The Yellala of the Congo. 287 
good bird’s eye view of the scene. The old river- 
valley, shown by the scarp of the rocks, must have 
presented gigantic features, and the height of the 
trough-walls, at least a thousand feet, gives the 
Yellala a certain beauty and grandeur. The site 
is apparently the highest axis of the dividing ridge 
separating the maritime lowlands from the inner 
plateau. Looking eastward the land smoothens, 
the dorsa fall more gently towards the counter- 
slope, and there are none of the “ Morros” which 
we have traversed. 
With the members of the Congo Expedition, I 
was somewhat startled by the contrast between the 
apparently shrunken volume of waters and the vast 
breadth of the lower river; hence Professor 
Smith’s theory of underground caverns and com¬ 
munications, in fact of a subterraneous river, a 
favourite hobby in those days. But there is not 
a trace of limestone formation around, nor is there 
the hollow echo which inevitably would result from 
such a tunnel. Evidently the difference is to be 
accounted for by the rapidity of the torrent, the 
effect of abnormal slope deceiving the eye. At 
the Mosi-wa-tunya Falls the gigantic Zambeze, 
from a breadth of a thousand yards suddenly 
plunges into a trough only forty-five to sixty feet 
wide : the same is the case with the Brazilian 
Sao Francisco, which, a mile wide above the 
Cachoeira de Paulo Affonso, is choked to a 
