Vol. 59.] ROCKS PROM SOUTHERN ABYSSINIA. 293 



One of these, extinct, but perfect in form, 40 miles south of Addis 

 Abbeba, is Mount Saquala, from which specimens of rocks have been 

 brought. Farther west, beyond the Didesa Kiver, older rock in places 

 underlies basalts or similar volcanic deposits, but the main mass 

 of the mountains is composed of disturbed and contorted schistose 

 and granitic rocks. 



The collection contains some specimens of structural interest, and 

 further illustrates certain petrological features which we may perhaps 

 associate with East African geology. Thus the presence of some 

 pressure-modified Archaean rocks at certain zones, the abundance of 

 volcanic masses, and the occurrence among these of soda-bearing 

 types, will be gathered from the following account. 



II. Crystalline Kocks. 



I have grouped the granites and diorites with the gneisses and 

 schists, as some specimens exhibit transitional characters. These 

 rocks occurred mainly in two areas — one, south-west of Berbera ; 

 the other around the head- waters of the tributaries of the Blue 

 Nile. At both localities the Archaean rocks seem to have been 

 thrust up, and some of the specimens are crushed. Westward of 

 the range bounding the coastal plain, they form a floor largely 

 covered by overlying sandstones, limestones, and volcanic rocks of 

 later epochs. According to Dr. Kcettlitz, they often exhibit a high 

 dip with contortions and foldings. 



(1) Granites. 



The granites, both coarse- and fine-grained, some with porphyritic 

 crystals, consist of quartz, felspar (often plagioclase or microcline), 

 biotite, and occasionally apatite or zircon. In crystals formed of a 

 micropegmatitic growth of two felspars, the alternate, roughly- 

 parallel, irregular streaks are generally plagioclase or microcline. A 

 grey granite with black biotite (from between Dabus and Jem Jem) 

 encloses patches (2 inches or more across) of a close-grained 

 blackish hornblende-schist or iluxion-diorite, as if one magma, and 

 it is doubtful which, had intruded into the other. 



Probably a dyke or vein is represented by a specimen from the 

 same place, of a rather modified microcrystalline garnet-aplite. 

 The felspar shows evidence of strain in its wavy cleavage-planes ; 

 and aggregates of white mica are doubtless derived from an original 

 constituent, the residuum of which may have formed the small 

 associated garnets. Other indications of pressure are found in 

 some granites, although the most marked are in certain gneisses. 



(2) Gneisses. 



Some are normal, but many are pressure-modified, and certain 

 of these are finely speckled (the * pepper-and-salt ' type). One 

 hornblende-gneiss allied to a diabase, crushed and reconstituted, 

 and other much modified specimens come from near Jibuli (Gibeli), 



