Vol.51.] ON THE GEOLOGY OF MOUNT RUWENZ0RI. 673 



16,500 feet, but the highest specimens collected are from 9800 feet 

 on the western and from 13,000 feet on the eastern side. 



The eastern section follows up the valley of the Nyarnwamba. 

 It commences on a small lake (Kianja) a little north of Lake 

 Ruisamba. At the height of 3240 feet it rises thence to the level of 

 the gneiss plateau at 5200 feet. From this point to the 6000-foot 

 contour there is a thick series of gneisses : one specimen of these 

 [154] is a whitish gneiss of medium grain, containing muscovite, 

 biotite, microcline, and numerous rounded blebs of quartz. The 

 arrangement of the inclusions in the last-named of these minerals 

 shows that they have been subjected to great strain. In this gneiss 

 occur some beds of black, foliated epidiorite. At the level of 6000 feet 

 the gneisses are succeeded by dark mica-schist. At 7000 feet the 

 steep slope begins, but the whole surface is covered by bamboo- 

 jungle, and no outcrop of rock could be found. At 10,000 feet the 

 bamboos give place to peat-bogs, which are equally fatal to 

 geological work. At 13,000 feet the base of a cliff is reached, the 

 rock of which is a coarse granitoid gneiss. The specimen collected 

 [140] consists in the main of a mass of schillerized orthoclase ; 

 quartz and microcline also occur in the rock, and it may be only a 

 very coarse-grained variety of that previously described [154] below 

 6000 feet. This rock appears to continue to the height of 15,000 

 feet. The summit of the ridge is here 16,500 feet high. 



Crossing to the western side and descending to the valley of 

 Butagu, the section is resumed at the height of 9800 feet; for, 

 though the height of 13,000 feet was reached, the whole slope was 

 covered by peat and vegetation. From 7500 to 9800 feet the rocks 

 are a series of schists; the specimen selected as a type [111] is a 

 chlorite-schist, crowded with granules of magnetite, and containing 

 ciwstals of muscovite and biotite and nodules of orthoclase. The 

 whole rock has been intensely altered. There is a trace in it of 

 what appears to have been a former foliation crossing that which 

 now dominates in the rock. The highest mass of schist seen also 

 shows signs of intense alteration, but the rock is of a somewhat 

 different type. It is a contorted nodular mica-schist. At the base 

 of the steep slopes of schist occurs a dyke of a basic rock about 

 100 yards in width ; it occurs at the height of 7400 feet. The 

 rock may be called an epidiorite, though in places there is no 

 plagioclase left in it. Where least altered it consists of plagioclase 

 and secondary hornblende ; other specimens consist of crystals of 

 hornblende, needles of actinolite in a quartz-matrix, with a little 

 orthoclase. Rocks of the same type occur elsewhere on Ruwenzori 

 in each of the other sections and at about the samo horizon ; thus 

 in the Yeria valley they are seen between 8000 and 9000 feet, and 

 in Kivata at 9000 feet. There is nothing exactly like them in the 

 Archaean series farther east, but they remind us closely, both in 

 macroscopic and microscopic characters, of some of the altered basic 

 igneous rocks of Mashonaland. 1 The most typical rock is a true 



1 A. R. Sawyer, 'GoldGelds of Mashonaland,' 1894, p. 14. 



