CHAPTER XVIII

THE VOLCANIC ROCKS OF VANUA LEVU (_continued_)

OLIVINE CLASS

SUB-CLASS II

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

THIS sub-class includes the plagioclase-olivine-basalts. Although these rocks are not the most numerous of the basic rocks, they are well represented in the island, being in great part confined to the western half, and being especially characteristic of the districts of Wainunu and Solevu and of the mountains of Seatura and Naivaka. It will be seen from the Synopsis that this sub-class is split up into two divisions, according to the relative abundance of the olivine. Many of the rocks are grey basalts with the olivine more or less hematised; but the majority are blackish with the olivine usually more or less serpentinised. In the typical blackish rocks there is a little dark opaque interstitial glass. In the grey basalts the groundmass is as a rule holo-crystalline. The specific gravity ranges generally from 2·8 to 3.

It will be noticed in the scheme that the “prismatic” sub-orders, where the pyroxene of the groundmass is for the most part in prisms, are scarcely represented. The “ophitic” sub-orders are poorly represented, since they only include about 10 per cent. of the total. The ophitic olivine-basalts are indeed mostly confined to the division where the olivine is scanty, and the felspars of the groundmass are for the most part not parallel, the plexus arrangement, as will be often pointed out, being almost essential for the development of the ophitic structure. With the basaltic andesites, which cannot always be sharply separated from the basalts with scanty olivine, the proportion of ophitic rocks is much higher, probably about 20 per cent. The pyroxene in the olivine-basalts is nearly always augite, intergrowths with rhombic pyroxene being only occasionally observed.

I. DIVISION OF THE OLIVINE-BASALTS

CHARACTERS.—Abundant olivine.

FORMULA.—_Oliv, matr, cop._

These rocks are characterised by abundant olivine usually as phenocrysts but sometimes represented in the groundmass. When a basalt presents much of this mineral in an ordinary hand-specimen and displays at least five or six phenocrysts in a slide, it is placed in this division. Olivine-basalts of this character are well exhibited in the hills around Solevu Bay and in the neighbouring Seatovo Range. They are also fairly represented on the northern slopes of Mount Seatura, on the coast between the Wainunu River and Nandi Bay, and on the Wainunu basaltic tableland. In the Ulu-i-ndali Range, which lies east of the Wainunu estuary, they are especially frequent. Whilst confined mostly to the portion of the island west of Savu-savu Bay, they occur sporadically in other localities to the eastward, as in Na Suva-suva Hill and in some parts of the Natewa Peninsula. The grey basalts, which form one-third of the total, are chiefly characteristic of the hill of Ulu-i-ndali, of the Solevu district, and of the northern slopes of Seatura. Whilst the blackish basalts usually compose the flows, the grey basalts form dykes and volcanic necks.

Two-thirds of these olivine-basalts belong to the order presenting flow-structure and almost all (28 out of 29) are included in the sub-order exhibiting granular augite in the groundmass. The ophitic structure is displayed in only one case; and the prismatic form of the augites is never a predominant feature.

1. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Abundant olivine. Felspar-lathes of the groundmass not in flow arrangement. Pyroxene of the groundmass granular. Phenocrysts of glassy plagioclase.

DESCRIPTION.—Dark-brown or blackish rocks. Sp. gr. 2·88 to 2·93. Phenocrysts of pyroxene occur in fair quantity in addition to those of the olivine and plagioclase. The groundmass displays a plexus of felspars and augite-granules with much magnetite in grains and irregular patches. The interstitial glass is scanty or almost absent. The olivine phenocrysts, of which the larger are 3 to 4 mm. in size, are as a rule hematised at the borders and in the fissures, and are sometimes partially serpentinised. In some cases small crystals of olivine are enclosed in the pyroxene-phenocrysts. The plagioclase phenocrysts do not usually exceed 2 mm. in size. They give lamellar extinctions of 15°-28°, and are often cross-macled. They generally contain magma-inclusions, which may be arranged in zones, and they sometimes inclose small pyroxene crystals. Their borders are often eroded. The pyroxene-phenocrysts, which frequently are 3 to 4 mm. in size, give extinctions of 30° and over, and may be described as composed of brown augite. It is only at times that intergrowths of rhombic pyroxene occur. They are often twinned and are sometimes eroded and may contain magma and other inclusions. The felspars of the groundmass, which for the most part form a plexus, are small and stout, their average length varying from ·08 to ·13 mm., whilst they frequently display lamellar twinning and give extinctions of about 15° (oligoclase-andesine). The pyroxene-granules of the groundmass, which are of brown augite, vary in average size from ·02 to ·04 mm.

