Vol. 56.] GEOLOGY OF MOUNT KENYA. 211 



(b) The Basic Dykes. 



The dykes of the second group are more basic in character, and 

 range from a basalt with a little olivine to a coarsely crystalline 

 dolerite rich in olivine. 



The rock of this series, which, judging from its specific gravity, is 

 the least basic, is a dark brown, compact basalt, collected by Mr. C. 

 W. Hobley during the British East Africa Company's expedition 

 to Mount Kenya in 1891. It was obtained in a ravine at the 

 height of about 7000 feet on the southern slope of the mountain. 

 The specific gravity is 2*74. Under the microscope the rock 

 appears as a felted mass of lath-shaped plagioclases, which are cut 

 across in all directions. Some platy felspars are also present. 

 Granules of a pale brown to greyish pyroxene and of olivine 

 are abundant ; and numerous very thin acicular crystals, which 

 Mr. Prior refers to apatite, traverse the plagioclases. 



The basic dykes of the Teleki Valley district have a higher 

 specific gravity, and contain more olivine and magnetite than the 

 specimen collected by Mr. Hobley. These more basic dykes are 

 well exposed near the snout of the Lewis Glacier, and are of con- 

 siderable width ; in the centre they are coarsely crystalline, and may 

 be called olivine-dolerites. The dykes which have yielded specimens 

 510 & 511 may be taken as types of this group. 



No. 510, from the centre of a dyke below the snout of the 

 Lewis Glacier, consists largely of plagioclase-laths : these, from their 

 high extinction- angle, may be referred to labradorite. Augite is 

 abundant, in nests of small prismatic crystals and in rounded 

 phenocrysts. Olivine in small altered crystals is also plentiful. The 

 specific gravity of the rock is 2*8. 



A larger dyke from the same locality (No. 511) is a black rock 

 weathering dark brown ; it contains numerous tabular crystals of 

 plagioclase, roughly parallel in arrangement, and large crystals 

 of black pyroxene which are recognizable in hand-specimens. 

 The specific gravity is 2*97. Examined under the microscope, the 

 plagioclase is seen to occur in two forms. The olivine is idiomorphic. 

 The augite is titaniferous, with a pleochroic outer zone. This rock 

 is the most basic of the Kenya dykes from which specimens have 

 been collected so far. 



3. The Lavas. 

 The Mount Kenya lavas belong to three groups : — 



(a) The Kenytes or Lavas of the Nepheline-syenite 



Series. 



At various points on the south-western flanks of the central peak 

 of Mount Kenya are a series of flat-topped, massive crags, which 

 can be recognized from Laikipia. These crags are remnants of the 

 greatest series of lava-flows that came from the central vent of Kenya. 

 The rocks which form these crags are well exposed, for instance, on 

 the Lewis Col and on the Teleki Bidge, where they rest upon beds of 



