Wherever a plain sloping towards a narrow opening in hills or higher ground exists, there we have the conditions requisite for the formation of an African sponge.
The vegetation, not being of a heathy or peat-forming kind, falls down, rots, and then forms rich black loam.

In many cases a mass of this loam, two or three feet thick, rests on a bed of pure river sand, which is revealed by crabs and other aquatic animals bringing it to the surface.

At present, in the dry season, the black loam is cracked in all directions, and the cracks are often as much as three inches wide, and very deep.

The whole surface has now fallen down, and rests on the sand, but when the rains come, the first supply is nearly all absorbed in the sand.

The black loam forms soft slush, and floats on the sand.