Vol. 70.] MIOCENE OF THE TICTORIA STANZA, ETC. 147 



of the Gongogongo Range a broad zone of amphibolite l extends 

 for 5 miles through the district of Sakwa down to the Kuja River. 

 Exposures are extremely rare, for the rock is greatly altered, and 

 weathers to a considerable thickness of red-brown clay. Wide 

 valleys opening out to the north-west alternate with steep grassy 

 ridges. 



North of the Kuja extends for many miles the wide plateau of 

 Kainagambo, composed of a granitic horn blende -gneiss. 2 Its 

 essentially level surface, and the fact that the Kuja has carved 

 deep meanders into it, coupled with the presence of old river-gravels 

 containing big quartzite -pebbles derived from the Kisii Highlands, 

 serve to indicate that this plateau is essentially an old peneplain in 

 which the Kuja has cut down its meanders to 300 feet below its 

 surface, owing to the rejuvenation of its course, to which further 

 reference will be made later on. The formation of the rjeneplain 

 must have taken place at the time ay hen the lake stood at the 

 4000-foot level, but slightly beloYV the le\-el of this gneiss plateau. 



I left the district of Sakwa, and entered that of Kamagambo by 

 a wooden bridge which has been constructed bY" native labour, 

 replacing a ford immediately above some picturesque waterfalls at 

 4347 feet. Rapids are characteristic for several miles downwards 

 from this part of the river's course. The gneiss weathers readily 

 and deeply into pale yellow-brown or orange sands, or else into 

 greenish- white clays ; and the contrast is striking, betYveen the 

 gentle slope of the gneiss on the right bank and the steeper 

 inclination of the more resistant amphibolite on the left. 



Before one reaches the top of the grassy gneiss plateau a black 

 peaty soil is noticeable in several places ; but, on the actual summit 



1 The rock near the junction with the biotite-gneiss is a green, extremely 

 close-grained zoisite -amphibolite, consisting of a very fine aggregate of 

 granular zoisite, clinozoisite, and abundant bladed green hornblende in a 

 quartzose matrix, with a few granules of magnetite. Some indistinct 

 pseudomorphs after primary hornblende consist of secondary hornblende and 

 zoisite, with magnetite-granules and some chlorite enclosing calcite. Clearer 

 areas, consisting of aggregates of quartz, zoisite, and flakelets of white mica, 

 may possibly represent original felspars. Chlorite enclosing calcite in 

 lamellae is probably after biotite. Calcite is diffused through the slide, and 

 a Little epidote is present. The original rock may have been a not very 

 basic, rather finely crystalline basalt. 



2 A gneissose banding is suggested by successive wavy zones of quartz 

 alternating with felspar, and by the hornblendes lying with their long axes 

 parallel to these zones. Orthoclase predominates in allotriomorphic crystals 

 (measuring up to 5 mm. in longest diameter), turbid with dusty decomposition- 

 products and flakelets of white mica ; it is altered in patches to clear albite, 

 especially marginally, where it is sometimes brecciated. Green hornblende 

 (measuring up to 2 mm. in longest diameter) is both primary and secondary ; 

 the primary is pale green, somewhat bent, containing apatite and magnetite, 

 and alters along the cleavages into a yellow serpentinous mineral with some 

 epidote. The secondary hornblende occurs only in association with quartz : 

 it is fresh and unaltered, and sometimes twinned (a, pale straw ; j3, sap-green ; 

 y, blue-green). Quartz is abundant as a fine granulitic mosaic, showing 

 undulose extinction. Leucoxene is present in rather big dusky grains, with 

 their long axes parallel to the planes of schistosity. Magnetite is accessory 

 in crushed and drawn-out grains. 



