CHAPTER XX

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

HYPERSTHENE-AUGITE CLASS

II. SUB-CLASS. HYPERSTHENE-AUGITE-ANDESITES

FORMULA.—_Plag, hypersth-aug, matr._

CHARACTERS.—The pyroxene phenocrysts usually are represented by separate crystals of the monoclinic as well as the rhombic type, and the two forms are often associated in the same crystal. The monoclinic form prevails in the groundmass in most cases.

REMARKS.—It is not possible to draw a sharp line between the augite and the hypersthene-augite-andesites; but where two or three phenocrysts of the rhombic type occur in a slide the rock may be placed in this division. Between this variety and that where rhombic pyroxene prevails, both among the phenocrysts and in the groundmass, numerous intermediate kinds exist. These rocks mostly occur in agglomerates and form small and large dykes or sills, but rarely are found in flows. They are distributed over most of the island except in the western portion (the basaltic districts of Wainunu, Seatura, and Solevu), but reappear again in the Mbua peninsula in places, as at Mount Koroma.

The pale yellow rhombic pyroxene is uniform in its optical behaviour. The prisms are noticeably pleochroic, being nearly colourless when lying across the long axis of the lower nicol and pale yellow when parallel with it. The intergrowths with monoclinic pyroxene often take the form of lamellar bands, whilst in some cases a nucleus of the one (usually rhombic) is invested by a growth of the other.

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

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

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

1 genus (_Vitr._) } 2 " (_Opac._) } See Synopsis, p. 247. 3 " (_Magn._) } 4 " (_Parv._) }

Nearly all of the rocks of this sub-order that are represented in my collection belong to the genus (1) with phenocrysts of glassy plagioclase. They vary considerably in appearance and in colour (black to grey), and occur under very different conditions, as in “necks,” old flows, large intrusive masses, dykes, agglomerates, &c. Their specific gravity has rather a wide range according to the degree of basicity. In the heavier rocks where the rhombic pyroxene is scanty, it is usually 2·7 to 2·8. In the others, where rhombic pyroxene is more predominant and where the felspar is less basic, it is 2·6 to 2·7.

In the slide small phenocrysts of plagioclase and pyroxene occur in a groundmass of felspar-lathes and pyroxene granules, whilst there is as a rule a fair amount of residual glass. The plagioclase phenocrysts, which give extinctions in different rocks of acid and basic andesine and contain abundant magma-inclusions, are generally one to two mm. in size. The pyroxene phenocrysts are small, and may be represented by separate crystals of the monoclinic and rhombic kinds, or by crystals displaying intergrowths of the two sorts. The pyroxene granules vary much in size and are evidently in great part of augite. In most of the rocks the felspar-lathes are less than ·1 mm. in length. In those where the length is ·1 to ·2 mm. they are sometimes stout and display a few lamellæ, yielding extinctions corresponding to those of the phenocrysts.

A singular dark grey almost holocrystalline doleritic rock (sp. gr. 2·85) is exposed in the Thulanga Ridge (p. 211). It shows no plagioclase phenocrysts, but those of pyroxene are numerous, which, however, do not exceed 2 mm. in size, so that the rock would be referred to genus 4 of this sub-order. It appears to be a doleritic form of the plutonic rock found at Nawi in this neighbourhood (p. 211). The pyroxene phenocrysts are mostly of brownish-yellow augite, but rhombic pyroxene, either as separate crystals or as intergrowths, is not uncommon. The plagioclase lathes are long and fairly stout, giving at times lamellar extinctions of 20°. Their average length is ·3 mm., and it is to their large size that the doleritic texture is due. The pyroxenes of the groundmass are similarly coarse (·2 mm.), and include both monoclinic and rhombic forms, the latter infrequent. There is a slight tendency to semi-ophitic behaviour in places; but generally these pyroxenes are irregular in shape or rudely prismatic.

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

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

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

5 genus (_Vitr._) } 6 " (_Opac._) } See Synopsis. 7 " (_Magn._) } 8 " (_Parv._) }

This sub-order includes rocks varying much in appearance, but all alike in the presence of prismatic pyroxene in the groundmass and in the absence of flow-arrangement of the felspar-lathes. They belong to the two genera, with glassy and opaque plagioclase phenocrysts. These crystals are not usually over 2 mm. in size and are of medium andesine (15°-20°). The pyroxene phenocrysts are small and may be entirely of monoclinic or of rhombic pyroxene, or the two may be associated either as lamellar intergrowths, or by displaying an eroded nucleus (generally rhombic) around which a regular crystal of monoclinic pyroxene has grown. The felspar-lathes are in some cases less than ·1 mm. long, and in others ·1-·2 mm. The pyroxene prisms of the groundmass average ·01-·04 mm. in length, and give both straight and oblique extinctions, the last prevailing. The specific gravity ranges from 2·55 to 2·75 according to the degree of basicity and amount of interstitial glass, which is usually in fair quantity.

