CONTENTS
CHAPTER I
GENERAL INTRODUCTORY REMARKS ON SOME OF THE LEADING PHYSICAL
FEATURES OF THE ISLAND
Its remarkable shape, [1].—Its building up, [2].—Study of its profile, [3].—Mount Seatura.—Regions of acid andesites.—Basaltic tablelands.—Great ridge-mountains, [5].—Boundary of the regions of basic and acid rocks, [6].—Its primary features, the dacitic peak, the basaltic plateau, and the ridge-mountain
Pages [1-6]
CHAPTER II
ON THE EVIDENCE OF EMERGENCE OR OF UPHEAVAL AT THE SEA-BORDERS
Elevated coral reefs scantily represented, [7].—Apparent absence of coral reefs in the early stages of the emergence, [8].—Elevated reefs confined to the coast and its vicinity.—Detailed examination of the sea-borders, [9].—Silicified corals and siliceous concretions the only evidence in many localities of the upraised reefs, [13].—The relations of the mangrove-belt to the reef-flat, [14].—Indications of a very gradual movement of emergence in our own time, [15].—The rate of advance of the mangroves, [16].—Conclusions, [19]
Pages [7-20]
CHAPTER III
THE HOT SPRINGS OF VANUA LEVU
The thermal springs of other parts of the group, [21].—The hot springs of the Wainunu valley, [22].—The boiling springs of Savu-savu, [25].—Analyses of the water, [28].—The hot springs of other localities, [31].—Distribution of the springs, [35].—The algæ and siliceous deposits, [37].—The cold and thermal springs of Hawaii and Etna, [38].—Infiltration, the source of the springs, [39].—A view negatived by Prof. Suess.—List of the hot springs of Vanua Levu, [40].—Summary of the chapter, [42]
Pages [21-42]
CHAPTER IV
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES OF
VANUA LEVU
Naivaka, [43].—Korolevu Hill, [45].—Bomb formation of Navingiri, [46].—Remarkable section near Korolevu, [48].—Wailea Bay to Lekutu, [50].—Mount Koroma, 51.—Mount Sesaleka, [53].—The Mbua-Lekutu Divide, [55].—The Mbua and Ndama plains, [55].—The shell-bed of the Mbua river, [58].—Lekumbi Point, [60]
Pages [43-60]
CHAPTER V
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
Mount Seatura, [61].—Its eastern slopes, [63].—Its western slopes, 64.—Its northern slopes, [65].—Ascents to the summit, [66].—The Ndriti Basin, [67].—A huge crateral cavity, [68].—Its dykes of propylite, [69].—Seatura a basaltic mountain of the Hawaiian order, [72].—The Seatovo Range, [73].—Solevu Bay, 75.—Koro-i-rea, [77].—Nandi Bay, [78].—Na Savu Tableland, [79]
Pages [61-81]
CHAPTER VI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The basaltic plateau of Wainunu, [82].—Its margins covered by pteropod and foraminiferous ooze-rocks, [86].—The hill of Ulu-i-ndali, [87].—Kumbulau Peninsula, [90].—The basaltic flow of Kiombo Point, [92].—Soni-soni Island, [93].—Yanawai coast, [95]
Pages [82-97]
CHAPTER VII
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Ndrandramea district, [98].—Its mountains and hills of acid andesites, [100].—Ngaingai, 101.—Ndrandramea, [102].—Soloa Levu, [103].—The underlying altered acid andesites, [106].—Section of the district, [107].—The magnetic peak of Navuningumu, [108].—The Mbenutha Cliffs and their pteropod and foraminiferous beds, [109]
Pages [98-112]
CHAPTER VIII
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
Mount Vatu Kaisia and district, [113].—The Nandronandranu district, [117].—Nganga-turuturu cliffs, [119].—Ndrawa district, [120].—Tavia ranges, [121].—Na Raro, [123].—Its Ascent, [125].—Na Raro Gap, [127]
Pages [113-127]
CHAPTER IX
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The basaltic plains of Sarawanga, [129].—Tembe-ni-ndio and its foraminiferal limestones, [131].—The basaltic plains of Ndreketi, [132].—The Nawavi Range, [135].—Nanduri, [136].—Tambia district, [137].—The basaltic plains of Lambasa, [138]
Pages [128-139]
CHAPTER X
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Va Lili Range, [140].—Its Nambuni spur, [144].—Originally submerged and covered with palagonite-tuffs and agglomerates, [145].—The Waisali Saddle, [146].—Narengali district, [147].—Nakambuta, [148].—The valleys of the Ndreke-ni-wai, [150].—Their origin, [151]
Pages [140-152]
CHAPTER XI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Korotini Range, [153].—Traverse from Waisali to Sealevu, [154].—Traverse from Mbale-mbale to Vandrani, [156].—Traverse from Vatu-kawa to Vandrani, [160].—Traverse from Nukumbolo to Sueni, [161].—The Sueni valley, [163].—General inference concerning the range, [164]
Pages [153-165]
CHAPTER XII
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Koro-mbasanga Range, [166].—The Sokena Ridge, [169].—Lovo valley, [169].—Mount Mbatini, [172].—The Vuinandi Gap, [175].—The Thambeyu or Mount Thurston Ranges, [176].—Structure of Thambeyu, [177].—The Avuka Range, [179]
Pages [166-180]
CHAPTER XIII
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Valanga Range, [181].—Its western flank, [183].—Ngone Hill, [183].—Valley of Na Kula, [184].—The Mariko Range, [185].—Savu-savu Peninsula, [189].—Naindi Bay, [192].—The Salt Lake, [194]
Pages [181-196]
CHAPTER XIV
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Natewa Peninsula, [197].—Viene district, [198].—Lea district, 199.—Waikawa Mountains, [201].—Ndreke-ni-wai coast, [203].—Waikatakata, [203].—Mount Freeland or the Ngala Range, [204].—Traverse from Tunuloa to Ndevo, [205].—Coast from Ndevo to Mbutha Bay, [205]
Pages [197-206]
CHAPTER XV
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The north-east portion of the island from Mount Thurston to Undu Point, [207].—Coast between Vuinandi and Tawaki, [208].—The corresponding inland region, [209].—The gabbro of Nawi, [211].—Uthulanga Ridge, [211].—Ascent of Mount Vungalei or Ndrukau, [213].—Nailotha, [214].—Exposure of altered trachytes and quartz-porphyries at its base, [215].—From Nandongo to Vanuavou, [216].—From Ngelemumu to Wainikoro, [217].—Sea border between Lambasa and Mbuthai-sau, [218].—Coast between Mbuthai-sau and the Wainikoro and Langa-langa Rivers, [219].—Coast between the Langa-langa River and Thawaro Bay, [221].—The Globigerina clay of Visongo, [221].—Vui-na-Savu River, [222].—Some General inferences, [223]
Pages [207-223]
CHAPTER XVI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Wainikoro and Kalikoso Plains, [224].—Vaka-lalatha Lake, [225].—Its floating islands, [226].—A region of acid rocks, [227].—Silicified corals and limonite, [228].—Tawaki district, [229].—Thawaro district, [230].—Mount Thuku, [231].—Undu Point, [232].—General characters of the Undu Promontory, [233]
Pages [224-234]
CHAPTER XVII
THE VOLCANIC ROCKS OF VANUA LEVU
Their varied character, [235].—Their classification, [236].—Descriptive formula, [237].—Synopsis, [239].—Orders of the Olivine-Basalts, [241].—Orders of the Augite-Andesites, [245].—Orders of the Hypersthene-Augite-Andesites, [247].—Description of the Plutonic Rocks, [249]
Pages [235-251]
CHAPTER XVIII
THE VOLCANIC ROCKS OF VANUA LEVU (continued)
The Olivine Basalts
Pages [252-265]
CHAPTER XIX
THE VOLCANIC ROCKS OF VANUA LEVU (continued)
The Augite-Andesites
Pages [266-284]
CHAPTER XX
THE VOLCANIC ROCKS OF VANUA LEVU (continued)
The Hypersthene-Augite-Andesites
Pages [285-292]
CHAPTER XXI
THE VOLCANIC ROCKS OF VANUA LEVU (continued)
THE ACID ANDESITES, TRACHYTES, QUARTZ-PORPHYRIES.
The Hornblende-Andesites of Fiji, [293].—Occurrence of Dacites in Fiji, [294].—Suggestion of “felsitic andesite” as a rock-name, [295].—The Acid Andesites of Vanua Levu, [295].—The Hypersthene-Andesites, [296].—The Hornblende-Hypersthene-Andesites, [298].—The Quartz-Andesites or Dacites, [302].—Tabular comparison of the Acid Andesites, [304].—The characters of the Rhombic Pyroxene, [306].—Magmatic Paramorphism, [306].—The Oligoclase Trachytes, [308].—Quartz-Porphyries and Rhyolitic rocks, [309]
Pages [293-311]
CHAPTER XXII
THE VOLCANIC ROCKS OF VANUA LEVU (continued)
Basic pitchstones and basic glasses, [312].—Volcanic Agglomerates, [314]
Pages [312-316]
CHAPTER XXIII
CALCAREOUS FORMATIONS, VOLCANIC MUDS, PALAGONITE-TUFFS
General Character, [317].—Coral Limestones, [318].—Foraminiferal Limestones, [319].—Pteropod-oozes, [320].—Foraminiferous Volcanic Muds, [321].—Samples, [322].—Altered kinds, [324].—Submarine Palagonite-tuffs of mixed composition, [326].—Samples, [330].—Altered Basic Tuffs, [332].—Submarine Basic Pumice Tuffs, [333].—“Crush-tuffs” formed of basic glass and palagonite, [334].—Zeolitic Palagonite-Tuffs, [334].—Palagonite-marls, [335].—Acid Pumice Tuffs, [336]
Pages [317-336]
CHAPTER XXIV
PALAGONITE
Its abundance in a fragmental condition in Vanua Levu, [337].—Its occurrence in deep-sea deposits, [338].—Modes of formation in situ, [338].—In the upper portion of a basaltic flow, [339].—In the groundmass of hemi-crystalline basaltic rocks, [339].—In veins in a basic tuff-agglomerate, [340].—In the fissures of a basaltic dyke, [341].—In the matrix of pitch-stone agglomerates, [349].—In “crush-tuffs,” [341].—Regarded as a solidified magma-residuum of low fusibility, [342].—Its connection with crushing, [342].—Bunsen’s experiment, [343].—Rosenbusch and Renard, [344].—The Nandua series of beds, [345].—Suggested explanation of the origin of palagonite, [346].—Type of basalt associated with palagonite, [347].—Hydration and disintegration of palagonite, [348]
Pages [337-349]
CHAPTER XXV
SILICIFIED CORALS, FLINTS, LIMONITE
Mode of occurrence of the silicified corals, [351].—Their character and structure, [352].—Flints, nodules of Chalcedony, Agates, etc., [353].—Other siliceous concretions, [354].—Jasper, [355].—Deposits of Limonite, [356].—Magnetic Iron-sand, [357].—Suggested explanation of the silicification of the corals, [358].—Note on a silicified Tree-fern, [360]
Pages [350-360]
CHAPTER XXVI
MAGNETIC ROCKS
Previous observations, [361].—Magnetic Polarity usually caused by atmospheric electricity, [362].—Displayed by both acid and basic rocks, [364].—Very frequent in Vanua Levu, [365].—Its relation to specific weight, [366].—The influence of locality, [367].—Frequently observed in mountain peaks, [367].—Description of the peaks, [368].—Measurement of the polarity of rocks, [370]
Pages [361-371]
CHAPTER XXVII
SOME CONCLUSIONS AND THEIR BEARINGS
Vanua Levu, a composite island formed during a long period of emergence, [372].—The submarine plateau probably produced by basaltic flows, [373].—The distribution of the volcanic rocks, [374].—Comparison with Iceland, [374].—The mountain-ridges, [375].—The emergence of the Fiji Islands, [376].—Wichmann’s view of the early continental condition not supported, [376].—Age and character of the emergence, [377].—The evidence of the Lau Group and of the Tongan Islands, [378].—Two principal stages of the emergence, [379].—Relative antiquity of the Hawaiian, Fijian, and Tongan Islands as indicated by their floras, [379].—Islands have always been islands, [380].—The hypothesis of a Pacific continent not yet needed, [381].—The great dilemma, [381].—Much remains to be learned of the possibilities of means of dispersal in the past and in the present, [382]
Pages [372-382]
APPENDIX.
(1) Note on microscopical examination of stone-axes.
(2) Note on the ascent of the tide in the Ndreketi River.
(3) Note on the “talasinga” districts.
INDEX [385]
LIST OF ILLUSTRATIONS
| Plates | ||
| TO FACE PAGE | ||
|---|---|---|
| Na Raro (2,420 feet) from the south-west, a peak of acid andesite | [Frontispiece] | |
| Ndrandramea (1,800 feet) from the south-east, a peak of acid andesite rising about a thousand feet from its base | ||
| The Ndrandramea District from the westward | [98] | |
| Mount Tavia (2,210 feet) from Vatu Kaisia | [108] | |
| The magnetic peak of Navuningumu (1,931 feet) from the south | ||
| Mbenutha Cliffs, showing volcanic agglomerates overlying tuffs and clays, containing shells of pteropods and foraminifera, which are raised 1,100 feet above the sea | [111] | |
| Duniua Lagoon, representing an old mouth of the Ndreke-ni-wai | [153] | |
| Lithographs | ||
| Vanua Levu, Fiji Islands | [1] | |
| Fiji Islands | [373] | |
| Figures | ||
| PAGE | ||
| Profiles of Vanua Levu as Viewed from the South. Graphically Represented on a Horizontal Scale of about 16 miles to the inch | [4] | |
| Korolevu Hill (800 feet) from Wailea Bay | [46] | |
| Profile and Geological Section of the western end of Vanua Levu from the Wainunu estuary across the summit of the basaltic mountain of Seatura to the edge of the submarine platform off the Ndama coast as limited by the 100-fathom line | [62] | |
| Profile, looking north from off the mouth of the Wainunu River | [83] | |
| Rough plan of the Ndrandramea district in Vanua Levu; made with prismatic compass and aneroid by H. B. Guppy | [99] | |
| Profiles of Ngaingai and Wawa Levu from Nambuna to the south-west. Both are dacitic mountains | [101] | |
| Profile and Geological Section of Vanua Levu, across the island from the Sarawanga (north) coast to the Yanawai (south) coast | [107] | |
| Profile-sketch of the Vatu Kaisia district from S.S.E. | [113] | |
| Section of the Vatu Kaisia district | [115] | |
| Profiles of Na Raro | [124] | |
| Profile-sketches of the Va-Lili Range | [141] | |
| Profile-sketch of the mountainous axis of Vanua Levu | [167] | |
| Koro-mbasanga from the north-north-east | [167] | |
| Mount Mbatini from the top of Koro-mbasanga | [173] | |
| View from Muanaira on the south coast of Natewa Bay | [173] | |
| Ideal Section of Thambeyu | [177] | |
| Diagram illustrating the two sets of felspar-lathes in a dyke | [238] | |
| Magma-lakelet, ·25 mm. in size, magnified 290 diameters, from a basalt at Navingiri | [339] | |
| Showing fragments of glass with eroded borders and of plagioclase with more even edges in a matrix of palagonite traversed by cracks | [342] | |
| Diagram showing the succession of deposits below the Nandua tea-estate | [345] | |
VANUA LEVU,
FIJI ISLANDS.
Drawn on a scale of 25 miles to 3 inches by H. B. Guppy, M.B.
Based on the Admiralty Surveys, but most of the topographical
details of the interior have been supplied from the author’s observations
with the aneroid and prismatic compass in 1897-99. It is
merely intended to illustrate his general account of the physical
and geological characters of the island and is very far from
complete. (see introduction.)
OBSERVATIONS OF A NATURALIST IN THE PACIFIC
CHAPTER I
GENERAL INTRODUCTORY REMARKS ON SOME OF THE LEADING
PHYSICAL FEATURES OF THE ISLAND
The remarkable shape of this island at once attracts the attention: and indeed it is in its irregular outline and in the occurrence over a large portion of its surface of submarine tuffs and agglomerates that will be found a key to the study of its history. With an extreme length of 98 miles, an average breadth of 15 to 20 miles, and a maximum elevation of nearly 3,500 feet, it has an area, estimated at 2,400 square miles, comparable with that of the county of Devon.
Whilst its peculiarly long and narrow dimensions are to be associated with the narrowing of the submarine basaltic platform, from which it rises together with the other large island of Viti Levu, its extremely irregular shape is closely connected with the composite mode of its origin. We have here exemplified the process of the building up of a continental island in the great area of emergence of the Western Pacific, that region which displays at various heights above the sea the ancient reefs and the underlying deposits of the Solomon Islands, New Hebrides, Fiji, Tonga, &c. But this process of construction has never been completed, and is at present suspended; yet it is in its incomplete condition that Vanua Levu possesses its importance for the investigation of this subject.
This island has in fact been formed by the union of a number of smaller volcanic islands during a long protracted period of emergence. These original islands are indicated approximately by the 1,800-feet contour-level in the accompanying map. There is, however, no reason for supposing that the movement of emergence has altogether ceased. In the course of ages the extensive submarine plateau, from which it rises, will be laid bare; and the small surrounding islands that are situated upon it, such as Yanganga, Kia, Mali, Rambi, Kioa, &c., will be included in the area of Vanua Levu.[[1]]
Excluding for the moment the effects of denudation, which have been very great, we shall be able to discern some of the stages of the building-up of the island during the emergence or upheaval by looking at the map and reversing the process in imagination. A subsidence of only 50 feet would cause the Natewa Peninsula to be isolated by a sea-passage along the line of the Salt Lake; whilst several islands would be formed along the northern and southern coasts, and the Naivaka Peninsula would become detached. If the subsidence extended to 300 feet, the sea would flow over a large portion of the island, where it would regain what was not many ages since its own, an area of basaltic plains, which by their prolongation under the sea form the great submarine plateau. A subsidence of 1,000 feet would break up the remaining elevated axis of the island into a number of lesser portions; and after a total lowering of 1,800 feet there would exist only a few scattered islands, the arrangement of which would show but little relation to the present form of Vanua Levu. At either end of the area there would arise from the sea the isolated volcanic peaks of Seatura and Ngala (Mount Freeland); and between them would be situated four or five long narrow islands, together with a group of small islands and islets where Na Raro and the other acid andesite mountains of the Ndrandramea district now lie.
As might be partly expected, there is in the surface-configuration of the interior of Vanua Levu an absence of that simplicity of contour which exists in a volcanic island of supra-marine formation, as for instance in the large island of Hawaii where the three great volcanic mountains of Mauna Kea, Mauna Loa and Hualalai together with the older Kohala range, determine the form of the whole island’s surface.[[2]] Here in Vanua Levu there is, on the contrary, but little order amongst its physical features. The rivers often run obliquely with the sea-border, whilst mountains frequently rise at the coast and plains lie far inland, and the view of the elevated interior, as obtained from one of the peaks, presents in many parts a series of mountain-ridges running athwart the island’s axis.
A study of the profile of the island is an important preliminary step to its more detailed examination. One may ramble over a particular region of it for weeks, as I have done, without getting any satisfactory idea of the true configuration of the surface. In a locality densely wooded and occupied by steep mountain ridges and deep gorges, the field of view is often very limited; but seen from the deck of a passing ship the main features of the island assume their true proportions and relations, and much that was uncertain is in this manner made plain. The profile here given has been constructed from a number of others, and represents in a graphic fashion Vanua Levu as viewed from the southward. I have here sacrificed smaller details and occasionally some degree of accuracy in small matters in order to bring out the principal features of the island.
At and near the extreme western extremity rise the conspicuous hills of Sesaleka (1,370 feet), Naivaka (1,651 feet) and Koroma (1,384 feet), all of them formed of basic volcanic materials.[[3]] Naivaka, which is connected with the main island by a narrow isthmus, only about 30 feet in height, is probably one of the most recent additions to the island’s area; and it is at the same time one of the most recent of the numerous volcanic vents that once existed. The leading feature, however, of this end of Vanua Levu is the great mountain of Seatura (2,812 feet), which occupies a large part of the Mbua province and monopolises most of the landscape whilst largely determining the form of the western extremity of the island. It is a basaltic mountain of the Mauna Loa type, its long eastern slope descending gently at an angle of three or four degrees for about ten miles to the mouth of the Wainunu River. In its deeply eroded radial valleys and gorges, and in other respects, it is not unlike the island of Tahiti, as described by Dana.
The Ndrandramea region to the eastward, which I have named after one of its best known peaks, has a profile of a very different character. Its broken outline indicates the existence of numerous mountains and hills of acid andesites, occasionally dacitic. Although some of them attain a height of 2,000 feet and over their tops alone are seen from seaward. Between the foot of these mountains and the south coast extends a great plateau of columnar basalt, incrusted at its borders with submarine deposits, which descends coastward with a very gentle slope, the fall in about five miles being only about 300 feet (1,100 to 800 feet). It terminates abruptly opposite the elevated headland of Ulu-i-ndali, a range, composed mainly of grey olivine-basalts, which is not indicated in the profile.
Profiles of Vanua Levu as Viewed from the South. Graphically
Represented on a Horizontal Scale of about 16 miles to the inch.
From Naithombothombo Point to Undu Point, representing the length of the island (98 miles).
The Natewa Peninsula from the Salt Lake to Kumbalau Point.
The two conical peaks of Vatu Kaisia (1,880 feet) and Na Raro (2,420 feet), which rise up so unexpectedly in the region immediately east of the Ndrandramea district, are also of acid andesitic rocks, in the last case approaching the dacitic type. They lie within the borders of the area of basic tuffs, basic agglomerates, and basic massive rocks, that here begins and extends eastward to Mount Thurston and a little beyond. East of Na Raro there is a gap or break in the profile, where the greatest elevation is probably not over 800 feet; and on its farther side rises up the mountain of Va Lili (2,930 feet), a lofty inland ridge that lies towards the southern coast. Palagonite-tuffs and agglomerates are the prevailing surface-formations in this district.
Eastwards from Va Lili extends for eight or nine miles a lofty, level-topped, and almost peakless range, which I have called the Korotini Table-land, after the towns once situated on its southern slopes. Its outline is shown in the background of the view facing page [153]. It is, however, not so level-topped as it appears to be; but the gradual variations in elevation between 2,000 and 3,000 feet, when spread over a length of some miles, are more or less lost in the general outline of the range as viewed from the coast. Basic agglomerates are principally exposed on the lower slopes; whilst higher up, reaching often to the summit of the table-land, occur palagonite-tuffs containing tests of foraminifera and molluscan shells, massive basic rocks being exposed in places.
The level profile of the Korotini tableland gives place, as one proceeds eastward, to the broken outline of the several lofty peaks of Mariko (2,890 feet), Mbatini (3,437 feet), Thambeyu (3,124 feet) and others.[[4]] Each of these peaks marks one of the bold mountain-ridges that form such a striking feature in the surface-configuration of this part of the island. On the slopes of these ridges, and often also on their summits, appear basic agglomerates and palagonitic tuffs and clays often inclosing tests of foraminifera; whilst exposed in the gorges and protruding at times through the tuffs and agglomerates on the crests of the ridges are displayed massive basic rocks of the type of the hypersthene-augite andesites.
East of Thambeyu the level sinks to about 1,000 feet above the sea, and beyond rises an irregular group of hills and mountains which attain their greatest height in Nailotha,[[5]] 2,481 feet above the sea. We are now near the limit of the area of basic rocks. Following the profile as it slopes away, marked by occasional peaks and breaks, towards Undu Point, we pass at first over a district where basic rocks are mixed with those of more acid type; but before we reach Mount Thuku we enter the district of oligoclase-trachytes, quartz-porphyries, and rhyolitic tuffs, that extends to the extremity of the Undu promontory.
