The Basaltic Lowlands of Sarawanga and Ndreketi.
One of the most striking features of the north side of the island is the extensive undulating plain that stretches from the Lekutu river to near Sealevu on the head-waters of the Ndreketi, a distance of almost 30 miles. In its western half this plain slopes gradually to the sea-coast, where it is bordered by a broad belt of mangroves. In its eastern half, from the mouth of the Ndreketi eastward, the lofty Nawavi coast range intervenes between it and the sea-shore. Its breadth varies usually between 4 and 6 miles, and its elevation, though it reaches a maximum of about 300 feet, is as a rule between 100 and 200 feet above the sea.
Over nearly all its area it presents the dried-up and scantily vegetated appearance of the “talasinga” regions. It is an open country mostly clear of forest; and it is to this character as well as to its peculiar vegetation that it in some measure owes its barren look. Amongst the bracken, grass, and tall reeds (Eulalia japonica) that clothe much of its surface flourish the Pandanus, the Casuarina, and the Cycad, which give a special physiognomy to the whole area; whilst several sea-side plants, as Ipomea pes capræ, Morinda citrifolia, Cerbera odollam, &c., have spread themselves far and wide over its extent. It is traversed by the rivers Ndreketi, Sarawanga, and Lekutu, the two first named being navigable for several miles, as the tide ascends a long way from the coast.
In its essential characters this region corresponds with the Mbua and Ndama plains at the west end of the island, which have been previously described. Wherever the rivers have worn channels of any depth, basaltic rocks, sometimes columnar in structure, are exposed; and over most of its surface the same rocks are displayed, often much decomposed and developing a spheroidal character, or lying in large blocks all around. Overlying the basaltic rocks in various localities occur foraminiferous clays and other submarine deposits. This great region of plains is partially divided into two by the projecting mass of the dacitic district of Ndrandramea, the slopes of which descend to within 3 or 4 miles of the coast between the Sarawanga and Ndreketi rivers. For convenience of description I will deal with these two sub-regions separately under the names of the Sarawanga and Ndreketi plains.
The Basaltic Plains of Sarawanga.— These plains extend about 6 miles inland to the village of Tembe-ni-ndio on the head-waters of the Sarawanga river. The prevailing type of basalt in this region is a porphyritic olivine-basalt showing a few large crystals of glassy plagioclase and having a specific gravity of 2·84 to 2·9. They are neither vesicular nor scoriaceous and are referred to genera 25 and 37 of the olivine class. The felspar-lathes of the groundmass average ·2 mm. in length, and there is a little interstitial glass. They cannot often be distinguished in their characters from the olivine-basalt displayed in vertical columns, 4 to 5 feet in diameter, on the lower slopes of Seatura at the back of Tembe-ni-ndio (page [63]). It is highly probable that most of the basalts of these plains belong to lava-flows that descended from the great Seatura vent. In the lowlands it is much decomposed, and a spheroidal structure is frequently developed during the disintegrating process, just as has been noticed in the case of the Mbua and Ndama plains on the west side of Seatura. The rounded blocks that commonly occur on the surface may be regarded in each instance as the nucleus of a weathering spheroidal mass. When this rock is exposed unaltered in the streams it is usually massive or non-columnar.
There is a less common type of basalt in this region which perhaps may represent the upper portion of these basaltic flows. I found it exposed in the bed of the Selesele river about half-way between Lekutu and Sarawanga and about 2 miles inland, where it formed vertical columns 1½ feet across. It differs principally in the presence of a few small amygdules and in the greater amount of interstitial glass. The columnar basalt that Dana in the “Geology of the United States Exploring Expedition” describes as occurring at the mouth of the same river probably belongs to the same flow. He remarks that a few hundred yards back from the “Watering-place” there is an exposure of columnar basalt, the columns being vertical, 1 to 2½ feet in diameter, and usually six-sided.
The incrusting submarine deposits found in patches over these plains are generally calcareous clay-rocks containing tests of formaminifera and often also univalve, bivalve, and pteropod shells. They are referred to the foraminiferous mud-rocks described on page [321]. Such deposits are properly dark-coloured; but as exposed at and near the surface they have often lost by hydration most of their lime, and have acquired by the removal of the iron oxides a whitish or pale-yellow appearance, whilst they have a peculiar soapy “feel,” on account of which they are generally known as “soapstone” amongst the whites. Streams flowing through such districts have a somewhat milky colour. These deposits are extensively represented on the slopes of the Sarawanga valley, and especially to the east of the town of that name. They are well displayed on the way from Sarawanga to Tembe-ni-ndio, and are also to be seen on the surface of the plains between Lekutu and the Mbua-Lekutu watershed to the southward.
In the vicinity of Sarawanga they attain an elevation of 200 feet above the sea; but they may be traced in patches up to 500 feet on the adjacent slopes of the acid andesite region of Ndrandramea. Near the river, and less than 100 feet above the sea, these deposits are in one place overlain by an agglomerate formed of large blocks, 1 to 2 feet across, of these Ndrandramea andesites and dacites. In another place, near the town of Sarawanga, I found them exposed in the river-bank, where they were covered over by a coarse palagonitic bedded tuff, dipping gently eastward and somewhat calcareous. From the character of the shells of marine univalves inclosed in this tuff, it appears to have been formed in shallow water.
