198 



PROCEEDINGS OF SECTION C. 



HuAHEiNE. — ^A dense greyish-green rock. It consists mostly 

 of nepheline. Some of the crystals, though small, are distinctly 

 idiom orphic ; they are associated with a quantity of irregular 

 segerine, which almost surrounds them and forms irregular patches. 

 These patches are embedded in a fine-grained clear matrix, which 

 is composed of granular allotriomorphic nepheline. In the clear 

 matrix there are small grains of magnetite often with cossjTite or 

 katoforite ; or the amphibole may occur alone in angular grains 

 in the clear matrix. 



The rock forms conspicuous lava flows crowning Pahiraia, and 

 apparently other hills such as Tahateao. An analysis showed that 

 it had the following composition : — 



100-05 



A basalt occurs largely as boulders in the stream, but it could 

 not be found in situ. It contains much coarse and idiomorphic 

 olivine, a little idiomorphic augite, felspar that is probably labra- 

 dorite, in small and ill-defined crystals. There is a great deal of 

 dusty magnetite and minute granular augite between the felspars. 



Boulders of a highly vesicular rock with large inclusions of 

 olivine were very common. This rock appears to correspond with 

 the felspathic picrite of Lacroix. The olivine masses are granular, 

 and are embedded in a coarse ground mass consisting mainly of 

 augite with microlites of labradorite and ilmenite. 



Raiatea. — ^Mt. Tapioi. — A dense greyish-green rock without 

 conspicuous phenocrysts. In section it is seen that about nine- 

 tenths of the rock consists of felspar in crystals, which are occasion- 

 ally 1 m.m. long, cracked transversely and often bent. The 

 aegirine is very pale green, with an extinction angle of 40-45 degrees ; 

 magnetite is quite abundant and is often associated with the 

 pjTToxene, A little apatite in relatively large prisms. The small 

 quantity of nepheline that is present is slightly decomposed and 

 much cracked. It is intercalated between the felspar prisms, and 

 quite allotriomorphic. The rock agrees closely with that described 

 by Lacroix from Vairao in Tahiti. The composition of the rock is 

 given on the following page. (PI. xviii., fig. 2 and PI. xix.). 



A lava flow of this rock crowns the top of Mt. Tapioi, which is 

 968 feet high. The lava flow is nearly 200 feet thick, and has a 

 characteristic steep escapement. It rests on phonolitic scoria and 



