Vol. 70.] MIOCENE OF THE VICTORIA NTANZA, ETC. 151 



o£ the quartzite-plateau (over 6000 feet high) the view to the 

 westward over Kitembe shows its flat-topped outliers above the 

 gneissic peneplain at their foot, and the west-south-western slope 

 of their surface is a dip-slope. Turning thence north-westwards, 

 I left the Kuja basin and descended to the Yawi River (a tributary 

 of the Kiana), over white quartzite down to the amygdaloidal 

 dolerite, here exposed over a wide surface ; and the symmetrical 

 cone of Saria marks another apophysis, with an aureole of white 

 quartzite, similar to the occurrence of the Kinsunsi cone. From 

 Saria a steep descent to the Nyanchoba led to the basement-floor 

 of pink granitic biotite-gneiss l below the dolerite: it 

 decomposes to a red-brown soil. 



Crossing a bastion of the doleritic plateau of Merinde, I descended 

 again to the biotite-gneiss, traversing a wide valley (tributary to 

 the Kiana). On its northern slope, just at the western foot of 

 Xvachwa, the gneiss is directly overlain by the dolerite, which 

 shows massive jointing and no signs of flow or columnar structure. 

 This doleritic spur of Nyachwa forms the left side of the valley of 

 the Riana, with its gold-bearing sands. Kisii Boma, the present 

 centre of administration, lies on the lower northern slopes of 

 Nyachwa, at the height of 5705 feet. On the summit, I noticed 

 firmly embedded in the dolerite a lump of white quartzite (12 x 6 

 inches) : this had doubtless been floated off by the intrusive 

 dolerite from its overlying cover of quartzite, which has been 

 long ago denuded away. Probably the dolerite never reached the 

 surface when molten, but insinuated itself as a sill beneath the 

 ancient sandstones, which must have extended far to the west and 

 north beyond their present limits. 



During the two days which I had to spend at Kisii Boma before 

 returning to the lake-shores, I marched across to the Manga 

 Escarpment, which stretches in commanding cliffs far to the north- 

 north-east (PL XXIII, fig. 2). The lower ground at its base 

 comprises the thickly-populated district of Kitutu in the basin 



1 The rock encloses numerous allotriomorphic crystals (measuring up to 7 mm. 

 in longest diameter) of microcline-microperthite with perthitic net- 

 work and Carlsbad twinning, quite fresh and slightly dusty, and often outlined 

 with haematitic dust ; occasional inclusions of biotite, plagioclase, and quartz- 

 granules, and sometimes thin veins and blebs of green hornblende. Plagio- 

 clase (andesine, AbsAna) is also present in smaller crystals (up to 4 mm.), 

 with a tendency to idiomorphism ; often it is turbid centrally with flakelets 

 of white mica, zoisite, epidote, and calcite, and occasional patches and thin 

 veins of secondary hornblende, but showing cleared margins of secondary 

 felspar in optical continuity — sometimes enclosing quartz-granules. Biotite 

 (up to 2 mm.) is fairly abundant, often greenish, enclosing apatite and zircon, 

 and associated with magnetite; it is altering to chlorite, epidote, and 

 lenticidar aggregates of carbonates — in one case a sagenite-web is evident. A 

 few prisms of brown hornblende are present, on which biotite is moulded. 

 Magnetite and sphene are also accessory, and a small grain of kyanite was 

 observed. Quartz is rather abundant and interstitial, mostly in large grains 

 containing strings of minute fluid inclusions ; it is occasionally brecciated 

 marginally, but shows only a slight degree of undulose extinction ; sometimes 

 it encloses isolated flakelets of white mica. 



Q. J. G. S. No. 278. m 



