Note on the changes produced through the hydration of palagonite.
Most of that which is detailed below is not according to my views palagonitisation, but the effect of hydration in the disintegration of this material. The initial molecular condition and the other characters which represent potentially the palagonitic change are not connected with hydration; but are concerned with the causes before explained that led to the formation of a basic glass of such an unstable constitution. Indeed, there is good reason to believe that the changes to be now described may be observed under the ordinary influences of weathering in a wet region.
The early stages of alteration are well displayed in some of the tuffs formed mainly of basic vacuolar glass, the submarine character of which is often indicated by a few tests of foraminifera. Whilst the glass retains its original bottle-green colour, it loses the clean sharp conchoidal edges and displays rough and uneven or granular borders. With a high power the surface of the fragment is seen to be minutely pitted or pock-marked in places, the shallow circular pits, less than ·01 mm. in diameter, being sometimes arranged in a row like a number of overlapping rain-prints. This process proceeds until all the surface is affected, and from this cause there is often an appearance of polygonal markings. The pock-marking, however, continues; and as the pits encroach more and more on each other an irregularly wrinkled rough surface results. Up to this time the glass has retained much of its original colour; but its clearness is replaced by turbidity, and collections of very minute rounded, rod-shaped, and irregular granules, composed of a colourless feebly polarising material, are displayed here and there in its substance, whilst some of the previously empty vacuoles are now filled with water.
In the next stage the hydration of the iron-oxides begins, and the glass becomes opaque and yellowish or reddish-brown, and has a more granular appearance, polarising feebly. Cracks now traverse the substance, and penetrate into the vacuoles, which, as they become filled with the alteration products, whether palagonitic, zeolitic, or siliceous, become ruptured and curiously distorted. The hydration and consequent disintegration continue until the deep stain of the iron-oxide is removed, and a semi-pulverulent whitish material remains. This is the history of the little bleached powdery patches so common in basic tuffs, each representing originally a lapillus of basic pumice. This powder when examined with the microscope is shown to be made up of fine granular and tubercular materials which lose much of their distinctness when mounted in Canada balsam. It is not affected by boiling in HCl, and contains usually an abundance of minute siliceous oval amygdules that have been freed in the last stage of the disintegration of the palagonite.
Such is the story of the degradation of the palagonite daily in operation in the basic tuffs of this island. From this source is doubtless derived much of the finest constituents of the submarine clays so common over Vanua Levu.
Supplementary note on the occurrence of palagonite in the glassy matrix of pitchstone-agglomerates and in rubbly pitchstones.—In my last revision of the proofs I find that I have not laid sufficient stress on the production of palagonite under these conditions. The evidence of crushing is often very evident, and especial references to this point will be found in the index under “Pitchstone,” and on page [334] under “Crush-tuffs.”
CHAPTER XXV
SILICIFIED CORALS AND FLINTS
Silicified corals, together with siliceous minerals (quartz, chalcedony, jasper, &c.) and siliceous concretions are evidently widely distributed in these islands. Kleinschmidt in his journal refers to large blocks of flint on the island of Ono, from which the natives used to obtain their musket-flints,[[133]] and he collected from this island as well as from Viti Levu, Ovalau, &c., numerous specimens of these and other siliceous minerals and rocks, such as hornstone, chalcedony and jasper, which were examined by Wichmann and described in his paper.[[134]] Mr. Andrews observed silicified corals on the summits and higher slopes of Vanua Mbalavu.[[135]] The Fijian name for flints, “ngiwa” (thunderbolt) or “vatu-ngiwa” (stone-thunderbolt), affords a good instance of that curious superstition connected with the origin of these stones, which came also under my notice in the Solomon Islands,[[136]] and in fact is widely spread.
In Vanua Levu these siliceous rocks and minerals are in places abundant. They are especially frequent on the surface of the extensive low plains on the north side of the island which constitute the basins of the Sarawanga, Ndreketi, Wailevu, and Lambasa rivers; but it is in the low-lying district of Kalikoso, in the north-eastern part, that they exist in the greatest quantity. They do not occur usually at greater elevations than 300 feet, and are found as a rule at much lower levels.
It must be understood that reference is not here made to quartz-veins, such as are found in certain localities and of which mention is made on pages [106], [116]. It is not with the ordinary products of contact or general metamorphism that we have here to deal; but with the remarkable surface-collections of silicified corals, nodules and flints of chalcedony, fragments of white quartz-rock, bits of jasper, and certain curious siliceous concretions, that occur often in association with fragments of limonite in these low-lying regions. All the siliceous materials above named have, as the microscope indicates, a common character, chalcedonic silica in a greater or less degree being the basis of all of them, whether coral, flint, white quartz-rock, or jasper. It soon became apparent whilst examining these districts that one general condition prevailed whilst this extensive deposition of silica and the formation of the beds of limonite were in progress. It cannot, however, be pretended that these processes are actually in operation on the plains now. Except in the case of the limonite in a few localities the processes have been suspended; but they were in active operation not long ago: and an examination of the general characters of the districts will probably disclose some of the conditions under which these products have been formed.
On the surface of the Kalikoso plains, where these materials are most abundant, we find silicified corals associated with fragments and nodules of chalcedony, flints, white quartz-rock, limonite, concretions of carbonate of iron, &c., in the low-lying and often swampy district around the fresh-water lake, the whole region being only elevated between 20 and 60 feet above the sea. This is an area of decomposing acid rocks (quartz-porphyries, trachytes).[[137]] On the other hand in most of the regions where these materials occur on the surface we have areas of basic rocks (basalts and basaltic andesites) incrusted in places with submarine tuffs and foraminiferous clays, the volcanic rocks undergoing extensive disintegration. Such for instance are the Lekutu, Sarawanga, Ndreketi, and Lambasa plains. In the Lambasa plains, which are described in this connection on page [139], we find besides the corals and flints and nodules of chalcedony, fragments of jasper. In the Sarawanga and Lekutu lowlands, we find silicified corals and limonite; but here the crystallised silica of the corals contains a large quantity of water, whilst in its lesser degree of hardness and in its low specific gravity it comes near to semi-opal. In these and other localities, as in the level country around Ndranimako on the right side of the Yanawai estuary, we find curious concretions of the same kind of hydrous silica more or less crystalline. These concretions are described below.
It may be remarked that nearly all the districts in which the silicified corals and concretions, siliceous minerals, and limonite occur, are scantily vegetated “talasinga” lands[[138]] with reddish soil. Except in the instance of the Kalikoso plains, the swamps and lakes have as a rule long since disappeared, their sites being alone indicated by the limonite on the surface. In the Mbua plains, however, there are occasional small ponds and swamps, and there is no doubt that the limonite so bountifully represented on the dry districts is still in process of formation.
Before drawing some general inferences as to the conditions under which this deposition of silica and iron took place, I will refer to the characters of the materials thus produced.
