Vol. 56.] GEOLOGY OF MOUNT KENYA. 207 



area between the western and southern aretes and the adjacent part 

 of the upper Alpine zone. 



As the completion of a detailed report on the geological structure 

 of British East Africa has been delayed, it seems advisable to 

 publish it in instalments. I gladly express my indebtedness to 

 Mr. C. W. Hobley, who kindly gave me the rock-specimens that he 

 collected on Mount Kenya in 1891 ; and to Mr. G. T. Prior for much 

 valuable assistance in the determination of some minerals in the 

 rock-slides, and in consulting the fine collection of foreign rock- 

 slides in the Mineralogical Department of the Natural History 

 Museum. I am also indebted to Mr. Prior for kindly undertaking 

 to see these pages through the press : a task which I am unable 

 to fulfil myself, in consequence of my somewhat hurried departure 

 for Australia. 



II. Physical Geography of Mount Kenya. 1 



Mount Kenya consists of three zones : (1) a long forest-clad slope 

 of volcanic ash, extending to a height of about 10,000 feet; (2) the 

 open moors and valleys of the lower Alpine zone, covered by an open 

 scrub of Alchemilla Johnstoni and a previously unrecorded species 

 of arborescent Erica ; and (3) the upper Alpine zone, formed of 

 grassy meadows and valleys cut in volcanic ash below bluffs of 

 coarse agglomerate. Above this upper zone is the steep rugged 

 central peak, where the only vegetation consists of tufts of grass 

 and Helichrysum cymosum with patches of lichen. 



The altitude of Mount Kenva has been variously estimated as from 

 17,200 feet (Smith) up to 23,000 feet (Peters). L. von Hohnel's 

 determination by triangulation from Ndoro made the height 19,029 

 feet, a figure which I have accepted, as my own observations roughly 

 agreed with it. 



The three main zones of Mount Kenya are characterized by different 

 geological features. The long slope of the forest-belt consists in 

 the main of volcanic ash, though the remains of secondary parasitic 

 craters probably occur on it. The Alpine zone consists of coarser 

 ash, agglomerates, and tuffs, interbedded with lava-flows and 

 traversed by numerous dykes, with the remains of some secondary 

 centres of eruption. The third zone, or central peak, consists of the 

 plug which choked the central vent, and the beds of agglomerates and 

 thick proximal ends of the great lava-flows. 



III. Petrography. 



The rocks of which Mount Kenya is built may be divided into four 

 groups : (1) the rocks of the central core, (2) the dykes of the 

 Alpine zone, (3) the lavas, and (4) the pyroclastic rocks. 



1 For a short account of the physiography of Mount Kenya, see 'The 

 Glacial Geology of Mount Kenya' Quart. Journ. Geol. Soc. vol. 1 (1894) 

 p. 515, and ' The Great Eift Valley ' 1896, pp. 166 et seqq. 



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