I State The Fact
Exactly As It Opened To My Own Mind, For It Was Only Now That I Apprehended
The True Form Of The River Systems And Continent.
I had seen the various
rivers of this country on the western side flowing from the subtending ridges
into
The centre, and had received information from natives and Arabs
that most of the rivers on the eastern side of the same great region
took a somewhat similar course from an elevated ridge there,
and that all united in two main drains, the one flowing to the north
and the other to the south, and that the northern drain found its way out
by the Congo to the west, and the southern by the Zambesi to the east.
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 any thing 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
toward the centre of the country, and their strike nearly corresponded
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 hundred 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 lie. 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. This 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 favored 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 lie
in a perfectly horizontal position on extensive areas, held in their substance
angular fragments, containing algae 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 hills in the central parts,
composed of clay and sandstone schists, with the ripple mark distinct,
in which no fossils appear; 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.*
-
* 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 Roderick Murchison
thus writes:
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