388 Account of the 'Dona Lake in Africa. 



derable depth at present (July), from the evaporation of the 

 water, and many places are dry now which are covered in the 

 winter and spring. The trona is deposited in cakes at the bot- 

 tom of the lake, when the saturation is at a certain extent. The 

 cakes are of various degrees of thickness, from the finest film to 

 several inches. The thickest I could find was not more than 

 |ths of an inch; but at the beginning of winter, when the water 

 begins to increase, it is of the thickness I have mentioned, and 

 it is then said to be ripe. The surface next the earth is not 

 unequal from crystallization, but rough to the feel with numer- 

 ous rounded asperities. That next the water is generally found 

 studded with small beautiful cubical crystals of muriate of soda; 

 the line of junction is always distinct, and the one is easily re- 

 moved from the other. The upper surface, when not covered 

 with the muriate, is composed of a congeries of small tabular 

 pieces, joined in every position ; when the mass is broken, there 

 is a fine display of acicular crystals, often radiated. The sur- 

 face of the water is covered in many places with large thin 

 sheets of a carbo-muriate of soda, giving the whole the ap- 

 pearance of a lake partially frozen over : film after film forms, 

 till the whole gets of considerable thickness. The soil of the 

 lake is a dark brown sand, approaching to black, of a viscid 

 consistence, and slimy smell, and on the lately uncovered sur- 

 face, near the banks, a black substance, like mineral tar, is 

 seen oozing out. 



The water begins to increase in the winter, and in the spring 

 it is at the maximum. The trona is best about the commence- 

 ment of winter, but disappears entirely in the spring. 



The lake has diminished considerably in size within the last 

 few years, and if care be not taken, the diminution will soon be 

 much greater : plants are making rapid encroachments, and 

 very shallow banks are observable in many places. On inquiry, 

 we found the quantity of trona had not sensibly diminished for 

 the last ten years ; perhaps it may appear so, from there al- 

 ways being sufficient to answer every demand. The quantity 

 annually exported amounts to between 400 and 500 camel- 

 loads, each equal to about 4 cwt. — a large quantity, when the 

 size of the lake is considered. It is removed only when a de- 

 mand comes ; then a man wades in, breaks it off in large sheets, 

 which he easily does; hands it to others outside, who are 

 ready to remove all foreign matters, and pack it in the setose 

 bases of the palm-leaves. The water in the valley is good, 

 and if a well be dug on the very border of the lake, the water 

 is also good, and sensibly free from saline impregnation." 



