CHAPTER XXI

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

ACID ANDESITES

_Previous observations on the Hornblende-Andesites of Fiji_

THESE rocks were first described by Wichmann[112] from specimens obtained by Kleinschmidt from the mountain of Mbuke Levu in Kandavu. These Kandavu rocks had a microfelsitic base, the porphyritic brown hornblende having usually black borders in which a change into epidote was observed. Rhombic pyroxene was only noted as an occasional constituent of a rock from Ono. Renard[113] described these rocks from the vicinity of Ngaloa Harbour in Kandavu and remarked that bronzite was of more common occurrence than the monoclinic pyroxene. In the groundmass were numerous felspar and augite microliths, whilst there was a porphyritic development of plagioclase, hornblende, biotite, and pyroxene. The hornblende phenocrysts played an important part in the rock-composition, being surrounded by a black zone of magnetite or bordered by a bacillary aggregate of small pyroxene prisms, parallel and colourless or greenish, with extinction-angles of 40°. There was often also a development of biotite in the heart of the mineral, the whole hornblende section being sometimes thus transformed.

Mr. Eakle[114] has more recently described the hornblende-andesites from Mbuke Levu in Kandavu. As the result of his examination of a collection of volcanic rocks made by the Agassiz expedition in Viti Levu, Kandavu, Mbenga, Totoya, Malolo, Yasawa group, and in several other small islands, he inferred that the hornblende-andesites are much more limited in their occurrence in Fiji than the augite-andesites; whilst hypersthene-andesite was only represented in the collection from Vomo-lailai near Waia in the Yasawas. The specimens from Waia had a microfelsitic base with pseudomorphs of hornblende and some augite. Mr. E. C. Andrews[115] in his account of his collection of volcanic rocks, made mostly in the Lau Group and Taviuni, makes no special reference to hornblende-andesites, the andesites being mainly augitic, rhombic pyroxene also occurring as a common porphyritic constituent.

It may be inferred from the above and from my own observations in Vanua Levu below given that hornblende-andesites have a relatively limited distribution in Fiji. They are not generally distributed as in the case of the augitic and basaltic andesites; but are confined to certain localities in Viti Levu,[116] Vanua Levu, Ovalau, Kandavu, Ono, Malolo, Yasawa Islands, etc.

_The occurrence of quartz-andesites or dacites in Fiji._—In connection with the existence of these rocks in Vanua Levu, it is noteworthy that except in Mr. Eakle’s paper there is no reference in any of these writings to the occurrence of quartz-andesites in Fiji. Wichmann expressly states that the rocks he examined were free from quartz, and that up to his time (1882) no quartz-bearing younger eruptive rocks were known from the South Seas. Mr. Eakle in 1899 described a holo-crystalline andesite with a felsitic aspect from Malolo and another similar looking rock from Vatu Mbulo, in the same sub-group of the Fijian Islands, showing quartz both in the phenocrysts and in the microcrystalline groundmass, concerning which he observed that it was perhaps more of a dacite than an andesite. Dacites were found by me in 1884 in the island of Fauro in the Solomon group,[117] and it is probable that they are of more frequent occurrence in the Pacific than has been generally supposed. As shown immediately below, they are represented in Vanua Levu; and the extent of their distribution in the island depends on the limits we assign to the definition of the term “dacite.”

If we restrict the term to a hornblende-andesite carrying porphyritic quartz and displaying a microfelsitic groundmass, such rocks, though they form some of the highest peaks in the Ndrandramea district, namely Ngaingai and Wawa-levu, would not be very frequent in Vanua Levu. If, however, a microfelsitic groundmass is alone necessary to constitute a “dacite,” the great majority of the acid andesites of the island would fall under this designation.[118] This has long been a controverted point in petrology. If I adopted the last procedure, my general classification for the andesites would fall into confusion and many rocks without any quartz would be included in the dacites.

In the Synopsis it will be seen that my classification of the andesites is as far as concerns the great groups based on the mineral and not on the structural characters. There are three sub-classes closely allied to each other, the hypersthene-andesites, the hornblende-hypersthene-andesites, and the quartz-hornblende-hypersthene-andesites or dacites, which cannot be distinguished at their boundaries by their petrological characters or by their different modes of occurrence. These groups of rocks which include all the acid andesites of the island will now be dealt with.