Two of the four species, where the felspar-lathes are less than ·1 mm. and between ·1 and ·2 mm. in length, are represented in this collection.

2. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Abundant olivine. Felspar-lathes of the groundmass not in flow-arrangement. Pyroxene of the groundmass granular. Phenocrysts of plagioclase opaque white.

DESCRIPTION.—Grey compact-looking rocks; sp. gr. 2·83 to 2·9. Interstitial glass scanty. The olivine phenocrysts, which range up to 5 mm. in size, are more or less hematised; and in extreme cases of this alteration, where schiller-planes are formed, the hand-specimen appears to carry brown mica. There are sometimes small grains of olivine (·1 mm.) in the groundmass. The plagioclase-phenocrysts, varying in size from 2 to 4 mm., owe their opacity partly to their composite character, when they present an aggregate of smaller clear crystals, and partly to multiple macling. They give extinctions of acid labradorite (25°-32°). Pyroxene-phenocrysts, when present, are scanty, pale brown, not over 2 mm. in size, and give the large extinctions (+30°) of augite. In their absence small augites (·2 mm.) are frequent. Fine granules (·01-·03 mm.) of similar augite, together with magnetite, abound in the groundmass. The felspars of the groundmass are fairly stout and lathe-like and show at times a few twin-lamellæ which give extinctions of 13° to 20° (medium andesine).

Species represented:

(_a_) felspar lathes ·1-·2 mm. (_b_) felspar lathes ·2-·3 mm.

3. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Abundant olivine. Felspar-lathes of the groundmass not in flow-arrangement. Pyroxene of the groundmass granular. No plagioclase phenocrysts. Large phenocrysts of olivine and pyroxene over 2 mm. in size.

DESCRIPTION.—A remarkable blackish-grey rock. Although somewhat scoriaceous, it has a sp. gr. of 2·91. It displays large phenocrysts of olivine and pyroxene, 4 to 8 mm. in size, in a coarse-textured groundmass of stout felspars, augite granules, magnetite and a little glass. The olivine is extensively hematised. The pyroxene phenocrysts are of brownish-yellow augite with regular outlines and giving angles of extinction up to 40°. The broad lamellar felspars of the groundmass, which are on the average ·3 to ·4 mm. long, give extinctions indicating both acid labradorite (23° to 28°) and medium andesine (16° or 17°). The abundant augite-grains average ·05 mm. in size; but the prism form occurs at times.

The species with felspar-lathes ·3 to ·5 mm. long is alone represented.

4. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Abundant olivine. In the groundmass the felspar-lathes are not in flow-arrangement and the pyroxene is granular. No plagioclase phenocrysts. Small phenocrysts of olivine and occasionally pyroxene under 2 mm.

DESCRIPTION.—A grey coarse-grained rock. Sp. gr. 2·9. It displays abundant small phenocrysts of olivine, all less than a millimetre in size (·2-·8 mm.), which are hematised in the fissures and at the borders. The felspar-lathes, which display a few twin-lamellæ giving extinctions of 16°-18° (medium andesine), vary greatly in size. The smaller are ·1 to ·3 mm. and the larger ·3 to ·5 mm. long; but the two are connected by felspars of intermediate length. The abundant augite granules average ·037 mm. in breadth. Pyroxene phenocrysts are not represented in the slide. From its coarsely crystalline texture this rock merits the field-name of a grey doleritic basalt; and except in the arrangement of the felspar-lathes it does not differ from the grey doleritic basalts of genus 16.

12. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine abundant. Felspars of the groundmass not in flow-arrangement. Pyroxene of the groundmass ophitic or semi-ophitic. No plagioclase phenocrysts. Small phenocrysts (under 2 mm.) of olivine and occasionally a few of pyroxene.

DESCRIPTION.—A dark greenish-brown rock, with sp. gr. 2·91, showing abundant micro-porphyritic olivine in a groundmass consisting of ophitic pale-brown augite inclosing the felspar-lathes, together with small olivines, patches of magnetite, and a little altered interstitial glass. The olivine-phenocrysts are about ·5 mm. in average size and are more or less hematised. The felspar-lathes, which average ·15 mm. in length, often show twin-lamellæ that give extinctions of 23°-30° (andesine-labradorite).

The species with felspar-lathes ·1-·2 mm. long is alone represented.

13. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine abundant. Felspars of the groundmass in flow-arrangement. Pyroxene of the groundmass granular. Phenocrysts of glassy plagioclase.

DESCRIPTION.—Dark grey or dark brown rocks with sp. gr. 2·88 to 2·99. Phenocrysts of olivine, pyroxene, and plagioclase occur in a groundmass showing partially parallel felspar-lathes, abundant pyroxene grains, and fine magnetite, residual glass being scanty or absent. The olivine-phenocrysts do not exceed 3 or 4 mm. in size and in some rocks are less than 1 mm. They are usually more or less serpentinised and hematised at the borders and in the cracks; but sometimes they are almost fresh and present regular outlines. In some rocks the olivine also occurs as grains (·3 mm.) in the groundmass. When the phenocrysts of olivine have blackish borders they are surrounded by a halo, as though the crystal had attracted the magnetite from the groundmass immediately around. The plagioclase phenocrysts vary from 1 to 3 mm. in size. They often contain abundant magma-inclusions and give lamellar extinctions of 15° to 25° (basic andesine). In some rocks they are rudely parallel. The pyroxene phenocrysts, which are of pale brownish-yellow augite giving extinctions of over 30°, do not usually exceed 3 mm. They present regular octagonal cross-sections and sometimes display lamellar twinning. Occasionally there is a suspicion of intergrowth with rhombic pyroxene. The felspar-lathes, which according to the species vary much in length, at times show a few lamellæ. The augite grains of the groundmass are abundant and are as a rule about ·02 mm. in size; but in some rocks they are larger and in others smaller.

This genus may be divided into two sub-genera, the porphyritic sub-genus where the felspar phenocrysts are larger than 3 mm., and the non-porphyritic where they are smaller. All four species, as indicated by the length of the felspar-lathes, are represented.

14. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine abundant. Felspars of the groundmass in flow-arrangement. Pyroxene of the groundmass granular. Opaque plagioclase phenocrysts.

DESCRIPTION.—Dark grey rocks, with sp. gr. 2·9 to 2·93, showing phenocrysts of olivine and pyroxene with opaque whitish phenocrysts of plagioclase in a groundmass of felspar lathes, pyroxene grains, and magnetite, with occasional fine olivine. The olivine phenocrysts, which are sometimes 5 or 6 mm. in size, are often deeply eroded. They are at times so extensively hematised along the schiller-planes that they appear like brown mica. The plagioclase phenocrysts owe their opacity in part to their consisting of an aggregate of lesser crystals which are clear and glassy and give lamellar extinctions of 20° to 30° (andesine labradorite). They do not usually exceed 3 mm. and are sometimes scanty. The pyroxene phenocrysts, which are at times infrequent, may be 5 mm. in size. They are of pale yellowish-brown augite, giving extinctions of 40°. The felspar-lathes are rarely lamellar; but in one such case the angle of extinction was 17° (medium andesine). The grains of augite in the groundmass average ·02 mm. in diameter.

