512 SIR E. MUECRTSON'S EXPLANATION. 



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, 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 De- 

 cember 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 lacustrine conditions 

 in the central part of South Africa, as proved by fresh-water and terrestrial fossils, 

 Sir Eoderick 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 nourished, at a time when not a single animal was 

 similar to any living thing which now inhabits the surface of our globe. The pres- 

 ent 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 de- 

 files have been suddenly formed in the subtending ridges through which some riv- 

 ers escape outward. 



"Travelers 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 mount- 

 ains also form, in a general sense, its flanks only." — President's Address, Royal Ge- 

 ographical Society, 1852, p. cxxiii. 



