(1) THE DISTRICT EXTENDING TWO AND A HALF MILES WEST OF TAWAKI
When proceeding eastward along the north coast of Natewa Bay one enters the region of acid rocks between 2 and 2½ miles west of Tawaki. Here the country is much broken, picturesque hills with bare precipitous faces rising up near the coast, one of which named Natoto has a rudely conical and truncated form. Grey oligoclase-trachytes having a specific gravity of 2·4 and possessing the characters described on page 308, prevail in the district extending west of Tawaki. Sometimes they occur in mass; but they often form agglomerates. A singular pitchstone-agglomerate occurs at the coast at the foot of Natoto. The pitchstone, which has a specific gravity of 2·48, is a semi-vitreous form of a hypersthene-augite andesite. It shows abundant small pyroxene prisms in its glassy groundmass and is referred to the prismatic sub-order (5) described on page 289.
A high range of hills, forming the backbone of this part of the island, extends eastward for about five miles from Thawaro and Tawaki. It is named “Naithombothombo” in the Admiralty 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 few 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 Tawaki 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[104] (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.