i 4 4 A NATURALIST IN THE PACIFIC chap, 



inclosing porphyritic crystals of plagioclase, augite, and olivine,, 

 which is described on page 313. Here also occurs an agglomerate 

 made up of blocks of a semi-vitreous basaltic andesite (sp. gr. 278), 

 showing prismatic pyroxene in the groundmass, and referred to 

 genus 20 of the augite-andesites. 



The upper part of the hill displays the same agglomerate, and 

 a tuff-agglomerate in which small fragments of the basaltic andesite 

 are inclosed in a matrix largely made up of fine debris of basic 

 glass. There protrudes through these detrital rocks at the top of 

 the hill a broad dyke-like mass of the same basaltic andesite that 

 forms the agglomerate around ; and the structure of the hill is 

 thus displayed as that of an old volcanic neck. It has evidently 

 an axis of massive basaltic rocks, more or less covered over with 

 agglomerates and tuffs. 



(4) The Nambuni Spur. — This singular spur runs down to 

 the coast between Waiwai and Wailevu ; but it is partly separated 

 by a deep gap from the main range. It attains a height of 550 to 

 600 feet, and has two little peaks which the natives call Vatu- 

 tolutolu and Vatu-tangitangiri. Its position is shown in one of 

 the profile-sketches of Va-lili, given on page 141. The crest of the 

 spur is formed by a dyke-like mass, 25 to 30 feet thick, which is 

 composed of a basic agglomerate passing down into a palagonite- 

 tuff. The blocks of the agglomerate are composed of a semi- 

 vitreous basaltic andesite, showing minute felspar-lathes in flow- 

 arrangement in an abundant smoky glass, the fine pyroxene being 

 not differentiated. The tuff, into which the agglomerate passes 

 down is non-calcareous, and displays no organic remains. It is, 

 however, composed of fragments, which do not generally exceed a 

 millimetre in size, of palagonitised vacuolar glass, basic andesites, 

 plagioclase, monoclinic and rhombic pyroxene, &c. 



This dyke-like mass forms the axis of the ridge and protrudes 

 vertically about 100 feet, the bulk of the spur being composed of a 

 compacted brecciated palagonite-tuff made up mainly of fragments 

 a centimetre in size, of a basic vacuolar glass, sometimes fibrillar, 

 which is extensively palagonitised. 



The filling up of a fissure in a mass of tuff-breccia by palagonite- 

 tuffs and agglomerates probably occurred during the submergence, 

 the original dyke-rock having been removed by marine erosion. 

 After the emergence the subaerial denuding agencies reshaped the 

 surface, and as a result of the less yielding character of the materials 

 filling the fissure, they protrude as a dyke-like mass from the 

 crest. 



