DESCRIPTION OF A TRONA LAKE. 261 



although rarely in isolated rocks, ridges, and cliffs. From 

 these details it appears that the general basis of the Desert 

 consists of secondary rocks, principally sandstone and 

 limestone. 



Description of a Trona or Natron Laie.— Natron or trona, 

 as already mentioned, is found in various parts of the Desert, 

 but principally in its eastern half. Dr. Oudney describes, 

 in the following terms, in a letter to us, afterward printed 

 in Denham's Travels, the wadey Trona he passed through 

 in his journey from Tripoli to Mourzouk : — ^^ Monday, July 8. 

 — We entered the wadey Trona early this morning, on 

 the north-east side. Near where we entered there are a 

 cluster of date palms, and a small lake, from which impure 

 trona is obtamed. On the western side the trona lake is 

 surrounded with date-trees, and its marshy borders are 

 covered on almost all sides by grass and a tall juncas. It 

 is about half a mile long, and nearly 200 yards wide. At 

 present it is of inconsiderable depth, from the evaporation 

 qf the water ; for many places are dry now, which are 

 covered in the winter and spring. The trona crystallizes at 

 the bottom of the lake when the water is sufficiently satu- 

 rated ; for when the water is in large quantities it eats the 

 trona, as the people say. The cakes vary in thickness 

 from a fine film to several inches (two or three). The 

 thickest at present is not more than three-fourths of an 

 inch ; but in the winter, when the water begins to increase, 

 it is of the thickness I have mentioned. The surface next 

 the ground is not unequal from crystallization, but rough to 

 the feel from numerous small rounded asperities. That 

 next the water is generally found studded with numerous 

 small, beautiful cubical crystals of muriate of soda ; the 

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

 removed from the other. When not covered with muriate 

 of soda, the upper surface shows a congeries of small 

 tabular pieces joined in every direction. When the mass is 

 broken, there is a fine display of reticular crystals, often 

 finely radiated. The surface of the water is covered in 

 many places with large thin sheets of salt, giving the whole 

 ihe appearance of a lake partially frozen over ; film after 

 film forms till the whole becomes of great thickness. Thus 

 may be observed, on ^he same space, trona and cubical 



