Art. XXXIX.—The Geology of Tahiti.
[Read before the Otago Institute, 1st December, 1914.]
All those islands of the Central Pacific which have been examined geologically are either formed of coral rock or they are of volcanic origin. No sediments other than those that have the nature of coral detritus or of volcanic tuffs are known, with the possible exception of the coal-seams that are said to occur at the Island of Rapa.
In a few of the islands the occurrence of plutonic rocks has been recorded. Such records are, however, at the least, doubtful at Borabora, Maupiti, and at the Marquesas. At Sunday Island, in the Kermadec Group, there are certainly blocks of granite of considerable size and of some number embedded in a volcanic breccia.
In the Island of Tahiti, however, a series of plutonic rocks has been definitely proved to exist. The first record of these appears to be contained in Cuzent's work on Tahiti in 1860. Afterwards, in the year 1898, specimens were found in old collections previously stored in “l'ancien musée colonial,” in Paris. These, like Cuzent's specimens, had been collected in the valley of the Papenoo, in which the Tuoru River flows.
Professor Lacroix at once realized the interest attached to these rocks, and in 1901 he induced M. Seurat to search for them. This distinguished biologist was at that time sent on a zoological mission to Tahiti in connection with the pearl-shell industry. The success of M. Seurat's geological work is described in the following words: “M. Seurat à procédé à l'exploration hérisée de difficultés de toute la vallée de Papenoo et particulièrement de sa partie haute. Il est parvenu ainsi à trouver le gisement en place de la roche en question.”*
With the aid of M. le Capitaine Courtet, who had at a previous time made a survey of the valley, a good map was drawn, and on it the geological information obtained by M. Seurat was inserted. This map was of the greatest value, as it was in all respects more accurate and more detailed than the official map used in Tahiti. So far as the lower course of the river was concerned, it appeared to be absolutely correct. It was not until we reached the upper part of the valley that any discrepancies were found between the map and the actual courses of the streams.
Professor Lacroix had hoped that the work of M. Seurat would have enabled him to decide the vexed point as to whether the plutonic rocks were the remnant of an ancient eroded land-mass, or whether they were masses intrusive into the basaltic series of which the island is almost entirely composed, in the same way as the gabbros are intrusive into the lavas of the Hebrides. In this important respect the results were inconclusive, for he says, “Malheureusement, je ne suis que poser ce problème. M. Seurat m'a dit n'avoir pas vu de blocs de couleur clair dans les tufs basaltiques, mais les researches précises seraient nécessaires pour élucider ce problème.”†
Matters resting in this rather unsatisfactory state, advantage was taken of a visit to Tahiti in August, 1913, made with the aid of a grant from the
[Footnote] * Lacroix, “Les roches alcalines de Tahiti,” Bull. Soc. Géol. de France, 4° série, t. x, 1910, p. 92.
[Footnote] † Lacroix, loc. cit., p. 97.
Australasian Association for the Advancement of Science, for research on the alkaline rocks of Australasia.
Letters written previously to the Governor, M. Léon Géraud, received replies which assured me of every assistance in the project of ascending the Papenoo Valley, which was said to be “hérisée de difficultes” and “il n'existe pas de sentier bien tracé et on doit se frayer soiměme son chemin.”
On my arrival in the colony M. Géraud kindly gave me a letter to the chief of the Village Papenoo, situated at the mouth of the River Tuoru, and he promptly furnished us with the necessary guides It was extremely fortunate that one of these, Teaeo by name, had accompanied M. Seurat when he made his geological collections in the valley. This enabled me to find his localities with the utmost certainty and with the least loss of time.
The entrance to the Papenoo Valley is about 300 metres wide, and its sides rise precipitously to a height of 100 metres at first, but they ascend gradually towards the interior of the island. Its floor is covered with gravels, in which boulders of plutonic rocks of some variety are quite common. These vary from pure white types to dark theralites, many of which contain conspicuous crystals of hornblende and augite. Mixed with these there is a great variety of basalts, and a few of tinguaite and monchiquite.
The river maintains a wide floor as far as Tiamii (see map of valley), about 9 kilometres from its mouth. Here the Tamauu tributary branches off from it. This stream drains the east and north of Orofena, the highest peak of the island. In its gravels there were no boulders of plutonic rock. Above this point the Papenoo Valley narrows rapidly, and in many places flows over the native rock.
Close to Tiamii an area of plutonic rock is indicated by Seurat. The guide Teaeo showed me a large boulder in the forest, which he said was the outcrop discovered by Seurat. It was clearly a transported boulder, and I could find no outcrop of plutonic rock near it, though basalt occurred in situ 100 metres farther up the stream.
The same was found to be true at the mouth of the Navenave and Pihoi, other localities where plutonic rocks had been reported in situ by Seurat. In both of these places large boulders of plutonic rocks were to be seen in number, but no rock other than basaltic breccia could be found in place. From a point a little beyond the mouth of the Pihoi our track led some distance above the bed of the Tuoru across the small stream Teti. Here again the only plutonic material that was found consisted of large boulders evidently water-borne. The bed of the Tuoru was not seen near this place, so I am not able to say whether the plutonic occurrence recorded by Seurat at this place is correct, though, as far as appearances went, it appeared unlikely that it was so.