The silicified corals include massive corals of the Astraean and “Porites” kinds and branching specimens of the Madrepore type or habit. The former are rarely larger than 7 or 8 inches across and are merely fragments. The latter are always portions of branches, never exceeding 3 or 4 inches in length. In the last case it is sometimes possible to show, as in the case of a specimen found on the Kalikoso plains, that before silicification occurred the dead fragment of branching coral had been extensively eroded by solvent agencies and had been penetrated by burrowing molluscs. The larger blocks of massive corals have usually been extensively chipped by the natives in obtaining flints. In past times they were carried from one place to another, the result being that occasionally they were brought to me in the mountain-villages, all showing evidence of their having supplied flints to a past generation.
These corals are as a rule completely silicified. When a massive specimen is broken across it is not infrequently found that whilst the coral structure is preserved in its outer part, the inner portion is composed of a compact seemingly structureless mass of bluish-white or pale-grey flint, which has the characteristic microscopical appearance of chalcedony and a specific gravity of 2·59.[[139]] It is from the more compact parts of the silicified massive corals that the “worked” flints found on the surface were obtained, though in some of them, as in the case of a “scraper” in my collection, the traces of coral structure are still apparent to the eye. Wichmann observed in the case of the silicified corals from Fiji that the whole petrifying process appears to consist in the saturation of the coral with silica, the coral structure being usually distinct, whilst the septa, often still calcitic, show the points of the calcite crystals projecting into the chalcedony which forms the mass. Lime however rarely occurs in the silicified corals of Vanua Levu. It was only in the case of one or two localities that the corals displayed any effervescence with an acid. In the microscope slide the massive specimens appear to be entirely of chalcedonic silica, the outlines of the cells and of the septa being indicated by ferruginous material. In a specimen of Porites by my side the crystallization of the silica has advanced beyond the chalcedonic stage and the coral is composed entirely of minute quartz-crystals, ·2 to ·4 mm. in size, often irregular, but sometimes forming doubly-terminated prisms. This has produced a somewhat crumbling rock, which is easily powdered by the finger; and in this case, therefore, the complete crystallization of the silica is resulting in the disintegration of the silicified coral.
The ordinary silicified massive corals of Vanua Levu, where the replacement by chalcedonic silica is complete, though the structure is preserved, have a hardness of about 6 and a specific gravity of 2·54, and yield but little water in the closed tube. Occasionally, however, as in the Sarawanga plains and in the Lekutu lowlands we find silicified fragments of branching corals which are easily scratched with a knife and have a hardness of 3 to 4 and a specific gravity of 2·3. The fractured surface is milk-white or reddish, and looks like semi-opal. When powdered and heated in a closed tube, the material loses one fourth or one fifth of its weight of water, the finest dust (passing away in the steam) being deposited on the sides of the glass. In the slide there is displayed a finely granular crypto-crystalline structure with in places a somewhat coarser quartz-mosaic, whilst chalcedonic quartz fills minute cracks in the mass. No coral structure is preserved. Numerous points coloured by iron oxide occur in the section, and minute dust-like inclusions abound, which are doubtless water-pores. I have described on a later page certain concretions found associated with these silicified corals which though formed of the same crypto-crystalline hydrous silica, are apparently silicified portions of nullipore-rocks.
The fragments of flint that occur commonly on the surface in these districts are, as above remarked, derived from the hard silicified coral-masses. Nodules of chalcedony, having all the appearance of having originated in cavities, are also very frequent. They may take the mamillary, agate, or onyx form, some of the agates when polished making beautiful specimens. These nodules are of all sizes up to 3 or 4 inches across. Some of them are hollow and lined with clear quartz-crystals, whilst with others the cavity may be completely filled by interlocking quartz-crystals. The outer surface of one of the agates displays markings showing in relief casts of the “cups” of a minute-celled coral.
Mingled with the other siliceous materials on the surface of the Kalikoso and Lambasa plains are found fragments of a whitish quartz-rock, having a specific gravity of 2·53-2·57, being therefore markedly lighter than quartzite (2·63-2·67) which it somewhat resembles. It usually occurs as small hand-specimens; but in the vicinity of Mbati-ni-kama I found blocks, 12 to 15 inches across, lying in the river-bed. Under the microscope it displays a fine radio-globular aggregate of chalcedonic quartz.
Mention has already been made of the siliceous concretions, composed mainly of hydrous crypto-crystalline silica, which are associated with the silicified coral fragments formed of the same kind of silica on the surface of the plains of Mbua, Lekutu, and Sarawanga. They also occur in the Ndranimako lowlands on the right side of the Yanawai estuary, and in the more elevated inland districts of the Wainunu and Na Savu table-lands at elevations of 650 to 770 feet above the sea. They take the form of irregular nodules, or of flat uneven “cakes,” usually two or three inches in size. They are as a rule reddish, but sometimes pink and white. Their hardness is only 3 to 4, and they are easily scratched with a knife; and when powdered and heated in a closed tube, they lose about one fourth of their weight of water. Under the microscope they exhibit a grey crypto-crystalline groundmass showing very finely granular crystalline silica with the cracks and small cavities filled with more brightly polarising chalcedonic quartz. But they differ as regards their other components and also in their mode of occurrence; and it is highly probable that the history of their origin is not always the same.
Those associated with the silicified corals on the Sarawanga and Lekutu lowlands show no structure in the slide that gives me a clue as to their origin; but they may perhaps represent old Nullipore nodules. Those around Ndranimako are coloured deep red; and whilst some give no indication as to their source, others are transitional in character, and display in the sections traces of the vacuolar semi-vitreous basic rock of which the original fragment was composed. The same red siliceous concretions form the pebbles and gravel in the stream-beds on the surface of the Na Savu table-land, 700 feet above the sea. These red flint-like nodules of Ndranimako and Na Savu somewhat resemble the jasper of the island; but they are sharply distinguished by their microscopic characters, by being easily scratched with a knife, and by the large amount of water which they contain. Rolled stones, which were found in the shallow stream-courses on the surface of the Wainunu table-land 750 to 800 feet above the sea, exhibit in the sections, in spite of the general silicification of the groundmass, the outlines of the original phenocrysts of felspar, and abundant skeletal magnetite rods, such as would characterise a semi-vitreous basic rock. It is evident that in the basaltic districts of the Na Savu and Wainunu table-lands these concretions have been formed under certain conditions by the decomposition of the silicates of basic rocks. But these conditions do not exist now; and I infer that the silicified rocks, which occur only in fragments on the surface, represent the silicification that occurred during the emergence of the land ages since.
Occasionally one comes upon in the mountain districts, as in the vicinity of Ndrawa, large solitary blocks 2 to 4 feet across of a whitish chert-like rock which has a hardness of 5 or 6, the harder variety having a specific gravity of about 2·58 and the softer, which yields a fair amount of water, a specific gravity of about 2·46. I noticed such solitary masses also on the Mbua plains. The first-named locality is dacitic and the last basaltic. They exhibit in the slides a patchy appearance, showing in some places finely granular crypto-crystalline silica and in others a coarser mosaic of chalcedonic quartz. Apart from the absence of any definite coral structure, I can only surmise that they were originally masses of reef-limestone. Their elevation even in the mountainous districts was not over 400 or 500 feet above the sea.
