144 dk. f. Oswald on the [June 19 14, 



VI. Geology of the Country between the Victoria 

 Nyanza and the Kisii Highlands. 



The gneisses and amphibolites forming the foundation upon which 

 the Miocene beds repose, together with the nepheline-basalts which 

 cap them, have already been described, and a reference has been 

 made to the old augite-andesite that occurs east of Kikongo. 

 This augite-andesite, 1 together with its agglomerate, has a consider- 

 able distribution, extending for 11 miles from near Kikongo east- 

 wards to G-ongogongo, and from Nonnia south-eastwards for at any 

 rate 9 miles and probably much farther. The hills and ridges of 

 this andesite have the same north-westerly and south-easterly strike 

 as the rest of the gneissic area, yet there is no evidence of cata- 

 clastic structure in the rock, although it has been much altered 

 (the augite, in particular, being uralitized), and it is eveiywhere 

 traversed by thin veins of quartz. 



The steep-sided hills of the andesite often display rugged crags 

 and precipices, especially above the Grogo Falls on the Kuja River, 

 in the Kodondo Cliffs, and in the striking hill of Nvakuru 

 (4673 feet) with its twin peaks. This area has evidently under- 

 gone a great amount of denudation, whence we may infer that the 

 rock now exposed was probably rather deep-seated originally. 



An interesting circumstance is the occurrence at Metamala of the 

 andesitic agglomerate weathering into picturesque rugged crags of 

 bare rock, crowded with rounded masses (2 feet or more in diameter) 

 of grey quartz-augite diorite, 2 and also of microgranite, gneiss, 



1 The rock is least altered at Kodondo (5 miles east of Kikongo). Here it is 

 compact and greenish grey, displaying splintery fracture, crowded with tabular 

 idiomorphs (measuring up tD 4 mm.), dusty and saussuritized, of plagio- 

 clase (andesine, Ab 3 An 2 ), and abundant idiomorphs (up to 3 mm.) of pale- 

 green augite (diopside), much uralitized, the uralite in turn altering to 

 chlorite and epidote. The augite is sometimes twinned, and contains apatite ; 

 when adjacent to a quartz -vein it is margined by a secondary brownish-green 

 hornblende. Biotite is represented by a few pseudomorphs of chlorite with 

 magnetite and a little epidote ; originally it was partly resorbed. Magnetite 

 occurs in large grains enclosing apatite, as well as small plagioclases and 

 augites. Apatite and pyrite are accessory. The ground-mass is micro- 

 poecilitic, the texture being coarser round the bigger augites. Thin irregular 

 veins of quartz traverse the rock. 



At Nyakuru (3 miles south-east of Kodondo) the rock is nearly identical ; 

 but the augite has here been entirely uralitized, and the uralite has also been 

 partly altered to chlorite ; some original biotite, however, still remains un- 

 altered, with a resorption-border of magnetite and actinolite. 



At the Ogo Ford on the Kiija River (6 miles east-north-east of Kodondo) 

 the rock is also very similar, but has undergone alteration to a greater degree, 

 the uralitized augite having been replaced by chlorite, or by epidote and 

 calcite, or even entirely by calcite. Yet the plagioclase is fresher than in the 

 previous two specimens. 



2 In this rock plagioclase (oligoclase-andesine, Ab-An 3 ) predomi- 

 nates in tabular crystals (measuring up to 1 cm. in longest diameter), with a 

 strong tendency to idiomorphism : it is occasionally intergrown with ortho- 

 clase — zoning is seldom visible, and is confined to the margin, which is often 

 composed of fresher felspar ; it is sometimes partly intergrown with quartz 

 peripherally, and contains chlorite along cracks, but is not so decomposed as 