THE ACID-ANDESITES OF VANUA LEVU

(_Comprising the hypersthene-andesites; hornblende-hypersthene-andesites; and quartz-hornblende-hypersthene-andesites or dacites_)

These rocks compose in mass numerous isolated hills that rise up abruptly in the interior of the central portion of the island. Such hills, or mountains, as they might be often termed, usually attain a height of from 700 to 1200 feet above the surrounding country, and possess precipitous slopes and frequently perpendicular cliff-faces. In the geological description of the island, I have referred in detail to these mountains, when speaking of Na Raro, Vatu Kaisia, Ndrandramea, Ngaingai, etc.; and illustrations of some of them are included in this work. It may, however, be here remarked that they are as a rule rudely conical with rounded or peaked summits. The ground-plan is generally elliptical in outline; and in consequence the profile often varies from different points of view, so that as in the case of Na Raro, it is that of a sharp conical peak when the mountain is viewed “end-on,” or of a broad truncated mass when seen from the side. A similar change of form is to be noticed in the illustrations of Ndrandramea. No traces of crateral cavities came under my notice. The rocks are neither vesicular nor scoriaceous, and are usually massive; but exhibit at times a rudely columnar structure.

Each hill or mountain has its peculiar variety of these rocks. This is well shown in the Ndrandramea district. Thus the rocks of Ngaingai and of Wawa-levu in carrying porphyritic quartz differ from those of all the other hills around. Those of Soloa Levu are distinguished by the orthophyric groundmass and by the absence of hornblende. Those of Mount Ndrandramea again have no porphyritic quartz, but little hornblende, and possess a micro-felsitic groundmass. The rocks of Na Raro and Vatu Kaisia differ as regards specific gravity, the “grain” of the felsitic groundmass, the presence of phenocrysts of rhombic pyroxene, etc. The characters of these rocks from various localities are contrasted in the table given on a later page, whilst the different sub-classes to which they belong are described in detail below.

SUB-CLASS HYPERSTHENE-ANDESITES

These are dark and light grey rocks, sometimes granitoid in appearance. They pass on the one hand into the hypersthene-augite-andesites before described and on the other into the hornblende hypersthene-andesites to be subsequently dealt with. From the former they are distinguished by the great predominance of rhombic pyroxene both as phenocrysts and in the groundmass; whilst from the latter they are separated by the absence of brown hornblende or its pseudomorphs. These rocks are found in the Ndrandramea, Valanga, and Vunimbua districts. They may form isolated dome-shaped hills as in that of Soloa Levu, or they may constitute the deeper-seated rocks of the region from which these hills arise, as in the Ndrandramea district. In their general mode of occurrence, however, they cannot be treated apart from the allied hornblende-hypersthene-andesites and the dacites.

This sub-class may be divided like the hypersthene-augite-andesites into four orders according to the character of the groundmass; and these are enumerated in the Synopsis. Only the orthophyric and felsitic orders are represented in my collection. Of the former the most typical rocks are those composing the hill of Soloa Levu which is described on page 103.

These Soloa Levu rocks are lightish grey and granitoid in aspect, with specific gravity of 2·54-2·62, and displaying abundant porphyritic crystals of pyroxene, 2-3 mm. in size. In the slides they show a large number of plagioclase phenocrysts together with those of pyroxene in a relatively scanty groundmass, for the most part orthophyric in texture and without residual glass.... The plagioclase phenocrysts, which are not usually over 2 or 3 mm. in size, are often tabular and show distinct zone-lines. Though they are traversed by minute cracks and have frequently a semi-saussuritic appearance arising partly from change-products and partly from the abundance of colourless inclusions, they yield clear lamellar extinctions of medium and basic andesine (15°-25°).... The pyroxene phenocrysts, which are not much altered, are in most cases long pale-yellow rhombic prisms with rounded ends, behaving optically as described on page 285; but intergrowths with monoclinic pyroxene may occur and even separate crystals of augite.... The scanty groundmass, though in the main formed of short and broad felspars, ·12 mm. long, of the orthophyric type, displays in places a rude mosaic, apparently of quartz and felspar. It also shows abundant small pyroxenes in the form of small prisms (·05 mm. in length), giving extinctions nearly always straight but occasionally oblique (30°-35°).

As examples of the felsitic order of these rocks, most of which are altered like the propylites, I will first take the case of those deep-seated rocks that are exposed in the river-bed above Nambuna in the Ndrandramea district. In the least altered state they are dark grey and mottled, and have a specific gravity of 2·66-2·69. In section they display tabular zoned plagioclase phenocrysts, usually more or less occupied by alteration products, but at times giving lamellar extinctions of basic andesine (20°-25°). The rhombic pyroxene is more or less replaced by chloritoid pseudomorphs; whilst the “grain” of the mosaic is often coarse (·03 mm.), and much of it is evidently quartz. The more advanced stages of alteration of these rocks are described in the account of the district given on page 106.... Similar rocks, showing pyrites, occur amongst the blocks of Vunimbua River; but here the rhombic pyroxene is mostly converted into bastite, and the groundmass is in part trachytic as well as felsitic in texture. The specific gravity is 2·7.