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

(_Felspar-lathes in flow-arrangement._)

13. GENUS

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

CHARACTERS.—Glassy plagioclase phenocrysts.

DESCRIPTION.—This is a group of rocks that comes near the basaltic andesites represented in genera 1 and 13 of the augite sub-class; and to the more basic kinds the terms of basaltic andesite is equally applicable. These rocks, however, differ in the prevalence of rhombic pyroxene, which occurs as phenocrysts, but always accompanied by monoclinic pyroxene, whether as separate crystals or as inter-growths. Such rocks are intermediate between those of the augite and rhombic pyroxene classes. They are particularly characteristic of the Savu-savu peninsula, but they are also found in other scattered localities. Sometimes they appear to form ancient flows, and at other times intrusive masses and dykes; but they are rarely scoriaceous.

Almost all the rocks in my collection referred to this genus belong to the species where the felspar-lathes of the groundmass are ·02-·1 mm. long. They are generally blackish or dark-brown, and the specific gravity ranges usually from 2·72 to 2·83. They display in the slide a fair number of plagioclase and pyroxene phenocrysts in a groundmass of felspar-lathes, pyroxene granules, and magnetite, the interstitial glass being scanty or moderate in amount.... The plagioclase phenocrysts are rarely as large as 3 mm., so that most of the rocks belong to the non-porphyritic group of the genus. These phenocrysts, which are often zoned, give extinctions of basic andesine (15°-25°). They contain magma-inclusions, sometimes in abundance, which are arranged in zone-lines.... The pyroxene phenocrysts are small, the two kinds being always represented in the same slide. In some cases separate crystals occur, and in others the two are associated as intergrowths, but in most cases separate and compound crystals occur in the same section. Not infrequently the phenocryst is an aggregate of lesser crystals of the two pyroxenes. The monoclinic form is a brownish yellow augite with large extinctions and often twinned. The felspar-lathes of the groundmass, which usually average ·05 or ·06 mm. in length, are either narrow and untwinned, or they may be stouter and display simple and at times lamellar twinning, giving extinctions of medium andesine.... The granules of pyroxene are generally ·01-·02 mm. in size; but occasional prism-forms occur which give sometimes the straight extinction of rhombic pyroxene and at other times the large oblique extinctions of the augite type.

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

(_Felspar-lathes in flow-arrangement_)

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

17 genus (_Vitr._) } 18 " (_Opac._) } See Synopsis. 19 " (_Magn._) } 20 " (_Parv._) }

The rocks of this sub-order that are represented in my collection admit easily of a general description, and since the diagnoses of the genera are given in the Synopsis, there will be no need to separately describe each genus.

Almost without exception these rocks form a constituent of agglomerates in various parts of the island; and they occur in this condition in some of the highest mountains, as Mariko, Thambeyu, and Koro-mbasanga. The exception refers to a low mound-like hill, apparently a “volcanic neck,” that rises from the basaltic plains west of Mbua (see page 58).

In about half of the specimens the rocks are referred to section 10, where the plagioclase phenocrysts are either small and very scanty or are absent altogether. In a fair number these phenocrysts are opaque (genus 18); whilst in a few they are glassy (genus 17). The rocks are typically blackish or dark grey, and often have a pitchstone-like appearance, the groundmass being frequently semi-vitreous in character. Vesicular and scoriaceous rocks occur at times.

In all cases the felspar-lathes and pyroxene prisms are more or less in flow-arrangement; whilst pyroxene granules, if present, are subordinate. The felspar-lathes, which are either simple or once-twinned and give extinctions of acid and medium andesine, are usually small, and average in different sections ·05-·08 mm. in length. The pyroxene prisms are pale brown and are ·03 or ·04 mm. long. Most of them give oblique extinctions of over 25; but in the same slide some give straight extinctions; the proportion varies in different rocks. The pyroxene phenocrysts in all the specimens are small (not over 2 mm.), and are rhombic and monoclinic. In most sections the two forms are represented by separate crystals and are also associated in the same phenocryst. Those of rhombic pyroxene have often dark borders. There is a considerable amount of a pale brown glass in the groundmass, more or less devitrified. The specific gravity varies considerably, but is as a rule between 2·55 and 2·75, the more basic rocks containing augite in preponderance and basic andesine, whilst the less basic possess a large proportion of rhombic pyroxene and display oligoclase-andesine. Sometimes, as in the case of a rock composing an agglomerate east of Nanduri, where the porphyritic plagioclase is opaque and there is some degree of alteration, the rock looks very like a porphyrite.

