The river as it flows down the valley from Sueni to Koro-utari traverses a region of basic agglomerates and agglomerate-tuffs. These deposits, as they are displayed in the hill-slopes lying W.S.W. and at the back of Sueni, are composed of blocks of the size of the fist of a vesicular basaltic andesite; whilst the large masses on the surface are made of the same, but non-vesicular, rock. The blocks in the agglomerates between Sueni and Koro-utari range usually from a few inches to a foot in diameter. A specimen obtained from one of them is made of a partly vitreous basaltic andesite; whilst in another case the rock is an altered basic andesite, the glassy groundmass being largely impregnated with colloid silica looking like opal under the lens.[74]
Nearly a mile below Sueni, within a space of less than 60 yards, there are exposed at the river-side in the agglomerates three vertical or nearly vertical dykes, 4 to 6 feet in thickness. They trend roughly N.E. and S.W., and are non-columnar, except in the case of the one farthest up the river, which has rude, transverse joints.[75] The rocks composing these dykes are somewhat doleritic basaltic andesites, olivine being very rare or absent. The two highest, which are only 15 to 20 feet apart, are made of similar rocks characterised by abundant interstitial glass, and having a sp. gr. of 2·78. The rock of the third dyke, about 50 yards farther down the river, has but scanty glass in the groundmass, the sp. gr. being 2·89. The differences between the two types represented in the three dykes are mainly concerned with the degree of crystallisation, and it is probable that though not contemporaneous they were derived from the same fluid magma which, as we may infer from the proximity and lie of the dykes, was situated at no great depth.[76]
GENERAL INFERENCE RESPECTING THE KORO-TINI RANGE.—If we can imagine a line of vents, protruding in some cases above the surface of the sea, that were ultimately worn down to a common level through marine-erosion, and were then largely covered over with submarine tuffs and agglomerates, we should have in our mind’s eye the first and most important stage in the formation of this range. If we then assume that there followed a period of emergence characterised by a renewal on a very extensive scale of marine-erosion, during which the agglomerates were mainly formed, and that since that period the sub-aerial denuding agencies have been for ages in operation, we shall, I think, obtain some idea of the history of the Koro-tini Range.