Vol. 56.] GEOLOGY OF MOUNT KENYA. 215 



(b) The Phonolites. 



The lavas of the phonolite-group are less conspicuous than the 

 kenytes, as they form the lower slopes of the upper Alpine valleys, 

 which were obscured by moraine and snow. The phonolites may be 

 seen on both banks of the Teleki Valley, and one of the best 

 exposures is on the lower western slopes of Mount Hohnel. 



The rock (No. 490) in hand-specimens is dark, speckled, and 

 greyish-green, and is very fissile. In the centre of the flow it is 

 compact, and under the microscope resembles more closely a dyke 

 than a flow. This rock consists of much the same material as the 

 dyke on the western ridge of Mount Hohnel. Its structure is 

 trachytic ; the groundmass includes abundant, small, lath-shaped 

 felspars, which are irregular in arrangement, and often occur in 

 radial groups. The pyroxenes are small, and consist of asgyrine in 

 needles and short crystals, often arranged in moss-like aggregates. 

 Nepheline is not very abundant, but occurs in larger crystals than 

 the other constituents. (PI. XII, fig. 1.) 



(c) The Basalts. 



The basalt-flows of Mount Kenya occur in two main areas. 

 There is one series of very fissile olivine-basalts which occur in the 

 forest-zone. Mr. Hobley collected on the southern slopes a specimen 

 of a very vesicular basalt composed mainly of lath-shaped plagio- 

 clases, with a well-marked fluxion- structure around the empty 

 vesicles. The groundmass is very dense, from the abundance of 

 small crystalline granules and grains of magnetite. Olivine occurs 

 both as granules and as small, corroded, rounded crystals. 



On the western foot of Kenya a fissile olivine-basalt is one of the 

 commonest constituents of the tuffs and gravels ; and, as the pebbles 

 appear to have travelled a shorter distance than the kenytes, it is 

 probable that some dykes or flows of this rock occur in the forest- 

 zone. These fissile basalts have a specific gravity of 2*7. 



The second set of basaltic lavas occur in the neighbourhood of 

 Mount Hohnel and the Teleki Valley. The best example of this rock 

 forms crags of columnar basalt, wherein the columns are some 

 20 feet high and are often curved. The rock in the upper and 

 lower surfaces of the flow is massive, and the lower belt contains 

 a few inclusions of the kenytes. 



No. 517 is a heavy 7 , black, fine-grained basalt of specific gravity 

 3 # 09 ; the section shows no phenocrysts of plagioclase. Olivine is 

 abundant, and occurs both in idiomorphic crystals and in rounded 

 inclusions, which, though corroded, are not serpentinized ; in one 

 case the olivine forms a mere shell surrounding an included fragment 

 of the groundmass. The groundmass consists of microliths of 

 plagioclase and grains of pyroxene, olivine, and magnetite. 



The rock in an adjacent part of this flow is coarser in grain, and 

 there the chief minerals are olivine, plagioclase, and titaniferous 

 augite, while magnetite occurs in large skeleton- crystals. 