(In the last survey of my collection I have found a solitary specimen from an agglomerate in the Mbua-Lekutu “divide,” which must be referred to the order with felspar-lathes in flow-arrangement. It is a pale grey rock showing abundant macroscopic pyroxene prisms, 2 mm. long, mostly rhombic but showing also intergrowths with monoclinic pyroxene. The felspar-lathes do not average more than ·1 mm., and there is a quantity of small prisms of rhombic pyroxene in the groundmass, which also contains a little residual glass.)

SUB-CLASS HORNBLENDE-HYPERSTHENE-ANDESITES

This is an extensive group which includes the rocks forming several of the hills in the Ndrandramea district as well as the isolated peaks of Na Raro, Vatu Kaisia, etc. It passes on the one side into the Hypersthene-Andesites before described and on the other into the Hornblende-Hypersthene-Quartz-Andesites, the Dacites of this island.

Of the four orders established in the Synopsis (page 236) according to the general method there adopted, the first, where the groundmass exhibits felspar-lathes not in flow-arrangement, is not represented in my collection.

SECOND ORDER OF THE HORNBLENDE-HYPERSTHENE-ANDESITES

(_Felspar-lathes in flow-arrangement_)

This order is only represented by three rocks, all of which belong to the prismatic sub-order where the pyroxene of the groundmass is prismatic and not granular.

Two of these rocks are very similar in appearance and character, though coming from different localities on the opposite sides of Savu-savu Bay, one from the agglomerate of Vatu-ndamu in the Kumbulau peninsula (page 91), the other from an intrusive mass in the vicinity of Urata (page 184). They are dark grey, with specific gravity 2·6 to 2·7, and display macroscopic crystals of hornblende and pyroxene. In the slide they exhibit in addition numerous phenocrysts of plagioclase, 1 to 2 mm. in size, in a groundmass showing small felspar-lathes (less than ·1 mm. in length) in partial flow-arrangement and numerous pyroxene prisms (·05 mm. long) giving straight extinctions, together with a little residual glass.... The plagioclase phenocrysts, which give extinctions of medium and basic andesine, are often tabular and display zone-lines. They contain abundant pale inclusions arranged zone-wise.... The hornblende phenocrysts are dark brown, markedly pleochroic, and give extinctions up to 12 degrees. They have dark resorption borders and are sometimes deeply corroded. They show in various stages the remarkable conversion at the borders into fine pyroxene, which is described on page 306.... The pyroxene phenocrysts are more numerous in the Urata rock. They are for the most part of the pale yellow feebly pleochroic rhombic type that prevails in the island. A few phenocrysts of pale augite (ext. 35°) may occur in the same slide; whilst the two pyroxenes may be associated as intergrowths.

A crypto-crystalline variety of these rocks, where the felspar-lathes and rhombic pyroxene prisms of the groundmass are only in part differentiated, is found on the hills of Ndreke-ni-wai on the shores of Natewa Bay (page 201). It is a pale-grey open-textured rock, displaying numerous small macroscopic crystals of hornblende.

THIRD ORDER OF THE HORNBLENDE-HYPERSTHENE-ANDESITES

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

These rocks occur generally as agglomerates and are more particularly characteristic of the district between the Mariko Range and the Salt Lake. They belong for the most part to the prismatic sub-order of the group and to the section with plagioclase phenocrysts, and fall naturally into two divisions corresponding to the two genera with _glassy_ and _opaque_ phenocrysts. The last named would be regarded by some as porphyrites. The specific gravity of the specimens ranges from 2·52 to 2·7.

The plagioclase phenocrysts, 1 to 2 mm. in size, give extinctions indicating in some rocks oligoclase-andesine (10°-15°) and in others basic andesine (15°-25°). Their opacity in the porphyrites is sometimes due to multiple macling, but more usually it arises from the numerous fine cracks filled with decomposition products that traverse them. The phenocrysts of dark brown hornblende are generally abundant and give extinctions of 15 degrees. They as a rule have dark resorption borders in which the process of conversion into fine pyroxene is in active operation. The pyroxene phenocrysts are scanty and in most cases rhombic; but intergrowths with augite and separate crystals of the last-named may occur. In the altered rocks or porphyrites they are largely replaced by bastite and viridite. The felspars of the groundmass are broad and often rectangular and may give lamellar extinctions of oligoclase-andesine. The pyroxene in the groundmass of the porphyrites is often partly decomposed. It is as a rule prismatic. A little interstitial glass, altered in the porphyrites, is generally present.

FOURTH ORDER OF THE HORNBLENDE-HYPERSTHENE-ANDESITES

(_Groundmass felsitic, displaying a granular mosaic structure_)

These are light and dark grey rocks showing usually macroscopic pyroxene and hornblende. They vary considerably in appearance from the open-textured rock to that with a granitoid coarsely crystalline aspect. They generally carry brown hornblende phenocrysts, but frequently these are represented by pseudomorphs; and they all have a felsitic groundmass. They are only separated by the absence of porphyritic quartz from the dacites of Vanua Levu, which are treated in the next sub-class. They present all stages from the crypto-crystalline to the holo-crystalline condition, but all show a groundmass which may be scanty in the more coarsely crystalline rocks.

