DISTRICT BETWEEN MOUNT SESALEKA, THOMBO-THOMBO POINT, AND VATU-KAROKARO HILL.—This is a broken country with several abruptly rising lesser hills. Starting from Koro-vatu and crossing the Thombo-thombo promontory, I reached the coast of Mbua Bay near Navunievu. Basic tuffs and agglomerates prevailed on the way, the last containing blocks of a scoriaceous basaltic lava bearing olivine. The massive rocks exposed belong in some cases to genus 13 of the olivine-basalts as described on page 256, being dark grey and having a specific gravity of 2·88, and in other cases to genus 16, species B, of the augite-andesites when they are lighter in colour and have a specific gravity of 2·77. In both cases the interstitial glass is scanty.
I ascended Vatui, one of the numerous small hills of the district. It is 450 feet high and is capped by a bare mass of tuff-agglomerate, 40 to 50 feet high and containing fragments of vesicular basic lava. This mass is pierced by a dyke, 18 inches thick, which is inclined to the N.NE. at a high angle of 60 or 65 degrees with the horizon. This dyke is composed of a compact olivine-basalt which is remarkable for the prevalence of small augite prisms in the groundmass. It is described on page 265 under genus 44 of the olivine-basalts. Hand-specimens are magnetic and display polarity, which is due, as pointed out in Chapter XXVI., to the exposed situation of the peak.
Vatui in its characters is evidently typical of the other lesser hills around, which, as viewed from below, possess bare tops and precipitous declivities of the same formation. All the hills in the district including Sesaleka are capped by these basic tuffs and tuff-agglomerates; and doubtless as in the case of Sesaleka these deposits are all submarine. This is true also of Vatu-karokaro, a hill 600 feet high, overlooking Mbua Bay and about two miles east of Sesaleka. In the lower part of this hill is exposed a dark compact basaltic andesite, referred to genus 13, species B, of the augite-andesites (sp. gr. 2·83), whilst blocks of a black olivine-basalt (sp. gr. 2·91) occur in the agglomerate of the summit. These hills may all be regarded as “volcanic necks” or the stumps of volcanic cones, probably submarine.