Vol. 59.] ROCKS FROM SOUTHERN ABYSSINIA. 305 



belonites, and contains grains of iron -oxide and a few crystals of 

 augite and of felspar, some much corroded, others perfect with 

 albite-twinning. The perlitic cracks in the glass are narrow, and 

 sometimes polarize brightly. The specific gravity (tested on two 

 occasions by a Walker's balance) was found to be between 2*33 and 

 2-38, the variation being probably due to the cracked and splintery 

 character of the specimen. Thus the rock seems to be an andesitic 

 pitchstone rather than a tachylyte. Other basalts occur (one layer 

 100 feet thick), then a bed of white quartz-sand, and one of a slightly 

 quartzose red clay, which may be a basaltic tuff or possibly an 

 ordinary sediment. 



(2) Mount Yoel consists largely of igneous rocks, but furnishes 

 one specimen of a white quartz-sand. Decomposed felstoncs are 

 represented by two rocks at the foot of the mountain (one on the 

 south side). In one fresher porphyrite, just above the plain on 

 the Baramidia (?) side, the groundmass consists mainly of lath- 

 shaped plagioclase, with needles and patches of iron-oxide. A black 

 basalt with a ferruginous glass comes from the same side. A red 

 rotten rock may be also either a basalt or a basic ash. A compact, 

 jointed basalt comes from the summit. 



(3) Gib be Hill is formed of volcanic rocks, the specimens being- 

 basalts, sometimes glassy, often vesicular or amygdaloidal. One of 

 these, rather rotten, forms a ' bed near the summit.' Another dark- 

 red weathered rock becomes disintegrated and veined, with develop- 

 ment (in amygdales and veins) of crystals, clear, colourless, often 

 with penetration-twins. These at first sight appear to be cubic, 

 but are really rhombohedral, with low double refraction, and I 

 would refer them to chabazite. 



(4) Between Gewaba and Allali(?). — These rocks are com- 

 pact basalts, microcrystalline (with a second pyroxene or a little 

 altered olivine) or glassy, sometimes spheroidal, as is shown by 

 a flake which has shelled off one specimen. The surface of one 

 fragment is smoothed and ridged, as if worn by blown sand. 



(5) Blocks in river-bed, Djemma(?). — One is a dull black 

 rock, compact, but jointed and cracked. The finely-speckled glass 

 contains a few small felspars and augite-crystals, and exhibits 

 perlitic cracks as ill-defined brownish lines, a structure not very 

 frequently found in a magma-basalt. A second block, blackish and 

 compact, but crowded with spherical amygdales, is found to consist 

 of a brown glass, with abundant granular iron-oxide, crossed by 

 perlitic cracks, which are whitish or colourless lines, polarizing 

 brightly. It is doubtless a very basic andesite. A similar rock 

 comes from Gewaba (?), from the lowest exposed bed. 



(6) Some water from a hot spring south-west of Addis Abbeba 

 (collected on February21st, 1899) was brought back. The subjoined 

 analysis was made by Mr. W. L. Alton in the Chemical Laboratory 

 of University College (London), through the kindness of Sir William 

 Ramsay, to both of whom I am greatly indebted : — 



Q. J. G. S. No. 234. y 



