CHAPTER XVI

DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES (_continued_)

THE NORTH-EAST PORTION OF THE ISLAND (_continued_)

THE WAINIKORO AND KALIKOSO PLAINS

THESE extensive inland plains occupy a considerable area in this part of the island. I estimate that there is an area of about 20 square miles that does not exceed an elevation of 100 feet above the sea and is often much less. Of the two villages situated in the midst of these plains, about 5 miles inland, Kalikoso is about 30 feet and Wainikoro is scarcely 20 feet above the sea; whilst much of the surrounding district is similarly low. Taking the 300-feet contour-line as a guide, this low-lying region, as shown in the map, would be much larger. The plains are backed on the south by the high mountain-range of Vungalei and Nailotha. On the north the coast-ranges, which attain a height of 1,100 or 1,200 feet, separate them from the sea-border; whilst on the west they are shut off from the Mbuthai-sau valley by a line of hills, of which the minimum elevation is about 700 feet. This region is occupied by the scanty open vegetation characteristic of the “talasinga” or “sun-burnt” districts. The tall “Ngasau” reed is common; the Pandanus trees are frequent; and amongst the bushes and small trees are represented “Dodonæa viscosa,” “Morinda citrifolia,” and a species of “Hibbertia.”

Different rivers and streams, rising in the mountains on the south side of the plains, traverse this area; and after breaking through the coast-ranges reach the sea. They acquire an exaggerated size from the circumstance that they are in great part tidal estuaries. The tide ascends them for several miles reaching behind the coast-ranges into the plains; whilst the dense mangroves, which line their lower courses amongst the hills, extend beyond into the low-lying level districts farther inland. Boats can follow up the winding course of the Wainikoro River until they reach the village of that name, which lies about five and a half miles in a direct line from the coast and nearly in the centre of this part of the island. The mangroves extend up to the village. The Langa-langa River, which is of much smaller size, is similarly navigable for three or four miles. The mangroves that line its banks spread out in broad tracts when in ascending the river we emerge from the hills into the plains. Above the influence of the tide it dwindles into a stream ridiculously small. The same remarks apply to the river two miles to the eastward. The Vui-na-savu River near the eastern margin of this low-lying region can be ascended by boats into the heart of the island.

From the foregoing remarks it will be expected that some portion of this low-lying inland region will be occupied by swamps. This is in truth the case. About one and a half miles north-east from the village of Kalikoso is a small fresh-water lake, about 100 yards across, which lies in a slight depression in the plains and is surrounded by a wide margin of swampy ground, occupied by reeds and sedges, in which one sinks knee-deep when approaching the banks. The level of the surface of the lake is not over 25 feet above the sea, and only a foot or so below that of the plains around. Since the depth, as I ascertained it, is 15 to 18 feet, it follows that the bottom of the lake is only a few feet above the high-tide level. The mangroves extend to within a mile of its border; and it is possible that though lying about five miles inland, it may be indirectly affected by unusually high tides. It would be interesting to determine whether the water is ever brackish.

This small lake is, or was, regarded with superstitious awe by the natives on account of the floating islands that it contains. Different legends are connected with it, and the Fijians have given it the name of “Vaka-lalatha,” in allusion to the drifting of the islands from one side of the lake to the other, the small trees growing upon them acting “in the manner of sails.” Mr. Horne, who was in this locality in 1878, refers to the lake in his book, _A Year in Fiji_ (p. 24); but does not appear to have seen it. Mr. Thomson[102] visited it in 1880, and describes it as containing a single floating island, a quarter to a half an acre in extent. The island was in existence, the natives said, in the days of their great-grandfathers, a statement indicating that the people of the district had no reason to doubt its antiquity. A chief, who formed one of the party, dived under the island.

When I visited this lake in 1899 there were three floating islands, named by the natives “Wanga levu” and “Wanga lailai,” that is, “Large canoe” and “Small canoe.” The largest was 90 or 100 feet long, whilst the two smaller were about 50 feet long, the breadth being less than half the length. They are composed of a dense growth of reeds and sedges supporting small living trees 10 to 17 feet in height, swamp ferns, and other smaller vegetation, the whole forming fairly solid standing-ground, and doubtless possessing considerable thickness.

