3 to Salt Lakes paet h 



St. Julian the mud, also, rested on the gravel : hence 

 the depressions must have been formed anteriorly to, 

 or contemporaneously with, the spreading out of the 

 gravel. I was informed that one small salina occurs in 

 an alluvial plain within the valley of the Rio Negro, 

 and therefore its origin must be subsequent to the exca- 

 vation of that valley. When I visited the salina, fifteen 

 miles above the town, the salt was beginning to crystal- 

 lise, and on the muddy bottom there were lying many 

 crystals, generally placed cross-ways of sulphate of soda 

 (as ascertained by Mr. Reeks), and embedded in the mud, 

 numerous crystals of sulphate of lime, from one to three 

 inches in length : M. d'Orbigny 1 states that some of 

 these crystals are acicular and more than even nine 

 inches in length ; others are macled and of great purity : 

 those I found all contained some sand in their centres. 

 As the black and fetid sand overlies the gravel, and 

 that overlies the regular tertiary strata, I think there 

 can be no doubt that these remarkable crystals of sul- 

 phate of lime have been deposited from the waters of 

 the lake. The inhabitants call the crystals of selenite, 

 the padre del sal, and those of the sulphate of soda, 

 the madre del sal; they assured me that both are found 

 under the same circumstances in several of the neigh- 

 bouring salinas ; and that the sulphate of soda is an- 

 nually dissolved, and is always crystallised before the 

 common salt on the muddy bottom. 2 The association 

 of gypsum and salt in this case, as well as in the super- 

 ficial deposits of Iquique, appears to me interesting, 

 considering how generally these substances are associated 

 in the older stratified formations. 



1 ' Voyage Geolog.' p. 64. 



2 This is what might have been expected ; for M. Ballard asserts 

 ('Acad, des Sciences,' Oct. 7, 1844) that sulphate of soda is precipi- 

 tated from solution more readily from water containing muriate 

 of soda in excess, than from pure water. 