Fragments of jasper, which are associated with nodules of chalcedony and silicified corals in the Lambasa plains, are also to be found as pebbles and small blocks in the mountain streams of the Ndrawa, Ndrandramea, and Lea districts, together with bits of chalcedony and quartz-crystals. They do not occur, or are of rare occurrence, in the recently emerged Kalikoso district and probably belong to an earlier stage in the history of the island’s emergence from the sea. They have a hardness of 6 to 7, not being scratched by a knife, and a specific gravity of 2·65 to 2·70; whilst but little water is given off in the closed tube. They are a variety of chalcedony, rendered opaque by the large quantity of red oxide of iron that it contains, and are really, therefore, iron-flints. The microscopical section in one case displays in the clear spaces a beautiful globular aggregate, each globule having a nucleus of the iron oxide and giving a black cross in polarised light. In another case the globular structure is less perfect, and the chalcedonic groundmass is penetrated by a multitude of fine cracks filled with iron oxide.
The deposits of limonite vary in character in different localities, and evidently they have not all the same history. The soil of the low-lying plains around Wai-ni-koro and Kalikoso, and especially in the vicinity of the fresh-water lake, is often coloured a deep ochreous red. Small fragments of an earthy yellowish-brown limonite occur on the surface in quantity and are particularly abundant near the lake. They yield much water when heated. In some places in this district, as in the country traversed between Wai-ni-koro and Kalikoso, the surface is strewn with a number of small round concretions of the size of small marbles (6 to 12 mm.) which are composed of a mixture of carbonate of iron and limonite, but show no recognisable structures. They are somewhat friable and give off much water when heated, whilst they effervesce freely in hot hydrochloric acid. It is probable that some of the earthy limonite of the Kalikoso district contained originally iron carbonate and has been produced from concretions such as I have just described.
The variety of limonite found in fragments on the surface of the plains of Mbua, Lekutu, and Sarawanga, at elevations usually of 100 or 200 feet above the sea, is a heavy compact kind with a specific gravity of 3 to 3·5, and closely resembling red hematite. Since, however, it is lighter in weight and still contains a little water, it may be regarded as in the transition stage. It occurs as portions of cake-like masses varying usually from a third of an inch to rather over an inch in thickness. As a rule it is found in localities where no lakes or swamps now exist and may be associated, as in the Sarawanga and Lekutu plains, with silicified corals and siliceous concretions; but in some cases, as in that of the Mbua plains, ponds and swamps are still scantily represented in the vicinity, and the water of the stagnant streams is deeply coloured with iron (see page [56]).
Ironstone gravel occurs in great quantity strewn over the surface of the basaltic table-lands, especially in the case of that between the Wainunu and the Yanawai rivers. The smaller gravel varies usually between one eighth and one third of an inch in size, the larger fragments being about an inch. The specific gravity is 3·1 to 3·2. The material forming the finer gravel dissolves with but little effervescence and scanty residue in hot hydrochloric acid; it gives off water and is evidently impure limonite. The larger fragments, 1 to 2 inches in size, represent the partial conversion into limonite of a basic volcanic rock with much glass in the groundmass which formed probably the surface of the basaltic flows of the plateaux. There must be an enormous amount of this iron-stone in the island. The finer gravel has a concretionary character, some of the pieces appearing like bits of stick that have been converted into limonite. It seems to have been formed during the disintegration of the rock on the moist surface of these densely wooded basaltic plateaux; the process was not accomplished in ponds or swamps, but was carried out on ordinary damp ground.
It must be observed in the above connection that the soil in the areas of basalt and basaltic andesites, which occupy a large portion of the surface of the island, contains a large amount of fine magnetic iron-sand. After heavy rains the foot paths glisten with this fine material which has been washed out on the ground. This is especially the case in the extensive scantily vegetated “talasinga” regions where the basaltic rocks are disintegrating for a considerable depth. The river-sand of these areas, after a little washing, yields about 75 per cent. of magnetic iron grains which give in some cases a slight titanium reaction. The amount of magnetic iron-sand in these rivers, as for instance in the Yanawai and the Wainunu, must be very great. In the beds of the small sluggish streams on the surface of the Wainunu table-land the amount is also very large.
Any explanation of the origin of the extensive silicification evidenced by the occurrence of silicified corals and siliceous concretions on the surface in various parts of the lower regions of the island will have to include that of the formation of the limonite fragments so often accompanying them. The necessary conditions would, I think, be afforded by an emerging land-surface during the consolidation of the exposed calcareous muds and the subsequent draining of the new surface. On parts of the newly formed land, there would follow the successive stages of sea-water, brackish, and fresh-water swamps, such as are clearly indicated by the abundance of silicified coral fragments that strew the surface of the low-lying and often swampy districts around the fresh water lake of Kalikoso.
In such a locality as that of Kalikoso, there were no doubt at the time of the emergence large tracts covered with chalky calcareous mud derived from reef-debris; and it was during the consolidation of this mud in the recently reclaimed area that the fragments of coral imbedded in it became silicified. In these cases where the imbedded corals were already much decayed, it is probable that the empty cavities thus produced were filled with silica, and that in this manner the nodules of chalcedony were produced. Here and there a pebble or a larger block of a volcanic rock would have been inclosed in the mud; and in this case also silica largely replaced the original material of the stone. I imagine that with the evaporation of the water in the mud during the drying and consolidating processes the proportion of silica in solution would attain a degree of super-saturation and that the silicification would hence be brought about.
With the consolidation of the mud the deposition of silica ceased; and in the case of any coral fragments, where the transformation was not completed, decay would often commence. In the instance of some bits of coral found imbedded in foraminiferous mud-rock in the Lambasa plains the process of the change had been suspended, and the fragments were in a state of decay, and coloured red by iron oxide. If silicification occurred in a submarine deposit only after it became a portion of an old land-surface we ought not to find incompletely silicified corals inclosed in it. For these reasons I do not consider that silicification would occur in the case of submarine deposits long after they have been raised above the sea.
On the other hand it would seem that the deposition of silica in the hard parts of dead organisms does not proceed in the shallow-water calcareous mud of coral reef coasts previous to emergence. Silicified corals have never as far as I know been found under such conditions. Nor could the coral fragments now lying on the Kalikoso plains, often only elevated some 20 or 30 feet above the sea, have undergone this change whilst exposed on the land-surface as they now lie. They must have been inclosed in some material containing abundant free silica; and it is reasonable to suppose that this material was the chalky mud of the reef-flats on which they once lived. If this is admitted, then it follows that since, as above assumed, silicification does not occur in such a mud either before upheaval or long after it has been raised above the sea, it must take place in the intermediate period, or in other words whilst the recently exposed submarine deposits are consolidating and drying.
