These form a group of hard compact rocks, the fragmental character of which is not always apparent in hand-specimens, microscopical examination of thin sections being usually required for the determination of their true nature. They are commonly exposed on the southern flanks of the Korotini Range at the back of Vatu-kawa and Nukumbolo. They are composed of compacted fragments, varying in different localities from 1 to 5 or 6 mm. in size, of a variety of semi-vitreous basic rocks, the matrix being scanty but often containing zeolites and secondary silica, whilst occasionally secondary calcite is developed. They contain no organic remains, and palagonite when present is usually scanty, whilst viridite and similar materials represent the decomposition of the pyroxene.... A hard breccia-tuff found on the flanks of Mariko and in one or two other localities contains vesicular fragments, where the steam-holes are filled with opal or chalcedony, and the cracks traversing the matrix are also filled with this mineral.
The alteration of these tuffs has evidently arisen from a variety of causes. In some cases the change appears to be purely interstitial. In other times it has arisen from contact-metamorphism, or from hydro-thermal agencies, as in the case of the altered tuffs near the hot springs at Nukumbolo (p. [161]).
Submarine Basic Pumice Tuffs
These deposits, which, however, are not of frequent occurrence, are interstratified with volcanic mud-rocks in certain localities, as at the Mbenutha Cliffs (p. [110]), and in the vicinity of the hill of Korolevu (p. [47]). They indicate periods of volcanic activity during the deposition of the foraminiferous muds, with which they are associated, when the fine materials ejected from sub-aerial vents fell into the seas around.
Such tuffs are more or less compact and usually fine in texture. When the glass fragments are but slightly altered, the tuff-rock is dark grey; but when the palagonitic change is well advanced, it becomes pale and yellowish. They are made up chiefly of small fragments of a bottle-green basic glass, which are as a rule vacuolar and sometimes fibrillar; but it never happens that the pumiceous character is as pronounced as in acid pumice; and in some cases the vacuoles or steam-pores are to be observed only in the minority of the fragments displayed in a slide. The size of the glass fragments is as a rule small, in some tuffs averaging only ·1 mm. and in others ·5 mm.; but occasionally they may be 1 or 2 mm. in diameter.
Fragments of minerals (plagioclase and pyroxene) corresponding in size to the glass fragments are as a rule well represented, forming a fourth or a third of the mass. A little fine detritus of a semi-vitreous basic rock also occurs. Tested in an acid several of the tuffs either do not effervesce or give an indication of a small percentage of carbonate of lime; whilst others effervesce freely. They usually display a few minute tests of foraminifera of the “Globigerina” type, the cavities of which are filled with fine palagonitic debris.
The palagonitic alteration is to be noticed in all cases; but it varies considerably in its extent. In the dark grey tuffs it affects the margins only of a few of the glass-fragments. In the paler tuffs it has extended more into their substance, although the alteration is never more than partial. The pale greyish material filling up the interspaces is composed of disintegrated palagonite. The steam-pores or vacuoles are sometimes empty; and at other times, especially where the palagonitic change has begun, they are filled with a granular alteration product. The glass fuses readily; but is not affected by acids. It is clear and isotropic, showing however a few scattered microliths. These tuffs correspond with the hyalomelan-tuff from the island of Munia in this group as described by Wichmann; but in that instance no mention is made of inclosed tests of foraminifera.
“Crush-Tuffs” formed of Basic Glass
This is a remarkable group of compacted tuff-like rocks which as hand-specimens would be generally regarded as pitchstone-tuffs. Their detrital origin is, however, often very doubtful. They are composed of fragments of basic glass, carrying plagioclase phenocrysts, with the interspaces occupied by palagonite and by the finer debris of the glass and felspar. The larger glass fragments, which vary in different rocks from 1 or 2 to 4 or 5 mm. in size, have been crushed in situ, the broken portions often remaining more or less in position. These fragments are invested by palagonite and have eroded borders, as shown in the figure on page [342]. The glass is bottle-green, non-vacuolar, fuses readily, and only at times displays incipient crystallisation. The explanation of the origin of these rocks is attempted in [Chapter XXIV.] They contain neither carbonate of lime nor organic remains. The most typical example is present in a bed underlying a pitchstone-agglomerate near Narengali (see page [149]). It is not uncommon to find evidence of crushing in the glassy matrix of a pitchstone-agglomerate or of rubbly pitchstone, as in the Va Lili Ridge (142), the Korotini Bluff (157), and Mount Soloa Levu (313); and here also palagonite has been produced around the crushed fragments.