Marshall. — Geology of Tahiti. 367 



boundaries, indicating, of course, that it was the last mineral to crystallize. 

 A little dark-brown biotite is associated with the olivine. There is no 

 iron-ore apart from that associated with the olivine. (For analysis, see 

 page 371.) 



It will at once be evident that the discovery of these types considerably 

 extends the series previously described by Lacroix, who has already com- 

 mented on the close relationship between the various members. An ex- 

 tension of the series in the basic direction was clearly foreseen by him, for 

 he recognized that an ultra-basic plutonic would probably exist to represent 

 the picrite which he had distinguished among the basalts. This picrite. 

 however, contains some feldspar, and it is thus less completely ultra-basic 

 in a mineralogical sense than the peridotite just described. 



In the acid direction the series of Lacroix is still further extended, for 

 the most acid type described by him — the syenite nepMlinique d biotite- — 

 contains only 52-25 per cent, of silica, whereas the syenite now found in 

 situ in the bed of the Tahinu contains 61-06 per cent, of silica, notwith- 

 standing the considerable amount of pyrite in the rock. This syenite con- 

 tains a slight excess of potash over soda — a feature that was noticed bv 

 Lacroix also in his syenite nephelinique a biotite, though in all the other 

 rocks of the series there is a considerable excess of soda over potash. 



It is thus apparent that in this small area there is a plutonic series 

 extending from a syenite with potash in excess of soda, and consisting 

 almost entirely of alkaline feldspar, through various types of monzonites, 

 essexites, theralitet, and gabbros to a peridotite with absolutely no feldspar. 



Professor Lacroix, in 1910, mentioned a specimen of granite which had 

 been collected at Tautira, on the north side of the Peninsula of Taiarabu. 

 Pere Alain, of L'Ecole des Freres, at Tahiti, kindly gave me a specimen 

 of a similar granite which had been given to him as coming from Tautira. 

 As Lacroix said, " S'il provient du sol de File il y aurait Id un fait d'une 

 grande importance. ." A visit was accordingly made to Tautira, and a 

 specimen of the granite was shown to various Natives, who all agreed that 

 they had never seen .such a rock anywhere in the district. The chief, 

 Ori a Ori, in particular, was emphatic in the statement that he had not 

 seen a rock of this nature in the mountains, or anywhere else in the country. 

 A diligent search was afterwards made in the river-gravels and on the sea- 

 beach for boulders or pebbles of granite, but entirely without success. It 

 seems that this granite, if it ever really came from Tautira, had been 

 previously brought there as ballast or for some other purpose by some 

 /isiting vessel. 



Though no granite pebbles were found at Tautira, pebbles of plutonic 

 rock were not infrequent in the gravels of the Tautira Eiver. These were 

 found to be specimens of a highly feldspathic olivine gabbro similar in many 

 respects to the specimens from the Papenoo Valley. 



Dyke Rocks. 



Specimens of tinguaites, monchiquites, and camptonites have already 

 been described from the material brought from the Papenoo Valley by 

 M. Seurat. I found that boulders of tinguaite were not common in the 

 material of the river-bed, and I was unable to find any in place. One of 

 my guides — Mauri — however, told me that the roches vertes were to be 

 found in some abundance in the gravels of the upper part of the Teti Valley, 

 but I had no time to attempt to verify his statements. Boulders of mon- 

 chiquite were quite common among the boulders of the Papenoo Valley. 



