Ascending the lower slopes of the range up to 800 feet one finds the altered rocks still exposed in the stream-courses; but the changes exhibited are not always the same. A specimen from 500 feet looks like a tuff, but in the slide it appears as a semi-vitreous augite-andesite, its substance being penetrated by fine veins of chalcedonic quartz and opal, whilst the same material is developed within the larger plagioclase crystals. Another specimen from 800 feet, which is apparently a tuff, contains so much lime that it effervesces freely with an acid. It was composed originally of fragments of a hemi-crystalline basic rock, of which the plagioclase phenocrysts have been replaced by calcite; whilst the augite and interstitial glass is now represented by viridite and a chloritic mineral. It is to be inferred that at some time hot springs were very numerous in the district between Nukumbolo and the lower slopes of the range, those at Nukumbolo, as far as I know, alone existing in our time.
From a height of 1,100 or 1,200 feet the mountain slopes rise steeply to the summit rather over 2,000 feet in elevation. At the foot are exposed in situ aphanitic augite-andesites,[[71]] which in some specimens show a little alteration in the chalcedonic quartz filling minute cracks, and in one case there is an irregular cavity, ¾ inch across, filled with milk-white opal. Another rock exposed at the foot of the steep ascent is a semi-vitreous basaltic-andesite, doleritic in texture and ophitic structure, but apparently not much changed.[[72]] At 1,700 feet is displayed a vesicular basic andesite, semi-vitreous in character, and above this I found a porphyritic basaltic andesite.
The summit of the range is 1½ or 2 miles in breadth and is relatively level, its undulating surface varying in elevation between 1,900 and 2,200 feet. The prevailing rocks exposed on this elevated plateau are vitreous pitchstone-like rocks finely vesicular and scoriaceous, the cavities being filled either with aragonite or with opal. The basic glass, of which they are formed, shows incipient crystals, and begins to fuse in an ordinary flame. One specimen obtained here is a doleritic basaltic andesite, slightly ophitic and containing a fair amount of residual glass.[[73]] However, the vitreous and scoriaceous character of most of the rocks on the summit is very remarkable. (Similar rocks occur on the top of Mount Thambeyu where the slopes of the mountain are covered with submarine tuffs and agglomerates.) There is a precipitous descent on the north side of the range to Sueni at its foot, massive basaltic andesites being exposed at first, whilst basic tuffs and agglomerates are displayed lower down.
The special features of this traverse of the range are the alteration of the tuffs and massive rocks between Nukumbolo and the lower southern slopes, the variation in character of the basic rocks in the upper southern slopes, the occurrence of vitreous vesicular and scoriaceous rocks on the summit, and the restriction of the ordinary basic agglomerates and tuffs to the northern slopes. Any attempt on my part to explain the structure of this part of the range from the data here given would be futile without comparing them with those obtained from other parts of the range. It will be subsequently pointed out that the difficulties will be in part removed if it is assumed that the submarine palagonite-tuffs and agglomerates, that so often cover the flanks of the mountains to their summits, have been in this case largely stripped off by the denuding agencies.
(5) The Sueni Valley.—My acquaintance with the extreme eastern part of the Koro-tini Range is restricted to the descent of the picturesque valley from Sueni to Koro-utari. It is occupied by a tributary of the Lambasa River, and is bounded on the east side by the lofty slopes of the main range, and to the westward by a mountainous spur that projects far into the Lambasa plains. Sueni lies by the river-side in the midst of mountains which rise steeply on most sides to heights of 2,000 feet and over, and often display precipitous bare faces apparently of volcanic agglomerates. Numerous waterfalls may be observed on their flanks, which, as in other localities, doubtless indicate the occurrence of large intrusive dykes. Sueni is situated about 300 feet above the sea, the descent to Koro-utari at the mouth of the valley, a distance of 3 to 4 miles in a direct line, being about 150 feet.
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.