Several objections at once occur with reference to this explanation of the silicification of corals in this island; but much more investigation is needed to establish any view on the subject. In the Kalikoso plains, however, we have a critical locality for the pursuit of this inquiry. Concretions of carbonate of iron and deposits of earthy limonite are here associated with silicified corals on the surface of a level and often swampy district around a freshwater lake in a region which is only elevated 20 to 60 feet above the sea. We are dealing here with an area of land that has emerged in comparatively recent times as far as the history of the island is concerned. The element of time is limited, and the problem is not complicated, as it would be in the case of an old land-surface, raised some hundreds of feet above the sea, by the intrusion of many other disturbing agencies. Nature has simplified matters here for the inquirer.
The evidence of recent emergence with regard to the whole island is discussed in [Chapter II.], and need not be again referred to here; whilst the general description of the Kalikoso district is given in [Chapter XVI]. In this connection it may be remarked that before their emergence the Kalikoso plains were covered by the waters of a large irregular sea-water lagoon or lake, which though more or less surrounded by hills had free communication with the sea on the north along the line of the passages now occupied by the Wai-ni-koro and Langa-langa rivers. Both massive and branching corals then thrived in the waters of the lagoon. There is no ground for supposing that during the emergence there was an intermediate stage characterised by brine-ponds and salt-swamps. The drainage from the slopes of the mountains to the southward would have prevented it. Whilst this change of level was in operation, brackish water collected in the deeper part of the original lagoon, forming a lake which as evidenced by the present distribution of limonite on the surface of the plains was then far larger than it is now. As the plains became exposed large flats covered with chalky mud in which dead corals were more or less imbedded were bared; and there and then as the drying and consolidation proceeded silicification took place in the manner before surmised. This deposit was of no great thickness, and has been since removed by the denuding agencies, whilst the silicified corals remain behind.
When in the Solomon Islands I was unable to find the source of the chalcedonic worked flints of such frequent occurrence in that region. In my general work on those islands (pp. [77] to [80]) reference is made to this subject. It will probably be shown that there as in Fiji most of the flints are silicified corals.
In conclusion it may be remarked that those who object to the explanation of the origin of silicified corals advanced in this chapter will be able to find support for their alternative hypothesis in many facts detailed in these pages. Vanua Levu, for instance, abounds in hot springs; and Mr. Andrews might regard this fact as giving strength to his view that the silicified corals of Vanua Mbalavu in this group owe their condition to the agency of superheated water derived from volcanic rocks, more especially since hot springs are found on the island. Such an explanation could not, I think, apply to the extensive area of the Kalikoso plains where the silicified corals are associated with limonite on the surface of a recently emerged area. If these changes had been induced by hydrothermal action, one ought to find evidence of this in those localities in Vanua Levu where the hot springs issue from foraminiferous clay deposits, as in the vicinity of Vuni-moli; but no traces of such a transformation came under my notice. Wichmann does not advance any explanation of the silicification of the corals; but he considers that the “hornstones,” which he obtained from Fiji, rocks corresponding to the chert-like rocks described by me on page [355], are the products of disintegration of the basic andesites. I have already pointed out that certain siliceous nodules have probably this origin. It is also likely that some of the jasper of Vanua Levu has been thus formed.
Note on a silicified Fern Rhizome.—This is a specimen, about three inches long, picked up by a native in a stream near Sueni in the centre of the island. It has the appearance of being a portion of the stem or rhizome of a tree-fern, and is permeated in its entirety by chalcedonic quartz, the fibro-cellular structure being still preserved. No other specimen of the kind came under my notice. The probability of the occurrence of silicified plant-remains in the pumice-tuffs of the Undu Promontory is pointed out on page [233].
CHAPTER XXVI
MAGNETIC ROCKS
The literature on the subject of the magnetism of rocks is very extensive,[[140]] and even if I was capable of doing so, any attempt to deal generally with this complicated phenomenon would be out of place here. Zirkel in his characteristically thorough fashion has reviewed the subject in his general work on petrography, but since the date of the last publication of that book, 1893-94, the literature has been much increased and the subject has from time to time been opened up in scientific periodicals, occasionally in ignorance of the labours of those that have gone before. Here, the local magnetisation of rocks is alone considered, the general question of earth magnetism not being entered into.
According to Zirkel one of the earliest known observations of this phenomenon was made by Bouguer, the French geographer, whilst he was engaged in the measurement of a degree of the meridian in the vicinity of Quito in 1742. Alexander von Humboldt, however, was one of the first to attract general attention to this subject by the announcement of his discovery in 1796 of a “great magnetic mountain” in the heart of Germany. He was then director-general of the mines in two Franconian principalities; and in order to awaken the interest of German physicists and mineralogists in this matter, he announced his discovery with an air of mystery, and did not disclose the locality for many months. He then placed his specimens in the mining-office at Bayreuth to be sold at so much by weight for the relief of poor miners. His plan succeeded, and this young savant who had yet before him his great career, had soon enlisted the interests of several of the noted scientific men in Germany, including Werner the mineralogist, Voigt the mathematician, Blumenbach the naturalist, Charpentier, and others. The amount of attention that this subject then excited can be inferred from the pages of the “Intelligenzblatt der Allgemeine Literatur-zeitung” for 1796-1797 and from the contemporary publications. It has been almost forgotten now, and the matter is indeed often approached “de novo.”
However, although by these means the data became largely increased, no generally accepted explanation resulted. Opposing views continued at various times to be advanced; and it has only in recent years come to be recognised that the magnetic polarity[[141]] of rocks in exposed situations, as in the mountain-peak or in the crested spur, often arises from atmospheric electricity independently of the inductive action of terrestrial magnetism. This is the conclusion to which the later evidence given by Zirkel is directed and was that which Oddone and Sella formed from their study of the magnetic rocks of the Central Alps. It is not, however, always necessary to suppose that the affected rocks have been struck by lightning, although Sella and Folgheraiter have shown that this is the result of such a contact. They may be found, as indicated by Mr. Harker, in mountainous localities where thunder-storms are remarkably rare, and where the peaks act, it is suggested, as natural conductors. It is easy to show, remarks the same author, that no lapse of time is required for rocks in exposed situations to become magnetised. The stones of cairns erected a few years before on the mountain-tops of the Isle of Skye become invariably highly magnetic; whilst the loose stones lying on the ground display this property to a much less degree. Nor is it requisite that the rocks affected should be basic volcanic rocks. It has long been known that granites, trachytes, &c., can possess magnetic polarity[[142]]; and the existence of this quality among acid volcanic rocks is well shown in the case of the dacites in Vanua Levu, rocks which compose some of the isolated mountain-peaks.
