80 A NATURALIST IN THE PACIFIC chap, 



waters of the Wainunu and Sarawanga rivers in part take their rise 

 but have little or no eroding power. 



It is not easy to obtain a good general view of the district of 

 the falls on account of the dense forest-growth. When making the 

 traverse from Tambu-lotu to Ndawa-thumi, it is observed that 

 there is here a singular hollow, about half a mile in length, which 

 receives the falls at the western end. The river crosses this hollow 

 and is at once received into the gorge below, but there is no stream 

 to explain the origin of the cavity. On its north side the cliffs of 

 agglomerate rise to a height of 150 to 200 feet from their base, but 

 on the south the sides are much lower. Here there seem to be the 

 remains of the crater of the ancient vent from which all the tuffs and 

 agglomerates of the district were derived. We must look for their 

 origin in the vicinity, and the only evidence of a crateral cavity is 

 this streamless hollow extending east from the falls of Na Savu. 



With reference to the basic tuffs and agglomerates of this 

 plateau it may be observed that they cover the massive basic rocks 

 and are probably not over 100 or 150 feet in maximum thickness. 

 They are well exposed where the streams cut into the borders of 

 the plateau. The tuffs are sometimes bedded and slightly inclined, 

 and they may be fine or coarse grained. They are more or less 

 palagonitised hyalomelane-tuffs, being composed mainly of frag- 

 ments of a basic glass, often finely vesicular and even fibrillar, the 

 vacuoles being filled with different materials, whilst the palagoniti- 

 sation is well advanced. Sometimes they have a brecciated 

 appearance, and in that case when the alteration of the basic glass 

 is very extensive we find angular fragments, 1 to 2 inches across, 

 of a greenish palagonite imbedded in a pale matrix of palagonitic 

 debris, the whole rock having a soapy feel and a steatitic appear- 

 ance. This is well shown on the sides of the stream-course at 

 Ndawathumi which lies at the border of the table-land. These 

 tuffs effervesce but slightly with an acid. 



The basic agglomerate is displayed in the face of the falls and 

 in the gorges. The blocks are as a rule composed of semi-vitreous 

 basaltic andesites of varying type, showing no olivine and con- 

 taining a fair amount of smoky glass in the groundmass. At times 

 they are scoriaceous and display amygdules of calcite or a zeolite. 

 In places the rock shows large phenocrysts of plagioclase and 

 a semi-ophitic groundmass, when it is referred to the porphyritic 

 group of genus 9 of the augite-class. In a few of the scoriaceous 

 blocks the augite of the groundmass is for the most part prismatic 

 and rarely granular (genus 5). 



