This peak, 1,880 feet in height, starts up suddenly in the mountainous interior of the island. Being situated in the valley of the Yanawai river, which opens to the south, it forms a conspicuous landmark for vessels off the south coast; but from most other points of view, on account of its peculiar situation, it is usually difficult and often impossible to obtain even a glimpse of it.[[53]] From its remarkable blunt-topped conical shape it has received the not very appropriate name of Marling Spike in the Admiralty charts. The natives name it Vatu Kaisia, the first word signifying “rock,” whilst the second is the name of a demon.
Profile-sketch of the Vatu Kaisia district from S.S.E.
Some idea may be formed of its situation and of the character of the neighbouring country from the profile-sketch and photograph here produced. I was unable for reasons given below to take a photograph of the mountain itself, as it was either too near or too far away. Vatu Kaisia is approached either from Ndrawa on the north or from Ndawara on the south, the ascent being best made from the west side. The regions traversed on the way are so densely wooded that the mountain does not become visible until the traveller is right upon it. He becomes suddenly aware that there is some huge mass close to him looming above his head through the trees; and it is with a feeling of awe that he first looks upon a mountain that although only a few hundred yards away nearly escaped his search. He is startled by its proximity, and wonders what strange forces have been at work to place it there; but his view is transitory, and whether proceeding north or south he sees it no more, unless he essays to climb its slopes.
Vatu Kaisia lies not in the centre but towards the west side of the Yanawai valley the river flowing as an impetuous stream around the foot of its eastern slope. In the profile-sketch the mountain itself conceals the peculiar feature of its position, which is, however, shown in an exaggerated form in the geological section below. On its west side rises a broad ridge running south which in places is not much higher than the basaltic plateau of Wainunu to the west of it. This ridge is only separated from Vatu Kaisia by a dark narrow gorge not many hundred yards in width, across which my natives were able to make themselves heard when near the summit. The mountain rises 1,100 or 1,200 feet above the gorge on its west side, which is 700 feet above the sea, and some 1,400 or 1,500 feet above the Yanawai river on the east, which is 300 or 400 feet above the sea. It possesses two peaks, of which the western one is smaller and lateral and has a height of 1,600 or 1,650 feet, whilst the eastern is the main peak and rises to 1,880 feet. The saddle between the peaks has an elevation of about 1,500 feet. It is very difficult to obtain a distant view of the two peaks, which lie about N.W. and S.E. with each other. They are either merged into one as in the view from the south, or else the highest portion of the main peak is alone visible.
On the lower slopes of the mountain as high as 1,100 or 1,200 feet is exposed a porphyritic doleritic basalt showing semi-ophitic augite and abundant interstitial glass. Its specific gravity is 2·8, but there is no olivine. It belongs to a type of basalt described under genus 9, sub-genus A, of the augite-andesites. The upper double-peaked portion rises precipitously, displaying bare rocky cliff-faces with a drop of 100 or 150 feet, and formed in mass of a grey andesitic rock with a specific gravity of 2·71 and showing abundant small porphyritic crystals of hornblende and rhombic pyroxene. It represents a type of the hornblende-hypersthene-andesites described on page [301]. I was unable, through want of a rope-ladder, to accomplish the last hundred feet of the summit; but the general uniformity of structure was evident. No detrital rocks came under my observation.
That the porphyritic basalt represents a later flow around this old andesitic mountain is indicated amongst other things by this absence of tuffs and agglomerates. Vatu Kaisia is undoubtedly the core of an ancient cone of hornblende-andesite, and as in the case of Mount Soloa Levu, which is formed of somewhat similar andesites (see page [103]), it has been more or less completely surrounded by later basaltic flows. Vatu Kaisia and Soloa Levu occupy similar positions with respect to the great basaltic table-land of Wainunu, the first lying just within its eastern border, the second lying partly within its western margin.
The structure of the ridge immediately west of Vatu Kaisia lends support to this view of the formation of this region. The ridge is here, it is true, elevated a hundred feet or so above the table-land which is about 1,000 feet above the sea; but whilst on its slopes facing the mountain the same porphyritic basalt prevails, there is a limited exposure on its top of the same rock (sp. gr. 2·68), differing only in the larger size of its porphyritic crystals of hornblende and rhombic pyroxene.
Vatu Kaisia