Shortly above the junction with the Teti the Tuoru forks into the Maroto and the Tahinu. My guide (Teaeo) asserted that Seurat went no farther than the junction with the Teti. Lacroix, however, states that Seurat went for some distance along the bed of the Maroto, and found that the plutonic boulders soon disappeared, and that the upper part of the valley is constituted entirely of basalt. There must be some mistake here, for at the junction of the Tahinu and Maroto there is nothing but volcanic rock in situ, while a little farther up the Maroto plutonic rock is to be seen forming the bed of the stream, and it continues to form its banks for some distance.
It thus appears that in all but one of the localities where Seurat is stated to have found plutonic rock in situ I was unable to find any, and this one I was not able to visit.
In the bed of the Tahinu, about a mile above the point where the river joins the Maroto, there is an outcrop of white syenite which extends for about 100 yards along the bed of the stream. This rock is seamed most irregularly by thick and thin dykes of a dark rock of a diabasic nature. Two hundred metres above the syenite outcrop the Tahinu again branches, and the right-hand branch retains the name Tahinu. In the bed of this stream no boulders of plutonic rock could be seen; all the boulders had a basaltic appearance. In the bed of the left-hand branch, the Terefaatautau, plutonic boulders of various natures were quite frequent. This branch was followed for a short distance, but no rock was seen in situ. The proportion of boulders of plutonic rock slightly decreased, and the stream in this part of its course was relatively open, and not enclosed in a gorge.
The Maroto branch was not followed for some distance from its junction with the Tahinu, but it was found that about half a kilometre above that point it cuts through a plutonic rock (gabbro). A short distance farther on a small tributary which joins it on the right contained no boulders of basalt, but a great variety of large boulders of plutonic rock. Another half a kilometre farther on the stream showed a large exposure of peridotite. A little distance farther on there was close to its right bank an occurrence of a highly porphyritic theralite containing large crystals of hornblende. Above this point the stream contained as many boulders of basalt as of plutonic rock. Nearly a kilometre farther on, and at a short distance from the right bank, there was a large outflow of spring-water of a strongly ferruginous character. The water was highly charged with a gas which had no smell and would not support combustion; it was probably carbon dioxide.
The rock near the spring was much decomposed; it contained large crystals, and appeared to be a theralite. This spring was said by my guide to be Vai Apaaoa, which in Lacroix' map is placed on the left bank of the Maroto. All the ground near this spring is saturated with spring-water, and the soil is everywhere red with iron oxide. The water of a neighbouring stream is as great in volume, and contains ferruginous matter to as great an extent, as Vai Apaaoa, and it is evident that there is close at hand another spring as large as the one that we saw. The water of these springs, which is quite cold, tinges the whole of the water of the Maroto a ferruginous tint. The water of the Tahinu is tinged in a similar manner near the outcrop of syenite, and spring-water is everywhere escaping in some quantity near the main stream. This evolution of spring-water may account for the fact that the syenite contains a considerable quantity of pyrite. Above its junction with the Terefaatautau the water of the Tahinu was quite clear, and in the Terefaatautau there was very little discoloration due to spring-water. It appears, then, that the evolution of spring-water occurs only in the area of plutonic rock, and, so far as observed, the escape of the spring-water was greatest near the margin of the plutonic rock.
On the left side of the Maroto, opposite the spring Vai Apaaoa, no plutonic rock was seen on the side of the hill which forms the watershed between the Tuoru and the Tamanu.
From the summit of this watershed at Tetiairi Pass a good view was obtained of the whole of the central basin of the island. The general form of this basin was now disclosed, and was seen to fully justify the expression of Meinicke: “Liegt er an der Westseite eines grossen runden Bergkranzes der das Thal des oberen Papenoo flusses umschliesst.”*
[Footnote] * Meinicke, “Inseln des stillen Oceans,” Zweiter teil, p. 164. Leipzig, 1875.
From the point where the Navenave joins the Tuoru the whole of the basin of the Upper Papenoo, about 5 kilometres in diameter, is enclosed by a rampart of astounding mountain-peaks, which, however, do not actually include Orofena (2,230 metres). The highest peaks of this rampart certainly nowhere rise to a greater height than the 1,800 metres of Tetufera (Plate VII, fig. 1), and the 1,476 metres of Tamaiti, yet their acclivities are most abrupt. Even at the locality where syenite outcrops in the Tuoru bed the elevation is only 320 metres above sea-level, and for some distance toward the mountain the slope is but slight. Along this nearly circular wall the summits rise in remarkably sharp aiguille peaks (Plate VII, fig. 2), and in two places the wall is pierced by relatively low passes, one of which—Urufaa—is only 884 metres above sea-level, and leads to the lake Vaihiria, on the south-eastern slopes of the island.
