CHAPTER XIX

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

AUGITE CLASS

SUB-CLASS II

AUGITE-ANDESITES INCLUDING THE BASALTIC ANDESITES (_Plag, aug, matr._)

THIS sub-class, which comprises 40 per cent. of the volcanic rocks, is characterised by the absence of olivine on the one hand, and by the rarity or absence of rhombic pyroxene on the other. On the basic side it shades into the olivine-basalts through the basaltic andesites, and on the acid side by intermediate stages into the hypersthene augite andesites; and for these reasons it is not always possible to draw a sharp line of distinction. In cases where a hand-specimen displays no macroscopic olivine and where a solitary small phenocryst of olivine is alone observed in the slide, it should be referred to this sub-class; and here also all doubtful specimens as regards the occurrence of olivine should be placed. When the question of the occurrence of rhombic pyroxene arises, it should be remembered that the great prevalence of monoclinic pyroxene amongst the phenocrysts and the practical absence of rhombic pyroxene from the groundmass are essential characteristics of this sub-class. Rhombic pyroxene is only indicated at times by intergrowths in the phenocrysts.

The basaltic andesites enter into the formation of old “flows,” as in the Mbua and Ndama plains. The other rocks enter principally into the composition of dykes, necks, and agglomerates.

I. GRANULAR SUB-ORDER (AUGITE-ANDESITES)

FORMULA.—_Aug, matr, non-flu, gran._

1. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is granular. Phenocrysts of glassy plagioclase.

DESCRIPTION.—These rocks frequently form dykes; and it is probable that most of the instances where the nature of the exposure could not be ascertained also fall into this category. They are dark-brown or blackish, and their sp. gr. ranges, except in the semi-vitreous rocks, from 2·7 to 2·83. They are sometimes vesicular, and rocks with abundant interstitial glass are common. They admit of grouping into two sub-genera according to the size of the plagioclase phenocrysts:

(_a_) Porphyritic, where the average size is 3 mm. or over. (_b_) Non-porphyritic where the average size is less than 3 mm.

Nearly all the rocks in my collection belong to the second group.

In the sections they display phenocrysts of plagioclase and occasionally of pyroxene in a groundmass formed of a plexus of felspar-lathes, augite granules, magnetite, and usually a fair amount of smoky more or less opaque interstitial glass.... The felspar phenocrysts, which are sometimes abundant, give lamellar extinctions of andesine labradorite (15° to 30°). They are frequently small (1 to 2 mm.) and contain often many magma-inclusions. Whilst the corroded aspect of some indicate that they belong to an earlier period, the aggregate character and regular outlines of others suggest that they have been produced in position.... Pyroxene phenocrysts are absent in half the rocks. When present they are generally small and of a pale augite which gives extinctions of 30°. Their size does not usually exceed 2 mm.; and they may consist of single crystals (sometimes twinned) or of an aggregate of smaller crystals. At times there is a suspicion of intergrowth with rhombic pyroxene; but no phenocrysts formed alone of that mineral occur.... The augite granules of the groundmass as a rule vary from ·02 to ·04 mm. in diameter. Occasional prism-forms giving large extinctions occur.... The felspar-lathes vary much in length in different rocks. In some they average as little as ·05 mm., and in others as much as ·2 mm.; but the doleritic type with yet longer lathes is not represented in the collection except among the altered rocks. Most of the lathes show a single median twin-line, and when broader they display twin-lamellæ. The angles of extinction indicate acid and basic andesine.

Three out of the eight species distinguished by the length of the felspar-lathes occur in my collection, that with the longest lathes (·3-·5 mm.) being not represented.

2. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is granular. Opaque plagioclase phenocrysts.

DESCRIPTION.—This genus may be divided into two groups according to the size of the plagioclase phenocrysts, the first “porphyritic,” where they average 3 mm. and over, the second “non-porphyritic,” where they are smaller than 3 mm., usually not over 2 mm. The former would include some of the “porphyrites,” and to this only one of the rocks sliced is to be referred. All the rest belong to the non-porphyritic type; and several of them are rocks that have undergone the propylitic change, as indicated by the formation of pyrites, chlorite, calcite, and other alteration-products.

(_a_) PORPHYRITIC SUB-GENUS.—A greyish rock, with sp. gr. 2·78, showing abundant porphyritic opaque plagioclase (4 to 7 mm.), from the vicinity of Satulaki. These phenocrysts are often aggregates of lesser crystals, or they may be extensively cross-macled. They are traversed by numerous fine cracks and show much dust-like included material. They are in part corroded by the magma and give evidence of fracture in their present position, the re-union being sometimes effected by the growth of new substance. Their lamellar extinctions (10° to 20°) are those of andesine. The groundmass displays a plexus of stout felspar-lathes, averaging ·1 mm. long, with the meshes occupied by coarse augite granules, ·03 to ·05 mm., with little, if any, interstitial glass. The felspars are often lamellar and give extinctions like those of the phenocrysts.

