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.