One finds occasional reference to the highly magnetic character of the rocks in oceanic islands of volcanic origin, but the nature of the property is not always described; and it is sometimes not possible to gather from the data given whether the magnetism affects the whole mountain mass, when it would be of the regional kind, due probably to induction, or whether it is the simple magnetic quality that almost all basic volcanic rocks possess on account of the fine magnetite disseminated through the rock, or whether there is evidence of a deposit of magnetite in the vicinity, or whether it is a mere surface phenomenon confined to the bare rocks of peaks and ridges, when such rocks, whether gabbro, granite, basalt, trachyte, or dacite, display magnetic polarity. Dana, with regard to the basaltic mountain of Tahiti, remarks that the compass was often rendered useless by the local attraction of the rocks, bearings taken being found to vary two to three points on changing the position of the instrument.[[143]] Major Haig says that the compass becomes perfectly useless anywhere in the neighbourhood of one of the mountain-masses or extinct craters in Mauritius, and attributes this effect to the magnetite in the basalt.[[144]]
On the summit of Mauna Loa in Hawaii, at the edge of the great crater and in the vicinity of the site where Commodore Wilkes carried out his pendulum observations in 1840, I found my compass-needle greatly affected by local attraction, but I neglected to inquire further into the matter. Judging from my sojourn of twenty-three days on this mountain-top, thunder-storms are of very rare occurrence there; but the electric condition of the air is at times very evident, and its physiological effects are somewhat distressing. My blanket at night crackled in my hands and emitted sparks, so that I could trace with my finger the letter A in phosphorescent hues on its surface.
That lightning is directly responsible in some instances for the magnetic polarity of rocks in mountain-peaks is also well established. It has been illustrated in an indirect fashion only last year in the disaster on the Wetterhorn. Rocks partially fused by thunderbolts and displaying polarity occur on the summit of the Riffelhorn and on one of the peaks of Monte Rosa, and fulgurites have been also obtained from Mont Blanc.... It is not always easy to explain, however, isolated cases of polaric rocks where no signs of fusion occur. Whilst descending into the Valle del Bove from the Etna Observatory, I picked up four small volcanic bombs of basic lava, of which one displayed polarity, the poles being situated at the sides of the bomb. Zirkel quotes the observation of Naumann on the summit of the volcano of Moryoshi in Japan. Here out of a number of lava-blocks lying about only one exhibited marked polarity, whilst the rest showed no signs of it.
Before dealing with the polaric rocks of Vanua Levu, I will refer to two localities in other parts of the group where magnetic rocks have been observed. During the Wilkes’ expedition in 1840,[[145]] Lieutenant Underwood observed great local attraction at Naikovu, a rock 90 feet high of volcanic formation lying off the south end of Nairai Island. He found a “deviation” of 13¼ points (149 degrees) at the top of the rock, whilst at the foot near the water the needle gave correct bearings. In the Sailing Directions for the Pacific Islands, published in 1900, the “deflection” at the summit is said to be 87 degrees. It is stated by Mr. Eakle in his paper (quoted on p. [293]) on the rocks collected by the recent Agassiz expedition that this rock is composed of an augite-andesite.... I have learned from Mr. Alex. Barrack that there are some highly magnetic rocks on the west coast of Viti Levu in the vicinity of Likuri Harbour in the Nandronga district. It is said that specimens sent down to the colonies were found to contain 95 per cent. of magnetite.
It is very probable that the results obtained by me for Vanua Levu can be generally applied to the other large islands of the group. The observations were made during my various geological journeys and deal only with certain aspects of this interesting subject.
The first feature in this connection is the frequency with which simple magnetism is displayed by the acid as well as basic volcanic rocks of this island. About 95 per cent. of the volcanic rocks collected attract both ends of the needle.[[146]] This property of volcanic rocks is well known, and is to be attributed to the magnetite in the groundmass.[[147]] On examining the character of the non-magnetic rocks it appears that almost all belong to two groups where magnetite might be expected to be scanty. The first includes the pitchstones or basic glasses, sometimes fresh, at other times more or less palagonitised. The second comprises the highly altered basic rocks, where the ferro-magnesian silicates have been replaced by viridite, calcite, and pyrites. It is not, however, to be implied that rocks of these two kinds will not sometimes attract the needle. Many do not, and those in my collection that do so act feebly.
Coming to the magnetic polarity displayed by some of these rocks, when the ordinary hand-specimen behaves like a magnet in attracting one pole of the needle and repelling the other, it is to be at first observed that a rock can become polaric without being previously magnetic. Dr. Folgheraiter has observed polarity in the case of fragments of ancient bricks and pottery; and he has described the same effect in the masonry of a house struck by lightning. In one or two of the Vanua Levu acid rocks showing polarity this can be also premised since magnetite is present in very slight degree.
Polarity is very frequent among the volcanic rocks of this island. Out of 520 specimens in my collection, which was made without any reference to this matter, 80, or 15 per cent. are polaric. Of these seven-eighths are basic and the rest are acid rocks; but this proportion is partly accounted for by the far greater prevalence of basic rocks in the island. The basic rocks showing polarity include some of the heaviest olivine-basalts with a specific weight of 3·0, as well as some of the lighter augite-andesites with specific weight of 2·7. They comprise the coarse textured dolerite as well as the vitreous pitchstone and include both scoriaceous and amygdaloidal rocks. The polaric acid rocks are mostly referable to the dacites, with a specific gravity of 2·5 to 2·6.
Humboldt remarked long ago that there is no direct relation between the degree of polarity and the specific weight. This is well brought out in the table subjoined; but it should be at once observed that there is an indirect relation. Although when we arrange the rocks in a series according to their specific weight we find no corresponding relation in the amounts of the polarity, we observe that the extent to which polarity can be developed is markedly greater in basic than in acid rocks. From this it may be inferred that the degree of intensity of the exciting cause required to give polaric powers of a certain value to an acid rock, like a dacite, would be much greater than that necessary to endow a basalt with equal powers. We should not expect to find the same amount of polarity in the bare rocky peaks of two adjacent mountains, where one was of dacite and the other of basalt; and, other things being equal, if two mountains had been exposed for ages to the same conditions, we should regard the polaric powers of the two as nature’s equivalent values for the work of atmospheric electricity, on the two rocks in question. We have two such mountains in Vanua Levu in the case of the adjacent peaks of Ngaingai (2,448 feet) and Navuningumu (1,931 feet) which are about 2¼ miles apart and possess similar bare rocky pointed summits. I take it that the polaric power of 25° of the dacite (sp. gr. 2·57) in the first case is equal to the power of 90° of the basaltic andesite (sp. gr. 2·82) in the other. In the dacitic peak of Ngaingai and in the basaltic peak of Navuningumu we can measure what work atmospheric electricity can accomplish in the course of ages in the magnetisation of rocks. The other conditions being taken as about the same, the main determining difference is to be found in the rock-characters.
In the table on the opposite page we have a series of volcanic rocks placed according to their specific weights, which range from 2·5 to 3·0, and in the second column are shown their relative polaric powers as indicated by the number of degrees the north end of the magnetic needle is repelled by the corresponding pole in the hand-specimen. For this purpose a magnetic needle 2½ inches in length (strictly speaking 6·5 centimetres) was employed, a card marked in degrees being placed beneath. The north pole of the stone was placed in contact with the north end of the needle, and after the needle had become stationary in its new position a reading was taken.
