline silica. In others a recrystallisation of the calcitic material is in progress, as described on page 131; and the matrix presents in places a mosaic of calcite.

These rocks have therefore been subject to some degree of alteration, the causes probably lying within the mass. They are, however, far from frequent. They are best represented in the upper valley of the Sarawanga River in the vicinity of Tembe-ni-ndio, where they reach to a height of about 250 feet above the sea. The greatest elevation at which I found them was in the mountainous interior of the Waikawa Promontory, where they occur at a height of 1,100 feet above the sea.

As samples, the results of the examination of two rocks from the Tembe-ni-ndio district are here appended:—

A.

Carbonate of lime 46 per cent. {Fragments of palagonite and of semi-vitreous {basic rocks 30 " " Residue {Minerals 12 " " {Secondary silica replacing the carbonate of {lime in the organic remains 12 " " --- 100

It displays to the naked eye fragments of shells including pteropods, and numerous tests of foraminifera 2 or 3 millimetres in diameter. In the section it displays in addition coral debris and a considerable quantity of rounded and sub-angular pieces of palagonite and semi-vitreous basic rocks, usually less than a millimetre across, with smaller fragments of minerals (pyroxene and plagioclase), and much calcitic material in the matrix.

B.

Carbonate of lime 25 per cent. Debris of palagonite and of basic volcanic rocks 45 " " Minerals 10 " " Siliceous casts of foraminifera 20 " " --- 100

The organic remains mainly consist of tests of foraminifera, many of which occur in the residue as colourless siliceous casts. The fragments, whether of minerals or of volcanic rocks, are usually less than half a millimetre across. The foraminifera are mostly small and of the “Globigerina” type; but there is a cast of a tube of some boring-mollusc, and fragments of shells also occur.

PTEROPOD-OOZE ROCKS

These rocks are bluish-grey when not exposed; but through the hydration accompanying exposure they become much lighter in colour. They are crowded with pteropod shells, and contain also small gasteropod and lamellibranchiate shells together with tests of foraminifera both microscopic and macroscopic. They yield between 30 and 40 per cent. of carbonate of lime, the residue being made up of disintegrated palagonitic debris and fine clayey material derived from the same source, together with a fair amount of mineral fragments (10 per cent.) which include plagioclase, pyroxene, and brown hornblende, and measure in the case of the larger fragments between ·1 and ·4 mm. in diameter. Such rocks are somewhat friable and correspond with the pteropod-ooze rocks of the Solomon Islands; but they are not very frequent, being best represented on the flanks of the basaltic table-land between the Wainunu and Yanawai rivers, as in the vicinity of the Nandua tea-plantation where they extend up to 500 feet above the sea. In this particular locality (see page 345) they overlie horizontally-stratified tuffs and clays, composed of the debris of a basic glass usually vacuolar but now for the most part converted into palagonite, and showing a few small tests of foraminifera.

These deposits are always either surface or incrusting formations. The circumstance of their passing down into characteristic palagonite-formations is repeated in the case of the Tembe-ni-ndio limestones, as observed on page 131; and there is no doubt that much of their non-calcareous material is derived from the disintegration of palagonite.

_Sample of pteropod-ooze rock from below the Nandua tea-plantation._

Carbonate of lime 38 per cent. {Palagonitic debris and clayey material 51 " " Residue {Minerals 8 " " {Casts of foraminifera 3 " " --- 100

Fine clayey material makes up the greater part (72 per cent.) of the residue. It presents the microscopic characters of material derived from the degradation of palagonite. Amongst the mineral fragments, of which the larger are ·2 to ·3 mm. in size, occur brown hornblende, pyroxene, felspar, magnetite, &c. The casts of foraminifera are usually glauconitic, but a few are of crystalline silica. A number of curious little pellets of palagonite, oblong or oval in form, occur in the residue. Their size is ·3 to ·6 mm., and they apparently represent the minute amygdules of palagonite that occupy the vacuoles in an altered basic glass.

FORAMINIFEROUS VOLCANIC MUD-ROCKS

These deposits, which represent the “volcanic muds” forming around the coasts of volcanic islands, are more or less consolidated clay-rocks. They contain in varying numbers the tests of foraminifera with occasionally pteropod-shells. The former are usually minute and of the “Globigerina” type; but in some rocks larger bottom-forms prevail. The original colour of these deposits is bluish-grey, but as generally displayed they are pale-brown and considerably affected by hydration and are known as “soapstones” in the group. The ultimate effect of exposure is the production of a whitish or yellowish soapy rock that has lost all the carbonate of lime and all the organic remains and breaks down easily in the fingers. Such a crumbling material, when examined with a high power, cannot be distinguished from the products of the final disintegration of palagonite as described on page 348.

