CHAPTER I

GENERAL INTRODUCTORY REMARKS ON SOME OF THE LEADING PHYSICAL FEATURES OF THE ISLAND

THE remarkable shape of this island at once attracts the attention: and indeed it is in its irregular outline and in the occurrence over a large portion of its surface of submarine tuffs and agglomerates that will be found a key to the study of its history. With an extreme length of 98 miles, an average breadth of 15 to 20 miles, and a maximum elevation of nearly 3,500 feet, it has an area, estimated at 2,400 square miles, comparable with that of the county of Devon.

Whilst its peculiarly long and narrow dimensions are to be associated with the narrowing of the submarine basaltic platform, from which it rises together with the other large island of Viti Levu, its extremely irregular shape is closely connected with the composite mode of its origin. We have here exemplified the process of the building up of a continental island in the great area of emergence of the Western Pacific, that region which displays at various heights above the sea the ancient reefs and the underlying deposits of the Solomon Islands, New Hebrides, Fiji, Tonga, &c. But this process of construction has never been completed, and is at present suspended; yet it is in its incomplete condition that Vanua Levu possesses its importance for the investigation of this subject.

This island has in fact been formed by the union of a number of smaller volcanic islands during a long protracted period of emergence. These original islands are indicated approximately by the 1,800-feet contour-level in the accompanying map. There is, however, no reason for supposing that the movement of emergence has altogether ceased. In the course of ages the extensive submarine plateau, from which it rises, will be laid bare; and the small surrounding islands that are situated upon it, such as Yanganga, Kia, Mali, Rambi, Kioa, &c., will be included in the area of Vanua Levu.[1]

Excluding for the moment the effects of denudation, which have been very great, we shall be able to discern some of the stages of the building-up of the island during the emergence or upheaval by looking at the map and reversing the process in imagination. A subsidence of only 50 feet would cause the Natewa Peninsula to be isolated by a sea-passage along the line of the Salt Lake; whilst several islands would be formed along the northern and southern coasts, and the Naivaka Peninsula would become detached. If the subsidence extended to 300 feet, the sea would flow over a large portion of the island, where it would regain what was not many ages since its own, an area of basaltic plains, which by their prolongation under the sea form the great submarine plateau. A subsidence of 1,000 feet would break up the remaining elevated axis of the island into a number of lesser portions; and after a total lowering of 1,800 feet there would exist only a few scattered islands, the arrangement of which would show but little relation to the present form of Vanua Levu. At either end of the area there would arise from the sea the isolated volcanic peaks of Seatura and Ngala (Mount Freeland); and between them would be situated four or five long narrow islands, together with a group of small islands and islets where Na Raro and the other acid andesite mountains of the Ndrandramea district now lie.

As might be partly expected, there is in the surface-configuration of the interior of Vanua Levu an absence of that simplicity of contour which exists in a volcanic island of supra-marine formation, as for instance in the large island of Hawaii where the three great volcanic mountains of Mauna Kea, Mauna Loa and Hualalai together with the older Kohala range, determine the form of the whole island’s surface.[2] Here in Vanua Levu there is, on the contrary, but little order amongst its physical features. The rivers often run obliquely with the sea-border, whilst mountains frequently rise at the coast and plains lie far inland, and the view of the elevated interior, as obtained from one of the peaks, presents in many parts a series of mountain-ridges running athwart the island’s axis.

A study of the profile of the island is an important preliminary step to its more detailed examination. One may ramble over a particular region of it for weeks, as I have done, without getting any satisfactory idea of the true configuration of the surface. In a locality densely wooded and occupied by steep mountain ridges and deep gorges, the field of view is often very limited; but seen from the deck of a passing ship the main features of the island assume their true proportions and relations, and much that was uncertain is in this manner made plain. The profile here given has been constructed from a number of others, and represents in a graphic fashion Vanua Levu as viewed from the southward. I have here sacrificed smaller details and occasionally some degree of accuracy in small matters in order to bring out the principal features of the island.

At and near the extreme western extremity rise the conspicuous hills of Sesaleka (1,370 feet), Naivaka (1,651 feet) and Koroma (1,384 feet), all of them formed of basic volcanic materials.[3] Naivaka, which is connected with the main island by a narrow isthmus, only about 30 feet in height, is probably one of the most recent additions to the island’s area; and it is at the same time one of the most recent of the numerous volcanic vents that once existed. The leading feature, however, of this end of Vanua Levu is the great mountain of Seatura (2,812 feet), which occupies a large part of the Mbua province and monopolises most of the landscape whilst largely determining the form of the western extremity of the island. It is a basaltic mountain of the Mauna Loa type, its long eastern slope descending gently at an angle of three or four degrees for about ten miles to the mouth of the Wainunu River. In its deeply eroded radial valleys and gorges, and in other respects, it is not unlike the island of Tahiti, as described by Dana.