These rocks are characteristic of some of the hills of the Ndrandramea district and of the isolated peaks of Vatu Kaisia and Na Raro. They include a large proportion of the acid andesites of the island, and all belong to the prismatic sub-order with prismatic pyroxene in the groundmass, and to the section with plagioclase phenocrysts. Their specific gravity ranges usually from 2·55 in the more acid and less crystalline types to 2·74 in the most crystalline and basic kinds.

In the typical slides they display phenocrysts of plagioclase, pyroxene, and brown hornblende in a microfelsitic groundmass formed evidently of felspar and quartz together with much prismatic pyroxene. They may be conveniently divided into three species according to the size of the “grain” of the groundmass.

Of the first species, where the “grain” is less than ·01 mm., the rocks of Mount Ndrandramea are typical. They have a crypto-crystalline groundmass where the felsitic structure is in process of development and where the pyroxene prisms or microliths are very minute. The plagioclase phenocrysts (1 mm. in size) give extinctions of acid and medium andesine (10°-20°), and are tabular, zoned, and contain abundant pale inclusions. The hornblende phenocrysts, except in the case of the rocks at the foot of the hill, are represented by pseudomorphs in various stages of dispersion, so that this character is likely to be overlooked. The pyroxene phenocrysts are in most cases of the pale yellow feebly pleochroic rhombic type, but they may present intergrowths of both the monoclinic and rhombic forms. These are light grey rocks with a specific gravity increasing as one descends from the summit, where it is about 2·5, to the base where it is 2·7, a change corresponding with increase of the ferro-magnesian minerals and with the more crystalline structure of the groundmass.

The second species, where the “grain” of the mosaic is between ·01 and ·02 mm., is represented by the Vatu Kaisia rock and by that exposed in the opposite side of the gorge. They are granitoid in appearance and have a specific gravity of 2·68 to 2·71. Large porphyritic crystals of pyroxene and hornblende, the last sometimes 7 mm. in length, occur in a dark grey base. In the slide these phenocrysts together with those of plagioclase are displayed in a somewhat scanty holo-crystalline groundmass, where the “grain” of the mosaic averages ·012 mm. The plagioclase phenocrysts are zoned, and give in different crystals extinctions in some cases of oligoclase-andesine (10°-12°) and in others of andesine-labradorite (25°-30°). The hornblende phenocrysts in their pseudomorphism illustrate the various stages of the process of conversion into fine granular and prismatic pyroxene. The least altered crystals have dark resorption borders and are at times deeply corroded. The pyroxene phenocrysts are for the most part rhombic; but intergrowths with the monoclinic form occur. The pyroxene of the groundmass consists for the most part of small rhombic prisms averaging ·05 mm. in length.

The third species, where the groundmass may be described as a coarse mosaic with a “grain” between ·02 and ·03 mm., is represented by the rocks of the peak of Na Raro and of Mount Thokasinga in the Ndrandramea district.

The Na Raro rocks are light grey with a specific gravity in the unweathered state of about 2·6, and display macroscopic crystals of glassy plagioclase and hornblende. In the slide they exhibit tabular phenocrysts of the plagioclase, together with dark pseudomorphs after hornblende, in a coarsely felsitic groundmass (grain ·022 mm.) where a little very fine prismatic pyroxene, apparently rhombic, occurs. There is also a little altered interstitial glass. The plagioclase phenocrysts are zoned and give extinctions of 15° to 25° (acid and basic andesine), whilst they often show magma inclusions. An interesting feature of these rocks is concerned with the rarity or absence of phenocrysts of pyroxene. They are to be seen, however, in the process of being built up within the substance of the hornblende pseudomorphs, which consist entirely of minute prisms and granules of pyroxene and of fine magnetite. The process seems to consist in the formation of parallel layers of rhombic and monoclinic pyroxene.

The Thoka-singa rocks are more basic (spec. grav. 2·72 to 2·74), and in the scanty holo-crystalline groundmass approach the plutonic type. They are dark grey granitoid rocks displaying abundant macroscopic pyroxene crystals 2 to 3 mm. long. The original hornblende phenocrysts are only represented by traces of pseudomorphs of fine pyroxene and magnetite, the process of dispersion, described on page 307, being almost completed. The pyroxene phenocrysts are mostly rhombic; but intergrowths with the monoclinic form occur. The “grain” of the mosaic of the groundmass is coarse (·023 mm.), and there is a fair amount of prismatic with a little granular pyroxene, the prisms, ·04 mm. long, giving usually straight extinctions, whilst the granules are apparently monoclinic.

