Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1 / Vanua Levu, Fiji

by H. B. Guppy

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

Sub-Class Quartz-Hornblende-Hypersthene-Andesites or Dacites

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

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

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

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

Tabular Comparison of the Acid Andesites of Vanua Levu.

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

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

Locality.			Sp. Gr.
Ngaingai, summit			2·57
Ngaingai, lower part			2·57
Wawa-levu, 1,500 feet			2·61
Wawa-levu, 1,500 feet			2·61
Wawa-levu, west side of base but not on mountain			2·57
Ndrandramea, summit			2·44
Ndrandramea, 1,600 feet			2·58
Ndrandramea, 1,200 feet			2·68
Ndrandramea, saddle at base			2·71
Kalakala, near base			2·61
Navuningumu near base			2·65
			2·48
Thokasinga			2·74
Thokasinga			2·72
Na Raro, upper part			2·58
Na Raro			2·58
Vatu Kaisia			2·71
Vatu Kaisia, opposite to			2·68
Soloa-levu, upper part			2·54
Soloa-levu, upper part			2·57
Soloa-levu, near base			2·62
Soloa-levu, near base			2·61