Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1
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Title: Observations of a Naturalist in the Pacific Between 1896 and 1899, Volume 1
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OBSERVATIONS OF A NATURALIST IN THE
PACIFIC BETWEEN 1896 AND 1899
[Illustration: [Image: Publisher]
[Illustration: NA RARO (2,420 feet) from the south-west, a peak of acid
andesite.]
[Illustration: NDRANDRAMEA (1,800 feet) from the south-east, a peak of
acid andesite rising about a thousand feet from its base.]
[_Frontispiece._
OBSERVATIONS OF
A NATURALIST IN
THE PACIFIC BETWEEN
1896 AND 1899
BY
H. B. GUPPY, M.B., F.R.S.E.
VOLUME I
_VANUA LEVU, FIJI_
_A description of its leading Physical and Geological characters_
London
MACMILLAN AND CO., LIMITED
NEW YORK; THE MACMILLAN COMPANY
1903
_All rights reserved_
RICHARD CLAY AND SONS, LIMITED.
BREAD STREET HILL, E.C., AND
BUNGAY, SUFFOLK.
Dedication
TO THE FIJIAN PEOPLE
PREFACE
DURING a sojourn in the Pacific, which covered a period of rather over a
year in Hawaii (1896-97), and of two years and three months in Fiji
(1897-99), my attention was mainly confined to the study of
plant-distribution and to the examination of the geological structure of
Vanua Levu.
With Hillebrand’s “Flora of Hawaii” always in my hands I roamed over the
large island of Hawaii, ascending the three principal mountains of Mauna
Kea, Mauna Loa, and Hualalai, and in the case of my second ascent of
Mauna Loa spending twenty-three days alone on its summit. Similarly in
Fiji, Seemann’s “Flora Vitiensis” was my counsellor and guide in the
matter of plants.
In Hawaii I was in a land of active sub-aerial volcanoes, and I paid my
devotions at all the altars of “Pele,” their presiding deity. In Fiji I
trod upon the surface of submarine volcanoes that emerged ages since
from the ocean and still retain their coverings of sea-deposits. Both in
Hawaii and Fiji I lived much among the people; and though my chief
interest lay in the comparison of these two types of volcanic islands, I
could not but be drawn to the kindly natives whose hospitality I so long
enjoyed.
Destiny led me to Vanua Levu in the following fashion. With the relief
party to take me down from Mauna Loa there arrived a well-known German
naturalist who, like myself, had been interested in coral-reef
investigations. We discussed this warm topic at an elevation of nearly
14,000 feet above the sea, with the thermometer at 20° F. As we sipped
our hot coffee and listened to the occasional “boom” from the bottom of
the great crater, at the edge of which we were camped, I remarked to my
friend that I was thinking of spending some months in Samoa. To this he
good-humouredly replied that I might leave Samoa to his countrymen and
describe one of the large islands of Fiji. International rivalry over
that group of islands was then rather keen. However, Dr. K. went to
Samoa, and I have now completed this volume on the geology of Vanua
Levu, Fiji.
It will not be necessary to lay stress here on the difficulties and
hardships connected with the exploration of little known tropical
regions. Many will be familiar with all that these imply, where the
rainfall ranges from 100 to 250 inches, where the forests are dense,
where tracks are few and swollen rivers are numerous, and where the
torrent’s bed presents often the only road.
The only extensive geological collections made in Fiji previous to my
visit were those of Kleinschmidt in 1876-78, which together with a small
collection previously made by Dr. Gräffe were examined by Dr. A.
Wichmann. These rocks were obtained from Viti Levu, Kandavu, Ovalau,
etc., but not from Vanua Levu. Dr. Wichmann’s paper of 1882, descriptive
of these collections, presents us with the results of one of the
earliest studies by modern methods of research of the volcanic rocks of
the Pacific Islands. It is to this investigator that we are indebted for
the establishment of the occurrence of plutonic rocks, such as granites,
gabbros, diorites, in Viti Levu.
Although, as far as I can ascertain, few, if any, rocks have been
specially described from Vanua Levu, this island was visited by Dana in
1840 when attached to the United States Exploring Expedition under
Wilkes. His observations on its geology were published in his volume on
the geology of the expedition. Although not extensive they are valuable
from their reference to his discovery of trachytic and rhyolitic rocks
as well as acid pumice-tuffs in the island. It is singular that his
observations have apparently been overlooked by all his successors.
Wichmann with this discovery unknown to him remarked on the seeming
absence of quartz-bearing recent eruptive rocks from the South Seas.
When the “Challenger” Expedition visited the group in 1875 some
geological collections were made which were described by Prof. Renard in
the second volume on the “Physics and Chemistry” of the expedition. No
collections, however, were made in Vanua Levu. In 1878 Mr. John Horne,
Director of the Botanic Gardens at Mauritius, made some important
observations on the geological structure of this island and of other
parts of the group, which he published in his account of the islands
given in “A Year in Fiji.” No collections were obtained by him; but
prominence is given to his observations by Dr. Wichmann and others. Like
Dana in the case of the acid volcanic rocks, Mr. Horne has forestalled
me in his conclusion that Vanua Levu amongst the other larger islands
has been formed mainly of the products of submarine eruptions.
