5 : THE TANGANYIKA PROBLEM. 



of high and ancient plateaux. Each so-called rift, how- 

 ems far more correctly conceived as a plait-valley, 

 or what I shall call in future a " eurycolpic fold/' the word, 

 from the Greek evpv/co\7rcK, simply suggesting the fact that 

 valleys having the peculiar characters of "'graben may 

 be caused by folding due to lateral pressure : and, as we 

 have seen, this sort of folding has occurred all along the 

 main central " graben " series in Africa. Throughout this 

 series the valleys seem invariably to have arisen as by- 

 products of such folding, and not through the vertical 

 subsidence of strips of an ancient plateau- 

 Having thus discussed the structural features of the 

 main eurycolpic depression, we may now endeavour to 

 ascertain the number, extent and inter-relationships of 

 these remarkable folds. In attempting to do this we are 

 greatly indebted to a valuable contribution to the subject 

 by Dr. Kohlschutter, who has recently returned from the 

 German Gravitational Survey in East Central Atrica. 

 During the course of the investigations of this survev it 

 was found that the great Xyassa valley, after passing to 

 the north of the group of small volcanic cones in its floor, 

 divides, giving off first a branch to the north-east which 

 becomes wider, and finally appears to open out and 

 disappear among the hills of Ussangu and Uhehe. The 

 more westerly branch of the valley of Xyassa is directly 

 continuous with the valley of Lake Rukwa. and in the 

 vicinity of this lake the valley sends off another branch 

 which runs to the south and appears actually to join the 

 Nyassa valley near the region of Mount Waller. North- 

 ward the Rukwa valley has been found to run at an acute 

 angle into the great depression of Tanganyika itself, and 

 it appears extremely probable that it actually crosses the 

 Tanganyika trough in the region of Karama, appearing 



