296 miss c. a. raisin on [3tlay 1903, 



crowded crystals of augite and of iron-oxide. The specific gravity of 

 three specimens (tested by a Walker's balance) is respectively 2*95, 

 3*09, 3*18, which is higher than that of an ordinary tachylyte 

 (2-71 to 2-86). Thus, notwithstanding the presence of felspar (which, 

 as Prof. Bonney has shown, is always potential and sometimes 

 actual in the rock from Limburg itself *), the specimens really 

 belong to the limburgites. 



Olivine is a distinct constituent, although not abundant in a few 

 other basalts, which often have a purplish tinge, and resemble the 

 Niedermendig and allied rocks.. The two most typical come from 

 the Saquala district. One from the hillside between that mountain 

 and Galanda Lotha is minutely microcrystalline, and contains larger 

 crystals of augite and plagioclase, with a ferruginous olivine. This 

 is bright brown, with metallic iridescence, in the hand-specimen ; 

 and under the microscope it resembles the brown-edged crystal 

 changing to iron-peroxide figured by A. Rosiwal. 2 Several felspar- 

 crystals have undergone partial melting-down, forming an outer 

 cloudy or gelatinous-looking part, in which the iron-oxide has been 

 deposited along the plagioclase-bands, by infiltration from the 

 groundmass. A specimen from Tsingari is similar, but it contains 

 more plagioclase, is more coarsely crystalline, and is lighter in colour 

 than an ordinary basalt. 



(ii) Basalts proper. — These are mostly microcrystalline, but 

 in a few a little residual glass may occur. Several contain some 

 ferruginous olivine, and occasionally plagioclase with a cloudy appear- 

 ance, or granular patches caused by a melting-down of small crystals. 

 Among these basalts from Somaliland are a purplish vesicular rock 

 from the river-bed at Hamas, a specimen from near Addi Adeya 

 from a talus (but ' tiers of rock probably similar appear above '), 

 and one from the Hill of Sobolo (5854 feet), where a ' chalk-like 

 rock fills the interstices.' 3 Several microcrystalline basalts come 

 from near Jigjiga, one perhaps rather to the east, but two in situ, 

 one ' from apparently a small intrusive sill' in the pass, and another 

 with sandstones or grits ' on the road to Fantalli and Harrar.' Also 

 associated with grits was a basalt from the hill close to the east 

 side of the camp at Garsa (Karsa). Similar rocks were taken ' from 

 a dyke in the valley west of Colluby,' where the expedition camped, 

 and from the pass high up before getting to camp, and some distance 

 west at the Laga-Hardim Camp, both in the lower and the upper 

 series of rocks. Basalts were collected from the second brook- 

 cutting going from camp into Addis Abbeba, from between that 

 town and Akaki (' exposed among softer tuff-rock '), and in situ at 

 Galanda Lotha towards Mount Saquala. 



A slice cut from a dark-grey finely-speckled basalt from Tsingari 

 contains a few microporphyritic felspars, partly corroded and 



1 Geol. Mag. 1901, p. 411. 



2 ' Beitrage zur geol. Kenntniss des ostlichen Afrika ' pt. ii, Denkschr. d. k. 

 Akad. Wissensch. Wien, vol. lviii (1891) pi. ii, fig. 4. 



3 See p. 303. 



