Vol. 70.] MIOCENE OF THE VICTOKIA NYANZA, ETC. 141 



uniform texture and composition, it is, on the contrary, extremely 

 variable, exactly like other nepheline-lavas. The most typical 

 variety is a specimen 1 which I collected south of Homa Bay, on 

 the Rungwena Plateau, belonging to the Ruri system. It presents 

 many points of similarity to the nepheline-basalt of Fogo in the 

 Cape Verde Islands. 



The regularity of the slope from the central mass, right down 

 to the last lava-streams at Nira and Kikongo is very distinct, when 

 viewed from the lake or from the Kuja Plain. In several cases, it 

 is clear that many of the now isolated hills that rise above the 

 Bay of Karungu have been carved out of the plateau by denu- 

 dation, whether they are flat-topped hills such as Omangi (4532 feet) 

 or conical peaks such as Tigra (5031 feet) and Okai (4857 feet), 

 which still show a lava -flow in section capping their actual 

 summits. The lava 2 on Nira Hill, for instance, was clearly once 

 in continuity, not with Nundowat on the east, but with flat- topped 

 Omangi on the north-north-east, but it is now separated by a wide 

 and deep valley. On the other hand, it is also evident that there 

 were several subsidiary points of eruption, even so far south as the 

 peak of Nundowat (4479 feet), almost at the southern limit of 

 the plateau, overlooking the Miocene deposits. It is a boss of huge 



1 The black compact rock, with irregular fracture, contains numerous crystals. 

 of pale-green augite (diopside) measuring up to 11 mm. in longest diameter, 

 with frequent twinning, both simple, lamellar, and herring-bone, often zoned, 

 and sometimes bordered by augite of a darker green ; occasionally enclosing 

 magnetite -grains, a nepheline-crystal, or some brown glass full of microlites. 

 A few rather small crystals of olivine have been serpentinized and stained 

 yellow by iron oxide, and in several cases are surrounded by a wreath of 

 green augite and magnetite-granules. Nepheline is present in large fresh 

 idiomorphs, enclosing augite-needles and magnetite, as well as in small quad- 

 ratic crystals. Magnetite is abundant in large hexagonal crystals, down 

 to quite small dimensions. Apatite is fairly frequent, enclosing numerous, 

 minute, purple -black, longitudinal rods. The ground -mass consists of a 

 close felt of augite- and magnetite-granules, with occasional clear streaks of 

 allotriomorphic nepheline, into which larger needles of green augite project. 



2 This rock is very similar to the Rungwena basalt, ,and contains similar 

 augites but no porphyritic nepheline ; and, since no olivine is present, it would 

 be properly termed a nephelinite rather than a nepheline-basalt. On the 

 other hand, it contains some pale-brown, nearly resorbed hornblende- 

 crystals, sometimes surrounded by a wreath of augites with occasional small 

 flakes of biotite. In the clear streaks of allotriomorphic nepheline in the 

 ground-mass occur some instances where allotriomorphic sanidine seems 

 also to be present ; but the twinning is much less sharp than usual. 



C. Uhlig collected some specimens of the lava 1600 yards inland from 

 Karungu, and here, according to the description by M. Groldschlag (' Beitriige 

 zur Kenntnis der Geologie & Petrographie Ostafrikas I.' Centralblatt f. Min. 

 1912, pp. 567 & 593), it proved to be nephelinite, since no olivine was present ; 

 it contained allotriomorphic nepheline, like my specimens collected a little 

 farther south. Uhlig's three specimens were taken all close one to the other, 

 but showed considerable variety : in one case nepheline occurs mostly in the 

 ground-mass, and is only occasionally idiomorphic ; the proportion of nepheline- 

 crystals increases in the second specimen ; and, in the third specimen, the 

 nepheline is almost exclusively in the form of idiomorphs, while the ground- 

 mass contains some glass in this specimen alone. 



