392 PKOF. P. MARSHALL OX THE SEQUENCE OF [Dec. 1914, 



Ground-mass : labradorite-inicrolites measuring *13 x "015 mm., 

 76 per cent. ; augite, '12 x "02 mm., 10 per cent. ; magnetite, 

 '012 mm., 14 per cent. 



(3) Basalt. — Anorthoclase-phenocrysts, .5 mm. long, with much 

 included matter, especially augite, which may be in micrographic 

 intergrowth, and some oligocla.se, which may be zoned ; augite 

 greenish, especially in the centre, slightly violet, with some pleo- 

 chroism, sharply idiomorphic, 1 mm. long, but not abundant ; 

 olivine-grains, usually rounded, 1 mm. in diameter, not abundant. 



Ground-mass: labradorite-microlites measuring 2 x "02 mm., 

 62 per cent. ; augite, "008 x *001 mm., 29 per cent. ; magnetite, 

 •008 mm., 9 per cent. 



(2) Phonolite. — No phenociysts ; sanidine - microlites, 

 *2 x # 01 mm., 64 per cent. ; segirine-augite, 43° extinction. 

 *26 x "02 mm., 21 per cent. ; magnetite, "02 x '05 mm., 5 per cent. 

 The structure of the microlites of sanidine and segirine-augite is 

 interwoven. 



(1) Trachyte. — Anorthoclase-phenocrysts, abundant, 5 mm. 

 long. 



Ground-mass almost solely composed of anorthoclase-microlites, 

 •26 X '06 mm. ; a little magnetite. Structure trachytoid. 



Summary of Petrographical Features. 



These short descriptions of the structure and the mineral compo- 

 sition of the rocks of this series may be briefly summarized as 

 follows : — 



The lowest lava is entirety composed of anorthoclase. It is 

 succeeded by a phonolite in which sanidine is the predominant 

 mineral, although segirine-augite and magnetite attain some im- 

 portance. A series of basalts follows. The lower of these contain 

 conspicuous, much-corroded xenocrysts of anorthoclase. The fifth 

 of these basalts (No. 6) is especially basic, and in particular contains 

 much more olivine than any other rock in the whole series. 



This series of ten basalts is succeeded by a lava of a wholly 

 different type, belonging to the class that I have previously termed 

 kaiwekite (No. 13). It has a coarsely porphyritic structure, and 

 hornblende of a barkevikitic type forms the largest crystals in it. 

 The crystals of this mineral are deeply resorbed, and were evidently 

 formed under conditions that were wholly different from those 

 under which extrusion of the lava took place. The augite, too, is 

 widely different from that of the basalts, for it has distinctly the 

 violet tinge associated with the presence of titanium ; while it shows 

 at times a small border of segirine. The felspar-phenocrysts also 

 include anorthoclase as well as labradorite. 



There is a sudden transition from this to the next lava, which 

 contains no phenociysts, while the minerals of the ground-mass arc 



