At intervals all along the coast between Ndevo and Nuku-ndamu, passing on the way the villages of Koro-i-vonu, Tuvumila and Kanakana, fine and coarse basic tuffs, often calcareous, are to be seen exposed in the sea-cliffs and in the low hills behind. They are bedded and dip 5° to 10° W.S.W. near Ndevo and 15° to 20° S. b W. south of Kanakana. The lower slopes of the Ngala mountains here approach the coast, and it is highly probable that the submarine tuffs which form the sea-border extend a considerable distance inland and to some height above the sea. Between Nuku-ndamu and Tukavesi, where the mountains rise close to the beach, occur basic agglomerates and agglomerate-tuffs, derived from basaltic andesites. A specimen of the last named represents an uncommon type of basaltic rock (sp. gr. 2·85), which on account of the character of the felspars and of the pyroxene of the groundmass is referred to the prismatic sub-order of the orthophyric order of the hypersthene-augite andesites (page [290]).
It may be inferred from the foregoing remarks that whilst the main elevated mass of the Ngala Range is composed of altered basic andesites, the product of ancient eruptions, the basic agglomerates, tuffs, and clays, which occur on the lower slopes and in the outlying spurs, are of later date. These tuffs and clays are evidently of submarine origin, at least in the lower levels; but although fossiliferous deposits were not observed at greater elevations than 300 feet above the sea, it is probable that future investigators will find them at much higher levels. Their discovery, as before noticed, at an elevation of 1,100 feet in the adjacent Waikawa mountains, renders it likely that as in the case of the mountainous districts of the main portion of the island the whole of the Natewa peninsula was at one time submerged, or at least all the region excepting the summit of Mount Ngala.
CHAPTER XV
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The North-east Portion of the Island
This large area, which extends for a distance of nearly forty miles from the eastern slopes of the Mount Thurston Range to Undu Point, forms the region closing in Natewa Bay on the north. It would be difficult to imagine an area of this size with a greater variety of surface or showing such a lack of arrangement of its principal features. The hills and mountains on the north side gather at the coast, and extensive inland plains, raised but a few feet above the sea and strewn with silicified corals, occupy a portion of its interior. A long valley with a very small gradient extends nearly across its breadth; and the rivers are for the most part tidal estuaries fed, except in one or two cases, by insignificant streams. There is, however, a lofty range of ridge-mountains in its broadest part attaining a height of 2,500 feet; whilst away to the east stretches the great Undu Promontory singular for the straightness of its form.
Volcanic rocks of acid types, such as oligoclase-trachytes, quartz-porphyries, and white pumice-tuffs prevail in the northern part between Undu Point and the promontory opposite Mali Island. In the southern part, from the foot of Mount Thurston and Vuinandi to the vicinity of Tawaki, massive rocks and tuffs and agglomerates of basic characters predominate. Although my acquaintance with this area is incomplete, the data below given will be sufficient to enable a general idea to be formed of its structure. The more conspicuous features in its geology will gradually come into prominence as the various localities visited are described.
The Southern Sea-border between Vuinandi and the Vicinity of Tawaki
The basic rocks, which characterise this long extent of coast, give place about two miles west of Tawaki to the acid rocks. I will proceed methodically with the description from Vuinandi eastward.
(a) The coast between Vuinandi and Nakarambo.—Along this coast, spurs from the Mount Thurston Range reach the borders of Natewa Bay, forming a succession of small bays a mile or so across, on the shores of which are situated the villages and towns of Vuinandi, Ndaku-ndaku, Korotasere, and Nakarambo. The basaltic rocks characteristic of the vicinity of Vuinandi are predominant here; but agglomerates and tuffs are of rare occurrence, the prevailing rocks exposed in the spurs being fine textured basaltic andesites (sp. gr. 2·82). A calcareous tuff-clay is, however, to be observed in the east point of Vuinandi Bay, where it is apparently penetrated by a dyke of the above mentioned rock. In Ndaku-ndaku Bay there issue from the shore-flat between the tide-marks some hot springs which are referred to on page [34].
(b) The coast between Nakarambo and Waimotu.—In this tract we meet with rocks of a somewhat different character, though still basic in their type. Through the agglomerates and agglomerate-tuffs of the spurs protrude coarsely crystalline grey pyroxene-andesites.[[91]] They are to some degree altered, and are characterised by their abundant phenocrysts of plagioclase and of rhombic and monoclinic pyroxene, the groundmass being relatively scanty. The agglomerates are formed of similar materials. There is an interesting exposure in the point east of Nakarambo. My specimens from this locality were unfortunately lost; but in my notes reference is made to the coarsely crystalline grey andesite above noticed, to the later intrusion of a dark amygdaloidal rock, and to an altered calcareous tuff.