In the profile of the island attached to this work the Seatura slope is well shown; but that of the Wainunu table-land being seen from the south is represented only by a level contour-line at the base of the Ndrandramea mountains. The two great series of basaltic flows, though closely approaching in a direction at right angles to each other, do not come into actual contact, and the intervening space is now occupied by the valley of the Wainunu River. In the accompanying rude outline-sketch of this region, as seen from off the mouth of the Wainunu estuary, the relation of this valley to the two great series of basaltic flows is clearly shown. On the left is the foot of the Seatura basaltic slope; on the right is the Wainunu basaltic table-land; and between them lie the estuary and valley of the Wainunu, at the back of which appears the “Na Savu” table-land, formed of basic tuffs and agglomerates. Behind all there rise up suddenly the Ndrandramea mountains formed of acid andesites; whilst in the foreground to the right is the hill of Ulu-i-ndali, which is composed in the mass of a grey basalt of a type quite different from the blackish basaltic rocks of the Seatura slope and of the Wainunu table-land. It was from this view off the mouth of the estuary that I received my first lesson in studying the structural formation of the island. I kept it always in my mind’s eye, and for months in an almost unmapped region it was my only guide.
Profile, looking north from off the mouth of the Wainunu River.
The gradual slope of the Wainunu table-land from an elevation of 1,100 or 1,200 feet in the interior to 700 or 800 feet near the coast has already been referred to. Beyond this lower limit it descends much more rapidly and within less than a mile it terminates at Masusu in a steep-sided declivity 300 feet high opposite Ulu-i-ndali, and in a gentler slope on the eastern side in the Ndranimako district. Its somewhat undulating surface is well wooded; but on account of the small gradient the small streams on the table-land do not excavate deep channels, but flow slowly along in shallow courses and often stagnate in swampy land where the interesting “Scirpodendron costatum,” the giant-sedge, flourishes. In their beds occur reddish flinty concretions, up to 3 inches across in size, and magnetic iron sand in great abundance. A sample of this sand roughly washed on the spot contains 77 per cent. of magnetic iron.[[44]]
Basaltic rocks, often exhibiting a columnar structure, are exposed at intervals on the surface and slopes of this table-land all over its area. Now and then when traversing this region one comes upon a tract strewn with large blocks, amongst which occur fragments of huge columns 3 to 4 feet in diameter; but it is on the steep southern slopes of the plateau in the vicinity of Ndavutu and Masusu that the most extensive exposures of columnar basalt are to be found. Here there have been large clearings made for the tea-plantations, and portions of columns 2 to 3 feet in thickness are scattered all over the slopes and surface of Masusu.
A very interesting exposure occurs on the southern edge of the Masusu flat facing Ulu-i-ndali. Here there is displayed in the face of a waterfall a mass of basalt about 40 feet deep, formed of regular cross-jointed columns, 3 to 4 feet in diameter and often pentagonal in shape, which are almost perpendicular, being inclined about five degrees from the vertical. But in the upper portion of the fall the columns are smaller (2 to 3 feet across) and become arched and nearly horizontal. This was the only section of the inner mass of the basaltic flows that I found, and here the columns are almost vertical. In this locality several other exposures of the columnar basalt occur; but they are all at the surface and the columns are nearly horizontal or very much inclined from the vertical, being often pentagonal in form, 2 to 3 feet across, and sometimes curved with joints 10 to 20 feet in length.
Neither vesicular nor scoriaceous rocks came under my notice in this region, and the presence of pteropod-ooze deposits and of foraminiferous clays and tuffs on the slopes of the basaltic tableland indicates that the flows were submarine. The common character of a sub-aërial basaltic flow, where there are large vertical columns below and smaller radiating columns above, did not present itself; and it is probable that the singular arrangement of the columns in the upper portion of these flows may be connected with the conditions of depth under which the flows took place.
It is apparent from the description given by Dana of the columnar basalt of Tahiti[[45]] that it was formed under different conditions from those under which the basaltic flows of Wainunu and Seatura were formed. The columns composing a cliff 500 feet high in the Matavai valley were 10 to 20 inches across. A bluff, 200 to 300 feet high, in another part of the valley, was made up of columns 5 to 8 inches in width. The tallest cliff displayed in places converging and curved columns, which is attributed to the unequal cooling of the interior of the mass; but it is evident from a diagram given by the author that the columns were not inclined at a large angle from the perpendicular.[[46]] He also refers to some prisms of a grey basalt exposed just below the Wailuku Falls near Hilo in the large island of Hawaii which were 8 feet in diameter and were surmounted by others only 1 to 4 feet across.
The basalts of the Wainunu table-land are blackish and non-vesicular, with a density of 2·87 to 2·90. They all carry olivine and microporphyritic plagioclase, and display a little interstitial glass, and the felspar-lathes are usually in plexus-arrangement, being stout and often showing twin lamellæ. But the rocks exhibit important variations in different localities as regards the amount of olivine, the length of the felspar-lathes, the presence or absence of the ophitic character, &c., and they are grouped in different genera of the olivine class (1, 13, 25, 33). Probably the type of genus 25, with scanty olivine and granular augite, would prevail.
From the varying size of the felspars of the groundmass it is apparent that the flows are not all of the same character. At Masusu, where the rock is doleritic in texture, they average from ·25 to ·3 mm. in length. A mile further north, they are about ·17 mm. long, and two miles more to the north they average only ·1 mm. in length. It is probable that a semi-vitreous basaltic andesite (spec. grav. 2·73), that shows no olivine and is referred to the porphyritic sub-genus of genus 9 of the augite-andesites, which is exposed in the stream-courses near the base of the dacitic mountains of the interior, is the product of a later eruption. Occasionally one finds, as at Thongea in the Wainunu valley, a basalt rich in olivine (spec. grav. 2·95), the felspars of the base averaging ·1 mm. in length. It may be remarked here that one cannot draw a sharp distinction between the basalts of this region and those of the adjacent eastern slope of Seatura. Their specific gravity is about the same (2·87 to 2·90); but the coarse texture of the Masusu basalts did not come under my notice in the last locality, where the felspars of the groundmass average ·18 mm. in length or about two-thirds the length of those of the Masusu rocks.