These polaric rocks came under my notice over most of the island. They are infrequent in the district between Undu Point and the Wai-ni-koro River, where, however, acid tuffs are largely exposed; and I did not find them in the Natewa Peninsula east of Lea, their absence from my collections made in the Mount Freeland range being remarkable. But it is probable that this is due to the surface conditions, since dense wood covers the slopes, and bare rocky peaks are rarely to be seen.
With regard to the influence of locality on the occurrence of polaric rocks, the results may thus be classified. About one-third are found in the exposed rocky peaks of hills and mountains. Another third are found where the rocks are bared in headlands, coast cliffs, inland-bluffs, ridge-tops, and in the open basaltic plains where trees are scanty. On the other hand, a third occur in situations, as in wooded districts where the rock exposure is scanty, when it is not easy to explain the polarity, unless it was developed in clear districts that have since become covered with forest.
Table showing the Relation between the Specific Gravity and the
Polarity of Volcanic Rocks.
| Character of rock. | Specific gravity. | Amount of polarity.[[148]] |
|---|---|---|
| Dark olivine-basalt | 3·00 | 10° |
| Grey olivine-basalt | 2·94 | 29° |
| Grey olivine-basalt | 2·92 | 7° |
| Dark olivine-basalt | 2·90 | 10° |
| Dark olivine-basalt | 2·87 | 30° |
| Basaltic andesite | 2·82 | 90° |
| Basaltic andesite | 2·77 | 5° |
| Pyroxene-andesite | 2·72 | 38° |
| Dacite | 2·61 | 17° |
| Dacite | 2·59 | 5° |
| Dacite | 2·57 | 25° |
| Dacite | 2·50 | 14° |
In no place did any evidence of the direct action of lightning come under my notice. Mr. S. Skinner who kindly looked at a few of these rocks says that he found no trace of fulgurites in them. It is probable that here as in the mountains of Skye, as described by Mr. Harker, these effects are the result of the general influence of atmospheric electricity independently of the direct agency of lightning. The frequency of polaric rocks in the highest peaks of the island is very remarkable. Generally speaking, all the bare summits of the mountains are polaric. In my experience there is no exception. All the rocks obtained from the actual summits show polarity. The variety of rocks thus affected is suggestive; and this chapter may be concluded with a brief reference to their mode of occurrence on some of the mountain-peaks.
In Mbatini, 3,437 feet in height, which is the highest mountain of Vanua Levu, the pyroxene-andesite of which the bare rocky peak is composed is somewhat weathered and has a polaric or repellent power of 28°. Specimens of rock obtained below the top show no polarity, the mountain being well wooded except at the summit. In the adjacent mountain of Koro-mbasanga,[[149]] the polaric rocks are limited to those exposed in the peak which is bared of vegetation. The rocks in question are tuff-agglomerates, the small blocks of pyroxene-andesite standing out from the tuff having a polaric force of 14° or 15°. This effect has been produced in greatest intensity in the isolated peak of Navuningumu (1,931 feet) in the Ndrandramea region. Here the bare summit is formed of a semi-vitreous, slightly vesicular, basaltic andesite with a specific gravity of 2·82 in its present condition.[[150]] This rock is powerfully polaric, and rendered the compass useless, the deviation generally to the westward varying from 20° to 50°. I place its repellent force at about 90°, hand specimens affecting the magnetic needle at a distance of 13 or 14 inches. None of the various rocks obtained from the wooded slopes below displayed polarity.
The neighbouring mountain of Ngaingai is composed entirely of dacite having a specific weight of 2·57. The highest point of the summit, 2,448 feet above the sea, is bare and rocky, and the stone here is markedly polaric, the repellent force being about 25°. Specimens from the lower wooded slopes show no polarity. Near by rises the hill of Ndrandramea, which is composed in mass of acid andesites or dacitic rocks. The summit (1,800 feet) is scantily vegetated, and here the somewhat weathered rock which has a specific weight of 2·44 (probably near 2·5 in the fresh condition) has a polaric force of 14°. Specimens of a more compact rock taken from the wooded slopes 300 feet below the summit (sp. gr. 2·58) and from 700 feet below the top (sp. gr. 2·68) showed no such effect; but a specimen taken from a mass of agglomerate in the last locality repels the needle 12°. Its specific gravity is 2·61, and no doubt the mass had been originally a portion of an exposed cliff-face.[[151]]
The summit of Mariko (2,890 feet), the Drayton Peak of the chart, is formed of a rubbly agglomerate of a compact basic andesite. Though it displays bare rock-faces, the actual peak has a soil-cap at least 18 inches deep and supports small trees and shrubs. Notwithstanding this, I found when standing on the peak that my compass was very noticeably affected, the pull being to the eastward, whilst the amount of deviation increased from 11° to 16° when changing from the sitting to the standing position. Specimens of blocks from the agglomerate forming a rock-face 10 feet below the summit possessed polaric powers of 12° and 5°. Others of the same rock exposed in a cliff-face 450 feet below had a weak repellent power of only 4°.... As in the case of Mariko, the top of Thambeyu (2,700 feet) is vegetated; and beneath the smaller trees blocks of polaric rocks lie on the surface. One of these, a pyroxene-andesite (sp. gr. 2·72), from which I obtained a specimen, has a polaric power of 38°. In another case, that of an amygdaloidal rock of the same character, the repellent power is 14°.
I might mention several other polaric peaks, but it will be sufficient to refer to one or two other localities. In the mountainous basaltic district around Solevu Bay the peaks are usually polaric. Specimens from the top of Uli-i-matua, 1,100 feet, have a repellent power of 15°. The three-peaked hill of Koro-tolu-tolu appears to be in the mass of polaric basalt from the foot to the summit, having a repellent power varying from 4° to 30°, the most active specimens being obtained from the lower slopes, which, however, are scantily covered with trees. Samples of the grey basalt from Koro-i-rea show polaric powers of 3° to 7°.
As examples of the numerous lesser hills with bare rocky polaric summits I will first take Bare-poll Hill facing Soni-soni Island. This hill is only about 150 feet above the sea, its top being formed by two large masses of a basic andesite lava with a glassy groundmass, incrusted with agglomerate, the whole representing a volcanic “neck.” A specimen of the rock masses has a repellent force of 22°. Another instance is afforded by Vatui, a hill 450 feet in height situated south of Mount Sesaleka. Its summit is capped by a naked mass of tuff-agglomerate pierced by a dyke 18 inches thick of an olivine-basalt, with a specific gravity of 2·90 and a polaric power of 10°.
A somewhat suggestive example is afforded by the hill of Na Suva-suva, 1,110 feet high, which overlooks Naindi Harbour to the east of Savu-savu Bay. It is only occupied by trees in its upper part, and a specimen of the olivine-basalt, of which the hill is composed, that was obtained from the wooded summit, shows no polarity; whilst another from the slopes, two-thirds of the way up, which had been cleared of trees, has a repellent force of 10°. The polarity of the olivine-basalt from the well-wooded slopes of Ulu-i-ndali, a range 1,100 feet in height on the east side of the Wainunu estuary, is not so easily explained; the intensity varies from 8° to 28°. Ngalau-levu, a hill 1,650 feet in height, rising behind Lea on the south coast of Natewa Bay, is polaric in its upper portion. Specimens of a hemicrystalline basic andesite, somewhat scoriaceous, which form the agglomerate of the rocky summit, have a repellent force of 18°, whilst a similar rock from the agglomerate of an exposed spur two-thirds of the way up the hill has a force of as much as 38°. A curious case of polarity is exhibited in a bare tuff overlooking the Vui-na-Savu River between Rauriko and Vitina. It is composed of a much weathered whitish trachytic rock, which in appearance affords no promise of polarity, but has the power of repelling the magnetic needle 2° to 3°.
