1082 A TREATISE ON METAMORPHISM. 



occur from them in consequence of change of temperature and pressure 

 than from descending solutions. 



When one attempts to apply these general statements to specific metals, 

 experimental data are lacking. It is undoubtedly true that decreasing tem- 

 perature and pressure are much more influential in the precipitation of some 

 metals than of others. Until experimental work has determined how the 

 various economic metals respond to changing temperature and pressure, it 

 is impracticable to specify the ores in which precipitation is more strongly 

 favored by decrease of temperature and pressure. One would expect that 

 precipitation as a consequence of changing temperature and pressure would 

 tend to give a somewhat orderly vertical distribution of the various metal- 

 liferous ores. 



As an illustration of the influence of a decrease of temperature Ave 

 may take the cases of gold and silver. It is well known that gold is 

 precipitated by cuprous chloride, according to the following reaction: 



AuCl 3 +3CuCl=Au+3CuCl 2 



Silver is precipitated by ferrous sulphate according to the following 

 reaction : 



Ag 2 S0 4 +2FeS0 4 =2Ag+Fe 2 ( S0 4 ) 3 



Under given conditions, if a sufficient amount of time be allowed, these 

 reactions will proceed until equilibrium is reached. Stokes has shown that 

 if the temperature is decreased, after equilibrium is reached, these reactions 

 will proceed further, but if the temperature is increased the reactions 

 reverse and the gold and silver are dissolved. Therefore, where for a 

 given temperature these solutions are saturated with silver and gold, and 

 the temperature is decreased, precipitation of the metals will result. 



PRECIPITATION BY MINGLING OF SOLUTIONS. 



Precipitation in the trunk channels is produced by reactions caused by 

 the mingling of various solutions. The mingled solutions may be gaseous 

 and aqueous, or all aqueous. Precipitation by reactions between aqueous 

 solutions and gases is especially likely to take place near the level of ground 

 water, where gases from the belt of weathering mingle with the solutions 

 of the belt of cementation. 



Precipitation by reactions between aqueous solutions is especially likely 

 to take place at the intersections of trunk channels, where aqueous solutions 



