220 PROF. J. W. GREGORY ON THE [May IOOO,. 



basalts of the lower slopes, which form so large a proportion of the 

 gravels near Ndoro, may possibly be older than the basalts of the 

 upper Alpine zone. Petrographically the two basalts agree, but the 

 dense jungles of the forest-zone prevent any definite correlation by 

 stratigraphical evidence. It might be suggested, therefore, that the 

 basalts of the lower forest-zone belong to an earlier series of basic 

 eruptions, which were pierced by the core of nepheline-syenite, and 

 covered by the central pile of the kenyte-series ; after which a 

 minor basaltic series closed the volcanic history of the mountain. 

 Cases of such subsidiary recurrence of basalt are known ; thus 

 among the Kainozoic volcanoes of Scotland Sir Archibald Geikie * 

 has remarked that ' in the case of the Tertiary volcanic series [of 

 Scotland] there is evidence that after the acid protrusions a final 

 uprise of basic material occurred.' The basalts of Pantelleria may 

 be another illustration of this tendency. But, other than the 

 general rule of eruptive sequence, I know of nothing to suggest the 

 occurrence on Mount Kenya of a double series of basalts ; and at 

 present there is no evidence of any basalts on the mountain earlier 

 than the kenyte-series. Though the basalts whose relative age is 

 known are younger than the phonolites and kenytes, it does not 

 seem necessary to assume any great lapse of age between them ; for 

 it is possible that all the lavas represented in the Teleki Valley 

 quadrant may belong to one series. They may all have resulted by 

 differentiation from the olivine -anorthoclase- nepheline -syenite 

 magma, from which were erupted first phonolites, then kenytes, 

 and finally olivine-basalts. 



In Pantelleria, though no definite passage has been described 

 from the pantellerites to the basalts, the sequence as determined 

 by E. Fcerstner 2 agrees with that of Mount Kenya ; for it began 

 with phonolites, which were followed by pantellerites, and it 

 closed with basalts. In the nepheline-syenite area of Christiania y 

 as described by Brogger, the passage in the dyke-series from tin- 

 guaites rich in nepheline to sblvsbergites free from nepheline, and 

 the similar passage in the deep-seated rocks from the nepheline- 

 bearing foyaites to the nephelineless hedrumite are comparable to 

 the passage from the nepheline-syenites and phonolites of Mount 

 Kenya to the kenytes in which nepheline has not been developed. 



VI. Kenya and Kilima Njaro. 



The position of Mount Kenya in the East African volcanic series 

 can be more conveniently considered after a description of the 

 adjacent volcanic areas ; but it is advisable to include some com- 

 parison of the mountain with the other great extinct East African 

 volcano, Kilima Njaro. Unfortunately, however, the exact geo- 

 logical history of Kilima Njaro has not yet been determined. 

 Extensive rock-collections have been made there and carefully 



1 « Ancient Volcanoes of Great Britain ' vol. ii (1897) p. 477. 



2 ' Nota preliminare sulla Geologia dell' Isola di Pantelleria ' Boll. Com. 

 Geol. Ital. vol. xii (1881) pp. 539-544. 