The central part of this impressive amphitheatre is occupied by a low hill—Ahititera—about 850 metres high (Plate VII, fig. 1). Its surface is rounded and smooth compared with the steep precipices and aiguille peaks of the surrounding mountains. This hill—Ahititera—is drained on its two sides by the Terefaatautau and the Maroto respectively. Its summit consists of a rocky mass, which, unfortunately, was not seen before our viewpoint was reached. Our food-supply and other considerations did not allow of time for visiting this peak, from which as a central point the surrounding landscape must be wonderful. Specimens, too, were not obtained from it. The guide Teaeo, whom I found most accurate in all of his topographical statements, assured me that it was formed of a roche grenue, and his information was strongly supported by the appearance and weathering forms of the rock.
The topography of the country strongly suggested the opinion that this relatively low conical hill was composed of plutonic rock, as was actually the case so far as that part of it which I had visited was concerned. The essentially different nature of the surrounding hills suggested also that they were formed of a different kind of rock-series. This opinion is supported by the fact that in the Upper Tahinu, judging by the nature of the boulders in its bed, nothing but basalt outcrops. In the Upper Navenave and Pihoi there is said by M. Seurat to be no indication of the outcrop of plutonic rock. In the main stream I found no plutonic material in situ until the white syenite was reached on the east side of the hill Ahititera, and in the gravels of the Terefaatautau and Maroto boulders of plutonic rock became less numerous as the stream-beds were ascended.
It thus appears that, so far as my observations go, the whole area of plutonic rock is practically confined between the beds of the Tahinu, Terefaatautau, and Maroto Streams—that is to say, the area of the hill Ahititera.
The plutonic rocks of the Papenoo Valley, of Tahiti, have already been shown by Lacroix to be of special and somewhat peculiar interest, for they include alkaline rocks which here, as elsewhere, display a great variety. Lacroix compares them with those of Madagascar, from which island he has described a similar series. All but one of Lacroix' types contain a feldspathoid mineral, principally nepheline; and they all lie between nepheline syenite and essexitic gabbro. He classes them as follows:* Syénites néphéliniques, monzonites néphéliniques, gabbros néphéliniques, gabbros essexitiques.
[Footnote] * Lacroix, loc. cit., p. 121.
None of these types were, however, found in situ, though the large boulders that were found suggested that this was the case. It was my good fortune to extend this series at both ends by additional specimens of plutonic rock actually found in situ. On the one hand, in the bed of the Tahinu, as mentioned before, there was a syenite wanting in nepheline, and of a more acid character than any of the rocks described by Lacroix. On the other hand, there was a peridotite in the bed of the Maroto which in a sense corresponds among the plutonic types to the picrite mentioned by Lacroix as occurring in the volcanic series. The plutonic rocks of the interior of Tahiti thus form, a very complete series from acid to peridotite types, through a great variety of alkaline varieties.
Hand-specimens of this rock are nearly quite white, though they show a few plates of biotite, and contain a great many very minute crystals of pyrite.
In section the rock is found to consist almost solely of an alkaline feldspar in allotriomorphic crystals. The albite twinning is very general, but irregular, and the mineral is almost certainly anorthoclase or a fine perthite. It is considerably decomposed, and in some cases the product of decomposition is distinctly muscovite. There is a little biotite in rather large crystals, with intense pleochroism, from pale-straw colour to nearly black. Small crystals of pyrite are scattered through the rock rather plentifully. Sphene, which is so abundant in the nepheline syenites and monzonites, and to some extent in the theralites, is in this rock almost entirely wanting. (For analysis, see page 371.)
This was found in a small gorge where an ill-defined pig-hunting track of the Natives crossed the Maroto, apparently about half a mile above the point where the Maroto joins the Tahinu. In hand-specimens the rock is moderately coarse-grained, the feldspar predominant over the augite, but a quantity of magnetite is to be seen.
In section the feldspar is found to be bytownite. It is in small grains, and its crystal outline is seldom, complete. All the feldspar is fresh and well twinned. The augite has a very slight violet tint, and is not noticeably pleochroic. Ilmenite is abundant in the section. It occurs in sharp crystals, bordered by a thin reaction rim. Frequently there is a small amount of biotite associated with the ilmenite, but this mineral is not to be seen elsewhere in the section. No sphene was seen.
A black rock, in hand-specimens showing a small amount of serpentine, and clearly granular in structure. In section, augite is found to be the most abundant mineral. It does not show the distinct violet colour which characterizes this mineral in the great majority of plutonic rocks of this island. The crystals are of moderate size, and show few regular crystallographic planes. Olivine is fairly abundant. It is much fractured, and the crevices are filled with magnetite. The olivine does not show crystal boundaries. Associated with it there are patches of colourless crypto-crystalline matter, in which a few grains of iron-ore can be seen. This crypto-crystalline matter is very fine-grained, and has a low refractive index, with very high birefringence. Where it borders the augite it has sharp clear-cut
boundaries, indicating, of course, that it was the last mineral to crystallize. A little dark-brown biotite is associated with the olivine. There, is no iron-ore apart from that associated with the olivine. (For analysis, see page 371.)
It will at once be evident that the discovery of these types considerably extends the series previously described by Lacroix, who has already commented on the close relationship between the various members. An extension of the series in the basic direction was clearly foreseen by him, for he recognized that an ultra-basic plutonic would probably exist to represent the picrite which he had distinguished among the basalts. This picrite. however, contains some feldspar, and it is thus less completely ultra-basic in a mineralogical sense than the peridotite just described.
