We now left the shores of the lake, ascending a little, but had a difficult march this morning in order to avoid the many small boggy inlets and natron-lagoons which are formed by the lake, and wind along through the sand-hills. With regard to these natron-lakes, which, after the report of Major Denham, have led to many erroneous conjectures respecting lake Tsád, I have to observe that the natron or soda is not originally contained in the water, but in the ground, and that all the water of lake Tsád is fresh; but when a small quantity of water, after the lake has retired from the highest point of its inundation, remains in a basin the soil of which is filled with soda, the water of course becomes impregnated with this quality. The consequence is, that there are many basins round lake Tsád which, according to the season, are either fresh or brackish; for the soda contained in the ground has very little effect so long as the basin is deep, and does not begin to make itself felt till the water becomes shallow. Of this same character seems to be Lake Bóro in Kánem, which I shall mention hereafter. I here remind the reader of what I have stated above with regard to the importance of the natron-trade between Bórnu and Núpe or Nýffi. Having no guide—for who would willingly trust himself in the hands of such lawless robbers as our companions?—we found it rather difficult work to get out of this labyrinth of lagoons; and after a few miles we came to a narrow but very boggy inlet, which it was thought necessary to cross.