In the district between the Ndranimako and the Yanawai rivers basic tuffs and “soapstone” prevail. In this locality, and especially in the vicinity of Ndranimako, siliceous concretions 2 to 3 inches across, occur in places on the surface. Their nature is described in Chapter XXV.
From the foregoing remarks it may be inferred that the sea-border between the Kumbulau Peninsula and the Yanawai River is formed of submarine deposits overlying basic rocks which probably represent ancient flows. Some of the deposits are largely formed of glassy erupted materials, which have been converted into palagonite. Others again are more characteristic sedimentary formations accumulated in relatively deep water.
DESCRIPTION OF THE GEOLOGICAL AND GENERAL PHYSICAL FEATURES (_continued_)
THIS hilly region of acid andesites is a continuation of the mountainous backbone of the island, being separated from the basaltic mountain of Seatura by the saddle formed by the Na Savu table-land. These acid andesites exhibit in nearly all cases a felsitic groundmass and phenocrysts of plagioclase and rhombic pyroxene; whilst many of them are characterised by brown hornblende more or less pseudomorphosed in the manner described on page 306, and a few display porphyritic quartz. Although these rocks have a common facies, they vary considerably among themselves; and it is difficult to find a term that would strictly include them all. A general description of their characters is given in the chapter on the Acid Andesites.
In this interesting region a number of hills or mountains formed in mass of acid andesites rise up abruptly without any regular arrangement within an area measuring 5 by 6 miles, and elevated 600 to 1,000 feet above the sea. Of these hills, thirteen in all, nine range in height between 1,600 and 2,500 feet above the sea, none of the others rising less than 1,000 feet above that level. But the actual height of each hill above the country at its base is much less than this. The height of the hill-mass, in five or six of the largest, ranges between 900 and 1,200 feet, whilst in the smaller hills it varies between 400 and 800 feet. (See accompanying plan.)
[Illustration: The NDRANDRAMEA District from the westward. The hills and mountains are of acid andesites and dacites. The foreground is elevated about 450 feet above the sea.]