The only species represented is that with the felspar-lathes ·1 to ·2 mm. long.

15. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Abundant olivine. In the groundmass the felspars are in flow-arrangement and the pyroxene is granular. No plagioclase phenocrysts, but large phenocrysts, over 2 mm., of olivine and pyroxene.

DESCRIPTION.—This genus includes the most basic rocks represented in my collection. They are compact heavy blackish rocks with sp. gr. 3 to 3·1, and display large porphyritic crystals of olivine and pyroxene often 3 or 4 mm. in size. The olivine phenocrysts may be fairly fresh with clean outlines, or they may be deeply eroded and stained by iron oxide, or they may be passing into serpentine. The pyroxene phenocrysts may be either eroded or possess regular outlines. They are of pale brown augite and give extinctions over 30°. The pyroxene granules, which average ·02 mm. and are very abundant, are of the same augite. The felspar-lathes are relatively scanty. In the two rocks here included they average in length ·06 mm. and ·08 mm. When lamellæ can be recognised they give extinctions of 30°-40° (labradorite). The untwinned lathes give extinctions of 20°-28° (labradorite).

The only species represented is that with felspar-lathes less than ·1 mm. in length.

16. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Abundant olivine. In the groundmass the felspar-lathes are in flow arrangement and the pyroxene is granular. There are no plagioclase phenocrysts; but there are numerous small phenocrysts, under 2 mm. in size, of olivine and occasionally a few of pyroxene.

DESCRIPTION.—As a highly basic genus this ranks next to the preceding one, the specific gravity of the rocks ranging from 2·91 to 3·01. All the four species indicated by the varying length of the felspar-lathes are represented in my collection. The rocks of the first two, with the felspars averaging less than ·1 mm. and between ·1 and ·2 mm., are compact aphanitic basalts, only displaying an occasional small phenocryst of augite and blackish-grey or bluish-black in colour. Those of the last two species, with the average length of the felspar-lathes ·2-·3 mm. and ·3-·5 mm. respectively, are lightish-grey coarse-textured rocks of the doleritic type. In all the rocks no pyroxene phenocrysts are displayed in the slide; and the olivine phenocrysts, which are very numerous, do not usually exceed 1 mm., though occasionally the average size is 1·3 mm., and not infrequently it is only ·5 mm. In some cases where the larger olivine phenocrysts lie athwart the current of the felspar-lathes, the smaller (·5 mm.) lie with their long axes parallel to the flow. The olivine is either fresh, or it may be beginning to serpentinise in the cracks, or it may be in part hematised. The crystals may have regular outlines, or they may be rounded and sometimes deeply eroded. The pyroxene granules of the groundmass are of pale-brown augite, and average ·01 to ·03 mm. in size. Occasionally a few prism forms occur, giving extinctions of 30° to 40°. In the case of the more compact rocks, with the felspar-lathes averaging less than ·2 mm. in length, lamellar twinning is but scantily to be noticed in the lathes, which give extinctions measured from the long axis of 20° to 25° and by the twin lamellæ of 30° to 35°, indicative of acid labradorite in both instances. With the coarser doleritic grey basalts, where the felspar-lathes are stouter and have an average length exceeding ·2 mm., lamellar twinning is more frequent; the extinctions afforded by the lamellæ range between 15° and 25° (medium andesine). Residual glass is scanty in these rocks, and in the grey dolerites it is often difficult to recognise any.

2. DIVISION OF THE OLIVINE-BASALTS

CHARACTERS.—Scanty olivine.

FORMULA.—_Oliv, matr, pauc._

25. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine scanty. Felspars (lathes and prisms) of the groundmass not in flow-arrangement. Pyroxene of the groundmass granular. Glassy plagioclase phenocrysts.

