Reports and Proceedings — Geological Society of London. 139 



craters occur in it. The Alpine zone consists of coarser asb, 

 agglomerates, and tuifs, interbedrled with lava-flows and traversed 

 by uuaierous 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, of beds of agglomerate, and the thick 

 proximal ends of the great lava-flows. 



The rocks of the central core comprise an olivine-anorthoclase- 

 nepheline- syenite with gegyrine, and a black glassy lava with 

 numerous white phenocrysls of anorthoclase ; this rock is allied in 

 some respects to the pantellerites, but receives a special name both 

 here and in its occurrence in lava-flows. 



The dyke-rocks fall into two categories — a series of phonolites, 

 and one of basalts and dolerites. One, at least, of the dykes is 

 compound, including both these rock-types. The basic group ranges 

 from basalts with little olivine to coarsely crystalline dolerite rich 

 in that mineral. 



The lavas belong to three main groups : (1) those of the 

 nepheline-syenite series, (2) the phonolites, and (3) the basalts. 

 The oldest lavas appear to be the phonolites, but they and the 

 first group of lavas seem to have overlapped in age ; the olivine- 

 basalts, which reach the surface from a zone of secondary craters 

 in the Alpine zone, are the latest in date. The first group of lavas 

 are rhyolitic in aspect, and consist of a brown or green glass in 

 which are embedded phenocrysts of anorthoclase, and green needles 

 which show the optical characters of aigyrine. As these rocks do 

 not conform to Rosenbusch's definition of pantellerites, a new name 

 is proposed for them ; the author considers that the pantellerites 

 may be liparitic equivalents of the dacites, while these Kenya 

 rocks correspond with the nepheline-syenites. The structure of the 

 phonolites is trachytic, the groundmass including abundant small, 

 lath-shaped plagioclases ; nepheline is not very abundant, and occurs 

 in larger crystals than the other constituents, the chief of which is 

 segyrine. The basalts are often vesicular and columnar, and contain 

 olivine, augite, magnetite, and apatite often in a glassy groundmass. 



The pyroclastic rocks vary in texture from coarse to fine ; they 

 contain blocks of all types of the lavas, and also large crystals of 

 anorthoclase. 



The nepheline-syenite is the most deeply-seated, holocrystalline 

 rock found on Kenya; and the mountain appears to represent 

 a single, ancient, dissected, volcanic mass, with a shorter geological 

 history and a narrower range of rock-types than its great companion 

 volcano, Kilima Njaro. The author gives reasons for considering 

 that all the difi'erent rock-types present may have resulted by 

 differentiation from the olivine - anorthoclase - nepheline - syenite 

 magma, although this involves the conclusion that in some cases the 

 basic materials must have concentrated in the glass and solidified 

 after the formation of the felspars. 



4. " Contributions to the Geology of British East Africa. Part Hi : 

 The Blaeolite-Syenite and Fourchites intrusive in the Coast Series." 

 By J. W. Gregory, D.Sc, F.G.S. 



