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.
(3) SEA-BORDER BETWEEN THE LANGA-LANGA RIVER AND THAWARO BAY
The next part of the coast which I visited was that opposite Tutu Island. Here a range of high hills, 1,100 or 1,200 feet in height, sends a lofty spur to the sea, in the precipitous faces of which are exposed breccia-tuffs and agglomerates derived from acid rocks. A specimen of the tuffs shows, besides fragments of altered rhyolitic or trachytic rocks, portions of decomposing pumice, the vacuoles and tubular cavities of which are filled with alteration products. The blocks in the agglomerates are altered oligoclase-trachytes, both compact and vesicular. These deposits are non-calcareous and rarely display bedding; but in one place there was a rude arrangement of the materials, the dip being to the north-west. I crossed this coast range where it is only 600 feet above the sea, and descended into the Kalikoso plains in the vicinity of Numbu.