The saline incrustations, near Bahia Blanca, are not confined to, though most abundant on, the low muddy flats; for I noticed some on a calcareous plain between thirty and forty feet above the sea, and even a little occurs in still higher valleys.Low alluvial tracts in the valleys of the Rivers Negro and Colorado are also encrusted, and in the latter valley such spaces appeared to be occasionally overflowed by the river.I observed saline incrustations in some of the valleys of Southern Patagonia.At Port Desire a low, flat, muddy valley was thickly incrusted by salts, which on analysis by Mr.T.Reeks, are found to consist of a mixture of sulphate and muriate of soda, with carbonate of lime and earthy matter.On the western side of the continent, the southern coasts are much too humid for this phenomenon;but in Northern Chile I again met with similar incrustations.On the hardened mud, in parts of the broad, flat-bottomed valley of Copiapo, the saline matter encrusts the ground to the thickness of some inches:
specimens, sent by Mr.Bingley to Apothecaries' Hall for analysis, were said to consist of carbonate and sulphate of soda.Much sulphate of soda is found in the desert of Atacama.In all parts of South America, the saline incrustations occur most frequently on low damp surfaces of mud, where the climate is rather dry; and these low surfaces have, in almost every case, been upraised above the level of the sea, within the recent period.
SALT-LAKES OF PATAGONIA AND LA PLATA.
Salinas, or natural salt-lakes, occur in various formations on the eastern side of the continent,--in the argillaceo-calcareous deposit of the Pampas, in the sandstone of the Rio Negro, 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 S.Julian is the most southerly point (latitude 49 degrees to 50 degrees) at which salinas are known to occur.(According to Azara "Travels" volume 1 page 56, there are salt-lakes as far north as Chaco (latitude 25 degrees), on the banks of the Vermejo.The salt-lakes of Siberia appear (Pallas "Travels"English Translation volume 1 page 284) to occur in very similar depressions to those of Patagonia.) The depressions, in which these salt-lakes lie, are from a few feet to sixty metres, as asserted by M.d'Orbigny, below the surface of the surrounding plains ("Voyage Geolog." page 63.); and, according to this same author, near the Rio Negro they all trend, either in the 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 elevations above the sea; they are often several leagues in diameter; they are generally very shallow, but there is a deep one in a quartz-rock formation near C.Blanco.In the wet season, the whole, or a part, of the salt is dissolved, 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.
(Professor Ehrenberg examined some of this muddy sand, but was unable to find in it any infusoria.) In a salina, situated about fifteen miles above the town of El Carmen 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 S.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 excavation of that valley.