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.

	Sp. Gr.	Plagioclase.	Quartz.	Hornblende or its pseudomorph.	Rhombic Pyroxene.	Monoclinic Pyroxene.	Structure.		Pyroxene.
Locality.		Phenocrysts.					Groundmass.
	Sp. Gr.	Plagioclase.	Quartz.	Hornblende or its pseudomorph.	Rhombic Pyroxene.	Monoclinic 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	?
Navuningumu near base	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
Navuningumu near base			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

Tabular Comparison of the Acid Andesites of Vanua Levu. continued

Locality.	Plagioclase.	Quartz.	Hornblende or its pseudomorph.	Rhombic Pyroxene.	Monoclinic Pyroxene.
Locality.		Phenocrysts.
Ngaingai, summit	+	+	+ pseudom.
Ngaingai, lower part	+	+	+ pseudom.
Wawa-levu, 1,500 feet	+	+	+ pseudom.
Wawa-levu, 1,500 feet	+	+	+ pseudom.
Wawa-levu, west side of base but not on mountain	+	+	+ pseudom. in great part	+
Ndrandramea, summit	+		+ only traces of pseudom.	+	+ scanty
Ndrandramea, 1,600 feet	+		+ only traces of pseudom.	+	+ scanty
Ndrandramea, 1,200 feet	+		?	+	?
Ndrandramea, saddle at base	+		+	+	?
Kalakala, near base	+		+ pseudom. in part	+
Navuningumu near base	+		+ in part pseudom.	+	+ scanty
Navuningumu near base	+	+	+	+
Thokasinga	+		+ pseudom.	+	+
Thokasinga	+			+	+
Na Raro, upper part	+		+ pseudom.
Na Raro	+		+ pseudom.
Vatu Kaisia	+		+ in part pseudom.	+
Vatu Kaisia, opposite to	+		+ in part pseudom.	+
Soloa-levu, upper part	+			+	+
Soloa-levu, upper part	+			+	+
Soloa-levu, near base	+			+	+
Soloa-levu, near base	+			+	+

Tabular Comparison of the Acid Andesites of Vanua Levu. continued

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.	Groundmass.
	Structure.		Pyroxene.
	Felsitic.	Orthophyric.	Rhombic.	Monoclinic.
Ngaingai, summit	+ ·014		+ ·03
Ngaingai, lower part	+ ·013		+ ·02
Wawa-levu, 1,500 feet	+ ·015		+ ·05
Wawa-levu, 1,500 feet	+ ·014		+ ·06
Wawa-levu, west side of base but not on mountain	+ ·022		+ ·04
Ndrandramea, summit	+ ·007 hemi-crystalline		+ ·01
Ndrandramea, 1,600 feet	+ ·008 hemi-crystalline		+ ·01
Ndrandramea, 1,200 feet	+ ·008 hemi-crystalline		+ ·01
Ndrandramea, saddle at base	+ ·009		+ ·01
Kalakala, near base	+ ·02		+ ·01
Navuningumu near base	+ ·02		+ ·03	?
Navuningumu near base	+ ·018		?
Thokasinga	+ ·024		+ ·03	+
Thokasinga	+ ·023		+ ·04	+
Na Raro, upper part	+ ·02		+ ·01 scanty
Na Raro	+ ·022		+ ·01 scanty
Vatu Kaisia	+ ·013		+ ·05
Vatu Kaisia, opposite to	+ ·01		+ ·05
Soloa-levu, upper part		+	+ ·06
Soloa-levu, upper part		+	+ ·05
Soloa-levu, near base		+	+ ·06	+ scanty
Soloa-levu, near base		+	+ ·06	+ scanty

Note on the Rhombic Pyroxene of the three foregoing sub-classes of the Acid Andesites.—The term “hypersthene” has been here used as a convenient expression equivalent to “rhombic pyroxene.” The mineral is always a little pleochroic and is never colourless, and it is only in very rare cases that the term “enstatite” could be used. As a matter of fact there is practically only one form of rhombic pyroxene represented in my collections whether in acid or basic andesites or in hemi-crystalline and plutonic rocks. In the acid andesites it occurs not only as phenocrysts but also as minute prisms forming a constituent of the groundmass.

