﻿I860.] 



FORBES BOLITIA AND rEllTT. 



13 



sively employed for this purpose at Arica and Tacna. In the quarries 

 of this rock near Tacna I discovered a mineral very much resembling 

 allophane in external characters, but differing in only containing 

 28-49 per cent, of water : it occurs in fissures in the trachytes, 

 forming veins of from a line up to some inches in thickness, and is 

 probably derived from the decomposition of the felspathic element of 

 the trachyte by the action of water. 



2. Saline Formations. — Later in age than the Tertiary deposits, 

 the saline formations so characteristic of this part of South America 

 are not, as frequently supposed, merely confined to the country 

 surrounding the port of Iquique, but appear at intervals scattered 

 over the whole of that portion of the western coast on which no rain 

 falls, extending further north than the limits of the map accompany- 

 ing this memoir ; whilst to the south they run entirely through the 

 desert of Atacama, and even show signs of their existence further 

 south than Copiapo in Chile, thus stretching more than 550 miles 

 north and south ; their greatest development appears, however, 

 between latitudes 19° and 25° S. 



They are generally superficial, but occasionally reach to some 

 small depth below the surface, and then may be entirely covered 

 over by diluvial detritus ; they always, however, show signs of their 

 existence by the saline efflorescence seen on the surface of the ground, 

 which often covers vast plains as a white crj^stalline incrustation, the 

 dust from which, entering the nostrils and mouth of the traveller, 

 causes much annoyance, whilst at the same time the eyes are 

 equally suffering from the intensely brilliant reflection of the rays of 

 a tropical sun. 



The salts forming these " Salinas," as they are generally termed, 

 are combinations of the alkaline and earthy bases soda, lime, mag- 

 nesia, and alumina, with hydrochloric, sulphuric, nitric, and carbonic 

 acids, and occasionally with boracic, hydriodic, and hydrobromic 

 acids, — and in combination present themselves as the following 

 minerals in a more or less pure state : — Common salt, epsom-salt, 

 glauber-salt, thenordite, glauberite, soda-alum, magnesia-alum, gyp- 

 sum, anhydrite, along with chloride of calcium, iodide and bromide 

 of sodium, carbonate and nitrate of soda, and in some places borate 

 of lime and borax. 



"With the exception of the boracic acid compounds, the presence of 

 which (as subsequently will be attempted to be proved) is due to 

 volcanic causes, all the mineral substances found in these " Salinas " 

 are such as would be left on evaporating sea-water, or by the mutual 

 reactions of the saline matter thus left on evaporation on the lime, 

 alumina, and organic matter found in the adjacent rocks, soil, and 

 shell-beds ; and as we have indisputable evidence of the recent 

 elevation of the whole of this coast, and bearing in mind likewise 

 that no rain falls in these regions, it appears very reasonable to 

 suppose that all these saline deposits owe their origin to lagoons of 

 salt water, the communication of which with the sea has been cut 

 off by the rising of the land. When studying the structure of the 

 mountain-ranges near the coast, it was also observed that, at all the 



