148 A NATURALIST IN THE PACIFIC chap. 



mainly confined to the filling of the fissures with chalcedonic 

 quartz, minute nests of the same material occurring in the ground- 

 mass. At other times the small augite granules are also decom- 

 posing. The specific gravity varies from 2*64 to 273 ; the rocks 

 being referred to genus 16, species A, of the augite-andesites. 

 Occasional detached masses of a propylitic basic andesite, 

 displaying porphyritic plagioclase and pyroxene, also occur in this 

 gorge, the felspar phenocrysts being largely occupied by calcitic 

 and other alteration products, whilst much viridite occurs in the 

 groundmass. It exhibits both monoclinic and rhombic pyroxene ; 

 and on account of the prism form of the groundmass pyroxene it 

 is placed in the 2nd sub-order of the hypersthene-augite andesites. 

 These altered rocks are deep-seated intrusive masses that were 

 originally covered over by the basic agglomerates and palagonite- 

 tuffs exposed in the sides of the gorge. 



Below the gorge there is an extensive exposure in the sides 

 and bed of the river of light-coloured calcareous tuffs which were 

 originally composed of palagonitic materials ; but owing partly to 

 hydration, and partly to other secondary changes, the original 

 structure is much disguised. 



Crossing the river in the midst of these tuffs there is a dyke, 1 5 

 feet thick, formed of a propylitic basaltic andesite, a semi-vitreous 

 rock in which calcitic and zeolitic materials have been developed in 

 quantity. The dyke, which is not columnar, is steeply inclined at 

 an angle of 45° to the north-east. . . . Further down the river-valley 

 as far as Narengali, occur basic tuff-agglomerates. 



The Tract of Nakambuta 



This is a tract of broken country that projects from the 

 mountainous backbone of the island (between the Va-lili and Koro- 

 tini ranges) into the heart of the Ndreketi plains in the vicinity of 

 Natua. As limited by the 300-feet contour line, it is indicated in 

 the map attached to this work. Its general level varies between 

 300 and 600 feet in elevation ; but a number of isolated peaks are 

 included within this area. More than one of these hills attain a 

 height of 1,000 feet, Nakambuta a very conspicuous hill being as 

 much as 1,500 feet. Basaltic andesites with basic agglomerates 

 and palagonite-tuffs prevail. 



Towards Natua the basaltic andesites, which are often much 

 decomposed, are of the doleritic type referred to in the account of 

 the Ndreketi Plains on page 133. Inland, towards Narengali and 



