156 dr. f. Oswald ox the [June 1 9 14, 



this consists of a flow of phonolite, 1 overlying a cellular, yellowish 

 earthy, calcareous tuff, much veined with calcite, and containing 

 fragments of nepheline-crystals, a little biotite, some magnetite- 

 granules, and a little zeolite. It is somewhat reddened and baked 

 near the plane of contact. 



Between the Northern Awach and the Kavirondo Grulf at Kendu 

 is a hill of brown clay, its low slopes measuring no more than 5°. 

 As we reach the summit, a crater-lake, Lake Simbi (PL XXIV), 

 is suddenly disclosed to view, slightly elliptical in outline, the long 

 axis running west-north-west and east-south-east and measuring 

 about three-quarters of a mile, while the width of the lake is a 

 little more than half of this. The river rises to between 70 and 

 100 feet above the surface of the lake, which stands somewhat 

 below the level of the plain of the Awach on the south, but 

 slightly above that of the JSTyanza on the north. 



The solid rock only appears in two places on the south-western 

 shore, and consists of the micaceous sandstone which I subse- 

 quently found well developed along the coast west of the Awach. 

 Elsewhere, the rim of the lake is composed of a brown clay dipping 

 everywhere away from the centre of the lake at an angle of 30° to 

 40°. In places the clay contains blocks of gneiss (which occurs 

 in situ only a couple of miles away to the east) and pebbles of 

 quartz, but in no case were there any fragments of lava. On the 

 edge of the eastern rim I found a block of travertinous limestone, 

 measuring 3 x li feet, containing a fragment of bone, and clearly 

 belonging to the same series as the Pliocene beds west of the Awach 

 (see below, p. 158). This block was indurated, and presented the 

 appearance of having been subjected to heat. The outward dip of 

 the clay of the lake-margin conveyed the impression that Lake 

 Simbi marks the site of an explosion-crater similar to those of the 

 Eifel. Probably it was formed within the memory of man, for 

 the natives told rne of a legend that its site was once occupied by 

 a hill crowned by a homestead belonging to a very wicked chief, 

 and that it was destroyed and disappeared in a single night. The 

 water of the lake is yellow-green, due to the presence of confervae, 

 and jelly-like lumps of an emerald-green JSfostoc occur along the 

 margin. It is strongly alkaline and also bitter, with the fishy taste 

 of carbonate of soda. 3 



1 The rock is pale grey, compact, and fine -grained, exhibiting irregular 

 f racture and a honeycombed surface, with some indications of lines of flow ; 

 occasional white felspars (up to 4 mm.) are visible. Under the microscope the 

 felspar (sanidine) occurs in a few large phenocrysts with occasional Carlsbad 

 twinning, but it has been much altered to a soda-zeolite. The ground-mass 

 consists of a very fine-grained felt of abundant small nepheline-crystals, 

 sanidine-microlites and gramdes, and needles of very pale-green augite 

 in mossy aggregates and patches stained brown by iron-oxide, in a glassy 

 base. 



- Prof F. Stanley Kipping, F.R.S., has kindly examined my sample of the 

 water, and informs me that, while sodium carbonate is the main component, 

 iron salts, together with sulphates and silica, are present in appreciable 

 quantities : smaller quantities of calcium and magnesium salts and phosphates 

 are also present. 



