Chap. XXXIX. FRESH-WATER LAKES AND NATRON. 53 
We now left the shores of the lake, as- Saturdav 
cending a little, but had a difficult march Se pt- 27th - 
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 Tsad, 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 Tsad is fresh ; but when a small quan- 
tity 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 Tsad 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 Boro in K&nem, 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 Bornu and Niipe or Nyffi. 
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 
Kanem. 3d. Beri. Some people going from Ngegimi to Beri 
sleep the first night in Turra, second night in Balaya. 
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