ii6 A NATURALIST IN THE PACrFIC chap. 



glass. It is referred to genus 16, species A, sub-species i, of the 

 augite-andesites (page 280). 



The probable structure of this district is shown in the geological 

 section here given. It is assumed from the limited exposure ot 

 the same rock on the top of the ridge that the basaltic flows which 

 surrounded the lower portion of Vatu Kaisia at the same time 

 covered over another similar peak lying immediately west of it. 

 Through stream-erosion Vatu Kaisia has now been isolated on its 

 west side ; and since the basaltic rocks rise to about the same 

 height on both sides of the gorge thus produced, the original 

 surface was probably as indicated by the dotted line in the 

 diagram. 



By following the summit of the ridge, as it runs south on the 

 right side of the Yanawai valley towards Ndawara, some interesting 

 rocks are observed. For the first mile from the camping-place 

 opposite Vatu Kaisia the elevation increased from' 1,100 to 1,300 

 feet, and blocks of a blackish basaltic andesite (sp, gr. 276) lay on 

 the ground. About a mile further on fragments of white quartz- 

 rock appeared on the surface having been thrown out of a shaft 

 close to the track which had been sunk to a depth of 15 or 20 feet 

 by a gold miner ^ a few years before. I could not descend the 

 shaft to examine it : but the specimens picked up are evidently a 

 white vein-quartz, some of them having a striated " slickenside " 

 surface on one side.^ There is evidently a "contact" in this 

 locality, probably of a basaltic rock with an acid andesite. 



Leaving the shaft, the track proceeds southward and eastward 

 and one descends gradually from a height of 1,100 feet down to the 

 Yanawai river where the elevation is only about 1 50 feet above the 

 sea. Occasional blocks of basaltic rocks lie on the surface of the 

 ridge, and in one locality there is exposed a curious-looking 

 agglomerate formed of fragments of a greenish altered augite- 

 andesite, somewhat scoriaceous, the cavities being filled with a 

 zeolite. At the crossing of the river a black basalt (sp. gr. 2"82) 

 occurs in situ ; whilst loose blocks of basalt and of an acid 

 andesite occur in the river-bed. Continuing the journey from the 

 Yanawai crossing to Ndawara near the mouth of the river, one 

 follows the track across a range of hills, 500 to 600 feet in height, 

 basaltic rocks prevailing on the surface. 



^ Alluvial gold has long been known to occur in the bed of the Yanawai 

 below Vatu Kaisia ; but it has never been found in paying quantity. 



2 Under the microscope it is shown to be granular in structure, exhibiting a 

 mosaic of irregular quartz grains. 



