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 
4 