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

These rocks are infrequent. They compose in mass the adjacent mountains of Ngaingai and Wawa Levu in the Ndrandramea district, and appear also on the lower slopes of the neighbouring mountain of Navuningumu. They differ chiefly from the hornblende-hypersthene-andesites in the presence of porphyritic quartz, which, however, is not as a rule abundant. In their general origin and affinities and in their mode of occurrence they cannot be separated from the two sub-classes of hypersthene-andesites and hornblende-hypersthene-andesites before described. They all belong to the felsitic order of the sub-class, and all are referred to the sub-order with prismatic pyroxene and to the section with plagioclase phenocrysts.

They are light grey rocks, with a specific gravity of 2·57 to 2·61, showing usually dark pseudomorphs after hornblende and a little porphyritic quartz. In the slide they display these pseudomorphs and quartz crystals, associated with abundant plagioclase phenocrysts, in a felsitic groundmass, evidently a mixture of felspar and quartz, with fine pyroxene, mostly prismatic and rhombic. Pyroxene phenocrysts are absent or rare; but they may be seen in process of formation in the substance of the hornblende-pseudomorphs. It is only at times, as in the instance of the Navuningumu rock, that the brown hornblende phenocrysts are in part unchanged and that complete pyroxene phenocrysts occur. In such cases the last may be entirely rhombic, or may exhibit at times intergrowths with the monoclinic form.

The plagioclase phenocrysts, 2 mm. in size, are often tabular and zoned and give two sets of extinctions, indicating acid and basic andesine. Their abundant inclusions are arranged in zones. The quartz-crystals, 1 to 2 mm. in size, present hexagonal sections with rounded angles. They are sometimes traversed by cracks occupied by iron oxide films. The pseudomorphs after hornblende, which consist of fine pyroxene mixed with magnetite, exhibit often the building up in their interior of pyroxene phenocrysts, apparently rhombic, by long parallel rows of stout prisms. In other cases the pseudomorphs display the different stages of dispersion. The fine pyroxene of the groundmass consists mostly of rhombic prisms (·02-·06 mm. long) with some granules. The “grain” of the groundmass is usually between ·01 and ·02 mm. There is little or no residual glass.

The rocks of Wawa Levu and Ngaingai are closely similar, but they differ in the size of the prismatic pyroxene of the groundmass, which is coarser in the first-named mountain (·055 mm. long) than it is in the second (·025 mm. long). In both the “grain” of the mosaic is about the same (·014 mm.).

TABULAR COMPARISON OF THE ACID ANDESITES OF VANUA LEVU.

NOTE.—The figures in the columns headed “Felsitic” and “Rhombic” refer to the size in millimetres of the felsitic “grain” and the larger pyroxene prisms.

Column headings:

P: Plagioclase. Q: Quartz. H: Hornblende or its pseudomorph. R: Rhombic Pyroxene. M: Monoclinic Pyroxene. O: Orthophyric. Rh: Rhombic. Mo: Monoclinic.