These deposits are frequently displayed in the lower regions up to elevations of 300 feet above the sea, incrusting the basaltic plains of Lekutu, Sarawanga, Ndreketi, and Lambasa, that occupy such a large area of the north side of the island, and exhibiting there in the prevailing horizontality of their beds the same indication that is presented by the vertical position of the columns of the under-lying basalt, namely, the comparative absence of disturbance during the emergence. At the foot of the mountains these deposits are interstratified with and finally overlaid by coarse palagonite-tuffs, also containing marine remains; and these are in their turn covered over by the agglomerates that often enter so largely into the composition of the mountainous backbone of the island. Such beds form apparently the lowest of a series which begins with the foraminiferous clay and ends with the agglomerate. But in some places, as has been noticed in the cases of the pteropod-ooze rock of Nandua, and of the shelly and foraminiferous limestone of Tembe-ni-ndio, beds composed largely of characteristic palagonite lie beneath.

In the elevated interior of the island these volcanic mud-rocks are usually concealed by the tuffs and agglomerates. Occasionally, however, they are to be seen exposed by landslips high up the flanks of the mountains, as on the slopes of Thambeyu 1,000 feet above the sea. An interesting exposure of them is displayed in the heart of the island in the face of the Mbenutha cliffs where the elevation is about 1,100 feet. Here they are overlaid by a thick bed of agglomerate; and tuff-beds largely made up of vacuolar basic glass debris and showing a few foraminifera are interstratified with them; but they exhibit signs of considerable disturbance (see page 109).

These deposits contain between 5 and 25 per cent. of carbonate of lime, and as a rule about 90 per cent. of the residue consists of fine clayey material derived from the final degradation of palagonite and of basic rocks. The mineral fragments (plagioclase, augite, rhombic pyroxene, and occasionally hornblende) vary much in amount, their average proportion being 13 or 14 per cent. of the mass. Their size is usually less than ·2 mm. and does not exceed ·4 mm. Casts of foraminifera are nearly always present in the residue and form generally 3 or 4 per cent. of the whole deposit. Sometimes they are black and glauconitic; but more frequently they are white and composed of chalcedonic silica. Such casts represent on a small scale the results of the same silicifying operation to which the flints and silicified corals that occur so frequently on the surface in some localities owe their origin (see Chapter XXV.).

With regard to the age of these volcanic mud-rocks of Vanua Levu, it is most likely that as in the case of similar deposits in Viti Levu, which were examined by Mr. H. B. Brady, they are of post-tertiary origin. Samples of the Suva “soapstone” containing 5 or 6 per cent. of lime and displaying shells of foraminifera, pteropods, and other molluscs, were obtained from different heights up to 100 feet above the sea. Since 87 out of the 92 species of foraminifera represented in the deposits are known to be living now in the Pacific, Mr. Brady had no hesitation in assigning the beds to the post-tertiary epoch.[126]

SAMPLES OF THE VOLCANIC MUD-ROCKS

A. _From districts west of Ndranimako, 100 feet above the sea._

Carbonate of lime 20 per cent. {Fine debris of palagonite and { semi-vitreous basic rocks 62 " " Residue {Minerals 14 " " {Casts of foraminifera 4 " " --- 100

The organic remains consist mainly of tests of minute foraminifera of the “Globigerina” type, casts of which, both glauconitic and chalcedonic, occur in the residue. About 88 per cent. of the residue consist of fine clayey materials less than ·25 mm. in size. The mineral fragments, which average about ·1 mm. in diameter, are mostly of felspar with a little pyroxene and brown hornblende.

B. _From the Mbenutha Cliffs, 1,100 feet above the sea._

Carbonate of lime 15 per cent. { Fine material mainly derived from { the degradation of palagonite 60 " " Residue { Minerals 23 " " { Casts of foraminifera 2 " " --- 100

This rock is somewhat hard, so that the proportion of fine clayey material, which is however large, cannot be accurately determined. It shows in places dark streaks composed of an abundance of minute and often perfect tabular crystals of zoned plagioclase and prisms of rhombic pyroxene, the size in neither case exceeding half a millimetre, both of them being derived from the acid andesites of the neighbourhood. In the slide it displays minute tests of foraminifera of the “Globigerina” type in a matrix formed mainly of palagonitic debris, fragments of minerals and semi-vitreous basic rocks. The larger fragments of the minerals and of the volcanic rocks do not exceed ·15 mm.; but most of the material is very fine. The tests of the foraminifera are sometimes filled with the matrix, but often they are entirely of calcite and exhibit in polarised light a dark cross.

