140 de. e. Oswald on the [June 1 9 14, 



once islands when the lake stood at a higher level. Still farther 

 south the bays of Gurekeri and Mohuru are just as obviously 

 drowned valleys, and this characteristic becomes increasingly pro- 

 nounced in the gulfs farther southwards as far as Mwanza. But 

 the lake at the present day is falling in level, and therefore 

 these drowned valleys are no longer so deepry submerged as they 

 once were. 



This gneiss 1 weathers into large bare blocks, usually rounded, 

 but sometimes angular, often longer than broad, and vividly recalling 

 the granite-tors of Devon and Cornwall. Both in this and in the 

 other instances of granitic gneiss which I observed in the Nyanza 

 province, the gneiss is so much fresher than the schists, that it is 

 probable that they are of later date, as in Rhodesia, and that they 

 have undergone only a slight amount of pressure. Most of the 

 occurrences present the appearance of having originally been 

 intrusive granites. . 



V. The Basalt-Plateau oe Gwasi. 



The Miocene beds are capped by the terminal lava-flows belong- 

 ing to the basaltic system of Gwasi (6384 feet), the central peak 

 of which rises 17 miles north of the southern tongues of the lava- 

 streams that have buried up the sediments. On sailing round this 

 massif of Gwasi, from the Kavirondo Gulf to Karungu, I was 

 struck by the resemblance of the scenery to that of the basalt- 

 plateau of Skye and the Western Highlands, the lofty cliffs con- 

 sisting of flow upon flow of columnar black basalt like Ben More in 

 Mull; while the flat- topped outliers, now detached by extensive 

 denudation, are the exact counterparts of Macleod's Tables in Skye. 

 The outlvin^ islands of Mfwansranu on the north-west and of the 

 symmetrical pyramid of Xagodaluru on the south-west once (as 

 shown by the soundings) stood in connexion with the main ma>>. 

 The neighbouring systems of Gembe (6208 feet) on the north, of 

 Ruri (5583 feet) with its many cones on the north-east, and even 

 of Homa (5742 feet) still farther north-east, consist of the same 

 nepheline-basalt or nephelinite, and form a geological unity. 



Although the basalt, at a superficial glance, appears to be of very 



1 The felspar and quartz prevail over the ferromagnesian constituents - r 

 the orthoclase allotriomorphs (up to 5 mm. in length) are often turbid 

 centrally with decomposition-products, and enclose an occasional granule of 

 kyanite and zircon — the cleared margin often shows undulose extinction, with 

 patches of fresh microeline developing at its expense. Plagioclase- 

 (oligoclase-andesine, Ab-An 3 ) is quite subordinate. Quartz occurs in granu- 

 litic mosaics, with rough parallelism bordering the orthoclase-crystals, and 

 here sometimes shows a myrmekitic interg-rowth. Green hornblende is- 

 present in larger crystals (up to 4 mm. in length) than the biotite with which 

 it is associated, but is not so frequent, and shows the usual pleochroism 

 (a, pale straw; jS, sap-green; y, blue-green). Biotite occurs in greenish 

 flakes, enclosing apatite and small zircons with pleochroic halves. Sphene 

 is accessory in pale-brown, rather large crystals, sometimes bordering the 

 rare grains of magnetite; and a few granules of pyrite are present. 