The Ndrandramea region to the eastward, which I have named after one of its best known peaks, has a profile of a very different character. Its broken outline indicates the existence of numerous mountains and hills of acid andesites, occasionally dacitic. Although some of them attain a height of 2,000 feet and over their tops alone are seen from seaward. Between the foot of these mountains and the south coast extends a great plateau of columnar basalt, incrusted at its borders with submarine deposits, which descends coastward with a very gentle slope, the fall in about five miles being only about 300 feet (1,100 to 800 feet). It terminates abruptly opposite the elevated headland of Ulu-i-ndali, a range, composed mainly of grey olivine-basalts, which is not indicated in the profile.

PROFILES OF VANUA LEVU AS VIEWED FROM THE SOUTH. GRAPHICALLY REPRESENTED ON A HORIZONTAL SCALE OF ABOUT 16 MILES TO THE INCH.

The two conical peaks of Vatu Kaisia (1,880 feet) and Na Raro (2,420 feet), which rise up so unexpectedly in the region immediately east of the Ndrandramea district, are also of acid andesitic rocks, in the last case approaching the dacitic type. They lie within the borders of the area of basic tuffs, basic agglomerates, and basic massive rocks, that here begins and extends eastward to Mount Thurston and a little beyond. East of Na Raro there is a gap or break in the profile, where the greatest elevation is probably not over 800 feet; and on its farther side rises up the mountain of Va Lili (2,930 feet), a lofty inland ridge that lies towards the southern coast. Palagonite-tuffs and agglomerates are the prevailing surface-formations in this district.

Eastwards from Va Lili extends for eight or nine miles a lofty, level-topped, and almost peakless range, which I have called the Korotini Table-land, after the towns once situated on its southern slopes. Its outline is shown in the background of the view facing page 153. It is, however, not so level-topped as it appears to be; but the gradual variations in elevation between 2,000 and 3,000 feet, when spread over a length of some miles, are more or less lost in the general outline of the range as viewed from the coast. Basic agglomerates are principally exposed on the lower slopes; whilst higher up, reaching often to the summit of the table-land, occur palagonite-tuffs containing tests of foraminifera and molluscan shells, massive basic rocks being exposed in places.

The level profile of the Korotini tableland gives place, as one proceeds eastward, to the broken outline of the several lofty peaks of Mariko (2,890 feet), Mbatini (3,437 feet), Thambeyu (3,124 feet) and others.[4] Each of these peaks marks one of the bold mountain-ridges that form such a striking feature in the surface-configuration of this part of the island. On the slopes of these ridges, and often also on their summits, appear basic agglomerates and palagonitic tuffs and clays often inclosing tests of foraminifera; whilst exposed in the gorges and protruding at times through the tuffs and agglomerates on the crests of the ridges are displayed massive basic rocks of the type of the hypersthene-augite andesites.

East of Thambeyu the level sinks to about 1,000 feet above the sea, and beyond rises an irregular group of hills and mountains which attain their greatest height in Nailotha,[5] 2,481 feet above the sea. We are now near the limit of the area of basic rocks. Following the profile as it slopes away, marked by occasional peaks and breaks, towards Undu Point, we pass at first over a district where basic rocks are mixed with those of more acid type; but before we reach Mount Thuku we enter the district of oligoclase-trachytes, quartz-porphyries, and rhyolitic tuffs, that extends to the extremity of the Undu promontory.

There remains to be noticed the profile of the Natewa Peninsula. As shown in the diagram, this level begins at a few feet above the sea in the vicinity of the Salt Lake; and as it proceeds eastward it attains a level of 1,960 feet in Ngalau-levu and of 1,540 feet in the Waikawa promontory, finally culminating, as it nears Kumbulau Point, in a mountainous district which attains its greatest elevation of 2,740 feet above the sea in the lofty ridge of Ngala, the Mount Freeland of the chart. Altered basic rocks prevail in this peninsula; but more acid andesites also occur, and foraminiferous tuffs and clays are exposed on the slopes, reaching to over 1,000 feet above the sea.

I will conclude this reference to the profile of the island with the remark that if I had neglected to indicate here the close connection that exists between the nature of the surface-configuration and the character of the prevailing rocks I should have ignored a means of investigation which has proved of the greatest value. The rock and surface characters go together. The inland plateau now upheaved 1,000 feet above the sea, was built up by submarine flows of basaltic lava. The isolated conical peak that so unexpectedly intrudes itself into the view is the dacitic core of some submarine volcano long since stripped of most of its fragmental coverings. The lofty mountain-ridges that run athwart the island’s breadth, with their summits usually in the rain-clouds received their coverings of tuffs and agglomerates ages ago when they were submerged; and now they rise to heights of over 3,000 feet above the sea. Bound up with the mysterious origin of these great ridges is the history of the island of Vanua Levu.

These preliminary remarks are only intended to serve as a general introduction to the detailed description of the island and its formations. The closing chapter is devoted to a summary of the principal results of my investigations.