XII THE SOKENA RIDGE 169 



angular and rounded fragments, up to a centimetre in size, of the 

 same andesites, the interstitial material being formed of fine 

 detritus and palagonitic debris. 



Though the agglomerates of the peak of Koro-mbasanga are 

 composed of a variety of rocks, all the rocks are to be referred to 

 the pyroxene-andesites with specific gravity below 2*8 but above 

 27. They are therefore less basic than the olivine-basalts and 

 basaltic andesites of the Koro-tini range, where the density is 

 usually 2*8 and over. Their variation, however, is more consistent 

 with the characters of an agglomerate formed by marine erosion. 

 The same may be said of the sorting of the blocks according to 

 their size and of the occasional occurrence in the matrix of small 

 rounded pebbles. That these deposits of agglomerates were 

 formed under the sea is indicated also by their overlying submarine 

 sedimentary tuffs near the base of the mountain. 



(2) The Sokena Ridge. — To the west of Koro-mbasanga, 

 and forming a spur of the same range, is the flat-topped hill of 

 Sokena, which rises about 1,100 feet above the country at its base 

 and about 1,600 feet above the sea. From a distance it has the 

 appearance of being formed in its higher portion of nearly hori- 

 zontal strata dipping gently northward. In its upper part it 

 terminates in a line of cliffs about 200 feet in height, and there is a 

 similar line of cliffs lower down the slopes. These cliffs are com- 

 posed of bedded fine and coarse non-calcareous tuffs, dipping about 

 10° N.N.W., in which are imbedded without any arrangement 

 blocks, ranging in size from 2 or 3 inches to 3 or 4 feet, of a 

 remarkable blackish pitchstone-like rock displaying opaque plagio- 

 clase phenocrysts. It is referred to genus 18 (see page 289) of 

 the hypersthene-augite andesites, both rhombic and monoclinic 

 pyroxene being represented in the phenocrysts and in the ground- 

 mass where they take the form of minute prisms ('03 mm.). There 

 is a considerable amount of pale brown glass. A rock very similar 

 occurs in the Thambeyu agglomerates (see page 178). The tuffs 

 are formed largely of palagonitic materials, the angular fragments 

 in the coarser beds being | to 2 centimetres in size, the palagonite 

 being often vacuolar but much affected by hydration. 



These tuffs and agglomerates of the Sokena cliffs apparently 

 :ontain no organic remains. They appear to have accumulated 

 jnder water as the result of the eruptions of a neighbouring vent 

 A^ithout the intermediate agency of marine erosion. 



(3) The Ascent of the Lovo Valley. — About two miles 

 :o the east of the peak of Koro-mbasanga the picturesque Lovo 