There remains to be noticed the profile of the Natewa Peninsula. As shown in the diagram, this level begins at a few feet above the sea in the vicinity of the Salt Lake; and as it proceeds eastward it attains a level of 1,960 feet in Ngalau-levu and of 1,540 feet in the Waikawa promontory, finally culminating, as it nears Kumbulau Point, in a mountainous district which attains its greatest elevation of 2,740 feet above the sea in the lofty ridge of Ngala, the Mount Freeland of the chart. Altered basic rocks prevail in this peninsula; but more acid andesites also occur, and foraminiferous tuffs and clays are exposed on the slopes, reaching to over 1,000 feet above the sea.
I will conclude this reference to the profile of the island with the remark that if I had neglected to indicate here the close connection that exists between the nature of the surface-configuration and the character of the prevailing rocks I should have ignored a means of investigation which has proved of the greatest value. The rock and surface characters go together. The inland plateau now upheaved 1,000 feet above the sea, was built up by submarine flows of basaltic lava. The isolated conical peak that so unexpectedly intrudes itself into the view is the dacitic core of some submarine volcano long since stripped of most of its fragmental coverings. The lofty mountain-ridges that run athwart the island’s breadth, with their summits usually in the rain-clouds received their coverings of tuffs and agglomerates ages ago when they were submerged; and now they rise to heights of over 3,000 feet above the sea. Bound up with the mysterious origin of these great ridges is the history of the island of Vanua Levu.
These preliminary remarks are only intended to serve as a general introduction to the detailed description of the island and its formations. The closing chapter is devoted to a summary of the principal results of my investigations.
CHAPTER II.
ON THE EVIDENCE OF EMERGENCE OR OF UPHEAVAL AT THE SEA-BORDERS.
One would have expected that in an island where submarine muds and tuffs are of such common occurrence at the surface, extending from the sea-border to elevations of 2,000 feet and over, upraised coral reefs would be also frequent and extensive. But it is remarkable that the uplifted masses of reef-limestone, so characteristic of the islands of the Lau Group, are here very scantily represented. It is certainly true that the fossiliferous volcanic muds that form the foundations of coral reefs are often exposed at and near the coast; but the elevated reefs that ought to be found reposing on them are rarely to be observed.
It is not to be inferred, however, that in a region so remarkable for the great development of reef-formations coral reefs did not then thrive in these localities, but rather that such a long period has elapsed since the emergence of the present sea-border that the upraised coral reefs at and near the coast have long since been in a great part stripped off by the denuding agencies. Notwithstanding this, it is evident that coral reefs could never have been very extensive at the sea-border during the last stages of the emergence; whilst they do not appear to have existed at all during the early periods of the history of the island.
In this connection it may be observed that hard compact limestones of any kind are rarely to be found, and only in a scanty fashion. The extensive development of dolomites and hard limestones, described by Mr. Andrews and others in the valley of the Singatoka in Viti Levu, is not a character of Vanua Levu. The foraminiferous and pteropod clays, which exist in the interior and often in the heart of the island, are not overlaid by ancient reef-limestones, but by great masses of volcanic agglomerate and coarse fossiliferous tuffs, the foraminiferous muds in their turn covering the core of massive volcanic rocks. There were no signs of coral-reef fragments in the volcanic agglomerates in any locality examined, notwithstanding that these agglomerates are so intimately associated with the fossiliferous tuffs and clays that their submarine origin could not be doubted.
The conditions for reef-formation evidently did not exist in that early stage of the island’s history, when the foraminiferous tuffs and clays, now occurring at elevations of 2,000 feet and over, were being deposited on the sea-bottom. At some time or other, however, these high mountain-slopes, previous to their emergence from the sea, must have been within the limits of the zone of reef-building corals. If reefs had been formed along those ancient coasts, or on the original shoals, they would have been in some cases preserved, as in the case of the foraminiferous tuffs and clays, by a covering of volcanic agglomerate. These soft submarine deposits have been in this manner saved from the destructive effects of denudation over a large part of the island whether on the higher slopes or at the lower levels; but no trace of reef-formation ever came under my notice in the higher regions of the interior. This is a puzzling point that will have to be considered in connection with the origin of the great mountain ridges, one of the most difficult problems in the history of the building-up of Vanua Levu.
I will now refer to the evidence of the latest stage of the upheaval of the island as indicated at and near the sea-border by the scantily occurring upraised reefs. The elevated reefs are mostly to be found on the south coast between Fawn Harbour and Na Viavia Islet off Harman’s or Savu-savu Point. Na Viavia Islet itself is 300 or 400 yards in length and is formed of much honeycombed reef-limestone, which is raised 10 or 12 feet above the high-water line. Proceeding eastward along the south coast of the Savu-savu promontory we next come upon uplifted reefs in a curiously isolated hill that rises on the coast between Naithekoro and Naindi Bay. This hill is about 250 feet in height and is composed in the mass of coral limestone. About 100 feet above the sea-level it exhibits an erosion-line, above which it rises precipitously to the summit. The west point of Naindi Bay is formed of reef-limestone reaching to a height of 40 to 50 feet and displaying in position massive corals, “Fungiæ,” and “Tridacna” shells. Near its base, four to five feet above the present high-water level, it shows an erosion line. This limestone overlies a rock in which blocks of volcanic rocks, five to six inches across, are imbedded in a calcareous matrix.
Raised coral limestone occurs at intervals on the coast between Naindi Bay and the mouth of the Salt Lake Passage, usually forming low islets, of which the smaller about 12 feet in height often assume, through the erosion of the sea at their base, that peculiar mushroom-shape, so characteristic of upheaval on reef-bound coasts. The passage into the Salt Lake lies in a slightly elevated reef-mass; and the islet which rises up in its centre to about a foot above the water-level is mainly formed of coral blocks, although I did not find any remains of coral on the low neck of land intervening between the Salt Lake and Natewa Bay. Eastward from the Salt Lake Passage to Nanutha in the vicinity of Fawn Harbour low cliffs of coral limestone, six to eight feet high and occasionally displaying massive corals in position, most frequently constitute the sea-border, rarely, however, extending more than a few paces inland or attaining there a greater elevation than 12 or 15 feet.
This limitation of the upraised reef-belt to the immediate vicinity of the coast is true of all this district. It is only when the sea-border is low and swampy that it is found 100 or 200 yards inland; and in any case as one follows it inland it soon gives place to the fossiliferous mud-rocks and tuffs of the interior. It should be noted that the upraised reefs of this region were rarely observed at greater heights than 20 feet above the sea, in fact usually at a much lower level. The exceptional occurrence in mass of reef-limestone at a height of 250 feet in a coast hill between Naithekoro and Naindi therefore lends colour to the idea that the elevated reefs formerly extended farther inland and that they have been stripped off by denudation.
On the north coast of the Natewa Peninsula elevated reefs are of very rare occurrence. I walked along the whole of that coast from the head of Natewa Bay to within four miles of Kumbulau Point and only found them in the locality, one to one and a half miles west of the mouth of the river Ndreke-ni-wai. Here there were two islets, 20 to 25 feet high and lying close to the shore, which were formed entirely of coral-rock, massive corals occurring in position in their lower part. Although, however, upraised reefs are so scantily to be found on this coast, other proofs of upheaval are to be observed in the fossiliferous tuffs exposed occasionally by the beach. On the east coast of this peninsula, between Ndevo and Loa, submarine tuffs and sandstones, at times fossiliferous, were alone noticed.
Upraised reefs are also very rare on the north coast of Natewa Bay. Here again I traversed the whole coast from the head of the bay to Undu Point, a distance as the crow flies of about 50 miles; but I find no record in my notes of any elevated reef-formations. However the calcareous nature of the volcanic tuffs exposed in places at the coast indicate emergence. The extreme rarity, if not the absence, of upraised reefs on this long stretch of coast, which is usually bordered by shore-reefs, is very remarkable, more especially since there is extensive evidence of upheaval in the plains of Kalikoso in the interior, as indicated in the succeeding paragraph.
On the other side of Undu Point, between that headland and Lambasa, elevated reefs did not come under my observation, although in the low-lying inland district of the Kalikoso lake silicified corals are scattered about in quantity at an elevation of 20 or 30 feet above the sea. But the emergence of the sea-border is shown in the occurrence of a “Globigerina” sedimentary tuff near Visongo at a height of 200 feet (see page [221]), and by the occasionally calcareous character of the pumice-tuffs that mainly compose the coast cliffs. Near Nukundamu these tuffs of the shore cliffs inclose subangular fragments of massive corals of the size of a walnut; whilst in a cutting between Mbuthai-sau and Lambasa, about 50 feet above the sea, I observed bits of coral limestone in a basic tuff. Mr. Horne refers to seams or layers of coral limestone occurring in the volcanic agglomerate of the coast cliffs between Lambasa and Tutu Island.[[6]] Since his experience of this coast was mostly confined to a passage in a canoe along the shore, it is very probable that he only saw the beds of white pumice-tuffs that prevail in places on this coast. I found no beds of coral limestone in the shore-agglomerates of this coast, nor does Dana in his description of the pumiceous formation of the cliffs of Mali Point make any reference to them.[[7]]
Along the stretch of 50 miles of coast between Lambasa and Naivaka upraised reefs are of infrequent occurrence. However between Lambasa and Wailevu, coral limestone is extensively exposed in a low range of hills a mile or two inland but not over 100 feet above the sea. No elevated reefs came under my notice between the mouth of the Wailevu river and Nanduri Bay. That a small upheaval has been recently in progress in this part of the coast is indicated by two circumstances. In the first place an erosion-line about a couple of feet[[8]] above the high-water line, and a few paces removed from it, is displayed in the volcanic tuff of the point bordering the reef-flat on the east side of Nanduri Bay. In the next place there exist at different places in the midst of the mangrove-belt extensive bare mud-flats, sometimes several hundred yards across, which are only covered by the higher tides. These flats are quite bare of mangrove or any other vegetation and are often cracked on the surface and sun-dried and firm to walk upon.[[9]] These naked mud-flats in the midst of the mangrove tracts are peculiar to this part of the coast. Their general level must be between one and two feet above that of the mangrove belt in other parts of the island; and I infer that a slight upheaval or emergence has led to the death of the mangroves in these situations.
I know little of the coast between Nanduri Bay and the mouth of the Ndreketi River. At two localities where I landed no elevated reef-formation was observed. Dana referring to the coast opposite Mathuata Island alludes only to the volcanic agglomerates. The low mangrove-bordered coast between the mouths of the Ndreketi and Lekutu rivers was not actually visited by me; but I traversed the region behind the broad mangrove-belt, and found occasionally in the tuffs and muds exposed in the river-banks marine-shells and foraminiferous tests, indicating an elevation of a few feet. I examined much of the coast between Lekutu and the extremity of the Naivaka peninsula, but came upon no upraised reef-rocks. In the low isthmus, 20 to 30 feet high, which connects this peninsula with the main island only volcanic rocks came under my notice. A palagonitic tufaceous sandstone exposed in the cliffs on the north coast of Naivaka contains a little carbonate of lime, and being probably a submarine deposit it implies an emergence of the sea-border.
Although I have been able to produce but scanty evidence of uplifted reefs on the north coast of Vanua Levu, it is probable, judging from the heights given in the Admiralty Sailing Directions, that such formations exist in a few of the numerous low islands and islets that front this coast. Some of these islands and islets, which are often not much more than reef-patches largely reclaimed by the mangroves, will be noticed below when considering the question of the extension of the mangrove belts since the survey of Commodore Wilkes in 1840.
Neither on the south coast of the peninsula of Naivaka nor on the west coast of the Sesaleka promontory did upraised reefs come under my observation; but my acquaintance with the last locality is very scanty. The emergence of the Sesaleka promontory is however indicated by the occurrence inland at heights of at least 700 feet of palagonitic tuffs, occasionally containing foraminifera.
With the long tract of coast between Naithombothombo Point and Solevu Bay, I am fairly well acquainted. However, with the doubtful exception of Lekumbi Point, no elevated reef-formations were observed. Evidence of an emergence of a few feet, and of a very extensive seaward advance of the land-surface in recent times, is afforded by a curious bed of marine shells exposed in the banks of the Mbua River, nearly two miles inland and in the vicinity of the Wesleyan Mission station. This is described on page [58]. The submergence at some period of the watershed between the Mbua and Lekutu districts is indicated by the presence of microscopic foraminifera in the hyalomelan tuffs that are exposed in the dividing ridge.
Along the whole coast between the mouth of the Mbua River and Solevu Bay, there are but few if any traces of upheaval. Even volcanic tuffs are of rare occurrence, and there is only the case of the formation of Lekumbi Point to be here referred to. This singular low cape is described on page [60]. Here it is sufficient to remark that it is monopolised by the mangroves except at the outer part where the swampy ground passes into the dry sandy soil of a reef-islet, occupied by the usual littoral vegetation, and raised only a foot or two above the high-water level. It exhibits on the beach the bedded sand-rock so often found on coral islets, but this in itself is no evidence of emergence.
Neither on the shores of Wainunu Bay nor in the Kumbulau peninsula were upraised reefs observed, although the presence in places of submarine tuffs inland and near the coast affords evidence of elevation. The same remark applies to the coasts of Savu-savu Bay.
I have little doubt that the absence of elevated reefs on the coasts of by far the greater part of the island is the result largely of denudation. In this case we have to explain why an island in a region of coral reefs exhibits on the surface of its interior submarine tuffs and clays in most localities, whilst uplifted reefs are very rarely to be found at the coast or in fact anywhere. This view receives support from the existence of traces of old elevated reefs in different parts of the island. These traces are afforded by the occurrence on the surface in different localities of silicified fragments of coral associated with concretions of chalcedony, bits of flints and hornstones, jasper, impure siliceous nodules, &c. The localities may be at the coast or a mile or two inland, and are not usually more than 100 or 200 feet above the sea. This subject is treated with some detail in [Chapter XXV]. Here I may say that such localities are confined mostly to the open, low, undulating districts on the north side of the island. Silicified corals are not always present with the fragments of chalcedony and other siliceous concretions that are found so frequently in these situations; but from their association in the plains of Kalikoso, where the silicification of corals may almost be observed in operation, the previous existence of corals may be more than suspected in localities where only the other siliceous materials are observed.
I pass on now to some general considerations regarding the relations of the mangrove-belt to the sea-border and the character of the slope of the land-surface as compared with that of the submarine platform. An accurate conception respecting these matters will help one to avoid some pitfalls in forming an estimate of the character of the movement of emergence which this region has experienced.
Beginning with the mangrove-belt, some curious preliminary reflections arise, when we endeavour to look back into the past stages of the history of a mangrove tract in an area of emergence. We might perhaps expect to find the remains of such a belt in the upraised sea-borders; or if no traces existed, we ought to find in some places an extension inland of the reef-flat on which the mangroves at one time flourished. If a rapid movement of emergence is now in progress, the mangroves ought to cover the whole or greater part of the reef-flat; and in the mangrove tract of an emerging area we might look for signs of central decay and marginal growth, the mangroves dying in the middle of the tract and flourishing at the advancing margins.
When, however, we look at the mangrove-belt, as it at present exists around much of the coast of this island, we find that, except in the vicinity of the mouths of rivers, there extends beyond it a considerable extent of bare reef-flat, varying usually between 200 and 1,000 yards in width, and covered by the rising tide. There is no evidence of recent emergence in this condition of things. This relation between the mangrove-belt and the reef-flat indicates a state of equilibrium which might have been established long ago. It is the normal relation that exists between reef and mangrove growth; and it excludes all but very gradual movements of upheaval or emergence of the sea-border. It is not always easy to see why there should be this fine adjustment between the rapidly-growing mangrove and the slowly-growing reef. Under normal conditions, however, that is to say, when the land is stationary or when the change of level is of a very gradual nature, the reclaiming agency of the mangrove receives a check, and this relation between the mangrove-belt and the outer reef-flat is maintained.
Actual acquaintance with such localities soon forced me to the conclusion that whilst a gradual emergence or upheaval of 3 or 4 feet in a century would not materially affect the relation between the mangrove-belt and the reef-flat, a sudden or rapid change of level of that amount would destroy the mangroves around the whole island. There is some evidence, however, of there having been a rapid upheaval of this kind in different parts of the coast: and it follows, therefore, if this movement was general, that the present mangrove-belts date only from the last upheaval. But this elevation may have occurred ages ago; and the equilibrium between mangrove-belt and reef-flat may have been long since established. Accordingly, the breadth of the mangrove-belt can afford no indication of the period that has since elapsed. From data referred to below, it is evident that the mangrove-belt, taking its average width, away from the estuaries, at about 500 yards, might have been formed in two or three centuries, whilst a thousand years or more may have passed since it assumed its present relation to the reef-flat. If, therefore, upheaval is in progress, it must be of a very gradual character, since the normal relation of mangrove-belt to reef-flat now prevails.
There are indeed signs of such a gradual movement of emergence or of elevation being in operation on the north coast of Vanua Levu at the present time. I have before referred (page [11]) to the extensive bare mud-flats in the midst of the mangrove-belt between Nanduri and Lambasa, which are well represented on the Tambia coast and in Nanduri Bay. They are only covered by the higher tides, and in the intervals their surfaces are dried and cracked by exposure to the sun. Here we have the central decay and the marginal growth which would be expected in a mangrove tract situated in a gradually rising area.
An indirect indication of such a slow upheaval on the north coast is to be found in the circumstance that the great submarine platform, which reaches seaward to the line of barrier-reefs, 15 to 20 miles away, passes gradually, as it extends landward, into the low-lying plains that constitute the sea-border between Lekutu and Ravi-ravi Point. As shown in the profile-section on p. [62], these low coast districts are prolonged inland, with an average rise of between 20 and 30 feet in a mile, to the heart of the island; and we have here an extension inland of the slope of the submarine platform. These broad inland plains, and I may here include those behind Lambasa, are covered over much of their surface with submarine tuffs and clays in such a manner that we may almost trace their continuity at the coast with similar deposits now in actual formation beyond the low-water level on the surface of the submarine platform.
A glance at the map of the island, where these inland plains are indicated by the 300 feet of the contour-line, will make this point more clear. These plains are traversed by the Sarawanga, Ndreketi, Wailevu, and Lambasa rivers; and so slight is the fall that cutters usually ascend the rivers for several miles, whilst the tide extends for a considerable distance up their courses. That the emergence of the inland plains of Kalikoso in the eastern part of the island is comparatively recent there can be but little doubt. In that locality as described on page [224], the low marshy land, surrounding the fresh-water lake of Vakalalatha, although five miles inland, is only elevated 20 to 30 feet or less above the sea, and silicified corals are scattered over its surface.
There is one other method of ascertaining the character and amount of elevation that may be still in progress in this island namely the comparison of the results of surveys of the coasts at different periods. In this manner data may be obtained as regards the growth of the mangrove belt, changes in size of the low reef-islets and islands, and alterations in depth. For this purpose I have employed the charts of the north and west coasts of the island made by Commodore Wilkes in 1840[[10]] and the Admiralty charts 379 and 382 as completed from the survey of these coasts by Commander Combe in 1895-96.
It was not easy to make many good comparisons in the case of the advance of the mangrove-belt of the main coast. There certainly has been no great advance seaward of the margin of the mangroves in this half century. The average amount probably lies between the estimate obtained for the coast opposite Mathuata Island, where there has either been no change or an advance of only 100 yards or so, and that for the advance seaward of the mangrove promontory of Lekutu which amounts to 500 or 600 yards. In this last case, however, much of the extension may be due to the advance of the mangroves on the mud brought down by the Lekutu river, so that, as far as these data show, the average advance of the belt of mangroves on this coast between 1840 and 1895 would appear to be slight.[[11]]
On the other hand, the mangrove-borders of the several low islands and islets, mainly formed of reef-débris, that lie off the coast, have often extended themselves during this period in a marked degree. The results of my comparisons are given below, the rate of advance being obtained by halving the increase in length or breadth as measured between the mangrove-borders, the breadth being used in the long islands.
Advance of the Mangrove-Borders of Low Islands on the North Coast of
Vanua Levu between 1840 and 1895.
| Thukini, or Gibson Island of Wilkes | 700 to | 800 | yards | |
| Nangano, or Piner’s Island of Wilkes | 300 to | 400 | " | |
| Nandongo, or Nuvera of Wilkes | 500 | " | ||
| Talailau (two new islands) | 400 to | 900 | " | |
| Nukunuku or Clark’s Island of Wilkes | Not much change. | |||
| Thakavi, or Day’s Island of Wilkes | ||||
It will be noticed that the islands of the Talailau Reef are not marked in the chart of 1840; they are both low mangrove islands, the largest being slightly under a mile long and the smallest a little under half a mile. In Nukuira Island, the Vatou of Wilkes, there has been a decrease of about two-thirds of a mile during this period. The difference between Thukini in 1840 and in 1895 is very noticeable. In the time of Wilkes the mangroves only occupied about one-third of the reef-patch. Now they occupy about two-thirds, the area of the reef-patch remaining much about the same. Taking the minus and plus values of all the islands here measured, the average rate of the advance of the mangrove-margins during this half-century may be placed at about 250 yards in the case of these reef-islands, which would amount to a mile in 400 years.
It is probable that a long island like Ndongo, which is about four miles in length, has been formed by the union of smaller mangrove islands. Therefore, taking half its maximum breadth of a mile as a guide, it would at this average rate of growth require two centuries for its formation. But since the extension of the mangroves depends on the growth of the reef-patch, which takes place on the average at a much slower rate, it follows that this can only be a minimum limit for the age of this island. We can only assume that if the reef-patch had suddenly appeared 200 years ago, Ndongo Island could by this time have acquired its present dimensions. It does not follow that the mangrove border has been continuously advancing. A hundred years ago there may have been a state of equilibrium between the growth of the mangrove and the reef-patch, which does not now exist. All we can say of some of these low islands is that the mangroves have been rapidly extending their margins during the last half century, and that the normal adjustment between reef-growth and mangrove-growth, which must have once existed, does not now prevail.
There is evidence of the shoaling of the ship channel amongst these islands to the extent of about a fathom during this period.[[12]] The usual depth immediately around the patches, on which the islands have been formed, is 8 to 10 fathoms. If, therefore, the shoaling is a general process, it is to be inferred that although the outward growth of the reef-patches would be usually very slow, probably not over fifty yards in a century, there must be times when, in shallowing depths, the growth of the reef-patch would be comparatively rapid; and it is at such times that the adjustment between the relations of mangrove and reef-patch would be upset so that the advance of the mangroves would be for a time unrestricted.
It is, therefore, apparent that the rate of growth of one of these low islands is not to be determined by the rate of growth of the mangrove-tract occupying the surface. The subject is a complicated one; but I think enough has been said to show that the destructive agencies do not prevail on this great submarine platform on the north coast of Vanua Levu.
If the data here adduced of the increase of the low islands, of the shoaling of the channels, and of the advance of the delta of the Lekutu river,[[13]] are well founded, all the islands, islets, and reef-patches that lie along this north coast will be united to each other and to the main island within a thousand years.
The facts here produced do not directly indicate a movement of upheaval but they are quite consistent with the conclusion that the great movement of elevation which has built up Vanua Levu by the union of several smaller islands is still in operation at its coasts. To assume that there is now in progress at the sea-border the same process of island-building which has produced Vanua Levu, as we now see it, is to assume a uniformity in nature’s methods which is disregarded by the hypothesis that the great submarine platform, from which the large islands of Viti Levu and Vanua Levu now arise, represents the work of marine erosion into the flanks of the upheaved islands since the last elevation. The origin of this submarine platform is dealt with in [Chapter XXVII]. Here it may be remarked that I regard it as older than the islands that rise from it.
However, this movement of upheaval is so gradual that the utmost one can expect to do by the comparison of surveys made half a century apart is to show the lack of evidence of the destructive agency of erosion. As far as the comparison admits of judging, there seems to have been no important change on the coasts of the western end of the island during this period. The low neck of land connecting Naivaka with the main island, if we take the low-water line in the Admiralty chart as the limit, had much the same breadth at the time of both surveys. The depths in Mbua Bay remain about the same, with perhaps a shoaling of less than a fathom in places. There are two cays awash in the Admiralty plan of this bay which were described as sand-spits in the time of Wilkes. The promontory of Lekumbi could scarcely have been expected to show any extension during this time, since there are depths of 10 to 16 fathoms close to its extremity; and there is in fact no difference of critical importance indicated in the charts.
