Chap. XXIV. SIR E. MURCHISON'S EXPLANATION. 475 



too were a portion of the original bottom, and fossils may yet be 

 foimd 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, and perhaps the Nile. The rains seem to 

 follow the course of the sun, for they fall in October and Novem- 

 ber, when the sun passes over this zone on his way south. On 

 reaching the tropic of Capricorn in December, it is dry; and 

 December and January are the months in which injurious droughts 

 are most dreaded near that tropic (from Kolobeng to Linyanti). 

 As he returns again to the north, in February, March, and April, 

 we have the great rains of the year ; and the plains, which in 

 October and November were well moistened, and 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. The two rivers rise in the same region ; 

 but there is a difference in the period of flood, possibly from their 



* After dwelling upon the geological structure of the Cape Colony as 

 developed by Mr. A. Bain, and the existence in very remote periods of lacus- 

 trine conditions 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 

 countless 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 geo- 

 graphical 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. President's Address, Iioycd G ' cograjjhical Society, 1852. 



