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.