220 A NATURALIST IN THE PACIFIC CHAP.. 



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. 1 The small quartz crystals, which are I 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. 2 The pumice-tuffs and 

 agglomerates are well exposed at the coast between Narikosa 

 Point and the Langa-langa River. In the vicinity of Songom- 

 biau, 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 scoriace- 

 ous. 3 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 



1 Described on p. 310. 



2 These rocks are described on p. 308. 



3 It contains small phenocrysts of plagioclase (medium andesine), and or 

 augite and rhombic pyroxene, and is referred to genus i of the hypersthene- 

 augite andesites. 