(_b_) NON-PORPHYRITIC SUB-GENUS.—Reference will first be made to some of the propylitic rocks of the dykes of the Ndriti Basin which belong to this group (see p. 70). They are greenish or greyish, with sp. gr. 2·76 to 2·8, and often sparkle with pyrites and contain secondary calcite, sometimes to such an extent that they might be taken at first sight for impure limestones.

The small opaque plagioclase phenocrysts (under 2 mm.), that they contain, are more evident in the slide than in the hand-specimen, and scarcely give a macroscopic character to the rock. They give extinctions (10° to 30°) ranging from those of acid andesine to acid labradorite, and are traversed by numerous cracks occupied by calcitic and other alteration products. The few pyroxene phenocrysts that once existed are now entirely represented by chloritic pseudomorphs. The groundmass displays a doleritic texture, exhibiting a plexus of long felspar-lathes, ·2 to ·4 mm. in average length, which often present a false resemblance to a flow-arrangement from their aggregation into bundles. They are often clouded by secondary products, but occasionally give lamellar extinctions (20° to 30°) indicating andesine labradorite. The rest of the groundmass is greatly altered, the granular augite and the interstitial glass, which originally existed in fair amount, being replaced by calcite, chlorite, pyrites, and occasionally epidote, so that the rock mass appears largely impregnated with alteration products. In addition there is much secondary magnetite, and in some cases there are a few minute cavities filled with chalcedonic silica and zeolites.

Reference may here be made to a singular rock from Ruku-ruku Bay, which resembles the Ndriti rocks in its propylitic alteration, but the felspar-lathes of the groundmass, ·21 mm. in length, give the small extinctions of oligoclase. Spec. grav. 2·61.

Most of the prevailing rocks of Mount Freeland belong to this sub-genus. They are dark grey and show small opaque plagioclase phenocrysts 1 or 2 mm. in size. They usually, however, are more or less altered, the change being often of the propylitic type, calcite, chloritic material, viridite, and occasionally pyrites occurring as alteration products. The specific gravity of the altered rocks is 2·61-2·69; that of the least affected is about 2·76. They all, however, belong to the same genus, displaying small phenocrysts of plagioclase and augite in a groundmass composed of minute stoutish felspar-lathes (·03-·06 mm.), augite granules, magnetite, and a little residual glass. The plagioclase phenocrysts owe their opacity partly to the numerous fine cracks traversing them and partly to the alteration products. The pyroxene phenocrysts, which are mostly of pale yellow augite, display at times intergrowths of rhombic pyroxene.

4. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is granular. There are no plagioclase phenocrysts, and those of augite when present are small (under 2 mm.).

DESCRIPTION.—Two groups of these rocks occur in my collection. In the one there are vesicular and scoriaceous rocks forming dykes near Nukunase and near the village of Ndriti. They display a plexus of felspar-lathes with abundant smoky more or less devitrified glass, the augite granules not being always differentiated. The felspar-lathes vary from ·1 to ·2 mm. in average length, and when lamellar give extinctions of basic andesine (25°). There are no pyroxene phenocrysts, and the augite granules when present average ·02 mm. in size.

In the other group are included some propylites from the dykes of the Ndriti Basin. They are greyish or greenish rocks, have a sp. gr. of 2·72 to 2·76, sparkle often with pyrites, and contain so much secondary calcite that they effervesce freely with an acid. Except in the rarity or absence of plagioclase phenocrysts, they come near to the propylitic rocks described under genus 2. They usually display a doleritic groundmass exhibiting long felspar-lathes, ·2 to ·33 mm. in length, which present the same pseudo-flow arrangement from their being gathered into bundles. The alteration corresponds precisely to that previously described, chlorite, epidote, pyrites, &c., occurring in quantity as secondary products.

II PRISMATIC SUB-ORDER OF THE AUGITE-ANDESITES WHERE THE FELSPAR-LATHES ARE NOT IN FLOW-ARRANGEMENT

FORMULA.—_Aug, matr, non-flu, prism._

The augite-andesites, which display in the groundmass a plexus of felspar-lathes and much prismatic pyroxene, are not frequent in my collection. About half of the specimens belong to agglomerates, whilst the rest are of the massive type, none apparently being obtained from dykes. They admit of the same classification as that generally adopted for the “granular” sub-orders; and it must be not forgotten that granular pyroxene also occurs but is not predominant.

5. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is for the most part prismatic. Plagioclase phenocrysts glassy.

DESCRIPTION.—Except as regards the prismatic pyroxene these rocks do not differ much from the “granular” augite-andesites. Those before me show phenocrysts of plagioclase and sometimes of augite in a groundmass displaying a mesh-work of felspar-lathes, prismatic pyroxene, and much interstitial glass.... The plagioclase phenocrysts, 1 to 3 mm. in size, show abundant magma-inclusions arranged either zone-wise or parallel to the twinning-planes. They are often eroded.... The phenocrysts of augite, which give extinctions of over 30°, are often rounded and display glass and other inclusions.... The prismatic pyroxenes of the groundmass vary in average length from ·03 to ·08 mm. They have the peculiar pale muddy brown hue characteristic of the prismatic augite in these rocks, and give oblique extinctions up to 30° and over. They may be short and broad or long and slender, and when there is much glass in the rock they resemble the felspar-lathes in their forked and imperfect extremities. Granular pyroxene occurs, but is subordinate.... The felspar-lathes, ·1 mm. long, are rarely lamellar, and give extinctions measured from their long axis of 20° (basic andesine).

6. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes are not in flow-arrangement and the augite is for the most part prismatic. The plagioclase phenocrysts are opaque.

DESCRIPTION.—Light and dark grey rocks displaying abundant opaque plagioclase phenocrysts not exceeding 2·5 mm. They are somewhat altered, one of the specimens having a sp. gr. of 2·68.

In the section they exhibit phenocrysts of plagioclase, and occasionally of augite in a groundmass of felspar-lathes, pyroxene prisms and granules (the former predominating), with a fair amount of altered interstitial glass.... The plagioclase phenocrysts owe their opacity to the great number of fine and sometimes parallel cracks filled with alteration products, that traverse them. Although much of their original material has often disappeared, they still display the lamellar twinning of medium andesine (15° to 20°).... The phenocrysts of pale yellowish augite, which give the large extinctions of that mineral, exhibit but little alteration, although lying in the same slide with those of the plagioclase.... The pyroxene prisms of the groundmass are of the same yellowish augite. They are broad with rounded extremities and are arranged in a loose plexus.... The felspars of the groundmass, which average ·1 mm. in length and give extinctions of medium andesine, are either lathe-shaped or short and broad when they display lamellæ. The last-named approach the orthophyric type, and such rocks come near the porphyrites; but I do not feel justified in placing them in a separate orthophyric order.

III OPHITIC SUB-ORDER OF THE AUGITE-ANDESITES WITH THE FELSPARS OF THE GROUNDMASS NOT IN FLOW-ARRANGEMENT.

FORMULA.—_Aug, matr, non-flu, oph._

These rocks form generally ancient flows. They are for the most part semi-ophitic, large ophitic “plates” being uncommon.

9. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—The felspar-lathes and prisms of the groundmass are not in flow-arrangement. The augite of the groundmass is ophitic or semi-ophitic. Glassy plagioclase phenocrysts.

This genus may be divided into two sub-genera,

(_a_) Porphyritic, where the average size of the plagioclase phenocrysts is 3 mm. and over.

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

A. PORPHYRITIC SUB-GENUS

DESCRIPTION.—Coarse-looking brownish-black porphyritic rocks displaying large plagioclase crystals that often show a play of colours. Their sp. gr. is about 2·8. None of the rocks in my collection are vesicular. On account of the considerable porphyritic development of the plagioclase, the groundmass is relatively diminished, the large phenocrysts occupying about a third of the mass. They form ancient basaltic flows more especially in the vicinity of the isolated hills and mountains of acid andesite, as around Vatu Kaisia; whilst they may enter into the formation of the low basaltic plains as in the region west and south of the Ndreketi River. They are, however, limited in their extent and occurrence. From the large amount of glass in the groundmass, they may be inferred to belong to flows formed under different conditions from those under which the great basaltic plateaux were formed, where the rock contains but scanty interstitial glass.

