﻿I860.] 



FORBES BOLIVIA AND PERU. 



L5 



be expected to result from the luxuriant vegetation around such a 

 tropical swamp, and from the abundant marine plants in the lagoon 

 itself, would realize the conditions of the French artificial nitre-beds, 

 substituting only carbonate of soda for the carbonate of potash there 

 used : we may consequently, with all fairness, expect a similar result 

 in the production of nitrate of soda on a still larger scale. 



This view appears much strengthened by the occurrence of wood, 

 reeds, or rushes, and other vegetable matter in the nitre-grounds at 

 but little below the surface, as well as from the general position of 

 the nitrate of soda in the saline deposit, as it invariably occurs in 

 the margin or outer edge of these, representing the shelving sides of 

 the hollow or lagoon-basin, the central part of which is composed of 

 layers of sea-salt only, frequently several feet in thickness. 



In seeking for nitrate of soda, the searchers always look to the 

 rising edge of such salt-basins, and further judge of the probability 

 of finding the nitrate from a peculiar moist or clammy state of the 

 ground, which is due to the presence of the chloride of calcium pro- 

 duced by the decomposition above explained. 



The quantity of sulphates, and more especially of sulphate of lime, 

 included invariably in these deposits might, at first sight, appear to 

 the observer too great to suppose it due only to the evaporation of 

 the sea- water ; but I believe that this impression will be dissipated 

 when he sees the enormous amount of gypsum removed in the form 

 of hard white cakes, or sedimentary crust, from the boilers of the 

 large distilling machines in use along this arid coast for producing 

 from the water of the sea a supply of fresh water for the main- 

 tenance of the inhabitants, beasts of burden, and even the locomotive 

 engines of the railways along this coast. It appears not necessary 

 to suppose, as has been put forth, that the sulphates present have 

 been formed by volcanic exhalations acting upon the beds of salt. 

 The boracic acid compounds met with appear, however, to be due to 

 this cause ; and the borate of lime met with in such large quantities 

 appears to be indirectly produced by the condensed vapours of 

 volcanic fumeroles, many of which are still in full activity in this 

 district. 



The gaseous exhalations of these fumeroles have, I believe, never 

 been submitted to a chemical examination ; so that the presence of 

 boracic acid has not actually been proved ; it may,- however, be in- 

 ferred from the general resemblance which these fumeroles bear to 

 those of Tuscany, the Lipari Islands, &c, where it is known to exist. 



The borate of lime is found only on the more elevated part of 

 this saline district, occurring on the eastern side of the same, where 

 the rising ground begins to form the western slope of the adjacent 

 cordilleras. 



As volcanic action is developed on a grand scale in this moun- 

 tain-range, such solfataras or fumeroles, forming lateral orifices on 

 the side of the mountains, are very common ; and we may expect that 

 the waters coming down this slope carry with them in solution the 

 boracic acid contained in the condensed vapours of these solfataras, 

 which, coming into contact with the lime of decomposed porphyry- 



