Vol. 59.] KOCKS I'KOil SOUTHERN ABYSSINIA. 295 



(?) zoisite, within a mosaic in which the orientation is partly due to 

 pressure. In another rock from beyond Jem Jem (containing a 

 carbonate), the hornblende is ragged and fibrous at the ends, and, 

 like the biotite, has undergone alteration, but whether by pressure 

 is less certain in this example. Two rocks in the hand-specimens 

 are like a typical gabbro. One, fairly coarse (from Govie, where the 

 rock is said to extend for several miles, and from Gauti), is composed 

 of felspar mostly fresh (probably labradorite), of dark-green, well- 

 cleaved hornblende resembling altered diallage (often enclosing 

 felspar), with epidote, apatite, and iron-oxide. The other (from the 

 Didesa River), consisting of kaolinized felspar and of a platy green 

 hornblende, partly recrystallized or aggregate, occurs in a junction- 

 specimen with a speckled diabase (both containing epidote), but 

 the passage is somewhat gradual, as if either they were parts of 

 one mass, or the intruding diabase had carried off pieces of the 

 gabbro. 



A dark-greenish rock from the top of the hill between Donkoro 

 and Bojji, resembles a picrite, with its large platy crystals (J inch 

 long) lustre-mottled. These are found to consist of green pleochroic 

 hornblende, sometimes paler at the exterior. They enclose grains 

 of a well-cleaved white augite, often brown-stained at the edges, 

 which resembles that described by Prof. Bonney in the ' picrite ' 

 of Porthlisky, where olivine is also present. 1 In parts of this 

 Abyssinian slice, the augite is intergrown with the hornblende as a 

 granular aggregate, or in an almost micro-ophitic arrangement; 

 and in places the augite is changing to hornblende, that is, the rock 

 is a form of hornblende-pyroxenite. 



III. Volcanic liocxs. 



The collection includes a large number of volcanic rocks, most 

 of them from Southern Abyssinia. The great majority are basalts 

 and andesites, but phonolites or allied rocks occur, with one loose 

 specimen of obsidian and some tuffs. 



(1) Basalts, 

 (i) Olivine -basalt. — A blackish rock from the basaltic region 

 between Gudr and Toki is compact, but crowded with large crystals, 

 sometimes half an inch long. These are of augite or of olivine, the 

 latter often a transparent peridote, but sometimes possibly fayalite. 

 One specimen of the rock, with more groundmass, contains small 

 whitish amygdales and lath-shaped felspar-microliths. In the 

 olivine in another slice, curved cracks in one crystal slightly 

 resemble perlitic spheroids within cross-jointed blocks, and the glassy 

 magma extends along some cracks. The augite is brownish with a 

 puce-coloured edge, and encloses some olivine. The groundmass is 

 colourless, partly glassy, but includes felspar, and contains small 



1 Quart. Journ. Geol. Soc. vol. xli (1885) p. 519 & pi. xvi, fig. 5. The 

 colourless augite is more or less distinct in the boulders from Anglesey, ibid. 

 p. 518. 



