358 W. H. HUDLESTON^ E8Q., M.A.^ F.K.S.^ ON THE ORIGIN 



crystallines in Peninsular Africa helps us to understand tlie 

 sandy and unfossiliferous nature of the bulk of its sedimentary 

 rocks. What we now see aie merely the eroded stumps of 

 crystalline masses which once towered in the air, but which 

 have been riven for ages by equatorial storms and rains, and 

 their material distributed by torrents, rivers, and backwaters, 

 so as to help to level up the surface. In this particular case 

 the crystalline system has been invaded by an enormous 

 extent of volcanic extravasations, and if we wish to discover 

 the age of the Great East African Central Chain, as it now 

 exists, we must endeavour to ascertain the period during which 

 these phenomena have been in operation. The origin of Lake 

 Tanganyika itself depends upon these considerations. That 

 this period is post-Jurassic, there can be little doubt, for the 

 strip of Jurassic rock near Mombasa is traversed by dykes, 

 which seem to be connected with the general mass of extra- 

 vasated matter on the central plateau. It is probable, how- 

 ever, that a much later date may be assigned. In this con- 

 nection I woukl refer to Ih: Gregory,* who places the first 

 plateau-eruptions in the Cretaceous, probably towards the close 

 of that period, as is the case with the great basaltic outpourings 

 of Western India. From this time up to the Pleistocene there 

 have been, according to this author, a succession of eruptions 

 and coast-movements, and he places the first series of Eift- 

 Yalley faults (Graben) in the Upper Eocene and the second 

 series in the Pliocene. These statements are made, principally 

 with reference to the eastern arm of the Graben system, bub it 

 woukl probably apply also to the western arm in which Lake 

 Tanganyika is situated. It is pretty clear, however, that 

 volcanic eruptions have taken place, as we now know, down to 

 the present time, and that earth movements have continued, 

 for some of the fault scarps. Dr. Gregory observes, are so bare 

 and sharp that they must be of very recRnt date. 



Enough has now been said with regard to the anomalous 

 history and condition of the Great East African Central Chain 

 and its double string of lakes of depression, Tanganyika is 

 the largest and most peculiar of all these, and its origin is 

 intimately connected with the above considerations. We may 

 believe that its initiation may have taken place in early 

 Tertiary times, but that both its drainage area and also the 

 great Ivift in which it occurs have undergone some modification 

 owing to the instability of the earth's crust in that region. 



The Great Rift Valleij, p. 235. 



