V27 



III. MISCELLANEA. 



I. On^m o/'Saliferous Deposits. Salt-Lakes o/* Patagonia 

 and La Plata. 



Salinas or natural salt-lakes occur in various formations on the 

 eastern side of the continent of South America, in the argillaceo- 

 calcareous deposits of the Pampas, in the sandstone of the Rio Ne- 

 gro, where they are very numerous, in the pumiceous and other 

 beds of the Patagonian tertiary formation, and in small primary 

 districts in the midst of this latter formation. Port St. Julian is the 

 most southerly point (lat. 49° to 50°) at which salinas are known to 

 occur*. The depressions in which these salt-lakes lie are from a 

 few feet to sixty metres (as asserted by M. d'Orbigny, Voy. Geol., 

 p. 63) below the surface of the surrounding plains ; and according 

 to this same author, near the Rio Negro tliey all trend either in 

 N.E. and S.W. or in E. and W. lines coincident with the general 

 slope of the plain. These depressions in the plain generally have 

 one side lower than the others, but there are no outlets for drainage. 

 Under a less dry climate an outlet would soon have been formed, 

 and the salt washed away. The salinas occur at different eleva- 

 tions above the sea; they arc often several leagues in diameter; 

 they are generally very shallow, but there is a deep one in a quartz 

 rock formation near Cape Blanco. In the wet season the whole or 

 apart of the salt is dissolved, being re-deposited during the succeed- 

 ing dry season. In a large salina northward of the Rio Negro, the 

 salt at the bottom during the whole year is between two and three 

 feet in thickness. 



This salt rests almost always on a thick bed of black muddy sand, 

 which is fetid, probably from the decay of the burrowing worms in- 

 habiting it. [This mud in some places rests on gravel, and in one 

 case the salina is in an alluvial plain within the valley of the Rio Ne- 

 gro.] When I visited the salina about fifteen miles above the town 

 of El Carmen on the Rio Negro and three or four miles fiom the 

 banks of that river, the salt was beginning to crystallize; and on the 

 muddy bottom there were lying many crystals of sulphate of soda, 

 generally placed cross ways, and, imbedded in the mud, were nunie- 



* According to Azara (Travels, vol. i. p. 5G) there are salt-lakes as far north as 

 Chaco (lat. 25") on the hanks of the Vcnuojo. The salt-lakes of Sihcria appear 

 to occur in depressions very similar to those of Patagonia. — Pallas's Travels, EuaU 

 Tr., vol. i. p. 284, ° 



