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 plagioclase, but monoclinic and rhombic pyroxene are also well represented. 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 substance occur irregular patches formed of a colourless semi-isotropic mineral which is either zeolitic or a form of opal. Plagioclase and augite compose the mineral fragments, the former prevailing. Although these tuffs are derived from a vent that was probably the last in eruption in this island, they display considerable alteration which is mainly connected with the secondary changes affecting the palagonite since the deposition of these materials.
C. As an example of the banded tuffs composed of coarse and fine materials I will take a compact grey rock forming one of the horizontal beds in the natural section exposed near the hill of Korolevu (p. [49]). It is also an example of those tuffs which whilst not effervescing with an acid display a few casts of foraminifera in the slide. The alternating bands which are about a centimetre in thickness pass gradually into each other. The bands of finer materials are made up of sub-angular fragments, ·1 to ·2 mm. in size, of the dark opaque groundmass of a semi-vitreous basic rock and of a grey hemi-crystalline groundmass of an augite-andesite, together with palagonite more or less decomposed, and fragments of plagioclase and augite, whilst the interspaces are filled with the finer debris. The layers of coarse materials have much the same composition, the fragments varying usually between ·5 and 1 mm. in diameter, with occasional larger pieces of palagonite 2 to 5 mm. in size representing original lapilli of a vacuolar basic glass. The tests of the foraminifera, which occur scantily in the layers of fine material, are all minute. They are filled with palagonitic material.
D. The tuffs prevailing on the higher flanks of the mountainous backbone of the island are well represented by those exposed at an elevation of 1,200 feet above the sea on the south slope of the Korotini Range behind Mbale-mbale. It is a somewhat coarse-grained rather hard grey rock effervescing feebly with an acid. It is composed of sub-angular or partly rounded fragments, 1 to 4 mm. in size, of various basic rocks, and of rather smaller fragments of plagioclase and augite, the interstices being filled up with fine debris of the same materials, in which a few minute tests of foraminifera of the “Globigerina” type may be observed. The basic rocks of which the fragments are formed comprise the following: (a) a grey aphanitic augite-andesite, with but little interstitial glass, presenting a parallel arrangement of the minute felspar-lathes which have an average length of ·05 mm.; (b) a grey augite-andesite of coarser texture but in other respects similar, the felspar-lathes being about ·1 mm. in length, whilst there is a little microporphyritic plagioclase; (c) a semi-vitreous basaltic rock showing small porphyritic crystals of plagioclase and augite in a groundmass usually black and opaque, but sometimes smoky and displaying felspar microliths; (d) a vacuolar basic glass more or less palagonitised.
E. As a specimen of the calcareous tuffs those exposed on the south slope of the Korotini Range at an elevation of 1,850 feet may be given. They contain about 11 per cent. of carbonate of lime and inclose a few tests of foraminifera 1 to 2 mm. in diameter. The other constituents are fragments of semi-vitreous basic rocks and of palagonite, together with fragments of plagioclase and pyroxene crystals and of an amorphous siliceous mineral which behaves optically like chalcedonic silica. When the rock is gently rubbed down, minute fragments of this white mineral can be picked out. They have a wrinkled surface and an irregular form and are not affected by acids. In polarised light they display a rude mosaic or an imperfect radiate structure.
F. As specimens of the fossiliferous agglomerate-tuffs composed mainly of palagonite, those exposed on the high mountain slopes of the Korotini Range at heights of 2,000 feet may be here cited. They are described on p. [154].
Note.—The examples of mixed tuffs above given represent only some of the principal types of these deposits. Short descriptions of others will be found in the detailed account of the geology of the island.