330 A NATURALIST IN THE PACIFIC chap. 



basic pumice occur in the deposit, sub-aerial eruptions are directly 

 indicated. When an extensive exposure of these tuffs occurs, as 

 in the case of the Nganga-turuturu Cliffs and in that of the section 

 displayed near the hill of Korolevu (page 48), all three agencies 

 are often illustrated. 



Samples of the Submarine Basic Tuffs of Mixed 



Composition 



A. As examples of the non-fossiliferous tuffs where the palagonite 

 constituents do not predominate, I will take those exposed in beds 

 in the coast cliffs and in the low hills in the vicinity of Na Tokalau 

 in the Kumbulau peninsula (page 90). They are grey in colour 

 and have the texture of a sandstone, being more or less compacted 

 and showing no effervescence with an acid. They are composed 

 of fragments of basic rocks and of minerals, varying in size from 

 •5 to 1 mm., in a scanty matrix made up of fine detritus of the 

 same materials and of palagonitic debris. The fragments of 

 volcanic rocks are rounded and sub-angular, and formed mainly of 

 a basaltic rock, with a black opaque groundmass showing some 

 small plagioclase crystals and in places more or less palagonitised. 

 There are also portions of a hemi-crystalline basic rock showing 

 small augite crystals. The minerals entire and in fragments, 

 which make up quite a third of the mass, are mostly of plagio- 

 clase, but monoclinic and rhombic pyroxene are also well repre- 

 sented. It is evident that through the alteration of the palagonitic 

 constituents, which were probably more frequent when the tuffs 

 were deposited, the structure of the matrix is somewhat disguised. 



B. A yellowish grey tuff composing the cliffs on the north coast 

 of Naivaka affords a good example of a tuff where the palagonitic 

 materials predominate. It is somewhat fine-textured and displays 

 a tendency to lamination. The powdered material effervesces 

 slightly with an acid. In the slide it is exhibited as composed 

 mainly of palagonite and of fragments of minerals, the latter 

 making up about one-third of the whole and ranging in size 

 usually between - 2 and "5 mm. A number of more or less parallel 

 fine cracks, filled with calcite and traversing also the inclosed 

 crystals of plagioclase, together with small fragments of basic 

 rocks are displayed in the section. There are a few fragments of 

 semi-vitreous basic rocks, as just indicated, but the palagonite is 

 the principal constituent. It shows numerous minute amygdules 

 occupying the original vacuoles of the basic glass ; and in its 



