VII MBENUTHA CLIFFS 



109 



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 I 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 

 :ontain 15 or 16 per cent of lime. They inclose pteropod shells in 

 places and show many minute foraminiferous tests of the pelagic 

 :ype. Their composition is given on page 323 ; but it may be 

 lere remarked that the residue is made up mainly of palagonitic 

 iebris, fine clayey material and minerals. The mineral fragments 

 brm about 20 per cent of the mass, and consist principally of 

 jlassy plagioclase, with some rhombic pyroxene, and magnetite, 

 ;heir size averaging -i mm. The tuff-sandstones interstratified with 

 he clays contain only 2 or 3 per cent of lime, and show only a few 



