(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.
The headland, known as Narikosa Point, which lies between the mouths of the Wainikoro and Langa-langa rivers, terminates in a rocky spur formed by a large intrusive mass or dyke of a reddish oligoclase-trachyte altered in character and displaying in places a rudely columnar structure.[[100]] The pumice-tuffs and agglomerates are well exposed at the coast between Narikosa Point and the Langa-langa River. In the vicinity of Songombiau, a village here situated, the tuffs are penetrated by dykes. One dyke at the back of the village is 4 or 5 feet thick and has vitreous margins. It is composed of a darkish pyroxene-andesite which in the interior of the intrusion in hemi-crystalline and a little vesicular, but in the margins it is more glassy and rather scoriaceous.[[101]] Though not a basic rock in itself, its specific gravity in the compact state being only about 2·55, it is relatively basic when contrasted with the rhyolitic and trachytic rocks of the district, which have when compact a specific gravity of not over 2·3.
Pumice-tuffs and agglomerates appear in the cliff-faces of the hills on either side of the lower course of the Langa-langa River from the vicinity of Kalikoso to the sea. At one place about 3 miles up the river a dyke of trachyte, much altered and showing columns 12 to 18 inches across, was exposed at the bank.