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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nor. 21, 



large saline deposits, the chain of hills to the westward or sea side of 

 the " Salinas " is of such a formation as might on elevation he ex- 

 pected to enclose a series of lagoons, which, hy means of the breaks 

 or lateral openings in the chain itself, could for a longer or shorter 

 period keep up a tidal or occasional communication with the sea when 

 high, which thus would pour in a fresh supply of salt water to make 

 up for the loss sustained in the lagoons from the evaporation produced 

 by the heat of a tropical sun. It is therefore not necessary to suppose 

 that the great amount of saline matter generally present in these 

 deposits is due to the salts contained in an amount of sea-water 

 merely equal to the quantity originally contained in the lagoon, or, 

 in other words, to the cubical contents of the lagoons themselves. 



The occurrence of salt at different places along the coast at very 

 small elevations above the sea, previously noticed, is no doubt due 

 merely to the tidal infiltration of sea-water into the porous shingle 

 and other beds, and its subsequent evaporation ; and must not be 

 confounded with the much greater and more elevated saline deposits 

 further inland, which are met with at three very different altitudes 

 above the sea, as follows : — 



(1) At, approximated, from 2500 to 3500 feet, 



(2) „ " 7000 to 8000 feet, 



(3) „ 12,500 feet 



above the present sea-level, and which appear to indicate three di- 

 stinct and important changes in level in this part of South America. 



(1.) The deposits situated at about 2500 to 3500 feet above the 

 present sea-level include the important beds of nitrate of soda so 

 extensively worked along this coast, and appear to run from latitude 

 19° southward into the northern part of the Desert of Atacama, 

 showing themselves, according to the configuration of the country, 

 at distances varying from 10 to 40 miles inland. 



When in this part of the country, I had not time to make a more 

 detailed examination of these saline deposits than was necessary to 

 enable me to arrive at a conclusion as to their mode of formation and 

 the origin of the nitrate of soda contained in them. 



All the data that I coidd obtain appeared fully to confirm the " la- 

 goon hypothesis " previously mentioned, and to prove that the ori- 

 ginal constituents of these beds had merely been such salts as would 

 result from the evaporation of sea-water. The nitrate of soda and 

 some other associated compounds are due to subsequent reactions, 

 and consequent decomposition of the salt of the original deposit, 

 mainly produced by the agency of carbonate of lime and decomposing 

 vegetable matter. 



The first step in the formation of nitrate of soda appears to be the 

 decomposition of the chloride of sodium, or salt, by carbonate of lime 

 (in the fonn of shell- sand, &c.) with the production of chloride of 

 calcium and carbonate of soda, both of which salts have been shown 

 to be present in quantity in the soil qf these nitrate -grounds. 



The carbonate of soda thus eliminated, when in contact with the 

 mixture of shell-sand and decomposing vegetable matter which may 



