Vol. 56.] GEOLOGY OP MOUNT KENYA. 213 



east-north-east of Mount Hohnel, and in the agglomerates of that 

 district. The rock consists of large anorthoclase-crystals embedded 

 in a black tachylytoid glass, which is apparently more basic than in 

 pantellerites. Examined under the microscope the glass appears 

 deep brown, and contains some vesicles. The anorthoclase-pheno- 

 crysts are large, and zonally constructed j they include apatites 

 and brown-glass inclusions. Some cleavage- fragments have been 

 measured by Mr. Prior, who determined the extinction on b as 

 8° to 9°, and on c as nearly straight. Olivine occurs in corroded 

 phenocrysts. The specific gravity of the rock, measured from a large 

 block which may contain a few vesicles, is 2-5. (PI. XI, fig. 2.) 



To find a suitable name for these rocks is not easy. In the field 

 I called them rhyolites, but they differ from ordinary rhyolites 

 by the absence of quartz and by their great excess of soda. 

 Accordingly, it is natural to compare them with the rich soda- 

 bearing rhyolites of Pantelleria, especially as the porphyritic 

 pichstone is practically identical with a rock from Shoa, in Southern 

 Abyssinia, described by Eosiwal, 1 who named it ' vitrophyrischer 

 augit-trachyt (pantellerit)/ But this identification has been denied 

 by Rosenbusch, who maintains that a rock composed of anorthoclase- 

 phenocrysts in a glassy groundmass rich in soda is not necessarily 

 a pantellerite. 



According to Rosenbusch's definition, ' pantellerites are charac- 

 terized by the absolute predominance of the alkali-felspar, the total 

 absence of the lime-soda felspar, the development of aenigmatite 

 [cossyrite], diopside, SDgyrine, and arfvedsonitic amphibole as the 

 colouring constituents with the almost absolute suppression of mica ; 

 by the absence of magnetite, the rarity of quartz of the first 

 generation, the predominance of the glassy and microcrystalline 

 structures and the extreme rarity of microfelsitic structure ; and by 

 the abundant formation of the colouring constituents in the eruptive 

 period/ 2 This definition certainly does not describe accurately the 

 Mount Kenya lavas. They contain neither aenigmatite nor arfved- 

 sonitic amphibole, nor, in fact, any other amphibole. The positive 

 characters also are different : olivine is fairly abundant in some 

 slides ; the colour of the rock is dark sepia-brown, red, or grey, 

 rarely green, and one variety occurs with a tachylytoid groundmass. 



Rosenbusch objects that the Shoan rock described by Rosiwal 

 should be referred to pantellerite because • of its colour (brown 

 to grey) and its glassy, felsitic habit/ ' The anorthoclase alone/ he 

 continues, ' does not postulate the pantellerite character ; for that are 

 needed also the alkali-pyroxene and the alkali-amphibole, and the 

 remaining above-stated characters/ 



To include these Mount Kenya lavas in pantellerite would 

 necessitate a complete redefinition of that term. The only defini- 

 tion of pantellerite that would include the type-rock and these 



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

 vol lviii (1891) p. 518. 



2 'Mikroskop. Physiogr/ 3rd ed. vol. ii (1896) p. 612. 