In the acid direction the series of Lacroix is still further extended, for the most acid type described by him—the syénite néphélinique à biotite—contains only 52.25 per cent. of silica, whereas the syenite now found in situ in the bed of the Tahinu contains 61.06 per cent. of silica, notwithstanding the considerable amount of pyrite in the rock. This syenite contains a slight excess of potash over soda—a feature that was noticed by Lacroix also in his syénite néphélinique à biotite, though in all the other rocks of the series there is a considerable excess of soda over potash.
It is thus apparent that in this small area there is a plutonic series extending from a syenite with potash in excess of soda, and consisting almost entirely of alkaline feldspar, through various types of monzonites, essexites, theralites, and gabbros to a peridotite with absolutely no feldspar.
Professor Lacroix, in 1910, mentioned a specimen of granite which had been collected at Tautira, on the north side of the Peninsula of Taiarabu. Père Alain, of L'Ecole des Frères, at Tahiti, kindly gave me a specimen of a similar granite which had been given to him as coming from Tautira. As Lacroix said, “S'il provient du sol de l'île il y aurait Ià un fait d'une grande importance.” A visit was accordingly made to Tautira, and a specimen of the granite was shown to various Natives, who all agreed that they had never seen such a rock anywhere in the district. The chief, Ori a Ori, in particular, was emphatic in the statement that he had not seen a rock of this nature in the mountains, or anywhere else in the country. A diligent search was afterwards made in the river-gravels and on the sea-beach for boulders or pebbles of granite, but entirely without success. It seems that this granite, if it ever really came from Tautira, had been previously brought there as ballast or for some other purpose by some visiting vessel.
Though no granite pebbles were found at Tautira, pebbles of plutonic rock were not infrequent in the gravels of the Tautira River. These were found to be specimens of a highly feldspathic olivine gabbro similar in many respects to the specimens from the Papenoo Valley.
Specimens of tinguaites, monchiquites, and camptonites have already been described from the material brought from the Papenoo Valley by M. Seurat. I found that boulders of tinguaite were not common in the material of the river-bed, and I was unable to find any in place. One of my guides—Mauri—however, told me that the roches vertes were to be found in some abundance in the gravels of the upper part of the Teti Valley, but I had no time to attempt to verify his statements. Boulders of monchiquite were quite common among the boulders of the Papenoo Valley.
As stated by Lacroix, there is every variation between the rocks classed by him as monchiquites and camptonites; that is, between hornblende lamprophyres which have a basic feldspar in the finer constituents of the groundmass and those that have analcite. I did not find any dykes of monchiquite in situ.
Numerous dykes were found seaming the syenite. These, however, are of a diabasic type. The rocks contain an abundance of augite, always in small crystals with idiomorphic boundaries, and occasionally forming small group of five or six radiating individuals. Feldspar is not abundant. It has the moderate extinction-angle of andesine. There is a little serpentine, obviously derived from olivine. Some epidote occurs in somewhat irregular patches. There is much magnetite. A little brown hornblende is found where residual fluid matter has oozed into pores of the rock. A mineral is present in the form of small irregularly lath-shaped crystals with a high refractive index and high birefringence. It has the small extinction-angle of 6° and a pronounced longitudinal cleavage. It is almost colou less, and may be a variety of hornblende. Except for this type, no dyke rock was found in place.
The greater number of volcanic rocks that were found in Tahiti were basalts; in fact, this type appears to be the only one which occurs in the vicinity of Papeete, though, of course, its structure, and to some extent its composition, varies somewhat widely. An extremely coarse type with large idiomorphic crystals of augite is found in the bed of the small stream Rivière de la Reine, about 1 kilometre west of Papeete. Another type with large grains of olivine always surrounded with a red oxidized border is quarried for road-metal on the main road 3 kilometres west of the town. This rock contains small crystals of augite only in the groundmass, and very little labradorite feldspar, though iron-ores are plentiful. In the valley of the Papenoo boulders of basalt are more common than those of any other kind of rock. They are often vesicular, and in the vesicles there is a considerable variety of small crystals of zeolite.
One of the largest outcrops of basaltic rock that was seen occurred in the valley of the Papenoo, about 5 kilometres from its entrance. This outcrop extended from the Vai Rutu to Maoma. It forms cliffs rising to a height of over 100 metres above the floor of the valley, and shows a distinct columnar structure. In section the rock is found to consist of crystals of olivine, mostly idiomorphic, and of moderate size, embedded in a plexus of small brown crystals of augite. There are small grains of magnetite and a little residual dark-brown glass crowded with feathery crystals of ilmenite. In hand-specimens the rock is quite black, and the only mineral which can be distinguished is olivine. This rock is quite a typical limburgite, distinguished from the picrite of Lacroix by the entire absence of feldspar.
At the entrance to the Vaihi tributary of the Tuoru there is a zeolitic basalt in situ. It contains no olivine. The augite is of a pale-brown colour, in long slender crystals. The feldspar is in the form of very slender microlites arranged in the form of delicate feathery growths. This is certainly one of the older lava-flows of the island volcano.
