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 of 
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 
