| Specific gravity. | Character of rocks. | Number of specimens. | Polarity.[[152]] | |
|---|---|---|---|---|
| Range. | Average. | |||
| 2·50-2·59 | Dacites | 6 | 2°-25° | 11·6° |
| 2·60-2·69 | Dacites and augite-andesites | 5 | 3°-16° | 8·6° |
| 2·70-2·79 | Augite-andesites | 5 | 5°-38° | 13·2° |
| 2·80-2·89 | Basaltic andesites and olivine-basalts | 18 | 3°-90° | 15·0° |
| 2·90-3·00 | Olivine-basalts | 14 | 3°-29° | 12·7° |
It is, however, noteworthy, as indicated by the value of the average in each series that not one of them is a good series. They form curves which in each case present an extreme maximum variant which is suggestive of quite another degree of magnetising agency. This is also illustrated in the combined curve of all the results given above. The acid as well as the basic series are thus characterised, and the extreme maximum variants are in each instance afforded by the highest mountain peaks. It is probable that there is an accelerating ratio of magnetisation with increased elevation. However that may be, it appears evident from my observations on the two adjacent peaks of Ngaingai and Navuningumu that the limits of polarity acquired through atmospheric electricity without the direct action of lightning would be, as measured by the scale here employed, four times as great for a dacite (25°) as for a basaltic andesite (90°).
CHAPTER XXVII
SOME CONCLUSIONS AND THEIR BEARINGS
Vanua Levu is a composite island built up during a long period of emergence, that began probably in the later Tertiary period, by the union of a number of large and small islands of volcanic formation representing the products of submarine eruptions. It differs in this respect from Viti Levu which is much more massive in its profile and possesses a greater proportion of plutonic rocks. When, however, Viti Levu comes to be systematically examined, it is likely that traces of its composite origin will be found. The evidence seems to show that it is older than Vanua Levu; but they are both situated on the same submarine platform, the area of which is nearly equal to the combined areas of the two large islands that rise from it.
This platform, as indicated in the small plan of the group, is limited by the 100-fathom line in the charts; but since the reefs on their seaward slopes plunge down precipitously, such a line practically serves to delineate the margin of the plateau. It has been my object to show on previous pages[[153]] that this submarine platform is a basaltic plateau built up by submarine lava-flows and incrusted with coral reefs and their deposits. It has been pointed out that this platform passes gradually, as it proceeds landward, into the low-lying basaltic plains that constitute the sea-border in the western part of the island, where the breadth of the submerged plateau is greatest. The basaltic flows of the plains often display the almost vertical columns of slightly inclined flows. Their apparent termination at the sea-border, where they are in places covered over with submarine deposits, cannot, however, be accepted as their real limits. According to my view they extend several miles seaward and form the platform, as is shown in the sections on pages [62] and [107].
FIJI ISLANDS.
FROM ADMIRALTY CHART 780, CORRECTED TO 1901.
ALL DEPTHS BEYOND 100 FATHOMS ARE COLOURED BLACK.
HEIGHTS IN FEET, DEPTHS IN FATHOMS.
We have in the great basaltic mountain of Seatura, which forms the bulk of the western end of the island, a probable source of many of these basaltic flows; and the occurrence inland in the western half of Vanua Levu of elevated table-lands of basalt like that of Wainunu, which extend from the centre of the island to near the coast, afford testimony that the formation of these flows extends over a considerable period of the island’s history.
It is held by Professor Agassiz that these submarine platforms are the work of erosion into the flanks of the up-heaved islands.[[154]] In [Chapter II.] it has been pointed out that the eroding agencies are not actively in operation in our own day, and that there is good reason for the belief that the process of amalgamation by which Vanua Levu has been built up during a prolonged period of emergence, is not suspended at the present time. It is assumed that the uniformity in Nature’s methods has not been broken. If, however, we have here platforms of erosion, the coasts of Vanua Levu, as far as my interpretation goes, supply no evidence of it; and we have to imagine that a period of emergence extending over a geological age has been followed by a similarly vast period of erosion without much change in level.
Whatever agencies have been at work, the production of submarine platforms 10 to 20 miles in width must have been a stupendous operation; and we shall be obliged to inquire whether plateaux, either submarine or upheaved, occur in association with volcanic islands in other parts of the world, and under what conditions they have been formed. At least four hypotheses have been framed with regard to the submarine platforms of Fiji. There is first the original theory of subsidence of Darwin; but Vanua Levu, which presents one long story of emergence, offers nothing to support this view. There is the growth of a reef seaward on its own talus, as advanced by Murray. There is the theory of erosion of Agassiz. There is lastly my own idea of basaltic plateaux incrusted by reefs. We may therefore inquire as to the evidence afforded by Vanua Levu in favour of these views. Basaltic flows, in places covered by submarine deposits, form the low plains at its sea-border, where the platforms are broadest; and there rises a basaltic mountain of the Mauna Loa type, occupying most of the western end of the island. No one would be bold enough to place the limit of these basaltic flows at the water’s edge; and as is indicated in the sections, they probably extend for miles under the sea.