Note on the Average Polarity (Repellent Power) of the Volcanic
Rocks of Vanua Levu.
It would appear from the table given below that the difference in the average polarity of acid and basic rocks is not very great. The average for rocks with a specific gravity below 2·7 is about 10°; and that for heavier rocks is about 14°. The difference mainly lies at the maximum end of each series, the capacity for extreme polarity being, as before remarked, markedly greater in the basic rocks.
| Specific gravity. | Character of rocks. | Number of specimens. | Polarity.[[152]] | |
|---|---|---|---|---|
| Range. | Average. | |||
| 2·50-2·59 | Dacites | 6 | 2°-25° | 11·6° |
| 2·60-2·69 | Dacites and augite-andesites | 5 | 3°-16° | 8·6° |
| 2·70-2·79 | Augite-andesites | 5 | 5°-38° | 13·2° |
| 2·80-2·89 | Basaltic andesites and olivine-basalts | 18 | 3°-90° | 15·0° |
| 2·90-3·00 | Olivine-basalts | 14 | 3°-29° | 12·7° |
It is, however, noteworthy, as indicated by the value of the average in each series that not one of them is a good series. They form curves which in each case present an extreme maximum variant which is suggestive of quite another degree of magnetising agency. This is also illustrated in the combined curve of all the results given above. The acid as well as the basic series are thus characterised, and the extreme maximum variants are in each instance afforded by the highest mountain peaks. It is probable that there is an accelerating ratio of magnetisation with increased elevation. However that may be, it appears evident from my observations on the two adjacent peaks of Ngaingai and Navuningumu that the limits of polarity acquired through atmospheric electricity without the direct action of lightning would be, as measured by the scale here employed, four times as great for a dacite (25°) as for a basaltic andesite (90°).
CHAPTER XXVII
SOME CONCLUSIONS AND THEIR BEARINGS
Vanua Levu is a composite island built up during a long period of emergence, that began probably in the later Tertiary period, by the union of a number of large and small islands of volcanic formation representing the products of submarine eruptions. It differs in this respect from Viti Levu which is much more massive in its profile and possesses a greater proportion of plutonic rocks. When, however, Viti Levu comes to be systematically examined, it is likely that traces of its composite origin will be found. The evidence seems to show that it is older than Vanua Levu; but they are both situated on the same submarine platform, the area of which is nearly equal to the combined areas of the two large islands that rise from it.
This platform, as indicated in the small plan of the group, is limited by the 100-fathom line in the charts; but since the reefs on their seaward slopes plunge down precipitously, such a line practically serves to delineate the margin of the plateau. It has been my object to show on previous pages[[153]] that this submarine platform is a basaltic plateau built up by submarine lava-flows and incrusted with coral reefs and their deposits. It has been pointed out that this platform passes gradually, as it proceeds landward, into the low-lying basaltic plains that constitute the sea-border in the western part of the island, where the breadth of the submerged plateau is greatest. The basaltic flows of the plains often display the almost vertical columns of slightly inclined flows. Their apparent termination at the sea-border, where they are in places covered over with submarine deposits, cannot, however, be accepted as their real limits. According to my view they extend several miles seaward and form the platform, as is shown in the sections on pages [62] and [107].
FIJI ISLANDS.
FROM ADMIRALTY CHART 780, CORRECTED TO 1901.
ALL DEPTHS BEYOND 100 FATHOMS ARE COLOURED BLACK.
HEIGHTS IN FEET, DEPTHS IN FATHOMS.
We have in the great basaltic mountain of Seatura, which forms the bulk of the western end of the island, a probable source of many of these basaltic flows; and the occurrence inland in the western half of Vanua Levu of elevated table-lands of basalt like that of Wainunu, which extend from the centre of the island to near the coast, afford testimony that the formation of these flows extends over a considerable period of the island’s history.
It is held by Professor Agassiz that these submarine platforms are the work of erosion into the flanks of the up-heaved islands.[[154]] In [Chapter II.] it has been pointed out that the eroding agencies are not actively in operation in our own day, and that there is good reason for the belief that the process of amalgamation by which Vanua Levu has been built up during a prolonged period of emergence, is not suspended at the present time. It is assumed that the uniformity in Nature’s methods has not been broken. If, however, we have here platforms of erosion, the coasts of Vanua Levu, as far as my interpretation goes, supply no evidence of it; and we have to imagine that a period of emergence extending over a geological age has been followed by a similarly vast period of erosion without much change in level.
Whatever agencies have been at work, the production of submarine platforms 10 to 20 miles in width must have been a stupendous operation; and we shall be obliged to inquire whether plateaux, either submarine or upheaved, occur in association with volcanic islands in other parts of the world, and under what conditions they have been formed. At least four hypotheses have been framed with regard to the submarine platforms of Fiji. There is first the original theory of subsidence of Darwin; but Vanua Levu, which presents one long story of emergence, offers nothing to support this view. There is the growth of a reef seaward on its own talus, as advanced by Murray. There is the theory of erosion of Agassiz. There is lastly my own idea of basaltic plateaux incrusted by reefs. We may therefore inquire as to the evidence afforded by Vanua Levu in favour of these views. Basaltic flows, in places covered by submarine deposits, form the low plains at its sea-border, where the platforms are broadest; and there rises a basaltic mountain of the Mauna Loa type, occupying most of the western end of the island. No one would be bold enough to place the limit of these basaltic flows at the water’s edge; and as is indicated in the sections, they probably extend for miles under the sea.
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.
The period of emergence for this island may be divided into an earlier and into a later stage, the last corresponding to the age of emergence of the Lau Islands. The earlier stage, which may be termed the “Pre-Lau” stage, is represented by the deposits of the higher slopes of Vanua Levu, that is above 1,000 or 1,200 feet. This is really the critical epoch in the history of this group, and assuming that the movement of emergence has been fairly uniform over the archipelago we cannot but be astonished at the absence of all traces of ancient reefs in the earlier stage.
We may infer from the observations of Mr. Lister[[165]] that the islands of the Tonga Group represent the Lau stage of the emergence. They are similar in height and in general geological structure to the islands of Lau, that of Eua, for instance, which has an elevation of 1,100 feet, being formed of reef-limestones overlying volcanic tuffs. Dykes penetrate the tuffs but do not enter the incrusting calcareous strata. Mr. Harker,[[166]] after examining the collections of Mr. Lister, remarks that all the rocks excepting those from Falcon Island appear to be of submarine formation. The volcanic material, he adds, seems to have been almost exclusively of fragmental character. It would be rash, it is remarked, to refer all the rocks to a Recent age, and some of them may be found to go back far into Tertiary times.