The origin of these floating islands is probably to be found in the circumstance that the dense mass of swamp-vegetation lies on a rocky substratum, and that during some unusually heavy flood large portions of the swampy soil-cap became detached and floated up. A floating island occurs near the sea in the Lauthala district on the Lower Rewa in Viti Levu. The floating island in Derwentwater in the north of England is said to be “a blister of sublacustrine turf.” Those in Russia and Hungary, according to Mr. Hanusz, appear to be felted masses of tree-trunks, branches, and marsh-plants thinly covered with soil.[103]

I will preface my remarks on the geology of the Wainikoro and Kalikoso plains by observing that this region displays three conspicuous features. In the first place, it is a region of acid rocks, mostly altered tuffs, derived from quartz-porphyries, the alteration consisting in the deposition of quartz often of the chalcedonic type. In the second place, a silicifying process has been in operation here on an extensive scale, as evidenced by the abundance of silicified corals lying on the surface, especially in the district around the lake and by the abundance of concretions of chalcedony, of fragments of chalcedonic flints, and of portions of white chalcedonic quartz-rock that in places strew the ground. In the third place, earthy limonite, or bog iron ore, has been produced in quantity, particularly around the lake; and in places small round concretions of impure carbonate of iron cover the soil. Before referring more in detail to the different parts of this region, it may be remarked that the silicified corals, flints, iron-ore deposits, &c., of this and other parts of the island are dealt with in Chapter XXV.

This region of acid rocks extends about two miles to the west and south-west of the village of Wainikoro. Here prevail white acid tuffs often decomposing and altered by the formation of secondary quartz. They are derived from the degradation of quartz-porphyries or rhyolitic rocks. The surface is irregular but the elevation is small, the 100-feet level lying about one mile and the 200-feet level about two miles west of Wainikoro.

The villages of Wainikoro and Kalikoso are from two to three miles apart, the intervening district not attaining a greater elevation than 70 feet above the sea. Decomposing altered white acid tuffs here occur with occasional large blocks, a couple of tons in weight, of apparently a quartz-porphyry or trachyte with its structure disguised by silicification. Fragments of siliceous concretions, as chalcedony, chalcedonic flints, &c., lie on the surface, the soil being friable and of a deep ochreous red; whilst in places the ground is covered with round marble-sized concretions composed of a mixture of carbonate of iron and limonite which I have described on page 356. A few hundred yards to the north-west of Kalikoso, where there is a little rise, a decomposed quartz-porphyry is exposed displaying rounded crystals of quartz fractured in position, the matrix of the rock being impregnated with chalcedony. In one part of this mound there is a friable white rock, composed of a crumbling mass of small irregular quartz-grains, rarely showing crystal-faces. It appears to be disintegrated quartz-felsite.

In taking the track from Kalikoso to the village of Vungalei, which is distant about 2½ miles to the south-east, one traverses after the first mile, where the acid rocks terminate, a rather more elevated region which rises to a maximum height of 180 feet above the sea. Though the acid rocks give place to a semi-vitreous pyroxene-andesite as described on page 212, small fragments of chalcedonic flints are frequent on the surface over both areas. The district that intervenes between Kalikoso and the landing-place on the Langa-langa River, about two miles in length, is not more than 30 feet above the sea, and is crossed by small sluggish streams, in the beds of which occur numerous fragments of silicified corals, up to 3 or 4 inches in size, belonging to the Porites and Astræan type. In these stream-beds also occur bits of mamillated chalcedony and of onyx, flattish agates, 3 or 4 inches across, and pebbles of the compact pyroxene-andesite above alluded to, the last probably derived from the south side of the plains.

The plains extend in a north-east direction as far as Numbu, which lies between 2½ and 3 miles north-east of Kalikoso. The country between these two places is usually elevated between 60 and 100 feet and is never more than 130 feet above the sea. It is known as the Kuru-kuru district. The surface is strewn with the fragments of flints and of a white chalcedonic quartz-rock; whilst the soil is friable and deep-red in colour, limonite in abundant fragments occurring on the ground. Here and there one passes slabs of a hard white silicified tuff, small portions of which are frequent on the surface.