+------------+----+---------------------+---------------------------+ | | | Phenocrysts. | Groundmass. | | | +-+-+--------+-+------+-------------+-------------+ | Locality. | Sp.| | | | | | Structure. | Pyroxene. | | | Gr.|P|Q| H |R| M +-----------+-+------+------+ | | | | | | | | Felsitic. |O| Rh | Mo | +------------+----+-+-+--------+-+------+-----------+-+------+------+ |Ngaingai, | | | | | | | | | | | | summit |2·57|+|+| + | | | + ·014 | | + ·03| | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Ngaingai, | | | | | | | | | | | | lower part |2·57|+|+| + | | | + ·013 | | + ·02| | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Wawa-levu, | | | | | | | | | | | | 1,500 feet |2·61|+|+| + | | | + ·015 | | + ·05| | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Wawa-levu, | | | | | | | | | | | | 1,500 feet |2·61|+|+| + | | | + ·014 | | + ·06| | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Wawa-levu, | | | | | | | | | | | | west side |2·57|+|+| + |+| | + ·022 | | + ·04| | | of base | | | |pseudom.| | | | | | | | but not on | | | |in great| | | | | | | | mountain | | | | part | | | | | | | | | | | | | | | | | | | |Ndrandramea,| | | | | | | | | | | | summit |2·44|+| | + |+| + | + ·007 | | + ·01| | | | | | | only | |scanty| hemi- | | | | | | | | | traces | | |crystalline| | | | | | | | | of | | | | | | | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Ndrandramea,| | | | | | | | | | | | 1,600 feet |2·58|+| | + |+| + | + ·008 | | + ·01| | | | | | | only | |scanty| hemi- | | | | | | | | | traces | | |crystalline| | | | | | | | | of | | | | | | | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Ndrandramea,| | | | | | | | | | | | 1,200 feet |2·68|+| | ? |+| ? | + ·008 | | + ·01| | | | | | | | | | hemi- | | | | | | | | | | | |crystalline| | | | | | | | | | | | | | | | |Ndrandramea,| | | | | | | | | | | | saddle at |2·71|+| | + |+| ? | + ·009 | | + ·01| | | base | | | | | | | | | | | | | | | | | | | | | | | |Kalakala, |2·61|+| | + |+| | + ·02 | | + ·01| | | near base | | | |pseudom.| | | | | | | | | | | |in part | | | | | | | | | | | | | | | | | | | |Navuningumu{|2·65|+| | + |+| + | + ·02 | | + ·03| ? | | near base {| | | |in part | |scanty| | | | | | {| | | |pseudom.| | | | | | | | {|2·48|+|+| + |+| | + ·018 | | ? | | | | | | | | | | | | | | |Thokasinga |2·74|+| | + |+| + | + ·024 | | + ·03| + | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Thokasinga |2·72|+| | |+| + | + ·023 | | + ·04| + | | | | | | | | | | | | | |Na Raro, |2·58|+| | + | | | + ·02 | | + ·01| | | upper part | | | |pseudom.| | | | |scanty| | | | | | | | | | | | | | |Na Raro |2·58|+| | + | | | + ·022 | | + ·01| | | | | | |pseudom.| | | | |scanty| | | | | | | | | | | | | | |Vatu Kaisia |2·71|+| | + |+| | + ·013 | | + ·05| | | | | | |in part | | | | | | | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Vatu Kaisia,|2·68|+| | + |+| | + ·01 | | + ·05| | | opposite to| | | |in part | | | | | | | | | | | |pseudom.| | | | | | | | | | | | | | | | | | | |Soloa-levu, |2·54|+| | |+| + | |+| + ·06| | | upper part | | | | | | | | | | | | | | | | | | | | | | | |Soloa-levu, |2·57|+| | |+| + | |+| + ·05| | | upper part | | | | | | | | | | | | | | | | | | | | | | | |Soloa-levu, |2·62|+| | |+| + | |+| + ·06| + | | near base | | | | | | | | | |scanty| | | | | | | | | | | | | |Soloa-levu, |2·61|+| | |+| + | |+| + ·06| + | | near base | | | | | | | | | |scanty| +------------+----+-+-+--------+-+------+-----------+-+------+------+

_Note on the Rhombic Pyroxene of the three foregoing sub-classes of the Acid Andesites._—The term “hypersthene” has been here used as a convenient expression equivalent to “rhombic pyroxene.” The mineral is always a little pleochroic and is never colourless, and it is only in very rare cases that the term “enstatite” could be used. As a matter of fact there is practically only one form of rhombic pyroxene represented in my collections whether in acid or basic andesites or in hemi-crystalline and plutonic rocks. In the acid andesites it occurs not only as phenocrysts but also as minute prisms forming a constituent of the groundmass.

This mineral, when composing the phenocrysts, presents itself usually as single untwinned prisms which exhibit the typical octagonal cross-sections with much reduced prism-faces. The prismatic sections give straight extinctions; whilst with the cross-sections we obtain straight extinctions parallel with the pinakoid faces. The colour in transmitted light is pale brownish yellow. The pleochroism, though usually feeble, is quite distinct, the colour being pale yellow when the prism lies parallel with the long axis of the lower nicol, and almost white when it lies across. Not infrequently these phenocrysts behave abnormally and give small oblique extinctions. This is often the case when monoclinic pyroxene occurs in the same section. The association of the two pyroxenes in one crystal can in some cases be clearly recognised. At one time a plate of pyroxene exhibits itself as a coarse aggregate of the two pyroxenes. At other times the two occur as parallel intergrowths, as in the accompanying figure. But it is rarely that such intergrowths are so typically displayed, the reason of which has been supplied by Zirkel in his _Lehrbuch der Petrographie_; 2nd edit.: I. 271.

_Note on the “magmatic paramorphism”[119] of the hornblende phenocrysts._—Reference has before been made in the general description of these rocks to the dark alteration margins of the hornblende phenocrysts. The dark borders display the “bacillary” structure noticed by Renard in the case of some hornblende andesites from Kandavu in the same group of islands,[120] being composed of minute granules and parallel prisms of pyroxene and also of magnetite grains. With the Kandavu rocks Renard observed that the tiny pyroxenes were colourless or greenish and had an extinction angle of 40°. In the Vanua Levu rocks, however, there is a mixture in these dark margins of both monoclinic and rhombic pyroxene; and the process may be observed in all stages as it advances into the interior of the crystal, until a dark pseudomorph or paramorph of pyroxene and magnetite results. When the magnetite prevails, the pseudomorph may ultimately form a black patch in which the process is obscured. But when, as is generally the case, the pyroxenes are more frequent, it occurs as a dark grey mass.