C. _From between Natua and Mbatiri, about 290 feet above the sea._

Carbonate of lime 25 per cent. { Fine material derived from the degradation of { palagonite and of semi-vitreous basic rocks 62 " " Residue { Minerals 2 " " { Casts of foraminifera 11 " " --- 100

This is a relatively deep-water deposit, the foraminifera being minute and of the “Globigerina” type. About 90 per cent. of the residue consists of fine clayey material, with which the calcite is so intimately mixed that each particle is highly refractive and effervesces freely in an acid. The mineral fragments (pyroxene and felspar) are very scanty, the largest being less than ·25 mm. The white casts of foraminifera, composed of chalcedonic silica, form a conspicuous elements in the residue.

D. _From the vicinity of Mbatiri, 100 feet above the sea._

Carbonate of lime 4 per cent. Fine material derived from the degradation of palagonite 90 " " Minerals 2 " " Casts of foraminifera 4 " " --- 100

About 94 per cent. of the residue consists of fine clayey material. The fragments of minerals are very scanty and are all less than ·2 mm. in size. The casts of foraminifera are white and of chalcedonic silica. From the fineness of the materials and the small size and pelagic character of the foraminifera, this deposit may be regarded as formed in relatively deep water.

E. _From the eastern flank of the Wainunu table-land, 200 feet above the sea._—This is a shallow-water deposit and contains, besides small gasteropod shells, large flat tests of foraminifera 5 or 6 mm. in diameter. It possesses 24 per cent. of carbonate of lime, 62 per cent. of palagonitic debris, &c., and 14 per cent. of minerals.

ALTERED VOLCANIC MUD-ROCKS

This group includes compact hard foraminiferous usually dark-brown rocks, which exhibit evidence of alteration in their induration, in the presence of pyrites, and in the chalcedonic quartz filling fine cracks in the rock-mass. Occasionally special types of alteration occur, one of which will be referred to in the description of some of the rocks given below. The proportion of carbonate of lime is generally small; but sometimes it amounts to 10 per cent. or more.

They admit of being examined in thin sections; and their true nature is at times so much disguised that I have taken them at first for aphanitic basic andesites. In the slide they display a few scattered tests of foraminifera of pelagic habit in a matrix composed of the fine debris of palagonite and of basic rocks, together with fragments of plagioclase and pyroxene. Most of the material is very fine, and the size of the largest mineral fragments does not exceed ·2 mm.

Such rocks, however, are not very frequent. They may be displayed on the flanks of mountain-ranges buried beneath basic tuffs and agglomerates, as in the case of Mount Mariko (page 187) and of the mountain-slope behind Mbale-mbale (page 158); or they may be found at much lower levels as at Savarekareka Bay (page 190), where, however, they assume sometimes a peculiar character. Though in the last-named locality the alteration is possibly connected with thermal metamorphism, it is probable that in most instances it is a normal interstitial change occurring in beds of some antiquity which are covered over by a considerable thickness of later deposits. In places, where these rocks have been subjected to much hydration in the weathering process, they become red in colour, as is found on the flanks of Mount Mariko.

SAMPLES OF THE ALTERED VOLCANIC MUD-ROCKS

A. From between 400 and 500 feet above the sea on the south slope of the Mariko Range.... The characters and mode of occurrence of this rock are described on page 187.

B. From an elevation of 1,100 feet on the south slope of the Korotini Range.... The description of the locality will be found on page 160.

This rock is hard and compact and looks like an altered basic rock showing a few minute specks of pyrites. It is composed of fine palagonitic debris, and small fragments of semi-vitreous basic rocks and of crystals of pyroxene and felspar, none of the fragments exceeding ·2 mm. in diameter. Tests of minute foraminifera, filled with matrix and of the “Globigerina” type, occur very scantily. There is little or no carbonate of lime; but secondary silica, both colloid and crystalline, is present as an alteration product.

C. From the vicinity of Yaroi, 30 feet above the sea.... The locality is described on page 189.

This is a dark grey hard compact rock, containing probably between 10 and 15 per cent. of carbonate of lime, and looking like an altered limestone. In the section it displays minute tests of foraminifera of the “Globigerina” type in a matrix composed of fine disintegrated palagonitic material, impregnated with calcite and containing also fragments of minerals (augite and felspar), none of which exceed ·2 mm. in diameter. There are also a few similar-sized fragments of semi-vitreous basic rocks. Some fine cracks in the rock-mass are filled with a quartz mosaic. The tests of the foraminifera remain calcitic; but their cavities are filled either with the matrix or with calcite or with a colourless fibro-radiate mineral polarising in blackish-blue hues.

