Vol. 59.] KOCKS FliOM SOTJTHEKN ABYSSINIA. 



200 



larger crystals, rather scattered, not quite complete in outline, 

 exhibit straight or all-round extinction, and are probably nepheline. 

 In some of these a narrow irregular margin around the enclosed 

 crystal is a secondary product formed of an isotropic mineral, 

 probably sodalite. 1 The pyroxene which fits in between the other 

 constituents, and sometimes fringes the nepheline, is mainly segirine 

 or an segirine-augite. As in the last rock, much of this is somewhat 

 platy, with extinction about 30°, pleochroic from green to brownish 

 with the usual tints, and rather resembles aegirine-augite. Iron- 

 oxide is present, and one felspar encloses a few very minute 

 zircons. A microscopic vein is* partly filled with what is possibly 

 aragonite. The rock is a phonolitc with fluxional structure, and 

 somewhat resembles the solvsbergite from Edda Gijorgis (Central 

 Abyssinia) brought by Dr. Sadebeck. 2 



Among the ' rocks with a dip of 80° to the north-east, obtained 

 in ascending the hill close to the east side of the camp at Garsa,' 

 partly grits and sandstones, are pale fawn-coloured or whitish 

 felsi tic-looking rocks with blackish speckled patches. The ground- 

 mass (which possibly includes nepheline) passes at places into a 

 micrOpegmatitic intergrowth with a greenish or brownish mineral 

 forming moss-like groups. The greenish mineral includes riebeckite 

 (pleochroic from deep blue to greenish and pale yellow, with well- 

 marked cleavage and low extinction), and associated with this some 

 aegirine (pleochroic from grass-green to greenish-yellow, with low 

 extinction-angle and strong double refraction). The brown mineral, 

 although it may partly represent a cossyrite, is, much of it, 

 doubtless an alteration-product, and iron-oxide and small epidote 

 are present. Microporphyritic, generally oblong, crystals occur, 

 from the edge of which radial tufts grow inward, certainly a 

 secondary development, possibly due to subsequent -heating by 

 intrusion of an igneous magma. The crystals sometimes extinguish 

 straight, and, although the secondary- replacement renders the identifi- 

 cation of many uncertain, some at least are probably nepheline; 

 and -the roek was readily etched with hydrochloric acid, especially the 

 microporphyritic crystals, and these most strongly along the margin. 

 Also, with hydrofluosilicic acid, minute hexagonal crystals of sodium- 

 nuosilicate were formed in fair quantities. 3 The rock somewhat 

 resembles an segirine-tinguaite from Hot Springs (Arkansas), a 

 slice of which is in the collection at the British Museum (Natural 

 History). 



The rocks next described, although apparently not containing 

 nepheline, are certainly allied to the phonolites. 



Mount Saquala and its neighbourhood. — A dark-grey 

 rock in two specimens from this mountain, contains small por- 

 phyritic crystals of felspar (sanidine and anorthoclase). Other 

 microporphyritic crystals (highly refractive, and ironstained along 



1 I have to thank Prof. Bonney for the loan of additional slides containing 

 this mineral for comparison. 



- Described by Mr. Prior in Min. Mag. vol. xii (1900) p, 265 & pi. iii, fig. 4. 

 ■ 3 See JRosenbusch [transl. Iddings] 4th ed. (1898) pi. x, fig. 5. 



