300 miss c. a, raisin ox [May 1903, 



the edges and cracks) consist of a green, slightly-pleochroic, well- 

 cleaved augite, and of a yellowish mineral, probably olivine, 

 enclosing clear belonites. The groundmass consists largety of 

 small rectangular crystals like nepheline, but, as no hexagonal 

 sections can be seen, they are probably orthoclase (or anorthoclase). 

 These are embedded in a possibly quartzose base. A greenish and 

 a brownish mineral, intererystallized sometimes in a micro-ophitic 

 manner, form mainly an ill-defined network with rounded meshes. 

 The green mineral, pleochroic from bluish-green, extinguishing 

 sometimes at 3°, is doubtless riebeckite, and the brown seems 

 partly an alteration-product ; but some crystals, brownish or red- 

 brown, opaque within, and sometimes pleochroic almost to black, 

 are very probably cossyrite. The rock has a likeness to some 

 containing nepheline, and might be a soda-bearing augite-felsite or 

 porphyrite. 1 The two specimens labelled, one from the summit, the 

 other from the western edge of the crater, are alike, and thus afford 

 a presumption that they represent the general character of at least 

 part of the eruptive rock of this volcano. 



Thus several types of phonolites and their allies occur in the 

 collection. One rock from near Bilo approaches the solvsbergite of 

 Brcegger. A second and a third rock, from the same general locality, 

 are rather tinguaites. One contains very abundant scattered 

 nepheline, the other contains porphyritic felspars and riebeckite. 

 The rock from Garsa, with moss-like patches, including riebeckite, 

 is a possible tinguaite, and resembles the types from Adowa and 

 Axum. Lastly, the rocks from the crater and summit of the 

 volcanic mountain Saquala are closely allied, although containing 

 apparently no nepheline, resembling the ' phonolytic trachytes ' 

 described by Mr. Prior, from the ' Rif t- Valley ' district. 2 



A pale purplish-grey rock with close whitish bands, from Balchi 

 Hill, resembling a porphyrite in appearance, contains porphyritic 

 plagioclase within a minutely -microcrystalline, somewhat orientated 

 groundmass. Although hexagonal sections are rare, some are 

 found, and nepheline is probably present. Some very small 

 crystals of a pyroxene occur, also iron-oxide ; and a few minute 

 crystals may represent nosean, although the lines of small 

 black spots do not show the usual i grating ' arrangement, but are 

 inclined to be parallel. The rock is certainly fluxional ; it can only 

 be doubtfully placed near the phonolites. 



(3) Andesites and Porphy rites. 



Three rocks from the bed of the Hawash River, within 2 miles 

 of the base of Mount Saquala, westward of it, are grey and 

 compact. The glassy groundmass shows a fluidal structure, and 



1 I have to offer additional thanks to Mr. Prior for presenting me with an 

 advance-copy of his article on the Petrology of British East Africa, just as I 

 had completed this paper, and allowing me to compare some of the slices 

 described by him. The rock of Mount Saquala much resembles some of those 

 classed as phonolitic trachytes or quartz-trachytes. Thus it seems allied to 

 the earlier phonolitic lavas of the Kapte Plain, l-ather than to the more basic 

 nepheline-rocks of the active volcanoes of Doenyo Ngai and Mount Blgon. 



- Min. Mag. vol. xiii (1903) p. 228. 



