The inference to be drawn from the data above given concerning the Va-lili range seems clearly to be this. We have here indicated the emergence of a submarine mountain-ridge covered over with palagonite-tuffs and agglomerates, the last being uppermost. These coverings have been in places stripped off by the denuding agencies and the underlying massive basic rocks exposed. These rocks, however, vary much in texture, some being vitreous, as in the case of the pitchstones, others hemi-crystalline as in the case of the basaltic andesites; and it is to be gathered from this and other similar indications that different submarine vents were formed along a fissure or fissures at the sea-bottom. No evidence of subaerial eruptions came under my notice. After the vents became extinct they were buried beneath the palagonite-tuffs and agglomerates. During and after the emergence the denuding agencies reshaped the surface of the range and left but little of its original form.
Since it is my object to build up a theory of the origin of the ridge-mountains as I proceed with the systematic description of the island, it will be here convenient to follow up the preceding remarks on the Va-lili Range by a preliminary reference to the great ridge district lying east of it.
When a panoramic view of this region is obtained, one observes a series of lofty ridges more or less parallel and running about N.W. and S.E. There are the Va-lili, Narengali, and Sealevu ridge-mountains with lesser ridges between. The intervening valleys are elevated about 400 feet above the sea, whilst the mountains rise up to over 2,000 feet. In many localities this configuration of the surface would be attributed mainly to subaerial denudation. In this island I will endeavour to show that these mountain-ridges existed before the emergence. They do not owe their form to the rivers that flow through the valleys, though no doubt river-erosion has brought these features into greater relief.
In Vanua Levu, as there will be frequent occasion of showing, rivers often flow in valleys that they have not made. This is especially pointed out on page [151]; and it is necessary to emphasise it here, before proceeding farther with the description of the geological structure of the mountain-ridges.
The Waisali Saddle
This saddle, which connects the Va-lili and the Koro-tini ranges, has probably a minimum elevation of not over 1,200 or 1,300 feet. To understand this district thoroughly a regular survey is, however, necessary. It is only at times in this densely wooded range that a view of the surrounding country is obtained; but in spite of this drawback I was able by a diligent use of watch, aneroid, and prismatic compass, to obtain a fair general notion of the surface-configuration.
The track that proceeds westward from Waisali to Narengali leads also to the villages of Na Sinu and Sealevu. About 1½ or 2 miles from Waisali, the track branches off to the westward for Narengali and to the northward for Na Sinu and Sealevu. After half an hour’s walk along this last-named path, one comes to a place where at an elevation of about 900 feet it branches off to the left for Na Sinu, crossing the lowest part of the saddle, and to the right for Sealevu across the Koro-tini Range. It may here be remarked that since the natives are gradually abandoning their mountain-villages and are settling at the coast, many of the mountain-tracks used by me will before long be overgrown and forgotten.
In taking the path from Waisali to Narengali one soon enters the hilly country where large masses of basic tuffs and basic agglomerates, the last formed of blocks of a compact basaltic andesite, occur on the surface up to 700 or 750 feet above the sea. The rock just named has a specific gravity of 2·84, and since it displays rhombic pyroxene amongst its phenocrysts, it is placed in genus 1 of the hypersthene-augite andesites. Above this elevation, and as far as the top of the range, 1,800-1,900 feet above the sea, porphyritic basaltic andesites, having a specific gravity of 2·8, prevail at the surface. They display small porphyritic crystals of plagioclase, augite, and rhombic pyroxene in a groundmass composed of small felspar-lathes, prismatic pyroxene, and much smoky glass, and are referred to genus 5 of the same pyroxene andesites. It is probable, judging from one of these exposures, that such rocks are dyke-like masses: but on account of the thick soil-cap it is not possible to obtain a good view of them.
In the stream-courses occur large blocks of altered basaltic andesites of the propylitic type, having a specific gravity of 2·64 to 2·70, and exhibiting abundant alteration products, such as calcite, viridite, &c. These propylites, I presume, constitute the deeper portion of the range. It will often be necessary to distinguish between the altered basaltic andesites, such as are above referred to, and the relatively fresh rocks of the same type. The former are light coloured (sp. gr. 2·6 to 2·75), and are only exposed in gorges and stream-courses that deeply score the mountain-slopes. The latter are blackish (sp. gr. about 2·8), and at times penetrate the covering of tuffs and agglomerates.
Descending the opposite or north-west side of the saddle-range, one finds the same basic andesites, both fresh and altered, down to about 1,100 feet above the sea. Then the track leads one down a precipitous slope into the picturesque gorge traversed by the head-waters of the Narengali River. At its lower end the gorge opens out into the broad Narengali valley, and here the dense forest of the higher districts gives place to the scanty vegetation of the “talasinga” region.