A very interesting display of these surface marine deposits occurs in the upper part of the Sarawanga valley in the vicinity of Tembe-ni-ndio. Here we have fine and coarse calcareous palagonitic tuffs, containing tests of foraminifera, associated with impure foraminiferal limestones. They occur up to elevations of 300 feet above the sea on either side of the Sarawanga valley above this town, incrusting on the north side the lower dacitic slopes of the Ndrandramea district, and on the south side the lower basaltic slopes of Seatura. At the bottom of the valley, as in the rising ground between Tavua and Tembe-ni-ndio, they conceal in part the basaltic rocks of the district.
Near the last-named place, on the right bank of the Tembe-ni-ndio branch of the Sarawanga river, the foraminiferal limestones are displayed in low cliffs 15 to 20 feet in height. They are sometimes earthy when they contain about 25 per cent. of lime, and at other times more compact with about 45 per cent. of lime, the residue being composed of palagonitic materials, tiny fragments of minerals and of a basic rock, &c.[[59]] Large shells, of Ostræa and Cardium are also contained in these limestones, the valves being detached from each other. The oyster shells project from the weathered surface; and it is probable that the name of Tembe-ni-ndio, which signifies “the shell of the oyster,” may be thus explained. Underneath the foraminiferal limestones in this locality occur bedded coarse tufaceous sandstones, slightly inclined E.N.E., and inclosing waterworn gravel and pebbles. These low limestone cliffs, although about six miles inland, are not more than 120 or 130 feet above the sea. In their face there is evidence of an old erosion-line of the river 10 or 11 feet above its present level.
By following up this branch of the river for a little distance I came upon an exposure of nearly horizontal bedded palagonitic tuffs on its floor and sides. Here a coarse tuff, of which the larger fragments composing it range between 3 and 5 mm. in size, passes upward into a chocolate-coloured compact tuff-clay formed of the same materials, the larger averaging ·2 or ·3 mm. in size. These tuffs are made up chiefly of a palagonitised vacuolar basic glass, the vacuoles being filled with the alteration products. The lower coarse tuffs contain very little lime, probably not over 1 per cent., and exhibit no organic remains in the slide. The upper fine tuffs have 3 or 4 per cent. of lime, and inclose numerous minute tests of foraminifera of the globigerina type, their cavities being generally filled with palagonitic material.
Further up the valley about a mile above Tembe-ni-ndio, and about 250 feet above the sea, the impure foraminiferal limestones again appear; but they here exhibit an important difference in texture. In the groundmass of those of the lower locality, the calcite is granular and loosely arranged, or displays in an obscurely indicated mosaic the commencement of recrystallization. In the case of those of the upper locality the calcitic material of the groundmass has more completely recrystallized, and shows a fairly clear mosaic; whilst in one place the rock was overlain or rather incrusted above by a layer, 3 inches thick, of a white crystalline limestone, looking like statuary marble, and inclosing portions of a material like that of the rock beneath it. This last, when examined in the slide, exhibits itself as formed in mass of crystalline calcite, displaying a regular mosaic, and inclosing small fragments of palagonitised materials and of minerals (pyroxene) such as are abundant in the rock below. In places the grains of the mosaic are bordered by brown and black iron oxide. It would, therefore, appear that a metamorphism has been in operation here, and that the process which began with the recrystallization of the matrix in the lower rock is almost completed in the overlying thin layer where even most of the non-calcareous materials have disappeared. No evidence suggestive of contact-metamorphism came under my notice in this locality. These foraminiferal limestones are surface formations, and it was in the uppermost portion that the metamorphism was most complete. We here witness in operation the transformation of a rock containing 46 per cent. of carbonate of lime (the residue of minerals, palagonite, &c.), into a marble or crystalline limestone. I gather that as in the instance of several of our old British limestones the change is a purely interstitial one, and is not connected with thermal metamorphism.
These remarks on the basaltic plains of Sarawanga and on their incrusting submarine deposits may be concluded with a brief reference to the siliceous concretions, 2 or 3 inches across, the silicified portions of corals, and the fragments of clay iron-stone and limonite resembling hæmatite, that occur frequently on the surface. They are common on the plains south of Lekutu and between Lekutu and Sarawanga, and up to elevations of 200 feet in the foraminiferous clay district east of Sarawanga, where fragments looking like portions of the silicified branches of Madrepores are to be found; but they are not limited to such localities, and may occur also where the surface is formed of decomposed basaltic rock. (These matters are generally discussed in [Chapter XXV].)
