366 Transactions. 



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. 



Syenite. 



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 feld- 

 spar 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 decom- 

 position 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 .) 



Gabbro. 



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. 



Peridotite (Wehiite) . 



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 crystallo- 

 graphic planes. Olivine is fairly abundant. It is much fractured, and the 

 crevices are filled with magnetite. The olivine does not show crystal bound- 

 aries. Associated with it there are patches of colourless crypto-crystalline 

 matter, in which a few grains of iron-ore can be seen. This crypto-crystal- 

 line 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 



