208 PROF. J. W. GREGORY ON THE [May I9OCV 



1. The Rocks of the Central Core. 



The central peak of Kenya consists of a group of rock-pyramids, 

 formed mainly of the lava that plugged the main vent. This rock is 

 exposed on the main southern, south-western, and western aretes. 



The lowest exposure of any rock belonging to the central-core 

 series was on the northern face of the valley below the snout of the 

 Lewis Glacier. Unfortunately the field-relations of this rock were 

 obscured by talus and snow. This rock (No. 496 *), which is some- 

 what gabbro-like in aspect, is coarsely crystalline, and consists of 

 well-developed felspars, which are sometimes 15 mm. long and 

 3 mm. thick, separated by dark green minerals. The specific gravity 

 is 2*6. Examined microscopically, the rock is seen to be an olivine- 

 bearing nepheline-syenite. The rock is holocrystalline, hypidio- 

 morphic, and coarse-grained. (PI. XI, fig. 4.) 



The main constituent is felspar, which occurs in large idio- 

 morphic crystals twinned on the Carlsbad type, and in a mosaic 

 of small crystals, many of which are also twinned on the Carlsbad 

 type. The felspar is similar to that which occurs in the adjacent 

 tuffs, and is probably in the main anorthoclase. This mineral 

 was found in large isolated crystals in the tuffs of Mount Hohnel. 

 The crystals are of the same character as those previously described 

 by Mr. L. Fletcher & Prof. H. A. Miers 2 : they are described 

 on p. 216. (See also PI. XII, fig. 2, spec. No. 464.) The name 

 anorthoclase is retained instead of natronmikroklin, which 

 Brogger has shown holds priority ; 3 but the latter, for English 

 adoption, would require translation, and anorthoclase is more 

 convenient for international use. 



The second important constituent is nepheline, which occurs 

 in very large prisms (3 to 4 mm. in diameter) and in small grains. 

 The pyroxenes are bordered by a zone of aegyrine, which also occurs 

 in scattered grains. The amphibole is deep brown in colour 

 and has the pleochroism of barkevicite ; but some of it, which is 

 nearly opaque, Mr. Prior suggests may be allied to cossyrite. The 

 rock contains a considerable amount of a bright yellow, strongly 

 doubly-refractive, ferriferous olivine. Magnetite is scarce. Some 

 isotropic mineral, probably sodalite, is also present. 



The foregoing enumeration of the minerals shows that this rock 

 is dominated by its high percentage of soda, and as it is the most 

 deep-seated rock found on Mount Kenya it is natural to find the 

 eruptive rocks characterized by soda-minerals. 



Overlying this nepheline-syenite is a black glassy lava with 

 numerous white phenocrysts of anorthoclase. The rock occurs as 



1 The numbers in parentheses are those affixed to my African rock-specimens, 

 which will be presented to the Mineralogical Department of the Natural History 

 Museum. 



2 Min. Mag. vol. vii (1887) pp. 10-11 & 131-132. 



3 ' Das Ganggefolge des Laurdalits ' Die Eruptivgest. des Kristianiagebietes, 

 pt. iii, in Vidensk. Skrift. pt. i (1897) No. 6, p. 12. 