D. From the south shore of Savarekareka Bay.... The locality is described on page 190.

This is a bright green hard compact rock with flinty fracture and not effervescing with an acid. In the slide it shows a few casts of foraminifera of the “Globigerina” type in a matrix composed mainly of fine debris (·01-·04 mm.) of felspar and pyroxene with much greenish opaque amorphous alteration products. The abundance of pyroxene is remarkable. The material of the tests of the foraminifera is altogether replaced by a greenish yellow mineral, occurring in grains and radiating prisms, apparently epidote.

E. From an elevation of 950 feet on Mount Thambeyu.... The locality is described on page 177.

A hard dark grey rock containing 10 or 15 per cent. of carbonate of lime and showing fine specks of pyrites. In the slide are displayed numerous tests of foraminifera of varying size up to ·5 mm.; scattered patches of pyrites; fragments of a semi-vitreous basic rock, not exceeding ·15 mm. in diameter, and of plagioclase and pyroxene; in a matrix of the finest debris of the same materials impregnated with granules of calcite. The tests of the foraminifera are filled either with calcite, showing a black cross in polarised light, or with a zeolite, or with pyrites, or with the matrix.

SUBMARINE BASIC TUFFS OF MIXED COMPOSITION

These tuffs, which are composed not only of palagonitic materials but also of the fine detritus of usually semi-vitreous basic rocks, rank first in frequency amongst the volcanic sedimentary deposits of the island. In their character they pass on the one hand into the foraminiferous volcanic mud-rocks or clay rocks and on the other into the tuff-breccias and tuff-agglomerates. We have here a series beginning with the agglomerate and ending with the clay that represents in a general sense the successive stages of the degradation of the same materials.

These tuffs occur at all elevations from the sea-border, where they may form the shore-cliffs, to the upper slopes and summits of the mountain-ranges, where they are found at elevations between 2,000 and 2,500 feet above the sea. In the interior of the island they are generally to be observed underlying the basic agglomerates. Wherever an extensive exposure of the agglomerates exists in the mountainous districts, these tuffs are as a rule to be found at the base of the cliffs. The precipitous bluff of agglomerate, that so often gives a character to the mountainous interior, and the line of cliff of the same formation that runs along the slopes, represent the work of landslips, as is shown by the huge masses of agglomerate lying on the ground below. These “slips” are not uncommon, and are due to the undermining influence of the springs that percolate through the tuffs and clays underlying the agglomerates.

When the tuffs are well displayed they as a rule show stratification. The bedding may be indicated either by distinct parting-lines or by alternating bands of varying degrees of coarseness. That these deposits, when occurring in mass in the upland regions, are often horizontal or but slightly inclined, is evidenced by the Nganga-turuturu Cliffs, 1,200 feet above the sea, which are described in Chapter VIII., in the line of cliffs behind Sealevu (Chapter XI.), and high up the slopes of Mount Thambeyu (Chapter XII.) as high as 1,500 feet. This is also shown in the circumstance that the line of junction with the overlying agglomerate, except in rare cases, as in that of the Mbenutha Cliffs, is generally horizontal. It is, however, not uncommon to find the beds exposed on the mountain-flanks dipping away from the axis of the range at a small angle, as on the slopes behind Mbale-mbale and in the Sokena Cliffs. In the lower regions, where these deposits are associated with the volcanic mud-rocks on the basaltic plains, they are but slightly inclined. On the other hand, as in the Kumbulau district, the sea-cliffs for some miles may be composed of tuffs more or less steeply tilted.

These tuffs are generally more or less compacted and have a greyish colour; but as usually exposed in a weathered condition they are often pale brown or yellowish and are more friable. They may be uniform in structure, or they may display thin seams of a marl-like clay, or they may contain numerous lapilli of vesicular basic glass extensively palagonitised. Not uncommonly they contain larger fragments of basic rocks, and when these are at all frequent the terms “agglomerate-tuff” or “tuff-agglomerate” have been employed according to the preponderance of either material.

Many of these tuffs show no effervescence with an acid; and this is especially the case with specimens at all weathered. On the other hand there are just as many that contain a little carbonate of lime, not usually more than 3 or 4 per cent., but sometimes amounting to 12 or 13 per cent. It often happens in the case of a series of tuffs, apparently non-calcareous, that an occasional thin band of a fine clay-like rock contains a good percentage of lime. It is pointed out below, however, that the absence of effervescence does not necessarily imply the absence of foraminiferous tests.