Some of the principal points of this chapter may be thus summed up:—
(1) Upraised reef-limestones are of very limited occurrence. They occur at and near the coast and do not extend higher than 300 feet. Their scarcity at the sea-border is to be attributed to the denuding agencies.
(2) Since foraminiferous muds and sedimentary tuffs with marine organic remains occur at all elevations up to over 2000 feet, it is assumed that the absence of reef-limestones in the elevated interior indicates the paucity or absence of reef-growths in the early stages of the history of the island. The overlying agglomerates have often preserved from destruction the soft sedimentary deposits beneath; but they seem to have never covered over a coral reef.
(3) The relation between the mangrove-belt and the reef-flat indicates a state of equilibrium which might have been established long ago. If the movement of emergence is still in progress, it must therefore be of a very gradual nature, since the normal relation between the mangrove-belt and reef-flat now prevails.
(4) From the circumstance that the submarine platform passes with a uniform slope into the low-lying plains, covered with submarine deposits, it may be inferred that a very gradual emergence is now in operation.
(5) A comparison of the charts of Wilkes and of the British Admiralty shows that on the north coast of the island during the last half century the destructive agencies of marine erosion have not prevailed.
(6) The results of the comparison of the charts, whilst they do not directly imply a change of level, are quite consistent with the conclusion that the movement of emergence, which has been in operation probably since the later Tertiary period, is not suspended.
Note.—The extensive evidence of emergence presented by this island is treated in [Chapter XXVII.] in connection with the whole group. It is not always possible to avoid in such a discussion the use of terms such as “upheaval” and “subsidence,” although there is much to be said for the terms “negative” and “positive” employed by Suess. In the present chapter, however, I have avoided committing myself definitely to any view relating to the stability either of the land or of the sea, reserving the consideration of the subject for [Chapter XXVII.]
CHAPTER III
THE HOT SPRINGS OF VANUA LEVU
The abundance of hot springs in Vanua Levu, and in fact in the group generally, is not commonly known. In the earlier accounts of these islands those of Savu-savu are often alone referred to, not only for this island but for the whole archipelago. The United States Exploring Expedition under Wilkes spent six months in 1840 in making a survey of the whole group. Yet Dana, who was attached to the expedition, remarks that “the only trace of actual volcanic heat which the islands appear to contain is found at Savu-savu Bay.”[[14]] Horne in his excellent account of the group, which he visited in 1878, was among the first to direct attention to the abundance of hot springs there; but he does not enumerate many. Although he travelled extensively over Vanua Levu, he refers to only three in that island, namely, at Savu-savu, Wainunu, and Vunisawana.[[15]] It will be shown below that most of the thermal springs discovered by me might easily have been overlooked.
Before dealing with those of Vanua Levu I will mention the other localities in the group in which thermal springs are from various sources known to me. They probably form but a small proportion of those that actually exist; but the list can be readily extended by those acquainted with special parts of the archipelago. In Viti Levu they occur amongst other places at Wai Mbasanga, on the Singatoka river (Horne) and at Na Seivau on the Wai Ndina, where Macdonald in 1856 found temperatures of 106° and 140° Fahr. in two different springs.[[16]] Mr. Thiele in more recent years referred by hearsay to some hot springs on the Wai Ndina.[[17]] Kleinschmidt in 1876 visited a hot spring near the village of Nambualu in the island of Ono which rose up in the midst of a brook and had a temperature of about 100° Fahr.[[18]] The same naturalist in July of that year, when accompanied by Dr. Max Büchner, came upon a hot spring issuing among the mangroves at the coast about a mile from the village of Ndavingele in Kandavu. He did not take the temperature; but he says that Colonel Smythe (about 1860) observed the temperature to be 144° Fahr.[[19]] Different writers refer to extensive hot springs on the island of Ngau. They are placed near the beach, and close to an ordinary cool spring. Miss Gordon Cumming in At Home in Fiji gives an illustration of them. Horne mentions a hot spring on the island of Rambi. Andrews describes two others that bubble up through the limestone near the tidal zone in the southern part of Vanua Mbalavu. Both these springs are in close proximity to the junction line between the intruded andesite and the old reef rock. One of them, though not boiling, was hot enough to scald the skin.[[20]] This list is no doubt capable of being much extended, especially for Viti Levu and the Lau Group.
A description of the several systems of thermal springs of Vanua Levu will now be given.
1. The Hot Springs of the Lower Valley of the Wainunu River.—This is one of the most extensive systems of the kind in the island. The temperature of the various springs during my sojourn in this district in 1898 ranged from 100° to 130° Fahr. Those known to me are mostly situated in the lower part and at the mouth of the Ndavutu Creek, one of the tributaries of the Wainunu. They open usually on the river-bank, either close to the water or a few feet above it, but some of them find an exit under water at the bottom of the river. Natives allege that hot springs occur at intervals on the left bank and at the river-bottom along the whole length of the river below Ndavutu Creek. There is certainly a hot spring on the right side of the river’s mouth near Mr. Dyer’s house. It issues from the reef-flat and can only be observed at exceptionally low tides. There is also a hot spring which rises up at the edge of the stream at Thongea (Cogea) nearly a mile above Ndavutu. If the above statement of the natives is correct, as I believe it is, then these thermal springs issue along a line quite four geographical miles in length extending inland from the mouth of the Wainunu.
All the springs are situated in the tidal part of the river-valley, with the exception of that of Thongea, which is just above this limit. They are but little elevated above the sea-level, those exposed being usually not more than ten feet above the river and often much less. This is a region of basalt, the valley of the Wainunu lying, as described on page [82], in the fold between two great basaltic flows, and probably representing a line of weakness, along which the hot springs issue either from among loose blocks, or from the soil, or from a tufaceous sandstone. They deposit little if any of the siliceous sinter which is often found in the thermal waters of this island. This is due probably to their scanty exposure and to their low temperature. The density of the water is near that of fresh water, being not over 1001. The following temperatures may be useful for comparison with future observations:
| Thongea, | when not covered by the stream | July, | 1898, | 127° | F. | ||
| Ndavutu, | bath-spring at Mr. Barratt’s house | Usually | 100° | " | |||
| " | on left bank of the creek near the landing place | June, | 1898, | 126° | " | ||
| " | on left bank of creek near mouth | Dec. | " | 127° | " | ||
| " | pool in foot-path on left bank | June 2, | " | 112° | " | ||
| July 27, | " | 111° | " | ||||
| " | at bottom of main river in depth of 3 feet, close to the left bank and just above the mouthof the Ndavutu creek, self-registering Six thermometer used | July | " | 122° | " | ||
2. The Hot Springs of Natoarau and its Vicinity.—This thermal system lies in the lower valley of the Mbale-mbale branch of the river Ndreke-ni-wai. The principal springs are situated at Natoarau, a village about half a mile in a direct line from Mbale-mbale, about three miles from the coast, and only about fifty feet above the sea. They bubble up in pools near brooks, and extend at intervals over an area probably several hundred yards across. Five springs came under my notice; but there are doubtless several others in the low-lying and often swampy land of this district. No deposits were noticed, but the mode of occurrence and low temperature of the springs serve to explain this fact. The following temperature observations were made by me in March, 1899:—
| A. | Pool 4 feet across, with sides of stone, close to village | 126° | F. |
| B. | Pool 10 feet wide, a few paces from pool A | 114° | " |
| C. | Pool 12 feet wide, 100 yards from village, near the river | 103° | " |
| D. | Pool on the road to Mbale-mbale, mixed with surface water | 100° | " |
The natives and others often state that the thermal springs here and in other localities are much hotter in dry than in rainy weather. This is correct in a sense, because in wet weather the surface water would usually find access to the pools; but there is no reason to believe that the temperature of the water at the hole of exit varies at all from this cause. The temperature of pool A was taken at the bottom where the water bubbled up; and probably it represents the true degree of heat of these springs, since in the other cases observation of this point was not so easy. The weather was dry during this visit; but, three months before, I tested the temperature of this pool after heavy rain, when the district was flooded, and then I got a reading of 127° at the exit-hole of the spring.
Another thermal spring, which is distant about a mile from Natoarau, is known as Waitunutunu, that is, Warm Water. It lies about a third of a mile from the village of Nambuniseseri, between Mbale-mbale and Waisali, and is quite four miles inland and about 100 feet above the sea. The springs bubble up into a pool, about 12 feet across, which is close to a brook and had a temperature in March, 1899, of 109-112° F.
3. The Hot Springs of Nukumbolo.—The village of Nukumbolo, where the springs are situated, lies on the banks of a tributary of the Vatu-kawa branch of the river Ndreke-ni-wai, and is distant as the crow flies about six miles inland from the river’s mouth. The springs issue on a hill-slope from several places a few steps apart, and are removed about a hundred yards from the river, and from 20 to 30 feet above it. Their elevation above the sea would be about 130 feet. The temperature taken in the two hottest places was 157° F, in November 1898, and 158° in the following February. As in the case of the springs of Savu-savu and a few other localities, the rocks are coated with siliceous sinter mixed with carbonate of lime, and a gelatinous incrusting alga grows on the borders of tiny hollows bathed often in water of a temperature 137-140°, but thriving most where the temperature is 115-120°. The water runs down the slope into a series of pools made by the natives for bathing, the temperature of the lowest pool being 103-105° and of the highest 120°. This is one of the best localities I have seen in the island for the erection of thermal baths. The rock pierced by the springs is apparently a basic agglomerate-tuff. Large blocks of a hard and somewhat altered palagonitic tuff lie around the bathing pools.
4. The Boiling Springs of Savu-savu.—These springs figure in all the descriptions of the group, and they are also famous amongst the natives. Since they were described by Wilkes, who visited them in 1840, in his narrative of the United States Exploring Expedition, many accounts of them have been written by subsequent visitors; not infrequently they have been sketched as well as described; and several analyses of their waters have been made.[[21]] The accounts of these springs that lie before me extend at intervals over a period of nearly sixty years; but I shall allude to them only so far as they throw light on the history of the springs during this period.
The principal springs are situated in a slight hollow in a more or less level tract extending in from the beach, and are distant about 150 yards from the shore and about ten feet above the sea-level. They are five or six in number, and at the time of my visits in July and November, 1898, they were boiling briskly, the thermometer readings being 208-210° F., but the mercury probably fell two or three degrees in withdrawing the thermometer. When, as was the case when Wilkes visited this locality in 1840, there is but a slight appearance of boiling, brisk ebullition is produced by covering them over with leaves. The natives call this locality Na Kama, which signifies “the burning place,” and employed the springs extensively for cooking their food. Just as Wilkes describes, a freshwater brook runs past the springs and receives their outflow. The temperature of the brook immediately above the springs is that of an ordinary freshwater stream 75-76° F.; but below it is scalding. The account given by Wilkes of the spring and of the brook in 1840 applies to them in our own time. The small stones lying in the effluent channels of the springs are incrusted with siliceous sinter, and a green alga lines the sides, bathed generally in the steam but sometimes partially immersed in water only a few degrees below the boiling point. It is noteworthy that this alga which was flourishing in July was all dead in November.
The scalding water also oozes through the sand of the adjacent beach in abundance for a distance of at least some hundreds of yards. It is even stated that as far as Ndaku, a mile to the westward, the hot springs issue at intervals through the beach.[[22]] There are evidently also extensive submarine springs close to the beach; and probably Wilkes was not far from the truth when he remarked that the “whole area of half-a-mile square seems to be covered with hot springs.”
Off the beach, a few hundred yards to the westward of the springs, is a batch of dead reef formed of massive corals and only approachable from the shore at extreme low-tide when it is a little exposed. From numerous small holes and cracks in the dead-coral hot water issues almost at the boiling point (210° F). It is apparent that these springs have appeared at this particular spot since the corals grew. But it is remarkable that this has been apparently going on since the visit of Wilkes in 1840. He refers to a coral rock, distant one-third of a mile from the springs and 150 feet from the beach, through which boiling water was issuing in several places. This rock which was then 10 feet wide and 20 feet long, was at his visit exposed for three feet at low-tide and covered at high-tide.[[23]]
The geological characters of this locality are described on page [191]. I may here remark that if these thermal springs occupy the position of an old crater, it would require much imaginative power to restore it now. The off-lying small island of Nawi might by its situation appear to countenance this idea, but I found no special indication, when I examined it, in support of this view. From the geological character of the district, I would infer that if a crater once existed here it was submarine and that it has been long since obliterated by marine and aërial denudation. The boiling springs come up through apparently a rotten volcanic agglomerate. The slight hollow of three or four feet deep, in which they lie, was considered by Kleinschmidt to be an old crater cavity; but it is only 40 or 50 feet across, and in the earlier descriptions the hollow is described as surrounded by a mound of earth. As shown below, the natives themselves may be held responsible for many changes in the surface around the springs. There is, in fact, no trace of a crateral cavity in this district now.
I will now briefly notice the history of the boiling springs since 1840, when they were visited by Commodore Wilkes. At that time there were five springs, situated in a basin 40 feet across, and possessing a temperature of 200-210° F. Although there was scarcely any appearance of boiling, rapid ebullition could be excited by covering the springs with leaves and grass. The natives alleged that the springs had always been in the same condition. In 1863, when the Chief of Wainunu (Tui Wainunu) came to fight the Savu-savu people, he endeavoured but without success to choke up the springs by heaping earth over them. I was informed of this circumstance by Mr. A. H. Barrack, the owner of the springs. Miss Gordon Cumming also refers to it in her book At Home in Fiji. When this lady visited the springs in August, 1876, they were intermittent in their action, the highest making a fountain two to three feet high. According to the description of Kleinschmidt they were in the same intermittent condition in May of the same year. There were then four springs situated in a bowl-shaped hollow. The two larger springs were not constantly bubbling up, but displayed periodic ebullitions of about twenty minutes’ duration, the waters disappearing in the intervals. The other two springs were not then active. Horne, who visited this locality in 1878, refers to three or four principal springs situated in the centre of a hollow, which was surrounded by a mound of earth, the water boiling up to the height of about a foot.
About this time the springs entered for a while into a new phase of action and assumed the form of geysers. According to information received from Mr. A. H. Barrack and other old residents in Savu-savu, the waters spouted up to a height of from 40 to 60 feet, not vertically but at an angle. Each outburst, which lasted for ten or twenty minutes, was followed by a similar interval of repose, during which the springs dried up. This continued for a month or two, after which the springs gradually resumed their normal level. When I visited the springs in July and November, 1898, they were boiling briskly, attaining a height of a few inches, and showed no signs of intermittent action.
I come now to the different analyses that have been made of the water of these thermal springs of Savu-savu. Specimens have been analysed at different times by chemists in various parts of the world, in America, in Germany, in Australia, etc., and the results as far as known to me are now appended.
A. Analysis by Dr. C. T. Jackson of Boston, U.S., of the water
obtained in 1840 by the Wilkes Exploring Expedition.[[24]]
Specific Gravity 1·0097. Temperature 57° F.
The evaporation of a quantity equal to 1000 grains of distilled water gave 7·2 grains of salt, thus composed:—
| Chlorine | 3·577 | |
| Sodium | 1·665 | or Soda 2·238. |
| Magnesia | 0·440 | |
| Lime | 0·366 | |
| Silica and iron with a trace of phosphate of lime | 0·200 | |
| Carbonic acid | 0·493 | |
| 6·741 | ||
| Organic matter and loss | 0·459 | |
| 7·200 |
B. Analysis by Dr. Oscar Pieper of Hamburg of the water
obtained by Mr. Kleinschmidt in May, 1876.[[25]]
The report stated that the water was clear, neutral in reaction and salt-bitter in taste, brown flakes of hydrated iron oxide occurring in it after long standing. The dissolved salts amounted to “8·48 g. per litre,” and the remark is made that “the concentration is therefore not so great as in sea-water.” The solid constituents consisted in by far the greatest part of Natrium and Calcium chlorides. A quantitative determination, which on account of the small quantity of the water was confined to “eine Chlor und Kalkbestimmung,” gave this result:—
| Chlor (Chlorine) | 4·79 | g. per | litre. |
| Kalk (Lime) | 2·31 | " | " |
Reckoned as Chlornatrium (Kocksalz) and Chlorcalcium, these results were obtained:—
| Chlorcalcium (Calcium chloride) | 4·55 | g. per | litre. |
| Chlornatrium (Sodium chloride) | 3·09 | " | " |
Amongst other constituents found in small quantities were Sulphuric acid, Silicic acid (Kieselsäure), Potash, and Iron oxide. Iodine, Bromine, Nitrates, and Borates were completely wanting. “If this water,” says Dr. Pieper, “has healing properties, it does not owe them to its chemical composition.”
C. Analysis by Mr. H. Rocholl of sample obtained by Mr. H.
Stonehewer Cooper probably in 1877 or 1878.[[26]]
| Total | solids | at 212° F. | ·8796 | per | cent. |
| " | " | ignited | ·7726 | " | " |
| The residue consisted of— | |||||
| Free Sulphuric Acid (SO3) | ·0049 | " | " | ||
| Calcium sulphate | ·0260 | " | " | ||
| Calcium chloride | ·4355 | " | " | ||
| Magnesium chloride | ·0021 | " | " | ||
| Potassium chloride | ·0415 | " | " | ||
| Water | ·1070 | " | " | ||
| Sodium chloride | ·2641 | " | " | ||
| ·8811 | |||||
D. Analysis by Prof. Liversidge of the Sydney University of a sample of the water collected by Dr. Bromlow, R.N., about 1879.[[27]]
The specific gravity was 1·0064 at 60° F. The total solids in solution were 582·4 grains per gallon; but when heated to a dull red heat, the residue was 546·9 grains per gallon, the combined water having been driven off. Iodine and bromine were carefully sought for, but in vain. Four pints of the water were examined.
Composition.
| Per cent. in residue. | Parts per million of water. | Grains per gallon. | |
|---|---|---|---|
| Silica, insoluble | 1·681 | 133·3 | 9·20 |
| Silica, soluble | ·074 | 5·8 | ·40 |
| Alumina and traces of Iron sesquioxide | ·534 | 41·7 | 2·92 |
| Aluminium chloride | 1·646 | 128·6 | 9·00 |
| Phosphoric acid | traces | traces | traces |
| Calcium chloride | 46·754 | 3,652·9 | 255·70 |
| Calcium sulphate | 4·770 | 372·7 | 26·09 |
| Magnesium chloride | ·154 | 12·0 | ·84 |
| Sodium chloride | 42·171 | 3,294·8 | 230·64 |
| Potassium chloride | 1·756 | 137·2 | 9·60 |
| Carbonic acid | traces | traces | traces |
| Loss | ·460 | 34·0 | 2·52 |
| 100·000 | 7,813·0 | 546·91 |
Looking at the general character of these thermal springs of Savu-savu we may quote the remarks of Prof. Liversidge and Dr. Pieper that the salts in solution consist for the most part of chlorides, the chlorides of calcium and sodium largely prevailing.
Comparison of the analyses of the water of the Savu-savu thermal
springs, stated in grains per thousand of water.[[28]]
| Date collected. | Chlorine. | Sodium. | Calcium. | Calcium chloride. | Natrium chloride. | Total salts. | Density. | ||
|---|---|---|---|---|---|---|---|---|---|
| Dr. Jackson | 1840 | 3·57 | 1·66 | 0·36 | — | — | 7·20 | 1·009 | |
| Dr. Pieper | 1876 | 4·79 | — | 2·31 | 4·55 | 3·09 | 8·48 | — | |
| Mr. Rocholl | 1878 | — | — | — | 4·35 | 2·64 | 8·81 | — | |
| Prof. Liversidge | 1879 | (4·50) | (1·29) | (1·42) | 3·65 | 3·29 | 7·81 | 1·006 | |
| Sea-water, tropics | 19·46 | 11·08 | 0·46 | — | — | 35·00 | 1·02 | ||
| Date collected. | Chlorine. | Sodium. | Calcium. | |
|---|---|---|---|---|
| Dr. Jackson | 1840 | 3·57 | 1·66 | 0·36 |
| Dr. Pieper | 1876 | 4·79 | — | 2·31 |
| Mr. Rocholl | 1878 | — | — | — |
| Prof. Liversidge | 1879 | (4·50) | (1·29) | (1·42) |
| Sea-water, tropics | 19·46 | 11·08 | 0·46 | |
Comparison of the analyses of the water of the Savu-savu thermal
springs, stated in grains per thousand of water. continued
| Calcium chloride. | Natrium chloride. | Total salts. | Density. | |
|---|---|---|---|---|
| Dr. Jackson | — | — | 7·20 | 1·009 |
| Dr. Pieper | 4·55 | 3·09 | 8·48 | — |
| Mr. Rocholl | 4·35 | 2·64 | 8·81 | — |
| Prof. Liversidge | 3·65 | 3·29 | 7·81 | 1·006 |
| Sea-water, tropics | — | — | 35·00 | 1·02 |
It is to be inferred from the above that the quantity of salts in solution remains about the same, the proportion varying only in the four analyses, which extended over a period of forty years, between 7·2 and 8·8 grains per thousand grains of water. This is considerably less than the salts in solution in sea-water, namely 35 grains per thousand. The relative proportions of the salts, excepting those of calcium, do not vary more than we should expect in the case of analyses made by varying methods and probably with a varying degree of exactness.
Dana[[29]] considered from Dr. Jackson’s analysis that the water of the Savu-savu springs is probably of marine origin; but the absence of bromine and iodine, as especially remarked by Dr. Pieper and Prof. Liversidge does not support this view. We might also expect the proportion of the salts to each other to show a greater similarity to that in sea-water than they do. On the other hand the total volume of water discharged, not only by the springs proper but for several hundred yards along the beach, and also between the tide-marks and beyond, must be far greater than could be supplied by the rainfall of this portion of the Savu-savu peninsula, which is only one and a half to two miles across and 800 feet high. We must look, I think, for the source of these waters in deep subterranean streams or artesian basins that would be fed by the rains precipitated in the mountainous districts where the rainfall amounts to at least 200-300 inches in the year. This matter is further discussed in my general remarks on the hot-springs of this island (page [38]).
5. The Hot Springs near Ravuka.—These springs rise up in the centre of the breadth of the island about nine miles direct from the coast. They are about 200 feet above the sea and are situated on the Ndrawa branch of the Ndreketi River some two miles below the hamlet of Ravuka. They are on a small scale and ooze through a bed of rounded blocks and pebbles close to the water on the left bank. Their temperature in August, 1898, was 148° F. They are covered by the river when it is swollen by the rains, and very probably other hot-springs issue along the river-bottom. The conditions are not suitable for the formation of deposits.
6. The Hot Springs of Vuinasanga.—These thermal springs are also situated in the heart of the island on a tributary of the Ndreketi some three or four miles westward from Va Lili and about 150 feet above the sea. On each bank of the river four or five paces from the water and three or four feet above it, there is a small pool two to four feet wide. In June, 1899, the pool on the right bank had a temperature of 131° F., and that on the left bank of 134°. There were no deposits.
7. The Hot Springs on the South Side of the Nawavi Range.—These springs also lie within the borders of the valley of the Ndreketi. They may be “located” by describing them as lying a few miles inland from the north coast fronted by Mathuata Island. I did not visit them and have only learned of them from Mr. Thomson’s Mathuata paper.[[30]] That gentleman refers to them as two in number and situated at the back of the coast range about four miles inland from the village of Nangumu; but no particulars are given.
8. The Hot Springs of Vatuloaloa.—These springs lie on the Mathuata Coast in the neighbourhood of Mathuata Island. I have not seen them, but am indebted to Mr. Thomson for the particulars here given, which are taken from his paper above quoted. Mr. Thomson, who discovered them in 1880, named them the “Graçie” springs. They issue below high-water mark at Vatuloaloa, and had a temperature in 1880 of about 140° F. They are said to possess many valuable healing qualities.