The visit of Prof. A. Agassiz to Fiji in 1897-98 gave a fresh impetus to
its geological investigation. We are indebted to him not only for his
own extensive memoir on the islands and coral reefs of this group, but
also for the subsequent important explorations of Mr. E. C. Andrews and
Mr. B. Sawyer in Viti Levu and the Lau Islands. These two gentlemen have
since published a short paper on the caves of these islands. Mr. Eakle
has described the volcanic rocks collected during the visit of Prof.
Agassiz. It is, however, noteworthy that, although the collections were
made in Viti Levu, Kandavu and in many other of the smaller islands,
Vanua Levu is not represented. Mr. Eakle’s conclusion that basic
andesites and basalts are the characteristic rocks of the region, the
augite-andesites predominating, would apply to Vanua Levu in great part.
This island possesses also in fair amount hypersthene-andesites and
dacitic or felsitic andesites, which are very scantily represented in
the collections examined by Mr. Eakle. In connection with the
quartz-porphyries and trachytic rocks which also occur in Vanua Levu, it
should be observed that Mr. Andrews describes a rhyolite from Suva in
Viti Levu. Unlike Viti Levu, Vanua Levu displays but a small development
of plutonic rocks.
In conclusion it should be pointed out that much remains to be done in
the geological exploration of this island, and that I would have spent a
third year in this task much to my profit. Still I hope that a period of
two years devoted to its investigation will be regarded as some excuse
for a certain over-confidence in the expression of my opinions.
To enumerate all those from whom I received much kindness in these
islands would be a lengthy task. My indebtedness is very great to Bishop
Vidal, Father Rougier, and to various other members of the Roman
Catholic Mission, and I experienced similar favours at the hands of Mr.
Williams and other Wesleyan Missionaries in Vanua Levu. Mr. F. Spence
and Mrs. Spence showed me great kindness, and from Dr. Corney I received
valuable assistance on my arrival in the group. To the planters my debt
is equally great, more especially to Mr. Barratt, Mr. Dods, and Mr.
Mills.
In conclusion I would suggest the foundation of a “Fijian Society” for
the investigation of the islands, for the gathering together of all that
has been written about the group and its people, and for the advancement
of science.
HENRY BROUGHAM GUPPY.
_June, 1903._
_Note._—A type set of my geological collections representing the
massive rocks from this island has been kindly accepted by the
Curator of the Geological Museum, Jermyn Street.
LIST OF SOME OF THE PRINCIPAL AUTHORITIES QUOTED IN THIS BOOK
DANA, J. D., on the Geology of Fiji in vol. x, Geology, United States
Exploring Expedition Reports, Philadelphia, 1849.
KLEINSCHMIDT, T., “Reisen auf den Viti-Inseln,” Journal des Museum
Godeffroy, heft 14, Hamburg, 1879.
HORNE, J., “A Year in Fiji,” London, 1881.
WICHMANN, A., “Ein Beitrag zur Petrographie des Viti-Archipels,
Mineralogische und Petrographische, Mittheilungen,” band v, heft 1,
Wien, 1882.
RENARD, A., on andesites from Kandavu, “Report on the Petrology of
Oceanic Islands,” vol. ii of “Physics and Chemistry,” Challenger
Expedition, 1889.
AGASSIZ, A., “The Islands and Coral Reefs of Fiji,” Bulletin, Museum of
Comparative Zoology, Harvard College, vol. xxxiii, 1899, Cambridge,
Mass.
EAKLE, A. S., “Petrographical Notes on some rocks from the Fiji
Islands,” Proceedings, American Academy of Arts and Sciences, vol.
xxxiv, no. 21, May, 1899.
ANDREWS, E. C., Notes on the limestones and general geology of the Fiji
Islands, with special reference to the Lau Group. Based upon surveys
made for Alexander Agassiz. With a Preface by T. W. Edgeworth David.
Bulletin, Museum of Comparative Zoology, Harvard College; vol.
xxxviii, Cambridge, Mass. 1900.
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 }
} _Frontis-
Ndrandramea (1,800 feet) from the south-east, a peak of } piece_
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
[Illustration:
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.
[Illustration: From NAITHOMBOTHOMBO Point to UNDU Point, representing
the length of the island (98 miles).]
