Chap. XXIY. SIR R. MURCHISON’S EXPLANATION. 
475 
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 accoimt in some measure for the periodical 
floods of the Zambesi, and perhaps the Nile. The rains seem to 
foUow 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 m.uch 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. PresidenVs Address, Eoyal Geographical Society, 1852. 
