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



2. The Dyke-rocks. 



The dykes of the Alpine zone of Mount Kenya fall into two 

 categories : a series of phonolites and one of basalts and dolerites. 



(a) The Phonolites. 



As an example of the phonolites, we may take a dyke 4j feet 

 wide which cuts vertically across the agglomerate on the western 

 ridge of Mount Hohnel (No. 456). The rock is dark greenish, fine- 

 grained, and somewhat fissile. The specific gravity of a specimen 

 from the middle of the dyke is 2' 6. Under the microscope the 

 rock is seen to be composed mainly of felspar, in small lath- 

 shaped crystals, with well-marked fluxional arrangement. The 

 most conspicuous mineral is nepheline, which is abundant and 

 stands out with remarkable clearness ; the crystals are fresh, and the 

 larger prisms measure from *3 to '4 mm. in length. The pyroxenic 

 constituent is aegyrine, which occurs as plates with frayed ends and 

 as crystals with regular hexagonal sections ; the smaller segyrines 

 are often clustered like a framework around the nephelines. (PI. XI, 

 % 5.) 



Mr. Prior has shown me a slide of a phonolite from Risca (Grand 

 Canary), which is almost identical with this rock, and a second slide 

 from Chasna (Teneriffe), in which the resemblance, though less 

 complete, is close. 



The most striking feature of this dyke is its selvage, which has 

 the characters of a glassy basalt (No. 457). It consists of a black 

 basic glass in which are numerous circular vesicles and small 

 lath-shaped crystals of plagioclase : the larger crystals show twinning 

 on the albite-type, with symmetrical extinction in alternate lamellae 

 of about 35°, and may consequently be referred to labradorite. 1 

 Olivine is abundant, occurring in small idiomorphic crystals. The 

 glassy groundmass does not gelatinize when treated with cold 

 hydrochloric acid. (PI. XI, fig. 6.) 



This dyke is therefore compound, being made up of a central 

 sheet of phonolite bounded by basaltic selvages. 



As this phonolite is a dyke-rock, it may seem advisable to adopt 

 for it the name tinguaite; but in his original definition of that 

 rock Rosenbusch 2 laid stress on the absence of fluidal structure, 

 whereas the dyke from Mount Hohnel shows well- developed fluxion- 

 structure, which is absent from the phonolitic flows of Kenya. 

 In 1896, however, Rosenbusch 3 admitted fluidal structure in 

 tinguaites, and based the distinction between them and phouolite on 

 the aplitic instead of trachytic structure. But as the Mount Kenya 

 dykes are as trachytic as the flows, and as the two types are 

 microscopically indistinguishable, it seems advisable to accept the 

 name phonolite for both. 



1 The identification of this plagioclase has been kindly confirmed by Mr. Prior. 



2 « Mikroskop. Physiogr.' 2nd ed. vol. ii (1887) p. 628. 



3 Ibid. 3rd ed. vol. ii (1896) pp. 479-480. Brogger has described a fluidal 

 tinguaite from Lysebofjord, north of Laurvik, in 'Die Gesteine der Grrorudit- 

 Tinguait Serie ' Die Eruptivgest. des Kristianiagebietes, pt. i, in Vidensk. Skrift. 

 pt. i (1894) No. 4, p. 117. 



