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.

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