Coming to the magnetic polarity displayed by some of these rocks, when the ordinary hand-specimen behaves like a magnet in attracting one pole of the needle and repelling the other, it is to be at first observed that a rock can become polaric without being previously magnetic. Dr. Folgheraiter has observed polarity in the case of fragments of ancient bricks and pottery; and he has described the same effect in the masonry of a house struck by lightning. In one or two of the Vanua Levu acid rocks showing polarity this can be also premised since magnetite is present in very slight degree.
Polarity is very frequent among the volcanic rocks of this island. Out of 520 specimens in my collection, which was made without any reference to this matter, 80, or 15 per cent. are polaric. Of these seven-eighths are basic and the rest are acid rocks; but this proportion is partly accounted for by the far greater prevalence of basic rocks in the island. The basic rocks showing polarity include some of the heaviest olivine-basalts with a specific weight of 3·0, as well as some of the lighter augite-andesites with specific weight of 2·7. They comprise the coarse textured dolerite as well as the vitreous pitchstone and include both scoriaceous and amygdaloidal rocks. The polaric acid rocks are mostly referable to the dacites, with a specific gravity of 2·5 to 2·6.
Humboldt remarked long ago that there is no direct relation between the degree of polarity and the specific weight. This is well brought out in the table subjoined; but it should be at once observed that there is an indirect relation. Although when we arrange the rocks in a series according to their specific weight we find no corresponding relation in the amounts of the polarity, we observe that the extent to which polarity can be developed is markedly greater in basic than in acid rocks. From this it may be inferred that the degree of intensity of the exciting cause required to give polaric powers of a certain value to an acid rock, like a dacite, would be much greater than that necessary to endow a basalt with equal powers. We should not expect to find the same amount of polarity in the bare rocky peaks of two adjacent mountains, where one was of dacite and the other of basalt; and, other things being equal, if two mountains had been exposed for ages to the same conditions, we should regard the polaric powers of the two as nature’s equivalent values for the work of atmospheric electricity, on the two rocks in question. We have two such mountains in Vanua Levu in the case of the adjacent peaks of Ngaingai (2,448 feet) and Navuningumu (1,931 feet) which are about 2¼ miles apart and possess similar bare rocky pointed summits. I take it that the polaric power of 25° of the dacite (sp. gr. 2·57) in the first case is equal to the power of 90° of the basaltic andesite (sp. gr. 2·82) in the other. In the dacitic peak of Ngaingai and in the basaltic peak of Navuningumu we can measure what work atmospheric electricity can accomplish in the course of ages in the magnetisation of rocks. The other conditions being taken as about the same, the main determining difference is to be found in the rock-characters.
In the table on the opposite page we have a series of volcanic rocks placed according to their specific weights, which range from 2·5 to 3·0, and in the second column are shown their relative polaric powers as indicated by the number of degrees the north end of the magnetic needle is repelled by the corresponding pole in the hand-specimen. For this purpose a magnetic needle 2½ inches in length (strictly speaking 6·5 centimetres) was employed, a card marked in degrees being placed beneath. The north pole of the stone was placed in contact with the north end of the needle, and after the needle had become stationary in its new position a reading was taken.
These polaric rocks came under my notice over most of the island. They are infrequent in the district between Undu Point and the Wai-ni-koro River, where, however, acid tuffs are largely exposed; and I did not find them in the Natewa Peninsula east of Lea, their absence from my collections made in the Mount Freeland range being remarkable. But it is probable that this is due to the surface conditions, since dense wood covers the slopes, and bare rocky peaks are rarely to be seen.
With regard to the influence of locality on the occurrence of polaric rocks, the results may thus be classified. About one-third are found in the exposed rocky peaks of hills and mountains. Another third are found where the rocks are bared in headlands, coast cliffs, inland-bluffs, ridge-tops, and in the open basaltic plains where trees are scanty. On the other hand, a third occur in situations, as in wooded districts where the rock exposure is scanty, when it is not easy to explain the polarity, unless it was developed in clear districts that have since become covered with forest.
Table showing the Relation between the Specific Gravity and the
Polarity of Volcanic Rocks.
| Character of rock. | Specific gravity. | Amount of polarity.[[148]] |
|---|---|---|
| Dark olivine-basalt | 3·00 | 10° |
| Grey olivine-basalt | 2·94 | 29° |
| Grey olivine-basalt | 2·92 | 7° |
| Dark olivine-basalt | 2·90 | 10° |
| Dark olivine-basalt | 2·87 | 30° |
| Basaltic andesite | 2·82 | 90° |
| Basaltic andesite | 2·77 | 5° |
| Pyroxene-andesite | 2·72 | 38° |
| Dacite | 2·61 | 17° |
| Dacite | 2·59 | 5° |
| Dacite | 2·57 | 25° |
| Dacite | 2·50 | 14° |
In no place did any evidence of the direct action of lightning come under my notice. Mr. S. Skinner who kindly looked at a few of these rocks says that he found no trace of fulgurites in them. It is probable that here as in the mountains of Skye, as described by Mr. Harker, these effects are the result of the general influence of atmospheric electricity independently of the direct agency of lightning. The frequency of polaric rocks in the highest peaks of the island is very remarkable. Generally speaking, all the bare summits of the mountains are polaric. In my experience there is no exception. All the rocks obtained from the actual summits show polarity. The variety of rocks thus affected is suggestive; and this chapter may be concluded with a brief reference to their mode of occurrence on some of the mountain-peaks.