370 PRINCIPLES OF STRATIGRAPHY 



Sources of Boric Acid and Borates and of Nitrates. 



Boric acid and saline borates are commonly formed by volcanic 

 activities, as in the case of the Tuscan fumaroles, where ammonium 

 salts also occur, an association found in undoubted volcanic vents. 

 Leaching of rocks containing borosilicates (tourmaline, axinite, dato- 

 lite, etc.), as in the case of granites, mica schists, etc., also probably 

 accounts for some of the deposits of borates, and more rarely such 

 deposits originate from sea water, though Ochsenius and some other 

 writers attempt to refer all such deposits to a marine origin. When 

 the origin is undoubtedly a marine one, magnesium borates are the 

 result, while lake deposits, like those of California and Chile, con- 

 tain calcium borates, with nitrates near by. X'olcanic waters and 

 fumaroles, on the other hand, yield ammonium compounds along 

 with the borates. The sodium nitrate deposits of Chile and other 

 regions and the extensive potassium nitrate beds of Bolivia are like- 

 wise regarded by Ochsenius, Penrose, and others as of marine 

 origin, either directly or indirectly. Ochsenius derives the Chilean 

 nitrates from the mother liquors of salts deposited in the Andes, 

 which, flowing downward to the plains, have their chlorides partly 

 converted to carbonates by CO^ of volcanic origin. The nitrogen 

 is brought as ammoniacal dust from guano beds upon or near the 

 sea coast, the heavier phosphatic particles being left behind. The 

 sufficiency of the amount of ammoniacal dust thus carried has been 

 questioned, and it has been pointed out that carbonates are com- 

 paratively rare in the nitrate regions. 



Penrose (44 :i6) regards the nitrate fields as a former ocean bot- 

 tom and likewise derives his nitrogen from guano, the iodine from 

 decomposing sea weeds or from mineral springs and the accompany- 

 ing borates from the decomposition of rocks, containing boron-bear- 

 ing minerals. A volcanic origin has also been suggested for the 

 nitrogen (Clarke-io, 2d ed.:246). 



Summary. 



To sum up, it is apparent that salt deposits of to-day, when 

 not referable to a volcanic source, are only to a slight extent due to. 

 direct evaporation of the water on the sea coast, but that by far 

 the more prominent mode of formation is a secondary one, the salt 

 being derived from the enclosed connate waters of marine sediments 

 or from the products of alteration. In all cases a relatively arid 

 climate is necessary to permit extensive evaporation, so tliat salt 