My division of the long period occupied by the emergence of the Fiji Islands into two stages, the Lau stage corresponding to elevations of less than 1,000 or 1,200 feet, and the Pre-Lau stage which includes the earlier evidence of emergence found at heights exceeding these elevations and ranging up to 2,000 or 3,000 feet, may perhaps be applicable to other regions of emergence.
As bearing on the question of the isolation and antiquity of the Pacific Islands the following approximate results for the Hawaiian, Fijian, and Tongan floras may be here quoted.[[167]] These data are liable to correction; but they are near enough to the truth to be very suggestive. Of peculiar genera of flowering plants and ferns the Hawaiian Islands possess about 40, the Fiji Group about 16, and the Tongan Islands none. Of endemic species of flowering plants there are about 80 per cent. in Hawaii, about 50 per cent. in Fiji, and 3 or 4 per cent. in Tonga. Granting that there is much to be done yet in the investigation of these floras, it would be underrating the brilliant results of the labours of Hillebrand and Seemann to characterise their work as sampling. Let us suppose, however, that the floras of Hawaii, Fiji, and Tonga have been only sampled, the data above given would be still reliable. It is quite possible to obtain a botanical equivalent corresponding to the geological estimates of the relative ages of these islands; and taking the proportion of endemic plants as our guide, the Lau stage, as represented by the Tongan Islands, would have a value of 3 or 4, the Pre-Lau stage now exhibited in the earliest stage of emergence of Vanua Levu would have a value of 50, and the Hawaiian stage older than all would have a value of 80. These results are intended as suggestive and I hope to work out this subject in the second volume. They make the problem of the relative antiquity of these islands more mysterious than it even appeared before.
With regard to the vexed question of the light thrown on the past condition of these islands by the present state of their floras and faunas, it may be at once observed that my belief in the general principle that islands have always been islands has not been shaken by the results of the examination of the geological structure of Vanua Levu. In a correspondence in Nature about fifteen years ago it was suggested by me that this is the position we ought to take with regard to the stocking with plants of the islands of the Southern Ocean, such as Kerguelen; and I take the same standpoint for the islands of the Pacific. If the distribution of a particular group of plants or animals does not seem to accord with the present arrangement of the land, it is by far the safest plan, even after exhausting all likely modes of explanation, not to invoke the intervention of geographical changes. New possibilities of inter-communication, new ways of looking at old facts, and new discoveries of an unexpected nature come monthly before us in the progress of scientific research; and I scarcely think that our knowledge of any one group of organisms is ever sufficiently precise to justify a recourse to hypothetical alterations in the present relations of land and sea.
The hypothesis of a Pacific continent,[[168]] whether it takes a trans-oceanic form, as advocated by Von Ihering, Hutton, Baur and others, or whether it is represented by an island-continent isolated in mesozoic times, as suggested by Pilsbry, receives no support from the geological characters of Vanua Levu. Nor can I accept as regards Fiji Mr. Hedley’s theory of the Melanesian Plateau. There is no evidence that the various islands of the Fiji Group were ever amalgamated and no indication of a geological nature that they were ever joined to the Solomon Group. The Fijis, as we see them, have had an independent history, and the process at work is not one of disruption but of amalgamation, lesser islands being united to larger islands during the prolonged period of emergence. Mr. Hedley, however, has some weighty data on his side more especially zoological; but even here it would be wise to suspend one’s judgment. Though the great mass of botanical evidence is as respects Fiji opposed to such connections, the distribution of Dammara may, however, be fairly claimed on their behalf.
The dilemma into which such discussions lead us is aptly stated by Dr. Pilsbry. If we do not accept the hypothesis of a Pacific continent, we have to explain the cessation of the means of transportal in later geological times, since this is implied in the isolation necessary for the development of peculiar characters in a fauna or a flora. This was the dilemma that presented itself to me in studying the origin of the Fijian plants. Assuming on geological grounds that the insular condition had been always maintained I had to explain the differentiation in the inland plants, or in other words to account for the failure of the means of transportal that once existed. Since this subject bears directly on the past condition of the Fiji Islands, I may be pardoned for referring to it here. It belongs properly to the second volume which it is proposed to devote to the dispersal and distribution of Pacific plants; but as I contest the pre-existence of a Pacific continent, it is fitting though not necessary that this difficulty should be faced at once.
If we in imagination combine in a typical island the characters of the flora presented by islands of different elevation in the Pacific we get a result of this kind in an island of the height of Hawaii, nearly 14,000 feet. The littoral plants of such an island are found all over the coasts of the tropical Pacific, and for the explanation of this fact we look mainly to the agency of the ocean-currents. The plants of the mountain summit, belonging to the temperate genera of Geranium, Rubus, Ranunculus, Vaccinium, &c., are represented at least generically on the tops of the lofty ranges of the Pacific coasts and in the interior of the continents; and we find the explanation of the wide diffusion of such plants in the agency of the migrant birds that at no distant time, if not actually in our own time, were regular visitors to these mountain regions. The plants of the marsh, of the stream, and of the pond, belong often to species that occur in similar stations over a great portion of the world, such as species of Drosera, Ruppia, Potamogeton, &c.; and here the agency of wild duck and other waterfowl may be observed in active operation.
But when in such an island we regard the intermediate region between the uplands and the coast, usually the forest-zone, we find an area of change not only for the plants but also for the birds. It is here that the new genera of plants have been developed that distinguish the floras of the Pacific groups each from the others; and here also the migrant bird, having from some cause changed its ways, has given rise to new varieties and to new species. It is with this loss of the migratory powers of the birds of the forest-zone that I connect many of the important differences between the forest-floras of the different groups of the Pacific. At one time, it would seem, birds were far more active agents in dispersing seeds and fruits over these archipelagoes than they are at present; but it is not held that this is concerned with the extermination and extinction of the birds of these islands in the present day. The change dates far back and is far-reaching in its effects. It is assumed in this argument that the alpine plant and the plant of the pond and of the sea-shore preserve their characters by reason of the means of free dispersal that they still enjoy; and it is inferred that the plant of the forest-zone has varied more because opportunities of transportal of its kind no longer are afforded. Many a line of ancient migration is now broken.
It is suggested that in the past when birds were more generalised in type they were much more migratory in habits and that limitation of range has been associated with specialisation. The plants dispersed by the birds have undergone a parallel series of changes. At first widely distributed, as in the more generalised types of birds, they became localised in proportion as the birds to which they owed their means of dispersal lost their migratory ways; and both plant and bird began to vary. There is, I am convinced, a profound connection between birds and plants, of which we now perceive only the last of a long series of changes. This subject will be dealt with at length in the volume on plant-dispersal; and it is only referred to here to illustrate my contention that we have yet much to learn before it would be safe to look to hypothetical changes of sea and land to explain difficulties in distribution.