316 GEOLOGICAL FEATURES OF INTERIOR. Chap. XXIV. 



I could see clearly that these angular fragments formed the 

 bottom of the original lacustrine basin,, and that the traps, in 

 bursting through, had broken them off and preserved them. 

 There are, besides, ranges of hills in the central parts, com- 

 posed of clay and sandstone schists, with the ripple-mark 

 distinct, in which no fossils have been discovered ; but as they 

 are usually tilted away from the masses of horizontal trap, it is 

 probable that they too were a portion of the original bottom, 

 and fossils may yet be found in them. 



The characteristics of the rainy season in this wonderfully 

 humid region may account in some measure for the periodical 

 floods of the Zambesi. The rains seem to follow the course of 

 the sun, for they fall in October and November, when the sun 

 passes over this zone to the south. When the sun reaches the 

 tropic of Capricorn in December, a dry season ensues, and 

 injurious droughts are much dreaded in December and January. 

 As the sun returns again to the north, in February, March, 

 and April, the great rains of the year fall ; and the plains, 

 which in October and November had imbibed rain like 

 sponges, now become supersaturated, and pour forth those 

 floods of clear water which inundate the banks of the Zambesi. 

 Somewhat the same phenomenon probably causes the periodical 

 inundations of the Nile, and the difference in the period of 



in the central part of South Africa, as proved by freshwater and terrestrial fossils, 

 Sir Roderick Murchison thus writes : — 



" Such as South Africa is now, such have been her main features during count- 

 less past ages, anterior to the creation of the human race. For the old rocks 

 which form her outer fringe unquestionably circled round an interior marshy or 

 lacustrine country, in which the Dicynodon flourished, at a time when not a 

 single animal was similar to any living thing which now inhabits the surface of 

 our globe. The present central and meridian zone of waters, whether lakes or 

 marshes, extending from lake Tchad to lake Ngami, with hippopotami on their 

 banks, are therefore but the great modern residual geographical phenomena 

 of those of a mesozoic age. The differences, however, between the geological 

 past of Africa and her present state are enormous. Since that primeval time 

 the lands have been much elevated above the sea-level — eruptive rocks piercing 

 in parts through them ; deep rents and defiles have been suddenly formed in the 

 subtending ridges through which some rivers escape outwards. 



" Travellers will eventually ascertain whether the basin-shaped structure, which 

 is here announced as having been the great feature of the most ancient, as it is 

 of the actual geography of South Africa {i.e. from primeval times to the present 

 day), does, or does not, extend into Northern Africa. Looking at that much 

 broader portion of the continent, we have some reason to surmise that the higher 

 mountains also form, in a general sense, its flanks only." — p. cxxiii. Pre&ident't 

 Address, Royal Geographical Society, 1852. 