9. The Hot Springs of Nambuonu.—These springs are situated on the same part of the Mathuata coast as those of Vatuloaloa above referred to. I learned from Mr. Bulling of Undu Point that they issue from swampy ground half a mile inland. They were discovered accidentally by a Japanese who put his foot into them, the temperature being sufficiently high to scald the feet, but not at the boiling-point, probably about 140° F.
10. The Hot Springs near Tambia.—These extensive springs, situate 1½ to 2 miles inland, and rather under 100 feet above the sea, lie near the Mathuata or north coast of the island, some four miles west of the Wailevu river. They rise up in the midst of level country about a mile from the town of Tambia, and near the village of Ngovungovu. Although situated in the valley of the Tambia river, these springs are not adjacent to the river, and in this respect they differ from nearly all the inland hot springs. The hottest spring bubbles up into a pool 5 or 6 feet across, which had a temperature of 180° F., in March, 1899. Near by is a large deep pool, some 20 feet or more across, with a temperature of 100°. It receives the overflow from the smaller pool, and apparently hot water also bubbles up at the bottom. Around the smaller hottest pool there is a considerable deposit of what is mainly siliceous sinter. It incrusts the stones and also the oyster-shells lying about the pool in quantities, where they have been left by the natives after their contents had been cooked and eaten. Some of the shells are almost decayed away, the sinter for the most part alone remaining.
11. The Hot Springs of Vandrani.—These springs occur in the heart of the island, about 8 miles from the coast in a straight line, and about 270 feet above the sea. This is the greatest elevation, as far as I know, at which a hot spring exists in this island. Here they rise up near the base of the central mountain range, close to the head-waters of the Wailevu river which opens into Lambasa bay. The springs bubble up into a pool, a foot deep, on the left side of the river, four or five paces away from the water’s edge, and scarcely raised above it. They are covered over when the river is in flood. In February, 1899, the temperature recorded by my thermometer was 100° F.; but probably it was a few degrees higher at the bottom of the pool. I noticed no deposits.
12. The Hot Springs of Na Kama on the Wailevu River.—These boiling springs, which are of an extensive character, come up in half-a-dozen places on either bank of the river, and are from 5 to 6 miles inland, and about 90 feet above the sea. They are close to the water, and from 1 to 10 feet above it. The temperature of one small pool, where the water bubbled up briskly, was 204° F. in February, 1899. In another it was 194°. The water was probably at the boiling-point in some cases as it entered the pools, and in the others it could have been only a few degrees below it. The rocks of the district are agglomerates and tuffs. I have no recollection of deposits of any extent around the springs.
13. The Hot Springs of Vunimoli on the Lambasa River.—A few minutes’ walk from Vunimoli, and about 100 yards from the left bank of the river, these hot springs issue in a place named Vunimbele from the foraminiferous clay rock (soapstone) of the district. They are on the side of a ditch which communicates with the river. The natives have cut out of the soft rock small square basins which receive the waters. The temperature of the hottest spring in August, 1899, was 155° F. That of others was 140°. The conditions are not favourable for the formation of deposits. These springs lie about 8 miles inland and are rather over 100 feet above the sea. They are, however, small and unimportant, and the locality in which they occur is now overgrown with vegetation and not easy to discover.
14. The Hot Springs of Mbati-ni-kama on the Ngawa River.—These springs are situated in the Lambasa district about 7½ miles from the coast, and rather over 100 feet above the sea. They issue copiously from the volcanic agglomerate at a temperature of 161° F. (August, 1899), and are only removed a few paces from the river, and a foot or so above it. Algæ flourish in the water, and siliceous sinter incrusts the rocks.
15. The Hot Spring of Nandongo on the Head-Waters of the Wai-ni-koro River.—A few hundred yards from the village and elevated about 180 feet above the sea there is a small pool in the clay of the river bank, 2 or 3 feet above and close to the water, which in September, 1899, had a temperature of 97° F.
16. The Hot Springs of Natuvo on the North Coast of Natewa Bay.—About a mile east of Mbiagunu and near the village of Natuvo, there are two hot springs of small size which I visited in August, 1899. One that issued on the reef-flat from the coral-rock at a temperature of 136° F. was covered over towards high-tide. The other issued near by at a temperature of 131° from swampy ground a few paces among the trees.
17. The Hot Springs of Ndaku-ndaku on the North Coast of Natewa Bay.—At this place about 2 miles north of Vuinandi some hot springs rise through the reef-flat, which are only exposed at low tide. At the time of my visit they were covered over by the rising tide. The natives described them as not very hot and like the neighbouring hot springs of Natuvo.
18. The Hot Spring of Navakaravi, Natewa Bay.—The coast village thus named lies about one and a half miles north of Were-kamba. The hot spring is about a mile inland and not over 30 to 40 feet above the sea. It is reached after traversing a low and often swampy tract. The spring in August, 1899, issued from a little rise at a temperature of 133° Fahr., and formed a rivulet 18 inches across.
19. The Hot Springs of Vunisawana at the head of Natewa Bay.—Mr. Horne, who was in this locality in 1878, refers to these springs in his book A Year in Fiji. They had at one time, he remarks, a wide reputation for their curative qualities; but the people around became so poor on account of the hospitality that custom compelled them to extend to the numerous visitors that they buried up the springs. Mr. Horne was shown the site at the bottom of a muddy creek. I saw it in 1898. It lies 300 or 400 yards in from the beach and only a few feet above the sea. There were no signs of heat then; but I was told that when the stream close by is very low it sometimes is a little warm.
20. The Hot Spring of Ndreke-ni-wai on the South Coast of Natewa Bay.—This small spring issues between the tide-marks from an old reef-patch close to the shore and is only to be seen at low-water. Its temperature in May, 1898, was 130-135° Fahr.
21. The Hot Spring of Waikatakata on the South Coast of Natewa Bay.—This important spring lies about four miles east of the town of Natewa. It issues on a hill-slope about 400 yards from the beach and is some 25 or 30 feet above the sea; but it is so beset by undergrowth that the source is not easy to reach. Boulders and blocks of a basaltic rock lie about on the slope; and it is from under a huge boulder of five or six tons in weight that the spring emerges at a temperature of 148° Fahr. (April, 1898). There is a good volume of water, and a series of bathing pools of varying temperature could be readily made. Unlike most of the inland hot springs, it is not in connection with a stream or river.
22. The Hot Spring of Ndevo on the Coast opposite to Rambi.—I did not hear of any spring when in the locality; but I learned afterwards that near a stream on the beach there is a hot spring which is covered at high tide.
23. The Hot Spring of Navuni near Fawn Harbour.—This small spring is situated in a hilly district in a region where olivine-basalts prevail. I was indebted to Mr. Pickering for showing me its locality. It lies about three-quarters of a mile inland and about 100 feet above the sea. It issues from the volcanic agglomerate a few paces from the right bank of the Navuni stream and five or six feet above its level. In May, 1898, it had a temperature of 112-113° Fahr.
GENERAL REMARKS ON THE HOT SPRINGS
This island is therefore remarkable for the number of its hot springs. In the list given on page [40] I have enumerated 23 localities where they occur; but, as shown below, their number will probably in time be extensively increased.
On referring to the map it will be observed that the distribution of these springs is fairly general over two-thirds or three-fourths of the island. Taking this area at about 1,500 square miles and dividing it into squares with sides of eight miles, we should, if the springs were quite evenly dispersed, find a thermal system in every square. Even amongst the Fijians and among the white residents the number of hot springs will cause surprise. Only those of Savu-savu, Wainunu, Nukumbolo, Mbatini-kama, and Na Kama on the Wailevu river have been up to this time generally known. The reason of this is that most of them are insignificant, and with a temperature far below the boiling-point, and ooze up in unlikely and out-of-the way places, as by the water-side in little visited river-valleys, on the reef-flats of not much frequented coasts, and in swampy situations where they are likely to be overlooked. The natives only recognise as “Na Kama” the boiling or very hot springs; and it was only after much questioning that I could get them to tell me of some unimportant “wai katakata” (hot water) which they deemed to be far beneath my notice. The natives were keenly interested in my botanical and geological investigations; but they considered it to be beneath the dignity of a man who had seen the wonders of Na Savu-savu to spend some time looking for a half-forgotten thermal spring in a swamp. From this cause alone I no doubt failed to find several springs. All the boiling springs and those of very high temperature are probably known; but as is pointed out below it is more than likely that a large number of unimportant springs remain to be discovered in many a deserted inland valley and between the tide-marks along the very extensive reef-bound coasts.
As above remarked the hot springs did not come under my notice in all parts of the island. They are to all appearance wanting in the western or Mbua portion, and also in the Undu portion north of Natewa Bay. Taking the first-named region, it will be noticed that no hot springs are indicated in the map west of the Ndreketi and Wainunu rivers. I made inquiries wherever I went, but with no result. On my writing to Mr. Wittstock, of Mbaulailai, who is well acquainted with the Mbua peninsula, he informed me that if hot springs existed in that part of the island he would probably have known of them. In that portion of the island which ends in Undu Point I could neither discover nor hear of any thermal springs east of Lambasa on the north side, and of Lakemba on the south or Natewa Bay side; nor could Mr. Bulling, who has resided at Undu Point for many years, tell me of any springs in his neighbourhood.
On looking at the general map it will be observed that the hot springs are confined to the area of basic rocks, although they do not occur all over that area, not being indicated in the map to the west of the Ndreketi and Wainunu rivers. They are not known to occur in the region of dacites and acid andesites, as in the case of the Drandramea district; and they have not been found in the area of rhyolitic and trachytic rocks that extends from Undu Point to Mbuthai-sau on the north coast and to near Tawaki on the Natewa Bay side. The region of hot springs would be limited on the east by a line joining the Mbati-ni-kama springs with those of Nandongo on the Wainikoro river and Natuvo on the north shore of Natewa Bay. Such a line, though lying within it, roughly indicates the limit between the regions of basic and acid rocks.
The situation of the hot springs in the lower levels, and their non-discovery at elevations exceeding 300 feet above the sea, are facts of importance. In more than half the cases they arise close to and often on the banks of streams and rivers, occasionally indeed at the river-bottom; and no doubt numerous unknown thermal springs issue under water from the river beds. In about a third of the known cases the springs come up on the coast between the tide-marks, usually rising through the reef-flat. At times even they are to be observed below the low tide level; and one can scarcely doubt that there are a large number of undiscovered springs that are never exposed at the lowest tides. It is also very likely that a number of hot springs issuing between the tide-marks are still to be discovered without much difficulty.
The same may be said of inland hot springs. Looking at the insignificant character of many of them and noting their occurrence in places where they might easily be overlooked, it is highly probable, as before remarked, that a number of springs exist inland, which, though once known to the natives, are now forgotten. The interior of the island is very sparsely inhabited now; but there is evidence of a much more populous condition in old times. The present natives are fast losing the knowledge of the interior of the island which their forefathers possessed; and many tracts in the mountain districts are far removed from existing paths. From the haphazard manner in which I lighted upon thermal springs beside the head-waters of the Ndreketi, Wailevu, and Wai-ni-koro rivers, I cannot doubt that many more exist in similar localities not visited by me.
With regard to the distribution of the springs as respecting temperature, I cannot find any marked arrangement either in their grouping or in the amount of elevation. It is noticeable, however, that the three systems of hottest springs, that of Savu-savu (210°), that of Na Kama on the Wailevu river (204°), and that of Tambia (180°) are all less than 100 feet above the sea. Although the springs of highest temperature are confined generally, with the exception of those of Savu-savu, to the main mass of the island, it would seem that adjacent systems of springs may differ much in temperature. The springs of Vunimoli, for instance, have a maximum temperature of 155°, which is nearly 50° lower than that of Na Kama, three miles to the westward. Hot springs are more numerous in the region around Lambasa than in most other districts. Lastly, I may add that earthquakes are apparently more frequent in the Mbua district, where no thermal springs are known, than in any other part of the island.
With regard to the deposits formed around the springs, it may be observed that the circumstances are not usually suitable for their formation, as for instance when they rise through the reef-flat or in swampy localities. In those springs, however, where the temperature is over 150° F., and where the water spreads over a surface so as to facilitate evaporation, deposits of white sinter associated with algæ occur, as at Savu-savu, Tambia, and Nukumbolo. Its composition varies a little in different localities. At Savu-savu it is compact and laminated and formed almost entirely of hydrated amorphous or colloid silica. At Mbati-ni-kama the siliceous sinter is more friable, with a tendency to form opal. The sinter of the Nukumbolo springs resembles that of Savu-savu; but it also contains a good proportion of carbonate of lime (20 per cent.) in a granular form, and that of Tambia has the same characters. It is not unlikely that this lime is derived from the decayed shells, such as I have referred to in the case of the Tambia springs.... It may be here observed that Mr. Weed and others, who have studied the origin of siliceous sinter in the Yellowstone region and elsewhere, regard it as the secretion of algæ, mosses, &c., that grow in hot waters (American Journal of Science, vol. 37, 1889).
I come now to some general considerations respecting the hot springs of Vanua Levu. In the first place there is the singular fact that the inland hot springs nearly always make their appearance along the present lines of surface-drainage. But I do not gather that the hot springs are of more recent origin than the rivers and streams, by the side of which they rise. On the contrary the hot springs are probably far older. The conditions of subterranean drainage that favour the formation of springs at the surface, whether cold or thermal, would no doubt often determine the direction of surface drainage in a newly-formed land. Those familiar with modern volcanoes will recall the absence or rarity of streams and rivers, and the frequency often of cold and thermal springs at and near the coast, which are sometimes of such bulk at the exits that the expression “subterranean river” would be nearly appropriate. The presence of artesian reservoirs may also in some localities be safely assumed. I will here draw a little on my own experience of volcanic regions.
On the lava-bound coasts of the riverless southern portion of the large volcanic island of Hawaii, the subterranean waters issue as cold and thermal springs at numerous localities. At Punaluu, and at Ninoli, a mile to the westward, there are extensive freshwater springs at and near the beach which have a temperature of 64° F. all through the year,[[31]] those at Ninoli issuing as a large subterranean stream. East of Punaluu and at intervals along the Puna coast, springs of water, sometimes fresh and cold with a temperature occasionally as low as 64°, at other times mineral and thermal, but with a temperature not usually above 95°, issue at the surface or at the bottom of deep fissures in the old lava flows.... In Oahu, another island of the Hawaiian group, where the volcanic forces have been long extinct, artesian wells have been in extensive use for some years in the irrigation of the sugar-cane plantations. The last water-bearing strata are reached at depths of 400 to 500 feet.[[32]] The subterranean or artesian reservoirs are evidently therefore on a large scale; yet Oahu is scarcely one-third the size of Vanua Levu in Fiji.... Lastly, I will refer to the numerous subterranean streams that issue forth, as cold and thermal springs, from beneath the lavas near and at the Etna coast, as for instance in the vicinity of Acireale. The Etna slopes are in great part deforested, and in consequence soakage is relatively small, and after heavy rains much of the water runs off in the torrents. Whilst in this locality I was impressed with these facts, and I formed the opinion that in ancient times when Etna was well wooded the discharge of subterranean streams at the coast was far greater than at present.
For these reasons and on other grounds, amongst them notably the absence of recent crateral cavities, I infer that the numerous hot springs are the outflows of subterranean streams, fed originally by the “soakage” arising from a rainfall of at least 200 to 300 inches in the mountainous portions of the island. Such subterranean streams run probably at considerable depths, emerging, it is likely, as often under the sea as they do on the land.
Since writing the above I have read in the Journal of the Royal Geographical Society (November 1902), an abstract of a lecture by Prof. Suess on the subject of hot springs and volcanic phenomena. Thermal springs, he holds, are supplied by hypogene waters and do not receive their salts from the sea. Such springs, according to this view, being the survivals of volcanic activity, originate in the depths of the earth’s crust and bring water to the surface for the first time, not deriving it from infiltration. It seems almost impertinent to suggest a view opposed to that of such a high authority; but it appears to me that the frequent situation of the Vanua Levu thermal springs along the lines of surface-drainage requires an explanation that does not altogether exclude the agency of infiltration.
List of the Hot Springs of Vanua Levu, 1898-99.
| Locality. | Coast or inland. | Height above sea-level. | Near or far from streams. | Siliceous sinter. | Nature of the surface at the exit. | Temperature. |
|---|---|---|---|---|---|---|
| 1. Wainunu | Coast to four miles inland | Sea-level to 20 or 30 feet | River-side and under water | Little or none | Soil-cap, tuffs, &c. | 100°-130° F. |
| 2. Natoarau and vicinity | Three to four miles from coast | 50 to 100 feet | Near brooks and streams | Little or none | Soil-cap | 110°-126° F. |
| 3. Nukumbolo | Six miles inland | 130 feet | About 100 yards from river | Mixed with lime carbonate | Agglomerate-tuff | 157° F. |
| 4. Savu-savu | Beach and 150 yards inland | Sea-level to 10 feet | Inland springs near a brook | In fair quantity | Rotten volcanic agglomerate | 208°-210° F. |
| 5. Ravuka | Nine miles inland | 200 feet | River-side | None | Pebble bed | 148° F. |
| 6. Vuinasanga | Ten miles from north coast | 150 feet | River-side | None | Soil-cap | 131°-134° F. |
| 7. Foot of Nawavi Range | Four miles inland | Not known | Not known | Not known | Not known | Not known. |
| 8. Vatuloaloa | Coast below high-tide mark | Nil. | 140° F. | |||
| 9. Nambuonu | Half-mile inland | Probably slight | Not known | Not known | Swampy | 140° F. (probably). |
| 10. Tambia | Inland one and a-half to two miles | 90 feet | Not near a stream | Abundant | Soil-cap | 180° F. |
| 11. Vandrani | Eight miles from coast | 270 feet | River-side | None | Old river-bed | 100° F. |
| 12. Na Kama, Wailevu | Inland five to six miles | 90 feet | River-side | Probably little | Agglomerates and tuffs | 194°-204° F. |
| 13. Vunimoli | Inland eight miles | 120 feet | About 100 yards from river | None | Foramini ferous clay-rock | 140° F. 155° F. |
| 14. Mbati-ni-kama | Inland seven and a-half miles | 130 feet | River-side | In fair quantity | Volcanic agglomerate | 161° F. |
| 15. Nandongo | Inland four miles | 180 feet | River-side | None | Clay | 97° F. |
| 16. Natuvo | Coast and between the tide-marks | Nil and a few feet | None | Reef-flat and swampy ground | 131° F. 136° F. | |
| 17. Ndaku-ndaku | Between the tide-marks | Nil | None | Reef-flat | Not known. | |
| 18. Navakaravi | A mile inland | 30 to 40 feet | Swampy ground | None | Soil-cap | 133° F. |
| 19. Vunisawana | From beach 300 or 400 yards | A few feet | Near a brook | Not known | Soil-cap | Not known. |
| 20. Ndreke-ni-wai | Coast between the tide-marks | Nil | None | Old reef-patch | 130°-135° F. | |
| 21. Waikatakata | Inland 400 yards | 25 or 30 feet | Not near a stream | None | Rises beneath a boulder of basalt | 148° F. |
| 22. Ndevo | On coast below high-water level | Nil | Near a stream | Not known | Probably the reef-flat | Not known. |
| 23. Navuni | Inland three-quarters of a mile | 100 feet | Near a stream | Little or none | Volcanic agglomerate | 112°-113° F. |
| Locality. | Coast or inland. | Height above sea-level. | Near or far from streams. |
|---|---|---|---|
| 1. Wainunu | Coast to four miles inland | Sea-level to 20 or 30 feet | River-side and under water |
| 2. Natoarau and vicinity | Three to four miles from coast | 50 to 100 feet | Near brooks and streams |
| 3. Nukumbolo | Six miles inland | 130 feet | About 100 yards from river |
| 4. Savu-savu | Beach and 150 yards inland | Sea-level to 10 feet | Inland springs near a brook |
| 5. Ravuka | Nine miles inland | 200 feet | River-side |
| 6. Vuinasanga | Ten miles from north coast | 150 feet | River-side |
| 7. Foot of Nawavi Range | Four miles inland | Not known | Not known |
| 8. Vatuloaloa | Coast below high-tide mark | Nil. | |
| 9. Nambuonu | Half-mile inland | Probably slight | Not known |
| 10. Tambia | Inland one and a-half to two miles | 90 feet | Not near a stream |
| 11. Vandrani | Eight miles from coast | 270 feet | River-side |
| 12. Na Kama, Wailevu | Inland five to six miles | 90 feet | River-side |
| 13. Vunimoli | Inland eight miles | 120 feet | About 100 yards from river |
| 14. Mbati-ni-kama | Inland seven and a-half miles | 130 feet | River-side |
| 15. Nandongo | Inland four miles | 180 feet | River-side |
| 16. Natuvo | Coast and between the tide-marks | Nil and a few feet | |
| 17. Ndaku-ndaku | Between the tide-marks | Nil | |
| 18. Navakaravi | A mile inland | 30 to 40 feet | Swampy ground |
| 19. Vunisawana | From beach 300 or 400 yards | A few feet | Near a brook |
| 20. Ndreke-ni-wai | Coast between the tide-marks | Nil | |
| 21. Waikatakata | Inland 400 yards | 25 or 30 feet | Not near a stream |
| 22. Ndevo | On coast below high-water level | Nil | Near a stream |
| 23. Navuni | Inland three-quarters of a mile | 100 feet | Near a stream |
List of the Hot Springs of Vanua Levu, 1898-99. continued
| Locality. | Siliceous sinter. | Nature of the surface at the exit. | Temperature. |
|---|---|---|---|
| 1. Wainunu | Little or none | Soil-cap, tuffs, &c. | 100°-130° F. |
| 2. Natoarau and vicinity | Little or none | Soil-cap | 110°-126° F. |
| 3. Nukumbolo | Mixed with lime carbonate | Agglomerate-tuff | 157° F. |
| 4. Savu-savu | In fair quantity | Rotten volcanic agglomerate | 208°-210° F. |
| 5. Ravuka | None | Pebble bed | 148° F. |
| 6. Vuinasanga | None | Soil-cap | 131°-134° F. |
| 7. Foot of Nawavi Range | Not known | Not known | Not known. |
| 8. Vatuloaloa | 140° F. | ||
| 9. Nambuonu | Not known | Swampy | 140° F. (probably). |
| 10. Tambia | Abundant | Soil-cap | 180° F. |
| 11. Vandrani | None | Old river-bed | 100° F. |
| 12. Na Kama, Wailevu | Probably little | Agglomerates and tuffs | 194°-204° F. |
| 13. Vunimoli | None | Foramini ferous clay-rock | 140° F. 155° F. |
| 14. Mbati-ni-kama | In fair quantity | Volcanic agglomerate | 161° F. |
| 15. Nandongo | None | Clay | 97° F. |
| 16. Natuvo | None | Reef-flat and swampy ground | 131° F. 136° F. |
| 17. Ndaku-ndaku | None | Reef-flat | Not known. |
| 18. Navakaravi | None | Soil-cap | 133° F. |
| 19. Vunisawana | Not known | Soil-cap | Not known. |
| 20. Ndreke-ni-wai | None | Old reef-patch | 130°-135° F. |
| 21. Waikatakata | None | Rises beneath a boulder of basalt | 148° F. |
| 22. Ndevo | Not known | Probably the reef-flat | Not known. |
| 23. Navuni | Little or none | Volcanic agglomerate | 112°-113° F. |
Summary of the previous remarks on the hot springs of Vanua Levu.
(1) Hot springs have been recorded from 23 localities, but there are probably many undiscovered or forgotten.
(2) They are distributed over much of the island; but have not been observed in the Mbua or Western end and in the Undu extremity east of Lambasa and Lakemba.
(3) They are confined to the areas of basic rocks and are not known in the districts of dacites and other acid andesites or in those of quartz-porphyry and trachyte.
