THE TANGANYIKA PROBLEM. 43 



No. 3), and immediately behind these there are faulted up into 

 the air the imposing cliffs of Mount Waller itself. At the 

 foot of Mount Waller, the igneous base on which the two 

 thousand and odd feet of sandstones and conglomerate rest, 

 was exposed above the water line when I visited the region 

 in 1 896, and near this point to the north there were other sand- 

 stone ridges which like those to the south had been faulted 

 to a less extent than the Mount Waller mass, and bore on 

 their upper surfaces thick stratified beds of modern chalky 

 lake deposit. In these beds there were, moreover, fossilised 

 shells similar in all respects to those now living in Nyassa 

 and showing incontestably that the upthrust along the line 

 of the west coast of the lake had been going on and had 

 caused fresh faulting of the stratified material near the lake 

 shore, at any rate since Nyassa contained its modern fresh 

 water fauna. But besides thus giving evidence of modern 

 (post pleistocene) activity and continual rising of the cusps 

 of the great central African ridge, the phenomena I have 

 just described near Mount Waller in Nyassa are intensely 

 instructive in another sense. They show that where hori- 

 zontal strata overlie a region where folding of this sort is 

 going on, a typical faulted trough with vertical sides (a 

 so-called rift valley) can be produced by the rising of 

 the sides quite as well as by the falling in of a central 

 strip of land. Lateral compression of the earth's surface, 

 as by the shrinkage of the globe, would actually pro- 

 duce these effects, just as we can bend a piece ot 

 paper up into two folds with a valley in the middle ; and in 

 this case we see also that as the lateral pressure increases 

 the central valley in the paper tends to deepen at the same 

 time that the two ridges are rising. All the phenomena 

 encountered in the Nyassa region are at once perfectly 

 intelligible if we suppose that folding of the earth's surface 