The rock of this class previously described from Arue (Rep. Aust. Ass. Adv. Sci, vol. 13, 1912, p. 196) could not be found in situ. The road-cuttings
near at hand, where the road round the island passes over a projecting spur, show nothing but basalt lavas, and it becomes evident that the small stream in the bed of which the pebbles of hauynophyre are found have been transported from a considerable distance. This is at present the only lava rock of the island in which hornblende has been found.
Another rather different hauynophyre was, however, found in situ. This rock forms a high cliff at Tapahi, 20 kilometres east of Papeete. This rock contains a few grains of olivine, which have a thin border of iron oxide, and are entirely surrounded by augite. Pale-brown phenocrysts of augite occur, but they are not plentiful. The crystals of hauyne are of moderate size. They are dark blue in colour, but they weather readily to a pale-yellow tint. The hauyne contains a great abundance of inclusions of a black colour, which in many cases make the mineral quite opaque. The groundmass contains much augite in elongated microlites. Feldspar forms irregular microlites of small size: they are apparently anorthoclase. There is a little nepheline, quite allotriomorphic, and a great deal of magnetite.
A slip about 200 metres farther down the Tahinu than the outcrop of syenite discloses a highly weathered light-green rock, from which, however, a sound specimen was obtained. The greater part consists of small irregular crystals of feldspar (andesine) with much pale augite also in small crystals, and contains a number of inclusions of magnetite. There are some patches of irregular grains of brown hornblende. There are a few grains of olivine, which are much dusted with magnetite throughout, and are occasionally changed into a highly birefringent crypto-crystalline substance that could not be identified.
Another lava outcropping about 50 metres down-stream from the syenite outcrop proved to be a dolerite. It contains large crystals of pale-brown augite with a dark border. Labradorite feldspar is plentiful. Magnetite and ilmenite are abundant, and there is much olivine dusted with magnetite throughout.
These lavas have evidently been considerably affected by the high temperature due to the adjacent pipe of the volcano which was built up to a height of 2,000 metres above them after they had been ejected.
From the account given here it will be seen that the occurrence of plutonic rocks in the island of Tahiti is quite different from that indicated in the map published by Lacroix, which was based on the collections and observations of Seurat. The large exposure of these rocks over the greater part of the Upper Papenoo Valley which is shown in the map was not found by me. The actual exposure seen was situated almost entirely between the beds of the Tahinu and Maroto Streams. Here the ground has a relatively gentle slope, in marked contrast with the precipitous slopes of the hills around. The top of the conical hill—Ahititera—formed of these plutonic rocks does not rise to more than 850 metres above sea-level. The plan of the island shows that this area is almost exactly the centre of the whole island. Within this plutonic area there is found to be a great variety of rock types, actually varying from an acid syenite to a peridotite. The greater part of the area, however, judging at least from the preponderance of boulders, appears to be formed of the more alkaline types, from nepheline monzonite to theralite. This, however, has not yet been demonstrated by the actual location of the outcrop, partly because the tropical vegetation and weathered rock-matter largely obscured the outcrop, but mainly because of the lack of time available for work.
So far as the actual located outcrops are concerned, the extreme types appear to be marginal in their occurrence, while the main mass of the central hill—Ahititera—seemed to be composed of monzonitic rocks, judging by the boulders contained in the beds of the streams that radiate from the hill. It was most unfortunate that the slopes of Ahititera were not more closely examined, but as our route had led along a stream-valley in mountainous country covered with tropical forest it was not possible to get an idea of the topography of the country until the viewpoint from the pass Tetiairi had been reached, and it was then too late to return.
As stated previously, the hill Ahititera, over which the outcrop of plutonic rocks occurs, is the actual central area of the island. This hill is almost entirely surrounded by a circle of high mountain-peaks, composed apparently of lavas and breccias, and from these peaks the land slopes outward without steep average slopes in all directions. It is, however, radially furrowed by deep radiating valleys separated by the knife-like ridges so well described by Darwin and Dana. On the west side only is there any exception to this statement. There Aorai and Diaděme extend the high country somewhat to the west of the upper part of the Papenoo Valley.
When this arrangement of a small plutonic area surrounded by a circle of volcanic rocks, and of this central hill surrounded by a ring of lofty peaks, is borne in mind, only one conclusion as to the structure of the island can be arrived at. It appears obvious that the plutonic area marks the position of the much-denuded remnant of the plug filling the pipe through which the volcanic rocks were ejected. If this reasonable conclusion be accepted, it will be seen that the original volcano of which Tahiti was formed reached a height of at least 3,000 metres, for the dissected and worn remains still attain an elevation of 2,232 metres in Mount Orofena, which stands some distance back from what was probably the central orifice of the volcano. Here, then, we have the materials of a volcanic plug exposed by erosion and weathering at a depth of 2,000–2,500 metres below the summit of a volcano the ruined flanks of which still rise up in a mighty rampart to a height of 1,500 metres or more on every side.