Tests of foraminifera, often macroscopic bottom forms, together with shells of small gasteropods, are displayed at times; but they are as a rule in such cases not frequent. I found foraminiferous tuffs at considerable heights in some localities, as for instance between 2,000 and 2,500 feet on the slopes of Mount Thambeyu (page 178), at an elevation of 1,850 feet on the south slope of the Korotini Range above Vatu-kawa (page 158), and between 2,000 and 2,400 feet on the summit of the range between Waisali and Sealevu (page 154). In the last-named locality, where the tuffs are coarse and often of the nature of agglomerate-tuffs, they are highly fossiliferous; but such a character is exceptional.

The submarine origin of the tuffs can often be demonstrated in the absence of evidence of organic remains, as by their interstratification with foraminiferous clay rocks, such as we find at an elevation of 1,000 to 1,100 feet on the top of the “divide” between the Ndreketi and Lambasa basins. A single seam of marl-like rock displaying only a solitary test of a foraminifer in the slide may throw light on the origin of the coarser tuffs with which it is associated. The use of the microscope is essential in the case of some of the harder tuffs, where there has been a little alteration. Here casts of foraminifera may be observed, although no carbonate of lime is indicated by an acid. In some localities where no organic remains are evident in the tuff, fine waterworn gravel is to be noticed.

These deposits are composed as a rule of sub-angular fragments of semi-vitreous basic or basaltic rocks and of palagonite, together with fragments of plagioclase and pyroxene, the interspaces being filled with fine debris of the same materials. The relative proportion, however, of the three principal constituents varies considerably, the palagonite, for instance, being sometimes scanty and sometimes abundant. The size of the larger fragments in a tuff of the most common kind is about a millimetre; but deposits rather finer and rather coarser are also frequent. In the very coarse tuffs and in the breccia-tuffs, where the larger materials are mostly of palagonite, the larger fragments may be a centimetre in size and even more, the interspaces being filled with fine debris of the same character cemented together often by carbonate of lime.

The fragments of semi-vitreous basic rocks forming a regular constituent of these tuffs are usually dark and opaque and display a few plagioclase lathes. They correspond with the type of the semi-vitreous basalt or basaltic andesite, of which the blocks of the overlying agglomerates are as a rule composed and are doubtless derived from the same source. Fragments of unaltered basic glass are rarely to be observed in these tuffs. It is as a rule all converted into palagonite. This material presents itself in various stages of secondary alteration, from the compact greenish or yellowish waxy mass to a white friable pulverulent substance, which represents the last stage of degradation. These changes will be found described on page 348. It can, however, be stated here that they are mainly concerned with hydration. In the case of the lapilli of finely vesicular basic glass, that is, of basic pumice, which are inclosed in some of the tuffs, all stages of the secondary alteration of palagonite are often exhibited, and the last stage of the change is merely indicated by a white powdery patch containing a few minute siliceous amygdules. The puzzling little white patches so common in basic tuffs merely represent lapilli of basic pumice that have been palagonitised, and then bleached and disintegrated by hydration.

The minerals are more or less abundant and may constitute a third of the whole mass. They include plagioclase, augite, rhombic pyroxene, and magnetite, olivine being rare and scanty. Entire crystals of any size are infrequent. However, crystals of augite, 5 or 6 mm. in length, are found in the tuffs at Naivaka and of the coast cliffs near the Salt Lake Passage.... It may be observed that zeolitic minerals which are frequently developed in the tuffs consisting almost entirely of palagonite are not typical of the tuffs of mixed composition.

There is nothing suggestive of recent eruptions in any of these formations. They were formed ages since on the sea-floor at varying depths around volcanic vents. Sometimes a cone was able to rear itself above the level of the sea; but in most cases it rapidly succumbed to breaker-action. Three agencies, concerned with sub-aerial eruptions, submarine eruptions, and marine denudation, have co-operated in the production of these deposits, but their parts in the process have varied greatly. The last is indicated when the tuff is formed of a variety of basic rocks with but little palagonite. The tuffs containing much palagonite representing an original vacuolar basic glass are regarded as mainly the products of submarine-eruptions. In those cases where lapilli of altered basic pumice occur in the deposit, sub-aerial eruptions are directly indicated. When an extensive exposure of these tuffs occurs, as in the case of the Nganga-turuturu Cliffs and in that of the section displayed near the hill of Korolevu (page 48), all three agencies are often illustrated.