(4) They are always found at low elevations, never exceeding 300 feet.
(5) Whilst more than half are situated along river and stream courses, nearly all the remainder lie between the tide-marks.
(6) In only two localities is the temperature at or near the boiling-point. In one place it is 180° F., and in most of the other springs it ranges between 100° and 150°.
(7) Siliceous sinter is formed where the temperature is over 150°.
(8) As exemplified by the water of the Savu-savu springs the proportion of salts in solution (8 per 1000) is constant over many years; whilst in this fact and in the relative amounts of each salt there is a sharp distinction from the composition of sea-water.
(9) The hot springs are older than the streams and rivers, along which they are so frequently found.
It would appear that they are largely supplied from the “soakage” of the heavy rainfall in the mountains.
CHAPTER IV
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES OF VANUA LEVU.
In this chapter the detailed description of the island is commenced, beginning with the western extremity and proceeding eastward. Most of the petrological details are dealt with under their respective sections; but it has been found necessary also to frequently refer to them in this connection.
The Naivaka Peninsula.—This mountainous peninsula forms the conspicuous feature of the western extremity of Vanua Levu. Amongst all the mountains of the island its appearance from a distance gave most promise of displaying the products of recent volcanic eruptions; but as shown below it affords evidence of an antiquity nearly as great as that of the rest of the island, although there are reasons for believing that its eruptions took place during the last stage of the emergence.
Naivaka is connected with the adjacent relatively little elevated part of the main island by a low and narrow neck a little less than a mile in breadth. In its highest part, where it is only raised between 20 and 30 feet above the sea, this isthmus is formed of the basic volcanic rocks of the district; but about three fourths of its width are occupied by mangrove-swamps which are especially extensive on the south side.
Viewed from some miles to the eastward the mountain has a regular conical outline; but from the south, when seen from Ruku-ruku Bay, it has an elongated and a much more irregular profile, descending rapidly on the east side, but displaying a gradual and a fairly regular slope of about 10 degrees on the west side. The upper part of the mountain is in the form of a curve with the concavity facing south, the crest being more or less broken up into five or six peaks showing often precipitous and at times vertical rocky faces having a drop of from 100 to 300 feet, the highest peaks ranging from 1500 to 1658 feet above the sea.
All around the mountain, except on the upper steep portion on the south side where it is well-wooded, the slopes have the usual character of the “talasinga” districts, being occupied only by grass, ferns, cycads, and the ordinary scanty vegetation of such regions. Whilst on most sides the surface configuration is fairly regular and the ascent to the summit is more or less regular, on the south side bold spurs with valleys between them descend to the coast, and the central mass rises abruptly in the middle of the peninsula from a height between 300 and 500 feet above the sea. It is on this side that Naivaka has the appearance of having been originally a crateral mountain, of which, however, only the north segment in a much degraded condition now remains, whilst the other two-thirds have disappeared.
The prevailing rocks are a blackish compact olivine-basalt, having as a rule much smoky glass in the ground-mass and possessing a specific gravity of 2·92-2·94. They are referred to in the description of genus 25 of the olivine-basalts given on page [259].
These rocks compose the agglomerate and the agglomerate-tuffs that form the eastern portion of the summit and probably most of the elevated part of the mountain. Similar agglomerates occur along most of the north coast, the rock being in a few places scoriaceous or amygdaloidal; and they occur in huge fallen masses on the south side near the foot of the precipitous portion. The blocks in the agglomerate of the summit are usually six to eight inches across.
On the south-west side the massive rocks exposed are less basic with a specific gravity of 2·76 to 2·79. They are also more altered, the olivine being infrequent and the interstitial glass scanty. They differ besides in the parallel arrangement and in the length of the felspar-lathes (·18 mm.), which are on the average half as long again as those of the prevailing olivine basalts (·12 mm.). They are placed in a different order of these rocks and belong to genus 37 described on page [262].
Tuffs did not come frequently under my notice. At one part of the north coast the cliffs are formed of a palagonitic tuff-sandstone, effervescing with an acid, which is described on page [330]. Although no organic remains are to be noticed, it is probably a submarine deposit.
On a spur on the south-west side, at an elevation of 600 feet, there is exposed a hard red palagonitic tuff dipping away from the summit at an angle of 40°. It is mainly composed of the palagonitised débris of a vacuolar basic glass and incloses broken and entire crystals of plagioclase, augite, and olivine.
The augite crystals, which attain a length of five or six mm., project from the weathered surface and are easily detached, lying about in quantities on the ground in places. Although they are now imbedded in evidently a submarine tuff, these pyroxene crystals could only have been ejected as such from a subaerial vent; and it would therefore appear that they fell into the sea around the shores of a volcanic island in a state of activity. These crystals are often cracked and are as a rule not so perfect as those I have gathered from the slopes of Vesuvius, Stromboli, and Etna. They exhibit an unusual tabular form arising from the great development of the clinopinakoid at the expense of the orthopinakoid faces.
On the whole it may be inferred that the Naivaka volcano was submerged at the time of its origin, but that the eruptions continued after it began to show itself above the sea. In many of its features, especially in the character of the agglomerate that forms its upper portion, and in the palagonitic nature of the tuffs, Naivaka differs only from other elevated districts of the island, where organic remains occur, in the absence of such remains. Its form bears testimony to the extreme degradation we find in other districts, and the occurrence of foraminiferous tuffs high up the neighbouring slopes of Mount Sesaleka affords additional evidence of the original submergence of this district.
The Hill of Korolevu.[[33]]—About three miles east of Mount Naivaka there rises to a height of 800 feet, about a mile inland from the shores of Wailea Bay, the singular flat-topped hill of Korolevu. It displays vertical cliff-faces, with a drop often of 200 or 300 feet, which have become so deeply furrowed or fluted by the eroding atmospheric agencies that they appear at a distance to be made of columnar basalt. The hill is, however, formed in mass of a compacted tuff or agglomerate tuff built up of materials of a hyalomelan basic glass that has undergone partial conversion into palagonite. In the upper thirds these rocks show no bedding, but in the lower slopes on the seaward side they are bedded and dip to the north away from the summit at an angle of 15° or 20°. The form of this hill is well shown in the sketch attached, and there is little doubt that we have here an old volcanic “neck,” the remains of a submarine vent.
A specimen of the tuff from the summit is made up of compacted fragments, in size ranging up to one third of an inch, of a bottle-green vacuolar glass, which fuses readily in a lamp-flame and is not dissolved by hydrochloric acid. This glass is usually isotropic, but much of it is also palagonitic and feebly refractive, the vacuoles or steam-holes, which are often elongated, being in the last case filled with the same palagonitic material. Plagioclase crystals occur macroscopically in the glass; they are much eroded and contain numerous large inclusions both of the clear isotropic glass and of its palagonitised form.
Korolevu Hill (800 ft.) from Wailea Bay.
About a third of a mile west of the Korolevu hill rises the hill of Ngangaturuturu, 450 feet high, which presents a precipitous cliff-faced summit in which are exposed basic tuffs showing pyroxene crystals projecting from the weathered surface.
The Bomb Formation of Navingiri.—A mile north-west of Korolevu Hill, where the coast road crosses a spur at the back of Navingiri, a very curious formation is exposed at an elevation somewhat under 200 feet above the sea. Here there are to appearance a number of large more or less spherical volcanic bombs, two to three feet across and formed of a semi-vitreous scoriaceous basalt, imbedded in a hyalomelan-tuff displaying the same microscopical characters as in the case of the tuff forming the adjacent hill of Korolevu.
The ash is light grey in colour and rather friable; but where in contact with the bombs it becomes darker and is hardened. The steam pores of the bombs are round and not elongated; and as is usual with these bodies they increase in size from the outside, where they are very small (1 millimetre and less), to the centre, where they vary from two to five millimetres across. A vitreous border, about an inch in breadth, forms the outer shell of the bomb where it is in contact with the tuff. Some of the bombs are only two or three inches apart; and one of them shows evidence of fracture, fragments of the outer vitreous shell lying imbedded in disorder in the surrounding tuff.
Before entering into more detail it may be at once observed that the contiguity of some of the bombs to each other makes it at first difficult to view them as having been formed in the manner volcanic bombs are supposed to originate. Those who have seen the huge bombs lying scattered about on the summit of Vulcano in the Lipari Islands will appreciate the difficulty of imagining how these bombs can occur in such a close arrangement without having often shattered each other to fragments. However, Mr. Wittstock of Mbaulailai in a letter to me describes even larger bombs that came under his notice exposed on the surface in the Mbua district, their outer crust when broken looking “like the slag of a blast-furnace.”
The bomb-rock is a semi-vitreous basaltic andesite. It displays microporphyritic plagioclase in a ground-mass formed mainly of a smoky, almost isotropic glass, in which numbers of felspar microliths (·1 mm.) are developed, the augite being but slightly differentiated. Scattered about in the glass are little irregular patches, or “lakelets,” of residual magma composed of a yellowish feebly refractive material that I cannot distinguish from palagonite.
The ash, in which the bombs are imbedded, is a somewhat friable hyalomelan-tuff composed of fragments of basic glass often partially palagonitised, and usually 2 or 3 mm. in size. In it occur pumiceous lapilli of the same material up to 2 centimetres in diameter. The glass is markedly vacuolar, the cavities being either filled with gas or with alteration-products. The vacuoles are often drawn out into tubes, giving the glass a fibrillar appearance. The numerous plagioclase phenocrysts inclosed in the glass are much honeycombed and contain large inclosures of the glass, both altered and unchanged.
Although the line of contact is well defined in a hand-specimen, the two rocks cannot be separated along the junction. In a thin section, in which the union of the vitreous shell of the bomb with the ash is well shown, there is no defined line of demarcation, the non-vacuolar isotropic glass of the bomb being there broken up into fragments, with the interspaces filled with the partially palagonitised pumiceous ash. In the vitreous shell the felspar microliths are much less developed both in size and number than in the central portion of the bomb. Numerous cracks communicating with the round steam-pores, which are much larger than the vacuoles of the ash-glass, are filled with the same yellowish magma-exudation referred to in the case of the rock forming the centre of the bomb. Through the cracks this palagonite-material has found its way into the steam-pores.
It would appear from the above that the bombs were but partially consolidated when they fell into the bed of ash. The tuff is somewhat “baked” where it is in contact with the bombs; and there is evidence of a collision between the bombs in the fragments of the vitreous shell imbedded in the ash. Although the ash itself contains no organic remains, there occur, not many hundred yards away and at an elevation 100 feet higher above the sea, foraminiferous tuffs of basic glass which are described below. There is no indication of a crateral cavity in this locality; whilst the ancient “neck” represented by Korolevu Hill is a mile away. These bombs most probably after being ejected from some sub-aerial vent fell into the sea around, on the floor of which much basic pumice-ash had been previously deposited. Such masses as they sank would lose most of their original momentum.
Remarkable Section near Korolevu Hill.—Between the hills of Korolevu and Nganga-turuturu, at an elevation of about 300 feet above the sea, there is a singular exposure of tuffs horizontally stratified and forming a low escarpment or line of cliff about 15 feet high on the hill-side. These beds display the passage from basic tuffs below to relatively acid tuffs above, and they establish that in this locality the period of acid andesites followed that marked by the eruption of basalts and basaltic andesites. From their horizontal and undisturbed position, it may be inferred that these deposits began to be formed under the sea when the activity of the submarine basic vents was on the wane. In their composition and in the various degrees of coarseness of their materials, we can plainly discern the history of volcanic action in this locality.
A hard compacted palagonite-tuff makes up the lower half of the thickness of beds exposed, 15 feet in all. The greater portion of it has the uniform texture of a sedimentary rock, fine-grained below where the fragments are ·1 to ·3 mm. in size, and becoming coarser above where the larger measure 1 to 2 mm. It is composed of more or less angular fragments of a basic vacuolar isotropic glass, and of plagioclase and augite with much fine palagonitic débris. There is no effervescence with an acid; but in the upper part there are a few casts of foraminifera of the “globigerina” type, as indicated in the thin sections. Above this lies a bed of a similar basic tuff, having however a banded appearance from the arrangement of materials of different degrees of coarseness, the finer being ·1-·2 mm. in size, the coarser ·4-·8 mm. There is little or no carbonate of lime; but occasional tests of foraminifera of the type above mentioned occur in the slide. The basic tuffs here abruptly terminate. They represent the quiet deposition in water comparatively deep of the products of marine erosion, and of the finer ejectamenta of some distant subaerial vent.
Above the basic tuffs lie a series of tuffs, about 5 feet in thickness, and composed mainly of the debris of acid andesitic rocks of the hornblende-andesite type, such as occur in the Ndrandramea district. They mark a period of active eruption on the part of some neighbouring acid andesitic vent in this neighbourhood, which the subsequent explorer may be able to identify with some volcanic “neck.”
These tuffs are composed partly of fragments of a hemicrystalline hornblende-andesite and partly of crystals, broken and entire, of plagioclase, hornblende, rhombic pyroxene, and augite. The plagioclase is tabular, zoned, and glassy, and gives extinctions of oligoclase-andesine (6 to 12°). The hornblende is bottle green, markedly pleochroic, and gives extinctions up to 14°. The rhombic pyroxene has the characters described on page [301], in the case of the Ndrandramea rocks. The augite is less frequent, but the two pyroxenes are sometimes associated as intergrowths.
These acid tuffs do not effervesce with an acid, nor can any tests of foraminifera be observed in them; but since these organisms are represented in the basic tuffs below, it is highly probable that the whole series of these horizontal beds is submarine. The first or lowest bed of the acid tuffs indicates a somewhat violent volcanic outbreak in this neighbourhood, following the deposition of the basic tuffs. It is composed of loosely compacted subangular fragments, 1 to 3 millimetres in size, in which the macroscopic prisms of the rhombic pyroxene are especially frequent. It passes upward without interruption into a regularly grained sandstone formed of rounded and subangular fragments measuring ·3 to ·7 mm. across. Above this lies a quite distinct bed, a few inches thick, of a fine compact clay rock, where the mineral fragments measure only ·05 to ·12 mm. in diameter, hornblende being well represented, although the rhombic pyroxene is very scanty. Up to this time these beds of acid tuffs indicate a gradual defervescence of the volcanic activity that began with some violence, as shown by the characters of the lowest bed. Now another outbreak occurred, and overlying the clay-like bed we find a coarse tuff made up of fragments 2 to 5 millimetres across, and approaching in texture and appearance a subaerial tuff, but in other respects similar to those below it. It is the last and uppermost of this series of acid tuffs, and with it terminates an interesting record of the past in this region, the chief features of which may thus be summarised.
A prolonged period of quiet deposition of submarine basic tuffs, the products partly of marine erosion and partly of distant eruptions, was abruptly followed by the outbreak of a neighbouring vent during which tuffs formed of the debris of acid andesites were deposited. The gradual decrease in the degree of activity is plainly shown in the gradual diminution in size of these tuffs, until they acquire the fineness of a clay. Then another burst of activity from the same vent or vents occurred, and the record ends. Since that time there has been apparently an upheaval to an elevation of 300 feet above the sea. As, however, the beds are quite undisturbed, the emergence may have been due to the lowering of the sea-level, a subject which is discussed in [Chapter XXVII].
Coast between Wailea Bay and Lekutu.—The hills here often approach the coast, their spurs running down to the beach. In the low range, 250 to 300 feet high, east of Wailea Bay, are exposed palagonite-tuffs dipping gently north-east and composed of fragments of a vacuolar basic glass, more or less palagonitised, and of minerals (plagioclase, etc.) not exceeding 2 mm. in size. These deposits are apparently non-calcareous and show no organic remains.
Farther along the coast towards Nativi basic tuffs and agglomerates appear at the surface; but the underlying rock, exposed in position in the stream-courses and prevailing along much of the sea-border to Nativi and a mile or so beyond, is a vesicular semi-ophitic basaltic andesite with coarse doleritic texture and containing much interstitial smoky glass. (It belongs to the non-porphyritic group of genus 9 of the augite-andesites described on page [273].) Such rocks evidently represent ancient flows. They give place as one proceeds east to porphyritic semi-ophitic doleritic rocks of the same genus and to semi-vitreous basic rocks. About half a mile west of Nukunase a vesicular doleritic basaltic andesite forms a spur protruding at the coast. It is semi-ophitic and contains in the smoky glass of the groundmass little irregular cavities filled with a yellowish residual magma like palagonite in character. (It is referable to genus 12 of the augite-andesites, described on page [275].) A few paces west of this spur a vertical dyke, 20 feet wide and trending N.W. and S.E., appears on the beach. It is formed of a bluish scoriaceous basaltic andesite containing much glass in the groundmass and showing imperfectly developed felspar lathes. It is included in genus 4 of the augite-andesites described on page [270].
A little east of the spur there is another dyke apparently vertical and formed of a vesicular rather than a scoriaceous basaltic andesite referred to genus 1 of the augite-andesites (page [267]). It differs from the rock of the previous dyke in the presence of small plagioclase phenocrysts which contain abundant magma-inclusions; but it resembles it in the characters of the groundmass. This dyke is about 40 feet in thickness and trends N.E. and S.W.
It may be inferred from the foregoing remarks that there was at one time a volcanic vent in the district west of Nukunase. The lines representing the trend of the two dykes above noticed would if extended meet at a common focus a little way inland. The rocks of the dykes differ conspicuously from the prevailing doleritic rocks that form, as before remarked, the ancient flows, the average length of the felspar-lathes in the former being ·1-·2 mm., in the latter ·3-·4 mm. Both, however, belong probably to the same vent of which now the exact situation would not be easy to discover, on account of the re-shaping of the surface through the denuding agencies.
Mount Koroma.—The highest peak of the hills lying inland between Wailea Bay and Lekutu is named Koroma and attains a height of 1,384 feet. I did not ascend its slopes higher than 900 feet, and approached it from the Mbua or south side. Extensive plains, covered with the usual “talasinga” vegetation, reach inland from the shores of Mbua Bay to the foot of this range without attaining a greater elevation than 100 feet. This low district is drained by the Mbua river and its tributaries, the rock usually exposed at its surface being a decomposing porphyritic basaltic andesite. It is again referred to on page [56] in connection with the low-lying level region of this portion of the island of which it in fact forms a part.
A basic non-calcareous fine-grained tuff-sandstone is exposed in a stream at the foot of the south slope of Mount Koroma. Whilst crossing some low wooded outlying hills in this locality, I came suddenly upon what seemed like a desert in miniature, quite bare of vegetation and occupying an area of some acres. Here a porphyritic basic rock, from some cause unknown to me, has decomposed in the mass to a depth of 20 feet and more; and the result is a surface of white crumbling rock scored deeply by the rains and carved out by the denuding forces into miniature hills and dales. It is not improbable that a small crater in its last solfatara-stage once existed here; but the whitened disintegrated rocks alone remain, and we can now only hazard a conjecture as to the cause.
I found a variety of basic rocks exposed on the hill slopes up to 900 feet. The most frequent of the deeper-seated rocks which occurred in mass at this elevation, and as large blocks on the lower levels, is a dark grey rather altered hypersthene-augite-andesite, referred to genus 1 of that sub-class as described on page [286]. The specific gravity is 2·73, whilst the groundmass displays a little greenish altered glass. Another of the deeper rocks, exposed 500 feet up the slopes, is placed in the same sub-class, augite and rhombic pyroxene being porphyritically developed, separately and as intergrowths. The groundmass displays short stout felspars, augite, and a little altered glass. The rock is therefore referred to the orthophyric order described on page [290]. Spec. grav. 2·78.
Evidence of more recent surface lava-flows here exists. In one place I came upon such a bed 12 feet thick, compact in its upper half and slaggy or scoriaceous in its lower half. The rock is an aphanitic augite-andesite (spec. grav. 2·77) and belongs to species B, genus 16, of the augite-andesites, as described on page [281]. Its groundmass displays felspar-lathes in flow-arrangement with a little interstitial glass. Slaggy lava is not uncommon on these slopes. One specimen beside me is a semi-vitreous form of the deeper hypersthene-augite-andesites of this range.
There appears to be better evidence of sub-aerial lava-flows on the lower slopes of Mount Koroma than I found in any other part of the island. It should have been before remarked that one of these flows lies upon a bed of a hard reddish compact tuff, which appears in the thin section as an altered palagonite-tuff, containing fragments of minerals including both rhombic and monoclinic pyroxene, but showing neither lime nor organic remains. The larger fragments are 2 mm. in size. It seems likely that this flow ran into the sea during the emergence of this part of the island.
The prevalence of rocks of the hypersthene-augite-andesite type in Mount Koroma distinguishes this range from the surrounding regions of olivine-basalts and basaltic andesites. This district is well worth a detailed examination, and perhaps the remains of a crateral cavity may yet be found.
The Coast between Naivaka and Koro-ni-solo at the foot of the north slope of the Sesaleka Range.—Basaltic andesites, and olivine-basalts of the Naivaka type occur on this coast. A rock of more acid character, light grey and much altered, is exposed at the surface where the track crosses the headland projecting into Ruku-ruku Bay. It is one of the propylites referred to in my description of the second genus of the augite-andesites (p. [269]). The felspars of the groundmass give the small extinctions of oligoclase; and in this respect it differs from the other augite-andesites. Besides the altered plagioclase phenocrysts there is much microporphyritic augite but slightly changed. Calcitic and other alteration products occur in the interstitial glass.
Mount Sesaleka.—This is the name of the highest peak, 1,370 feet, of a remarkable ridge-shaped range, which is very precipitous on the east and north-east sides, where there is a sheer drop apparently of 500 or 600 feet, whilst on the other sides the slope is more gradual, especially on the north where there is a gentle descent to the sea. The actual summit is bare, rocky, and narrow. There is a curious native legend relating to a pond on the top of this hill. From what Mr. Wittstock tells me, it seems probable that there is a spring near the summit. Close to the top are the remains of an old “koro-ni-valu” or war-town; whilst numbers of shells of species of Cardium, Cypræa, and Strombus, such as would be used for food, lie about. Many years ago there was a prolonged siege of this stronghold, which is referred to here as indicating that the defenders had some independent water-supply.
In ascending from Koro-vatu on the west side basic agglomerates and agglomerate-tuffs were found exposed as far as half-way up. In the upper half occurred at first fine-grained calcareous tuffs, bedded and dipping gently down the slope, composed of palagonite-debris, mineral fragments and calcitic material and displaying a few macroscopic tests of foraminifera. These tuffs became non-calcareous and coarser as one approached the summit. A specimen obtained from the top is coarse-grained, being composed of fragments of basic glass, usually palagonitised, much augite, a little plagioclase and fresh olivine, but no tests of foraminifera, the size of the fragments being usually ·5-1·5 mm. Massive rocks were rarely exposed on this side; but half-way up in a stream course I came upon an exposure of a porphyritic olivine-basalt containing a fair amount of devitrified interstitial glass. Its specific gravity is 2·85 and it is referred to genus 25 of the olivine basalts (page [259]). I descended by a gentle slope to the north, coarse basic tuffs and agglomerates containing amygdaloidal fragments being displayed on the surface. In a stream at the foot, close to Koro-ni-solo, were blocks of a heavy compact olivine-basalt with specific gravity 2·96.
District between Mount Sesaleka, Thombo-thombo Point, and Vatu-karokaro Hill.—This is a broken country with several abruptly rising lesser hills. Starting from Koro-vatu and crossing the Thombo-thombo promontory, I reached the coast of Mbua Bay near Navunievu. Basic tuffs and agglomerates prevailed on the way, the last containing blocks of a scoriaceous basaltic lava bearing olivine. The massive rocks exposed belong in some cases to genus 13 of the olivine-basalts as described on page [256], being dark grey and having a specific gravity of 2·88, and in other cases to genus 16, species B, of the augite-andesites when they are lighter in colour and have a specific gravity of 2·77. In both cases the interstitial glass is scanty.
