In Mbatini, 3,437 feet in height, which is the highest mountain of Vanua Levu, the pyroxene-andesite of which the bare rocky peak is composed is somewhat weathered and has a polaric or repellent power of 28°. Specimens of rock obtained below the top show no polarity, the mountain being well wooded except at the summit. In the adjacent mountain of Koro-mbasanga,[[149]] the polaric rocks are limited to those exposed in the peak which is bared of vegetation. The rocks in question are tuff-agglomerates, the small blocks of pyroxene-andesite standing out from the tuff having a polaric force of 14° or 15°. This effect has been produced in greatest intensity in the isolated peak of Navuningumu (1,931 feet) in the Ndrandramea region. Here the bare summit is formed of a semi-vitreous, slightly vesicular, basaltic andesite with a specific gravity of 2·82 in its present condition.[[150]] This rock is powerfully polaric, and rendered the compass useless, the deviation generally to the westward varying from 20° to 50°. I place its repellent force at about 90°, hand specimens affecting the magnetic needle at a distance of 13 or 14 inches. None of the various rocks obtained from the wooded slopes below displayed polarity.
The neighbouring mountain of Ngaingai is composed entirely of dacite having a specific weight of 2·57. The highest point of the summit, 2,448 feet above the sea, is bare and rocky, and the stone here is markedly polaric, the repellent force being about 25°. Specimens from the lower wooded slopes show no polarity. Near by rises the hill of Ndrandramea, which is composed in mass of acid andesites or dacitic rocks. The summit (1,800 feet) is scantily vegetated, and here the somewhat weathered rock which has a specific weight of 2·44 (probably near 2·5 in the fresh condition) has a polaric force of 14°. Specimens of a more compact rock taken from the wooded slopes 300 feet below the summit (sp. gr. 2·58) and from 700 feet below the top (sp. gr. 2·68) showed no such effect; but a specimen taken from a mass of agglomerate in the last locality repels the needle 12°. Its specific gravity is 2·61, and no doubt the mass had been originally a portion of an exposed cliff-face.[[151]]
The summit of Mariko (2,890 feet), the Drayton Peak of the chart, is formed of a rubbly agglomerate of a compact basic andesite. Though it displays bare rock-faces, the actual peak has a soil-cap at least 18 inches deep and supports small trees and shrubs. Notwithstanding this, I found when standing on the peak that my compass was very noticeably affected, the pull being to the eastward, whilst the amount of deviation increased from 11° to 16° when changing from the sitting to the standing position. Specimens of blocks from the agglomerate forming a rock-face 10 feet below the summit possessed polaric powers of 12° and 5°. Others of the same rock exposed in a cliff-face 450 feet below had a weak repellent power of only 4°.... As in the case of Mariko, the top of Thambeyu (2,700 feet) is vegetated; and beneath the smaller trees blocks of polaric rocks lie on the surface. One of these, a pyroxene-andesite (sp. gr. 2·72), from which I obtained a specimen, has a polaric power of 38°. In another case, that of an amygdaloidal rock of the same character, the repellent power is 14°.
I might mention several other polaric peaks, but it will be sufficient to refer to one or two other localities. In the mountainous basaltic district around Solevu Bay the peaks are usually polaric. Specimens from the top of Uli-i-matua, 1,100 feet, have a repellent power of 15°. The three-peaked hill of Koro-tolu-tolu appears to be in the mass of polaric basalt from the foot to the summit, having a repellent power varying from 4° to 30°, the most active specimens being obtained from the lower slopes, which, however, are scantily covered with trees. Samples of the grey basalt from Koro-i-rea show polaric powers of 3° to 7°.
As examples of the numerous lesser hills with bare rocky polaric summits I will first take Bare-poll Hill facing Soni-soni Island. This hill is only about 150 feet above the sea, its top being formed by two large masses of a basic andesite lava with a glassy groundmass, incrusted with agglomerate, the whole representing a volcanic “neck.” A specimen of the rock masses has a repellent force of 22°. Another instance is afforded by Vatui, a hill 450 feet in height situated south of Mount Sesaleka. Its summit is capped by a naked mass of tuff-agglomerate pierced by a dyke 18 inches thick of an olivine-basalt, with a specific gravity of 2·90 and a polaric power of 10°.
A somewhat suggestive example is afforded by the hill of Na Suva-suva, 1,110 feet high, which overlooks Naindi Harbour to the east of Savu-savu Bay. It is only occupied by trees in its upper part, and a specimen of the olivine-basalt, of which the hill is composed, that was obtained from the wooded summit, shows no polarity; whilst another from the slopes, two-thirds of the way up, which had been cleared of trees, has a repellent force of 10°. The polarity of the olivine-basalt from the well-wooded slopes of Ulu-i-ndali, a range 1,100 feet in height on the east side of the Wainunu estuary, is not so easily explained; the intensity varies from 8° to 28°. Ngalau-levu, a hill 1,650 feet in height, rising behind Lea on the south coast of Natewa Bay, is polaric in its upper portion. Specimens of a hemicrystalline basic andesite, somewhat scoriaceous, which form the agglomerate of the rocky summit, have a repellent force of 18°, whilst a similar rock from the agglomerate of an exposed spur two-thirds of the way up the hill has a force of as much as 38°. A curious case of polarity is exhibited in a bare tuff overlooking the Vui-na-Savu River between Rauriko and Vitina. It is composed of a much weathered whitish trachytic rock, which in appearance affords no promise of polarity, but has the power of repelling the magnetic needle 2° to 3°.
Note on the Average Polarity (Repellent Power) of the Volcanic
Rocks of Vanua Levu.
It would appear from the table given below that the difference in the average polarity of acid and basic rocks is not very great. The average for rocks with a specific gravity below 2·7 is about 10°; and that for heavier rocks is about 14°. The difference mainly lies at the maximum end of each series, the capacity for extreme polarity being, as before remarked, markedly greater in the basic rocks.