If we look for an island which in its extensive palagonite-formations, in its basaltic table-lands and later basaltic flows, in its huge mountain-ridges, and in its evidence of submergence, most resembles Vanua Levu, we seem to find it in Iceland. It is in Iceland, I think, that we must expect an explanation of many of the puzzling features in the structure of Vanua Levu.
I pass on now to refer to some of the general points in the geology of this island, which have been dealt with in detail in the earlier chapters of this work. With regard first to the distribution of the volcanic rocks, it may be remarked that my materials do not lend themselves to making a geological map. The most comprehensive idea of the principal points in the geological structure will be obtained by reading the description of the profile given in [Chapter I]. There is, however, a method in the distribution of the rocks that may be again noticed here. The plutonic rocks are very scantily exposed, as is shown on page [249]; and they are not displayed at all in the western half of the island. The more basic eruptive rocks, the olivine-basalts and basaltic andesites, are mainly confined to the western half, that is, west of Nanduri on the north and of the Ndreke-ni-wai River on the south. Ordinary augite-andesites occur also in the western half; and together with the hypersthene-augite-andesites they are found over most of the rest of the island, excluding the north-east portion, east of Lambasa and Tawaki, where quartz-porphyries, oligoclase-trachytes, and acid pumice tuffs prevail. The acid andesites, including the hornblende-andesites and the dacites or felsitic andesites, are best represented in the Ndrandramea district in the midst of the basic rocks. They occur in the isolated peaks of Na Raro and Vatu Kaisia and in one or two other localities, as in the Valanga Range and on the shores of Natewa Bay in the vicinity of the Salt Lake. These peaks of acid andesites, as in the instances of Vatu Kaisia and Soloa Levu, are at times in part overwhelmed or surrounded by the basaltic flows. This singular feature of bosses of acid rocks in the midst of basaltic fields offers another point of resemblance between Iceland and Vanua Levu.
The mountain-types vary considerably, the ridge-mountains, however, being most characteristic of the island. The basaltic mountain of Seatura, though its lava-flows were evidently in the main submarine, belongs as before observed to the Mauna Loa type. In its radiating valleys and gorges and in other characters it recalls the description given by Dana of the island of Tahiti. The peaks of acid andesites, represented in the isolated hills and mountains of the Ndrandramea district, and in the solitary mountains of Vatu Kaisia and Na Raro, are the necks and stumps of submarine volcanoes dating back to the pre-basaltic period of the island. It is, however, in the great mountain-ridges of the central portion of the island, those of Va Lili, Korotini, Nawavi, Thambeyu, Mbatini, Mariko, &c., that we find, as just remarked, the most typical features of the internal topography of Vanua Levu.
Agglomerates overlying palagonite-tuffs and clays, that are usually foraminiferous and sometimes inclose molluscan shells, clothe the slopes of these mountain-ridges up to elevations of 2,500 feet and over above the sea. Most of these great ridges, now more or less covered over by these submarine deposits, represent lines of submerged vents, of which only a few raised their summits above the sea in the earliest stages in the history of the island. At this early period there were no coral reefs. Some of the ridges present a marked parallel arrangement, recalling the arrangement of the mountain-ridges and lesser chains of hills as described by Dr. Johnston-Lavis in the account of his visit to Iceland.[[155]] The description of Hekla (as given by Thoroddsen) as “an oblong ridge which has been fissured in the direction of its length and bears a row of craters along the fissure,”[[156]] comes very near to my conception of the original condition of these great mountain-ridges before the emergence. Dr. Johnston-Lavis sees in Hekla a type of volcanic mountain very different from that of Vesuvius and Etna. He regards it as a ridge marked by a number of parallel ridges and furrows, and built up along a main fissure with a number of subsidiary parallel fissures.
The part taken by palagonite in the composition of the finer deposits over the greater portion of Vanua Levu is another prominent characteristic of the island. Palagonite, as I have suggested in [Chapter XXIV.], is formed probably on the surface of submarine flows of an ophitic basaltic rock.
