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.