The Plutonic Rocks.
These rocks are very infrequent and are for the most part hypersthene-gabbros or norites, with a few representatives of diorites[[110]] without pyroxene. True plutonic rocks did not come under my observation in the western half of the island (west of Lambasa and Savu-savu), those of Mount Thoka-singa in the Ndrandramea district making the nearest approach (see p. [302]). The localities in which they were found are below enumerated:—
| No. | Nature of rock. | Locality. | Mode of occurrence. | Page. |
|---|---|---|---|---|
| 1 | Hypersthene-gabbro | Avuka Range between Lambasa and Mbuthai-sau | Probably forms the axis of the range. | [180] |
| 2 | Ditto | Nawi, at the head of the Vui-na-savu River | Deep-seated. | [211] |
| 3 | Ditto | Valanga Range between Savu-savu and Natewa Bays | Probably forms the axis of the range. | [182] |
| 4 | Ditto | Ridge at the head of Na Kula valley between Savu-savu and Natewa Bays. | Ditto. | [184] |
| 5 | Hornblende-gabbro. | Ditto | Ditto. | [184] |
| 6 | Diorite | Vunimbua River, on south side of the Mariko Range | Loose blocks in river-bed. | [182] |
| 7 | Ditto | Coast cliffs west of the Salt Lake Passage | Large block in agglomerate-tuff. | [193] |
The Hypersthene-gabbros.—These rocks also contain monoclinic pyroxene, and are the plutonic equivalents of the hypersthene-augite-andesites which as a rule prevail in the localities where these rocks occur. They are usually dark grey or steel-grey in colour with a specific gravity ranging from 2·7 to 2·84 and have a granitoid aspect. The following characters are common to all the specimens.
They display a mixture of plagioclase and pyroxene, the last filling up the spaces between the felspars and apparently of later formation. The plagioclase crystals, which are 1 to 2 mm. in size, are opaque; and since they are traversed by numerous fine fissures filled with dust-like decomposition products, their appearance is often semi-saussuritic. They are much cross-macled, are at times zoned, and give lamellar extinctions of andesine-labradorite (20°-30°).... The pyroxene includes both the rhombic and monoclinic forms, the last with extinction angles of over 30°. They may be associated or may occur as separate crystals, the rhombic prevailing in the less basic and the monoclinic in the more basic rocks. The rhombic pyroxene is usually more or less converted into bastite which by further change passes into a chloritic material; whilst the augite sometimes undergoes the diallagic change resulting from schillerisation.
Some special features are presented by rocks from different localities. That from Nawi is most basic and looks like a diallage-gabbro. That from the Valanga Range (sp. gr. 2·75) contains some quartz, apparently secondary and filling up the interspaces. The rock from the Na Kula Ridge shows traces of a groundmass; but it comes near the plutonic type.
A Hornblende-gabbro.—This granitoid rock, which is from the Na Kula Ridge, has a specific gravity of 2·72. Hand-specimens display large porphyritic crystals of hornblende (7 mm. long) in a base of opaque felspar and smaller hornblende. In the slide we observe besides the large crystals of plagioclase and hornblende a little pyroxene; but the mass of the rock consists of greenish-brown hornblende, plagioclase, and some secondary quartz, forming a coarse mosaic with a “grain” of about a millimetre. The hornblende is displayed in regular hexagonal sections, markedly pleochroic, and gives extinctions up to 12°. It shows no dark resorption borders; and the larger porphyritic crystals have the same characters. Almost all the plagioclase of the rock is traversed by numerous fine fissures, and often acquires a semi-saussuritic appearance from the presence of dust-like decomposition products. The lamellar extinctions indicate andesine-labradorite. The quartz occurs mostly in nests. The pyroxene is formed of large grains of both the monoclinic and rhombic types.
The Diorites.—The rock forming blocks in the Vunimbua River has a specific gravity of 2·78 to 2·8. It is a pretty rock showing long black blades of hornblende, 10 mm. in length, in an opaque felspar base. In the slide the hornblende, which is dark brown and markedly pleochroic, shows six-sided sections with characteristic prismatic cleavage lines, the longitudinal sections giving extinctions up to 15°. The borders in some cases display traces of resorption. The felspar (plagioclase) is in the form usually of broad regular crystals, 3 to 4 mm. in size, and giving extinctions of andesine-labradorite (28°); they are “clouded” through the presence of fine alteration products associated with numerous fissures. The relation between the hornblende and the plagioclase is not constant. This appears to be partially due to the occurrence of traces of a groundmass.
The diorite forming blocks in the agglomerate of the coast cliffs, west of the Salt Lake Passage, is a remarkable rock showing large blackish hornblende crystals, in the shape of blades 25 mm. long and 3 or 4 mm. broad, set in a base of opaque plagioclase felspar which surrounds the hornblende. The last-named is deep-brown, very pleochroic, yields extinctions up to 22°, and displays but little evidence of resorption. The plagioclase is irregular in shape and exhibits broad lamellæ giving extinctions of acid labradorite (28°-30°). It is traversed by numerous fine fissures filled with decomposition products and contains abundant dust-like materials. (Spec. grav. 2·8).
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.