18 I. C. KUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



exceptional conditions, and the point of concentration and deposition for 

 any of the substances present in solution. 



In the process just outlined, concentration by evaporation is the lead- 

 ing control, but chemical reactions, life, and variation in temperature, 

 escape of gases which enhance the solvent power of water, and other con- 

 ditions exert important influences. 



Of the secondary conditions, variation in temperature has received less 

 attention than perhaps any of the other modifying conditions enumer- 

 ated, but its importace is indicated on a grand scale by the precipitation 

 of sodium sulphate from the waters of Great Salt lake during the preva- 

 lence of abnormally cold seasons. 



The escape of gases from water when pressure is relieved, as in the 

 case of springs which have their sources deep in the earth, or when greater 

 surface of exposure to the air is brought about, as in water percolating 

 into caverns, is also an important and at times a controlling factor lead- 

 ing to precipitation. The best known illustration of this process is the 

 escape of carbon dioxide, which when present in water gives it enhanced 

 power to hold calcium bicarbonate in solution. The critical study of this 

 process from a geological point of view, and concerning other gases in 

 addition to carbon dioxide, remains to be undertaken, but gives promise 

 of furnishing instructive information. 



The results of concentration by evaporation fall into two groups, each 

 of which may be of commercial value. Solids are precipitated from solu- 

 tions which themselves remain as liquid concentrates or mother liquors, 

 and while yet in the open evaporating pans in which they were produced 

 are n some instances of economic importance as well as of scientific inter- 

 est. A place is thus provided in our general scheme of classification of 

 natural concentrates for the brines of dead seas and buried saline waters, 

 in some instances rich in bromides, iodine, etcetera, as well as rock-salt, 

 gypsum, anhydrite, and for saline deposits of varied character, such as 

 are mined at Stassfurt. The geographic distribution of the solid and 

 liquid products of concentration resulting from evaporation while still 

 present at the surface of the earth is controlled mainly by the distribution 

 of solar heat and of precipitation. The conditions favoring this produc- 

 tion and preservation increase from locality to locality with increase in 

 mean annual temperature and with decrease in precipitation, providing 

 the requisite conditions of relief of surface, such as the pressure of in- 

 closed basins, are present. With a knowledge of the climatic conditions 

 essential to the production of concentrates by evaporation and of the 

 nature of the receptacles required, a means is furnished for determining 

 the conditions under which buried fossil salts and brines of similar char- 



