72 GEOLOGY 



Solution and re-precipitation. Ore material is often leached out 

 of the surface-rock by water circulating slowly through it, and 

 carried on with the water until it reaches some substance which 

 causes a reaction that precipitates the ore material. This substance 

 may be a constituent of some rock which the circulating water en- 

 counters; but more commonly, the precipitation seems to be due 

 to the mingling of waters charged with different mineral substances, 

 the mingling inducing reactions resulting in the precipitation of 

 the ore. Precipitation, however, does not necessarily follow such 

 commingling. It is only when the mingling waters reduce the 

 solubility of the ore material sufficiently, that deposition takes 

 place. Changes of pressure and temperature also may enter into 

 the process. 



More concretely stated, the general process of underground 

 ore formation appears to be this: The permeating waters dissolve 

 the ore material disseminated through the rock, and carry it thence 

 into the main channels of circulation, usually the fissures, porous 

 parts, or cavernous spaces. If precipitating conditions are found 

 there, deposition takes place. The precipitating conditions may 

 be merely changes of physical state, such as cooling or relief of 

 pressure, but probably much more generally they consist in the 

 commingling and mutual reaction of waters that have pursued 

 different courses, and aro differently mineralized. . 



Location of greatest solvent action. Since solution precedes 

 deposition, the location of the greatest solvent action may be 

 noted. Water circulation is probably very slight below a depth 

 of two or three miles at most, and above that depth there is little 

 reason for supposing that the rocks of one horizon are more metal- 

 liferous than others of their kind. Thus there is no assignable 

 reason why the igneous or sedimentary rocks at the surface are not 

 as rich in ore material as the igneous rocks two or three miles l><>lo\v. 



Solvent action is probably greatest where the temperature and 

 pressure are highest, that is, in the deeper reaches of water circu- 

 lation; but the amount of water passing in and out of the deeper 

 zone is small compared with that of higher levels, and the total 

 solvent action is quite certainly much greater in the upper zone 

 than in the lower. At the same time, the solutions in the upper 