Silicified corals and earthy limonite are to be found in abundance scattered over the surface of the plains immediately surrounding the small lake of Vakalalatha. We also find lying on the ground in this district bits of chalcedony and onyx, portions of chalcedonic flints, and nodules of the size of the fist, which when fractured either disclose the regular layers of the agate or radiating crystals of quartz. The silicified corals occur usually in fragments not over 4 inches across, and include portions of branching corals of the Madrepore habit, bits of massive corals of the Astræan type, small rounded nobs of “Porites” just as one commonly observes on a reef-flat, &c. They have an ancient weathered look, and in some cases it is evident from the existence of boring-shells in the fractured end of a branching coral that it once lay as a dead fragment on the surface of a reef-flat.

In Chapter XXV. the characters of the silicified corals of the island are discussed in detail; and I have there advanced the view that the extensive silicification of the Kalikoso and Wainikoro plains took place during the consolidation of the calcareous muds of a reef-flat whilst the land was emerging. I have already alluded on page 222 to an area of silicification on the neighbouring sea-border between Visongo and Tutu. There can be no doubt that during the last stage of the emergence the present situation of the fresh-water lake near Kalikoso was occupied by the sea, which also extended far over the surrounding plains. The process of silicification would of necessity be restricted to the period that has since elapsed; and the discussion is therefore confined to the nature of the conditions under which this change was induced. As a factor in the process we cannot disregard the acid character of the rocks of the district.

THE UNDU PROMONTORY

The north-east portion of the island terminates in a long narrow promontory which I have named after Undu Point at its extremity. Commencing at Thawaro Bay and near Tawaki it runs in a straight line for a distance of between 13 and 14 miles, its breadth varying between 1½ and 2½ miles. Its greatest elevation of nearly 1,600 feet is attained at its western end; and it diminishes irregularly in height as one proceeds towards Undu Point, where a height of 400 feet is maintained about a mile from the cape. It is bordered by reefs sometimes a mile in breadth, and the reefs prolong the promontory for another three miles beyond Undu Point. As indicated by the 100-fathom line, the submarine contour corresponds to that of the land, and the extent of marine erosion during the existing relations of land and sea is evidently displayed in the breadth of the reefs. I found no sign of upraised reefs; and although diligent inquiries were made nothing could be learned of any hot springs.

It will be seen from the following remarks that pumice-tuffs, quartz-porphyries, and oligoclase-trachytes, are the prevailing rocks. On the north side they may be regarded as continuous with the acid rocks of the region extending from near Lambasa to Thawaro Bay. On the south side they commence in the vicinity of Tawaki.

(1) THE DISTRICT EXTENDING TWO AND A HALF MILES WEST OF TAWAKI

When proceeding eastward along the north coast of Natewa Bay one enters the region of acid rocks between 2 and 2½ miles west of Tawaki. Here the country is much broken, picturesque hills with bare precipitous faces rising up near the coast, one of which named Natoto has a rudely conical and truncated form. Grey oligoclase-trachytes having a specific gravity of 2·4 and possessing the characters described on page 308, prevail in the district extending west of Tawaki. Sometimes they occur in mass; but they often form agglomerates. A singular pitchstone-agglomerate occurs at the coast at the foot of Natoto. The pitchstone, which has a specific gravity of 2·48, is a semi-vitreous form of a hypersthene-augite andesite. It shows abundant small pyroxene prisms in its glassy groundmass and is referred to the prismatic sub-order (5) described on page 289.

(2) NAITHOMBOTHOMBO RANGE

A high range of hills, forming the backbone of this part of the island, extends eastward for about five miles from Thawaro and Tawaki. It is named “Naithombothombo” in the Admiralty chart. From it rise two conspicuous peaks, Thawaro Peak (1,573 feet) at its western end, where it overlooks the village of that name, and Mount Thuku (1,288 feet) near its eastern end. West of Thawaro Peak this range is connected with the hills beyond by a saddle 600 feet in height, which is ascended when crossing the promontory from Thawaro Bay to Tawaki.

(_a_) _Thawaro Peak._—This represents an old “volcanic neck” of agglomerate rising out of the tuffs that are exposed on its slopes to an elevation of about 600 feet. As viewed from the saddle above mentioned, the upper part of the hill presents bare precipitous sides, several hundred feet in height, of agglomerate, the blocks of which are composed of a compact hypersthene-augite andesite (sp. gr. 2·48). It displays a few small phenocrysts of medium andesine and of rhombic and monoclinic pyroxene; and is referred to the prismatic sub-order (5) described on page 289, characterised by prismatic pyroxene in the groundmass.