In the slide they show the large plagioclase phenocrysts together with a few small plates of ophitic augite in a groundmass displaying in an abundant smoky glass a loose plexus of long stout lathe-like plagioclase prisms partly wrapped around by lesser augites.... The plagioclase phenocrysts, which attain a size of 4 to 6 mm., give lamellar extinctions of basic andesine (20°-27°) and show concentric zone-lines with transmitted light. They often polarise in brilliant colours and are extensively cross-macled. They contain usually abundant inclusions of the magma sometimes arranged zone-wise, and are frequently eroded.... Non-ophitic pyroxene phenocrysts are uncommon. In the slide occur one or two small “plates,” 1 to 2 mm. in size, of ophitic pale-brown augite, and a number of lesser augites, ·2 to ·3 mm. in size, which in part wrap around the felspar-lathes and by their aggregation form imperfect ophitic “plates.”... The long stout felspar-lathes, which are on the average ·3 to ·45 mm. in length, give lamellar extinctions of 15° to 20° (medium andesine).... The copious smoky glass is rendered partially opaque by the abundant development of rods and skeletal crystals of magnetite, and shows the fibrous devitrification arising from the formation of incipient microliths. In some rocks there appear in the smoky glass brownish-yellow patches of the residual magma which under the microscope cannot be distinguished from palagonite.

All but one of the specimens belong to the species where the felspar-lathes average over ·3 mm. in length.

B. NON-PORPHYRITIC SUB-GENUS

DESCRIPTION.—Blackish-brown semi-ophitic rocks, sp. gr. 2·74-2·77, frequently of doleritic texture and showing a few small macroscopic plagioclase phenocrysts. They are sometimes vesicular, and form old flows in a few localities, as in the vicinity of Natua in the eastern part of the Ndreketi plains and in the coast district between Lekutu and Wailea Bay.

They display in the slide small plagioclase phenocrysts, often abundant, in a groundmass exhibiting a loose plexus of large lathe-shaped felspar prisms, together with occasional small ophitic “plates” of augite and numerous smaller semi-ophitic augites, whilst there is much interstitial smoky glass.... The plagioclase phenocrysts are as a rule 1 to 2 mm. in size and do not exceed 3 mm. They often contain abundant inclusions of the magma sometimes arranged schiller-fashion, and are frequently eroded. Their lamellar extinctions (15°-30°)indicate andesine labradorite.... The stout plagioclase lathes, which in most of my specimens range between ·2 and ·3 mm., and contain at times magma-inclusions, give the rocks their doleritic texture.... The occasional small ophitic “plates” of pale augite are not over 1 mm. in size and give extinctions of +30° from the single cleavage-lines. The lesser augites, ·2 mm. in size, are several times larger than typical granular pyroxenes (·02-·03 mm.), and adapt their form to the interspaces of the felspar-lathes which they in part invest.... The copious interstitial glass is generally smoky and sometimes quite opaque through the deposition of magnetite. It is never clear and isotropic, but displays fibrous devitrification and is usually a little altered.

These rocks come near to those of the previous sub-genus in several respects, but they differ conspicuously in their non-porphyritic character, in being sometimes vesicular, and in their general appearance. All the four species indicated by the length of the felspar-lathes are here represented; but the doleritic types with the lathes exceeding ·2 mm. are the most frequent.

10. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the augite is ophitic or semi-ophitic. The plagioclase phenocrysts are opaque.

This genus may be divided into two sub-genera, porphyritic and non-porphyritic, according to the average size of the plagioclase phenocrysts, whether above or below 3 mm.

A. PORPHYRITIC SUB-GENUS.—This is represented by a light grey porphyritic rock, with sp. gr. 2·75, from the lower part of Mount Freeland. It comes near to the porphyrites, and displays large opaque white phenocrysts of plagioclase 5 or 6 mm. long. It is a somewhat altered rock.

In the thin section it displays the plagioclase phenocrysts in a ground mass of doleritic and semi-ophitic texture showing a plexus of long felspar-lathes partly invested by small augites with a fair amount of altered greenish opaque interstitial glass.... The plagioclase phenocrysts, which give extinctions of 11° to 15° (acid andesine), are traversed by a network of fine cracks and contain a quantity of colourless dust-like inclusions and alteration-products. They are long and rectilinear in outline and are not much affected by the magma.... The felspar-lathes, which average ·26 mm. in length, are occasionally lamellar when the angle of extinction indicates acid andesine. Like the phenocrysts they contain many dust-like inclusions.... The augite may at times form an aggregate phenocryst of small size; but usually it occurs as semi-ophitic masses ·1 mm. in diameter.

B. NON-PORPHYRITIC SUB-GENUS.—The specimen representing this group is a coarse-grained greyish altered rock, sp. gr. 2·81, found in blocks near Waikatakata (p. 204), showing small somewhat opaque plagioclase phenocrysts. In the section these phenocrysts are displayed in numbers together with a few of augite. The groundmass, doleritic in texture in this species, displays a plexus of long stout felspar-lathes with numerous semi-ophitic lesser augites, chloritic pseudomorphs after pyroxene, and scanty interstitial altered glass.... The plagioclase phenocrysts, 2 to 3 mm. in size, give extinctions (22° to 28°) of basic andesine. They are traversed by many fine cracks and contain an abundance of colourless dust-like inclusions, apparently altered magma products.... The small pyroxene phenocrysts consist of aggregates of smaller crystals of pale augite. The lesser augites, ·1 mm. in size, partly invest the felspars.... The broad felspar-lathes, which average ·25 mm. in length and give lamellar extinctions of medium andesine (15°), contain inclusions similar to those of the phenocrysts.... The scanty interstitial glass is converted into viriditic and chloritic materials. Secondary calcite also occurs here and in the chloritic pseudomorphs.