DESCRIPTION.—About two-thirds of these rocks have a common facies, being closely similar in appearance, brownish-black in colour, and with spec. grav. usually between 2·84 and 2·92. They belong to the species with the felspar-lathes ·1 to ·2 mm. in length. They are essentially the rocks of the old submarine basaltic flows; and they are often columnar, the columns being 2 to 4 feet across. My remarks will mainly apply to this predominant group.

To the eye they are somewhat compact and show scattered porphyritic crystals of plagioclase. In the slide they display numerous phenocrysts of plagioclase, with a few of olivine and pyroxene, in a groundmass formed of stout lamellar felspar-lathes and small prisms forming a plexus with granular augite in the meshes. There is a good deal of magnetite and generally scanty residual glass. The plagioclase phenocrysts are usually 2 to 3 mm. in size, but they may be smaller (1 to 2 mm.) or larger (3 to 5 mm. or more) when the rock has a porphyritic appearance.[111] They are often cross-macled and at times show zoning. In many slides two kinds are distinguished by the extinctions which indicate in one case medium andesine (15° to 22°) and in the other acid labradorite (27° to 32°). They contain inclusions of the magma and are often eroded. The pyroxene phenocrysts are of pale brown augite, scanty and small, and give extinctions of 30° to 40°. They are sometimes twinned and may be eroded and contain inclusions of the magma. The olivine phenocrysts, which do not usually exceed 2 or 3 mm., are mostly rounded, but sometimes have the regular outlines, and are in various stages of serpentinisation. The felspars of the groundmass, which average ·17 mm. in length, are mostly stout and lamellar; but they exhibit all transitions from the lathe-shape with one or two lamellæ to broad multi-lamellar prisms where the breadth is half the length. They give lamellar extinctions averaging 15° to 18° (andesine). The augite granules of the groundmass are pale brown and average ·02 to ·03 mm. in size. In a few cases they are larger (·05 mm.) which is an indication of an approach to the ophitic type. In most slides occur a few small augites of prism-form, two or three times the size of the granules, which give extinctions of over 30°. Where the phenocrysts of augite are very scanty or absent, there exist large grains (·1 mm.) of an intermediate size. The magnetite is often abundant, occurring in crystals, rods, and irregular masses, the last associated often with the interstitial glass which is present in small quantities in most rocks, being greenish or brownish and showing fibrous devitrification.

In some localities semi-vitreous rocks referable to this genus are frequent. This is especially the case in the Naivaka peninsula, where the rocks show a fair amount of glass in the groundmass, the porphyritic augite being well developed, whilst the pyroxene of the groundmass is only in part differentiated. Three of the four species are here represented. Those with large felspar-lathes (·2-·3 mm. long) and coarse augite granules (·05) approach the semi-ophitic rocks included in genus 33.

26. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes and prisms are not in flow-arrangement and the pyroxene is granular. The plagioclase phenocrysts are opaque.

DESCRIPTION.—Dark grey porphyritic rocks, which, from the opacity of the felspar phenocrysts, look like porphyrites. They are not very frequent and occur mostly on the northern slopes of Mount Seatura. Two different types occur in my collection which may be regarded as sub-genera. In the most basic kind, where the sp. gr. is 2·86 to 2·89, the plagioclase phenocrysts, 2 to 3 mm. in size, owe their opacity chiefly to their aggregate structure. They give lamellar extinctions (15°-30°) of andesine labradorite. Porphyritic olivine is scanty and more or less hematised; but a fair amount of olivine grains, less than ·1 mm., occur in the groundmass. Pyroxene phenocrysts are scanty, but microporphyritic pale brown augite (·1 mm.) is frequent. In the groundmass are found stoutish felspar-lathes, averaging ·2 mm. long, together with an abundance of fine augite granules (·01-·02 mm.) and fine magnetite, the residual glass being scanty.... In the other type the sp. gr. is 2·75; and the plagioclase phenocrysts 4 to 6 mm. in size give extinctions of andesine and acid labradorite (10°-30°). There is an approach to the orthophyric structure in the groundmass, as is indicated by the number of short broad felspars, averaging ·2 mm. in length and giving lamellar extinctions of acid and basic andesine. The granular augite of the groundmass is coarse (·04 mm.), and occasional prism-forms give extinctions of 40°.

33. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes are not in flow-arrangement and the pyroxene is ophitic or semi-ophitic. Plagioclase phenocrysts glassy.

DESCRIPTION.—These brownish-black rocks are all of the semi-ophitic type. Although no ophitic “plates” occur in the slide, the augites of the groundmass have no longer the granular form, but are large, ·08-·1 mm. in size, and tend to invest the felspar-lathes. The specific gravity ranges from 2·78 to 2·86. As in other of the ophitic and semi-ophitic rocks of this collection (genera 9, 10, 12, of the augite-andesites), the large size of the felspar-lathes (·2-·3 mm. long) of the groundmass gives a doleritic texture in the slide. In most of the other characters these rocks approach those of genus 25 which possess felspar-lathes more than ·2 mm. in length. But they are more often semi-vitreous, and display a considerable amount of dark smoky glass showing numerous magnetite rods and skeletal crystals with fibrous devitrification. The plagioclase phenocrysts, which vary much in size in different rocks (in some 2 or 3 mm., in others 4 or 5 mm.), give extinctions of andesine labradorite (20°-35°). They are often eroded and contain numerous large black inclusions of the magma. The pyroxene phenocrysts, which are of pale-brown augite, often have an aggregate-structure, having been formed _in situ_. Others again consist of single crystals and have been much affected by the magma. The olivine phenocrysts, which are at times deeply eroded, are generally small and in part serpentinised.

37. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, flu, gran, phen, vitr._

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is granular. Glassy plagioclase phenocrysts.

DESCRIPTION.—Brownish-black rocks which cannot be distinguished, except in the flow-arrangement of the felspars of the groundmass, from those described under genus 25. Like them they enter into the formation of the basaltic plains of Sarawanga and Mbua and elsewhere. Most of the rocks of this genus group themselves into one type where the felspar-lathes average in length ·15-·21 mm. The sp. gr. is usually between 2·87 and 2·91. Though rarely porphyritic, such rocks display to the eye a few small scattered glassy phenocrysts of plagioclase and an occasional grain of olivine. It is to this type of the genus that the following description applies.

In the thin sections they display small plagioclase phenocrysts, with a few of olivine and occasionally of pyroxene, in a groundmass where the flow-arrangement of the felspar-lathes is well marked, the rest of the groundmass being made up of granular augite with magnetite and generally a little residual glass.... The plagioclase phenocrysts do not usually exceed 2 mm. in size and contain magma inclusions. Two kinds are often indicated in the same slide by the extinctions, namely, one of medium andesine (17°-22°), and the other of acid labradorite (28°-33°).... The pyroxene phenocrysts are of pale brown augite; but they are small (less than 2 mm.), scanty, and often absent when their place is taken by microporphyritic augite, ·2 mm. in size.... The olivine phenocrysts are generally small. Though sometimes showing the long hexagonal sections, they are often rounded and more or less serpentinised.... The felspars of the groundmass present more typical lathes than are to be observed in the non-parallel felspars of the rocks of genus 25. The twin-lamellæ, when present, are fewer; but give similar extinctions (15°-21°) of medium andesine.... The augite granules are, as a rule, ·02 or ·03 mm. in diameter; but occasional more prismatic forms occur, two or three times the length, which give extinctions of over 30°. The magnetite is abundant, and the scanty interstitial glass is green or brown and displays fibrous devitrification.

The following three species, as indicated by the length of the felspar-lathes, are represented in my collection:

(_a_) ·02-·1 mm. (_b_) ·1-·2 mm. (_c_) ·2-·3 mm.

38. GENUS OF THE OLIVINE-BASALTS

FORMULA.—_Oliv, matr, pauc, flu, gran, phen, opac._

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is granular. Opaque plagioclase phenocrysts.

