Yd. 56.] GEOLOGY OF MOUNT KENYA. 209 



a massive core, which appears to have plugged the central vent of 

 the mountain. 



The rock of this plug (No. 499) consists of a black glassy ground- 

 mass with scattered phenocrysts of glassy felspar, which have 

 rounded angles and lath-shaped sections, and range up to an inch 

 in length. This rock occurs on the western arete above Two-Tarn 

 Col, along the cliff that forms the right bank of the Lewis 

 Glacier, and on the ridge rising northward from the col above the 

 Lewis Glacier. Above these levels I could see no agglomerates, 

 except a few unimportant patches, and these appeared to extend 

 to the summit. 



Under the microscope the large phenocrysts of felspar constitute 

 the most striking feature of the rock ; these, though seldom poly- 

 synthetically twinned, are no doubt anorthoclase. 



The pyroxenic constituent is less abundant ; it occurs in two 

 forms. The sparsely scattered, small, very pale green phenocrysts, 

 with rounded and sometimes even elliptical outlines and a high 

 extinction-angle (over 30° from c) are augite. The second pyroxene 

 is aegyrine, which occurs in small patches of green grains, while 

 the black irregular grains which make the base appear dense are 

 possibly altered segyrine. 



Olivine occurs in corroded crystals, often partly altered to ser- 

 pentine. Apatite is present in well-developed prisms, included by 

 all the other constituents of the rock. 



The groundmass is crowded with felspar-microliths, small irre- 

 gular pyroxene-granules, and innumerable small black granules 

 which are possibly altered segyrine. The rock does not show any 

 pronounced fluxion -structure. (PI. XI, fig. 1.) 



The specimen (No. 499) upon which the foregoing description is 

 based was collected on the ridge rising northward from the eastern 

 end of the Lewis Glacier. Its specific gravity is 2*65. 



To give a name to this rock is not easy. The excess of soda- 

 minerals recalls the panteilerites, especially as Rosiwal x has pro- 

 visionally applied that name to an allied rock from Southern 

 Abyssinia. Mr. Prior, moreover, has called my attention to a slide 

 of somewhat similar rock from the eastern flank of Montagna 

 Grande (Pantelleria), which also contains porphyritic anorthoclase, 

 a ferriferous olivine, and a pale green augite, included in a f el- 

 spathic base full of minute irregular grains of pale green augite 

 and opacite. 



For the reasons, however, which are stated on pp. 213-214, it is 

 un advisable to include the Mount Kenya lavas among the pantei- 

 lerites, and the name of kenytes is accordingly proposed for them. 



1 ' Ueber Gesteine aus Schoa u. Assab ' Denkschr. k. Akad. Wissensch. Wien, 

 toI. lviii (1891) p. 518. 