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

12. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar lathes and prisms are not in flow-arrangement and the augite is ophitic or semi-ophitic. No plagioclase phenocrysts. Augite phenocrysts when present less than 2 mm. in size.

DESCRIPTION.—These rocks come near to the non-porphyritic group of genus 9; but differ in the absence or rarity of plagioclase phenocrysts, in their more frequently vesicular and scoriaceous character, and in the fresher condition of the rock. Their sp. gr. is about 2·77. They present themselves usually as blackish-brown doleritic rocks and form ancient flows in the coast-plains, sometimes exhibiting a columnar structure as in the crossing of the Ndreketi above Mbatiri. They are, however, of limited occurrence and are mostly represented in the Ndreketi plains and in the district between the Lekutu River and Wailea Bay.

Typically they display in thin sections no phenocrysts either of plagioclase or of pyroxene, and exhibit a plexus of usually long stout felspar-lathes partly invested by the lesser augites in a copious smoky glass.... The felspar-lathes, ·25 to ·4 mm. in length, give lamellar extinctions of 10° to 20° (andesine) and contain a few magma-inclusions.... The semi-ophitic augites, ·1 to ·2 mm. in size, are sometimes twinned.... The smoky glass polarises feebly and displays dark feathery aggregates of microliths. Within it are brownish-yellow semi-opaque patches of residual glass, which polarise faintly and behave like palagonite.

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

4. GRANULAR SUB-ORDER OF THE AUGITE-ANDESITES

FORMULA.—_Aug, matr, flu, gran._

13. GENUS

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

CHARACTERS.—In the groundmass the felspar-lathes or prisms are in flow-arrangement and the augite is granular. The plagioclase phenocrysts are glassy.

DESCRIPTION.—This genus readily splits up into two sub-genera, the non-porphyritic, where the plagioclase phenocrysts are less than 3 mm. in size, and the porphyritic where they are larger.

1. NON-PORPHYRITIC SUB-GENUS.—Dark-brown or blackish rocks displaying small plagioclase phenocrysts, usually only 1 or 2 mm. in size. Three of the four species defined by the length of the felspar-lathes are represented in my collection.

SPECIES A.—Felspar-lathes .02-.1 mm. in average length. This may again be sub-divided according to the degree of basicity of the rocks:—

(_a_) _Sub-species of greater basicity._—Sp. gr. 2·76 to 2·82.... Such rocks are represented in dykes and in the prevailing basic agglomerates. They are at times scoriaceous. The small plagioclase phenocrysts, which are fairly numerous, give lamellar extinctions of andesine labradorite (20 to 30°). Two kinds occur which may or may not be represented in the same slide. In the one the crystal is much corroded and contains abundant magma-inclusions. It belongs in such a case to an earlier period. In the other the outlines are clean and regular, and the crystal is often cross-macled to such an extent that it may be inferred from its unbroken condition to have been formed _in situ_. Augite phenocrysts when present are small and scanty, pale-yellow, idiomorphic, and giving extinctions of +30°. The felspar-lathes, which average ·06-·08 mm. in length, give extinctions indicating andesine labradorite. The augite granules are small (·01-·02 mm.). Interstitial glass, generally scanty, is sometimes abundant when it is smoky, showing fibrous devitrification, with irregular “lacunæ” filled with a brownish yellow opaque glass like palagonite.

(_b_) _Sub-species of lesser basicity._—Sp. gr. 2·65-2·70.... The remarks on the plagioclase phenocrysts of the more basic sub-species here apply, except that the lamellar extinctions indicate medium andesine (12°-20°). The characters of the augite phenocrysts and granules are in both groups the same; but in this case there is more frequently a suspicion of intergrowth with rhombic pyroxene. The felspar-lathes are very small, ·04 or ·05 mm, and give simple extinctions of acid andesine (5-10°). Interstitial glass exists in moderate amount.