SAMPLES OF THE SUBMARINE BASIC TUFFS OF MIXED COMPOSITION

A. As examples of the non-fossiliferous tuffs where the palagonite constituents do not predominate, I will take those exposed in beds in the coast cliffs and in the low hills in the vicinity of Na Tokalau in the Kumbulau peninsula (page 90). They are grey in colour and have the texture of a sandstone, being more or less compacted and showing no effervescence with an acid. They are composed of fragments of basic rocks and of minerals, varying in size from ·5 to 1 mm., in a scanty matrix made up of fine detritus of the same materials and of palagonitic debris. The fragments of volcanic rocks are rounded and sub-angular, and formed mainly of a basaltic rock, with a black opaque groundmass showing some small plagioclase crystals and in places more or less palagonitised. There are also portions of a hemi-crystalline basic rock showing small augite crystals. The minerals entire and in fragments, which make up quite a third of the mass, are mostly of plagioclase, but monoclinic and rhombic pyroxene are also well represented. It is evident that through the alteration of the palagonitic constituents, which were probably more frequent when the tuffs were deposited, the structure of the matrix is somewhat disguised.

B. A yellowish grey tuff composing the cliffs on the north coast of Naivaka affords a good example of a tuff where the palagonitic materials predominate. It is somewhat fine-textured and displays a tendency to lamination. The powdered material effervesces slightly with an acid. In the slide it is exhibited as composed mainly of palagonite and of fragments of minerals, the latter making up about one-third of the whole and ranging in size usually between ·2 and ·5 mm. A number of more or less parallel fine cracks, filled with calcite and traversing also the inclosed crystals of plagioclase, together with small fragments of basic rocks are displayed in the section. There are a few fragments of semi-vitreous basic rocks, as just indicated, but the palagonite is the principal constituent. It shows numerous minute amygdules occupying the original vacuoles of the basic glass; and in its substance occur irregular patches formed of a colourless semi-isotropic mineral which is either zeolitic or a form of opal. Plagioclase and augite compose the mineral fragments, the former prevailing. Although these tuffs are derived from a vent that was probably the last in eruption in this island, they display considerable alteration which is mainly connected with the secondary changes affecting the palagonite since the deposition of these materials.

C. As an example of the banded tuffs composed of coarse and fine materials I will take a compact grey rock forming one of the horizontal beds in the natural section exposed near the hill of Korolevu (p. 49). It is also an example of those tuffs which whilst not effervescing with an acid display a few casts of foraminifera in the slide. The alternating bands which are about a centimetre in thickness pass gradually into each other. The bands of finer materials are made up of sub-angular fragments, ·1 to ·2 mm. in size, of the dark opaque groundmass of a semi-vitreous basic rock and of a grey hemi-crystalline groundmass of an augite-andesite, together with palagonite more or less decomposed, and fragments of plagioclase and augite, whilst the interspaces are filled with the finer debris. The layers of coarse materials have much the same composition, the fragments varying usually between ·5 and 1 mm. in diameter, with occasional larger pieces of palagonite 2 to 5 mm. in size representing original lapilli of a vacuolar basic glass. The tests of the foraminifera, which occur scantily in the layers of fine material, are all minute. They are filled with palagonitic material.

D. The tuffs prevailing on the higher flanks of the mountainous backbone of the island are well represented by those exposed at an elevation of 1,200 feet above the sea on the south slope of the Korotini Range behind Mbale-mbale. It is a somewhat coarse-grained rather hard grey rock effervescing feebly with an acid. It is composed of sub-angular or partly rounded fragments, 1 to 4 mm. in size, of various basic rocks, and of rather smaller fragments of plagioclase and augite, the interstices being filled up with fine debris of the same materials, in which a few minute tests of foraminifera of the “Globigerina” type may be observed. The basic rocks of which the fragments are formed comprise the following: (_a_) a grey aphanitic augite-andesite, with but little interstitial glass, presenting a parallel arrangement of the minute felspar-lathes which have an average length of ·05 mm.; (_b_) a grey augite-andesite of coarser texture but in other respects similar, the felspar-lathes being about ·1 mm. in length, whilst there is a little microporphyritic plagioclase; (_c_) a semi-vitreous basaltic rock showing small porphyritic crystals of plagioclase and augite in a groundmass usually black and opaque, but sometimes smoky and displaying felspar microliths; (_d_) a vacuolar basic glass more or less palagonitised.

