Marshall. — Geology of Tahiti. 365 



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 cer- 

 tainly 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 sur- 

 rounding landscape must be wonderful. Specimens, too, were not obtained 

 from it. The ^uide Teaeo, whom I found most accurate in all of his topo- 

 graphical statements, assured me that it was formed of a roche grenne, 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 bill was composed of plu tonic rock, as was 

 actuallv the case so far as that part of it which I had visited was con- 

 cerned. 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 iact 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 Ahiti- 

 tera, 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. 



Petrology. 



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 feld- 

 spathoid mineral, principally nepheline ; and they all lie between nepheline 

 syenite and essexitic gabbro. He classes them as follows :* Syenites nephe- 

 liniques, monzonites nepheliniques, gabbros nepheliniques, gabbros essexitique, . 



* Lacroix, he. cit., p. 121. 