Finally follows the dispersion of the pseudomorph, which first becomes a loosely arranged aggregate of the two pyroxenes and magnetite, and then breaks down, and at length is only represented by small pale patches of its original constituents. These patches are easily recognisable, and in not a few rock-sections offer the only indication of the previously existing hornblende phenocryst. There can be little doubt that this is the source of much at least of the often abundant pyroxenes of the groundmass, which are usually most frequent in the vicinity of the patches.

In the earliest stage when the dark border alone exists, it is not easy to distinguish the one pyroxene from the other, the granules and prisms being colourless and very minute, less than ·01 in size. But in a far advanced stage of the paramorphism the granules and prisms become sometimes much larger, the first attaining a breadth of ·04 and ·05 mm. and the last a length of ·15 mm. Finally the interior of the paramorph is seen to be more or less completely composed of very pale brown augite and pale yellow rhombic pyroxene in coarse grains and prisms, the first distinguished by its oblique extinction of 30° to 35°, the last recognised by its straight extinction and feeble though distinct pleochroism.

Although as a rule the paramorph becomes dispersed and its pyroxene constituents are added to the groundmass, it sometimes exhibits a change of another character. In this case the outer portion is alone dispersed, whilst the growth of a single large crystal of pyroxene proceeds within the mass. In a later stage, when the dispersion of the outer part is complete, we have a fresh-looking pyroxene phenocryst with unformed edges, on the borders of which little granules and prisms of pyroxene may be seen arranging themselves, as if the crystal-building was still in progress, or rather as if it had been interrupted and left unfinished by the too rapid dispersion of the outer portions of the paramorph.

It will be gathered from the above that the source of the pyroxene of the groundmass is to be found in the magmatic paramorphism of the porphyritic hornblende. The hornblende is dark-brown, markedly pleochroic, and extinctions up to 15° are given in prismatic sections. It is well known that the conversion of a hornblende crystal into an aggregate of pyroxene prisms and magnetite was long since experimentally effected by Doelter and Hussak by immersing the hornblende in molten basalt, andesite, &c.[121] I would imagine that the transformation of the hornblende and the dispersion of the paramorph occurred under two conditions; in the first case whilst the “flow” was still in motion when the resulting pyroxene would be mixed up in the magma; in the second case after movement had ceased, but before consolidation of the groundmass, when a paramorph or pseudomorph would be formed.

OLIGOCLASE-TRACHYTES

The term “trachyte” is here applied in a general sense to a group of light-grey intrusive acid rocks, having a specific gravity when compact of 2·4 to 2·45 and showing phenocrysts of glassy felspar, but not of quartz. These rocks, which are especially characteristic of the districts around Tawaki and Mount Thuku and of the Wainikoro sea-border, are often open-textured and sometimes a little vesicular, whilst several of them exhibit some degree of alteration in the groundmass. In all cases they appear to be intrusions rather than surface-flows; and at times they display a columnar structure.[122]

The difference between the oligoclase-trachytes in various localities appears to be mainly concerned with the varying degrees of crystallisation. There are two principal varieties. In the most crystalline type there are small phenocrysts of glassy felspar and a few of pale augite, the angle of extinction of the last being over 30 degrees. The felspar phenocrysts, which contain but few inclusions and have sharp rectilinear outlines, in most cases show zoning and give lamellar extinction of 5° to 12° indicating oligoclase; but some of them have the tabular untwinned or simple twinned form of sanidine. The groundmass is in the main composed of minute felspar-lathes, less than ·1 mm. in length, arranged in a dense plexus, and giving nearly straight extinctions. But it also contains a number of scattered larger felspar-lathes averaging ·2 mm. in length and giving extinctions of 5° when simple, and of 8° to 10° when lamellar. There is also some small prismatic augite in the groundmass but often decomposing. The original interstitial glass is represented by numerous reddish-brown patches of devitrified glass.

In the second type of these trachytes, the rock is more open in texture and is at times vesicular, the specific gravity being usually less than 2·4. The general characters are much the same, but sanidine is better represented among the phenocrysts, and the groundmass is more blurred; but when the felspar-lathes are distinct they give an extinction either nearly straight or from 4° to 8°, according as they are simple or display lamellæ. The augite of the groundmass is scanty and more or less decomposed; whilst the interstitial glass when unaltered is in fair quantity and nearly isotropic.

