368 Transactions. 



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 ser- 

 pentine, 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 colourless, and may be a variety of hornblende. Except for this 

 type, no dyke rock was found in place. 



Volcanic Rocks. 

 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 

 Riviere 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 out- 

 crop 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 micro- 

 lites arranged in the form of delicate feathery growths. This is certainly 

 one of the older lava-flows of the island volcano. 



Haaynophyre. 



The rock of this class previously described from A rue (Rep. Aust. Ass. 

 Adv. Sci., vol. 13, 1912, p. 196) could not be found in situ. The road- 