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

Blackish or dark-grey rather compact rocks, sp. gr. 2·75-2·79, that cannot be readily divided into groups according to their basicity. They form dykes and volcanic “necks” and are sometimes scoriaceous. The small plagioclase phenocrysts, which are most evident in the slide, present the two kinds above described under Species A. They give lamellar extinctions varying from those of medium andesine to acid labradorite (15-30°). The augite phenocrysts, which are small and scanty, occasionally show intergrowths of rhombic pyroxene. The augite granules are generally ·02 to ·03 mm. in size, and here and there a prism form gives extinctions of +25°. The felspar-lathes which average ·11 to ·15 mm. long, are often rather stout, showing a few lamellæ that give extinctions of medium and basic andesine. Interstitial glass occurs in fair amount.

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

Blackish rocks with sp. gr. 2·75-2·84. The description of Species B applies here. The plagioclase phenocrysts are for the most part microporphyritic. The size of the augite granules is as above given.

2. PORPHYRITIC SUB-GENUS.—This group of rocks is mostly confined to the slopes and vicinity of Mount Seatura in the western part of the island, being prevalent in the Mbua and Ndama plains, and occurring also as dyke-rocks in the Nandi Gorge leading into the Ndriti Basin, and at and near the coasts of Wainunu Bay between the Tongalevu and Wainunu rivers. They come near in appearance to the porphyritic forms of the blackish olivine-basalts belonging to genera 13, 25, and 37 of the olivine rocks; but they differ in the absence of that mineral, in their lower density, and in other characters. They are the type to which the term “porphyritic basaltic andesite” is most frequently applied in the text when the ophitic structure is not displayed.

They are blackish rocks having a specific gravity of 2·71 to 2·81 and exhibiting large porphyritic crystals of plagioclase, but they vary in their minute structure on account of the different size of the felspars of the groundmass. Those forming dykes in the Nandi Gorge are often more or less propylitic in character. The felspar-lathes, which have an average length of ·2 to ·3 mm., sometimes show a few lamellæ giving extinctions of medium andesine (12°-20°). The plagioclase phenocrysts of the same andesine are 3 to 5 mm. in size. They are eroded and contain abundant magma inclusions. There are a few small phenocrysts of pale brown augite. The augite granules are ·03 or ·04 mm. in diameter, and there is a little dark opaque residual glass.

The rocks of the Mbua and Ndama plains have a specific gravity of 2·81. The plagioclase phenocrysts, which yield extinctions of basic andesine (21-27°), are sometimes a centimetre in length. They are traversed by cracks filled with dark altered glass or occupied by brownish films. The felspar-lathes, which average ·11 mm. in length, are often stout and lamellar and give extinctions like the phenocrysts. Augite phenocrysts are either absent or scanty; whilst the granules average ·02-·03 mm. in size. There is usually a little interstitial glass.

A rock, almost holocrystalline and 2·74 in density, which was obtained from the Tongalevu district in Wainunu Bay, approaches the orthophyric type in the character of the ground mass. The felspars are short (·06 mm.) and stout, and yield lamellar extinctions of oligoclase (5°-10°). The plagioclase phenocrysts are of basic andesine. Amongst the granules (·025 mm.) of pale brown augite occur prismatic forms giving oblique extinctions of +30°.

14. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes are in flow-arrangement and the augite is granular. The plagioclase phenocrysts are opaque.

DESCRIPTION.—Only two rocks are referred to this genus. One which is dark grey with a specific gravity of 2·72 is exposed in the gorge of the Mbutu-mbutu River below the falls of Na Savu. Flow-arrangement is displayed both by the felspar phenocrysts and lathes. The phenocrysts, 2 to 3 mm. in size, owe their opacity to the abundance of inclusions of brown glass. They are corroded and give extinctions of acid labradorite (26-32°). Pyroxene phenocrysts are scanty and small. The groundmass has a characteristic “pilotaxitic” appearance, the densely packed felspar-lathes averaging only ·05 mm. in length, whilst the pyroxene granules are ·01 mm. in size. Residual glass scanty.

The other rock is from the range behind Sueni. It shows large porphyritic crystals (5 or 6 mm.) of medium andesine which contain magma-inclusions in abundance. The average length of the felspar-lathe is ·08 mm. and the size of the augite granules is ·02 mm. There is but little glass. The rock is somewhat altered.

16. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes are in flow-arrangement and the augite is granular. Plagioclase phenocrysts are absent, or if present very scanty and not usually over 1 mm. in size. When present the augite phenocrysts are under 2 mm.

DESCRIPTION.—This is a very extensive genus, admitting considerable variation and including most of the aphanitic augite-andesites, where the felspar-lathes are as a rule very small (under ·1 mm. in length), as well as some of the doleritic types where they are very large (·2 to ·4 mm. long). In assigning a rock a place in this genus some regard must be paid to its macroscopic aspect as well as to the presence or absence of plagioclase phenocrysts. In many cases two or three small phenocrysts may be observed in a slide, under a millimetre in size; but they do not give a character to the naked-eye appearance of the rock, and such rocks cannot be distinguished from others that do not display them.

