216 THE GEOLOGY OP MOUNT KENYA. [May I9OO. 



4. The Pyroclastic Rocks. 



The fragmentary volcanic rocks of Mount Kenya consist of a 

 broad zone of volcanic ash, which forms the main foot-slope of the 

 mountain, extending on the west from Ndoro (at an altitude of 

 7100 feet) to the lower edge of the Alpine zone (alt. 10,100 feet). 

 In the lower part of this foot-slope occur beds of tuff and lava- 

 gravel, in which pebbles of fissile basalt predominate. 



In the Alpine zone the most prominent fragmentary rocks are 

 beds of coarse agglomerate, well exposed as rough crags and 

 pinnacles. The materials of these agglomerates are mainly coarse 

 blocks of kenyte and phonolite. 



The tuffs are best developed in the upper Alpine zone, where 

 they are well exposed on the cliffs of Mount Hohnel, a section of 

 which shows them interbedded with ash and flows of kenyte overlying 

 the main phonolitic beds at the base (fig. 1, p. 217). The tuffs are 

 often vesicular, with the cavities lined by zeolites. The most inter- 

 esting feature of these tuffs is the occurrence of large crystals of 

 anorthoclase, which have been kindly examined by Mr. Prior, who 

 reports as follows : — ' The crystals are apparently simple, with the 

 faces b (010), m (110), M (110), c (001), with x (l01), or in some 

 crystals y (201), all well developed. The cleavage-angle be, 

 measured by the reflecting goniometer, did not differ more than 2' 

 from 90°. The extinctions measured on cleavage-flakes are 



on b, 7° to 8°, obtuse positive bisectrix ; 

 on c, 1° to 2°, very fine twin lamellae. 



The material gives a strong sodium-flame." 



IV. The Stratigraphy op Mount Kenya. 



Turning from the petrography of the rocks to their stratigraphical 

 arrangement, we find that this is simple. The nepheline-syenite to 

 the north-west of the snout of the Lewis Glacier is the most deep- 

 seated holocrystalline rock on the mountain, and doubtless repre- 

 sents the original magma of the volcanic series. This rock probably 

 passes upward into the massive core of porphyritic kenyte which 

 forms the central peak of the mountain; but no actual passage 

 could be traced, and it is conceivable that the nepheline-syenite 

 may be an intrusion into the kenyte-plug. (See fig. 2, p. 217.) 



The upper Alpine zone, with its coarse agglomerates and radial 

 dykes, no doubt occupies the site of the crater-walls of the old 

 volcano, though the crater-edges must have been some thousands of 

 feet above the present height of this zone. The Alpine agglomerates 

 are traversed by numerous vertical dykes of olivine -basalt and 

 olivine-dolerite, and some flows of olivine-basalt also occur in it. 



The main lava-flows of Kenya are best exposed in the upper 

 Alpine zone. They slope at a dip of usually 8° to 15° from 

 the central plug. So far as my observations extended, the dip of 