The alteration observed in several of these oligoclase-trachytes is restricted chiefly to the interstitial glass in which secondary quartz and at times calcite and viridite are developed. Scarcely any of them are quite free from these changes.[123]

The pitchstone or vitreous form of these trachytes is displayed in the blocks of an agglomerate-tuff between Tawaki and Mount Thuku. It has a specific gravity of 2·36, is dark-brown, and has a conchoidal fracture. Phenocrysts of felspar, mostly oligoclase, with extinction-angles of 5° to 11°, and often penetrated by the magma, are inclosed in a semi-isotropic groundmass showing incipient development of felspar and other darker microliths. There are also a few small phenocrysts of pale augite.

QUARTZ PORPHYRIES AND RHYOLITIC ROCKS

Wichmann when he wrote in 1882 that no quartz-bearing younger eruptive rocks had hitherto been observed either in Fiji or in the South Sea Islands generally, had apparently overlooked Dana’s observations in the Fijian group. The American geologist[124] refers to a rock found on the north-east shores of Vanua Levu which exhibited in a greenish base thickly disseminated crystals of quartz (bipyramidal dodecahedrons, 1/8 of an inch in diameter) and glassy felspar, together with a few sphene crystals.

Quartz porphyries, akin to the rhyolites, are especially characteristic of the north-east part of the island, to which in fact they are entirely confined. They perhaps are best represented in the vicinity of Mount Thuku and in the neighbourhood of the mouth of the Wainikoro River. None of my specimens have the fresh appearance of the Lipari rhyolites and all are more or less altered. Their specific gravity does not exceed 2·4, and they are for the most part intrusive in character.

A rock frequently exposed between Tawaki and Mount Thuku[125] contains abundant phenocrysts of glassy felspar (oligoclase and sanidine) and quartz in a greenish opaque groundmass having a blurred microfelsitic structure. There appears to have been a secondary devitrification of the groundmass since consolidation. The porphyritic quartz crystals are rounded and about 2 mm. in diameter.

Another rock displayed in the coast-cliffs on the north side of Natewa Bay, a mile east of Mount Thuku, has a somewhat banded appearance. It shows crystals of quartz, more or less rounded and 3 to 4 mm. in diameter, together with phenocrysts of glassy felspar (oligoclase with lamellar extinction of 5° and sanidine). The groundmass displays traces of spherulites and is in places semi-isotropic; but for the most part it is microfelsitic.

The type of rock found in the Wainikoro district and in the adjacent sea-border, where it may be observed forming dykes in the pumice-tuffs, is light-grey and loose-textured with a specific gravity of 2·1. It exhibits small phenocrysts of quartz and of glassy felspar (oligoclase 5° to 12°, and sanidine), with, in one locality only, a scanty amount of dark green hornblende yielding extinctions up to 20°. The quartz crystals, which are 1 to 2 mm. in size, are sometimes bipyramidal; but are often rounded and have fused-like outer surfaces. The groundmass is semi-isotropic with a blurred aspect, and shows traces of spherulites and numerous crystallites, with occasional felspar-lathes giving a nearly straight extinction.

An extensively altered quartz porphyry of a different type is associated with other altered rocks at the base of Mount Nailotha. It has a specific gravity of 2·54, and displays large opaque crystals of plagioclase, 2 to 5 mm. in size with small quartz crystals 1 to 2 mm. across, in a grey compact matrix. The first-named shows the felspar to be mostly replaced by alteration products; but occasionally a lamellar extinction of 6° or 7° can be observed. The quartz crystals are rounded and penetrated by the magma, and contain numerous strings of fluid-cavities. The groundmass was originally spherulitic; but this structure is more or less disguised by the development of a mosaic of chalcedonic quartz. It shows some micro-porphyritic patches of viridite and calcite. A singular altered white rhyolitic rock is exposed on the north coast of Natewa Bay between Natasa and Sangani, where it is associated with altered tuffs. It is compact with a conchoidal fracture and has a specific gravity of 2·48. The hand-specimen has a banded appearance. Under the microscope it appears as a rhyolitic glass for the most part devitrified and rendered opaque by the formation of secondary silica. Much of it presents a microfelsitic structure, the bands appearing as semi-opaque streaks.

Glassy forms of the quartz porphyries or intrusive rhyolitic rocks are extensively represented in the pumice tuffs of the Undu Promontory and of the coasts between the Langa-langa river and Lambasa. These tuffs will be found described on page 336. Fragments of a grey rhyolitic glass looking like perlite are inclosed in the pumice-tuffs near the mouth of the Wainikoro River. Under the microscope it is displayed as a colourless glass inclosing phenocrysts of sanidine, oligoclase (ext. 4°), and quartz, the last with rounded outlines and a fused-like outer surface. The glass shows in places perlitic cracks; but it is mainly characterised by a vacuolar structure, the minute cavities being lengthened out in the direction of the flow and displaying eddy-currents around the phenocrysts. The elongated steam-cavities sometimes contain water, but are usually more or less filled with granular materials.