On the south-eastern slopes of the mountain between Ndawathumi (inland) and Korolevu (at the coast), somewhat similar basalts with scanty olivine are exposed (sp. gr. 2·86-2·91). Some of them show the felspar-lathes of the groundmass arranged in a plexus (genus 25), whilst others exhibit flow-structure (genus 37), the average length of the lathes varying in different localities between ·15 and ·21 mm. All display scanty residual glass. On the shores of Wainunu Bay between the Wainunu and Korolevu rivers occur porphyritic basaltic andesites with a considerable amount of glass in the groundmass. There is exposed on the right side of the mouth of the last-named river a highly basic variety of olivine-basalt with a specific gravity of 3·07. It is referred to genus 15 (described on page [258]), which includes the most basic rocks in my collection. There are in this rock no plagioclase phenocrysts and the felspar-lathes of the groundmass are relatively infrequent, whilst olivine and augite occur in abundance. There is little or no residual glass. In the district of Tongalevu blackish olivine-basalts and basaltic andesites of the usual character are found. In the Na Suva range, which lies two miles inland from the shores of Nasawana Bay and forms the southerly extension of the mountain, a somewhat compact variety of olivine-basalt (sp. gr. 2·92) prevails up to the summit, 1,550 feet above the sea. It is included in genus 37 of the olivine-basalts. In the length of the felspar-lathes (·15 mm.) it belongs to the Seatura type of these dark basalts.
(b) The Western Slopes of Seatura.—Here overlooking the plains north of the Ndama River the same olivine-basalts and porphyritic basaltic andesites occur. The vegetation is of the scanty “talasinga” character, and since there is little or no soil-cap the disintegration of the rocks has been very great, often extending to a depth of 10 or 12 feet. It is remarkable that this disintegration is most marked in the “talasinga” and similar scantily wooded districts of the mountain. On the densely wooded eastern and southern sides where there is a thick soil-cap, it is by no means so evident. Here on the western slopes have been carved out deep broad valleys and lofty spurs, the last in their turn furrowed on their flanks, without any apparent sufficient cause. The shallow streams at the bottom of the valleys appear quite incompetent to produce such great erosion; and doubtless these results are partly due to the action on the crumbling rock-surface of temporary torrents formed during the rains.
(c) The Northern Slopes of Seatura.—Here within the scantily vegetated “talasinga” region the conformation of the land is well displayed. Broad, deep and nearly parallel valleys, separated by level-topped spurs and occupied by the Lekutu and its tributaries, score the mountain’s slopes. The prevailing rocks are blackish-brown olivine-basalts and porphyritic basaltic andesites, such as occur around the other parts of Seatura; but grey olivine-basalts also occur, possessing opaque plagioclase-phenocrysts and looking like porphyrites. They are essentially holocrystalline and are probably more deeply situated than the other basaltic rocks. They are referred to genera 26 and 38 described on pages [261], [263], and have a specific gravity of 2·75-2·83. Dark doleritic basalts distinct from all the others are exposed in places.
A good idea of this region may be obtained by following the road westward from Tavua on the head-waters of the Sarawanga River to Wailevu on the westernmost tributary of the Lekutu River, a distance of about 6 miles. Leaving Tavua one at once begins to ascend and cross the long spur that descends from Seatura and divides the valleys of these two river-systems. On its slopes are exposed much decomposed blackish basalts possessing scanty olivine and showing large porphyritic crystals of plagioclase. They have a specific gravity of 2·84 and are assigned to the porphyritic sub-genus of genus 25 (page [259]). At the summit, 800 feet above the sea, occur blocks of a grey holocrystalline basalt with scanty olivine and semi-opaque plagioclase-phenocrysts referred to genus 26 and having a specific gravity of 2·76. It appears to form the axis of the spur. Descending to the main Lekutu River, just below Kavula, where the elevation is about 300 feet above the sea, one observes exposed in mass in the river-bed a dark semi-ophitic doleritic basalt similar to the doleritic rocks without olivine prevailing on the coast between Wailea Bay and Lekutu (see page [50]), but differing in the absence of felspar-phenocrysts. It displays a considerable amount of opaque interstitial glass and is assigned to genus 12 of the augite-andesites (page [275]). The specific gravity is 2·78, but there are a few minute irregular cavities in its substance.
