758 EEPOET— 1893. 



8. Geological Sketch of Central East Africa. By "Walcot Gibson, F.G.S. 



The tract of country described in this paper is situated in Equatorial East 

 Africa. It extends from the coast inland to the N.W. borders of Victoria 

 Nyanza. 



The small island of Mombasa, the starting-point of the expedition, lies fifty 

 miles north of the island of Pemba. A narrow creek, fordable at low water, 

 separates the island from the mainland. 



The sea cliffs are composed of coral rock, which also forms an inland belt 

 about two miles broad, with a general elevation of 50 feet, which sometimes rises 

 to 100 feet. A fringing reef borders the coast. The shore sand consists of 

 comminuted corals and shells mixed with rounded fragments of quartz, orthoclase, 

 garnets, and spUnters of clear blue cyanite. These constituents appear to be 

 derived from a submerged ridge, of which the Seychelles Islands are a remnant. 



The coral rock rests on a sedimentary series consisting of shales, limestones, 

 flaggy sandstones, grits, and conglomerates in descending order. The beds dip 

 gently to the east. They extend inland to the borders of the Taru Plain, a 

 distance of about forty-seven miles. 



The beds are of marine origin, ammonites and ichthyosaurian remains having 

 been found near Rabai and other localities. 



It is impossible to correlate these beds with any occurring in South Africa, 

 but they appear to form a belt running many miles north and south of Mombasa. 



The sedimentary beds rest unconformably on a metamorphic series, consisting 

 of gneisses, schists, and intrusive granites. The strike is N.N.W. and S.S.E., and 

 the dip is generally high. The beds are often intensely folded (Ndange River). 

 Biotite is the commonest mica, and orthoclase the predominant felspar. The 

 schists contain much cyanite, full of iron inclusions. Common garnets are 

 plentiful. Hornblendic rocks are remarkably scarce, the main mass being mica- 

 ceous. Graphite schists occur, and the Bura Hills are largely composed of a 

 crystalline limestone containing scales of graphite. No fossils could be detected. 

 Quartz veins and quartzites are only feebly developed. They form gently un- 

 dulating country or else nearly level plains (Taru, Serengeti) through which low 

 isolated hills of gneiss and granite protrude. 



It is evident that they have suffered enormous denudation. They no doubt 

 represent a complex metamorphosed series of sediments and intrusive rocks, but of 

 what geological age or ages it is impossible to state. 



The intrusive granites are generally pegmatites. Porphyritic granite covers a 

 large area in Kavirondo. Biotite is the essential mica, and a pink orthoclase the 

 predominant felspar. The relation of this large mass of granite to the gneisses 

 and schists could not be ascertained. 



The area covered by granite and metamorphic rocks is enormous. Fully two- 

 thirds of Central East Africa are composed of these rocks. The remaining portion 

 of the countiy, excepting the narrow coast belt of sedimentary rocks, is formed of 

 recent volcanic rocks. 



No traces of the fossiliferous sandstones and shales found by Professor Drummond 

 near Lake Tanganyika, and quite recently by Mr. Joseph Thomson to the west of 

 Lake Nyassa and around Lake Bangweolo, were detected. If further investigation 

 proves their absence from East Africa to be a fact, then we have in the deposits 

 around Lake Tanganyika the most northerly extension of the Karoo beds of South 

 Africa. 



Volcanic rocks form the grandest scenery in East Africa. They occur in two 

 forms, giving rise to two distinct types of scenery. They have either built up 

 tall isolated mountains like Kilimanjaro (19,718 ft.), Kenia (18,000 ft.), Elgon 

 (14,000 ft.), Chibchangani (12,000 ft.), besides numerous other smaller hills, or they 

 are arranged in lines running north and south. The lavas, tuffs, and ashes composing 

 the high central plateaux of Mau, Kamasia, and Lykipia have evidently issued 

 from a north and south fissure. The site of this fissure is now occupied by the 

 chain of lakes commencing with Naivasha on the south, and terminating northward 

 in Lake Baringo. Along this line recent eruptions, some still giving out steam, 



