POSITION OF EOCKS. 51 1 



flowing to the north and the other to the south, and that the north- 

 ern drain found its way out by the Congo to the west, and the 

 southern by the Zambesi to the east. I was thus on the water- 

 shed, or highest 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 con- 

 jectures of the speculative, we had extensive plains, over which 

 one may travel a month without seeing any thing 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 toward the centre of the country, and their strike nearly cor- 

 responded 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 generaliza- 

 tion which the observations led to was, that great 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 this igneous action, extending along both sides 

 of the continent, had tilted up the lateral rocks in the manner they 

 are now seen to lie. The greater energy and more extended 

 range of igneous action in those very remote periods when Africa 

 was formed, embracing all the flanks, imparted to it its present 

 very simple literal outline. This was the length to which I had 

 come. 



The trap rocks, which now constitute the " filling up" of the 

 great valley, were always a puzzle to me till favored with Sir 

 Roderick Murchison's explanation of the original form of the 

 continent, for then I could see clearly why these trap rocks, 

 which still lie in a perfectly horizontal position on extensive areas, 

 held in their substance angular fragments, containing alga3 of the 

 old schists, which form the bottom of the original lacustrine basin : 

 the traps, in bursting through, had broken them off 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 



