1 64 A NATURALIST IN THE PACIFIC chap. 



These deposits, as they are displayed in the hill-slopes lying 

 W.S.W. and at the back of Sueni, are composed of blocks of the 

 size of the fist of a vesicular basaltic andesite ; whilst the large 

 masses on the surface are made of the same, but non-vesicular, 

 rock. The blocks in the agglomerates between Sueni and Koro- 

 utari range usually from a few inches to a foot in diameter. A 

 specimen obtained from one of them is made of a partly vitreous 

 basaltic andesite ; whilst in another case the rock is an altered 

 basic andesite, the glassy groundmass being largely impregnated 

 with colloid silica looking like opal under the lens. 1 



Nearly a mile below Sueni, within a space of less than 60 yards, 

 there are exposed at the river-side in the agglomerates three 

 vertical or nearly vertical dykes, 4 to 6 feet in thickness. They 

 trend roughly N.E. and S.W., and are non-columnar, except in the 

 case of the one farthest up the river, which has rude, transverse 

 joints. 2 The rocks composing these dykes are somewhat doleritic 

 basaltic andesites, olivine being very rare or absent. The two 

 highest, which are only 1 5 to 20 feet apart, are made of similar 

 rocks characterised by abundant interstitial glass, and having a 

 sp. gr. of 278. The rock of the third dyke, about 50 yards farther 

 down the river, has but scanty glass in the groundmass, the sp. gr. 

 being 2-89. The differences between the two types represented in 

 the three dykes are mainly concerned with the degree of crystal- 

 lisation, and it is probable that though not contemporaneous they 

 were derived from the same fluid magma which, as we may infer 

 from the proximity and lie of the dykes, was situated at no great 

 depth. 3 



General Inference Respecting the Koro-tini Range. 

 — If we can imagine a line of vents, protruding in some cases 



1 Both these rocks belong to the hypersthene-augite andesites, showing 

 phenocrysts of both monoclinic and rhombic pyroxene. The first belongs to 

 the orthophyric order described on p. 290 ; whilst the second belongs to the 

 second order (genus 13, p. 287) where the felspars of the groundmass are lathe- 

 like and in flow arrangement. 



2 The highest dyke trends N. 48-"' E. and is inclined from the vertical about 

 1 5 N.W. The dyke, 5 or 6 yards below it, trends N. 30 E. and is vertical. 

 The dyke, 50 yards farther downtrends N. 35 E. and is inclined from the 

 vertical about 5 N.W. The inclination was only estimated. The bearings 

 are true. 



3 Both the types are referred to genus 1 of the augite-andesites, the olivine, 

 when present, being quite insufficient to give a character to the rock. They 

 however belong to different species according to the length of the felspar- 

 lathes, which in the doleritic rocks averages '2 mm. and in the other type 

 •08 mm. 