I ascended Vatui, one of the numerous small hills of the district. It is 450 feet high and is capped by a bare mass of tuff-agglomerate, 40 to 50 feet high and containing fragments of vesicular basic lava. This mass is pierced by a dyke, 18 inches thick, which is inclined to the N.NE. at a high angle of 60 or 65 degrees with the horizon. This dyke is composed of a compact olivine-basalt which is remarkable for the prevalence of small augite prisms in the groundmass. It is described on page [265] under genus 44 of the olivine-basalts. Hand-specimens are magnetic and display polarity, which is due, as pointed out in [Chapter XXVI.], to the exposed situation of the peak.
Vatui in its characters is evidently typical of the other lesser hills around, which, as viewed from below, possess bare tops and precipitous declivities of the same formation. All the hills in the district including Sesaleka are capped by these basic tuffs and tuff-agglomerates; and doubtless as in the case of Sesaleka these deposits are all submarine. This is true also of Vatu-karokaro, a hill 600 feet high, overlooking Mbua Bay and about two miles east of Sesaleka. In the lower part of this hill is exposed a dark compact basaltic andesite, referred to genus 13, species B, of the augite-andesites (sp. gr. 2·83), whilst blocks of a black olivine-basalt (sp. gr. 2·91) occur in the agglomerate of the summit. These hills may all be regarded as “volcanic necks” or the stumps of volcanic cones, probably submarine.
The Dividing Ridge between the Mbua and Lekutu Plains.—A level rolling “talasinga” district intervenes between Mbua Bay and the dividing ridge. The upper part of this ridge, which attains a height of about 500 feet above the sea, is composed of a hard grey sandstone-like tuff, effervescing feebly with an acid, which on examination proves to be formed in great part of fragments, ·07-·1 mm. in size, of a dark basic glass occasionally vacuolar. The rest of the deposit consists of similar-sized fragments of plagioclase and other minerals, and includes also a few tests of foraminifera of the “Globigerina” type.
The mass of the ridge, however, is composed of coarse tuffs and agglomerates of a different kind which have been covered over by the foraminiferous deposit just described. Thus there are exposed on the lower slopes, tuffs and agglomerates of a basic pitchstone formed of a brown glass containing a few felspar and pyroxene microliths. In the tuff the fragments are three to six mm. in size and show evidence of crushing in situ, the interstices being filled with debris of the same material more or less palagonitised,[[34]] but there is no carbonate of lime. Large masses of an agglomerate made up of blocks of an acid andesite occur higher up the slopes. The component rock belongs to an unusual type of hypersthene-andesite, specially noticed on page [297].
The interesting feature in this ridge lies in the testimony it affords that the extensive Mbua and Ndama basaltic plains, on which I was unable to discover any submarine deposits, were at one time submerged.
The Mbua and Ndama Plains.—These rolling plains are a striking feature in the western end of Vanua Levu. They have an arid barren look, are clothed with a scanty and peculiar vegetation, possess a dry crumbling soil often deeply stained by iron oxide, are traversed by rivers without tributaries descending from the wooded uplands of the interior, and in fact have well earned the name given to them by the natives of “talasinga” or sun-burnt land. Both Seemann and Horne have remarked on the South Australian aspect of these regions, which are characteristic of the lee and drier sides of the larger islands of the group. Covered for the most part with grass, ferns and reeds, these low-lying districts are dotted here and there with Casuarinas, Pandanus trees and Cycads, whilst such other trees and shrubs as Acacia Richii and Dodonæa viscosa, add to the variety and peculiarity of the vegetation. The origin of these “talasinga” districts is discussed in the last chapter.
The Mbua and Ndama plains form a continuous region extending three to five miles inland to the foot of the great mountain of Seatura, to the watershed between Mbua and Lekutu, and to the base of Mount Koroma; whilst it reaches along the sea border from the vicinity of Navunievu about four miles west of the Mbua River to beyond Seatovo a few miles south of the Ndama River. Their extent is defined in a general sense by the 300 feet contour line in the map. Their elevation, however, above the sea does not generally exceed 200 feet and is usually only 50 or 100 feet; but at the foot of Seatura they rise to between 300 and 400 feet. Whilst on the one side these plains form a continuation of the lower slopes of the great Seatura mountain, on the other side they are extended under the sea as the broad submarine platform, the edge of which, as defined by the 100-fathom line, lies eight to ten miles off the coast. It is pointed out on page [372] that this continuity of surface, both supra-marine and submarine, extends probably to the geological structure and that the submarine platform represents the extension under the sea of the basaltic flows of the plains.
The whole region of the plains is occupied by olivine-basalts and basaltic andesites, such as are found on the neighbouring lower slopes of the Seatura mountain. They are as a rule much decomposed, even at a depth of several feet below the surface. Typically, they are neither vesicular nor scoriaceous, and in this respect they possess the character of submarine lava-flows. The rolling surface of the plain is varied occasionally by small “rises” or hillocks marking apparently some secondary cone, of which the much degraded “wreck” alone remains. Here and there fragments of limonite, approaching hæmatite in its compact texture, lie in profusion on the soil, representing doubtless small swamps long since dried up, some of which still occur in the hollows of the plain. Mingled with these fragments are often pieces of siliceous rocks and concretions, such as are found in the other “talasinga” districts of the island, the description of which is given on pages [128], [132], &c.
I will now refer more in detail to some of the points alluded to in this short description of these plains. With reference first to the compact limonite, it should be remarked that it occurs on the surface either as fragments of hollow nodules two or three inches across, or as portions of flat “cakes” half to one inch thick. It is especially abundant in the district lying a mile or two on either side of the Navutua stream-course between Ndama and Mbua. Here the subsoil is charged with ferruginous matter, and the water of the series of stagnant pools in the bed of the stream is stained blood-red by iron-oxide, a circumstance that has naturally given rise to native legends of a corresponding hue. These fragments of iron ore, which lie between 100 and 150 feet above the sea, represent the final stage of a process which is now no doubt in operation on the bottom of the neighbouring pools and small swamps. Their presence on the surface goes to indicate that this open country has been for ages a land-surface free from forest, as it is in our own time.
In a similar manner, the extensive disintegration of the basaltic rocks that form these plains affords evidence of the great antiquity of these “talasinga” plains in their present unforested condition. The extent to which these rocks have weathered downwards is very remarkable. Between Ndama and Mbua they are decomposed to a depth often of eight or ten feet below the surface. This is well exhibited in the sides of deep channels excavated by the torrents during the rains. Here the spheroidal structure is well brought out in the disintegrating mass, all stages being displayed in the formation of the boulders that are scattered all over these plains.
In one locality, near the lower course of the Ndama river, a thickness of 25 feet of decomposed rock was exposed in a cliff-face. In this case the rock was a porphyritic basaltic andesite, the disintegrating process having affected the whole thickness of the large spheroidal masses with the exception of a hard central nucleus of the size of the fist. In one of these nuclei by my side it is apparent that during the extension of the weathering process the phenocrysts of glassy plagioclase become opaque long before the groundmass is affected. In this specimen the stage of disintegration as affecting the felspar phenocrysts is at least one and a half inches in advance of that affecting the groundmass.
This great disintegration of the basaltic rocks, which as pointed out on page [64] is also in progress on the slopes of the adjacent spurs of Mount Seatura, is more characteristic of the porphyritic basaltic andesites than of the olivine-basalts. It is to the spheroidal weathering that we must look for an explanation of the rounded boulders so frequent in these districts. It may also be inferred that the soil produced from this extensive decomposition of the rocks is often very deep. At the Wesleyan Mission Station at Mbua, on level ground nearly a hundred feet above the river, a well has been sunk to a depth of 20 feet in soil of this description; and away to the westward a similar thickness of soil produced by the same cause is in places to be observed.
Coming to the characters of the basaltic rocks of the Mbua and Ndama plains, it may be remarked that the prevailing rocks are the porphyritic basaltic andesites, having a specific gravity of 2·77 to 2·81, which are in most cases to be referred to genus 13 (porphyritic sub-genus) of the augite-andesites described on page [278]. They possess large phenocrysts of plagioclase and but little interstitial glass. The other rocks are olivine-basalts with specific gravity 2·88 to 2·90 and showing only a few small plagioclase-phenocrysts. They display a little residual glass and belong for the most part to genus 37 of the olivine basalts described on page [262]. In both these basaltic rocks the felspar-lathes are in flow-arrangement; but in the basaltic andesites they average ·11 mm. in length, whilst in the olivine-basalts they average ·18 mm.
The low mound-like “rises” in these plains, to which previous reference has been made, are not usually elevated more than 50 feet above the general surface. One of these hillocks that lies near the track from Mbua to Navunievu, about two miles from the Wesleyan Station, is composed of a remarkable semi-vitreous pyroxene-andesite passing upward into a rubbly rock of the same nature. The rock of this old volcanic neck is of an unusual type and is referred to the prismatic order of the hypersthene-augite andesites described on page [289]. Both the felspar and pyroxene prisms of the groundmass are in flow-arrangement. One of these mounds near the Mbua Wesleyan Station is apparently formed of the decomposing basaltic andesite of the district. On its surface are fragments of earthy limonite and siliceous rocks.
The rarity of submarine tuffs and clays on these plains is somewhat singular; but in the occurrence of foraminiferous tuffs high up the slopes of Sesaleka and on the crest of the Mbua-Lekutu dividing ridge we have evidence of the original submergence of all these lower regions. It is probable enough that the ages of exposure that have since witnessed the reduction of the solid basaltic rock to a crumbling mass several feet in depth were more than sufficient for the stripping off of most of the overlying submarine deposits. Such deposits are, however, common on the surface of the extensive “talasinga” plains that constitute much of the north side of the island.
The Shell-bed of the Mbua River.—Rather curious evidence of an emergence of a few feet and of a considerable advance of the delta of the Mbua river in comparatively recent times is afforded by a bed of marine shells exposed in the right bank of this river, about 200 yards below the boat-shed of the Wesleyan Mission Station and about two miles in a straight line from the sea. This bed, which is about a foot in thickness, is exposed for a distance of 70 or 80 yards. It slopes gradually seaward as one descends the river, being raised two or two and a half feet at its upper end above the river level at low tide, whilst at its lower end it is at about the water-level. The river-bank is here 15 or 16 feet high, and is composed in its upper half of a fine gravel of volcanic rocks mixed with earth, which below passes abruptly into a friable non-calcareous black mud-rock (not bedded and looking like consolidated swamp mud), in which the layer of shells is contained. These shells are, therefore, covered by deposits, 13 or 14 feet in thickness, of which the upper eight feet are formed of gravel and earth, and the rest of mud-rock. They are evidently gathered together on the slope of an old mud-flat.
The shells are all large marine bivalves, belonging to the genera Ostræa, Meleagrina, Cardium, Arca, &c., no freshwater shells occurring. They are often much decayed and have lost the ligaments. The valves are generally separate; but in some cases they are still in apposition, the cavity being then filled with the same black mud in which the shells are embedded. They lie about in all positions, some vertical, some horizontal, and are often piled on each other. In some cases large borers have perforated one or both of the valves; and here and there valves may be noticed with smaller oyster-shells attached to the inner surface. No vegetable remains were discovered with the exception of a single “stone” of the fruit of the Sea tree,[[35]] which is common in these islands, its empty almost indestructible stones occurring frequently in the drift stranded at the mouths of rivers.
At first sight one would look to human agency for the explanation of this shell-bed; but many of its features are inconsistent with such a view. If the shells had been originally collected by the aborigines for food, the absence of those of marine univalves of the genera Turbo, Strombus, Cypræa, &c., such as are much appreciated as food by natives, is inexplicable. The extent of the bed and its uniform thickness are characters that give no support to such an explanation. It represents, as I apprehend, an ancient shell-bank formed on a muddy bottom in comparatively shallow water near the mouth of a river. Since that time the Mbua River has cut through its old deposits, and the margin of its delta is now two miles to seaward, the intervening new land being formed of extensive mangrove-swamps in its lower part, whilst nearer the shell-bed there is much level land raised a few feet above the sea, on which the native town and different villages now stand. The amount of emergence here indicated since the time when this bank of shells was forming under the sea does not probably exceed a couple of fathoms.
Lekumbi Point.—This singular long and low promontory is between three and three-and-a-half miles in length and rather less than a mile in average width. It is monopolised by mangroves, except at the extremity where the swampy ground passes into the dry sandy soil occupied by the characteristic vegetation of coral beaches. This terminal portion, which is about a third of a mile in length and raised a couple of feet above high-water mark, was originally a reef-islet. The outer third of the cape, however, is cut off from the remainder by a narrow winding passage in the mangroves, which being 25 or 30 feet wide can be traversed by boats at and near high-water, and is often used to shorten the journey down the coast. The flowing tide rushes in at both entrances, and when the tide is ebbing it finds its way out at both exits, the passage presenting the readiest way of the filling and emptying of the interior swamps with the flow and ebb of the tide.
Before explaining the origin of this low tongue-shaped promontory of Lekumbi, it should be observed that it lies on a long projecting patch of coral reef which is continuous with the neighbouring shore-reefs. Depths of seven and eight fathoms are found off the sides and of 11 and 12 fathoms off the end of the reef-patch. This reef in its turn must have been built up on a submarine bank protruding from the coast. Such a bank may have originally been produced by the deposits brought down by the Ndama River which finds an exit through the mangroves near the base of the cape. With the exception, however, of the Lekutu River, none of the other Vanua Levu rivers have given rise to such tongues of land at their mouths. I am more inclined to hold that the submarine shoal, which underlies the present low cape of Lekumbi, indicates an old lava-flow from the great crateral valley of Seatura, opposite the mouth of which it lies. Traces of such flows are still to be found in that locality.
CHAPTER V
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Seatura Mountain.—In my description of the profile of this part of Vanua Levu, reference has already been made (p. [3]) to the great mass of this mountain which occupies five-sixths of the breadth of the island. Viewed from seaward it looks like a huge table-topped mountain-ridge, and as such it is represented in the Admiralty charts; but when its true contours are distinguished it appears, when defined by the 300-feet level in the map, as a somewhat rounded mass, measuring 12 miles in length and 10 miles in breadth and attaining a maximum height of 2,812 feet. Seen from the deck of a passing ship it displays more or less regular volcanic slopes, especially on the east, where there is a gradual descent at an angle of 3 or 4 degrees for some 10 miles, and on the north towards the Lekutu lowlands. It also shows a fairly regular descent towards Mbua Bay on the west. (See profile, p. [62].)
On the west side, however, there is a great gap in the mountain-mass (the Ndriti Gap), marking, as I hold, an old crateral cavity of large dimensions, and now occupied by the Ndama River and its tributaries.
The adjacent Seatovo Range to the southward obscures the profile of the mountain on the south; and it is in fact not at all easy for this reason to get a view with all the slopes displayed. It is only at times, when viewed in its complete mass with uninterrupted outlines, as from off the mouth of the Ndreketi River to the north-east, or when the symmetry of its long eastern slope is observed from Wainunu Bay that Seatura displays itself as a gentle-sloped mountain-mass of the Mauna Loa type. Dense forest clothes the greater part of it, except on the north and north-west, where it lies within the limits of the scantily vegetated “talasinga” region.
Profile and Geological Section of the western end of Vanua Levu from the Wainunu estuary across the summit of the basaltic mountain of Seatura to the edge of the submarine platform off the Ndama coast as limited by the 100-fathom line.
The slopes of this mountain are deeply furrowed by river-valleys which radiate like the spokes of a wheel from its central elevated mass. Down its northern slopes flow the Lekutu River and its tributaries and the principal tributaries of the Sarawanga River. The large western affluents of the Wainunu River descend from its eastern side, whilst the Korolevu, Tongalevu, and other small rivers flow south into Wainunu Bay, and the Ndama River drains its western slopes. In all these cases, excepting that of the Ndama River, the rivers have worn deep valleys into the mountain-mass, valleys of denudation that represent the work of ages. That of the Lekutu is a deep cut almost into the heart of the mountain; at Nandroro in this valley, which lies 6 to 7 miles inland and 800 feet above the sea, the hills rise steeply on either side of the river to an elevation of 1,100 and 1,200 feet and more. Some of the large tributaries of the Sarawanga and the Wainunu flow through gorge-like valleys 200 to 300 feet in depth. On the western slopes north of the Ndama river, the mountain-side presents an alternating series of lofty spurs and deep broad valleys. In fact, all around Seatura its slopes are deeply furrowed through the denudation and erosion of ages.
The rocks of this ancient volcanic mountain are almost all of the massive basic type, and except at the mouth of the Ndriti Gap hardly ever display a scoriaceous character. It is also noteworthy that no detrital rock, whether agglomerate, tuff, or tuff-clay came under my observation. The rocks exposed on the surface are mostly blackish brown olivine-basalts and porphyritic basaltic andesites, the former much prevailing. In the northern portion, however, grey olivine basalts of a different type occur. In the great crateral hollow, which I have named the Ndriti Gap, are displayed numerous dykes formed of highly altered basaltic rocks that may be classed among the propylites.
The dense forest that clothes the greater part of this mountain offers many serious hindrances to geological exploration. Except in the northern portion, views of the surroundings are very limited, and one has often to rely mainly on the aneroid and the compass to obtain correct ideas of the contours and general configuration. During most of the time spent in the southern part of the mountain, my work was greatly impeded by heavy rains, and from this cause and from the frequent necessity of following up the stream-courses and of crossing rivers in flood, I was usually wet through all the day.
(a) The Eastern Slopes of Seatura.—The basaltic flows, of which this mountain is principally composed, are best observed on the eastern side where the original volcanic slopes are preserved. Although the rivers have worn such deep valleys into the mountain sides, it is however not often that any great exposure of rock occurs, on account of the dense forest-growth over much of this region. It is only occasionally that the columnar structure of these old basaltic flows is displayed. It is especially well exhibited in the face of a waterfall, distant about two miles in a straight line from Tembenindio and elevated about 700 feet above the sea. Here there is an exposure to the extent of 25 feet of huge basaltic vertical columns, four to five feet across, and pentagonal in form. The rock is a blackish basalt with scanty olivine and a specific gravity of 2·87. It is referred to genus 25 of the olivine-basalts which is described on page [259]. Micro-phenocrysts of plagioclase and a few of augite occur, the olivine being mostly replaced by pseudomorphs. The felspar-lathes of the groundmass average ·18 mm. in length, and there is a little brown opaque interstitial glass. Boulders and fragments of a closely similar basalt, with a specific gravity of 2·9, lie about on the surface in this region. The Seatura slopes here abut on the plateau of Na Savu, formed largely of volcanic agglomerates, to be subsequently described.
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.
(f) The Ndriti Basin or Gap.—This great hollow in the side of Seatura, which I have named after the town in its midst, is apparently a crateral cavity now drained by the Ndama river, and its tributaries, and covered with dense forest to such a degree that a general view of the whole is impracticable. The glimpses, however, that one obtains of the mountain scenery are very grand, the town of Ndriti lying in the midst of mountains that rise almost on all sides of it except on the west. This great cavity is contracted at its mouth a little below the town and expands in its interior, where it must be two or three miles in width. Its floor is fairly level and is elevated only about 200 feet above the sea;[[36]] whilst its mountainous sides rise to 2,000 feet and over.
As shown in the map there are two breaks in the outline of this ancient crater, the one on the west through which the Ndama river flows, the other on the south where the dividing ridge, separating it from the Nandi Valley is under 700 feet in elevation. The Nandi Gorge, as I will term the last-named, is a narrow picturesque ravine leading through the mountains from Nandi to Ndriti. One follows up a rocky stream-course hemmed in by precipitous sides until the top of the gorge is reached, when the watershed is crossed, and the descent is then made to Ndriti by one of the tributary stream-courses of the Ndama river.
Two or three large rapid streams, after draining its mountainous slopes, unite within the basin to form the Ndama river, which, as it issues from its mouth, becomes a comparatively placid stream rolling sluggishly along to the sea, some five or six miles away, with an average drop of about thirty feet in a mile. In the course of ages the original configuration of this great hollow has doubtless been extensively modified by the denuding agencies. The rainfall on the mountain-slopes must be very great, probably not under 250 inches in the year[[37]]; and Ndriti, though only 200 feet above the sea, is in all probability on account of its situation one of the wettest places in the island. The rivers have evidently been important factors in reshaping the original cavity.
Nearly all the rocks exposed in situ in the beds of the rivers and streams in the floor of the great Ndriti basin, and for 300 or 400 feet up its sides are more or less highly altered basic rocks, to which the old and the new names of greenstone and propylite may be fitly applied. They often sparkle with pyrites, and not uncommonly effervesce with an acid, so that one is apt to imagine one’s self in a region of limestone. The degree of alteration varies considerably, those most altered being light-coloured and greenish, whilst the others are darker, the specific gravity ranging from 2·69 to 2·79. In spite of these differences almost all of them appear to belong to the same eruptive series, being as a rule sharply distinguished from the prevailing unaltered surface basaltic rocks of the slopes of Seatura by the size of the felspars of the groundmass, which average about ·3 mm. in length, whilst those of the basaltic rocks just alluded to average only ·17 or ·18 mm. long. These rocks are also well displayed in the sides of the Nandi Gorge; and from their mode of exposure by river-erosion, as well as from their relatively coarse crystalline texture, and from their alteration, it may be inferred that they are older and more deeply situated than any of the Seatura rocks before referred to. Whether these rocks, which extend over an area of some square miles, have been altered by solfataric action or contact-metamorphism,[[38]] I will not now say. The fact remains, however, that they are best exposed wherever the streams have worn deeply into the floor, and lower slopes of the great basin, or have cut down into the mountain-mass as in the case of the Nandi Gorge. The rocks that lie in loose blocks on the surface either at the bottom of the basin or on its slopes extending even to the very summit of the mountain (see page [67]), are characteristic blackish olivine-basalts of the type prevailing around the mountain’s slopes. These propylites are most frequently exposed as dykes in the beds of the rivers at the bottom of the basin. Such dykes vary from 4 to 6 feet in thickness, and they are very conspicuous when they stretch across the river’s breadth projecting more or less above the water. From their frequency it may be inferred that in many other small exposures, ill suited for displaying the mode of occurrence of the rock, we have also to deal with dykes. Judging from four dykes that were particularly examined, they are all vertical or nearly so, and all run in much the same direction, namely, N.N.W.—S.S.E. or N.W.—S.E., whether on the north or south side of the great basin. In one instance, a rudely columnar structure across the thickness of the dyke was observed. From their exposure in river-beds it was rarely possible to ascertain much more than is given above. However, in the bed of a river, a mile above Ndriti, there was an extensive exposure of a highly altered greenish rock which was crossed by a vertical dyke, 4 feet thick, formed of a dark grey less altered rock. I have referred these two propylites to two different genera of the augite-andesites, the dyke-rock to genus 2, and the other to genus 4. In the case of the dyke the rock is a little vesicular; whilst in the other it is densely charged with pyrites. Both have been subjected to the same alteration; but in a different degree; and it would thus seem that solfataric influences were here in operation before and after the intrusion of the dyke.
With reference to the characters of the alteration of these rocks of the Ndriti basin, it may be remarked that where the change is greatest the felspars of the groundmass are alone recognisable. The plagioclase phenocrysts are quite disguised by alteration products, and chlorite, viridite, epidote, calcite, pyrites, &c., occupy much of the groundmass. Other rocks are less affected and in a few the change is only slight.
With regard to the prevailing types of the propylites of the Ndriti Basin, it has already been observed that in most of them the felspar-lathes of the groundmass are unusually large, the average length being ·3 mm. From the rare occurrence of olivine in some of the rocks that are but slightly changed, it is to be inferred that most of them belong to the augite-andesites, and might be termed doleritic basaltic andesites. But in other respects they differ considerably, both as regards the presence or absence of flow-arrangement of the felspar-lathes, and in the occurrence and size of the plagioclase-phenocrysts, some having large porphyritic crystals, others small phenocrysts, and others none at all. Many of them contained a little interstitial glass. In my classification of the augite-andesites they are assigned to genera 2, 4, 16, &c., and additional particulars concerning their characters are given in the description of those genera. Judging from the average large size of the felspar-lathes it may be held that, although in other features they often differ, some of the general conditions under which they were produced were the same.
