THE TANGANYIKA PROBLEM. 49 



succession of ridges, or to a succession of cusps in trans- 

 verse section, and, when closely compared with the cross 

 section of Nyassa, the structure of the Great Central Range 

 in the region of the south end of Tanganyika will be found 

 to have the following peculiar features : — There are, first, 

 two lines of up-push, one on either side of Lake Rukwa, 

 then a more or less flat space, then a greater upraised ridge 

 flanking the eastern shores of Lake Tanganyika ; next there 

 is the depression of Tanganyika itself, and then another up- 

 raised ridge. In Nyassa the phenomena would be produced, 

 as we have seen, by a double fold in the earth's surface, like 

 that which can be made in a sheet of paper, while in the 

 region of the south end of Tanganyika the whole of the 

 phenomena are at once intelligible, if we suppose that four 

 wrinkles, four such folds, have been upraised. These are 

 not fanciful comparisons, but appear to me to represent, in 

 reality, exactly what has actually taken place. There is, in 

 the physical phenomena of all this Central African region, 

 an extraordinary simplicity, and a primitive boldness which 

 produces on the mind an indelible impression that we are 

 here dealing with a unique example of the initial stages in 

 the shaping of a continental mass, and not, as in most other 

 cases which confront the geologist, with the confused and 

 denuded relics of activities which have long since become 

 extinct. 



From the preceding examination of portions of the 

 great central " graben," it will be observed that the 

 conception which we thus gain of the nature of these 

 valleys, as phenomena incidental to mountain building, 

 is not by any means the same as the original conception 

 entertained by Suess, and more recently repeated by 

 others, all these authors having regarded the valleys as 

 the result of vertical falling in of land from the surface 



