OF THE HALOLIMNIC FAUNA OP LAKE TANGANYIKA. 319 



is probably fast dying out. There are yet some traces of 

 volcanic activity in the eastern arm of the Graben. where the 

 mountain Longonot (9,350 feet) still shows a fresh looking 

 crater and emits steam. The facts in regard to the existence of 

 modern volcanic action in the Graben system is of importance in 

 connection with any attempt to estimate the age of this mountain 

 plateau-system, in which the Graben themselves are situated. 



The second feature in connection with the equatorial portion 

 of the Great Central chain is the existence of the short, but 

 lofty Ruwenzori Range, whose southern extremity lies actually 

 on the equator. Whilst the axis of the volcanic chain of Mount 

 M'fumbiro lies at right angles to the northerly trend of the 

 great western Graben, that of the crystalline system of 

 Ruwenzori is approximately parallel to it. " These ranges, 

 which rival the Alps in magnitude and in the sublimity of their 

 scenery, lie along the eastern edge of the depression, and appear, 

 in fact, to stand out into it beyond what was originally its 

 eastern face."* We recognise the importance of the above 

 observation, as it tends to show that this portion of the Graben 

 is older than tlie Ruwenzori Range itself. The adjacent Victoria 

 Nyanza plateau is mainly composed of schists and gneiss, 

 and this class of rock usually terminates abruptly at the 

 eastern edge of the Graben where the depression ensues. But 

 opposite Ruwenzori, instead of being broken off at the edge of 

 the depression, the gneiss and schists are bent and piled upon 

 the steep flanks of the mountains themselves, which in their 

 more central portions are found to consists of massive old 

 amphibolites. These latter are, most probably, the base of the 

 Archaean, as developed throughout the greater part of Equatorial 

 Africa, and these amphibolites seem to have been thrust up 

 through the overlying gneissic and schistose layer.f 



Geology of British East Africa. — Having paid some attention 

 to the physical structure of the Central Range with its 

 associated Lake Chains, it would not be amiss just to glance at 

 the geology of the equatorial region of East Africa and its 

 relations to the remarkable system of Graben already partly 

 described. In this respect we cannot do better than follow 

 Gregory in his description of the region between the Indian 

 Ocean and the Victoria Ny an za, which includes the eastern arm 

 of the Graben system. The subjoined section, which lies 



* Moore, The Tanganyika Problem^ p. 94. He gives the altitude of 

 the highest peaks at about 16,500 feet. 



t Amphibolites are igneous rocks in which hornblende is a chief 

 constituent ; diorite is a common variety. — E. H, (Ed.) 



