The District between Nanduri Bay and Wailevu River
The sea-border between Nanduri Bay and Middle Point, nearly four miles to the east, consists of a fringe of lowland margined by the mangrove-belts and banked by a line of hills between a quarter and two-thirds of a mile inland. These hills form a continuation of the Nawavi coast range of mountains extending from Ravi-ravi Point to Nanduri. They attain their greatest height in the case of Ulu-i-sori, a cockscomb-like peak 1,141 feet above the sea. Another of these hills, Vatu-tangiri, is capped by a remarkable obelisk-like rock. Behind this coast range lies a hill with an elevation of nearly 1,400 feet.
The rocks exposed for the first mile or two along the coast east of Nanduri are agglomerates and basic tuffs. The blocks of the agglomerates, however, are made of an altered grey porphyritic rock which has the characters of a porphyrite of a rather acid type.[[63]] This composition of the agglomerate is quite exceptional and indicates the antiquity of the volcanic rocks in this locality. Farther along the coast the typical agglomerates occur, where the blocks, 3 to 10 inches across, are composed of the usual semi-vitreous black basaltic rock showing plagioclase phenocrysts. Nearer Middle Point a decaying doleritic basalt is displayed at the surface. It is similar to the prevailing rock of the Ndreketi plains, and is referred to the ophitic rocks forming genus 9 of the augite-andesites.
The elevated promontory of Middle Point is a prolongation of a spur of Ulu-i-sori. Where it is crossed by the road it is about 350 feet above the sea. On its west slopes are exposed yellowish-white tuff-like rocks, evidently the prevailing basic clay-tuffs which have become bleached through the hydration accompanying the weathering process. Beneath these deposits lies an amygdaloidal augite-andesite which is bared in places. The rock is semi-vitreous and the amygdules it contains are often a centimetre long. They are composed of a white mineral with fibro-radiate structure and made up of needle-like prisms. It gives off water, but it is not easily fused, and does not gelatinise in HCl.
From the top of the promontory the road strikes inland in an east-south-east direction for Tambia, passing inside the coast range, which is here 600 feet in height, and descending gradually through a region of basaltic andesite into the valley of the Tambia river. (This rock, which has a specific gravity of 2·84, displays more or less parallel stout felspar-lathes, ·23 mm. in length, and has a little interstitial glass. It belongs to genus 13 of the augite-andesites.) Low hills shut in the little valley on all sides except where the river breaks through the coast range. The town of Tambia is not over 100 feet above the sea. About a mile to the north exist hot springs of considerable extent which are described on page [32].
The road from Tambia to the Wailevu River traverses an undulating district varying from 100 to 300 feet above the sea. A basalt containing a little olivine, with a specific gravity of 2·91, is commonly exposed at the surface in a disintegrating condition. Here and there occur basic tuffs. In one locality, there is displayed a dyke-like mass in a small stream course, 200 feet above the sea, of an altered grey and compact andesite marked with parallel red streaks or bands. It is an aphanitic augite-andesite; and is to be referred to genus 13 of the augite sub-class. It displays closely crowded felspar-lathes, ·07 mm. in length, in flow-arrangement. The bands are due to the gathering of the residual glass in streaks parallel to the flow. Chalcedonic flints, some of them showing the agate-structure, together with fragments of silicified corals, are found occasionally on the surface in this district.