474 POSITION OF ROCKS. Chap. XXIV. 



drains, 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 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 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 great volcanic action had 

 taken place in ancient times, somewhat in the same way it does 

 now, at distances of not more than three hunched 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 he. 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. Tins 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 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 he hi a perfectly horizontal position on extensive areas, 

 held in their substance 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 lulls 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 



