chap. x. of La Plata and Patagonia. 309 



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

 occur. 1 The depressions, in which these salt-lakes lie, 

 are from a few feet to sixty metres, as asserted by 

 M. d'Orbigny, 2 below the surface of the surrounding 

 plains ; and, according to this same author, near the 

 Rio Negro they all trend, either in the NE. and SW. 

 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 elevations 

 above the sea; they are often several leagues in dia- 

 meter ; they are generally very shallow, but there is a 

 deep one in a quartz-rock formation near 0. Blanco. In 

 the wet season, the whole, or a part, of the salt is dis- 

 solved, being redeposited during the succeeding dry 

 season. At this period the appearance of the snow- 

 white expanse of salt crystallised in great cubes, is very 

 striking. 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. 



The salt rests almost always on a thick bed of black 

 muddy sand, which is fetid, probably from the decay 

 of the burrowing worms inhabiting it. 3 In a salina, 

 situated about fifteen miles above the town of El Car- 

 men on the Rio Negro, and three or four miles from 

 the banks of that river, I observed that this black mud 

 rested on gravel with a calcareous matrix, similar to 

 that spread over the whole surrounding plains : at Port 



1 According to Azara (' Travels,' vol. i. p. 56) there are salt lakes 

 as far north as Chaco (lat. 25°), on the banks of the Vermejo. The 

 salt lakes of Siberia appear (Pallas's ' Travels,' English Trans, vol. i. 

 p. 284) to occur in very similar depressions to those of Patagonia. 



2 ' Voyage Geolog.' p. 63. 



3 Prof. Ehrenberg examined some of this mudd} T sand, but was 

 unable to find in it any infusoria. 



