230 A NATURALIST IN THE PACIFIC chap. 



chart. From it rise two conspicuous peaks, Thawaro Peak (1,573 

 feet) at its western end, where it overlooks the village of that 

 name, and Mount Thuku (1,288 feet) near its eastern end. West 

 of Thawaro Peak this range is connected with the hills beyond by 

 a saddle 600 feet in height, which is ascended when crossing the 

 promontory from Thawaro Bay to Tawaki. 



(a) Thawaro Peak. — This represents an old " volcanic neck " 

 of agglomerate rising out of the tuffs that are exposed on its 

 slopes to an elevation of about 600 feet. As viewed from the 

 saddle above mentioned, the upper part of the hill presents bare 

 precipitous sides, several hundred feet in height, of agglomerate, 

 the blocks of which are composed of a compact hypersthene-augite 

 andesite (sp. gr. 2-48). It displays a (qw small phenocrysts of 

 medium andesine and of rhombic and monoclinic pyroxene ; and 

 is referred to the prismatic sub-order (5) described on page 289, 

 characterised by prismatic pyroxene in the groundmass. 



(b) South coast between Tazvaki and the foot of Mount Thuku. — 

 The tall cliffs that rise to a height of from 200 to 300 feet behind 

 Tawaki are composed of white tuffs and agglomerate-tuffs derived 

 from the acid rocks of the district. Eastward from Tawaki to 

 the base of Mount Thuku the coast scenery is particularly fine. 

 A little inland a line of hills, named " Na Kula," rises precipitously 

 to a height of 700 or 800 feet, and in the vertical sides are displayed 

 tuffs and agglomerates probably of the character of those above 

 noticed. Light-coloured tuffs are sometimes exposed at the coast 

 in which are inclosed fragments varying in size of a pitchstone l 

 (sp. gr. 2-36) approaching in structure a trachytic glass. At one 

 place the tuffs were evidently sedimentary and bedded, the dip 

 being about 15 N.W. 



The massive rock most frequently exposed at the coast 

 and on the hill-slopes between Tawaki and Mount Thuku is a 

 quartz porphyry 2 displaying abundant porphyritic crystals of 

 quartz and felspar in a groundmass originally semi-vitreous but 

 now obscurely felsitic in character. The shore-fiat for more 

 than half a mile west of Mount Thuku is strewn with great 

 numbers of detached columnar blocks, 12 to 15 inches across, 

 of a slightly vesicular oligoclase-trachyte of the type described 

 on page 308. 



(c) North coast between Thawaro Bay and the foot of Mount 

 Thuku. — Coarse and fine tuffs prevail at the coast and on the 

 neighbouring hill-slopes ; and in the bare rocky faces of the hills 



1 Described on p. 309. 2 Described on p. 310. 



