Mount Elgon

More than half of Mount Elgon lies in Uganda, and a fuller description of the mountain will be found under Uganda No. 170-00-024. Elgon is the fourth heighest mountain in East Africa rising to 4245 m with the upper slopes being heavily glaciated, as described in some detail by Searle (1952). It is extensive, the diameter approaching 90 km. Searle (1952) divided the succession in the Kenya section of the volcano into (a) Lower Pyroclastic Series and Lake Beds, (b) Middle Pyroclastic Series, (c) Lower Lava Series, (d) Series of the Caldera Rim and (e) Minor intrusions. The Lower and Middle Pyroclastic Series compose about 90% of the mapped area and comprise agglomerates, breccias and tuffs the last consisting of glass, altered nepheline, aegirine and aegirine-augite, iron oxides and much carbonate. The lava blocks of the agglomerates include nephelinites and melilitites with some oligoclase basalts. The lavas of the Lower Series range from melanephelinites through melilite-bearing nephelinites and melanephelinites to melilitites; no nephelinites, sensu stricto, are found on the eastern side of the mountain. The rock-forming minerals of the melanephelinites are pyroxene (augite, sometimes zoned to aegirine-augite), nepheline, magnetite, sparse perovskite and, in some rocks, biotite; olivine-free and olivine-bearing varieties were distinguished by Searle (1952). The melilite-bearing rocks generally also contain olivine and typically include phenocrysts of olivine and pyroxene up to 2 cm and 1.5 cm long respectively. Melilite forms crystals up to 2 mm in length and comprises >20% of some rocks; nepheline, magnetite and perovskite are also present. The Series of the Caldera Rim consists of a group of breccias, characterised by the presence of blocks of phonolitic nephelinite, and of lavas which occupy the caldera rim. The lavas contain sanidine in the groundmass and so were called phonolitic nephelinite by Odman (1931). The lavas also contain aegirine-augite, aegirine, a brown amphibole, apatite, titanite, magnetite and zeolites. There are melilite nephelinite dykes, locally forming swarms, and dykes, referred to as ‘bergalites’ by Odman (1931), consisting of melilite (45% in a mode quoted by Searle), nepheline, analcime, biotite, magnetite and glass. Xenoliths of wollastonite-bearing ijolite from tuffs and of pyroxenite from lavas are described by Rock (1976a) who gives mineral analyses.