THE TANGANYIKA PROBLEM . 
57 
supposed, and in the neighbourhood of the central range 
it in general possesses none of those characters which have 
so often been spoken of as showing that Central Africa is, 
geologically speaking, without a history. Granting that 
portions of the continent to the north and east may be the 
remains of a possibly paleozoic land-mass, even these ancient 
terrestrial areas are by no means without interest in a 
geological sense. Consider for a moment the characters of 
the Shiri highland district. It is in reality a tongue of 
the mountainous interior which stretches from the east of 
the Shiri river into the vast Zambesi swamps. Once 
over the steep forest-clad hills which bound the region to 
the south, we are in a hilly upland country, which in the 
configuration of its masses closely resembles the moun- 
tainous districts encountered in the interior of Sicily. The 
hills are of granitoid material, and wherever they enclose 
extensive valleys, these valleys are true rock valleys ; 
and are generally filled up to a certain level with the 
silt of old lakes, and ancient alluvium ; the strange flats 
produced in this way between the hills presenting a very 
curious and typical appearance (see illustration on page 59). 
From the absolutely horizontal character of the layers of 
mud and gravel and ancient lake deposit of which these 
plains are composed, it is evident that they have never been 
disturbed by local elevation or distortion ; and the mass of 
horizontal material encountered in such a valley is, in a 
sense, the measure of the disintegration which has gone 
on since the hills surrounding it assumed their present 
characters. If we could make a cutting, or sink an artesian 
well in the centre of such a valley, we should find it to 
be composed of older and older strata, and the age of these 
beds might, and probably would, be proclaimed by the nature 
of the animal remains we should encounter in the course 
