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.
The rocks exposed in the sides of the gorge are basic agglomerates overlying palagonitic tuffs of mixed composition and evidently sedimentary. On the bottom lie huge masses, some of them 70 or 80 tons in weight, of altered grey aphanitic or non-porphyritic augite-andesites, penetrated in some cases by thin veins of white quartz, and at times displaying a rudely columnar structure, the columns being 12 to 14 inches across. Sometimes the alteration is mainly confined to the filling of the fissures with chalcedonic quartz, minute nests of the same material occurring in the groundmass. At other times the small augite granules are also decomposing. The specific gravity varies from 2·64 to 2·73; the rocks being referred to genus 16, species A, of the augite-andesites. Occasional detached masses of a propylitic basic andesite, displaying porphyritic plagioclase and pyroxene, also occur in this gorge, the felspar phenocrysts being largely occupied by calcitic and other alteration products, whilst much viridite occurs in the groundmass. It exhibits both monoclinic and rhombic pyroxene; and on account of the prism form of the groundmass pyroxene it is placed in the 2nd sub-order of the hypersthene-augite andesites. These altered rocks are deep-seated intrusive masses that were originally covered over by the basic agglomerates and palagonite-tuffs exposed in the sides of the gorge.
Below the gorge there is an extensive exposure in the sides and bed of the river of light-coloured calcareous tuffs which were originally composed of palagonitic materials; but owing partly to hydration, and partly to other secondary changes, the original structure is much disguised.
Crossing the river in the midst of these tuffs there is a dyke, 15 feet thick, formed of a propylitic basaltic andesite, a semi-vitreous rock in which calcitic and zeolitic materials have been developed in quantity. The dyke, which is not columnar, is steeply inclined at an angle of 45° to the north-east.... Further down the river-valley as far as Narengali, occur basic tuff-agglomerates.