CHAPTER XI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Koro-tini Range or Table-land
The level-topped range that forms the mountainous backbone of the island for a distance of nearly 10 miles is one of the remarkable features of Vanua Levu.[[66]] In the general profile of the island it is named the Koro-tini Table-land on account of the level profile which it presents whether viewed from the north or from the south. But this is merely its appearance en masse. When it is examined in detail it is found that although much of the range has an elevation between 2,000 and 2,400 feet above the sea, it attains an elevation of about 3,000 feet in the case of two gently sloping peaks. With regard also to its table-top, it is necessary to remark that whilst in some portions of the range the summit is broad and level, in others it is much cut up into ridges, and in others again it presents a single narrow crest. Nor can we realise on looking at the profile the extent to which its slopes have been carved out by river-erosion, and we get no indication of the several lofty spurs that descend north and south far into the plains, as in the case of the spur west of Sueni and in that terminating in the Koro-tini Bluff. In the profile the eye ignores the details with which the investigator during many toilsome ascents has filled the pages of his note-books. To this extent it is useful in that it enables him to rise a little above the level of his facts, and permits him (to employ a figure-of-speech) to regard the style and general character of the edifice without being exclusively absorbed in the study of the bricks.
This range, which extends from a mile or two west of Sealevu to a couple of miles east of Sueni, is connected on the west with the Va-lili Range by the Waisali Saddle before described, and on the east with the Thambeyu or Mount Thurston Range by a broken chain of mountains, of which Koro-mbasanga is the most conspicuous. It is connected by an elevated col with Mount Mbatini and the Mariko Range to the southward. The name of Koro-tini has been applied to this range because it is familiar to the natives. It signifies “ten towns,” and was given to a once populous district on the slopes of the lofty bluff overlooking on the north the mouth of the Ndreke-ni-wai. I crossed the range in four places, namely, between Waisali and Sealevu, between Mbale-mbale and Vandrani, between Vatu-kawa and Vandrani, and between Nukumbolo and Sueni.
(1) Traverse of the Koro-tini Range from Waisali to Sealevu.—Starting from Waisali by the Narengali track, I ascended the east slope of the Waisali Saddle, as described on page [146], until an elevation of about 750 feet was reached, when my way lay to the northward across the Koro-tini Range to Sealevu. At 850 feet a singular altered tuff was displayed in position in a stream-course. It shows calcite and pyrites, and is interesting from the fact that although it is made up largely of basic glass the tuff does not seem to have undergone the palagonitic change.
Afterwards, there was a fairly steep ascent to the summit of the range, 2,400 feet above the sea, which has merely a ridge-like crest. Between an elevation of 1,400 feet and the top there are exposed at the surface compacted coarse and fine palagonite-tuffs and agglomerate-tuffs formed of the same materials. They contain often abundant organic remains, such as valves of “Cardium” and “Pecten” shells, macroscopic tests of Foraminifera, and some curious scale-like bodies, showing a concentric structure and about an inch across, which look like fish-scales. It is probable that these interesting rocks extend to a greater elevation than 2,400 feet, which was merely the highest level reached in the traverse, but is not the highest point of the range.
These deposits are made up in mass of a more or less palagonitised basic glass originally containing phenocrysts of plagioclase and pyroxene. The palagonitic process is nearly always far advanced; but it is seen in all its stages, the least altered materials fusing under the blow-pipe into a black glass. The fragments are usually sub-angular in the case of the coarse tuffs; but small rounded pebbles up to half an inch in size and fine water-worn gravel are not infrequent. The matrix is composed of palagonitic debris, portions of crystals of plagioclase and pyroxene, fine gravel, occasional tests of foraminifera; and it often contains a fair amount of carbonate of lime, in one specimen tested as much as 13 per cent. The amount of lime, however, varies, being in some places scanty.
The term “conglomerate” could not be applied to the coarser deposits, since the sub-angular and angular fragments always predominate. They could scarcely be deemed “breccias” on account of the mixture with pebbles and gravel. Their character is therefore intermediate between the two. I have used the expression “agglomerate-tuff” because it best describes their appearance. A specimen of such a rock presents a curious mixture, in the well-compacted mass, of angular and sub-rounded fragments of palagonite up to an inch in size, small pebbles and fine gravel of the same material, and detached valves of “Cardium,” entire and broken. One is forced to draw the inference that these materials accumulated in shallow water. They are such as might have been produced by the marine erosion of an emerging volcanic island endeavouring to hold its own above the waves. But from the occasional occurrence of blocks of a scoriaceous basaltic rock it would appear that during the formation of the deposits there were periods of eruption.
At times massive and comparatively fresh-looking basaltic rocks are exposed in situ on the mountain sides in the midst of these submarine deposits. A specimen obtained at 1,800 feet is a semi-ophitic porphyritic olivine-basalt with a specific gravity of 2·86 and showing a little interstitial glass. The mode of exposure did not admit of my ascertaining the exact relation of these rocks to the deposits. They are no doubt dyke-like masses representing the original fissures of eruption of a submarine vent; and during the emergence they were covered up with tuffs and deposits, the work of the marine erosion of the emerging land. These, however, are points on which light will be thrown when we come to examine other localities.
Descending the northern slopes of the range from the summit to Sealevu the general course was N.N.E. Several valleys were crossed, of which that occupied by the Na Sinu river was 600 feet in depth, the rivers and streams all flowing to the north-west into the Ndreketi basin. Basic tuffs and agglomerates were exposed at the surface all the way down to Sealevu, 400 feet above the sea.