These rocks range in specific gravity from 2·55 to 2·85. This large range is in the main concerned with different degrees of basicity depending on the character of the plagioclase, the relative abundance of the augite granules, &c.; but it is also connected with the amount of interstitial glass. The variety of plagioclase ranges between oligoclase and andesine labradorite. The fluidal structure is nearly always well-marked, and the closely packed felspar-lathes have often the peculiar “felted” appearance of many andesites. A little interstitial glass is present in most rocks.

Many, perhaps nearly all, of the rocks belong either to dykes or to larger intrusive masses. All the four species indicated by the length of the felspar-lathes are represented in my collection, especially the two with smallest felspars. They may again be split up into two sub-species according to the degree of basicity of the rocks.

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

(1) _Most basic sub-species_.... Sp. gr. 2·75-2·85. Dark-brown and dark-grey compact aphanitic rocks showing no plagioclase phenocrysts to the eye. When a few of these phenocrysts are present in a slide they are not usually much over 1 mm. in size, and give extinctions of andesine labradorite (20° to 30°). Augite phenocrysts are often absent, and when present are not over 1 mm. in size and are as a rule scanty, occasionally affording a suspicion of inter-growths with rhombic pyroxene. The felspar-lathes which display marked flow-structure, vary in average length in different rocks from ·05-·08 mm. Lamellar twinning is rare, the extinctions being those of oligoclase andesine (10° to 20°). The usual extinction, as measured from the long axis of the lathe, is 10° to 15° (medium andesine). The augite granules are small (·01-·02 mm.) and abundant. There is generally a little interstitial glass with small magnetite.

(2) _Least basic sub-species_.... Sp. gr. 2·55-2·75. Dark compact aphanitic rocks especially characteristic of the Ndrawa district. When plagioclase phenocrysts are present, they are very scanty and not generally over a millimetre in size, possessing rectangular clean outlines and showing but few inclusions. They may display carlsbad twinning and zoning, or albite twinning, when they give extinctions of oligoclase andesine (10°-15°). Pyroxene phenocrysts are either absent, or scanty and small, being usually of pale yellow augite with occasional indications of intergrowth with rhombic pyroxene. The felspar-lathes as a rule average ·07 or ·08 mm. and present a dense fluidal arrangement. They rarely display lamellar twinning and give extinctions measured from the long axis of oligoclase and oligoclase andesine (2°-8°). The pyroxene granules are very small, averaging ·01 mm. and less. There is also fine magnetite. A little interstitial glass is usually present. When abundant it is not generally smoky but shows clear fibrous devitrification.... One of the specimens, which is semi-vitreous, exhibits tube-like steam-pores drawn out to a length of 1-1½ centimetres. The felspar microliths are only ·02 mm. in length. The copious glass has the character above described.

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

This species may also be sub-divided into two sub-species (more basic and less basic). Since, however, all but one of the fifteen rocks belonging to the species are of the more basic kind my remarks will mainly apply to them. They are dark-brown or dark-grey compact aphanitic rocks, occasionally banded or streaky, in appearance, and ranging in specific gravity from 2·75 to 2·84. They occur in several districts, but are especially characteristic of the Ndrawa district. The plagioclase phenocrysts, if present, are very scanty and small (1 or 1½ mm.). They contain inclusions of the magma and give lamellar extinctions of andesine labradorite (20°-30°). Pyroxene phenocrysts do not generally occur. When present, they are small and of pale yellow augite yielding large extinctions. Occasionally micro-porphyritic augite is well represented. The felspar-lathes, which exhibit a well-marked flow-arrangement, are generally ·13 to ·15 mm. long. Lamellar twinning is uncommon, the extinctions measured from the long axis indicating basic andesine (10°-20°). The augite granules are abundant and small (·01-·02 mm.). Occasional prism-forms yield large extinctions. Magnetite is abundant, its grains corresponding in size to the augite granules. There is as a rule a little residual glass, which shows fibrous devitrification and is not smoky. The banded appearance of some of the rocks arises from the glass collecting in streaks rudely parallel and running in the direction of the “flow” of the felspar-lathes.

The only specimen in my collection of “sliced” rocks belonging to the less basic sub-species is an altered bluish-grey rock (sp. gr. 2·7) from the range between the Mbuthai-sau valley and the Wainikoro plains. Its long parallel untwinned felspar-lathes give the nearly straight extinctions of oligoclase. Fine cracks in the rock are filled with crystalline silica.

