The Zambesi flows above the Kebrabasa, through great beds of the same formation, and generally they are of hard crystalline rocks; and it is impossible to conjecture what the condition of the country was when the large pot-holes were formed up the hillsides, and the prodigious attrition that rounded the shingle was going on.

The land does not seem to have been submerged, because marine limestones (save in the exceptional cases noted) are wanting; and torrents cutting across the ancient river beds reveal fresh-water shells identical with those that now inhabit its fresh waters.

The calcareous tufa seems to be the most recent rock formed.

At the point of junction of the great southern prehistoric Nile with an ancient fresh-water lake near Buchap, and a few miles from Likatlong, a mound was formed in an eddy caused by some conical lias towards the east bank of this rent within its bed, and the dead animals were floated into the eddy and sank; their bones crop out of the white tufa, and they are so well preserved that even the black tartar on buffalo and zebra's teeth remain: they are of the present species of animals that now inhabit Africa.

This is the only case of fossils of these animals being found _in situ_.