(_b_) _South coast between Tawaki and the foot of Mount Thuku._—The tall cliffs that rise to a height of from 200 to 300 feet behind Tawaki are composed of white tuffs and agglomerate-tuffs derived from the acid rocks of the district. Eastward from Tawaki to the base of Mount Thuku the coast scenery is particularly fine. A little inland a line of hills, named “Na Kula,” rises precipitously to a height of 700 or 800 feet, and in the vertical sides are displayed tuffs and agglomerates probably of the character of those above noticed. Light-coloured tuffs are sometimes exposed at the coast in which are inclosed fragments varying in size of a pitchstone[104] (sp. gr. 2·36) approaching in structure a trachytic glass. At one place the tuffs were evidently sedimentary and bedded, the dip being about 15° N.W.

The massive rock most frequently exposed at the coast and on the hill-slopes between Tawaki and Mount Thuku is a quartz porphyry[105] displaying abundant porphyritic crystals of quartz and felspar in a groundmass originally semi-vitreous but now obscurely felsitic in character. The shore-flat for more than half a mile west of Mount Thuku is strewn with great numbers of detached columnar blocks, 12 to 15 inches across, of a slightly vesicular oligoclase-trachyte of the type described on page 308.

(_c_) _North coast between Thawaro Bay and the foot of Mount Thuku._—Coarse and fine tuffs prevail at the coast and on the neighbouring hill-slopes; and in the bare rocky faces of the hills inland they are also to be observed. The streams have worn deep gorges into their mass. Towards Thuku they are acid and pumiceous, and are evidently the products of eruption. Towards Thawaro, they are more basic and darker, and are in part at least to be attributed to marine degradation.

(_d_) _Mount Thuku._—My ascent of this hill, which is 1,288 feet in height, was made from the north coast. I found it to be composed from the foot to the summit of white pumiceous tuffs without any evident arrangement. It has a narrow top and shows no sign of a crateral cavity. The hills east and west, as viewed from its summit, are ridge-shaped and display nothing in their configuration at all suggestive of craters. The pumice-tuffs of Mount Thuku are non-calcareous, and exhibit greyish pumiceous lapilli in an abundant white matrix formed of fine pumice-debris. Under the microscope it shows the characteristic vacuolar and fibrillar structure; but the material has not the fresh appearance of ordinary pumice and the minute cavities are often filled with alteration products. The two rocky points on the north coast opposite the hill are formed in one case of a somewhat altered oligoclase-trachyte and in the other of a quartz-porphyry. Both no doubt represent intrusive masses, the almost horizontal columns, 12 to 15 inches in diameter, of the former indicating a nearly vertical dyke.

(3) THE UNDU PROMONTORY EAST OF MOUNT THUKU

East of Mount Thuku the hilly backbone of the promontory is of much less elevation. About three miles to the eastward the highest hill is 630 feet, and thence to within a mile or two of Undu Point the hills retain a height of 400 to 500 feet.

(_a_) _The north coast between Mount Thuku and the coast village of Nuku-ndamu._—On this stretch of coast, about five miles in length, the shore-cliffs are composed of white and pale-yellow, coarse and fine stratified pumice-tuffs, the beds being either horizontal or with a gentle dip northward. They are as a rule non-calcareous, and contain some quartz grains and small bits, 1 to 3 millimetres in size, of bottle green compact obsidian, much as one finds in Lipari pumice-tuffs. In general character, both naked-eye and microscopic, they correspond to the Mount Thuku pumice-tuffs above described. Large blocks of basic rocks are occasionally to be observed on this coast, sometimes probably indicating dykes, but in one place near Mount Thuku forming an agglomerate. The rock is a dark-grey augite-andesite with a specific gravity of 2·77. It is compact and has a hemi-crystalline groundmass.[106]

(_b_) _The south coast between Mount Thuku and Moala, a distance of about five miles._—Pumice-tuffs and agglomerates are displayed at the coast, the former often bedded and in one place having a dip of 35 or 40 degrees to the north. A quartz-porphyry, somewhat banded and a little altered, and displaying rounded quartz crystals 3 or 4 millimetres in size, is the prevailing massive rock exposed on the hill-slopes and occasionally at the coast. It is well exhibited about a mile east of Mount Thuku. Blocks of a grey oligoclase-trachyte also occur. These rocks are described on pages 308, 309.

