666 GEOLOGY OF SAMOA AND THE ERUPTIONS IN SAVAII, 



to do with the eruption, as shown by the HC1 evolved, and, it 

 should be mentioned, the rainy season, January to March, was 

 that of greatest activity. 



Many points of resemblance between Sanioan and Hawaiian 

 lavas have been dealt with. 



Part ii. — Petrology of the Samoan Lavas. 



Sp. S.l. Hyalopilitic Olivine Basalt. Loc. : New lava flow between 

 Matauto and Satapatu near Malaiola. (Plate lxiv., figs. 1-2). 



i. Handspecimen vesicular, consisting of two-thirds compact lava 

 and one-third large almond-shaped vesicles. Olivine is abundant 

 in small but macroscopic crystals of greenish colour. 



ii. Microscopic examination. — (1) Texture : (a) Crystallinity 

 hypocrystalline; (b) grain-size uneven and fine;, (c) fabric vesicular, 

 porphyritic; hyalopilitic base; and occasionally aggregates of 

 augite, optically continuous, embrace felspar ophitically. 



(2) Constituents. — Augite, felspar, and olivine occur in about 

 equal amount, and form over 90 % of the rock. The other con- 

 stituents, magnetite and ilmenite, form perhaps 5 % and glass 5 %. 



(3) The augite is greenish br-own in colour, and slightly 

 pleochroic, hence probably titaniferous. It occurs in small 

 idiomorphic grains and ophitic aggregates. The felspar is 

 plagioclase, having a maximum extinction of about 27° in 

 cross sections of microlites, and up to 40° in longitudinal 

 sections with symmetrical albite extinction, hence basic by- 

 townite. It occurs in lath-shaped and tabular sections, showing 

 prismatic, acicular, and tabular habit. It is twinned on the 

 Carlsbad and albite plans. Sometimes needles of it penetrate 

 augite. The olivine is in idiomorphic but highly corroded pheno- 

 crysts, containing primary magnetite inclusions. The ilmenite 

 occurs in tables, and the titaniferous magnetite in idiomorphic 

 grains. A few flakes of hematite are represented, and, as the 

 rock was barely two months old when taken, it is evidently a 

 primary constituent. A little glass is present in the ground mass. 