The plutonic rocks of the central plug of the volcano are mainly of an alkaline nature, lying between monzonite and theralite. The volcanic rocks are mainly basalts, though hauynophyres and phonolites also occur. There thus appears to be something of a discordance between the plutonic and volcanic types, and this even suggests that there is no community of origin between them. This discordance is of a specially marked nature mineralogically, for whereas almost all the plutonic rocks contain a large amount of nepheline, brown hornblende, and often biotite and sphene, the volcanic rocks contain practically none of them.
This difference is, however, far more marked mineralogically than chemically, as has been well shown by Lacroix. The analyses that he has published show quite clearly that the chemical differences between his gabbro néphélinique (theralite) and gobbro essexitique on the one hand, and of some of the basalts on the other, is quite slight. Again, the syénite néphélinique à amphibole and monzonite néphélinique on the one hand are also chemically equivalent to the hauynophyres on the other. The discovery related here of the plutonic rocks syenite and peridotite now provide equivalents for the phonolite and picrite of Lacroix among the volcanic rocks.
It thus appears that from a chemical standpoint there is a satisfactory equivalence between the plutonic and the volcanic rocks of this island.
To some extent at least the great mineralogical difference that has been mentioned can be explained. It appears that hornblende and mica do not crystallize in an igneous magma unless some mineralizer is present. Thus Doelter says, “Was die hornblende anbelangt, so wird vielfach die Notwendigkeit von Wasser und hoheren Druck angenommen. Keinenfalls ist ihre Bildung ohne Mitwirkung von Mineralisatoren moglich, ebensowenig wie bei Glimmer … Bezuglich der Hornblende hat Becke der Einfluss des Druckes auf ihre Bildung studiert und scheint jedenfalls wenigstens eine geringe Druckzunahme ihre Entstehung zu fordern.”*
At the time of an eruption of magma the water, and probably any other mineralizer, would escape, and there would then be no chance of hornblende forming. A new molecular arrangement within the magma would be the result. This might well end in the crystallization of the minerals of a normal basalt. It is noticeable that, while theralites and nephelinitoid monzonites are apparently the commonest types of plutonic rocks, basalts are by far the commonest types of volcanic rocks.
Lacroix has already shown that a clear relationship of a chemical nature exists between his monzonite néphélinique à biotite and the hauynophyre. It can be clearly seen from the accompanying table that the syenite mentioned in these pages is a chemical representative of the phonolite described by Lacroix as coming from Vairao. The peridotite also obviously belongs to the same chemical class as the picrite he has described. The table of analyses given here is taken from his paper, but the analyses made by me of the syenite and peridotite are added to it.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|A.||Syenite with pyrite and a little biotite.||Analysis, P. Marshall.|
|B.||Syénite néphélinique à biotite.||Analysis, M. Pisani.|
|C.||Syénite néphélinique à amphibole.|
|G.||Peridotite (wehrlite).||Analysis, P. Marshall.|
B, C, D, E, and F quoted from Lacroix, “Roches alcalines de Tahiti,” Bull. Soc. Géol. de France, 4e serie, t. x, 1910, p. 121.
[Footnote] * C. Doelter, “Petrogenesis, Braunschweig,” 1906, p. 146.
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A. Tinguaite. B. Camptonite. C. Microgabbro.
All analyses by H. Pisani, Lacroix (loc. cit.).
A. Phonolite. B, C, D, E. Hauynophyre. F, G. Basalt. H. Picrite. I. Limburgite.
A, B, C, D, F, G, H, analyses, H. Pisani, Lacroix (loc. cit.); E, analysis, P. Marshall (Rep. Aust. Ass. Adv. Sci., vol. 13, 1912, p. 197); I, analysis, P. Marshall.
Daly has lately, as is well known, suggested that the alkaline igneous rocks are due to the solution of masses or limestone in a magma of sub-alkaline composition. There appears to be no evidence in support of this theory so far as our knowledge of the alkaline rocks of the South Pacific islands allows us to form a judgment at the present time. Though volcanic tuffs and breccias are of common occurrence in the Pacific islands, and though they have been examined carefully in several places, no fragments of limestone or sedimentary rock have yet been discovered in them, nor has any material of this kind been found in the lava itself. It is true that Daly, in his summary of the field associations of alkaline rocks in different parts
of the world, inserts a query as to the possibility of carbonate rocks being cut by the eruptive rocks of Tahiti.*
All round this island the ocean-floor quickly falls to a depth of about 2,500 fathoms, and its surface is covered with volcanic mud, or with this material mixed with globerigina ooze or with red clay.† There appears to be no reason to suppose that before volcanic activity commenced on the site of Tahiti the ocean-floor at that particular spot was in any way different from its nature in the closely adjacent area. Over this area the ocean-floor is at such a depth that the calcium-carbonate remains of organisms have already been partly dissolved and the percentage of silica has become considerable.