(_c_) _The terminal portion of the promontory from Nuku-ndamu and Moala to Undu Point._—The same pumiceous tuffs, usually non-calcareous, form the shore-cliffs on the south coast from Moala to Mr. Bulling’s station at Ndothiu, which lies about 2 miles from the point. On the corresponding part of the north coast between Nuku-ndamu and Ndothiu these tuffs are often calcareous; and near the first-named place they contain sub-angular bits of coral of the size of a walnut. On the beach at Vunikondi in this locality they are overlain by nearly horizontal beds of basic lava, the upper surface of which when exposed displays the smooth, “ropy” crust of a lava flow. The rock is a dark slightly vesicular augite-andesite, hemi-crystalline in structure, and containing a fair amount of residual glass.[107] Since the underlying tuffs were evidently deposited on a sea-bottom, it follows that this is a submarine flow. I intended to revisit this locality, but was prevented. A detailed examination of it would be worth undertaking.

From Ndothiu to Undu Point, about 2 miles distant, the interior of the promontory has an undulating surface, the elevation being usually 200 or 300 feet and rising to 400 feet. On the coasts are exposed bedded pumiceous tuffs, steeply inclined and usually calcareous. As displayed in the hill-slopes of the interior they are horizontally stratified and as a rule non-calcareous. These deposits sometimes exhibit a spheroidal arrangement indicative of the proximity of a dyke. In one or two places at the coasts occur basic agglomerates, formed of the same augite-andesite lava-rock of which the Vunikondi beds are composed, but scoriaceous and containing more glass in the groundmass. In the hand-specimen beside me, the steam cavities are of all sizes, from that of a pin-prick to a third of an inch (8 mm.) and are generally elongated.

A careful search of the tuff-deposits in this part of the Undu promontory ought to result in the discovery of remains, both of plants and of marine mollusks. Mr. Chalmers informed me that fossilised tree-trunks occur on the coast near Vunikondi; but I was unfortunately not able to discover them.

* * * * *

BRIEF SUMMARY OF THE GENERAL CHARACTERS OF THE UNDU PROMONTORY FROM THAWARO AND TAWAKI TO UNDU POINT.—One suggestive negative feature of this region is to be found in the absence, as far as I could ascertain, of any trace of a crater. Here also, as in the area of acid rocks extending westward along the north side of the island to near Lambasa, hot springs are not to be found. The prevailing rocks are in the first place the pumice-tuffs, which not only as a rule form the coast-cliffs, but occur inland as high as the summit of Mount Thuku almost 1,300 feet above the sea. They were probably in great part ejected from sub-aërial vents, though no doubt, as in the vicinity of Undu Point, they were often deposited under the sea. The acid and basic tuffs in the vicinity of Tawaki and Thawaro are, as I imagine, largely derived from marine degradation. Next to the pumice-tuffs, massive quartz-porphyries and oligoclase-trachytes are the characteristic rocks. They are probably in most cases intrusive, and present themselves sometimes as vertical columnar dykes, evidently of considerable dimensions.

The basic rocks, however, are not unrepresented. They occur in one or two places as agglomerates, as in the vicinity of Mount Thuku on the north coast and near Undu Point; whilst they form “flows” overlying the pumiceous tuffs at Vunikondi. Occasional blocks lying on the surface on the north coast are indications of dykes. The basic rocks, nevertheless, take a very secondary part in the composition of the Undu Promontory. They are in most cases to be referred to the augite-andesites with a specific gravity 2·6 to 2·77; but some, as in the case of those forming the agglomerates of Thawaro Peak, are hypersthene-augite andesites with specific gravity of about 2·5. Olivine-basalts are not represented.

The vents, from which the materials forming this promontory were ejected, were arranged in a single straight line for a length of 14 miles. Along this linear fissure, which was probably submarine, vents were at different times formed; and though owing to sub-aerial and marine degradation the present surface has been since shaped and reshaped, their situation may still be recognised in the “necks” of tuff and agglomerate that form the peaks, and in the large dykes or sills of quartz-porphyry and oligoclase-trachyte.