SPECIES C, felspar-lathes ·2-·3 mm. long. SPECIES D, felspar-lathes ·3-·5 mm. long.

The rocks of these species in the collection are for the most part dyke-rocks of the more basic kind. They are blackish or dark-brown, almost doleritic in texture, and range in specific gravity from 2·77 to 2·87. At times they are vesicular or scoriaceous, as in the specimens from an agglomerate at Undu Point and from a flow or dyke at Vunikondi. The most typical of these rocks are those of some of the dykes of the Ndriti basin, which, however, display propylitic alteration in a varying degree. They would be described as semi-doleritic basalts without olivine or as non-porphyritic basaltic andesites. Plagioclase phenocrysts are typically absent, or they are scanty and not over 1 mm. in size. Augite phenocrysts are usually scanty and small. The felspar-lathes, which are more or less in flow-arrangement, are rather stout, and range in average length in different rocks from ·23 to ·35 mm. They often show a few twin-lamellæ which yield extinctions of medium and basic andesine (15-28°). The augite granules are large (·03 mm.) in the Ndriti rocks. Magnetites, usually corresponding in size to the augite granules, are abundant. Interstitial glass occurs often in fair quantity and is dark and semi-opaque.

At times there can be recognised a later generation of minute felspar microliths between the much larger lathes. They display a plexus rather than a flow-arrangement. Whilst the larger parallel lathes of the Vunikondi rock, above referred to, average ·23 mm. long, the felspar microliths of the interspaces average only ·03 mm. The significance of these two crops of felspars in the groundmass is discussed on page 237.

The only rock of the less basic sub-species in my collection is from a dyke near Vatua-karoa. It shows secondary calcite and viridite and other evidences of the propylitic change. The felspar-lathes, which average ·3 mm. in length, give extinctions of oligoclase (0-5°). The specific gravity is 2·72.

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

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

This sub-order includes dark-brown or blackish semi-vitreous rocks, all but one of which belong to the genus below described. Since the exception (which belongs to genus 17 of the synopsis) differs only in the presence of plagioclase phenocrysts, its separate description is not needed.

20. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes are in flow-arrangement, and the augite is prismatic. Plagioclase phenocrysts are absent or are very small and scanty, and pyroxene phenocrysts when present do not exceed 2 mm. in size.

DESCRIPTION.—These dark semi-vitreous rocks occur in agglomerates and as “necks” and dykes, and are at times scoriaceous. They are usually compact and aphanitic, showing few if any plagioclase phenocrysts and having a semi-conchoidal fracture. The specific gravity varies, being generally 2·6-2·65, but according to the degree of basicity and amount of glass it may be as low as 2·5 or as high as 2·77. In the less glassy condition, as in the case of a rock from the ridge east of Na Raro, the felspar-lathes are relatively scanty and the groundmass is mainly formed of augite prisms in flow-arrangement. The lathes are generally small, less than ·1 mm., and rarely over ·2 mm. Their extinctions are those of oligoclase and acid andesine. The pyroxene prisms, which give the large extinctions of augite, have a pale muddy-brown hue and are as a rule ·03-·07 mm. long. Granular pyroxene, if present, is subordinate in amount. The glass, which is always in good quantity and is sometimes abundant, displays fibrous devitrification. In a rock from the vicinity of Narengali a variolitic structure is exhibited in the form of sheaf-like aggregates of fibre-like felspars and skeleton prisms of pyroxene.

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

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

21. GENUS OF THE AUGITE-ANDESITES

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

CHARACTERS.—In the groundmass the felspar-lathes are in flow-arrangement and the augite is ophitic or semi-ophitic. Glassy plagioclase phenocrysts.

DESCRIPTION.—Dark rocks, sp. gr. 2·76-2·8, forming ancient flows and displaying at times a columnar structure as at Yanutha Point (page 123). The ophitic character is only in part developed, which may be connected with the flow-arrangement of the felspars. These rocks come near to the blackish ophitic basalts with scanty olivine (genus 33 of the olivine sub-class).

They all belong to the non-porphyritic sub-genus where the plagioclase phenocrysts are less than 3 mm. in size. These phenocrysts, which often contain abundant magma-inclusions, give extinctions of andesine labradorite (20°-30°). The augite phenocrysts are small and composite in character as often happens with these ophitic rocks. They sometimes invest the smallest felspar phenocrysts, and occasionally display intergrowths of rhombic pyroxene. The felspar lathes are ·1 to ·14 mm. in length, and give extinctions of medium and basic andesine. The augite granules are large (·02-·06 mm.), and tend to wrap around the lathes. Interstitial glass exists in fair amount.