East of the Avuka Range that limits the Lambasa plains on this side is the picturesque valley of Mbuthai-sau. This broad valley runs in a south-east direction into the heart of the island. So small is the gradient that the river flowing down it can be ascended in boats for some miles; whilst Ngele-mumu, a village situated between 5 and 6 miles up the valley, is not much over 50 feet above the sea. This valley in its lower part roughly divides the regions of acid and basic rocks that lie east and west of it respectively. It has, however, been above pointed out that the two regions overlap in the coast region on the west side of the valley. The two types of rocks are also associated in the coast hills immediately east of the valley, since when striking inland from Lloyd Point to the Mbuthai-sau sugar-cane district, one leaves behind the acid rocks at the coast and traverses a region of basic agglomerates. With these qualifications, therefore, the above line of demarcation holds good.
(2) THE SEA-BORDER BETWEEN THE MBUTHAI-SAU AND THE LANGA-LANGA RIVERS
The rocks predominating in this district are white and pale-yellow pumice-tuffs and pumice agglomerates, with quartz-porphyries and oligoclase-trachytes as intrusive masses. The light colour of the sea-cliffs, thus composed, makes them conspicuous from seaward. Their appearance evidently led Mr. Horne into an error in 1878 when he viewed this coast from his canoe during his sea-passage from Lambasa to Tutu. “The coast from Lambasa”—he says—“is a series of bold projecting bluffs of agglomerate interspersed with seams of coralline sandstone.”[98] The principal feature of this coast, however, is the prevalence of light-coloured pumice-tuffs, though it is not improbable that elevated reef-formations may occur in some localities.
I particularly examined the coast districts in the vicinity of the Wainikoro and Langa-Langa rivers. In a spur that descends to the right bank of the river, a little above the mouth of the Wainikoro, is exposed an open-textured rhyolite or quartz-porphyry containing, as described on page 310, phenocrysts of glassy felspar (oligoclase and sanidine), quartz, and a little hornblende. Some interesting exposures occur on the coasts between Narikosa Point (Nari-Roso Point of the chart) and the mouth of the Wainikoro. Here the pumice-tuffs and agglomerates are pierced by dykes, some of them vertical and 6 feet across and composed of a light-grey rhyolitic rock, similar except in its compact texture to the rock above mentioned.[99] The small quartz crystals, which are 1 to 2 millimetres in size, are sometimes bipyramidal. Many of them are rounded and have a fused-like surface. The pumice-tuff, which displays no effervescence with an acid, is composed mainly of finely pulverised vacuolar and fibrillar istropic colourless glass, and contains also small fragments of glassy felspar and small rounded quartz crystals, such as occur in the rock forming the dykes in these deposits. Imbedded in the tuffs in places are small masses, up to 4 inches in size, of a pretty grey vesicular rhyolite-glass exhibiting the intermediate condition between compact obsidian and pumice which is so characteristic of the rocks of Vulcano in the Lipari Islands. A more detailed account of this rock is given on page 311. In the pumice-agglomerates of this locality occur pale-yellow decayed and altered pumice blocks.