440 CHARACTER OF THE ROCKS. 



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 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 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 corresponded 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, til! 

 favoured 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 sub- 

 stance angular fragments, containing algse 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 distinct, in which no fossils 

 appear ; but as they are usually tilted away from the 

 masses of horizontal trap, it is probable that they too 