The question also arises as to whether any instances are known where basic magmas have invaded limestone and have acquired in consequence a local alkaline character on the borders of the magma. So far as New Zealand is concerned, we do not at present know of any district in this country where basic magmas have invaded limestone rocks. There is, however, a striking instance of a large acid batholite in contact with a limestone deeply buried at the time of the intrusion of the magma. This example has been described by Webb, Bell, and Clarke.‡ Here the great intrusive mass of granite of the Pikikiruna Boss is in contact with a crystalline limestone of Ordovician age. Basic rocks are the result of the contact, and patches of basic rock due apparently to absorption and solution of limestone are found at some distance within the granite. There appears to be no evidence, notwithstanding the obvious evolution of quantities of carbonic-acid gas, of any great disturbance of the magma, or of any influence of the absorbed limestone, on the whole mass of the granite, but only on localized portions of it. No development of alkaline types is noticed. Similarly, in the district of alkaline volcanic rocks at Dunedin, New Zealand, where a highly arenaceous limestone of Cainozoic age underlies the volcanic rocks, no evidence has so far been found that any solution of this material has taken place.
In the actual field occurrence of the plutonic rocks in the Papenoo Valley, Tahiti, the occurrence of acidic types and ultra-basic types within a short distance of one another appears to preclude the idea of absorption of calcareous matter. There is every reason to believe that the outcrop is at a level of at least 10,000 ft. to 15,000 ft. above that at which absorption has to be supposed. Upward movement through this distance would cause the complete mixing of the absorbed matter. Yet here we find a complete separation of the magma into rocks of which one at least cannot be regarded as a desilicated type.
The rock-specimens found on the beach at Tautira and in the gravels of the Tautira River included gabbros and phonolites. The Tautira River is the largest in the Taiarabu Peninsula, and, judging by the map, it appears to have a large circular basin in the centre of the peninsula. There can be no doubt that this peninsula was an independent volcano, and the Tautira River appears to have the same relation to it as the Papenoo has to the main island. In other words, the Tautira appears to have drained the original crater, and now that denudation is far advanced its basin includes
[Footnote] * Daly, “The Origin of Alkaline Rocks,” Bull. Geol. Soc. Amer., vol. 21, 1910, pp. 87–118.
[Footnote] † Agassiz, Mem. Mus. Comp. Zool., vol. 28, plates, pl. 202.
[Footnote] ‡ N.Z. Geol. Surv. Bull. No. 3 (new series), 1907, p. 73.
the denuded plug that fills the pipe through which magmatic matter rose to the crater. The variety of plutonic rocks found here, however, was quite small, a fact that can be explained by the smaller size of the Taiarabu volcano, and therefore by the less extent to which denudation has laid bare the deeper portions of the plug. There was, unfortunately, no opportunity of making geological observations in the valley of this river.
It will at once be seen that there is in Tahiti a splendid opportunity of studying the differentiation that has taken place in the magma that filled the pipe of a volcano of large dimensions at a depth of 2,000 metres below the actual crater, and of the correlation of these differentiates with the lavas that have issued from the crater from time to time. It happens, however, that a lack of time and means have prevented anything more than a mere locating of a few of the plutonic types, and but little has yet been done in finding the extent of the different lavas. The present paper merely calls attention to the excellence of this field for study.
It may, however, be noticed that the rocks of Tahiti appear to be merely typical of those of most of the volcanic islands of the Eastern Pacific. Thus, nothing but basic and alkaline rocks have up to the present been recorded from the following islands: Samoa, Rarotonga, Aitutaki, Mangaia, Raiatea. Huaheine, Mangareva, Pitcairn, Rurutu, and the Sandwich Islands.
The association of the rock types in Tahiti appears to have a close relationship to the series of alkaline and basic rock types at the Otago Peninsula, New Zealand. Here, as previously pointed out, there is a large area of white trachyte composed of almost pure feldspar, a large development of phonolites, and a great many basalts, as well as connecting types such as trachydolerites.* Some of the trachydolerites contain large crystals of brown hornblende which is largely resorbed, and is associated with the violet augite in exactly the same manner as in the theralites and essexites of Tahiti, and in similar rocks of other localities, such as Massachusetts. The composition of these trachydolerites is intermediate between the composition of the phonolites and that of the basalts.
At the north head of Otago Harbour a trachydolerite lava nearly always lies beneath a phonolite, which is in turn followed by basalts. The author has suggested that this association can be explained by the assumption that the lavas were supplied from a reservoir at an intermediate level, to which theralitic matter party crystallized was moved from time to time. Within this reservoir resorption of the crystals that had already formed might have taken place if the mineralizers of the magma escaped. Differentiation would then take place in this magma, with the result that phonolitic and basaltic matter might be separated and emitted at different eruptions.
Inspection of the old solidified plug of Tahiti shows that much differentiation of a magma can take place at depths of no more than 2,000 metres—in fact, that it may be so complete as to produce a syenite composed almost solely of feldspar as well as a non-feldspathic peridotite.
