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.