DESCRIPTION.—Grey rocks, sp. gr. 2·78 to 2·83, showing small opaque porphyritic crystals of plagioclase with a few phenocrysts of olivine and pyroxene in a groundmass of parallel felspar-lathes, augite granules, and magnetite, with very scanty, if any, interstitial glass.... The plagioclase phenocrysts, 2 to 3 mm. in size, are often aggregates of smaller crystals. They contain colourless granular inclusions and are sometimes zoned, giving extinctions of medium andesite (15°-18°), and of andesine labradorite (25°-29°).... The pyroxene phenocrysts are pale-brown, scanty, 2 to 3 mm. in size, often twinned and give the large extinctions of augite.... The olivine phenocrysts, which do not exceed 2 or 3 mm., are deeply eroded by the magma and are hematised and schillerised. Small grains also occur in the groundmass.... The felspar-lathes, which in the species here represented average ·15 mm. in length, are stout and give lamellar extinctions of andesine (18°-22°).... The augite granules are pale-brown and usually ·02-·03 mm. in diameter.

The only species represented in my collection is that with the felspar-lathes ·1-·2 mm. long.

40. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is granular. No plagioclase phenocrysts; but there are a few small phenocrysts of olivine and sometimes of pyroxene under 2 mm. in size.

DESCRIPTION.—Compact-looking non-porphyritic blackish-brown rocks, sp. gr. about 2·9. Occasionally a little vesicular. For the most part dyke-rocks.

In the slide are displayed a few small phenocrysts of olivine and pyroxene in a groundmass formed of more or less parallel felspar-lathes, augite granules, magnetite, sometimes in rods, and a little greenish devitrified residual glass.... The pyroxene phenocrysts are of pale-brown augite and are generally less than a millimetre in size. They may be single crystals or they may be formed of an aggregate of a few smaller crystals.... The olivine phenocrysts rarely exceed 2 mm. in size and are in part serpentinised.... The augite granules vary usually from ·01 to ·03 mm. in diameter.... The felspar-lathes of the rocks in this collection are large, often exceeding ·2 mm. in length, giving the rock a doleritic texture in the slide. In a single slide they may range from ·1 to ·6 mm. When lamellar they give extinctions of 15° to 25° (basic andesine).

Two species are represented in this collection:--

(_a_) with felspar-lathes ·2-·3 mm. (_b_) with felspar-lathes ·3-·5 mm.

44. GENUS OF THE OLIVINE-BASALTS

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

CHARACTERS.—Olivine scanty. In the groundmass the felspar-lathes are in flow-arrangement and the pyroxene is in great part prismatic. There are no phenocrysts of plagioclase; but there are a few small phenocrysts of olivine and sometimes of pyroxene, less than 2 mm. in size.

DESCRIPTION.—A dark grey compactish rock; sp. gr. 2·9; showing a little macroscopic olivine; forming a dyke in the tuffs on the summit of the hill of Vatui (p. 54).

In the section it exhibits a few phenocrysts of olivine (more or less serpentinised) and of augite in a groundmass formed of stout augite prisms and small augite granules with felspar-lathes, magnetite, and a little devitrified yellowish interstitial glass. The augite prisms and the felspar-lathes are in flow-arrangement.... The pyroxene phenocrysts, which are pale brown and give extinctions of over 30° from the single cleavage-lines, may be aggregates of five or six smaller crystals or single crystals presenting sometimes lamellar twinning. The first are doubtless formed _in situ_. The second though showing regular outlines may have a nucleus giving a different extinction and possessing eroded margins. The stout augite prisms of the groundmass, which are occasionally twinned, have an average length of ·2 to ·3 mm. and give angles of extinction with the long axis of 30° to 40°. The felspar-lathes average only ·07 mm. in length and afford extinctions, when untwinned, of 18° to 24° (acid labradorite).

The only species represented is that where the average length of the felspar-lathes is between ·02 and ·1 mm.