This mineral, when composing the phenocrysts, presents itself usually as single untwinned prisms which exhibit the typical octagonal cross-sections with much reduced prism-faces. The prismatic sections give straight extinctions; whilst with the cross-sections we obtain straight extinctions parallel with the pinakoid faces. The colour in transmitted light is pale brownish yellow. The pleochroism, though usually feeble, is quite distinct, the colour being pale yellow when the prism lies parallel with the long axis of the lower nicol, and almost white when it lies across. Not infrequently these phenocrysts behave abnormally and give small oblique extinctions. This is often the case when monoclinic pyroxene occurs in the same section. The association of the two pyroxenes in one crystal can in some cases be clearly recognised. At one time a plate of pyroxene exhibits itself as a coarse aggregate of the two pyroxenes. At other times the two occur as parallel intergrowths, as in the accompanying figure. But it is rarely that such intergrowths are so typically displayed, the reason of which has been supplied by Zirkel in his Lehrbuch der Petrographie; 2nd edit.: I. 271.

Note on the “magmatic paramorphism”[^[119]] of the hornblende phenocrysts.—Reference has before been made in the general description of these rocks to the dark alteration margins of the hornblende phenocrysts. The dark borders display the “bacillary” structure noticed by Renard in the case of some hornblende andesites from Kandavu in the same group of islands,[^[120]] being composed of minute granules and parallel prisms of pyroxene and also of magnetite grains. With the Kandavu rocks Renard observed that the tiny pyroxenes were colourless or greenish and had an extinction angle of 40°. In the Vanua Levu rocks, however, there is a mixture in these dark margins of both monoclinic and rhombic pyroxene; and the process may be observed in all stages as it advances into the interior of the crystal, until a dark pseudomorph or paramorph of pyroxene and magnetite results. When the magnetite prevails, the pseudomorph may ultimately form a black patch in which the process is obscured. But when, as is generally the case, the pyroxenes are more frequent, it occurs as a dark grey mass.

Finally follows the dispersion of the pseudomorph, which first becomes a loosely arranged aggregate of the two pyroxenes and magnetite, and then breaks down, and at length is only represented by small pale patches of its original constituents. These patches are easily recognisable, and in not a few rock-sections offer the only indication of the previously existing hornblende phenocryst. There can be little doubt that this is the source of much at least of the often abundant pyroxenes of the groundmass, which are usually most frequent in the vicinity of the patches.

In the earliest stage when the dark border alone exists, it is not easy to distinguish the one pyroxene from the other, the granules and prisms being colourless and very minute, less than ·01 in size. But in a far advanced stage of the paramorphism the granules and prisms become sometimes much larger, the first attaining a breadth of ·04 and ·05 mm. and the last a length of ·15 mm. Finally the interior of the paramorph is seen to be more or less completely composed of very pale brown augite and pale yellow rhombic pyroxene in coarse grains and prisms, the first distinguished by its oblique extinction of 30° to 35°, the last recognised by its straight extinction and feeble though distinct pleochroism.

Although as a rule the paramorph becomes dispersed and its pyroxene constituents are added to the groundmass, it sometimes exhibits a change of another character. In this case the outer portion is alone dispersed, whilst the growth of a single large crystal of pyroxene proceeds within the mass. In a later stage, when the dispersion of the outer part is complete, we have a fresh-looking pyroxene phenocryst with unformed edges, on the borders of which little granules and prisms of pyroxene may be seen arranging themselves, as if the crystal-building was still in progress, or rather as if it had been interrupted and left unfinished by the too rapid dispersion of the outer portions of the paramorph.

It will be gathered from the above that the source of the pyroxene of the groundmass is to be found in the magmatic paramorphism of the porphyritic hornblende. The hornblende is dark-brown, markedly pleochroic, and extinctions up to 15° are given in prismatic sections. It is well known that the conversion of a hornblende crystal into an aggregate of pyroxene prisms and magnetite was long since experimentally effected by Doelter and Hussak by immersing the hornblende in molten basalt, andesite, &c.[^[121]] I would imagine that the transformation of the hornblende and the dispersion of the paramorph occurred under two conditions; in the first case whilst the “flow” was still in motion when the resulting pyroxene would be mixed up in the magma; in the second case after movement had ceased, but before consolidation of the groundmass, when a paramorph or pseudomorph would be formed.