152 DE. F. OSWALD OX THE [June I914,. 



of the Megusi River, and is composed of the same sill of dolerite 

 as that which occurs at the base of the Vinyo Escarpment. As 

 one approaches the cliffs, about 700 feet high, of the Manga 

 Escarpment, it is soon evident that the}' consist of the same 

 current-bedded quartzitic sandstone as at Vinyo, much stained 

 and blotched with crimson haematite, and frequently displaying 

 ripple-marks and impressions of raindrops, as also an occasional 

 streak of quartz -pebbles. A close search for fossils was equally 

 fruitless. Here, however, the dip is 12° south-south-eastwards, 

 while at Vinyo it was 10° west- south -westwards : from my 

 lofty standpoint (6388 feet) it certainly seemed to me that the 

 intervening strata had been raised into a low dome by the intru- 

 sive dolerite. For this reason, the intrusion may, perhaps, be 

 regarded rather as a laccolite of low curvature than as a sill. The 

 haematitic staining seems to have proceeded from below, rising up 

 the vertical joints and extending along the bedding-planes : it 

 was, not improbably, due to pneumatolytic action accompanying 

 the intrusion of the dolerite. Wherever the iron-oxide is most 

 evident the weathering is greatest, and even swallow-holes (6 feet 

 in diameter) occur near the edge of the precipitous escarpment. 



The view to the south and east shows that the Kisii Highlands 

 form an extensive plateau of quartzitic sandstone, greatly dissected 

 by the Kuja and its tributaries flowing southwards, as well as by 

 the Northern Awach (the Awach Mateni) flowing northwards. 

 The plateau culminates in the distant heights of Chamonyeru 

 (7068 feet) in the east. 



The edge of the escarpment is itself a watershed between the 

 basins of the Kuja and the Megusi (a tributary of the Southern 

 Awach or Awach Madoung). The eastern streams which drain the 

 escarpment and flow to the Kuja are very chalybeate, deriving 

 the iron from the haematite of the quartzite. 



After leaving Kisii Boma on my march west-north- westwards 

 to Homa Bay, I left the dolerite behind at Nyachwa, and entered 

 again on the peneplain of biotite-gneiss which extends across the 

 deep Biana Valley, rising northwards in the rounded bluff of 

 Meriba and stretching up to the Kona Plateau and watershed, 

 whence the rivers run directly northwards to the Kavirondo Gulf. 

 This part of the old plateau, 12| miles from Kisii, is composed 

 of a crushed, pale brownish-grey porphyry, 1 showing similarity 

 with the Vinyo quartz-porphyry. It weathers readily to a yellow- 



1 The rock consists of a dusky mosaic of quartz -grains, with interstitial 

 sericite, as in the Vinyo rock ; and, although the spherulitic structure is not 

 so well marked, some instances can still be made out containing a nucleus 

 of sericite flakelets, but there has been considerable recrystallization : all 

 the felspathic constituent of the spherulites and ground-mass seems to have 

 been altered into quartz and sericitic mica. No phenocrysts are present, 

 but there are numerous small mossy aggregates of limonite. The rock has 

 been much fractured and veined by a mosaic of clear interlocking quartz- 

 granules ; and, where the crack has fractured one of the dusky quartz-grains, 

 the clear quartz of the vein is in optical continuity with it. 



