474 
POSITION OF ROCKS. 
Chap. XXIY. 
di’ains, the one flowing to the N. and the other to the S., and that 
the northern drain found its way out by the Congo to the W., 
and the southern by the Zambesi to the E. I was thus on the 
watershed, or liighest point, of these two great systems, but still 
not more than 4000 feet above the level of the sea, and 1000 feet 
lower than the top of the western ridge we had already crossed; 
yet, instead of lofty snow-clad mountains appearing to verify the 
conjectures of the speculative, we had extensive plains, over which 
one may travel a month without seeing anything higher than an 
ant-hill or a tree. I was not then aware that any one else had 
discovered the elevated trough form of the centre of Africa. 
I had observed that the old schistose rocks on the sides, dipped 
in towards the centre of the country, and their strike nearly corre¬ 
sponded with the major axis of the continent; and also that where 
the later erupted trap-rocks had been spread out in tabular masses 
over the central plateau, they had borne angular fragments of the 
older rocks in their substance; but the partial generalization 
which the observations led to, was, that gveat volcanic action had 
taken place in ancient times, somewhat in the same way it does 
now, at distances of not more than three hundred miles from the 
sea, and that tliis igneous action, extending along both sides of 
the continent, had tilted up the lateral rocks in the manner they 
are now seen to he. The greater energy, and more extended 
range of igneous action, in those very remote periods when Africa 
was formed, embracing aU the flanks, imparted to it its present 
very simple hteral outline. Tliis was the length to which I had 
come. 
The trap-rocks, which now constitute the filhng-up ” of the 
great valley, were always a puzzle to me, till favoured with Sh 
Eoderick Murchison’s explanation of the original form of the 
continent, for then I could see clearly why these trap-rocks, 
wliich still lie in a perfectly horizontal position on extensive areas, 
held in their substance angular fragments, containing algae of the 
old schists, wliich form the bottom of the original lacustrine basin: 
the traps, in bursting through, had broken them oif and preserved 
them. There are, besides, ranges of hills in the central parts, 
composed of clay and sandstone schists, with the ripple mark dis¬ 
tinct, in which no fossils appear; but as they are usually tilted 
away from the masses of horizontal trap, it is probable that they 
