218 PROF. J. W. GREGORY ON THE [May I9OO, 



these lavas is quaquaversal ; thus on the eastern ridge of the Hobley 

 Valley the rocks dip south-eastward; on the Lewis Col and its 

 main southern ridge they dip southward ; on the flanks of the 

 Teleki Valley they dip south-westward ; and on the north of the 

 Thomson Valley (the second valley northward from the Teleki 

 Valley) they dip north-westward. 



As an illustration of the arrangement of the lava-series in 

 the Teleki Valley area, we may consider the section exposed on 

 the western arete and face of Mount Hohnel. At the base are 

 greenish fissile phonolites, which form the slopes around Lake 

 Hohnel, the lower part of both flanks of the Teleki Valley and of 

 Phonolite Cwm, and the floor of the col at the lower end of the 

 western ridge of Mount Hohnel. This phonolite is not well exposed, 

 for the rock is easily denuded and weathers into smooth slopes, which 

 are mostly covered by talus, snow, and moraine-debris. Ascending 

 Mount Hohnel by the western ridge we find the following sequence. 

 Next above the phonolite comes : 



2. A thick flow of porphyritic kenyte wherein the anorthoclase-phenocrysts 

 are smaller and more crowded, and the groundmass is of a brighter red than in 

 the usual variety ; 



3. A thick bed of tuff, partly vesicular, and enclosing large crystals of 

 anorthoclase ; 



4. A flow of kenyte ; 



5. Coarse kenyte-agglomerate (no basalt-pebbles could be found in it) ; 



6 (in 5). A vertical compound dyke of phonolite with basic selvages ; the 

 dyke is 4-§ feet wide, and stands out as a wall 3 feet high ; 



7. Brown and buff volcanic ash over the agglomerate (5), dipping 8° south- 

 westward ; 



8. Flow of black kenyte, including red porphyritic bombs ; 



9. Alternations of coarse ash and agglomerate, weathering into crags (in- 

 cluding the ' Janda Pinnacle ') ; 



10. Porphyritic kenyte, with a dull pinkish groundmass ; 



11. Thin kenyte-flows interstratified with tuff; and 



12. A coarse flow of porphyritic kenyte capping the summit. 



The total thickness of the foregoing series (2 to 12 inclusive) 

 is about 1600 feet. 



V. The Geological History of Mount Kenya. 



Mount Kenya accordingly consists of a very ancient, denuded 

 volcano, which is built up of rocks belonging to three petrographical 

 types: (1) an olivine-bearing nepheline-syenite, with its kenytes; 

 (2) the phonolite-series of dykes and flows ; and (3) the olivine- 

 basalt series. 



At first, I was inclined to believe that the basalts were the 

 oldest members of the series : for Mr. C. W. Hobley had given me 

 in Mombasa a specimen of a vesicular basalt from the edge of the 

 forest-zone of the southern slopes of Kenya ; and, on the day after 

 we reached the Alpine zone, I found blocks of an apparently tachy- 

 lytic lava in the agglomerates. It accordingly seemed probable that 



