The whole of this space is paved with a bed of tufa,
more or less soft, according as it is covered with soil, or left exposed
to atmospheric influences.
Wherever ant-eaters make deep holes
in this ancient bottom, fresh-water shells are thrown out,
identical with those now existing in the Lake Ngami and the Zambesi.
The Barotse valley was another lake of a similar nature;
and one existed beyond Masiko, and a fourth near the Orange River.
The whole of these lakes were let out by means of cracks or fissures
made in the subtending sides by the upheaval of the country.
The fissure made at the Victoria Falls let out the water of this great valley,
and left a small patch in what was probably its deepest portion,
and is now called Lake Ngami. The Falls of Gonye furnished an outlet
to the lake of the Barotse valley, and so of the other great lakes
of remote times. The Congo also finds its way to the sea
through a narrow fissure, and so does the Orange River in the west;
while other rents made in the eastern ridge, as the Victoria Falls and those
to the east of Tanganyenka, allowed the central waters to drain eastward.
All the African lakes hitherto discovered are shallow, in consequence of being
the mere `residua' of very much larger ancient bodies of water.
There can be no doubt that this continent was, in former times,
very much more copiously supplied with water than at present,
but a natural process of drainage has been going on for ages.
Deep fissures are made, probably by the elevation of the land,
proofs of which are seen in modern shells imbedded in marly tufa
all round the coast-line.
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