Marshall. — Geology of Tahiti . 369 



cuttings near at hand, where the road round the island passes over a pro- 

 jecting spur, show nothing but basalt lavas, and it becomes evident that 

 the small stream in the bed of which the pebbles of hauyn.opb.yre 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 sit". 

 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 opacpie. 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. 



'. slip about 200 metres farther down the Tahinu than the outcrop of 

 syenite discloses a highly weathered light-green rock, from which, how- 

 ever, 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 entirelv 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 mainlv because 

 of the lack of time available for work. 



