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: 

“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 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 defiles have been suddenly formed in the subtending ridges through which some rivers 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 mountains also form, in a general sense, its flanks only.”—­President’s Address, Royal Geographical Society, 1852, p. cxxiii.

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 November, when the sun passes