The Basaltic Plains of the Ndreketi.—This low-lying region of rolling “talasinga” country now serves as the basin of the Ndreketi river, the largest of the rivers of Vanua Levu. It is usually elevated between 100 and 300 feet above the sea, and its limits are well defined by the 300 feet contour line in the map of the island. On the east it is separated from the basin common to the Wailevu and Lambasa rivers by the Sealevu Divide, which is described on p. [136]. On the west, as before observed, it is only in part distinguished from the basin of the Sarawanga by the spur descending from the dacitic mountains of Ndrandramea. It meets the coast in the vicinity of the mouth of the Ndreketi; but for two-thirds of its length it is cut off from the sea by the great Nawavi range. It supports the characteristic vegetation of the “talasinga” or sun-burnt land. Whilst the Pandanus and the Casuarina are most conspicuous amongst the trees, bushes, herbs, grasses and ferns predominate. Here the native Ginger and the native Turmeric with species of Tacca are frequently to be recognised, and the waste-land bushes of Dodonæa viscosa and Mussænda frondosa are abundantly to be found.
As in the Sarawanga plains, the basaltic rocks are here often overlain or incrusted by submarine deposits, the former exposed in all the deeper river-beds, the latter frequently displayed in the sides of their tributaries.
I will deal first with the basaltic rocks. In the places where the surface deposits have been stripped off, these rocks are generally exposed as decomposing boulders, the spheroidal structure being well developed in the weathering process. Not infrequently, however, a rudely columnar structure is exhibited where the rivers have cut deeply into the basalt. The columns that I observed were usually vertical. In the river-bed at the landing-place at Mbatiri, for instance, the columns are from 2½ to 3 feet across and vertical. As exposed in the river-crossing about a mile above this town they are 12 to 15 inches in diameter and also vertical. However, at Na Kalou, a coast village about 1½ miles east of the mouth of the Ndreketi, where there is an unexpected exposure of basalt, the columns, about a foot in diameter, are inclined at an angle of about 20° from the vertical and face to the north.
These rocks are, as a rule, compact, only showing a typical scoriaceous structure in the case of specimens obtained near the foot of Nakambuta, an isolated hill about three miles to the southward of Natua, which probably represents a vent of more recent times. Often, however, they have a pseudo-vesicular appearance, from the occurrence in the midst of the patches of interstitial glass of minute irregular cavities that seem to have been formed during the last stage of consolidation of the magma.
The prevailing type of basalts is a blackish, doleritic, semi-ophitic rock without olivine, with specific gravity 2·78 to 2·80. They are characterised by the length of the felspars of the groundmass (·22-·35 mm.), by the large size of the augite granules (·1-·3 mm.), and by the quantity of dark interstitial glass. They present two forms, one with and the other without plagioclase phenocrysts. The first kind is referred to genus 9 of the augite-andesites (page [272]), some of the specimens being referred to the porphyritic sub-genus, and others to the non-porphyritic sub-genus, according to the size of the plagioclase phenocrysts. The second kind, without felspar phenocrysts, belongs to genus 12 of the same class (page [275]). A good example of the porphyritic rocks is afforded in the large blocks lying in the stream-beds during the first half of the way from Ndreketi to Sarawanga.
It may be pointed out here that these doleritic, semi-ophitic basaltic andesites of the Ndreketi plains differ conspicuously from the prevailing type found on the slopes of Seatura, on the Sarawanga and Mbua plains, and on the Wainunu table-land. There we have, as a rule, olivine-basalts, having a specific gravity of 2·86 to 2·90, and showing but scanty interstitial glass, the felspars of the groundmass being on the average not over ·2 mm. in length, whilst the augite granules are, as a rule, only ·02-·03 mm. in diameter, and the ophitic structure is infrequent.
The submarine deposits, consisting of foraminiferous clays and coarser tuff-sandstones, the former being usually beneath, are found at intervals all over this area. They occur inland as far as Vuinasanga and Nareilangi, near the base of the mountains of Va Lili and Na Raro, reaching as high as 300 feet, their place being taken on the mountain slopes by coarser tuffs and agglomerates. When not weathered they are more or less calcareous, and contain occasionally marine molluscan shells, whilst palagonitic debris enter largely into their composition. The foraminiferous clays, often much bleached by hydration, are well represented around Mbatiri and in the districts between that town and Natua and Nareilangi. They are relatively deep-water deposits, and belong to the type described on page [323]. Others, again, as exposed in the banks of the river at Natua, are chocolate coloured and of the kind referred to in detail on page [335]. These foraminiferous clays in the region between Natua and Mbatiri are overlain in places by coarse, almost brecciated, tuffs, formed in part of the debris of acid andesites, such as compose the not far distant mountain of Na Raro.
Since the massive basaltic rocks are exposed in all the deeper rock channels of these plains, it is apparent that the overlying submarine deposits can possess no great thickness. Probably they are never 100 feet thick, and usually far less. In many places, through their denudation, the underlying basaltic rocks are exposed, and in a decomposing condition largely form the surface. These deposits as a rule display bedding, the beds being horizontal or at least only inclined 2 or 3 degrees. This horizontality is a nearly constant feature of these submarine beds, as they overlie the basaltic rocks of the plains; and it is a feature we should expect to find where there has been emergence rather than upheaval.
Siliceous concretions and silicified coral fragments, so characteristic of the surface of some of these plains of Vanua Levu, did not frequently come under my notice here. They, however, occur occasionally, as in the district between Nanduri and Natua.