On the right bank of the Ndama river, opposite Ndriti, there is a singular association of a vertical dyke of a bluish-grey basic andesite with a reddish scoriaceous lava, apparently a flow. The dyke is about 4 feet thick and runs N.W. and S.E., like the other dykes of the basin, exhibiting also a rudely columnar structure across its breadth. Where the two rocks are in contact, the dyke has a vitreous border half an inch thick, and an offshoot of the dyke, four inches wide, has penetrated the lava, acquiring at the same time a more glassy texture. The small size of the felspar-lathes of both rocks distinguishes them from the dyke rocks of the basin, where the felspars are twice as long. Both rocks show some degree of alteration.[[39]]
In following the valley of the Ndama River from Ndriti to Telana, about three miles farther down, one traverses a picturesque region. Emerging from the great basin the river flows through the rolling plains of the “talasinga” district. Near Ndriti, and occasionally on the way to Telana, is exposed a scoriaceous grey basaltic rock; and between two and three miles below Ndriti there is to be observed in the river-bed evidence of a comparatively recent flow of a highly basic scoriaceous lava from the ancient crater of the Ndriti basin. The rock, which is dark and fresh-looking, shows large porphyritic crystals of augite and olivine but no plagioclase, whilst the groundmass contains a little brown interstitial glass. Its characters will be found described under genus 3 of the olivine-basalts (p. [255]). Its specific gravity, notwithstanding its large empty steam-pores, is 2·91. It differs markedly from the basaltic rocks of the Seatura slopes and the Mbua and Ndama plains, in the great porphyritic development of augite and olivine, in the large size of the felspars and augite of the groundmass, and in its numerous steam-holes. But in the coarseness of its small felspars it belongs to the same type as the altered or propylitic basic rocks of the Ndriti basin. It is probably by some such lava flow from the old Ndriti crater that the submarine bank was formed off the adjacent coast on which the low Lekumbi promontory has been built up.
In the numerous dykes of the Ndriti basin and in the great alteration which their rocks have frequently undergone, we have evidence in support of the view that this is an old crateral cavity, an opinion that is supported by the indications of lava-flows that have issued, apparently in later times, from the mouth of the basin. Reference has already been made to the locality where a dyke-rock and the rock-mass, into which it has been intruded, are both propylitic; and from this and other facts, such as the varying degrees of alteration in different parts of the basin, it is to be inferred that in the last stage of the activity of this vent its bottom and sides were extensively affected by solfataric influences. Since that period, the configuration of the crater-basin has been greatly modified through the denuding agencies.
The absence, or at least the great rarity, of tuffs and agglomerates in the case of Seatura is remarkable. The mountain has evidently been built up in the mass by flows of basic lava; and from this source have no doubt in an important degree been derived the basaltic flows of the Ndama, Mbua, and Sarawanga plains, great streams of basalt that further seaward have helped to form the submarine platform extending several miles from the coast. The submarine tuffs and agglomerates that occur at various elevations, reaching as high as 1,200 feet above the sea, in the Sesaleka, Lekutu, Sarawanga, and Ndrandramea districts lying to the north-west, north, and east, did not come under my notice on the Seatura slopes. On the other hand, except in the few localities, where scoriaceous rocks occur, the general type of the basalts is such as we would expect to find in submarine flows. In no part of the island, however, is the antiquity of the land-surface so well attested by the disintegration of the basaltic flows, which extends here to depths of ten and even twenty feet. This is in favour not only of the sufficiency of time, but also of the ability of the denuding agencies to strip off the surface-deposits.
However this may be, it is evident that the mountain of Seatura possesses a history quite independent of that of the rest of the island. I have pointed out in [Chapter I.] that it represents a mountain of the Tahitian type. In its radiating valleys and in its basaltic character it much resembles the mountainous island of Tahiti, which Dana describes as a gently sloping cone of the Hawaiian order that through the erosion of ages has become a dissected mountain.[[40]]
The Seatovo Range.—This remarkably situated mountain-range, which I have named after a town at the foot of its western slope, extends from the valley of the Ndama River to Solevu Bay. It attains a maximum height of about 1,800 feet, and varies between this elevation and 1,500 feet until in the vicinity of Solevu, where it descends as a mountainous headland to the coast. Its summit is narrow and ridge-shaped, and although the whole range is not interrupted by gaps it has a composite origin. At its north end, where it is cut off from the Seatura Range by the Nandi Gorge it helps to close in the large Ndriti basin. Towards the south an offshoot proceeds eastward and shuts in Solevu Bay. But, although apparently all the rocks are basic, considerable variety prevails, and there are many puzzling points in the geological structure of this region.
At the place where this range abuts on the Ndama valley, below Ndriti, the grey scoriaceous basalt, before referred to, is exposed at its foot. However, the usual blackish basaltic rocks, often carrying a little olivine, form in mass the mountainous southern headland that culminates in Solevu Peak (Ulu-i-matua); and the same rocks prevail in the lower regions on the west side of the range from Vuia Point to the valley of the Ndama River. The southern portion will be described in the account of Solevu Bay; and I will now give the results of my journey across the summit of the range about half a mile south of the Leading Peak of the chart.
The eastern slopes are steep and often precipitous, whilst on the western side there is a more or less gentle descent to the lower levels, suggestive of a volcanic slope; and it is remarkable that whilst the rocks exposed on the precipitous eastern side for the lower two-thirds are sometimes markedly altered, on the western side they are comparatively unchanged. These facts at once suggest that we have here the western rim of a large crateral cavity, though the topography of this district is not sufficiently well shown in the chart to enable one to define its original limits. This inference is also supported by the occasional scoriaceous character of the rocks below referred to.
The most frequent rocks in the upper two-thirds of the range are grey porphyritic olivine-basalts, displaying opaque plagioclase phenocrysts and more or less hematised olivine, the specific gravity being about 2·9. They approach in characters the grey porphyritic olivine-basalts of the northern part of Seatura (pages [65], [66]); but differ amongst other features in the greater abundance of the olivine and in exhibiting flow-structure. They are usually almost holocrystalline, and are assigned for the most part to genus 14 of the olivine-basalts. They are extensively exposed in the stream-courses on the west side between 500 and 900 feet; and huge masses of the same rocks, but containing less olivine and more glass, and displaying much calcite, viridite, and other alteration products, are found near the base of the eastern slopes. The semi-vitreous condition of these rocks is represented in the large masses of a dark very scoriaceous porphyritic lava, possessing quite a cindery appearance, that occur on the narrow ridge-shaped summit. The groundmass shows a few scattered felspar microliths; but it is in the main composed of a dark opaque glass. Small cube-like crystals of chabazite line some of the cavities.
Other basic rocks are not infrequent and apparently represent dykes. Thus on the eastern side at 800 feet is exposed a dark-grey semi-ophitic doleritic rock (sp. gr. 2·77) assigned to genus 12 of the augite-andesites (page [275]). The felspar-lathes average ·3 mm. in length, and there is a little interstitial glass containing viriditic and calcitic alteration products, the same materials filling small rounded vesicular cavities. On the same slope between 1,000 and 1,200 feet, there are displayed fresh-looking compact non-porphyritic basaltic andesites (sp. gr. 2·84), where the felspar-lathes average ·2 mm. and the interstitial glass is scanty. They are referred to genus 16, species C, of the augite-andesites. On this side also between 600 and 800 feet occur blocks of a highly altered slightly vesicular augite-andesite showing a little microporphyritic plagioclase. It is assigned to genus 13, species B, of the augite-andesites. In one place where it is in position it is scoriaceous, the steam-holes being round, empty and one to five mm. in size. In the less glassy rock it displays numerous small irregular cavities either filled with fibrous viridite or calcite or showing concentric zones of the two minerals. The felspar-lathes are ·15-·2 mm. in length. In blocks near the foot of the eastern slope occur a blackish olivine-basalt (sp. gr. 2·88) of the prevailing Seatura type, possessing a little interstitial glass and felspar-lathes with an average length of ·2 mm. It belongs to genus 25 of the olivine-rocks.... On the western slopes at a height of 500 feet occurs a dark compact rock (sp. gr. 2·89) with abundant olivine which is referred to genus 1 of the olivine basalts. There is a little residual glass, the felspar-lathes averaging only ·08 mm. in length. A similar-looking rock is exposed at 1,400 feet, which displays felspar-lathes averaging ·2 mm. long (sp. gr. 2·9). It belongs to genus 37 of the same olivine class. Here also is assigned an aphanitic basalt, with a few scattered large plagioclase phenocrysts and felspar-lathes averaging ·15 mm. long, which is displayed near the base of the slope.
I could not satisfy myself as to the presence of tuffs on the slopes of this range. Some fine argillaceous rocks exposed half-way up on either side show no lime and contain no organic remains. One specimen beside me is certainly a disintegrated basic rock. No agglomerates came under my notice. In the absence or rarity of detrital rocks this part of the range resembles the adjacent mountain of Seatura.
Although olivine-basalts prevail in this part of the Seatovo Range there is great variety in their characters; and it does not appear possible to explain such a diversity except to assume that we have here an old crateral ridge which has again and again been penetrated by dykes and has since been greatly denuded. We have here one of those singular mountain-ridges that characterise the central portion of the island, but differing in this respect that the submarine tuffs and agglomerates, which there occur on the surface, even in the higher levels, are here absent.
Solevu Bay.—There are few localities in the island where so many kinds of basic rocks are displayed as around Solevu Bay. In addition to the prevailing blackish porphyritic basalts and basaltic andesites, there are grey porphyritic basalts, grey non-porphyritic basalts, black basalts with abundant large crystals of olivine, &c., all of which have their distinctive characters.
This picturesque bay is surrounded by hills. On the west side it is inclosed by the promontory forming the southern extension of the Seatovo range which, culminating in Ulu-i-matua, or the “Head-of-the-Strong” peak, descends at first steeply and then gradually to the coast, where it projects as Vulavulandre Point. On the east side is a broken line of hills, of which Koro-i-rea, the hill known to the natives as the “Town of the Albinos,” is the most conspicuous. Beyond it stretches the eastern point of the bay, which the Fijians call “Ua-nguru,” that is, “the noise of the waves.” On the shores lie the village of Nawaindo, “the running-stream,” and the once populous town of Solevu, which has given its name to the bay. Solevu, as its name indicates, is the place of the “great assembly.” In the background rises the three-peaked mountain of Koro-tolutolu, “the three towns,” which forms a continuation inland of the eastern arm of the bay, and joins the Seatovo Range at the head of it. Between these two ranges inclosing the bay lies the valley of Solevu, down which descends the Solevu River to the sea. In ascending this valley from the shore, one rises only about 100 feet above the sea for the first mile or two.
The promontory, which in the even-topped Ulu-i-matua or Solevu Peak, attains a height of 1,100 feet above the sea, displays on its summit and on its eastern slopes descending to the Solevu river, and on its western slopes reaching down to the coast at Vuia, more or less porphyritic blackish olivine-basalts of the usual type with specific gravity 2·88-2·90. These basaltic rocks contain scanty olivine and only a little interstitial glass. The felspars of the groundmass vary in different localities from ·11 to ·15 mm. in average length. The rocks belong to genus 37 of the olivine class which is described on page [262].
They are in the lower regions often decomposed to a considerable depth, the spheroidal structure being well displayed during the weathering process. Where this promontory terminates in the low Vulavulandre point, these rocks give place in part to grey porphyritic olivine-basalts, with specific gravity 2·79-2·83, which from the abundance of the macroscopic opaque felspar look like porphyrites. They come near to the rocks exposed on the north slopes of Seatura and in the Seatovo Range. At the end of the point they become scoriaceous and more vitreous; but with this exception they contain but little glass. They vary somewhat in character and are referred to genera 2 and 38 of the olivine-class.
The prevailing rock in the interior of the Ua-nguru promontory to the south of Koro-i-rea is the blackish porphyritic basalt, containing a little olivine, and often much decomposed; but at the point and on the east shores of Solevu Bay, there is a considerable variation in the character of the basic rocks, of which the two following are the most conspicuous. Near the village of Nawaindo, there is an apparent intrusion of a black lava-like basalt of high basicity (specific gravity 3·01) showing abundant large olivine crystals, five or six mm. across, with some porphyritic augite, but no macroscopic felspar. At the point the rock is somewhat scoriaceous, with calcite occasionally filling the cavities, whilst the olivine is so thoroughly hæmatised that it glistens like brown mica. The compact rock contains a little devitrified interstitial glass, the felspar-lathes being unusually small, their average length being only ·07 mm. It belongs to genus 15, the most basic of the genera of the olivine class represented in the island. The second rock to be noticed is a slightly altered compact basalt without olivine forming apparently a dyke near the coast about half way between the village of Solevu and Ua-nguru Point. It has a specific gravity of 2·84, the felspar lathes (·15 mm.) presenting a marked flow-arrangement, whilst there is a fair amount of altered residual glass in irregular spaces, a millimetre in size. The rock, on account of its joint-structure, could be easily worked as a building-stone. It is referred to genus 16, species B, of the augite andesites.
The hill of Koro-i-rea, which rises on the east side of the bay to a height of 850 feet, has a ridge-shaped summit. Its upper half is composed of a bluish-grey rock looking like a phonolite and usually compact, except at the top of the hill, where it is a little scoriaceous. It has, however, a specific gravity of 2·91 or 2·92, and is in fact a pretty grey olivine-basalt studded with small olivine crystals about a millimetre in size and showing no other phenocrysts. This type of olivine-basalt occurs also at Ulu-i-ndali on the east side of Wainunu Bay, but is rare in the island. It differs amongst other features from the porphyritic olivine-basalts of the northern part of Seatura and of the Seatovo range in the absence of plagioclase phenocrysts. There is apparently no interstitial glass, whilst the average length of the more or less parallel felspar-lathes is ·13 mm.[[41]] On the lower slopes of the hill the common blackish porphyritic basalt or basaltic andesite is exposed. In the grey-basaltic upper portion of this hill we have probably an old volcanic “neck.”
Following the line of hills inland from Koro-i-rea, we cross the intervening saddle 450 feet above the sea, and ascend the slopes of Koro-tolutolu, a ridge-shaped mountain backing Solevu Bay, and having, as its name indicates, three peaks, of which the highest is 1,280 feet above the sea. My observations indicate that this mountain is formed in mass of the common blackish-basalts described under genus 37, their specific gravity being 2·88 to 2·94. But Koro-tolutolu has also the peculiarity that it appears to be in mass magnetic. The rocks obtained from its summit, half-way up its western slopes, and near its foot on the same side, all display polarity, a character also of the rocks of the neighbouring hills of Ulu-i-matua and Koro-i-rea, but in their cases seemingly confined to the higher levels.[[42]]
Neither tuffs nor agglomerates came under my notice at Solevu Bay. This appears to be an ancient corner of the island, from which denudation has stripped off nearly everything that could guide us in speculating as to its past. Although the hills of Koro-i-rea and Koro-tolutolu doubtless represent old volcanic necks, the relation of Ulu-i-matua to the very differently composed northern part of the same range, as described on page [73], is extremely puzzling. Then again in the opposite sides of Solevu Bay we see exposed the remains of lava-flows that bear no relation to the present configuration of the surface. We may suspect, however, that most of the volcanic energy was displayed under the sea.
Nandi Bay.—Lying north of Solevu Bay, this bay is situated between spurs, descending to the coast from the mountainous interior. The valley extends a long distance inland without much change of level, the elevation 1½ miles from the coast being not over 100 feet above the sea. At its head is the Nandi Gorge, which leads into the Ndriti Basin, the great crateral cavity of Seatura. There are some remarkable lofty, isolated hills in this valley that would be well worth examining.
That the bay represents the site of an old volcanic centre is indicated by the occurrence on the shore of two basaltic dykes, one on either side of the village of Na Savu and 300 to 400 yards apart. The eastern dyke is perhaps 30 feet thick, whilst that to the west is scarcely half this thickness. They exhibit an imperfect columnar structure, the columns, which are 6 to 12 inches across, being inclined at an angle of 15° or 20° from the vertical in such a way that it may be inferred that the molten material was ejected from some subterranean focus lying to the northward (or inland) at an angle of 15° or 20° above the horizon. The basalt is a compact bluish-black rock with specific gravity 2·95-2·99. It contains abundant olivine but no other phenocrysts and very scanty interstitial glass, whilst the felspar-lathes average ·1 mm. in length. It is referred to genus 16 of the olivine basalts, and is remarkable for the flow arrangement not only of the felspar-lathes but also of the smaller olivine crystals.
Blackish basaltic rocks of the prevailing type are exposed on the surface of the broad spur, not over 500 feet in height, that divides the Nandi and Nasawana valleys and descends to the coast between the two bays thus named. They belong to genus 37 of the olivine-basalts and display a few small plagioclase phenocrysts. The felspar-lathes average ·2 mm. in length, and there is a little interstitial glass. Entering Nasawana Bay we find ourselves on the southern slopes of Seatura, of which the high Na Suva range that backs the bay is the southern extension.
The Table-land of Na Savu.—This remarkable plateau has an elevation varying usually between 700 and 800 feet above the sea and a maximum breadth of four or five miles. It is an area of basic agglomerates and basic tuffs and lies in the hollow between the basaltic mountain of Seatura and the acid andesitic hilly region of Ndrandramea. For the convenience of description I have named it after the picturesque falls of Na Savu[[43]] at its southern edge. These falls are celebrated in Fijian tradition; and from the brink in old time the native desirous of ending his life leapt into the gorge below.
After flowing sluggishly along on the surface of the table-land, the Mbutu-mbutu River arrives suddenly at the edge of a line of cliffs of volcanic agglomerate, that here form the southern border of the plateau, and with a volume 30 to 40 feet across, it plunges down into the ravine 150 feet below. As shown in the view from the gorge below, there is a break in the middle of the descent. These falls, however, are not easily accessible. They are best approached by proceeding from Wainunu to Ndawathumi and thence up the gorge of the Mbutu-mbutu River.
The surface of the plateau of Na Savu is densely wooded. In places it is marshy, and here thrives the Giant Sedge (Scirpodendron costatum). The Makita tree (Parinarium laurinum) also flourishes in the wet districts; and in the drier localities occur the Ndakua (Dammara vitiensis) and the Ndamanu (Calophyllum-burmanni) together with a palm of the genus Veitchia. Here on this level watershed between the basins of the Wainunu and Sarawanga rivers, the sluggish streams flow aimlessly along in but slightly eroded channels; and it is not always possible to determine the side of the island to which they ultimately direct their course. In their beds are pebbles and irregularly formed concretions of an impure reddish flint which I have described on page [354]. On the north and south sides the table-land is much excavated by the tributaries of the Sarawanga and Wainunu rivers. On the west where it meets the foot of the Seatura slope portions of columns of basaltic rocks appear on the surface, and deep gorges are worn by the large streams descending from the mountain. On the east towards Nuku-ni-tambua and Tambu-lotu, the surface is also much cut up. The preservation of this table-land in a region, where the denuding agencies are very active in their operations all around it, is to be attributed to its being a level watershed, where the head-waters of the Wainunu and Sarawanga rivers in part take their rise but have little or no eroding power.
It is not easy to obtain a good general view of the district of the falls on account of the dense forest-growth. When making the traverse from Tambu-lotu to Ndawa-thumi, it is observed that there is here a singular hollow, about half a mile in length, which receives the falls at the western end. The river crosses this hollow and is at once received into the gorge below, but there is no stream to explain the origin of the cavity. On its north side the cliffs of agglomerate rise to a height of 150 to 200 feet from their base, but on the south the sides are much lower. Here there seem to be the remains of the crater of the ancient vent from which all the tuffs and agglomerates of the district were derived. We must look for their origin in the vicinity, and the only evidence of a crateral cavity is this streamless hollow extending east from the falls of Na Savu.
With reference to the basic tuffs and agglomerates of this plateau it may be observed that they cover the massive basic rocks and are probably not over 100 or 150 feet in maximum thickness. They are well exposed where the streams cut into the borders of the plateau. The tuffs are sometimes bedded and slightly inclined, and they may be fine or coarse grained. They are more or less palagonitised hyalomelane-tuffs, being composed mainly of fragments of a basic glass, often finely vesicular and even fibrillar, the vacuoles being filled with different materials, whilst the palagonitisation is well advanced. Sometimes they have a brecciated appearance, and in that case when the alteration of the basic glass is very extensive we find angular fragments, 1 to 2 inches across, of a greenish palagonite imbedded in a pale matrix of palagonitic debris, the whole rock having a soapy feel and a steatitic appearance. This is well shown on the sides of the stream-course at Ndawathumi which lies at the border of the table-land. These tuffs effervesce but slightly with an acid.
The basic agglomerate is displayed in the face of the falls and in the gorges. The blocks are as a rule composed of semi-vitreous basaltic andesites of varying type, showing no olivine and containing a fair amount of smoky glass in the groundmass. At times they are scoriaceous and display amygdules of calcite or a zeolite. In places the rock shows large phenocrysts of plagioclase and a semi-ophitic groundmass, when it is referred to the porphyritic group of genus 9 of the augite-class. In a few of the scoriaceous blocks the augite of the groundmass is for the most part prismatic and rarely granular (genus 5).
The massive rocks underlying the agglomerates in the vicinity of Na Savu are aphanitic augite-andesites, differing in important characters from the rocks of the agglomerates. They probably represent ancient lava flows of the Na Savu vent. They are compact (sp. gr. 2·72-2·76), and display a groundmass formed of a felt of felspar-lathes, averaging ·05 or ·06 mm. only in length, and in flow-arrangement. That occurring just below the falls is almost aphanitic, but is referred to genus 13, species A, sub-species a, of the augite-andesites. The rock from the gorge below is of the same character, but on account of its opaque plagioclase phenocrysts it is referred to genus 14, and is described on p. [279].
In one place on the plateau a tuff-agglomerate is penetrated by veins, a few inches thick, formed apparently of a finely brecciated tuff of basic glass fragments in a palagonitic matrix. It is, however, pointed out on p. [340] that they were originally veins of basaltic glass which have been subjected to crushing, and that the palagonite has since been produced.
In concluding this description of the table-land of Na Savu, it may be inferred that the source of its basic tuffs and agglomerates is to be found in the same locality; and probably the original vent is now represented by the hollow extending eastward from the falls. With the exception of a large block of silicified coral found in the vicinity of Ndawathumi and of the impure flints of the surface of the plateau, which are described on pages [354], &c., no direct testimony of its submarine origin offered itself to me. The palagonitic characters of the tuffs afford, however, indirect evidence in this connection; and indeed the occurrence of submarine tuffs and limestones in the vicinity of Tembenindio on its lower northern slopes (see page [131]), and the existence at elevations of several hundred feet above the sea of fossiliferous tuffs and clays in the Wainunu and Ndrandramea districts to the eastward, afford strong presumptive evidence that the tuffs and agglomerates of the table-land were deposited under the sea, and I may add in a period subsequent to that of the formation of the great basaltic flows of Seatura and Wainunu.
CHAPTER VI
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES
(continued)
The Basaltic Plateau of Wainunu.—This table-land extends for a distance of seven miles from the base of the Ndrandramea mountains in the heart of the island, where it is elevated 1,100 to 1,200 feet above the sea, to the valley immediately north of the hill of Ulu-i-ndali, where within a short distance of its termination it still retains a height of 700 to 800 feet. Limited on the west by the valley of the Wainunu River and on the east by that of the Yanawai River, its breadth varies usually between four or five miles. It is best seen in profile when viewed from the south-west on the western shores of Wainunu Bay, between Korolevu and Nasawana, when it presents itself to the eye as a table-land, descending with a very gradual slope from the interior towards the coast. From such a point of view the two great basaltic slopes of Seatura and Wainunu may be seen together, the former descending eastward to the Wainunu valley at an angle of 3 or 4 degrees, the latter descending at right angles to it to the southward with a similar small gradient of 2 or 3 degrees.
