no A NATURALIST IN THE PACIFIC chap. 



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 con- 

 ditions. The clays represent the quiet deposition in fairly deep 

 water of fine materials derived from the degradation of acid ande- 

 sites 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 'i 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 altera- 

 tion 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. 



The bed of agglomerate, 60 to 70 feet thick, which overlies the 

 foraminiferous tuffs and clays exposed in the line of cliff extending 

 from Mbenu-tha to Mumu, is made up of subangular blocks, not 

 usually over 6 inches in diameter, of an acid andesite of the general 

 type found in the Ndrandramea region, but possessing a semi- 