E. As a specimen of the calcareous tuffs those exposed on the south slope of the Korotini Range at an elevation of 1,850 feet may be given. They contain about 11 per cent. of carbonate of lime and inclose a few tests of foraminifera 1 to 2 mm. in diameter. The other constituents are fragments of semi-vitreous basic rocks and of palagonite, together with fragments of plagioclase and pyroxene crystals and of an amorphous siliceous mineral which behaves optically like chalcedonic silica. When the rock is gently rubbed down, minute fragments of this white mineral can be picked out. They have a wrinkled surface and an irregular form and are not affected by acids. In polarised light they display a rude mosaic or an imperfect radiate structure.

F. As specimens of the fossiliferous agglomerate-tuffs composed mainly of palagonite, those exposed on the high mountain slopes of the Korotini Range at heights of 2,000 feet may be here cited. They are described on p. 154.

_Note._—The examples of mixed tuffs above given represent only some of the principal types of these deposits. Short descriptions of others will be found in the detailed account of the geology of the island.

ALTERED BASIC TUFFS OF MIXED COMPOSITION

These form a group of hard compact rocks, the fragmental character of which is not always apparent in hand-specimens, microscopical examination of thin sections being usually required for the determination of their true nature. They are commonly exposed on the southern flanks of the Korotini Range at the back of Vatu-kawa and Nukumbolo. They are composed of compacted fragments, varying in different localities from 1 to 5 or 6 mm. in size, of a variety of semi-vitreous basic rocks, the matrix being scanty but often containing zeolites and secondary silica, whilst occasionally secondary calcite is developed. They contain no organic remains, and palagonite when present is usually scanty, whilst viridite and similar materials represent the decomposition of the pyroxene.... A hard breccia-tuff found on the flanks of Mariko and in one or two other localities contains vesicular fragments, where the steam-holes are filled with opal or chalcedony, and the cracks traversing the matrix are also filled with this mineral.

The alteration of these tuffs has evidently arisen from a variety of causes. In some cases the change appears to be purely interstitial. In other times it has arisen from contact-metamorphism, or from hydro-thermal agencies, as in the case of the altered tuffs near the hot springs at Nukumbolo (p. 161).

SUBMARINE BASIC PUMICE TUFFS

These deposits, which, however, are not of frequent occurrence, are interstratified with volcanic mud-rocks in certain localities, as at the Mbenutha Cliffs (p. 110), and in the vicinity of the hill of Korolevu (p. 47). They indicate periods of volcanic activity during the deposition of the foraminiferous muds, with which they are associated, when the fine materials ejected from sub-aerial vents fell into the seas around.

Such tuffs are more or less compact and usually fine in texture. When the glass fragments are but slightly altered, the tuff-rock is dark grey; but when the palagonitic change is well advanced, it becomes pale and yellowish. They are made up chiefly of small fragments of a bottle-green basic glass, which are as a rule vacuolar and sometimes fibrillar; but it never happens that the pumiceous character is as pronounced as in acid pumice; and in some cases the vacuoles or steam-pores are to be observed only in the minority of the fragments displayed in a slide. The size of the glass fragments is as a rule small, in some tuffs averaging only ·1 mm. and in others ·5 mm.; but occasionally they may be 1 or 2 mm. in diameter.

Fragments of minerals (plagioclase and pyroxene) corresponding in size to the glass fragments are as a rule well represented, forming a fourth or a third of the mass. A little fine detritus of a semi-vitreous basic rock also occurs. Tested in an acid several of the tuffs either do not effervesce or give an indication of a small percentage of carbonate of lime; whilst others effervesce freely. They usually display a few minute tests of foraminifera of the “Globigerina” type, the cavities of which are filled with fine palagonitic debris.

The palagonitic alteration is to be noticed in all cases; but it varies considerably in its extent. In the dark grey tuffs it affects the margins only of a few of the glass-fragments. In the paler tuffs it has extended more into their substance, although the alteration is never more than partial. The pale greyish material filling up the interspaces is composed of disintegrated palagonite. The steam-pores or vacuoles are sometimes empty; and at other times, especially where the palagonitic change has begun, they are filled with a granular alteration product. The glass fuses readily; but is not affected by acids. It is clear and isotropic, showing however a few scattered microliths. These tuffs correspond with the hyalomelan-tuff from the island of Munia in this group as described by Wichmann; but in that instance no mention is made of inclosed tests of foraminifera.