This observation may therefore explain the frequent association of alkaline and basic rock types in the Pacific region, to which attention has previously been called, and for which a special explanation has been required. “Whether the different species of rocks in Tahiti have resulted
[Footnote] * Marshall, p., “The Geology of Dunedin,” Q.J.G.S., vol. 62, 1906, p. 388.
from magmatic differentiation is a question for the future…. The constant association of the main types in the islands visited truly supports this view.”*
Interest largely centres in these oceanic islands in connection with the structure and origin of the Pacific basin. Many attempts have been made within recent years to establish an association between coastal structure and the products of volcanic activity, notably between the coast lands of the Pacific types of Suess and the occurrence of andesitic rocks. This idea has been supported by Harker and Prior, amongst others, while Gregory and others have dissented from it. Thus Lacroix points out that the andesitic type is strictly circum-Pacific. This statement holds in the present state of our knowledge, as pointed out by Marshall, if, as seems reasonable, Tonga, Fiji, New Hebrides, Santa Cruz, Solomon, Caroline, and Marianne Islands are considered as circum-Pacific in their situation, a suggestion which is strongly supported by ocean soundings.
On the other hand, those islands which are situated in the central areas of the Pacific Ocean are, so far as known, constituted of the alkaline and basic facies which are referred to the Atlantic region. It is, however, possible that the Marquesas (biotite trachyte, Lacroix) and Easter Island (andesites, Helsch) are exceptions. In order to explain this contrast between the andesitic border and the alkaline basic central area, the suggestion of Supan that andesitic rocks are the product of eruption where rock-folding is in progress, and alkaline basic rocks where radial fractures have been formed, demands attention. There is, however, so little knowledge of the actual structure of the basement on which these island groups are situated that any stated conclusions are more likely to be guesses or the expression of the personal bias of an author than scientific deductions based on reliable premises.
Other questions arise. Is the basin of the Pacific Ocean the scar left by the moon when it separated from the earth, an infallen area, or a depression of an isostatic nature due to the high specific gravity of the material of the earth's crust in that region? The study of the petrology of these island groups ought to produce some evidence on this point confirmatory or otherwise of such pendulum experiments that have been made.
I do not know whether the first theory should or should not require any particular nature of the rocks forming the floor and the material immediately beneath the floor of the ocean-basin. If there is, any one would perhaps expect the rock to be of an ultra-basic nature. If the area is infallen, it would be reasonable to expect to find occasional fragments of sediments included in the tuffs or lava-flows, as at Mount Ruapehu, New Zealand, as well as at Auckland and Dunedin. Search has, however, failed to reveal any such fragments in the islands of the Society and Cook Groups that I have visited. It would thus appear that sedimentary rocks form a less important part of the earth's crust in the neighbourhood than in the areas of the great volcanic districts of New Zealand.
On the other hand, there is in the volcanoes of these islands a great predominance of basic types of rock. In the basic rocks feldspar is relatively unimportant, and is in almost all cases confined to the groundmass. In the phonolites, which are, however, found in much smaller quantity than
[Footnote] * Marshall, P., “Handbuch der Regionalen Geologie,” band vii, abt.2; also Rep. Aust. Ass. Sci., vol. 13, 1912, p. 201.
the basic rocks, feldspathoid minerals are often more abundant than the feldspars. Such facts as these, though possibly in the future they may require qualification or revision, suggest at least in the present state of our knowledge that the composition of sub-Pacific material is, on the whole, more basic than that of the other areas, and to that extent favours the idea of the isostatic equilibrium of the Pacific basin.
The distribution of plutonic rocks in the Papenoo Valley, in the centre of Tahiti, is somewhat different from that stated in Lacroix' paper, which was based on the field-work of Seurat.
So far as the observations of the author showed, the outcrop of these rocks is almost confined to the low hill Ahititera, which occupies the centre of the large circular basin of the Upper Papenoo Valley.
The hill Ahititera is the centre of the island, and from the crest of the mountain rampart which surrounds the Upper Papenoo the level of the land falls outward somewhat evenly on all sides to the sea-level, though the slopes are deeply seamed by the stream-valleys which radiate outwards in large numbers.
The situation of Ahititera is therefore the position of the central conduit of the original great volcano of Tahiti.
The series of volcanic plutonic rocks discovered by Lacroix is extended by the discovery of a highly feldspathic syenite as well as a peridotite in situ in addition to some of the types already described as Lacroix.
This series is regarded as the result of the differentiation of an essexitic magma.
Parallels of these plutonic types are found in the dykes and the volcanic rocks found in the island.
It is suggested that the constantly occurring association of basic and alkaline volcanic rocks in other Pacific islands, and at Dunedin, New Zealand, has resulted in a similar manner.
No evidence was found in support of Daly's theory that the alkaline are due to the solution of calcium-carbonate rock by a basic magma.
It is suggested that the frequent highly basic character of the volcanic rocks of the Central Pacific islands supports the theory that the floor of the Pacific Ocean has its low-lying position as a result of isostatic equilibrium.
Explanation of Plate VII.
Fig.1. View of upper basin of Papenoo Valley, Tahiti, from Tetiari Pass (800 m.), looking south. Rounded conical hill in middle distance, Ahititera (850 m.), formed of plutonic rock. Tetufera Peak (1,800 m.) in background, in the centre.
Fig.2. View of Orohe Peak from base of Ahititera Hill, Upper Papenoo Valley, Tahiti, looking north. Orohe Peak is formed of volcanic lava and breccia.