[Illustration: 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 } July " 122° "
mouthof the Ndavutu creek, self-registering }
Six thermometer used }
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 (SO_{3}) ·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| Parts per |Grains per|
| | residue. |million of water.| 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 |Chlo- | | |Calcium|Natrium|Total | |
| | col- |rine. |Sodium.|Calcium.|chlor- |chlor- |salts.|Density.|
| |lected.| | | | ide. | ide. | | |
|----------------+-------+------+-------+--------+-------+-------+------+--------+
|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 |
+------------------------+------+-------+--------+-------+-------+------+--------+
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 | Height |Near or |Siliceous |Nature |Tempera-
| inland. | above |far from | sinter. |of the |ture.
| |sea-level.|streams. | |surface |
| | | | |at the |
| | | | |exit. |
---------+----------+----------+----------+----------+---------+--------
1. Wai- |Coast to |Sea-level |River |Little or |Soil-cap,|100°-
nunu |four |to 20 or |-side and | none |tuffs, |130° F.
|miles |30 feet |under | | &c. |
|inland | |water | | |
| | | | | |
2. Nato |Three to |50 to 100 |Near |Little or |Soil-cap |110°-
-arau and|four |feet |brooks | none | |126° F.
vicinity|miles | |and | | |
|from coast| |streams | | |
| | | | | |
3. Nukum-|Six miles |130 feet |About 100 |Mixed with|Agglomer-|157° F.
bolo |inland | |yards | lime |ate-tuff |
| | |from | carbonate| |
| | |river | | |
| | | | | |
4. Savu |Beach and |Sea-level |Inland |In fair |Rotten |208°-
-savu |150 yards |to 10 feet|springs | quantity|volcanic |210° F.
|inland | |near a | |agglomer-|
| | |brook | |ate |
| | | | | |
5. Ravuka|Nine |200 feet |River |None |Pebble |148° F.
|miles | |-side | |bed |
|inland | | | | |
| | | | | |
6. Vuina-|Ten miles |150 feet |River |None |Soil-cap |131°-
sanga |from | |-side | | |134° F.
|north | | | | |
|coast | | | | |
| | | | | |
7. Foot |Four |Not known |Not known |Not known |Not known|Not
of Nawavi|miles | | | | |known.
Range |inland | | | | |
| | | | | |
8. Vatu- |Coast |_Nil._ | | | |140° F.
loaloa |below | | | | |
|high-tide | | | | |
|mark | | | | |
| | | | | |
9. Nambu-|Half-mile |Probably |Not known |Not known |Swampy |140° F.
onu |inland |slight | | | |(proba-
| | | | | |bly).
| | | | | |
10. Tam- |Inland |90 feet |Not near |Abundant |Soil-cap |180° F.
bia |one and | |a stream | | |
|a-half to | | | | |
|two miles| | | | |
| | | | | |
11. Vand-|Eight |270 feet |River |None |Old |100° F.
rani |miles | |-side | |river-bed|
|from | | | | |
|coast | | | | |
| | | | | |
12. Na |Inland |90 feet |River |Probably |Agglomer-|194°-
Kama, |five to | |-side | little |ates and |204° F.
Wailevu |six | | | |tuffs |
|miles | | | | |
| | | | | |
13. Vuni-|Inland |120 feet |About 100 |None |Foramini-|{140° F.
moli |eight | |yards | |ferous |{155° F.
|miles | |from | |clay-rock|
| | |river | | |
| | | | | |
14. Mbati|Inland |130 feet |River |In fair |Volcanic |161° F.
-ni-kama |seven and | |-side | quantity|agglomer-|
|a-half | | | |ate |
|miles | | | | |
| | | | | |
15. Nan- |Inland |180 feet |River |None |Clay |97° F.
dongo |four | |-side | | |
|miles | | | | |
| | | | | |
16. Nat- |Coast and |_Nil_ | |None |Reef-flat|{131° F.
uvo |between |and a few | | |and |{136° F.
|the tide- |feet | | |swampy |
|marks | | | |ground |
| | | | | |
17. Nda- |Between |_Nil_ | |None |Reef-flat|Not
ku-ndaku |the tide- | | | | |known.
|marks | | | | |
| | | | | |
18. Nava-|A mile |30 to 40 |Swampy |None |Soil-cap |133° F.
karavi |inland |feet |ground | | |
| | | | | |
19. Vuni-|From |A few feet|Near a |Not known |Soil-cap |Not
sawana |beach 300 | |brook | | |known.
|or 400 | | | | |
|yards | | | | |
| | | | | |
20. Ndr- |Coast |_Nil_ | |None |Old reef-|130°-
eke-ni |between | | | |patch |135° F.
-wai |the tide- | | | | |
|marks | | | | |
| | | | | |
21. Wai- |Inland |25 or 30 |Not near |None |Rises |148° F.
katakata |400 yards |feet |a stream | |beneath a|
| | | | |boulder |
| | | | |of basalt|
| | | | | |
22. Ndevo|On coast |_Nil_ |Near a |Not known |Probably |Not
|below | |stream | |the |known.
|high- | | | |reef-flat|
|water | | | | |
|level | | | | |
| | | | | |
23. Nav- |Inland |100 feet |Near a |Little or |Volcanic |112°-
uni |three- | |stream | none |agglomer-|113° F.
|quarters | | | |ate |
|of a | | | | |
|mile | | | | |
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.
[Illustration: 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.
[Illustration: 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
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