Mt. TAVIA (2,210 feet) from VATU KAISIA. It is probably formed of an acid andesite.
The magnetic peak of NAVUNINGUMU (1,931 feet) from the south. The summit represents a basaltic neck.
[Face p. 108.
The peak itself is formed of a dark-brown slightly vesicular semi-vitreous basaltic andesite, of which, in fact, for the upper 200 feet, the summit is composed. The rock is somewhat rubbly; and where it is exposed on the bare peak it is powerfully magnetic, displaying polarity in a marked degree, and rendering the compass useless (see page [368]). A specimen of the magnetic rock, which is a little vesicular, has a specific gravity of 2·82. It is referred to genus 1 of the augite-andesites described on page [267]. It displays in the slide porphyritic plagioclase, with a little augite, in a groundmass formed of a plexus of minute felspar-lathes (·06 mm. in length), and exhibiting a large amount of a brown opaque glass in which grains and rods of magnetite with a few pyroxene granules are developed. The magnetite in the groundmass, although abundant, is not in greater quantity than is usually found in semi-vitreous basaltic rocks without polarity.... This terminal mass of basic lava-rock evidently forms the “plug” of a volcanic pipe that pierces the acid andesitic rocks of the district; and from this ancient vent were doubtless ejected the basic tuffs and agglomerates that now cover the lower slopes of the mountain.
The conditions under which this volcano displayed its activity are further illustrated in a remarkable section exhibited on the east side of the mountain half a mile or more north of the summit. Here there is a line of bold cliffs, in which, as shown in the illustration, a bed of agglomerate, 60 or 70 feet thick, overlies a series of foraminiferous clays and tufaceous sandstones, which are elevated about 1,100 feet above the sea. The locality is named “Mbenu-tha” or “Rubbish-heap.” It is well known to the natives on account of its caves, which serve as a half-way resting-place on the road from Nambuna to Ndreketi. These caves have been produced by the more rapid weathering of the underlying clays and sandstones. The line of cliff extends northward to Mumu, the peak at that end of the range, and preserves there the same structure. The clays and tuff-sandstones are more or less stratified, and dip generally to the west or south-west at an angle perhaps of 20 degrees; but in more than one place they show signs of great disturbance, being contorted and steeply tilted.
The foraminiferous clays form a more or less compact rock and contain 15 or 16 per cent. of lime. They inclose pteropod shells in places and show many minute foraminiferous tests of the pelagic type. Their composition is given on page [323]; but it may be here remarked that the residue is made up mainly of palagonitic debris, fine clayey material and minerals. The mineral fragments form about 20 per cent. of the mass, and consist principally of glassy plagioclase, with some rhombic pyroxene, and magnetite, their size averaging ·1 mm. The tuff-sandstones interstratified with the clays contain only 2 or 3 per cent. of lime, and show only a few scattered microscopic tests of foraminifera. About two-thirds of the rock consist of fragments of a bottle green basic glass, vacuolar and but little altered, the rest being composed chiefly of glass debris, plagioclase, and a little pyroxene, the larger mineral and glass fragments averaging ·3 to ·5 mm. in size. They are in fact submarine hyalomelane tuffs very similar to those first met with at the foot of the mountain, which are referred to on page [108]. (They are described on page [333].)
These interbedded clays and tufaceous sandstones of the Mbenu-tha cliffs were deposited under somewhat different conditions. The clays represent the quiet deposition in fairly deep water of fine materials derived from the degradation of acid andesites as well as of basic rocks. The hyalomelane tuff-sandstones were formed more rapidly by the accumulation of fine volcanic ash consisting of fragments of a basic glass ejected from some neighbouring volcano that rose above the sea-surface.
Submarine hyalomelane-tuffs with basic agglomerates appear to be of common occurrence around the base of the Navuningumu mountain. As we leave the range behind and begin to descend the long spur that slopes northward to Ndreketi, we find for the first mile or two these agglomerates. But where the deeper rocks are exposed at an elevation of 600 feet, near the village of Singa-singa, there are displayed fine basic pumiceous tuffs and compact palagonitised clays containing little if any lime, the last, however, containing a few casts of microscopic foraminifera. The tuff is made up of minute fragments, the largest less than ·1 mm. in size, of a basic hyalomelane glass, which is vacuolar, and often fibrillar like ordinary pumice, and in places shows the early stage of alteration into palagonite. The clay principally consists of more or less palagonitised debris of the same basic glass, together with minute fragments of plagioclase and rhombic pyroxene. These tuffs and clays represent the two conditions of deposition above referred to, the last indicating a period of quiescence when the fine materials resulting from the degradation of both acid and basic andesites were slowly accumulating in deep water, the first denoting the activity of a neighbouring supra-marine vent from which fine dust and ash formed of basic pumice were ejected.