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

FORMULA.—_Hypersth-aug, matr, orth._

CHARACTERS.—Felspars of the groundmass short and broad.

Since the material is insufficient for the separate description of each genus, a general account of the order is alone given. These rocks are often represented in agglomerates or they occur as large blocks, either lying on the surface or imbedded in tuffs. Many of them are somewhat altered.

They are for the most part dark grey dull-looking rocks, with a specific gravity of 2·7 to 2·8, showing macroscopic plagioclase together with conspicuous pyroxene phenocrysts. The plagioclase phenocrysts are usually small (1 to 3 mm.), and give extinctions of medium andesine (15°-20°) and in some rocks of acid labradorite (30°). They are as a rule corroded and are penetrated by numerous fissures, whilst they contain a considerable amount of altered magma-inclusions with sometimes other alteration products. The pyroxene phenocrysts are from 2 to 4 mm. in size. Brownish-yellow augite, giving extinctions of over 30°, and pale-yellow rhombic pyroxene of the type before described occur generally in the same slide, and are frequently associated as intergrowths in the same crystal. They may have regular outlines or dark eroded borders, and at times they exhibit abundant dark opaque inclusions. The broad felspars of the groundmass are sometimes rectangular and give lamellar extinctions of medium and acid andesine (12°-17°). They vary in length in different rocks from ·05 to ·2 mm. and more. The pyroxene of the groundmass is generally granular and coarse (·02-·05 mm.). As indicated by the extinctions of occasional prism-forms it is composed of both augite and rhombic pyroxene, the former prevailing. The prismatic sub-order is also represented, and here the pyroxene of the groundmass is in great part prismatic, the length of the prisms not often exceeding ·05 mm., whilst both the monoclinic and rhombic kinds occur. Interstitial glass varies in amount, sometimes absent, sometimes scanty and viriditic, at other times abundant and opaque. Magnetite abounds in the groundmass much of it often being of secondary origin.

FOURTH ORDER, FELSITIC, OF THE HYPERSTHENE-AUGITE ANDESITES

FORMULA.—_Hypersth-aug, matr, fels._

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

This order is capable of subdivision, as in the other orders of the hypersthene-augite-andesites (see Synopsis, page 247); but since it is only represented by six of my rock-sections, I will confine the description to the general characters.

These rocks are dark-grey, sometimes granitoid in appearance, with specific gravity 2·65 to 2·75. They usually show some alteration, arising from secondary changes within the rock-mass; and probably the felsitic or semi-mosaic appearance of the scanty groundmass is the result of such a secondary change. Such rocks in some respects approach the type of the gabbros. They are frequent on the north coast of Natewa Bay in the vicinity of Waimotu and also occur in the Valanga Ridge. They generally present themselves as deeper-seated massive rocks exposed by the stripping off of the superficial deposits.

There are as a rule more or less conspicuous phenocrysts, up to 3 mm. in size, of plagioclase and pyroxene, in a relatively scanty micro-felsitic groundmass, displaying a blurred mosaic, in which a few stout felspar-lathes can still be recognised, and composed apparently of felspar and crystalline silica. The “grain” of the mosaic may range in different rocks from ·005 mm. to ·02 mm. The pyroxene of the groundmass is largely decomposed, and the scanty residual glass is represented by viriditic materials.

The plagioclase phenocrysts, which give extinctions of medium and basic andesine (15°-25°), are often semi-opaque and corroded. They are traversed by numerous cracks and often contain many whitish alteration products, though the lamellar structure is usually well preserved. The pyroxene phenocrysts are composed of brownish-yellow augite (ext. + 30°) and pale rhombic pyroxene of the type described before, either as separate crystals or associated as intergrowths. The rhombic pyroxene crystals are often sub-rounded with dark borders; and as a rule the pyroxene phenocrysts are much fresher than the plagioclase phenocrysts. As far as can be ascertained, most of the original pyroxene of the groundmass was monoclinic with a little rhombic.