CHEMICAL CONCENTRATION 15 



The habitat, so to speak, of residual mineral and rock fragments, 

 which are left when the more soluble associated material of the original 

 rocks is removed, is the surface portion of land areas to the depth of the 

 zone of weathering. Below the surface of saturation, or the water table, 

 solution and deposition are active processes, but the former is far less 

 prominent than the latter, and leached mineral or rock masses are not 

 prominent. As is familiar to miners the world over, residual and ox- 

 idized minerals and ores are limited in depth by the upper surface of the 

 zone of saturation, and below that horizon deposition resulting in con- 

 centration and not depletion is the rule. The search for residual concen- 

 trates is thus limited by a definite and easily determinable horizon, 

 unless a recent or comparatively recent rise of the level of ground water 

 has occurred. 



SURFACE AND SUBSURFACE PRECIPITATES 



The substances taken into solution by percolating water in the zone of 

 weathering present great variety. In fact, all known chemical elements 

 are present in greater or less abundance in surface and subsurface water. 

 The zone of weathering may truthfully be said to be a vast laboratory in 

 which the concentration of mineral matter is brought about not only on 

 account of chemical inertia at the temperatures present, as in the pro- 

 duction of residual concentrates, but a great variety of chemical precipi- 

 tates also result and furnish examples of concentrated material fre- 

 quently of great commercial importance. • 



EVAPORATION 



The concentration of mineral matter other than residual material, at 

 the earth^s surface and in the zone of weathering, through the dominant 

 agency of chemical processes results principally from evaporation, de- 

 crease in temperature, and chemical reactions. 



Evaporation leads to the precipitation of mineral matter from water 

 solutions to some extent while the water is in the zone of weathering, but 

 principally from surface water, in part returned to the surface after 

 short subterranean journeys, as from the surface of the land and from 

 streams, lakes, seas, and the ocean. Eesulting from this process, which is 

 coextensive with the earth's surface, many and important chemical con- 

 centrates are produced. As a rule, the substances laid aside in this man- 

 ner, in more or less well defined accumulations, are such as are readily 

 soluble, but are present in such abundance that evaporation quickly pro- 

 duces a state of saturation, as, for example, in the case of calcium car- 

 bonate, sodium chloride, and sodium sulphate. 