On leaving Kavula one crosses another of the Seatura spurs at a level of 650 feet, descending then into a smaller river-valley occupied by a tributary of the Lekutu, on the banks of which lies the village of Nawai, 350 feet above the sea. Then another spur is crossed at an elevation of 450 feet and the descent is made into the valley of the Wailevu tributary of the Lekutu. Crossing the valley, which at the town of Wailevu is elevated 300 feet, one rises to a height of 700 feet and then descends into the Mbua plains. These three almost parallel valleys of the Lekutu and its two tributaries are worthy of a detailed examination.
The rocks on the surface between Kavula and Wailevu vary in character. Nearer Kavula there appears a blackish compact olivine-basalt (spec. grav. 2·88), showing a little microporphyritic plagioclase and belonging to genus 37 of the olivine rocks. Further on is exposed one of the holocrystalline grey olivine-basalts with porphyritic plagioclase-phenocrysts and specific gravity 2·83. It belongs to the type described in genus 38 of the rocks on page [263]. Nearer Wailevu there occurs a blackish porphyritic basalt with scanty olivine and specific gravity 2·81. It contains but little residual glass and is referred to the porphyritic sub-genus of genus 25. In some cliffs at the river-side close to Wailevu, there is displayed a semi-vitreous basaltic andesite, showing large porphyritic plagioclase crystals, 3 to 8 mm. Its low specific gravity (2·68) is to be attributed to the large amount of glass in the groundmass. There is a loose mesh-work of felspar-lathes, but the augite is not differentiated. Westward of Wailevu commence the decomposing basaltic rocks of the Mbua plains.
(d) Traverse of the Northern Part of the Summit of Seatura from Kavula South-West to Narawai.—The track first lay up the picturesque valley of the Lekutu River to Nandroro, 2½ miles distant and 800 feet above the sea. On the way blackish basaltic rocks of the prevailing Seatura type, with or without scanty olivine, were displayed often in a decomposing condition. At one place a characteristic grey olivine-basalt, showing opaque porphyritic plagioclase (sp. gr. 2·87), and looking like a porphyrite, was exposed. On account of the abundance of the olivine, it is placed in genus 2 of the olivine-rocks. After Nandroro the path lay up the steep mountain-side to a height of 1,500 feet: and afterwards across the summit of the northern part of Seatura, which is here about two miles in breadth. This elevated region is well wooded with here and there a patch of “talasinga” land; but it is by no means level, its elevation varying between 1,400 and 1,800 feet, and it soon became evident that we were crossing the heads of valleys, sometimes 200 or 300 feet in depth, that could only have been excavated by the torrential rains. These streamless valleys afford another indication of the denudation to which this ancient mountain has been subjected.
The rocks prevailing in this elevated northern portion of Seatura, at heights of 1,500 to 1,800 feet above the sea, are: (a) blackish basalts with scanty olivine, a little interstitial glass, and belonging to the porphyritic and non-porphyritic sub-genera of genus 25 of the olivine-rocks: (b) grey olivine-basalts with porphyritic opaque plagioclase, containing but little residual glass, but varying greatly in the amount of olivine and belonging to the genera 2 and 26 of the olivine-basalts; they would be classed, as far as appearance goes, as porphyrites; their specific gravity ranges 2·85 to 2·90. The rock exposures were, however, scanty; and but little information could be obtained of the mode of occurrence. No scoriaceous rocks were found except in the instance of a compact dark basalt without plagioclase phenocrysts, apparently a dyke rock, and belonging to genus 40 of the olivine-basalts.
(e) Ascent to the Summit of Seatura from Ndriti.—The town of Ndriti lies in the great gap in the south-west side of the mountain which has been previously mentioned as probably an old crateral cavity. After traversing a district of highly altered basic rocks or propylites, to be subsequently described, and reaching an elevation of about 400 feet above the sea, I came to the long slope that leads up to the summit. A dense forest hid everything from view, so that the compass and aneroid had alone to be relied on.
At first one traversed a series of step-like alternations of level ground and steep “rises,” until the old site of the village of Seatura, about 1,200 feet above the sea, was reached. There are some strange legends connected with this old mountain-village, which is now only indicated by little piles of stones and the debris of a wall, and was evidently abandoned long ago. We finally reached the summit by following up a spur or ridge in a northerly direction from Seatura. There was a precipitous descent on either side of the ridge with evidently a broad, deep valley to the eastward. The summit was rounded; but on account of the forest no view could be obtained. There was never any extensive exposure of rock noticed during the ascent; but all the way up occasional small blocks of a blackish olivine-basalt were observed on the surface, of the same general type as that found all around the mountain and referred to genus 37 in the synopsis.