CHEMICAL CONCENTRATION 17 



gravel deposits, it being in such situation that the solvent action of per- 

 colating water is least and where, also, the conditions favoring precipita- 

 tion from slowly percolating water are more favorable. 



The process of subsurface concentration of mineral matter, owing to 

 evaporation, leads to the hardening of rock outcrops, the production of 

 efflorescence, gypsum rosettes, similar incrustations in caverns, and is a 

 factor favoring the growth of stalactites and stalagmites. 



The process of concentrating readily soluble minerals, owing to the 

 evaporation of water from the surface of the land, is illustrated by the 

 efflorescence which forms in desert regions, where, owing to the drying of 

 porous material, water is drawn from below the surface by capillary 

 action and the mineral matter it holds in solution is precipitated as the 

 water evaporates. This snow, as it appears when viewed from a distance, 

 of hot arid lands, frequently contains sodium chloride, sulphate and car- 

 bonate, as well as boracic acid, calcium, etcetera, and is at times of com- 

 mercial value. ■ 



The rise and outflow of previously subsurface water from springs leads 

 to important changes in the conditions exerting an influence on the sol- 

 vent power of the emerging waters which favor concentration of mineral 

 matter in a variety of ways. Decrease of pressure, freedom of escape for 

 dissolved gases, decrease in temperature of water which comes from a 

 considerable or great depth, and a rise in temperature from the water of 

 hillside springs (the latter favoring the precipitation of calcium car- 

 bonate), exposure to the air thus preventing oxidation, and most of all 

 the emergence of the water into the light, thus favoring in a high degree 

 the growth of living organisms, and particularly of algae, lead to the 

 formation of abundant concentrates, the most common of which are cal- 

 cium carbonate, silica, gypsum, iron oxide, inorganic acids, etcetera. In 

 this complex process evaporation is an important factor, particularly in 

 the precipitation of silica by hot springs and geysers. 



During the transit of surface water supplied in part by springs and 

 seepage to inclosed lakes and the ocean, concentration by evaporation 

 continues, although precipitates from this cause in streams seem seldom, 

 if ever, to be found. 



The waters of streams on reaching inclosed lakes or inland seas evap- 

 orate and the mineral matter they contain is concentrated. Frequently, 

 as is well known, this process leads to precipitation and the formation of 

 deposits of calcium carbonate and calcium sulphate. A similar process 

 operates in lagoons shut off from the ocean, and the ocean itself is a vast 

 evaporating jar in which brine is concentrated both locally and under 



II — Bull. Geol. Soc. Am., Vol. 18, 1906 



