Note 17 — Chapter I · FROM MOURZUK TO KOUKA IN BORNOU.

The hills run nearly north and south, edging a little to the westward; they have numerous small bays or recesses, which produce a fine echo; many places with saline incrustations, and some of the large black patches like the frozen surface of a recently ploughed field. Almost all the salt formations are in low, protected situations; the water is near; and, often in the very centre, you have fine fresh springs.—From whence is this salt derived? I have already suggested that the air has a powerful effect, and is a principal agent. There is no reason to believe there are large subterranean salt beds; if these existed to any great extent, we should not have the fresh springs so prevalent.—W. O.