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.
By referring to the section across this part of the island, it will be observed that the basaltic lavas of this table-land must have issued from some fissure near the south side of the base of the Ndrandramea mountains. In crossing the head of this plateau on the way from Nambuna to Ndrawa one passes from the region of the acid andesites into that of the basalts. The track first skirts the base of Mount Wawa-Levu, where the prevailing altered dacitic rocks are exposed in a much decomposed condition in the stream-courses. Then there is a gradual ascent through somewhat broken country to reach the western slope of the table-land, and here are at first displayed the semi-vitreous basaltic andesites just referred to.
The Wainunu table-land is bisected in a singular fashion by the Ndavutu River. Since, however, the deep and often gorge-like channel of the river displays submarine deposits incrusting the basaltic slopes on its sides, it is evident that the break in the basaltic table-land existed in part at least before the emergence.
With regard to the total thickness of the basaltic flows of this plateau I have only a few data. In the bed of the Ndavutu River opposite Vunivuvundi, and about 400 feet above the sea, there is exposed a greyish porphyritic rock showing pyrites, apparently an altered andesite. If this is the bed-rock, the basaltic plateau in that locality would be 300 to 400 feet in thickness. This is rather over the thickness of the end of the table-land at Masusu.
I pass on now to consider briefly the submarine deposits that overlie the marginal slopes of this basaltic table-land in places. They are for the most part pteropod and foraminiferous ooze-rocks and are extensively represented on the surface and slopes of the Nandua flat to the north of Ndavutu, where they occur at all elevations up to 500 feet above the sea. They are also displayed on the eastern slopes overlooking the Yanawai but at rather lower heights; and little patches of them occur here and there in different places but not exceeding 500 feet in elevation. These friable clayey rocks, which contain from 30 to 40 per cent. of carbonate of lime, are described in detail on page [320]. It may however be remarked here that these deposits are but partly derived from the degradation of the submerged basaltic table-land or from the washings of a basaltic coast. They were formed in a clear sea-way, but probably at no great depth, at a time when the basaltic plateau was submerged below the level of breaker-action.
It is remarkable that these deposits do not repose directly on the basaltic rock. In one place below the Nandua tea-plantation, where there is a steep descent to the river of about 250 feet, the pteropod ooze-rock, which is exposed in the upper half, passes down into a chocolate-coloured marl that contains 5 per cent. of carbonate of lime and is horizontally bedded. It is composed in the main of fine palagonitic debris, with some fragments of minerals, &c., and contains a few microscopic tests of foraminifera. This deposit passes down into apparently a rock of pure palagonite. The succession of these beds and their characters are described more in detail on page [344]; and as indicated in the diagram there given it is to be inferred that a very extensive formation of palagonite has taken place on the surface of a submarine basaltic flow.
On a similar slope of the Nandua district, and about half a mile nearer Ndavutu, the pteropod ooze-rock overlies a coarse zeolitic palagonite-tuff composed in great part of fragments of a highly altered vacuolar basic glass, but without organic remains. These tuffs are horizontally stratified. Tuffs precisely similar occur on the northern slopes of Ulu-i-ndali three miles to the south. They are all described in detail on page [335].
Some miles up the valley of the Ndavutu River on the steep slope descending from Vunivuvundi to the river, and on the sides of the river lower down, are exposed dark palagonitic and sometimes calcareous clays and tuffs. I traced them as high as 450 feet above the sea where they were bedded and dipped gently to the west. In the river-channel they were mostly confined to the right bank, the slope on the other side being strewn with large fragments of columnar basalt. At the mouth of the Ndavutu River, there are exposed tufaceous sandstones and a tuff-conglomerate, probably in great part formed of palagonitic materials, but I have kept no specimens.
There is much that is puzzling about the tuffs of the region between Ndavutu and Vunivuvundi. The surface pteropod and foraminiferous ooze-rocks, that are found here and on the Yanawai or eastern border of the basaltic plateau and in other localities, offer no difficulties; but the origin of the palagonitic tuffs that in places lie beneath them is not so easy to explain. At Mr. Simpson’s old estate on the Nandua flat one finds numbers of huge blocks of columnar basalt scattered about on the slope descending to the river; and in places there is exposed in a small stream, up to a height of 500 feet, a fossiliferous ooze-rock containing marine shells. The ooze-rock is evidently an incrusting deposit; but when one goes down to the river-side, which is there about 200 feet above the sea, one finds displayed in situ in the river-bed an amygdaloidal basic lava with coarse tuffs and agglomerates a little lower down.
The Hill of Ulu-i-Ndali.—The meaning of the name of this hill is “Head of the rope.” It is noted on account of the dense growth of tall forest trees that clothes its surface, such as the Vesi (Afzelia bijuga), the Ndamanu (Calophyllum burmanni), the Ndakua (Dammara vitiensis), the Wathi-wathi (Sterculia sp.) &c.; and it may be that its name is connected with the launching of the large canoes that were at one time constructed on its slopes.
Ulu-i-ndali, which has a broad level summit 1,100 to 1,150 feet in height, rises on the left side of the mouth of the Wainunu estuary. Its relation to the surrounding region is partly shown in the rough sketch given on page [83]. It is separated from the basaltic table-land to the north by a deep and wide valley, the bottom of which is raised only a few feet above the sea; the small stream known as Ndawa-ndingo, that apparently flows through it, is merely a branch of the Wainunu estuary, the tide ascending it for some distance. This singular valley, like the main valley of the Wainunu, dates back in great part to the period preceding the emergence of this region. The steep basaltic slopes of Masusu, strewn with fragments of large columns, bound it on the north. On its south side are the lower slopes of Ulu-i-ndali which are composed of volcanic tuffs.
A long spur descends to the south from Ulu-i-ndali to form the rocky promontory of Vatu Vono or “Stone turtle,” so-named from the fanciful resemblance of the large rounded blocks of basalt on the shore to the backs of turtles. To the south-east extend the low tuff-formed Ravi-ravi plains which are but slightly elevated above the sea. The Ulu-i-ndali range is apparently connected by a “col” with a range of similar height to the eastward, the highest peak of which is about 3 miles distant.
A more or less coarse doleritic grey olivine-basalt forms the mass of this hill and is chiefly exposed in its upper portion. Around its slopes, extending from the coast usually halfway up the hill, are blackish-brown olivine-basalts; they differ amongst other points from the grey basalts—which are practically holocrystalline, in their greater amount of interstitial glass, to which, doubtless, is due their dark colour. These dark basalts also occur scantily on the summit; but from their greater prevalence on the lower slopes and from some other of their characters, it may be inferred that they are in the main formed at the surface. Outside all, on the north and south sides of the hill, are exposed coarse tuffs composed of fragments of palagonitised vacuolar basic glass and containing much secondary zeolitic and calcitic materials. They are purely of eruptive origin, and although containing no organic remains were doubtless, as in the case of the precisely similar tuffs of the neighbouring district of Nandua, deposited under the sea. A description of their characters is given on page [335]. Such tuffs extend as high as 300 feet above the sea on the north-west slopes, where there are exposures, 10 to 12 feet in thickness, in the dry stream courses; and here they may be seen overlying the basalt and rudely bedded, dipping away from the summit at an angle of 15 degrees.
The grey olivine-basalts of Ulu-i-ndali, which often look like clinkstone, range generally in specific gravity from 2·9 to 2·95. They contain microporphyritic olivine in abundance, which is usually more or less hæmatised and in extreme cases of the change looks like brown mica. Most of them are referred to genus 16 of the olivine class and their characters will be found described on page [258]. The felspar-lathes are stout and show sometimes lamellar twinning, and on account of their large size (·2 to ·5 mm in average length) the rock acquires a doleritic texture. They display as a rule a flow arrangement around the olivine crystals. Augite granules occur in great abundance, and there is rarely any interstitial glass.
These grey olivine-basalts are as a rule non-vesicular, but rocks with minute irregular cavities, though without glass, occur scantily on the upper slopes. They come near to the grey olivine-basalts of the hill of Koro-i-rea in the Solevu district, as described on page [77]; but they differ in their doleritic or coarser texture, the felspar-lathes in the last-named locality being much smaller, their average length being ·12 mm.
The blackish basalts, mostly characteristic of the lower slopes of Ulu-i-ndali, vary somewhat in character; but they may on the whole be regarded as surface forms of the more deeply situated grey basalts which are practically holocrystalline. The rock of this kind that prevails on the south and west sides has a specific gravity of 2·96. It is referred to the same genus (16) as the grey basalts, but differs from them in the circumstance that the microporphyritic olivine is serpentinised and not hæmatised, and in the occurrence of a fair amount of devitrified interstitial glass, to which probably the dark colour of the rock is due.... The dark aphanitic basalt, with flinty fracture and a specific gravity of 3·00, that is displayed in Vatu Vono Point, is merely a compact surface variety of the more coarse-textured grey basalts, being referred to the same genus. Here there is a great abundance of microporphyritic olivine in a groundmass of parallel felspar-lathes and augite grains; but the felspars are unusually small, averaging ·1 mm. in length; and there is a much larger amount of fine magnetite than in the grey basalts. There seems to be no interstitial glass; and the olivine when not fresh is usually serpentinised but occasionally hæmatised.
The dark basalts of Ulu-i-ndali when they occur on its upper slopes become ophitic. A specimen lying beside me has a specific gravity of 2·91. Allowing for the structural differences, it appears as an ophitic surface variety of the deeper seated grey basalts. A description of it is given under genus 12 on page [256], of which it forms the type.
From the data above given, the hill of Ulu-i-ndali is to be regarded as the basal portion of a submarine volcano still retaining part of its ash-coverings. The grey doleritic basalts probably represent the core and the dark fine-grained basalts represent the flows of this ancient vent.
The Kumbulau Peninsula.—South-east of Ulu-i-ndali stretches a remarkable “talasinga” district which for convenience I will call the peninsula of Kumbulau. Its south or seaward border is broken and hilly, and presents an irregular line of hills 300 to 470 feet in height, extending from Kumbulau Point to Soni-soni Island, which is almost connected with the coast. The rest of the peninsula is a low-lying and often marshy plain, which, though elevated in some places 20 to 25 feet above the sea, is usually much lower. On the north-east side of the isthmus is the narrow Nandi inlet, bordered by low mangrove-belts, which represents the broad channel that in a very recent period of the island’s history cut through the present neck of the peninsula between the head of the Nandi inlet and Ravi-ravi.
Stratified and often steeply inclined tuff-sandstones and clays, more or less basic and palagonitic in character, form together with basaltic agglomerates the prevailing rocks of the peninsula, whether in the hilly portion or in the plains. They belong to the basic tuffs of mixed composition described on page [330]; and though the agency of eruptions can be recognised in their components they are also the products of marine erosion.
Some of the hills represent volcanic “necks”; whilst the low narrow promontory between Kiombo and Soni-soni Island has been formed by an old basaltic flow.
I will begin the description of this peninsula with the eastern extremity north of Kumbulau Point, the interior of which is cut up into ridgy hills 300 to 350 feet in height. On its eastern coast are exposed volcanic agglomerates, composed of large blocks, which from their dimensions given below would weigh between one-third and two-thirds of a ton, a size indicating the immediate vicinity of the vent, now obliterated, from which they were originally ejected. Near Kumbulau Point the blocks, which are made of basaltic andesite, measure five or six cubic feet. Further north in the vicinity of Vatu-Ndamu, the precipitous coast cliffs are composed of agglomerates, the large blocks of which, often ten cubic feet in dimension, are formed, not of the prevailing basaltic andesites, as in other parts of the peninsula, but of a grey hornblende-andesite. This singular appearance of an acid andesite in a region of basic rocks has no doubt given rise to the native name of Vatu-Ndamu, “the red or brown stone.” It belongs to the second order of the hornblende-hypersthene-andesites, and is described on page [298].
Proceeding along the south coast westward from Kumbulau Point, before arriving at the village of Na Tokalau we pass from the district of agglomerates into that of the bedded tufaceous sandstones and clays which are exposed all along the coast to Kiombo about three miles away. The transition is indicated by the agglomerates becoming interstratified with the tuff-beds. These sedimentary tuffs are as a rule steeply inclined at angles of 20 to 40 degrees, the prevailing direction of the dip being to the north-east, its uniformity for such a length of coast being noteworthy. These beds however are occasionally “crumpled”; and here and there a globular structure is developed.
The hills of this region of sedimentary tuffs between Na Tokalau and Kiombo are the highest of the peninsula. They usually attain a height of 400 feet, but do not reach 500 feet. From each of them descends to the coast a spur terminating in a rocky point; whilst between these points lie low sandy flats, where the native villages of Levuka, Kiombo, &c., are situated. The tuff-rocks extend to the top of the hills behind Na Tokalau, and probably this will be found true of most of the other hills. Agglomerates are not common in the district. In the point west of Na Tokalau, however, they are overlaid by basaltic agglomerates, some of the blocks being scoriaceous. In the point east of Levuka, a chocolate-coloured somewhat calcareous tuff-clay occurs interstratified in thin beds with the coarser deposits.
The general characters of these tuff-sandstones and tuff-clays have already been briefly referred to. The former are much more prevalent and non-calcareous; the latter are sometimes a little calcareous and look like marl, and may perhaps contain a few tests of foraminifera. Both are formed of the debris of basic rocks and are more or less palagonitic. The coarser deposits are described as sample A on page [330]. At times these tuffs are composed of much coarser fragments of the same materials, some of them a centimetre in size. A type of tuff intermediate in character is not uncommon.
The promontory that lies between Kiombo and Soni-soni Island has been formed by a remarkable basaltic flow. The low tongue, about 50 feet high and 200 to 300 yards across, in which it terminates, was originally severed by a passage worn by the sea from the main portion; but it is now joined by a low tract only 2 or 3 feet above the beach and partly occupied by mangroves.
The structure of the flow is well exhibited in the shore-flat and coast-cliffs west of Kiombo, and extending to the end of the point. The waves have here cut into its mass and exposed its structure. Its lower part, as exposed in the shore-flat, is made of a compact hemicrystalline basalt; whilst its upper portion, as displayed in the cliffs, 30 or 35 feet in height, is composed of vitreous and semi-vitreous forms of the same rock looking like pitchstone. The upper vitreous part is sometimes massive; but usually it is rubbly, with a tendency to form spheroidal masses. All transitions can there be traced between the hemicrystalline rock of the shore-flat and the vitreous rock of the cliffs.
The rock of the shore-flat, which has a specific gravity of 2·83, is a blackish porphyritic basalt with scanty olivine, and on account of the semi-ophitic character of the augites of the groundmass it is placed in genus 33 of the olivine class. The plagioclase phenocrysts are 3 to 5 mm. in size. About half of the groundmass is made up of felspar-lathes (·17 mm. long) and large augites (·11 mm.), the rest consisting of a smoky devitrified glass containing a few irregular “lacunæ” filled with the residual magma in the form of a reddish-brown opaque palagonite-like material. The rock intermediate between the lower and upper portions of the flow is also intermediate in character, having a specific gravity of 2·77, whilst quite three-fourths of the groundmass are of smoky glass.
The vitreous rocks of the cliffs, though usually rubbly in appearance, have also the aspect in places of brecciated pitchstone tuffs with the interstices filled with waxy palagonite; but the microscopical examination shows that we have not to deal with a rock of detrital origin. We have here the effects of the breaking up and crushing in situ of a dark-brown isotropic basic glass[[47]] carrying porphyritic plagioclase. The interspaces then became partially filled with the finer fragments of the glass and of the crushed felspar; but they were in the main occupied by a still liquid magma which penetrated into the cracks of the glass-fragments and into those of the felspars, where the fractured portions in some cases remained in position. There it has become devitrified and often palagonitised. Whether this liquid magma was produced by a partial remelting resulting from the heat developed during the crushing of the glassy upper portion of the flow during the contracting process, or whether it was squeezed upwards from the less consolidated lower portion, I cannot determine, although the last supposition seems more probable. At all events the edges of the glass-fragments are peculiarly eroded as if by the magma. (The bearing of these facts on the origin of palagonite is discussed in [Chapter XXIV.])
I infer that this flow has descended from the hills west of Kiombo. Huge masses of agglomerate are exposed in the lower third of the hill marked “470 feet” in the chart, and immediately north of the town. Fine clayey tuffs are exposed in the hill at the back and to the westward of this place; but the locality requires a more detailed examination. The absence to all appearance of vesicular and scoriaceous rocks in the case of this basaltic flow is remarkable. This would not have been expected in the case of a supra-marine flow; and indeed the testimony of the tuffs of this peninsula sufficiently indicates that during their deposition the whole district was submerged.
The future inquirer will doubtless discover some old volcanic “necks” in the hills of this peninsula. One such hill overlooks the Soni-soni inlet about a mile west of Kiombo. It is a singular isolated hill which I have named Bare-poll Peak for descriptive purposes. In my notes its height is stated as 120 feet, but it appeared to me to be rather higher than this. It is capped by two huge masses, 14 or 15 feet high, of a dark grey slightly scoriaceous augite-andesite with a cryptocrystalline groundmass, which apparently form the uppermost portion of a volcanic “neck” or pipe. According to the size of these rock-masses the “neck” would have a circumference of 80 or 90 feet. These masses are in part incrusted with agglomerate.
The adjacent island of Soni-soni, which is almost joined by the mangrove-belt to the adjoining coast, probably represents one of the numerous small vents that were once active in this region. Its single peak is 460 feet in height. As there did not seem much prospect of finding rocks exposed on its upper part, its slopes being densely covered with tall reeds, my examination was confined to the lower portion during a walk around the island. On its east and north sides occur rocks of much the same character as those exposed in the neighbouring low promontory to the east of it. In addition to agglomerates and basaltic andesites occurred a rubbly pitchstone composed of fragments, up to a centimetre in size, of an opaque brown glass displaying a few phenocrysts of plagioclase and pyroxene, the interstices being filled with crushed fragments of the phenocrysts and finer glass debris. This rock is allied to the “crush-tuffs” described on page [334]. It may be added that the basic tuffs are more frequent on the west and south sides of the island.
The low island of Na Vatu in the midst of the Soni-soni inlet is about 250 feet across and only 3 or 4 feet above the ordinary high-tide level. In 1898, when I visited it, this tiny island possessed about 20 houses and a population of 60 or 70 persons, and I gather from Hazlewood’s account of these islands that Na Vatu was crowded with houses more than half a century ago. It was apparently in the first place a sand-key, and is protected against the wash of the waves by a low sea-wall formed of large blocks of stone.
An interesting exposure of bedded tuffs and clays is displayed at Ravi-ravi on the west side of the peninsula. A broad shore-flat has been formed by the marine erosion of a line of coast composed of these deposits. The strike is well exhibited, the dip being about 30 degrees N. by W. Here there are alternating beds, a few inches thick, of coarse and fine tufaceous sandstones, sometimes calcareous, with marls or calcareous clays. The mineral fragments of the coarser rocks are composed of plagioclase, augite and rhombic pyroxene, the last being abundant and giving a more acid character to these deposits. The calcareous fragments appear to be principally shell debris. The marl is in part composed of much finer detritus of the same minerals. The other materials of these deposits are derived from the degradation of basic andesitic rocks, and include also a little palagonite. To the westward of Ravi-ravi these beds show signs of disturbance, being steeply tilted to the N.W. Agglomerates also occur in the disturbed area.
The history of the Kumbulau peninsula is evidently the history of the eruptive phases of a number of more or less submerged small vents and of the periods of great marine erosion that followed during the emergence of this part of the island. The absence or rarity of dykes is remarkable; but most of the hills would represent volcanic “necks” whether of massive rock, tuff, or agglomerate.
The District Between the Kumbulau Peninsula and the Yanawai River.—Between Nandi Inlet and the village of Rewa the sea-border is low and often swampy, whilst occasional spurs descend from the inland range into the swamps without reaching the coast. Pebbles of “soapstone” (foraminiferous mud-rock) occur in streams and are no doubt derived from the incrusting deposits of the neighbouring hill slopes. In one stream-bed in the swamps is exposed in situ a remarkable chocolate-coloured rock that looks like a greasy pitchstone or a palagonite-rock. It is however of detrital origin, and is composed in mass of minute fragments of a basic, sometimes vacuolar, glass in great part converted into palagonite; whilst there are a number of broken crystals of olivine and plagioclase. Through the palagonitic alteration the fragmental character is somewhat obscured, zeolites being extensively developed in the interstices. A little lime occurs and there is a suspicion of foraminifera. The deposit belongs to the group of palagonite marls described on page [335]. The deeper rocks of the district are represented in a spur by an altered augite-andesite, originally hemicrystalline and containing much granular epidote.
Proceeding northward from the village of Rewa, one crosses another spur descending from the inland range. It is formed in mass of a dark doleritic olivine-basalt (spec. grav. 2·91) characterised by the length of the felspar-lathes (·28 mm.), possessing a little interstitial glass, and referred to genus 25 of the olivine class. It probably represents an ancient flow. Its surface is incrusted, as high as the road ascends, nearly 200 feet above the sea, by fine and coarse palagonite-tuffs; whilst the pebbles of foraminiferous mud-rock in the stream indicate the existence of incrusting marine deposits further up the slopes. The road then leads down into a low-lying undulating district that forms the sea border as far as the mouth of the Yanawai, and reaches about two miles inland without exceeding an elevation of 100 feet, although low hills occur here and there. This region is fronted by mangrove swamps and is traversed by the Matasawalevu and Ndranimako streams. It is a district of basic tuffs and foraminiferous clays, which, as shown below, extend up the slopes of the basaltic Wainunu table-land that lies behind. The soil in all the low country between Rewa and the Yanawai is red, heavy, wet, and clayey; and affords a contrast to the dry friable soil of the Kumbulau and Kiombo region to the southward.
The Navakavura plain lying north of Rewa deserves especial mention. It is a low, swampy district which a mile inland is raised only 20 or 30 feet above the sea, and is mostly occupied by casuarina and pandanus trees. Red argillaceous rocks, representing more or less decomposed palagonite coarse and fine tuffs, are exposed in the banks of the streams. Some of them were originally made up of fragments of basic glass which after being palagonitised became much disintegrated. A typical specimen by my side has a soapy feel and looks like a lump of red clay. Microscopical examination shows that it is composed in mass of palagonite, but in an extreme stage of the alteration process.
After traversing the Navakavura plain, one crosses a low hill rather over 100 feet above the sea before descending to Ndranimako. On the hill are exposed reddish clay-rocks, much weathered, but showing vegetable remains and a few univalve and bivalve shells. Extensive submarine deposits occur in the inland district west of Ndranimako. They are the usual foraminiferous clay-rocks or “soapstones,” and in places they contain pteropod shells. They are well displayed in river-banks, and in the hill-slopes on either side; but they are probably of no great thickness since in one locality named Na Savu, nearly two miles west of Ndranimako, the underlying basaltic rock is exposed in the bed of a gully, the sides being of “soapstone.” These deposits were formed in comparatively deep water.[[48]] The greatest elevation at which they were observed was about 100 feet; but this was as high as I reached in the ascent of the river. According to the natives, who are very observant in such matters, these submarine deposits extend up the slopes of the adjacent Wainunu plateau. On page [86] reference is made to their occurrence on the slopes of this basaltic table-land, 1½ or 2 miles farther north.
In the district between the Ndranimako and the Yanawai rivers basic tuffs and “soapstone” prevail. In this locality, and especially in the vicinity of Ndranimako, siliceous concretions 2 to 3 inches across, occur in places on the surface. Their nature is described in [Chapter XXV].
From the foregoing remarks it may be inferred that the sea-border between the Kumbulau Peninsula and the Yanawai River is formed of submarine deposits overlying basic rocks which probably represent ancient flows. Some of the deposits are largely formed of glassy erupted materials, which have been converted into palagonite. Others again are more characteristic sedimentary formations accumulated in relatively deep water.