Summary of Geuloo-ical Observations. 



>5 



All the gneiss and granitic rocks are strikingly conformable in their 

 structure and composition, and their characters agree, broadly speaking, with 

 the descriptions already given hy those writers who have occupied themselves 

 with the crystalline formations of South and Central Africa, thus further 

 showing the prevailing uniformity in the constitution of the extensive archsean 

 plateau. 



In the first archwan zone, that is, between the shores of the lake and the 

 overlying primitive formations near Mitiana, the mica-schists seem to prevail, 

 these being associated with gneiss and thick quartzite beds. The mica of the 

 micaceous schists is the mnscovite variety, which occurs in large lamina?, thus 

 forming rocks of a very marked schistose type, l)ut always highly reddened, and 

 often somewhat disintegrated by aerial denudation. 



When we pass beyond the palaeozoic and thus re-enter the archaean zone, 

 we find the region between Kasiba and Muyongo constituted entirely of a large- 

 grained granite, which appears to form a range running about south to north. 

 In this granite are noticed hydiomorphic forms of felspar, which in their greatest 

 development reach two inches and more. In the granular mass of the rock 

 qiuxrtz abounds, while the hiotite mica is, as a rule, relatively scarce. Throughout 

 the whole region the granite is always profoundly metamorj)hized, a fact which 

 contrasts with the relatively fresh aspect of the rock in the granitic outcrops of 

 the palaeozoic zone. 



At Muyongo the mica-schists again become associated with gneiss, the 

 latter predominating. Such association, always accompanied by quartzite beds, 

 and in places by minute hiotite mica-schists, and by talc-schists, is continued right 

 up into the Ruwenzori Range, into the constituents of which it largely enters. 



The gneiss is of a schistose character which is never very distinct, hence is 

 to be considered as a granitic gneiss, the micaceous element of which is hiotite, 

 and presents an always more or less pronounced kataclastic structure. Charac- 

 teristic of this rock are everywhere the really considerable abundance of the 

 ferruginous minerals, such as magnetite, ibneaite, and hematite (the changes of 

 which explain the frequent reddish surface of gneiss), and the constant presence 

 of microcline, which becomes the prevailing, one may even say often the 

 exclusive, felspar variety in this rock. This indeed is a fact which has already 

 been recorded in other parts of Central and South Africa. 



In the neighbourhood of Ruwenzori biotite gneiss, either normal or with 

 a predominance of microcline, is partly replaced by amphibolic gneisses. In 

 several districts, but especially in the granitic range between Kasiba and 

 Muyongo, are noticed outcrops of 2)egmatite and micro-granite ; here the 

 pegmatite has never the coarse-grained structure comparable to that of granite, 



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