The age of the more recent of the deposits of this island, the fossiliferous tuffs, the pteropod-ooze rocks, and the foraminiferous muds, cannot be far different from that of the same deposits in other parts of the group, since it is apparent that the same general movement of emergence has affected both of the two larger islands. Professor Martin of Leyden informed Dr. Wichmann that the fossil shells found in the tuffs of Viti Levu, Ovalau, and other islands were Tertiary but not older than the Miocene.[[157]] Dr. Dall, after examining the fossil mollusks collected by Professor Agassiz from the elevated limestones of Fiji, confirmed the impression formed by the latter as to their late Tertiary age. None of the genera were extinct, and the fossils were in his opinion younger than Eocene and either Miocene or Pliocene.[[158]] The Rev. J. E. Tenison-Woods described as extinct Tertiary fossils, some corals and mollusks from the interior of Ovalau.[[159]] Mr. H. B. Brady, basing his conclusions on the character of the foraminifera, assigned a Post-Tertiary date to the Suva “soapstone” taken at elevations up to 100 feet in that neighbourhood.[[160]] Professor David referring to some fossil teeth of Carcharodon and to a fossil Tridacna found at Walu Bay infers that the deposits are at least as old as Pliocene but not as old as the earlier Tertiaries.[[161]] Since, as pointed out by Professor David, the latest movements of emergence have taken place in recent geological time, these various observations go to show that whilst the latest exposure of deposits has occurred in recent time the mass of the fossiliferous deposits date back to the Pliocene and the Miocene periods.
According to Wichmann these islands were in a continental condition during the Palæozoic and Mesozoic periods, and it was only in the later Tertiary age that the movement of subsidence began that prepared the way for the formation of the more recent deposits. The submergence during the Tertiary period and the subsequent emergence are facts that cannot be gainsaid; but we may ask where is the evidence of the continental condition during the earlier periods. There is little in the results obtained from Vanua Levu that directly supports such an hypothesis. Under such circumstances one ought to have discovered in the deposits of this island some evidence of this early condition, and there should be found in the fauna and flora some traces of the original organisms. According to Hedley there is some indication of a continental condition in the molluscan fauna, and he quotes Fairmaire as regarding the Coleoptera as of a continental character; but no one, that I am aware of, has found any direct evidence of the Pre-Tertiary periods in this group. It is in harmony with the geological characters to assume that these islands made their first appearance during the Tertiary epoch.
Coming to the subject of the movements whether of land or sea that led to the appearance of these islands, we shall not be begging the question if we speak of their “emergence.” There is no doubt as to there having been during and since the Tertiary epoch an emergence of some thousands of feet, allowing for the original depth of the foraminiferous deposits now found at elevations of over 2,000 feet above the sea. In [Chapter II.] it is shown that there is good ground for the belief that these changes of level have not altogether ceased. Of what nature, we may ask, is this movement. We have before us the grand conception of Suess that the emergence of the land in the different phases of geological time has been produced by the general lowering of the level of the ocean arising from local subsidences of the earth’s crust. This view in the case of the recent calcareous formations of the Pacific is applied to the terraces of the Loyalty Islands;[[162]] and it follows that it is also applicable to the elevated calcareous deposits of the islands of the Western Pacific as a whole, as in the case of the Tongan Islands, the New Hebrides, the Solomon Group, &c. Such a general change of level ought to be represented in the large island of Hawaii in the North Pacific, since it could not be confined to one locality in this ocean. There is no evidence of emergence, as far as I know, presented by this island. During my sojourn there, I examined much of its coasts. Now the antiquity of the flora of this group is sufficiently attested by the circumstance that it ranks first among the oceanic groups of the Pacific for the number of endemic plants that it possesses; and the same conclusions may be drawn from the insects and the birds. There is no evidence in this group, one of the most ancient of the Pacific archipelagoes, of that great movement of emergence, which is abundantly demonstrated over the Western Pacific.
The standpoint is therefore taken that the movement of emergence which began in the Tertiary period and is probably still in operation is confined to the southern portion of the tropical Pacific. Speaking of the time of the Fijian emergence, Professor Agassiz observes that “it is not unnatural to assume that it was coincident with the elevation of Northern Queensland, and that the area of elevation included New Guinea, the islands to the east of it as far as New Caledonia, and as far east as the most distant of the Paumotus, and extended northward of that line to include the Gilbert, Ellice, Marshall, and Caroline Islands.”[[163]]
From the report of Mr. Andrews[[164]] it is evident that in the Lau Islands of the Fiji Group volcanic outbreaks have taken place since the last upheaval. He describes in the case of Mango and other islands the manner in which cliffs of limestone form inliers in flows of andesitic lava. In the history of these islands he first distinguishes the period of calcareous deposits, when the bedded limestones forming the submarine plateau were laid down. Then followed a period of volcanism during which masses of volcanic materials were erupted along the axis of elevation. Alternating epochs of upheaval and stable equilibrium ensued, during the last of which the reefs grew outwards and formed the terraces now so characteristic of the profiles of the islands. After the last upheaval the volcanic forces became again active. There is much of special interest in the account given by Mr. Andrews of the Lau Group. The blocks of limestone included in the volcanic agglomerates distinguish the Lau detrital rocks from those of Vanua Levu. There is no evidence that coral reefs existed during the early stages of the emergence of Vanua Levu to be obtained from the submarine formations found on the higher levels, 1,000 to 2,500 feet above the sea.