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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



rocks, or the shell-sands of the plains "below, would combine readily 

 to form the nodules of borate of lime here met with. 



It has been suggested that the nitrate of soda likewise owes 

 its origin to similar causes ; I consider, however, this view to be 

 untenable where the vapours themselves, from the great amount 

 of sulphurous and hydrosulphuxie acid gases which they contain, 

 are of so eminently deoxidizing a nature as to decompose any 

 nitric fumes evolved by such volcanic action. I therefore believe 

 the nitrate of soda wholly due to the chemical action previously ex- 

 pounded. 



The saline deposits of this series do not rest directly on the 

 rock itself, but on a beach more or less level, or hollowed out into 

 lagoon-basins, and composed, as the present and the raised sea-beach 

 previously described, of the debris of the adjacent porphyri tic, dioritic, 

 and volcanic rocks. 



The deposits explored for nitrate extend from the river of Pisagua 

 southward to Patillos, a distance of about 110 miles; but latterly 

 new and extensive deposits have also been worked further south, 

 inland from Tocopilla. There is, however, no doubt that they exist 

 along the whole coast-line depicted on the accompanying map, at 

 from 10 to 50 miles inland ; the borate deposits, however, appear to 

 recede from the coast, as they occur more to the south, and strike 

 to the eastward, following the line of volcanic action, indicating 

 thereby their connexion with the same. 



(2.) The series of saline deposits next in elevation are situated 

 at from 7000 to 8000 feet above the level of the sea, and are de- 

 veloped on a grand scale in the northern part of the desert of 

 Atacama, — the great " Salina de Atacama " extending 100 miles or 

 more from S.E. to N.W., with a breadth of 20 to 30 miles, and 

 the lesser " Salina de Punta Negro " still further south (about 30 

 miles long and 12 broad) — two examples of immense salt-plains, 

 apparently resulting from the drying up of such lagoons as those 

 before described. 



Not having made a personal examination of these, I am not in a 

 position to give any detailed account of them ; in fact, they are only 

 known in name and extent, and have never been examined. 



(3.) At an altitude of about 13,000 feet above the sea, saline matter 

 is found to occur in a manner similar to that of the last-mentioned 

 deposits. In Section No. 2 (PI. II.), at " Laguna Blanca," extensive 

 plains and salt-lagoons are found, — the latter still existing as lagoons, 

 since they are now situated on the extreme borders of the rainless 

 region, whereby the loss from evaporation is supplied, in part, by the 

 rain which falls ; and thus we generally find extensive plains covered 

 by white crystalline salt, forming the circumference of some small and 

 generally shallow lake, deserving only the name of a swamp except 

 in the rainy season of the year. This saline formation, I believe, is 

 seen more or less developed all the way to Oruro, and thence over the 

 saline plains of Sora-Sora, it extends much further south, but, like 

 the last, has not as yet formed a subject for more minute examina- 

 tion, and. from its occurring in districts exposed to a heavy annual 