“CRUSH-TUFFS” FORMED OF BASIC GLASS

This is a remarkable group of compacted tuff-like rocks which as hand-specimens would be generally regarded as pitchstone-tuffs. Their detrital origin is, however, often very doubtful. They are composed of fragments of basic glass, carrying plagioclase phenocrysts, with the interspaces occupied by palagonite and by the finer debris of the glass and felspar. The larger glass fragments, which vary in different rocks from 1 or 2 to 4 or 5 mm. in size, have been crushed _in situ_, the broken portions often remaining more or less in position. These fragments are invested by palagonite and have eroded borders, as shown in the figure on page 342. The glass is bottle-green, non-vacuolar, fuses readily, and only at times displays incipient crystallisation. The explanation of the origin of these rocks is attempted in Chapter XXIV. They contain neither carbonate of lime nor organic remains. The most typical example is present in a bed underlying a pitchstone-agglomerate near Narengali (see page 149). It is not uncommon to find evidence of crushing in the glassy matrix of a pitchstone-agglomerate or of rubbly pitchstone, as in the Va Lili Ridge (142), the Korotini Bluff (157), and Mount Soloa Levu (313); and here also palagonite has been produced around the crushed fragments.

COARSE ZEOLITIC PALAGONITE-TUFFS

These deposits represent coarse kinds of the submarine tuffs of basic glass, in which the palagonitic change is far advanced, and where zeolites and at times secondary calcite have been produced in abundance as a result of the alteration. They present themselves in the mass as mottled grey rocks which when examined in thin sections are seen to be composed in great part of fragments of more or less palagonitised vacuolar basic glass, whilst zeolites are extensively developed in numerous irregular cavities and in the interspaces. Although displaying no organic remains, their submarine character is indicated as at Nandua by the circumstance of their occurring as horizontal beds overlaid by pteropod-ooze deposits, or as at Tembe-ni-ndio by their forming part of a series of horizontal beds with a shelly limestone and a foraminiferous palagonite clay overlying them.

The fragments of bottle green basic glass vary usually between 1 and 4 millimetres. They were originally vacuolar and at times fibrillar from the lengthening out of the minute steam-pores; but through the palagonitic change these characters have been often disguised, and it is only at times that the unaltered glass is observed. Plagioclase and sometimes augite and occasionally olivine formed phenocrysts in the original glass. The zeolites, which include chabazite and natrolite, may be so extensively developed that they make up a fourth or a fifth of the rock mass. One may observe them in cavities where the walls are lined by fibrous natrolite with the cube-like crystals of chabazite occupying the interior. The calcite is usually subordinate to the zeolites, but sometimes the tuff contains as much as 10 per cent. of this mineral, which is evidently of secondary origin.... The history of these tuffs in the district of Nandua and Ulu-i-ndali is no doubt applicable to these deposits in other localities. They are the products of submarine eruptions which shattered into fragments the extensive palagonite crusts of flows of basaltic lava. In Chapter XXIV. I have attempted to show how palagonite is formed on a large scale in the case of such submarine displays of volcanic activity.

CHOCOLATE-COLOURED FORAMINIFEROUS PALAGONITE-MARLS

We have here hard, somewhat calcareous, clay-rocks which consist in great part (nine-tenths) of fine palagonite debris with some fragments of minerals and a little fine detritus of semi-vitreous basic rocks. Some hand-specimens would be taken for pure palagonite; but the fragmental nature appears at once in the slide. This is especially the case with a rock exposed in a stream-course near Rewa on the shores of Savu-savu Bay (see page 95). The materials composing them are exceedingly fine, the largest fragments not usually exceeding ·2 mm. As a rule they contain a little carbonate of lime and sometimes as much as 10 per cent., whilst a few tests of minute foraminifera are to be noticed in the slide. These deposits are horizontally bedded, and underlie a pteropod-ooze rock at Nandua and a shelly impure limestone at Tembe-ni-ndio. They are not very frequent, and sometimes approach in characters the volcanic-mud rocks, which, however, are much more mixed in composition. I regard them in the main as sedimentary deposits derived from the disintegration of the palagonitised vitreous surface of a submarine basaltic flow. They pass downward at Nandua, as described on page 345, into a rock of pure palagonite; and they are only to be found in localities where basaltic plains or plateaux are covered over with submarine deposits.

ACID PUMICE TUFFS

The general characters of these deposits are described on pages 10, 218, 220, 222, 223, 231, 233, &c. Such tuffs are restricted to the north-east part of the island east of Lambasa and Tawaki, and are well displayed in the coast cliffs. They are pale yellow or whitish, and are usually non-calcareous. They are composed of the debris of a vacuolar and fibrillar isotropic glass, nearly colourless and in some localities altered. Small crystals of quartz and of glassy felspar with bits of obsidian (up to 3 mm.) and lapilli of rhyolitic glass are inclosed in them. In places inclosed pieces of coral and coral rock indicate submarine deposition.